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RSC Green Chemistry
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Editor-in-Chief:
Professor James Clark, Department of Chemistry, University of York, UK
Series Editors:
Professor George A Kraus, Department of Chemistry, Iowa State University, Ames,
Iowa, USA
Professor Andrzej Stankiewicz, Delft University of Technology, The Netherlands
Professor Peter Siedl, Federal University of Rio de Janeiro, Brazil
Professor Yuan Kou, Peking University, China
Titles in the Series:
1: The Future of Glycerol: New Uses of a Versatile Raw Material
2: Alternative Solvents for Green Chemistry
3: Eco-Friendly Synthesis of Fine Chemicals
4: Sustainable Solutions for Modern Economies
5: Chemical Reactions and Processes under Flow Conditions
6: Radical Reactions in Aqueous Media
7: Aqueous Microwave Chemistry
8: The Future of Glycerol: 2nd Edition
9: Transportation Biofuels: Novel Pathways for the Production of Ethanol,
Biogas and Biodiesel
10: Alternatives to Conventional Food Processing
11: Green Trends in Insect Control
12: A Handbook of Applied Biopolymer Technology: Synthesis, Degradation
and Applications
13: Challenges in Green Analytical Chemistry
14: Advanced Oil Crop Biorefineries
15: Enantioselective Homogeneous Supported Catalysis
16: Natural Polymers Volume 1: Composites
17: Natural Polymers Volume 2: Nanocomposites
18: Integrated Forest Biorefineries
19: Sustainable Preparation of Metal Nanoparticles: Methods and
Applications
20: Alternative Solvents for Green Chemistry: 2nd Edition
21: Natural Product Extraction: Principles and Applications
22: Element Recovery and Sustainability
23: Green Materials for Sustainable Water Remediation and Treatment
24: The Economic Utilisation of Food Co-Products
25: Biomass for Sustainable Applications: Pollution Remediation and
Energy
26: From C-H to C-C Bonds: Cross-Dehydrogenative-Coupling
27: Renewable Resources for Biorefineries
28: Transition Metal Catalysis in Aerobic Alcohol Oxidation
29: Green Materials from Plant Oils
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30: Polyhydroxyalkanoates (PHAs) Based Blends, Composites and
Nanocomposites
31: Ball Milling Towards Green Synthesis: Applications, Projects,
Challenges
32: Porous Carbon Materials from Sustainable Precursors
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Porous Carbon Materials from
Sustainable Precursors
Edited by
Robin J White
Universität Freiburg, FMF - Freiburger Materialforschungszentrum,
Freiburg im Breisgau and Institut für Anorganische und Analytische Chemie,
Freiburg, Germany
Email: robin.white@fmf.uni-freiburg.de
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RSC Green Chemistry No. 32
Print ISBN: 978-1-84973-832-3
PDF eISBN: 978-1-78262-227-7
ISSN: 1757-7039
A catalogue record for this book is available from the British Library
© The Royal Society of Chemistry 2015
All rights reserved
Apart from fair dealing for the purposes of research for non-commercial purposes or for
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Preface
We currently live in the era of the Anthropocene – the time period where the
actions and consequences of human society and practices are the predominant geological, environmental and climate driving forces. As a consequence,
the terms “sustainable” and “sustainability” are increasingly becoming common terms in the chemical industry and sociopolitical discussions as we
look to a future beyond unsustainable fossil-based resources. These terms,
at least in the public domain, are typically associated with the agricultural
industry but also with ever-increasing exposure in the context of energy, fuel
and consumer product provision. This latter point relates to societies dependence on fossil-fuel-derived products (e.g. gasoline, plastics, etc.) and their
ever-dwindling reserves. Therefore, human society is looking to the development of innovative and “green” technologies to address the challenge of
providing for an ever-increasing population. This must be approached without reducing the capability of future generations to live in the manner in
which the western world is accustomed and allow the developing world to
achieve the same standards. Underpinning a “sustainable” society will the
development of innovative materials ideally based on abundant precursors/
elements, synthesised in a “green” manner, providing the necessary application behaviour suitable to provide sustainable energy, chemicals and products for society.
Perhaps one of, if not the most, important elements of the sustainable
challenge, is carbon. In the context of our existing and hopefully a future
sustainable society, carbon will be present in many forms. For example, CO2
is the major product of the combustion of carbon-based fossil fuels that is
driving the greenhouse effect and associated changes in the global climate.
In a closed cycle, CO2 could be taken up by photosynthetic organisms (e.g.
green plants) and converted to solid sinks of carbon (e.g. biomass) ultimately
transforming back into the fossil deposits that underpin our current society.
RSC Green Chemistry No. 32
Porous Carbon Materials from Sustainable Precursors
Edited by Robin J White
© The Royal Society of Chemistry 2015
Published by the Royal Society of Chemistry, www.rsc.org
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Preface
However, the latter process unfortunately occurs on a time scale that is not
relevant to human society, with consumption occurring at a rate resulting
in the near exploitation of reserves by the end of the current century. Therefore, new cyclical, carbon-neutral energy and chemical provision schemes
are needed to allow the establishment of a sustainable society. As will be
briefly introduced in this book, alternative schemes such as the Biorefinery
and Methanol Economy have been proposed and aspects of each scheme are
gradually entering the energy, fuel and chemical market place. Notably, both
these alternative future economies rely on CO2 and its natural derivatives (e.g.
biomass). Furthermore, as for existing industrial practices, these new economies will require the development of increasingly efficient, active materials
to perform the conversion, catalysis and separation processes needed to synthesize and produce the myriad of (e.g. carbon-based) products that modern
society demands.
The establishment of a sustainable society represents one of the greatest
challenges facing human kind and will require the transition from an essentially resource consumptive economy to a new sustainable, carbon-neutral
(and even carbon-negative) approach based on renewable resources to deliver
the material, chemical and energy demands of a modern sustainable society. To establish sustainable chemical, energy and fuel provision, the development and ultimately implementation of innovative sustainable chemical
practices is required. With regard to current energy and chemical provision,
fossil-based industries (e.g. petrorefineries, coal-fired power stations, etc.) still
dominate, although their viability will dwindle over the coming century. To
counteract this reduction, future energy/chemical supply from renewable
energies (e.g. solar, water, wind, etc.) will dramatically increase over the same
time frame. This in itself creates challenges regarding cost-efficient energy
storage and transportation. In this regard, there are several options by which
to generate and store renewable energy provided by the sun, wind, water,
geothermal or biomass sources. These solutions will all be reliant on porous
materials technologies to allow for their efficient implementation and conversion and storage of photonic, thermal or kinetic energy into suitable chemical
energy-vector molecules. The materials developed to address this task should
be low cost, scalable, industrially and economically attractive, based on renewable and highly abundant resources, whilst of course achieving application
performances that exceed the performance of existing technologies.
In this regard, new porous materials (e.g. catalysts, separation media, etc.)
with improved properties relative to the current state of the art are required,
synthesised with as minimal a carbon footprint as possible. If these synthetic
approaches can be based on sustainable resources (e.g. biomass), abundant
elements (e.g. C, N, P, etc.) and sustainable synthetic techniques, this has the
potential to aid the overall “C” balance in the material or chemical synthesis
pathway. This book aims to demonstrate how this might be possible via a
number of emerging approaches, predominantly focusing on carbon-based
materials. Furthermore, from a materials-chemistry perspective, sustainable
biomass precursors appear to be excellence platform compounds from which
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Preface
ix
to synthesise a variety of carbon-based materials, particularly when one considers the range of naturally occurring nanostructures and also the range
of opportunities that molecules including the saccharides, polysaccharides,
nucleotides, and proteins offer, (e.g. in terms of functionality, self-assembly
properties, etc.). It is in this context that this book draws its inspiration.
The natural products of CO2 capture and recycling, namely biomass,
as this book will highlight, can be transformed into useful, applicable,
carbon-based porous materials. It is also important to highlight the significance of porous materials in both current energy and chemical provisions
(e.g. the petrorefinery) and the aforementioned alternative future provisions schemes. In the context of sustainable precursors and specifically
biomass-derived compounds, their direct conversion into carbon-based
materials typically requires high temperatures and/or activation agents,
resulting in the production of either nonporous or predominantly microporous, hydrophobic materials. As an example, such materials may not be
suitable for the aqueous phase catalysis required in the Biorefinery, although
they may find application in gas-separations challenges of the Methanol
Economy. Therefore, there is a need to develop new synthetic practices with
regard porous carbon materials that enable control over physicochemistry (e.g. surface functionality, conductivity, hydrophobicity, etc.) in tandem
with material texture and porosity (e.g. micro- vs. mesoporosity, hierarchical
structuring, particle morphology, etc.). The following chapters will introduce
to the reader solutions to this problem based on the use of sustainable precursors and technologies, allowing the preparation of a range of materials
with properties that have the potential to fill the “gap” between classical inorganic and organic materials (e.g. mesoporous silica vs. microporous carbon).
The book introduces approaches to this end using sustainable, predominantly biomass-derived precursors, with a particular focus on the two leading synthetic approaches; namely Starbon® technology and hydrothermal
carbonisation (HTC). The book features contributions from a global collective of up and coming young scientists revealing the wide range of materials
and applications that are possible using the aforementioned synthetic platforms and derivations therefrom. The book starts with an introduction from
the Editor, providing a context for the following chapters with regards to the
demands of future energy and chemical provision schemes, whilst highlighting the material demands of these cyclical economies. Part 1 provides
contributions on the topic of Starbon® technology, developed and elaborated
predominantly at the Green Chemistry Centre of Excellence, University of
York, (York, UK), demonstrating the exciting porous properties of these polysaccharide-derived materials, with promising application in aqueous-phase
heterogeneous catalysis (e.g. esterification of succinic acid) and separations
science (e.g. separation of polar sugar analytes). Part 2 covers the topic of
hydrothermal carbonisation as a platform for the conversion of biomass to
porous carbonaceous materials, a topic initially reinvigorated by researchers
from the Max Planck Institute for Colloids and Interfaces, (Golm, Germany),
which has now proliferated, as reflected by the authors assembled, to the
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Preface
different corners of the scientific globe. Hydrothermal carbon materials are
discussed in the context of porosity development (e.g. templating, gelation,
etc.) and functionality control (e.g. heteroatom doping), with the resulting
materials discussed in the context of applications in heterogeneous catalysis,
electrochemistry (e.g. battery electrodes, metal-free oxygen-reduction reaction, etc.) and gas sorption (e.g. CO2 capture). Part 3 introduces and discusses
the challenges and analytical techniques associated with the development
and characterisation of the innovative porous carbon materials discussed
in Parts 1 and 2 (e.g. gas sorption, microscopy, etc.). Finally, the book concludes in Part 4, with a brief review of more recent, emerging platforms for
the synthesis of porous carbons from sustainable precursors that are still
in their infancy (at the time of writing). Part 4 also provides an overview of
the commercial efforts underway to bring porous carbon materials sustainably from the laboratory curiosity to industrial-scale products (e.g. Starbon®
Technologies).
This book is aimed at a broad readership, encompassing advanced undergraduates, graduates, researchers and industrialists alike whose interests lie
in the topics of renewable energy, nanomaterials, sustainability, green chemistry, and functional/porous materials. The book brings together, for the first
time in one volume, the different approaches to porous carbons synthesised
from sustainable precursors and hopefully as such provides the reader with
an indepth account of the benefits and applications of converting biomass
and biomass-derived precursors into functional, porous carbon-based nanomaterials for a variety of increasingly topical applications.
The Editor would like to express his gratitude to all the contributing
authors who have given their time and effort to writing their respective chapters. The Editor would also like to thank the publication team of the Royal
Society of Chemistry for all their help in assistance in bringing this book to
print – it is very much appreciated.
Robin J. White
Freiburg, Germany
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Contents
Introduction
Chapter 1 The Search for Functional Porous Carbons
from Sustainable Precursors
Robin J. White
1.1 Introduction and Sustainable Precursors
1.2 Principles of Green Chemistry and Sustainability
1.3 Future Energy/Chemical Economies and Sustainable
Materials
1.3.1 The Methanol Economy
1.3.2 The Biorefinery Concept
1.4 A Brief History of (Porous) Carbon Materials from
­Sustainable Precursors
1.4.1 General Aspects of Porous Materials
1.4.2 Activated Carbons
1.4.3 Mesoporous Carbons
1.4.4 Carbon Aerogels and Related Materials
1.4.5 Graphitic Nanocarbons – Carbon Nanotubes
and Graphene
1.4.6 Ionic Liquids
1.4.7 Hierarchically Porous Carbons Synthesised
in Deep Eutectic Solvents
1.4.8 Exploitation of Polysaccharide Chiral
Nematic Phases
1.5 Overview and Outlook of the Book
References
RSC Green Chemistry No. 32
Porous Carbon Materials from Sustainable Precursors
Edited by Robin J White
© The Royal Society of Chemistry 2015
Published by the Royal Society of Chemistry, www.rsc.org
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Part 1: Starbons®
Chapter 2 From Polysaccharides to Starbons®
Vitaliy L. Budarin, Peter S. Shuttleworth,
Robin J. White and James H. Clark
2.1
2.2
2.3
2.4
2.5
Introduction
Porous Polysaccharide-Derived Materials
First-Generation Starbons® – from Starch to Carbon
Second-Generation Starbons®
2.4.1 Pectin-Derived Starbons®
2.4.2 Chitosan-Derived Starbons®
2.4.3 Alginic Acid-derived Starbons®
The Synthesis of Starbons – Mechanistic
Considerations
2.6 Outlook and Conclusions
Acknowledgments
References
Chapter 3 Porous Carbonaceous Materials in Catalytic Applications
Rick A. D. Arancon, Duncan Macquarrie and Rafael Luque
3.1 Introduction
3.2 Biomass-Derived Porous Carbonaceous Materials
3.3 Sulfonated Starbons® and Carbonaceous
Materials as Solid Acids
3.4 Other Routes to the Introduction of Mesoporosity
and Associated Applications
3.5 Ordered Porous Carbonaceous Materials
3.6 Application in Hydrogenation Reactions
3.7 Biofuel Synthesis
3.8 Photocatalysis
3.9 Conclusions and Prospects
Acknowledgments
References
Chapter 4 Application of Carbonaceous Materials in Separation
Science
Andrew S. Marriott, Carla António and Jane Thomas-Oates
4.1 Introduction
4.2 Background to High-Performance Liquid
­Chromatography (HPLC) and Introduction
of Porous Carbon Stationary Phases
4.2.1 Overview
4.2.2 Efficiency of Column Separation
4.2.3 Requirements for the “Ideal” Stationary-Phase
Material
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4.3 Introduction of Porous Carbon Stationary Phases
4.3.1 Porous Graphitic Carbon and its Application
4.3.2 Chromatographic Applications of Porous
Graphitic Carbon
4.3.3 Synthesis and Drawbacks of Porous Graphitic
Carbon
4.4 Sustainable Porous Carbons in Separation Science
4.4.1 Starbons®
4.4.2 Alginate-Derived Mesoporous Carbon
Spheres (AMCS)
4.5 Does a Sustainable Porous Carbon Need to be
Graphitic?
4.6 Other Sustainable Carbons in Chromatography
4.6.1 “Chocolate” Hydrophilic Interaction Liquid
­Chromatography (HILIC)
4.6.2 Carbon Coating of Silica Particles
4.7 Future Perspectives
4.8 Conclusions
References
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Part 2: Hydrothermal Carbonisation (HTC)
Chapter 5 Hydrothermal Carbonisation (HTC): History,
State-of-the-Art and Chemistry
Adam Marinovic, Filoklis D. Pileidis and
Maria-Magdalena Titirici
5.1 Introduction
5.2 State-of-the-Art
5.3 Humins and Associated Materials
5.4 Societal and Commercial Aspects
5.5 Chemistry behind the Formation of HTC Materials
5.6 Outlook and Conclusions
References
Chapter 6 Porous Hydrothermal Carbon Materials,
Nanoparticles, Hybrids and Composites
Nicolas Brun, Shu-Hong Yu and Robin J. White
6.1 Introduction
6.2 Activated Hydrothermal Carbons
6.3 Porous HTC via Hard Templating: Premade
Sacrificial Inorganic Moulds
6.3.1 Silica-Based Hard Templates
6.3.2 Nonsilica-Based Hard Templates
6.4 Porous HTC via Soft Templating
6.4.1 Supramolecular Self-Assemblies: From
OMCs to Hybrid Hollow Spheres
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6.5 O
il-in-Water Macroemulsions: From Hybrid
Hollow Spheres to Carbo-HIPEs
6.5.1 Diluted Macroemulsions
6.5.2 Concentrated Macroemulsions
6.6 Polystyrene Latex Dispersions: From Hollow
Spheres to Coral-Like Structures
6.7 Template-Free Hydrothermal Carbon Hydrogels
and Related Dried Gels
6.7.1 Salt-Mediated Hydrothermal Gelation
Approaches
6.7.2 Ovalbumin-Derived Gelation Approach
6.7.3 Phenolic-Derived Gelation Approaches
6.7.4 Carbon Nanotubes-Assisted Structure
Formation
6.8 Porous Carbons from Direct Hydrothermal
Treatment of Natural Systems
6.8.1 Natural Scaffolds with in situ Hard Templates
6.8.2 Natural Scaffolds without in situ Hard
Templates
6.9 Biomass-Derived HTC Nanodots and
Nanocomposites
6.10 Summary and Outlook
References
Chapter 7 Hydrothermal Carbon Materials for Heterogeneous
Catalysis
Li Zhao, Pei-Wen Xiao and Bao-Hang Han
7.1 Introduction
7.2 Heteroatom Functionalised HTC Materials in
­Heterogeneous Catalysis
7.3 Nitrogen-Containing Carbons in Catalysis
7.4 Sulfur-Doped Carbons for Catalysis
7.5 Other Heteroatom-Doped Carbons in Catalysis
7.6 HTC-Supported Metal Complexes or
Nanoparticle-Based Catalysis
7.7 HTC in Photocatalysis
7.8 Other Catalysis
7.9 Conclusions
References
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Chapter 8 HTC-Derived Materials in Energy and Sequestration
­Applications
Rezan Demir-Cakan and Marta Sevilla
8.1
8.2
8.3
Introduction – Energy Storage
Electrodes in Supercapacitors
Electrocatalysts in Fuel Cells
8.3.1 Anode Catalyst Supports in Direct Methanol
Fuel Cells
8.3.2 Catalysts for the Oxygen-Reduction
Reaction (ORR)
8.4 Electrodes in Rechargeable Batteries
8.4.1 Li-Ion Batteries
8.4.2 Anode Materials
8.4.3 Cathode Materials
8.4.4 Na-Ion Batteries
8.4.5 Li–S Batteries
8.5 CO2 Capture
8.6 Conclusion
References
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Part 3: Characterisation of Porous Carbonaceous Solids
Chapter 9 Porosity Characterisation of Carbon Materials
Jens Weber
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9.1 Introduction and Definitions
9.2 Definitions
9.3 Methods
9.3.1 Gas Adsorption Techniques
9.3.2 Scattering and Diffraction Methods
9.3.3 Microscopy
9.3.4 Other Methods
9.4 Conclusion
References
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Chapter 10 Bulk and Surface Analysis of Carbonaceous Materials
Peter S. Shuttleworth, Niki Baccile, Robin J. White,
Eric Nectoux and Vitaliy L. Budarin
10.1
10.2
Introduction
Thermal Gravimetric Analysis
10.2.1 Introduction
10.2.2 Results and Discussion
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10.3
10.4
10.5
10.6
10.7
X-Ray Photoelectron Spectroscopy (XPS)
10.3.1 Introduction
10.3.2 Elemental Analysis
10.3.3 High-Resolution C1s Spectra
10.3.4 High-Resolution O1s Spectra
10.3.5 Discussion
Infrared (IR) Spectroscopy
10.4.1 Introduction
10.4.2 Experimental
Boehm Titration
10.5.1 Introduction
10.5.2 Experimental
10.5.3 Results and Discussion
Bromination
10.6.1 Introduction
10.6.2 Experimental
10.6.3 Results and Discussion
Solid-State Nuclear Magnetic Resonance (ssNMR)
10.7.1 Introduction
10.7.2 Fullerenes and Nanotubes
10.7.3 Lignin, Cellulose and Their Chars from
Pyrolysis
10.7.4 Carbonaceous Materials Prepared via
Hydrothermal Processing
10.8 Linear Solvation Energy Relationship Analysis
Using 19F MAS NMR Spectroscopic Probes
10.8.1 Validation of 19F MAS NMR Spectroscopic
Probing Method
10.8.2 Theoretical Background of 19F MAS NMR
Probe Spectroscopy
10.8.3 Estimation of the Electromagnetic Term
(δshielding)
10.8.4 Estimation Adsorption of Aliphatic and
Aromatic Reporter Molecules
10.9 Conclusion
References
Chapter 11 Microscopy and Related Techniques in the Analysis
of Porous Carbonaceous Materials
Shiori Kubo and Noriko Yoshizawa
11.1
11.2
Introduction
Tutorial Overview of a TEM Technique
11.2.1 TEM as a Visualisation Tool
11.2.2 TEM as a Tool for Analysing Nanostructure
of Porous Carbonaceous Materials
11.2.3 Electron Tomography – “3D-TEM”
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11.3 E
xamples of Microscopy Analyses of Porous
Carbonaceous Materials
11.3.1 Ultramicrotome and TEM in the Analysis
of ­Nanostructured Porous Carbonaceous
Materials
11.3.2 TEM Tomography in the Analysis of
­Nanostructured Carbonaceous Materials
11.4 Conclusion
References
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Part 4: Commercialisation
Chapter 12 Other Approaches and the Commercialisation
of Sustainable Carbonaceous Material Technology
Robin J. White, Vitaliy L. Budarin and Peter S. Shuttleworth
12.1 Introduction
12.2 Other Innovative Approaches to Porous Carbons
from Sustainable Precursors
12.2.1 Bacterial Cellulose
12.2.2 Filamentous Fungi
12.2.3 Gelatin
12.2.4 Silk Cocoon
12.2.5 Flavonoids and Tannin
12.2.6 Lignin
12.2.7 Ionic Liquids as a Solvent in Hydrothermal
­Carbonisation
12.3 Commercialisation of Sustainable Carbon
Materials
12.3.1 Starbon® Technologies Ltd
12.3.2 Hydrothermal Carbonisation
12.4 Summary and Outlook
Acknowledgments
References
Subject Index
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INTRODUCTION
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CHAPTER 1
The Search for Functional
Porous Carbons from
Sustainable Precursors
ROBIN J. WHITE
Universität Freiburg, FMF - Freiburger Materialforschungszentrum,
Stefan-Meier-Straße 21, 79104 Freiburg im Breisgau. Institut für
Anorganische und Analytische Chemie, Albertstrasse 21,
79104 Freiburg, Germany
E-mail: robin.white@fmf.uni-freiburg.de
1.1
Introduction and Sustainable Precursors
We currently live in the era of the “Anthropocene” – the interval of global environmental change, induced as a consequence of human activity; predominantly the result of fossil-fuel combustion and the emission of greenhouse
gases (GHG) (e.g. CO2, CH4, etc.).1–3 We must limit this change and ultimately
reverse the direction of these emissions. Therefore, there is a need to transit
from a petro- to alternative energy/chemical delivery schemes – perhaps best
epitomised by the German concept of the “Energiewende”.4–9 To stay below
a mean global temperature increase of 2 °C (considered “safe”) Germany –
an example of a developed nation – has calculated that it must reduce its
own CO2 emissions by ca. 95% across all sectors including transportation,
energy and chemicals (relative to levels in 1990) by 2050.10 Put simply, this
represents one of the greatest challenges facing our society and will require
the transition from an essentially resource consumptive economy to a new
sustainable, carbon-neutral (and even carbon-negative) approach based on
RSC Green Chemistry No. 32
Porous Carbon Materials from Sustainable Precursors
Edited by Robin J White
© The Royal Society of Chemistry 2015
Published by the Royal Society of Chemistry, www.rsc.org
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renewable resources to deliver the material, chemical and energy demands of
modern sustainable society. This transition will require scientists to develop
and ultimately implement sustainable chemical practices in the production
of energy and chemical products. To increase the efficiency of synthesis and
production, new nanomaterials (e.g. catalysts, separation media, etc.) with
improved properties relative to the current state-of-the-art will be required.
At this point, it is important to note that the production of such materials will
have to be conducted in a manner which reduces the carbon footprint of a
given product. Therefore, if new nanomaterials can be synthesised based on
renewable resources, abundant elements and sustainable synthetic routes,
this can potentially aid the overall “C” balance in the material or chemical
synthesis pathway, provided the synthetic routes themselves offer advantages (e.g. reductions) in terms of energy and resource consumption.
With regard to our current energy and chemical provision, fossil-based
industries (e.g. petrorefineries, coal-fired power stations, etc.) still dominate, although their viability will dwindle over the coming century. Many
nations are considering future energy/chemical supply and it is the general
consensus that the contribution to worldwide consumption from renewable
energies (e.g. solar, water, wind, etc.) should dramatically increase during the
same time frame. This in itself creates challenges regarding cost-efficient
energy storage and transportation. In relation to liquid energy fuels (e.g. for
transportation) and feed stocks for the chemical industry, alternative platform chemicals will have to be explored in the near future. Independently of
source, chemical compounds – derived from fossil or renewable resource – will
at some point in their life cycle, require a chemical transformation, conversion
and/or separation. Here, the use of industrially applicable nanomaterials
(i.e. catalysts) is an absolute necessity, to provide efficient access (i.e. via lower-energy pathways) to the range of precursors and compounds required to
meet growing consumer demand, particularly in the developing world.
Therefore, to address future energy demand of an ever-increasing global
population and to address the environmental consequences of a fossil fuelbased society, very innovative and increasingly sustainable solutions must be
found.11 In this regard, there are several options by which to generate and
store renewable energy provided by the sun, wind, water, geothermal or biomass sources. These solutions will all be reliant on materials technologies to
allow for their efficient implementation and conversion and storage of photonic, thermal or kinetic energy into suitable chemical energy vector molecules – e.g. an artificial photosynthesis.12 The materials developed to address
this task should be low cost, scalable, industrially and economically attractive,
based on renewable and highly abundant resources, whilst of course achieving application performances in renewable energy conversion or environmental applications that exceed the performance of existing technologies.
Concurrently with the opening of fossil-fuel exploitation and the development of associated industries during the first quarter of the 20th century,
the majority of carbon-based materials have been typically synthesised using
fossil-derived precursors (excluding some activated carbons). During the same
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period there was also extensive academic and industrial interest in the exploitation of renewable lignocellulosic biomass for the production fuels, chemicals
and materials. As will be discussed in Part 2, a Nobel Prize was awarded to
Friedrich Bergius for his contribution to the invention and development of
chemical high-pressure methods, particularly the production of synthetic fuel
from coal.13,14 He was also interested in the natural processes leading to the
formation of coal from biomass. And it is from this work that the first “hydrothermal carbonisation” experiments on cellulose were performed – a synthetic
coalification. However, as a consequence of the abundance of chemicals and
fuels being produced by the burgeoning petrochemical industries of the first
half of the 20th century, this work and indeed fuels and materials derived
from sustainable sources was largely forgotten. From a materials-chemistry
perspective at least, this seems rather odd particularly when one considers the
variety of naturally occurring nanostructures and also the range of opportunities that molecules including the saccharides, polysaccharides, nucleotides,
and proteins provide in the synthesis of new (predominantly carbon-based)
materials. Indeed Koopman et al. and others (including the US Department of
Energy) have commented on the significant potential of biomass-derived platform molecules in the context of future chemical and energy industries.15,16 It
is in this context that this book draws its inspiration.
After oxygen, carbon is the most abundant element within the natural
environment. Natural systems use these elements coupled with hydrogen as
the chemical platform for renewable energy storage (e.g. photosynthetic carbohydrates). Analogously, carbon-based materials are performing an increasingly more important role in renewable energy conversion technologies:
electrodes in energy-storage devices, electrocatalysis, photocatalysis, heterogeneous catalysis, biofuels, etc. (Figure 1.1).17,18 Carbon-based materials are
also extensively used in water purification, gas separation (e.g. CO2 capture)/
storage, and as a soil additive.19–25
The importance and potential of these carbon-based materials is best exemplified by the award of the some of the highest scientific awards to carbon
materials scientists including the 1996 Nobel Prize in Chemistry (fullerenes),
the 2008 Kavli Prize in Nanoscience (carbon nanotubes), and the 2010 Nobel
Prize in Physics (graphene). Generally speaking, modern carbon materials
and the carbon allotropes (e.g. nanotubes, fullerenes, graphene, etc.) are not
typically derived from renewable resources and require complex synthetic
approaches that are often difficult to scale, leading from a material chemistry
perspective, to condensed, hydrophobic carbon structures.27–31 Furthermore,
these structures are, as will be discussed throughout this book, arguably not
suitable for the challenges presented by future chemical/energy provision
schemes (e.g. the aqueous-phase-based chemistry of the Biorefinery).32–34
In the context of new material development, nanomaterials currently used
industrially are typically optimised for small hydrophobic molecule transformations (e.g. microporous zeolites, Pd/carbon black, etc.) and in certain sectors (e.g. pharmaceutical), they are based on precious metals (e.g. difficult to
recycle homogeneous catalysts), whilst the chemistry is normally performed
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Figure 1.1
Classical
and potential applications of porous carbon materials. Reproduced with permission from ref. 26.
in harmful, volatile organic solvents. Therefore, to increase efficiencies of
existing sectors and to elaborate sustainable energy and chemical provision
in the near future, the development of new functional, sustainable nanomaterials must be conducted in the context of the following challenges:
1. Based on sustainable, globally accessible precursors.
2. Capable of storing and converting stable CO2 (e.g. hydrogenation at low
temperatures and pressures).
3. Tuneability – to enable optimisation of structure, chemistry and activity for performance in a wide range of environments (e.g. aqueous,
acidic, organic and high-temperature gas phases).
4. Capable of alcohol/saccharide transformations (e.g. dehydrogenation).
5. Facilitate the substitution of precious metal catalyst with more sustainable alternatives (e.g. nonrare metals, metal-free, organo-catalysis).
6. Produced via industrially attractive, cost effective routes based on the
principles of Green Chemistry.
In this regard, nanomaterial synthesis based on abundant carbon-rich
biomass (and derived compounds) seems appropriate and as will be demonstrated throughout this book, is increasingly being considered as a promising route to porous, functionally tuneable, carbon-based nanomaterials.26
Green photosynthetic plants convert atmospheric CO2 and water to saccharides (e.g. glucose, amylose, cellulose), and poly(aromatics) (e.g. lignin) –
­lignocellulosic biomass – the natural CO2 sink or sequestration mode. If
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these natural precursors are converted to other (more stable) carbon forms
(e.g. nanomaterials), perhaps mimicking natural “coalification” processes,
this would represent in effect the sequestering of CO2 in (potentially useful)
solid materials.32,35,36 If renewable resource-derived nanomaterial synthesis
is achieved with as low a carbon footprint as possible, coupling the resulting
biomass-derived nanomaterials in a given chemical process (e.g. catalysis),
provided there are application advantages (e.g. improved yield, turn over
number, catalyst lifetime), would be a serious sustainable innovation. Furthermore, if such nanomaterials are used to mitigate GHG emissions (e.g.
as catalysts to couple H2 and CO2 to form CH3OH) or replace scarce, expensive rare metals, this would potentially represent a combined, synergistic
carbon capture and utilisation (CCU) and sustainable materials approach,
which could ultimately improve the CO2 balance of a given process. It is the
integration of these themes that provides an opportunity for the chemical
and energy industries to “future proof” themselves as fossil reserves become
increasingly unavailable. If, in such a scenario, abundant elements, renewable resources and sustainable synthetic pathways are used, then CO2 cycling
loops (e.g. biomass utilisation, biomass-to-fuels, methanol cycling) can be
established, utilising the materials component as a more permanent sequestration point, whilst also transforming this GHG from a liability to an energy
gas of the future.
With regard to sustainable precursors, biomass is the most abundant
renewable resource in the biosphere. Dry terrestrial biomass growth has
been estimated previously to be ca. 118 billion tonnes per year,37 with 14
billion tonnes coming from agricultural production of which ca. 80% is
essentially considered as waste. Therefore, there are significant volumes of
sustainable biomass (e.g. at relatively low costs) that can be used in the production of fuels, chemicals and materials. Biomass as a precursor for industrial practices will also become increasingly more attractive as the fossil
reserves continue to dwindle in the coming century, and can be envisaged to
be utilised in a number of ways. Perhaps the greatest option for sustainable
biomass conversion is the production of liquid fuels (“biofuels”), which has
the potential to radically reduce CO2 emissions in transportation sector. The
conversion of biomass to liquid fuels has been reported using gasification,
fermentation and catalytic liquefaction.38–42 One key aspect of the “Principles
of Green Chemistry” relates to the conversion of biomass to alternative platform chemicals for the synthesis of modern consumer chemical products
(e.g. pharmaceuticals, polymers, surfactants, etc.).43,44 In this regard, white
biotechnology (i.e. micro-organisms) can be used to enzymatically catalyse
the synthesis of high-purity materials (e.g. enantiomers) at typically low temperatures and pressures. The classic example of this approach is the production of polylactic acid.45 Whilst the areas of biofuel and platform chemicals
can be considered the dominant themes in the context of biomass conversion, the conversion of such sustainable precursors into carbon-based
materials and more specifically porous variants, is a rapidly growing area
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of research and indeed of commercial interest, as the potential properties
of carbon-based materials (e.g. metal-free electrocatalysts) are considered to
be beneficial in the development of sustainable technologies and energy/
chemical provision.26,46–49
It is perhaps appropriate to note that the materials chemist can draw inspiration from a variety of natural processes (e.g. coalification, photosynthesis)
and structures (e.g. crustacean shells, plant tissues, etc.) to produce the variety of complex nanostructures materials needed to address the energy and
chemical challenges of a future sustainable society. In this context, this book
aims to introduce the reader to the latest progress in the synthesis of porous
carbon materials from sustainable precursors via processes conforming to
the principles of Green Chemistry.32,33,43,44 The latest trends in the synthesis, characterisation and application of novel carbon materials are presented
with contributions from the leaders of this rapidly expanding field. In this
context, it is important to note the necessity in modern-day research and
development of global networks to generate and iteratively improve the innovative solutions required for a sustainable and clean future for all – and in the
context of this book, a sustainable base from which to develop present and
future carbon nanomaterials science.
1.2
Principles of Green Chemistry and Sustainability
It is generally agreed from both the political and the chemical viewpoint
that if society is to progress towards a sustainable and “greener” future,
chemists and chemical engineers must develop new routes, synthesis and
processes that enable the products desired by the modern age, in the most
efficient and clean a manner as possible. Green Chemistry defines the area
of chemical science research that aims to achieve this, challenging conventional theory and practices from the stand-point of improvement, innovation and resource preservation. Green Chemistry has been described
perhaps most elegantly by one of the main protagonists in the field. Paul
Anastas in his defining text “Green Chemistry: Theory & Practice” defined
Green Chemistry as:50
“… the utilisation of a set of principles that reduces or eliminates the use or
generation of hazardous substances in the design, manufacture and application
of chemical products”
However, if the modern chemical industry is to adopt “greener” chemical routes, processes and technologies, the advantages and benefits of
such strategies must be well developed, demonstrated and ultimately cost
effective. In essence, for a new synthesis or material to have a real impact
it must show an improvement on existing practice, whether from an economic or application viewpoint (ideally both), at every level of the product
supply chain. Therefore, reducing the environmental impact must stand
side by side with presenting a competitive market advantage, if the adoption of a “Green” route is to occur. Green Chemistry is therefore in essence
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a form of pollution prevention and has a certain synergy with the concept
of Sustainability:51
“Given reasonable assumption concerning progress in the technology and the
activities of a civilisation, a sustainable civilisation is one in which the net sum of
the daily activities of the people who comprise it, individually and collectively, can
be carried on into the indefinite future without undermining the ability of future
generations to leave with at least a comparably advantageous welfare.”
Therefore the goals and concepts of sustainability and Green Chemistry are
intertwined, a fact recognised by the United Nations as long ago as 1992, in
the Rio Declaration proposed at the Environment and Development summit:52
“Human beings are at the centre of concerns for sustainable development. They
are entitled to a healthy and productive life in harmony with nature.”
In 2002, Anastas and Kirchoff took these ideas one step further and suggested that:44
“The challenge of sustainability will be met with new technologies that provide
society with products we depend on in an environmentally responsible manner.”
Therefore, such definitions may be combined to produce what has been
described by socioeconomists as the “Triple Bottom Line” principle, which
defines that the sustainability of a particular chemical or chemical reaction can be divided into three facets: Social, Environmental and Economic
Factors.50 Therefore, when approaching a new chemical process, all three
aspects must be addressed if a process is to be considered green. In discussing the ideas and basis for the Green Chemistry movement, Anastas and Warner proposed the following twelve founding principles:
1.It is better to prevent waste than to treat or clean up waste after it is
formed.
2.Synthetic methods should be designed to maximise the incorporation
of all materials used in the process into the final product.
3.Wherever practicable, synthetic methodologies should be designed
to use and generate substances that possess little or no toxicity to
human health and the environment.
4.Chemical products should be designed to preserve efficacy of function while reducing toxicity.
5.The use of auxiliary substances (e.g. solvents, separation agents, etc.)
should be made unnecessary wherever possible and, innocuous when
used.
6.Energy requirements should be recognised for their environmental
and economic impacts and should be minimised. Synthetic methods
should be conducted at ambient temperature and pressure.
7.A raw material or feedstock should be renewable rather than depleting wherever technically and economically practicable.
8.Unnecessary derivatisation (i.e. blocking group, protection/deprotection, and temporary modification of physical/chemical processes)
should be avoided whenever possible.
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9.Catalytic reagents (being as selective as possible) are superior to stoichiometric reagents.
10.Chemical products should be designed so that at the end of their
function they do not persist in the environment and break down into
innocuous degradation products.
11.Analytical methodologies need to be further developed to allow for
real time in process monitoring and control prior to the formation of
hazardous substances.
12.Substances and the form of a substance used in a chemical process
should be chosen so as to minimise the potential for chemical accidents, including releases, explosions and fires.
These twelve principles have a number of primary aims – the main emphasis being the idea of reduction at every step in a process, see Figure 1.2.
Therefore, when approaching a chemical process or synthesis, the chemist
or chemical engineer should consider the twelve principles and ultimately
aim to reduce consumption and production of wastes during all synthetic
steps or processes. This approach applies equally to the synthesis of a new
chemical compound and solid material. A new process must therefore aim
to be:
●●
●●
●●
●●
●●
●●
●●
●●
atom and C efficient;
ideally “one step” (or as few steps as possible;
safe (avoidance of hazardous reagents or conditions);
environmentally acceptable and legal;
not reagent wasteful;
simple and elegant;
use readily available materials;
aim for 100% yield/conversion.
Therefore, with regard to the topic and content of this book, when considering the synthesis of a new porous carbon, the use of sustainable renewable
precursors and the utilisation/valorisation of waste(s) (e.g. biomass) appear
appropriate, whilst water or biosolvents (e.g. bioethanol) can be considered
suitable preparative media. These points combined with synthetic strategies
Figure 1.2
The
primary aim of Green Chemistry in the context of a chemical synthesis – to reduce. Adapted from ref. 53.
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that aim to use or incorporate the entire “C” of the reagents in the final product or products should hopefully help in reducing the carbon footprint of
material preparation and concurs with the ideas of Green Chemistry and sustainability. When the resulting materials are capable of demonstrating application benefits compared to state-of-the-art equivalents, this will emphasise
the potential of porous carbons synthesised from sustainable precursors.
1.3
Future
Energy/Chemical Economies and
Sustainable Materials
As mentioned earlier in this chapter, we are currently living in the era of the
Anthropocene and as such we have the opportunity to ultimately dictate the
manner in which we provide society with the energy and chemical products
that it desires. It is becoming increasingly clear, primarily as a result of the
observed consequences of human-driven climate change and the future
inaccessibility and finite nature of a current platform, the fossil fuels, that
a transition and development of alternative provision schemes are necessary. A number of proposals have been proposed over the last three decades
or so, including the “Hydrogen Economy”,54–56 “Methanol Economy”,57,58 the
“Biorefinery” concept33,41,59 and more recently an “Ammonia Economy”.60–62 All
these “economies” have their relative merits, public/political traction and it is
the author’s opinion that the Methanol and Ammonia economies have the
potential to overcome a number of limitations presented by a H2-based economy (e.g. transportation, infrastructure, safety, etc.), whilst CO2 and biomass
should be viewed as our carbon sources of the future. Essentially there is no
one “economy” that fits all, as factors associated with distribution networks,
resource (geo)-availability (e.g. biomass or wind energy potential) and public
opinion will affect the viability of a scenario in a given location. However, it is
important to note, independently of these points, that to address and establish any of these economies, the development of sustainable, functional and
applicable materials will be absolutely critical. In the following subsections,
two chemical and energy provision schemes receiving perhaps the most public traction and political attention will be briefly discussed in the context of
porous sustainable materials.
1.3.1
The Methanol Economy
This concept was proposed by Olah, Goeppert, and Prakash and describes
an energy/chemical economy based around the capture and recycling of
CO2 via its hydrogenation product, CH3OH (Scheme 1.1).57,58 This C1 alcohol would then act as the platform compound from which to access future
chemicals and energy.63–65 Under standard conditions CH3OH is a liquid and
is as a practical and safe molecular vector to transport H2, from example,
produced from renewable energy (e.g. via photocatalytic water splitting).
CH3OH is relatively easily stored and transported, meaning it can essentially
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Scheme 1.1 A
simplified depiction of the Anthropogenic Carbon Cycle based on
CH3OH cycling, as proposed by Olah, Prakash and Geppart, indicating
a (green) link to the biorefinery and materials production.
be “dropped in” to existing fuel infrastructure. Furthermore, it is an excellent fuel for internal combustion engines (octane No.: ≥ 100). Using simple
chemistry and well-established dehydration catalysis (e.g. zeolite catalysis),
methanol can be converted to dimethyl ether (CH3OCH3; DME) – a diesel
substitute (cetane No.: ≥ 55).66 The chemical technology used in CH3OH production can also employed to produce derivatives (e.g. methanol-to-gasoline
(Mobil’s MtG process)67 or syn-gas to hydrocarbons via Fischer–Tropsch (FT)
synthesis). Upgrading chemistry can also be used to make other products
(e.g. olefins, formaldehyde, acetic acid, etc.), including a variety of important
industrial compounds (e.g. ethylene, propylene)67 and subsequently a substantial amount of consumer plastics.
Elaboration of the Methanol Economy concept will require new, sustainable functional nanomaterials if atmospheric CO2 levels are ultimately to be
reduced (e.g. CO2 hydrogenation catalysts, fuel-cell electrodes, H2O splitting
catalysts, CO2 capture materials, CH3OH upgrading catalysts, etc.). Furthermore, when CO2-free electricity (e.g. wind, tidal, solar, etc.) is used to power
CO2 conversion/CH3OH production, then the resulting chemicals (e.g. a liquid fuel) – will be a store of C, H, and intermittent/surplus renewable electricity – overcoming limitations of a hydrogen-based economy (e.g. large
infrastructure adaptation, safety risks, low energy density, etc.). When renewable electricity (e.g. powering the electrolysis of H2O to H2 and O2) is used in
the production of CH3OH, then electrical energy stored in this liquid can be
converted to kinetic energy (e.g. the internal combustion engine) or viewed
as a chemical flow battery whereby electrical energy is recovered using fuel
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cells, recovering the chemically stored electrical energy at potentially higher
efficiencies.68 In this regard, direct methanol fuel cell (DMFC)-powered automobiles have been reported as 2.6–3.5 times more efficient than the combustion equivalent.69 Furthermore, if H2 can efficiently be extracted from CH3OH
or other chemical vectors (e.g. reforming of CH3OH to H2 and CO2), CO2 can
be viewed as an “energy gas” to transport H2 to fuel cells (e.g. polymer electrolyte membrane fuel cells) without invoking any CO shift chemistry or additional clean-up steps.70 The Methanol Economy therefore represents a closed
(i.e. noncarbon emitting) energy provision loop provided the resulting CO2 is
(efficiently) recycled (Scheme 1.1) analogous to the natural carbon cycle (and
indeed the Biorefinery concept).
The simplest of the alcohols, CH3OH can be efficiently synthesised based
on the mildly exothermic hydrogenation of CO2 (e.g. sourced from high-­
pressure fossil power plants) in the presence of a catalyst (eqn (1.1)). This
is significant, as CO2 capture/recycling will be the basis for future CH3OH
supplies, as fossil-based CH3OH production is phased out.
CO2 + 3H2 ←⎯→
CH3OH + H2 O Δ r H 298K = −48.97 kJ mol −1
cat
(1.1)
Promisingly, generally abundant Cu and Zn are used in the preparation
of Cu/ZnO-based catalysts for this hydrogenation, relying typically on Al2O3
(5–10 mol%) supported Cu nanoparticles (NPs) where ZnO is commonly
employed as a “chemical promoter”.71,72 Characterisation of the active site (e.g.
industrially used Cu/ZnO/Al2O3) suggest that combinations of crystal lattice
strain (e.g. at the Cu NP interface with ZnO), and a high number of crystal
defects (e.g. edge sites) leads to increased activity.73,74 This is of interest and
potentially provides scope to control crystallisation and induce specific
defect chemistry in the active phase to potentially enhance catalytic activity
(e.g. at low temperatures or pressures to convert CO2 captured from the air).
In terms of carbon materials development in this context, systems based on
carbon nanotubes (e.g. Pd/ZnO/CNT) have been proposed and are of particular interest as a result of the capability to reversibly adsorb large quantities
of H2.75,76
To meet the demands of other energy/transport sectors, CH3OH will have
to be “upgraded” to higher-energy hydrocarbons (e.g. aviation kerosene, high
octane petrol) – Mobil’s “methanol-to-gasoline” (MtG) process.67 Upgrading of
CH3OH proceeds via dehydration to DME, followed by further dehydration to
olefins (e.g. ethylene, propylene) – the “methanol-to-olefins” (MtO) process.77–79
Olefins are also converted using solid acid catalysts composed of abundant
elements (e.g. zeolites – H-ZSM-5, SAPO-17, SAPO-34, etc.) to produce complex hydrocarbon mixtures, the distribution (i.e. Cn) of which is determined
by a limited equilibrium and the catalyst employed, such that the formation
of heavy products is restricted, with kerosene the highest Cn product formed.
However, problems can arise regarding active-site accessibility and mass
transfer/diffusion limitations, which can result in pore and site blocking due
to coking/carbon formation. To overcome these problems there has been significant interest in the synthesis of zeolites featuring mesoporosity in order
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to enhance reagent/product diffusion to and from the catalytically active zeolite wall sites.80–84 The introduction of mesoporosity in zeolite crystals is also
extremely relevant in the conversion of large molecules (i.e. polysaccharides,
lignin – the Biorefinery).85,86 A full discussion regarding the development
and use of zeolite catalysts (e.g. in methanol upgrading) is beyond the scope
of this book; a review by Olsbye et al.67 and references indicated therein are
highly recommended. For the reader, it is important to note that to solve the
“sustainability” challenge of energy and chemical provision, society will need
more than purely carbon-based porous materials.
With regard to porous carbons produced from sustainable precursors,
nitrogen-doped carbonaceous aerogels synthesised from the hydrothermal
carbonisation of glucose and egg protein, ovalbumin, have been used as sacrificial hard templates in the preparation of meso-ZSM-5 single crystals featuring well-developed large-diameter mesoporosity (> 10 nm) (Scheme 1.2).87
The reported templates by White et al. are tuneable in terms of chemistry
and dimensions, meaning that mesoporosity properties can potentially be
directed in the zeolite crystal, whilst surface chemistry can be manipulated to
optimise zeolite precursor/carbon interaction for a given zeotype and enable
the formation of single-crystal particles. The templates were also reported
as being significantly more cost effective than previous attempt to utilise
carbon-based templates (e.g. carbon nanotubes). If the released combustion
products during the calcination step in the meso-ZSM-5 are recycled (e.g. to
CH3OH), then the cost and sustainability credentials of the approach can be
increased. This approach is of note as it demonstrates the tuneable chemistry
afforded by using sustainable precursors in the synthesis of porous carbons.
Chemists and chemical engineers are needed to integrate renewable energies with efficient, selective and cost-effective sustainable nanomaterials to
provide the basis for a sustainable CH3OH economy and society. Sustainable nanomaterials will be required to capture/activate/convert GHG into
Scheme 1.2 A
generalised synthesis of mesoporous zeolites (e.g., ZSM-5) – Based
on (1) and (2) biomass-derived nitrogen-doped carbon (NDC) monolith templates, followed by (3) NDC/(TPAOH)-zeolite preparation via
impregnation and hydrothermal treatment, and (4) template removal
via calcination to produce the templated mesoporous zeolite. Reproduced with permission from ref. 87.
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valuable products and easily transported energy-storage or feedstock chemicals (e.g. CH3OH). With regard to CO2 mitigation, and its recycling to CH3OH
(or higher fuels and associated chemical products), inputs of H2 and/or heat/
electrical energy are required – it is therefore necessary to ask where chemistry and indeed nanomaterials can be improved/developed to achieve this
goal in the most sustainable manner possible (e.g. carbon-based precious
metal-free catalysts for fuel cells, CO2 hydrogenation catalysts capable of
operating at low pressures, mesoporous zeolite synthesis for upgrading reactions). Furthermore, if renewable biomass can be utilised in nanomaterial
synthesis then further “C” sequestration benefits may result. Chapter 8 will
highlight specifically the use of porous carbons synthesised from sustainable precursors in electrochemical applications (e.g. metal-free electrocatalysts for the oxygen-reduction reaction at the fuel-cell cathode).
1.3.2
The Biorefinery Concept
Biomass, nature’s own CO2 sink, is considered (depending on the biomass
potential of a geographical location), an appropriate crude oil substitute
from which to produce future chemicals, materials and energy as it is typically available in high quantities, and renewable on relevant time scales.88
This biomass-based Biorefinery concept is described as the integration of different technologies to produce chemicals, biofuels, biomaterials and power
from renewable biomass (Scheme 1.3).32,33,89 Biomass is a complex biocomposite consisting of three dominant organopolymer components; cellulose,
hemicellulose and lignin – vis-à-vis lignocellulosic biomass. The ratio of these
components varies between each (land) biomass source, but as a general
principle, cellulose is the dominant component (e.g. ∼ 60 wt%), with hemicellulose and lignin present in equal amounts (e.g. ∼ 20 wt% each).34,41,90
Scheme 1.3 Simplified
representation of the Biorefinery concept and examples of
possible end products.
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Chapter 1
During the course of the 20th century commercially viable plants for cellulose hydrolysis were not cost competitive with the rapidly developing oil industry, mainly due to the low oil prices and the high efficiency of catalytic cracking
for the production of high-quality liquid fuels, hence the lack of development
with regards to using lignocellulosic biomass as a chemical feedstock.40,91–93
Marine algae and chitin are also interesting as they represent (excluding proteins) one of most accessible naturally occurring sources of N-containing
biomolecules (e.g. aminosugars, proteins) and their utilisation in chemical
schemes would have little impact on land or food use (Figure 1.3).94,95
In a 1st-generation Biorefinery, raw biomass (e.g. tree lignocellulosic biomass) is converted into material and bioenergy products. In subsequent iterations of this concept, fractionation/separation of the base feedstock (and
derivatives) will be required to allow access to increasingly more valuable
components. As cellulose and hemicellulose are the major components of
lignocellulosic material, their efficient, selective conversion into valuable
intermediates for the chemical industry will become increasingly important.
The conversion of polysaccharides (e.g. via hydrolysis/hydrogenation routes)
to fine chemicals (or fuels) may ultimately represent, alongside CH3OH
Figure 1.3
Simplified
molecular structures of common biomass components;
repeat units of the polysaccharides (a) cellulose, (b) chitin, (c) an exemplary structures of hemicellulose and (d) the complex polymer, lignin.
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upgrading chemistry, as an important process as the catalytic cracking
associated with modern oil refineries.96 Lignin will also become an increasingly important resource for aromatic compounds, vital in the production
of flavour compounds, medicines, polymers and plastics.97–100 In a 2nd-­
generation Biorefinery, lignocellulosics are converted into fuels, chemicals
and materials via routes with comparable to or greater efficiency than those
currently employed for the utilisation of nonrenewable feedstocks.59,101,102
The first step is feedstock fractionation into the principal cellulose, hemicellulose, and lignin components. In turn, each component has the potential to
generate its own Biorefinery stream and associated products. Fractionation
and typically fermentation are employed to generate complex aqueous based
products mixtures, which in turn must be separated and then converted via
chemical transformations into the desired products (e.g. hydrogenation,
hydrolysis, dehydrogenation, oxidation, amination, etc.).
In the context of sustainable nanomaterials, it is important to note that
current industrial chemical catalytic/separation technologies (i.e. for petrorefining small hydrophobic molecules in hydrophobic environments) will not
be suitable for the aqueous phase conversions of the Biorefinery. Therefore
to achieve full exploitation of each Biorefinery stream (e.g. cellulose-derived
streams), new tuneable, functional, nanomaterials (i.e. catalysts, separation
media, etc.) are required to operate on the often acidic, large (polar) molecule products streams of the Biorefinery.16 Furthermore, it is essential that
sustainable chemistry and technologies are applied in these conversions to
minimise the environmental footprint of each product, and this includes the
development of the sustainable nanomaterials to address these challenges.
It seems appropriate therefore that the feedstock of the Biorefinery, biomass,
is also utilised in the preparation of the nanomaterials needed to perform
the catalysis or separations in the overall production scheme. Chapter 3 will
highlight the application of porous carbons synthesised from sustainable
precursors in Biorefinery-related platform molecule transformations (e.g. the
esterification of succinic acid).
1.4
Brief History of (Porous) Carbon Materials
A
from Sustainable Precursors
Nature is the master of the self-assembly of organic polymers (e.g. polysaccharides, proteins, etc.) to form a variety of structural and storage roles in
higher plants and other biological structures. These “biomaterials” are
organised in natural systems at the nanoscale to macroscopic scale leading to the production of hierarchical materials of complex forms including
spirals, spheroids and skeletons. The natural elegance is perhaps exemplified by a single glycosidic bond; contrast the physiological roles of cellulose
(polymeric β(1 → 4)-d-glucose) and amylose (polymeric α(1 → 4)-d-glucose) in
the systems of plants. Cellulose provides dimensional stability in plant cells,
whilst amylose acts as an energy storage molecule in plant metabolism – the
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two polysaccharides are chemically identical but nature has utilised this difference in self-assembly as a consequence of the different glycosidic bonds to
generate structurally distinct materials. Therefore, as materials chemists can
we look to natural systems to aid us in our design of sustainable materials
to meet the challenges presented by future energy- and chemical-provision
schemes. This represents the main theme of the following chapters of this
book.
1.4.1
General Aspects of Porous Materials
Porous materials are considered a specific solid state, with small alterations
of the specific surface area and volume ratio significantly altering the physicochemical properties of a given material. Based on a classical definition,
a porous material is described as a solid matrix composed of an interconnected network of pores (sometime referred to as voids) filled with a fluid
(liquid or gas). The International Union of Pure and Applied Chemistry
(IUPAC) has separated the different pore sizes into three classes, with each
division relating to a specific pore size regime: microporous, mesoporous
and macroporous (Table 1.1).103
As will be discussed in more detail in Chapter 9, gas sorption is the most
common technique employed to determine features such as surface area
and porosity. In this regard, each pore-size regime relates to a specific nitrogen adsorption/desorption mechanism as manifested in the isotherm profile. In microporous materials, three-dimensional adsorbate condensation
proceeds within a strong electromagnetic field induced by the narrow pore
dimensions, resulting in the elimination of interphasic adsorbate–adsorbent
interactions and the system properties are close to a single phase. In mesoporous materials, adsorption occurs based on the formation of consecutive adsorbate layers, ultimately terminating as the phenomena of capillary
condensation occurs. Macropores have pore diameters that present porous
properties similar in character to conventional flat surfaces that cannot be
filled by capillary condensation.104
The varieties of pore sizes that are possible and associated adsorption
properties have led to the range of applications that have so far been explored
with porous materials and it is such features that will make them critical
in potential future (e.g. energy storage) applications. As a consequence in
part of strong Van-der-Waals interactions, micropores have been classically
utilised in liquid- and gas-phase adsorption applications. Going beyond the
Table 1.1
IUPAC classification of pore size and adsorption mechanism.
Pore type
Size regime
Condensation mechanism
Micropore
Mesopore
Macropore
<2 nm
≥2 ≤ 50 nm
>50 nm
Three-dimensional
Capillary
No condensation
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2 nm pore diameter boundary, into the mesopore domain, lowers the potential energy and escape surfaces, rendering materials particularly suitable for
liquid-phase applications including heterogeneous catalysis or chromatographic separation. The pore sizes in this region facilitate a high loading of
accessible active sites, whilst concurrently providing the pathways for efficient diffusion/mass transfer of the liquid-phase analyte or substrate. Going
further into the macropore domain, provides significantly enhanced system
filtering properties and flow/mass transfer/diffusion properties in the material. Typically in a synthetic material, there will be contributions to the porosity from all three types of pore sizes (Figure 1.4).
As will be highlighted during the following chapters of this book, a wide
variety of well-established technologies and applications are only possible
as a result of the coexistence of micro and mesopores. It is also important to
note that any future applications (e.g. energy generation/storage, gas storage/
capture) will only be efficiently possible with the suitable design of porosity
and, as will be alluded to during the course of the book, tailoring of suitable materials chemistry. In the context of porous structures, solid porous
materials possessing an inhomogeneous pore-size distribution are typically
inexpensive and relatively easily prepared. For example the classical conversion of nut shells or olive stones via thermochemical activation to produce,
predominantly microporous activated carbons, have been used for example
as water purification media for decades.106
The demand for novel, increasingly efficient porous materials to address
the challenges presented by tomorrow’s energy and chemical provision
schemes is increasing. The main drivers in this search for new materials
are derived from the general principles of increased selectivity and application efficiency, whilst more subtle aspects relating to the tuneability of
material bulk/surface chemistry are also becoming increasingly important.
However, if one is to take the material from the lab to the industrial scale,
then ultimately economics and sustainability points have to be addressed.
Furthermore, a key feature behind a specific materials efficiency in a given
application is the parameter of surface energy. This feature is particularly
relevant when describing mesoporous materials, as selectivity for these
Figure 1.4
Schematic
representation of (A) the three-dimensional and; (B) the
two-dimensional structure of a activated carbon. Reproduced with permission from ref. 105.
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materials is not based solely on pore size, but strongly influenced by specific
surface functionality/polarity. Therefore, the ability to produce materials, as
will be discussed throughout this book, with tuneable surface functionality
will have pronounced effects on, for example, the activity of a given heterogeneous catalyst (e.g. as a function of hydrophilic/hydrophobic surface group
ratio). This concept of “surface energy” leads to highly microporous materials
being strongly size selective, whilst the selectivity of mesoporous materials
will be defined by surface functionality and it is these two classes of materials that receive the majority of research attention.
Therefore, to reach the goals of “greener” sustainable nanotechnology and
indeed society in general, it is of critical nature to marry control of material
porosity and functionality together and ultimately allow the production of
designer, sustainable porous materials with properties that can easily be dictated for a specific application.
1.4.2
Activated Carbons
The preparation of porous carbons from sustainable precursors has until
relatively recently concerned only the synthesis of activated carbons
(ACs).26,47,48,107 This class of predominantly microporous materials is commercially available and has proven applications in water treatment,108,109
CO2 capture,110 energy storage (e.g. supercapacitors111), and perhaps most
typically heterogeneous catalysis.105,112 Almost 1 million tonnes per years
of ACs are produced. ACs for the aforementioned applications are typically
produced from low-cost renewable materials (e.g. coconut, wood and fruit
stones). Classical AC preparation, based on a thermochemical activation
step leads to generation of microporous materials with average pore diameters of < 2 nm (Table 1.2). Activation processes have been widely used to
obtain typically inexpensive and highly microporous carbons from various
organic precursors, although biomass (e.g. lignocellulosic) are conventionally the precursor of choice. As this topic has been addressed in a wide
range of literature, the reader is encouraged to refer to the extensive work of
Marsh and Rodriguez-Reinoso.113 Briefly, two different types of ACs can be
discriminated; (i) Physically (or thermally) ACs, prepared via selective gasification of individual carbon atoms using CO2 or steam at 800–900 °C; and (ii)
Chemically ACs, prepared based on the impregnation of suitable activating
reagents (e.g. NaOH) in the organic precursor followed by carbonisation,
typically in excess of 650 °C.
Whilst the following chapters predominantly address the synthesis of mesoporous carbon materials, it is worth noting that the introduction of well-defined, high surface area micropores to a carbon material can be particularly
beneficial in applications including gas and energy storage (e.g. supercapacitors).114,115 Industrial uses of ACs are somewhat limited in applications
including (large-molecule) catalysis, electrochemistry, fuel cells, biomedical
devices, personal protection and automotive components partially due to the
requirement for tuneable, mesoporous carbon materials. Furthermore, the
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Table 1.2
Examples of different routes to mesoporous carbon and their associated
textural properties.
Preparation method
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Vmeso
(cm3 g−1)
Vmeso/Vtotal
(%)
D (nm)
0.85
—
35
1.08
89
2.9
27,117
1.48
1.8
76
—
3.4
5.1
118
119
B
B.1
B.2
From polymer self-assembly
C-FDU-15
612
0.14
C-FDU-18-450
554
0.29
41
64
6.8
26
120
121
C
C.1
C.2
From polymer aerogel
Carbon aerogel 1215
Carbon cryogel 1107
2.44
1.54
90
88
6.2
6.2
122
122
D
D.1
Activation methods
Steam
356
invigoration
Activation with 1230
TiO2
0.8
81.8
5.1
123
0.55
52.9
SBET
(m2 g−1)
Group
Description
A
A.1
A.3.1
A.3.2
Template methods
Silica gel as
template
PGC
120
MCM-48 as
template
CMK-1
1800
SBA-15 as
template
CMK-3
1600
CMK-5
1850
A.1.1
A.2
A.2.1
A.3
D.2
—
Examples
116
124
top-down approach to the synthesis of ACs from natural precursors does not
provide the necessary control over material features including porosity, morphology and surface chemistry, whilst limited batch-to-batch reproducibility, final carbonisation yield and the use of harsh reagents during synthesis
(e.g. strong acids or bases) to generate porosity are other significant process
drawbacks.
1.4.3
Mesoporous Carbons
Regarding the synthesis of carbons materials featuring mesoporosity, there
are a number of common synthetic routes that are now relatively well established. These approaches can be divided into four main groups (Table 1.2).
A.Replication of a porous inorganic template – Hard templating.
B.Polymer blend self-assembly and carbonisation (i.e. composed of a carbonisable polymer and a pyrolysable polymer) – Soft templating.
C.Carbonisation of preformed organic polymer aerogel precursor.
D.Traditional chemical and physical activation of carbon (see Section 1.3.2).
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Chapter 1
At the time of writing, the Knox and Gilbert method developed in 1979 for
the production robust porous carbon microspheres on an industrial scale
is still one of the best examples of a hard templating strategy (Group A).116
In this strategy, a highly porous HPLC silica gel is impregnated with a phenol–formaldehyde mixture. Polymerisation of the organic component with
the silica gel pore system leads to an organic/inorganic hybrid. This material
is then carbonised at temperatures > 1000 °C (under N2 or Ar). The silica is
finally removed using strong alkali solution to form a glassy carbon replica,
hence the original name of “porous glassy carbon (PGC)”. PGC has excellent
textural properties suitable to act as stationary phase in chromatography;
high mesoporosity and large pore volume (≤0.85 cm3 g−1) dominate, with a
very small microporosity contribution (undesirable in chromatography and a
product of the high-temperature carbonisation step). More details regarding
the development of carbons for chromatography will be provided in Chapter
4. PGC has however a rather disordered pore structure, with limited scope for
manipulation of pore network or postchemical functionalisation.
The hard templating approach has since been extended to the replication
of a wide variety of different ordered inorganic hard templates to generate
ordered mesoporous carbons. However, this typically results in the loss of
regularly particle morphology and low micropore content. This approach
includes the following steps:
1.preparation of an inorganic template with controlled pore structure
(e.g. surfactant templated MCM-48);
2.impregnation/infiltration of the sacrificial template with monomer or
polymer precursor;
3.thermal carbonisation leading to crosslinking and condensation of the
organic precursors leading to the formation of the carbon material.
The last step here is typically performed at temperatures > 600 °C, to render the carbon product chemically resistant to the acidic or caustic solutions
employed in the dissolution of the inorganic matrix. This step is not only
resource intensive but also leads to increasingly more condensed, hydrophobic and homogeneous material chemistry, which is not amenable to postfunctionalisation and requires further processing to increase the wettability
of the material. The conductivity of such materials is often somewhat lacking
due to the amorphous nature of the resulting carbon.
The interest in the hard-template approach for the preparation of mesoporous carbons is thanks in part due to the wide range of mesophases and
morphologies potentially accessible. Their chemical inertness, stability
and inherent advantages over classical microporous ACs are also features
of significant interest. With regard to mesopore size and dimensions, Kruk
et al. first reported the replication of mesostructured MCM-48 in the synthesis of ordered mesoporous carbons (OMC; e.g. CMK-1).117 Interestingly
in this approach, MCM-48 has been impregnated with a sucrose or furfural
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alcohol – both sustainable carbon precursors – and subsequently converted
to graphitic-type material via sulfuric acid-catalysed thermal carbonisation.27 Dissolution of the silica template with NaOH or HF solution yielded
the mesostructured replicas with pore diameters in the lower mesopore
domain (ca. 3 nm). Further iterations of this approach using hard templates
with increased pore wall thickness (e.g. Santa Barbara SBA-15) has provided
the opportunity to increase mesopore diameter to > 4 nm (e.g. CMK-3).118,119
Whilst the ordered mesoporous carbons synthesised via nanocasting
approaches certainly deliver extremely attractive materials in terms of their
order and symmetry (Figure 1.5), they typically do not offer any macroporous
character (i.e. pore diameters > 50 nm). The introduction of pores with diameters > 50 nm within a hierarchical pore structure is advantageous, as such
voids offer rapid transport pathways for gases and liquids to the active sites
within the smaller pores. In this regard, a secondary macropore template is
often employed. The utilisation of polystyrene (latex) spheres has been used
by Baumann et al. to introduce 100 nm ordered macroporous domains to the
structure of 6 nm pore diameter mesoporous carbons.125
A full description and discussion of this extensive area is beyond the scope
of this book. Therefore, for further details regarding the preparation, mesophase structures and characterisation of ordered mesoporous carbons using
hard template routes, the reader is recommended to the reviews of Ryoo et
al.,28 Lu and Schüth,30 Yang and Zhao,29 and Hyeon et al.126 More recent innovations, importantly that have significantly reduced the number of process
steps and resource use, have employed soft-templating techniques based
on organic–organic self-assembly involving the combination of polymerisable precursors and block copolymer templates into ordered mesophases –
essentially an extension of the templating approaches designed for ordered
mesoporous silicas to organic polymers, followed by a thermal carbonisation
step to produce ordered mesoporous carbons.127 As a primary example of
Figure 1.5
TEM
images of CMK-1 (prepared from a sucrose or furfural alcohol precursor) viewed from the (111) direction (left), and CMK-2 viewed from
the (100) direction (right). Reproduced with permission from ref. 28.
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Chapter 1
this approach, Zhao et al. have reported a dilute aqueous route for the direct
synthesis of mesoporous polymer (FDU-14) and carbon (C-FDU-14) materials
via the self-assembly of P123 triblock copolymer templates, utilising resols
as carbon precursor.128
However, it should be noted that soft-templating approaches typically suffer from the same limitations as hard-template routes. The resulting carbons
typically have small mesopore size (D < 10 nm) and quite developed microporosity, whilst issues regarding mass transfer/diffusion limitations and
active-site accessibility generally remain an issue. In the context of porous
carbons prepared via soft templating and sustainable precursors, at the
time of writing only a few examples exist, namely the synthesis proposed by
White et al.129 and Naskar et al.,130 which both employed the Pluronic® F127
block copolymer as the soft template. White et al. utilised the hydrothermal
carbonisation approach to convert a fructose precursor to a hydrothermal
carbon in the presence of the F127 polymer and a pore-swelling agent (i.e.
1,3,5-trimethyl benzene), resulting in a cubic Im3m mesoporous structure.
Naskar et al. by contrast employed Kraft-processed hardwood lignin as
precursor and an evaporative self-assembly to produce a carbon material
exhibiting a degree of ordered porous structure. Further discussion of these
approaches will be made in Chapters 6 and 12, respectively.
During the course of this book, it should be noted that once a qualified
description of the decomposition mechanism(s) of a given sustainable biomass precursor have been established, then design of appropriate block copolymers (e.g. control of the hydrophobic/hydrophilic block ratio), should enable
the full elaboration of the soft-templating approach in the preparations of sustainable porous carbons. Furthermore, if the block copolymers used in these
processes are prepared using simple efficient methodologies, employing sustainable monomer precursors, this would mark major progress in this field;
even more so if the decompositions gases from the template removal and carbonisation steps can be recycled (e.g. CO, CO2, H2 to liquid fuels).
For an interesting and concise review regarding the use of the soft templates
in the preparation of ordered mesoporous carbon, the reader is referred to
the work of the Zhao29,128,131 and Dai groups,132 and more recently the work
of the Matyjaszewski group,133–135 as well as a number of other authoritative
works on the subject.127,136–138 With regard to these templating approaches,
the reader is encouraged to appreciate the aesthetic beauty of the ordered
mesoporous materials, which in principle can utilise sustainable precursors (e.g. saccharides) in their preparation. Conversely, these approaches are
somewhat wasteful, multistep and employ high temperatures and hazardous reagents (e.g. strong acids or bases in the hard-template-removal step),
whilst the synthesised porosity, whilst uniform, does not provide the most
efficient mass transfer/diffusion architectures; therefore, based on the aforementioned Green Chemistry principles, new synthetic routes are required to
produce porous carbons in as resource efficient a manner as possible and as
will be revealed throughout this book, this can potentially be achieved using
sustainable precursor and processes.
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1.4.4
25
Carbon Aerogels and Related Materials
An interesting method for the preparation of porous carbon materials without
the use of additional hard- or soft-templating approaches is the preparation
of carbon aerogels and related materials (e.g. xerogels), based on the thermal
carbonisation of organic/polymeric gel precursors. The IUPAC has defined
“aerogels” as nonfluid networks composed of interconnected colloidal particles as the dispersed phase in a gas (typically air).139,140 Aerogels are typically
lightweight materials, with their low densities arising from the solid phase
being composed of an interconnected three-dimensional structure of crosslinked primary particles.141–143 They typically have a density of ≥100 kg m−3
and surface areas in the region of 1000 m2 g−1. Whilst aerogels can either be
constructed of organic, inorganic or metallic components, microscopically,
they are composed of tenuous networks of clustered nanoparticles, resulting
in unique properties, including very high strength-to-weight and surfacearea-to-volume ratios.
In the context of porous materials, aerogels present all types of porosity (i.e.
micro-, meso-, and macroporosity), with the pore layering typically dependent
on the synthesis and subsequent treatment. Aerogels are known in a great
variety of compositions and are used in a manifold of high-end applications
including chromatography, adsorption, separation, gas storage, detectors,
heat insulation, as supports and ion-exchange materials.144,145 The reader is
referred to a number of informative reviews on the topic of aerogels.146–151 In
the context of this book, perhaps one of the most interesting applications of
organic/polymeric aerogels is their carbonisation to synthesise high surface
carbon aerogels (Table 1.2). As-synthesised organic aerogels (or indeed, xeroor cryogels) may be transformed into carbon aerogel equivalents via controlled
thermal annealing/carbonisation under a nonoxidising conditions (e.g. flowing Ar). The resulting carbons often have well-developed micro- and mesoporosity and associated large surface areas. The carbonised gels are composed
of interconnected nanosized primary particles – reflective of the parent gel.
The most common precursor gel phases are those derived from the polycondensation of resorcinol/formaldehyde (RF) mixtures. For RF gel-derived
carbon aerogels, micropores develop in the primary particles, with mesopores and macropores resulting from primary particle packing (i.e. the
voids originally occupied by solvent used in the synthesis of the organic gel
parent) (Figure 1.6). The porous dimensions of the parent organic gels are
not automatically transferred into the carbon aerogel as shrinkage, pore
closure, and pseudographitisation may occur during carbonisation. Significantly, the amount of micropores and mesopores in these carbon aerogels
in principle can be directed separately – a strong advantage of carbon gels
as porous carbon materials – with selection of parent organogel composition, drying method, curing time and temperature and of course the temperature employed to make the carbonised aerogel all exerting an influence over
the final material properties. For adsorption/energy applications, highly
condensed carbon aerogels (e.g. carbonisation at temperature >1000 °C)
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Figure 1.6
TEM
images of selected carbon aerogels reported by Schüth et al. Reproduced with permission from ref. 152.
combine beneficial adsorption properties and structural stability with high
thermal stability and in principle electronic conductivity,153–155 thus allowing
use in electrical/electrochemical applications (e.g. batteries, supercapacitors,
or conductive catalyst supports).156 For those applications, it is important to
have additional control over micro- and mesoporosity development – which
is a challenge based on the conventional RF-based system. This approach to
porous carbon synthesis is also limited with regard to the accessible physicochemical properties and functionalities afforded by phenolic-derived carbons. The highly aromatic/graphitic/hydrophobic structures whilst useful
for a variety of applications are not easily modified or the properties moderated for application optimisation. A significant volume of literature is now
related to the synthesis of carbon aerogels derived from organic RF gel systems and the reader is referred to a number of significant publications on the
topic.126,143–145,153,156,157 Importantly, as will be described in further detail in
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Chapters 2, 6, and 12, it is now possible to prepare a variety of carbon aerogels (including heteroatom-doped variants) based on the conversion of sustainable precursors including saccharides,158,159 polysaccharides,160–162 and
complimentarily to the original RF-derived aerogels, based on flavonoids,
tannins and lignin.130,163–169
1.4.5
Graphitic
Nanocarbons – Carbon Nanotubes
and Graphene
Perhaps of all the carbon nanomaterials, those composed of highly condensed graphitic nanostructures and specifically carbon nanotubes (CNTs),
and graphene/graphene oxides (GO), are the most famous and familiar in
the public domain. Conventionally, they are synthesised using organic precursors (typically sourced from petrochemical supplies) and appropriate
catalysis. This is a massive area of research and as these carbons are not conventionally porous, they will not be covered in detail in this book. However,
for completeness, a brief overview will be provided here highlighting those
reports demonstrating the synthesis of these carbon nanostructures utilising sustainable precursors. For further details, the reader is referred to a
number of topical reviews – CNTs,170–173 and graphene/GO.174–178
CNTs can be considered as a classical, demonstrative example of a carbon-based nanomaterial.179,180 The synthesis and mechanism of growth of
CNTs has been described extensively in the literature.180,181 Classically, synthesis uses well-established chemical vapour deposition (CVD) processes,
with the CNTs being prepared from a carbon precursor (e.g. methane, acetylene, xylene) and an appropriate catalyst (e.g. metal or mixed metal nanoparticles or surfaces), which are known to catalyse the decomposition of the
carbon precursor.182 Research in the area is still focused on the development
of methods to convert the carbon precursor in the most efficient manner
possible and optimisation of catalyst activity to obtain high yields and high
purity (i.e. “pristine”) CNTs (e.g. single, double or multiwall variations). With
regard to the preparation of CNTs from sustainable carbon precursors, there
are a number of reports that seek to reduce the overall CNT production costs
and ultimately improve the “green” credential of the given process and facilitate mass production.183
Unzipping the CNT, leads to the formation of graphene – which is in principle a one-atom thick planar sheet of sp2 bonded carbon atoms arranged in
a honeycomb fashion. The classical preparation of this carbon allotrope is
the exfoliation of graphite with “scotch tape”.184 With the interest of mass
production in mind, CVD processes have been investigated for graphene
synthesis but they suffer from using the high-quality, high-cost, single-crystal substrates, ultra-high-vacuum conditions and the intricate methods
required to separate the graphene layer from the substrate. Positively, these
approaches do lead to the production of large-area graphene films suitable
for high-value applications (e.g. electronics). GO, an oxidised graphene can
also be employed in graphene synthesis,185 and is perhaps one of the most
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attractive routes suited for large-scale production. In terms of chemistry,
the chemical reduction of GO to graphene is not necessarily 100% complete,
and therefore residual oxide moieties can exist, which in turn dramatically
alter the properties (e.g. electronic conductivity) of the synthesised graphene.
In terms of sustainable precursors, a carbon-based molecule with a low H
content is preferred for the production of high-quality CNT production, as
higher H contents are more likely to produce larger quantities of amorphous
carbon side products. In this context, sustainable precursors from biomass
for example, might appear suitable for this task (e.g. glucose, (C6H12O6); C : H
= 1 : 2; palm oil (C55H100O6); C : H = 1 : 2). As an alternative to CVD, spray pyrolysis is often used for the processing of liquid precursors whereby pyrolysis of
the carbon precursor and deposition occur in one step at high temperature.
In this regard, vegetable oils namely turpentine oil (C10H16),187,188 eucalyptus
(C10H18O),189 coconut,190 neem,191 and palm oil192–194 have all been employed
in the synthesis of CNTs using spray pyrolysis and carefully designed Fe/Co
zeolite catalysts, which in the case of eucalyptus oil led to the synthesis of
single-walled CNTs of ca. 0.79–1.71 nm diameter.189 This approach can also
be extended to multiwalled CNT synthesis with the addition of ferrocene. A
complimentary approach has utilised waste chicken fat oil (and a ferrocene
additive) as carbon precursor(s) and a mirror-polished p-type (100) Si wafer
substrate, for the synthesis of vertically aligned CNTs with good crystallinity
(ID/IG ratio of 0.63), a purity of 88.2%, and minimal amorphous carbon content (Figure 1.7).195 Camphor (C10H16O), a crystallised latex sourced from the
Cinnamomum camphor tree, has also been used by Kumar and Ando to produce CNTs in large quantities.196 A nanotube garden containing single-wall,
multiwall and aligned CNTs was produced from thermal decomposition of
camphor under Ar at 875 °C, using a low catalyst amount, whilst amorphous
carbon formation was found to be negligible.196
With regard to graphene production, Ruan et al. have demonstrated that
inexpensive carbon precursors including food (e.g. cookies, chocolate),
insects (e.g. cockroach legs) and waste (e.g. polystyrene, grass, dog faeces)
can all be used (without purification) in the synthesis of high-quality monolayer graphene.197 In this report, graphene was prepared from these precursors directly at the surface of Cu foils under a H2/Ar flowing atmosphere.
Graphene was formed as the carbon precursor decomposed after heating to
1050 °C, and decomposition production diffusion to the backside of the Cu
foil, leaving other elemental residues on the original surface. Although these
waste materials require pretreatment to remove moisture, the synthesis does
yield high-quality pristine graphene with few defects and 97% transparency.
High-quality graphene has also been prepared from lotus petals or hibiscus
flowers, based on thermal exfoliation under Ar at temperatures ≤1600 °C, in
the presence of catalytic quantities of nickel.198
As mentioned earlier, the reduction of GO to graphene is a potentially
cost-effective large-scale production method. However, it is important to
note that many of the chemicals (e.g. reducing agents, surfactants) required
for complete GO reduction in the aqueous phase can be considered toxic
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Figure 1.7
(A)
TGA and DTGA curves for chicken fat oil. (B)–(D) FE SEM images of
vertically aligned carbon nanotubes synthesised from waste chicken fat
on a Si substrate with increasing magnification. (E) HRTEM image of
multiwalled CNTs. Reproduced with permission from ref. 195.
and harmful. In this context, nontoxic reducing agents have been explored
including sustainable options such KOH,199 and biomolecules (e.g. protein).200 The use of protein is noteworthy as it can be employed as a universal adhesive in nanomaterial construction. With regard to biomolecules,
sugars (e.g. glucose, fructose) have also been used as reducing agents to
prepare glucose oxide graphene nanosheets in an aqueous ammonia solution, with glucose first being oxidised to aldonic acid by GO, followed by
conversion to a lactone and a large quantity of –OH and –C(O)OH groups.201
Dextran has been employed in the environmentally friendly synthesis of
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biocompatible reduced graphene oxide (RGO), whereby the polysaccharide
acts as a reducing agent and a surface functionalisation agent, rendering the
RGO water soluble and biocompatible.202 Similar biocompatible graphene
has also been synthesised using Ginkgo biloba extract as a reducing and
stabilising agent.203 Glycine, an inexpensive amino acid, has also been used
in the reduction of GO, whereby the amine groups covalently interact with
GO and, under reflux conditions, reduces GO to graphene.204 The use and
exploitation of sustainable precursors and molecules for the production of
RGO represents an interesting approach, particularly if the GO is derived
from sustainable sources, and may lead ultimately to a cost-effective and
sustainable synthesis of graphene-based materials. It should be noted in
the context of porous carbons that these one- and two-dimensional materials do not conventionally present a high surface area or defined porosity
(in the traditional sense) and hence numerous reports have focused on the
production of graphene aerogels and associated composite (e.g. with CNTs)
materials.205,206
1.4.6
Ionic Liquids
Ionic liquids (ILs) are generally considered as interesting, nonvolatile, potentially green solvents with tuneable solvation properties.207 However, their
exploitation as precursors in the synthesis of carbon materials has been a
relatively new occurrence. A number of research groups have been exploring
the possibility of using carbonisable, typically cyano-based, ILs as precursors
for the direct synthesis of nonporous and porous carbon including the synthesis of nitrogen-doped,208–211 and sulfur-doped variations.211 In the context
of carbon synthesis, ILs are nonvolatile and hence high pressures are not
necessary, thus rendering synthesis in principle relatively simple in comparison to other synthetic approaches. In this context, the synthesis of carbons
with high nitrogen contents and excellent electronic conductivity and oxidation stability has been reported.212
In this area, the groups of Dai, Antonietti and Thomas are demonstrating
the possibility to produce a variety of carbon-based materials featuring the
incorporation of wide range of heteroatoms including N, P, S and B. Regarding nitrogen-doping, contents > 10 wt.% have been reported, generating
electronic conductivity properties that are considered superior to graphite,
whilst oxidation resistance has also claimed to be improved (e.g. as compared to carbon nanotubes).208,211,213–215 In the context of porous materials,
ILs have the advantage of being a liquid with negligible vapour pressure,
providing scope for replication, impregnation and nanocasting, using the
classical techniques of hard and soft templating (Section 1.5.3).216,217 ILs
also have strong interactions with (e.g. inorganic) surfaces having good
wettability properties, which lead to the very popular use in the replication of inorganic structures. These physiochemical properties makes ILs
potentially very interesting material precursors, enabling relatively simple
processing and the possibility of shaping without the high pressures and
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associated safety issues. In the context of materials preparation, ILs can be
employed in well-established procedures including dip coating, printing,
electrospinning, electrospraying, and templating/nanocasting followed by
conversion to the corresponding (often heteroatom-doped) carbon via a
final pyrolysis step.209
Typical ILs involved in the preparation of carbon materials are based on
cations containing structural nitrogen (e.g. pyridinium, pyrrolidinium or
imidazolium – favoured for graphitic structure synthesis), while the anion
is preferentially cyano-based (e.g. dicyanamide, tetracyanoborate, etc.) as
this class of IL has been found to be the highest yielding carbon precursor with or without the application of pressure or nanoconfinement (Figure
1.8).211 For the aforementioned nitrogen-rich ILs (e.g. N,N-ethylmethyl-imidazolium-dicyanamide), carbonisation is proposed to proceed via a reverse
Menschutkin reaction or alkyl fragmentation, the nucleophilic attack of
dicyanamide on aromatic cations coupled with triazine from further cycloaddition reactions. These reactions in turn lead to formation of a “polymeric” branched carbonaceous precursor, which with continued heating
transforms into a graphitic nitrogen-doped carbon, however, more detailed
investigations are still pending to truly resolve this undoubtedly complex
carbonisation mechanism.208,210 Interestingly, reports have indicated that
by simply changing the anion chemistry it is possible to direct material
porosity and material yield.218
With regard to the development of porosity, typically the direct carbonisation of ILs results in the production of nonporous or microporous carbons.
Figure 1.8
(A)
Molecular structures of 3-methyl-N-butyl-pyridinium-dicyanamide
(3MBP-dca) and N,N-ethylmethyl-imidazolium-dicyanamide (EMIMdca); and (B) Examples of different ionic liquids precursors and their
behaviour in terms of solid (i.e. carbon) yield. Reproduced with permission from ref. 208.
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Chapter 1
Therefore strategies have been developed to introduce high surface area and
porosity at different length scales.208,209,219 The classical hard-templating or
nanocasting approaches (e.g. of SBA-15 or ludox nanoparticles) employed in
the preparation of ordered mesoporous carbons (e.g. the CMK series) have
been employed to produce high surface area IL-derived carbons (Figure
1.9).208,209,214,215 The excellent wetting behaviour of ILs on inorganic surfaces
enables a good impregnation of the typically silica-based, solid porous hard
templates. Subsequent heat treatment (typically in excess of 800 °C) yields
the IL-derived carbon/inorganic composite, with the latter being removed
using either strong acid or caustic solutions to yield, e.g. the inverse replica
in the case of SBA-15 or spherical pores with diameters reflective of the silica (ludox) nanoparticles employed in the synthesis. IL-derived carbons
produced from these approaches have found application in electrocatalysis
(i.e. metal-free oxygen-reduction reaction catalysts;220,221), the preparation of
binary and ternary nitrides,222 and as CO2 adsorbents.223
Regarding the green/sustainable credentials of producing carbon-based
materials from ILs, there are questions still outstanding regarding overall
Figure 1.9
TEM/SEM-images
of (A) porous alumina membrane, (B) SBA-15, (C) ­silica
monolith, and (D) Ludox templated N-doped carbon using 3MBP-dca or
­EMIM-dca as IL precursor. Reproduced with permission from ref. 224.
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carbon yield (solid yields are typically < 20%), the cost of the IL vs. the application benefit and also the use of hazardous compounds (e.g. aqueous ammonium dihydrogen fluoride in the template removal), representing a drawback
in the synthesis of high surface area materials. This discussion is made more
complex with the more recent use of poly(ILs) in material fabrication.225,226
There is also of course the question of how the IL is sourced and if they can
be produced sustainably.212 Independently of these points, the ability to
introduce a high quantity and indeed a variety of heteroatoms to the carbon
structure and therefore potentially tune the properties of the resulting material is certainly an attractive feature regarding materials chemistry.
1.4.7
Hierarchically
Porous Carbons Synthesised in Deep
Eutectic Solvents
One of the most common synthetic processes used in the synthesis of porous
carbons, as discussed in the previous sections, is the carbonisation of organic
polymers in the presence of a template or based on phase separation (e.g. RF
aerogels).141,227–230 Furthermore, hierarchically porous structures are known
to be beneficial in the majority of applications typically associated with carbon materials (e.g. adsorbents, filters, catalysts or electrodes).231–233 In this
context, the group of del Monte has demonstrated the use of eutectic mixtures (so-called deep eutectic solvents or DESs)234 as interesting alternatives
to standard templating approaches, for the synthesis of hierarchical carbons.235,236 DESs are defined as molecular complexes composed of a quaternary ammonium or phosphonium salts and suitable hydrogen-bond donors.
The charge delocalisation through the hydrogen-bonding donor (e.g. a halide
anion) and the hydrogen-donor moiety decrease the freezing point of the
mixture relative to the melting points of the individual components.
These solvents are quite similar to conventional ILs, being water insensitive, nonvolatile and biodegradable. However, the preparation of pure
eutectic mixtures is simpler compared to ILs, with no postsynthesis purification necessary, with eutectic mixtures based on inexpensive, readily
available components making them particularly applicable for large-scale
application. Inexpensive eutectic mixtures have been reported based on choline chloride (ChCl),238–240 sugar, urea, and salts,241–243 “natural deep eutectic solvents”,244,245 and low transition temperature mixtures.246 The use of
ChCl, natural carboxylic acids, amino acids, different sugars, and even water
in these examples, provides biodegradable and sustainable features to the
resulting eutectic mixtures, making them potentially useful “green” solvents.
With regard to synthesis of porous carbons, Carriazo et al. have reported
the use of DES mixtures of resorcinol and ChCl, or urea, resorcinol and ChCl
as precursors for the preparation of ultimately hierarchically porous carbons
(Figure 1.10).237 In this approach a phenolic resin is first prepared via polycondensation chemistry with formaldehyde, followed by a thermal carbonisation step under an inert atmosphere, leading to the synthesis of carbons
presenting bimodal porosity comprised of micropores and large mesopores
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Figure 1.10
Chapter 1
(Top
panel) Representative SEM micrographs of monolithic ­resorcinol–
formaldehyde gels synthesised in the presence of deep eutectic solvents (left, bar = 5 µm; right, bar = 2 µm); insets show images of the
monolithic RF gels. (Bottom panel) SEM micrographs of monolithic
carbons synthesised from these parent gel systems (left, bar = 1 µm;
right, bar = 1 µm); insets show TEM micrographs depicting the material nanostructure (left, bar = 50 nm; right, bar = 150 nm) and pictures
of the respective monolithic carbons. Reproduced with permission
from ref. 237.
(D > 10 nm).237 A bicontinuous porous network was synthesised constructed
from highly crosslinked primary particles that had aggregated into a stiff,
interconnected structure, reflective of a spinodal decomposition process
observed in other DES-based synthesises.247
The synthetic approach of the del Monte group has significant merit and the
number of DESs available should in principle enable a high degree of structural and compositional control over the resulting carbons, including macroand micropore dimension control,237,248 and chemical composition (e.g. N- or
P-doped).249,250 Therefore, relative to conventional polycondensation-based
hierarchical carbon synthesis routes, the use of DESs has the potential to
offer a more sustainable alternative by reducing (or even eliminating) the
residues and/or byproducts released as a consequence of the synthesis. However, this approach will become truly “green” when sustainable precursors
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(e.g. Biorefinery-derived phenolics, tannins or even hydroxymethyl furfural)
are employed as carbon precursors in the synthesis.
1.4.8
Exploitation of Polysaccharide Chiral Nematic Phases
The polysaccharides can be viewed, as will be discussed in another approach
in Part 1 of this book, as sustainable precursors for carbon materials. Furthermore, typical polysaccharides; (e.g. chitin, cellulose, amylose, etc.) are inherently chiral materials, arising from stereocentres in their structure. Many
nanocrystalline biomaterials are also known to have morphological chirality
at their surfaces, perhaps best exemplified by the screw-like morphology of
cellulose nanocrystals (CNCs). However, there are very few examples where
this inherent chirality has been exploited in the synthesis of chiral carbon
materials, although other materials including fullerenes,251 and chiral CNTs
have been reported.252 The successful synthesis of chiral porous carbons utilising the inherent stereochemistry of the precursor may open up opportunities in stereoselective adsorption/separation, sensing and stereoselective
catalysis, although this challenge has yet to be fully addressed. The carbonisation of materials prepared from biomolecules often leads to retention of
the morphology into the carbonised product,253–257 but as a consequence the
energetic, bond-breaking, bond-forming, and reorganisation processes that
occur with increasing temperature, the chirality of the parent precursor is
typically lost. However, it is worth noting that cellulose and chitin nanofibres have been shown to form lyotropic liquid-crystalline phases in water
that can be retained in films after water evaporation.258,259 In this regard, the
MacLachlan group have described the synthesis of a chiral nematic form of
carbon using such lyotropic cellulose phases as the precursor.260
In this approach the lyotropic phase of the CNCs were essentially
entrapped within a silica matrix prior to carbonisation, enabling a “fixing” of
the chiral nematic order of the cellulose (Figure 1.11).260,261 The CNCs phase
was combined with a silica precursor (Si(OMe)4) in water, followed by evaporation and drying in a petri dish, to obtain a composite CNCs/silica film.
Circular dichroism spectroscopy (CDS), SEM, and UV-visible spectroscopy
confirmed that the cellulose chiral nematic order was retained in the composite. Carbonisation at 900 °C transformed the polysaccharide into carbon
(∼30% yield) within the silica matrix, with the resulting materials appearing
iridescent, reflective of chiral nematic order (Figure 1.11(d)). Removal of the
inorganic component with NaOH(aq) etching, yielded freestanding semiconducting, amorphous carbon films composed of a mixture of sp2- (graphitic)
and sp3-hybridised carbon (Figure 1.11(d)). Gas adsorption measurements
indicated that the carbon was mesoporous with pores ∼3 nm in diameter
and with a high surface area (∼1500 m2 g–1). The porosity of the carbon films
could be modified by changing the proportion of cellulose to silica used in
the procedure (Figure 1.11(e)). The fine structure of the carbon materials was
reflective of the CNC ordering in the original film (Figures 1.11(f) and (g)).
This approach has since been extended to the preparation of chiral nematic
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Figure 1.11
Chapter 1
Synthesis
of chiral nematic mesoporous carbon. (a) CNC prepared by
hydrolysis with sulfuric acid is mixed with TMOS and slowly evaporated to form chiral nematic NCC–silica composite films. (b) CNC–­
silica composite films are pyrolysed in an inert atmosphere at 900
°C to generate carbon–silica composite films. (c) Silica is removed
from the carbon–silica composite films using 2 M NaOH to generate chiral nematic mesoporous carbon; and the porosity of different
CNC-­derived carbon samples. (d) Photograph of mesoporous carbon
sample CMC-3 (scale bar = 2 cm). (e) N2 adsorption isotherms of CMC1, CMC-3, and CMC-5. (f) TEM image of CMC-1 (scale bar = 200 nm).
(g) TEM image of CMC-3 (scale bar = 200 nm). Reproduced with permission from ref. 260.
mesoporous carbon films via the addition of glucose to the preparation of
the composite precursor.262 To prove the carbon product was indeed chiral
nematic, it was replicated into a thin silica equivalent that showed intense
reflections in the CDS relating to left-handed circularly polarised light that is
diagnostic of the chiral nematic order. Applications that utilise the chirality
of the carbon, however, have not yet been demonstrated.
The approach of the MacLachlan group represents an interesting step forward in the synthesis of porous carbons from sustainable precursors, with in
this case significant high-value potential. However, the approach still relies
on the use of a sacrificial inorganic component, whilst the synthesis is also
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step and resource intensive. Considering the high value potential applications (e.g. asymmetric synthesis), although not necessarily a completely sustainable and green approach to porous carbon synthesis, the benefits may
significantly outweigh these drawbacks.
1.5
Overview and Outlook of the Book
The preceding introductory chapter aimed to introduce the reader to a
number of synthetic approaches that have used sustainable precursors (e.g.
sucrose in CMK-1 preparation; activated carbons) or sustainable solvents (e.g.
hierarchical carbons synthesised in deep eutectic solvents), that present an
opportunity for sustainable precursor use (e.g. carbon aerogels) or exploitation of the special properties afforded by natural, biomass-derived compounds (e.g. inherent chirality in the preparation of chiral nematic carbon).
The use of ionic liquids in the preparation of heteroatom-doped carbons
was also introduced as this approach highlights the potential of doping the
carbon backbone to improve properties (e.g. electroconductivity, oxidation
stability) and generated excellent application performance (e.g. as metal-free
electrocatalysts), although the costs and low yields are potential drawbacks
from an economic and “carbon” utilisation standpoint.
However, for the observant reader it might be possible to discern a
“materials gap” in the field of (porous) carbon nanomaterials (Figure 1.12).
Regarding the approaches briefly discussed in Section 1.4, as a demonstrative example, mesoporosity has commonly involved sacrificial templating,
requiring a high carbonisation temperature to remove the organic template
or render the carbon structure inert to the chemical treatment during inorganic template removal. Likewise, whilst offering outstanding properties,
CNTs or graphene are also processed in a consumptive and high-temperature manner (e.g. chemical vapour deposition), often in low material yields,
whilst the products themselves are conventionally nonporous and not dispersible (e.g. in aqueous solution). Carbon aerogels are typically based on
condensed aromatic precursors (e.g. from RF-gel systems), which limits the
range of product functionalities open to the materials chemistry after the carbonisation step. Furthermore, high-temperature processing (≥1000 °C) also
commonly introduces extensive microporosity and inherently makes the carbon materials hydrophobic, which for some applications can be highly undesirable (e.g. chromatography or aqueous phase chemistry, respectively). Also,
as precursors, these synthetic approaches typically employ petrochemical-­
derived compounds. Whilst charcoals and activated carbons are “functional”
or “carbonaceous”, they are typically microporous and offer limited scope for
porous structure control.
Therefore, can the carbon materials chemist develop new functional carbons featuring high degrees of mesoporosity, avoiding the high temperature
and wasteful templating strategies of old, importantly employing uncondensed, sustainable biomass-derived precursors (Figure 1.12)? In this context, excluding classical microporous/activated carbons, a limited amount
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Figure 1.12
Chapter 1
Simplistic
representation of a perceived “materials gap” in carbon
materials synthesis (as indicated by “?”) with regard to porous structure compared to common porous carbon materials.
of research has been conducted regarding the synthesis of highly porous,
functional carbon (aqueous) materials based on sustainable, biomass-­
derived precursors. However, carbon synthesis (or indeed organic chemical
synthesis) from biomass is not a new topic, it has been relevant since the
nascence of civilisation, with the modern petrochemical era being a relatively recent arrival on the industrial landscape. As such, any new advances
in porous carbon synthesis from sustainable precursors would be of great
commercial and research interest as the final products would represent a
significant proportion of modern materials. These materials would be even
more relevant if they were applied successfully in future energy and chemical provisions schemes (e.g. the Biorefinery). The preparation of porous
carbons from sustainable precursors (excluding microporous/activated carbons) is a rapidly growing, topical area and is increasingly being recognised
not only in terms of application/economic advantages but also in terms of a
sustainable approach to nanomaterial synthesis. Up until relatively recently,
the main inhibitor in this area has been the lack of developed routes for the
transformation of sustainable, biomass precursors into useful porous carbon nanomaterials (e.g. without sacrificial templating and high-temperature
syntheses) and the demonstration of application benefits over commercial
equivalents. In this regard, the following chapters will introduce and focus
on the preparation of carbonaceous nanomaterials from sustainable precursors, predominantly sourced from biomass, via two similarly successful
aqueous-based synthetic approaches. As will be revealed throughout Parts 1
and 2 of this book, these strategies open up opportunities for multidimensional carbon-based nanomaterial design and the synthesis of functional,
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nanostructured porous materials, generating interesting and potentially
superior properties and application performance to traditional inorganic
solids and petrochemical-derived carbons.
As will be demonstrated throughout this book, the synthesis of carbon nanomaterials may in principle utilise natural, sustainable building blocks but may
also include the manipulation of inorganic and artificial organic components
at the nanoscale to generate the desired material property – akin to the development of natural systems. Thus, it is hoped that readers acquire a taste for the
opportunity to manipulate carbonaceous matter at the “nano” level allowing
in principle customisation of carbon nanostructure in terms of surface, texture
and nanoporosity for a specific application. From a materials-chemistry point
of view, the synthetic approaches discussed are also of significant interest. The
low-temperature syntheses provide highly functional carbonaceous materials, often coupled with selectable porosity and morphology – this is a serious
advantage as it allows materials evolution via the investigation and optimisation of structure–activity relationships for the range of applications presented
by future energy and chemical economies; e.g. fine tuning of materials hydrophobicity for aqueous-phase conversion chemistry; manipulation of the sp2/
aromatic carbon character of the material for a given polar analyte separation;
direction of “N” content and bonding motifs (e.g. pyrrolic, pyridinic, etc.) for
metal-free heteroatom-doped carbon-catalysed 2 or 4 e– oxygen-reduction reactions in fuel cells. The described sustainable porous carbons can be viewed
as a new class of chemically designed carbonaceous nanostructured materials
bridging the material divide between more classical “carbons” and traditional
inorganic materials. The platforms presented in this book are extremely flexible and opening opportunities for the future development of sol-gel carbon
chemistry.
It is obviously of interest to be “sustainable” in terms of carbon nanomaterials synthesis; however advantages in terms of future energy and chemical
provision (e.g. the Biorefinery or Methanol Economy) must also be demonstrated. This would represent a significant step forward in carbon nanomaterials science and could ultimately (e.g. when process/engineering dynamics
are appropriately established) result in a double “C” fixation (or CO2 sequestration), in both the material and chemical product – a linking of natural and
anthropogenic carbon cycles and transforming biomass (e.g. saccharides or
CO2) from liabilities to assets. This would mean that GHG emission reduction
can be achieved through the production of not only energy molecules/fuels/
chemicals but also via porous carbon nanomaterials from sustainable precursors.
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PART 1
STARBONS®
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CHAPTER 2
From Polysaccharides
to Starbons®
VITALIY L. BUDARINa, PETER S. SHUTTLEWORTHb, ROBIN J.
WHITE*c AND JAMES H. CLARKa
a
Green Chemistry Centre of Excellence, University of York, Department of
Chemistry, Heslington, York, YO10 5DD, UK; bDepartamento de Física de
Polímeros, Elastómeros y Aplicaciones Energéticas, Instituto de Ciencia
y Tecnología de Polímeros, CSIC, c/ Juan de la Cierva, 3, 28006, Madrid,
Spain; cUniversität Freiburg, FMF - Freiburger Materialforschungszentrum,
Stefan-Meier-Straße 21, 79104 Freiburg im Breisgau and Institut für
Anorganische und Analytische Chemie, Albertstrasse 21, 79104
Freiburg, Germany
*E-mail: robin.white@fmf.uni-freiburg.de
2.1 Introduction
Taking into account their natural abundance and general low cost, coupled
with a well-known capability to form thermoreversible “expanded” aqueous
gels, polysaccharides can be considered by the materials chemist as excellent precursors for the preparation of functional materials. Preservation of
the expanded gel phase can be achieved by careful drying to produce porous
polymer-based cryo-, xero- and aerogels, is one approach to the opening up
of the often compact polysaccharide state to generate high surface area, high
volume, typically hydroxyl rich functional porous phases.1 Furthermore, such
“gels” can be viewed as excellent precursors for the preparation of porous
carbon-based materials and it is in this context that this chapter is discussed.
The successful elaboration of “porous polysaccharide-to-carbon” synthetic schemes provides the opportunity to the green materials chemist to
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Porous Carbon Materials from Sustainable Precursors
Edited by Robin J White
© The Royal Society of Chemistry 2015
Published by the Royal Society of Chemistry, www.rsc.org
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Chapter 2
valorise typically low value and often waste biomass for the preparation of
new porous, higher-value media. As will be briefly discussed, whilst extremely
interesting materials in their own right, “soft” porous polysaccharides gels
often suffer from poor mechanical/chemical resistance and in turn applications can be rather limited although many of these structures are extremely
attractive as metal (nanoparticle) catalyst support media, although a number
of recent reports elude to the synthesis of cellulose and chitin-based aerogels
with exceptional mechanical properties.2–7 The theme of this chapter is the
transformation of such porous polysaccharide-based gels to produce more
stable porous carbonaceous forms to circumvent problems associated with,
e.g., chemical resistance/thermal stability. This opens new synthetic pathways to the synthesis of a variety of nanostructured sustainable carbon-based
materials, the properties of which can in principle be directly applied to a specific application. The highly functional and often, “noncondensed” chemistry of the porous polysaccharide precursors (e.g. amylose, starch, alginic acid,
chitosan, etc.) also enables the development of a carbon materials platform to
synthesise highly functional more condensed structures, the physiochemical,
bulk and surface properties can be tuned and directed. This renders materials
with properties that essentially fill a “materials” void between conventional
activated carbons and porous inorganic materials. As will be shown, this can
be achieved using relatively simple, controllable synthesis parameters.
The conversion of photosynthetic products – polysaccharide biomass – to
more thermochemically condensed, carbonised forms also potentially contributes to environmental benefits, as this process may represent a form
of carbon sequestration particularly if the polysaccharide is derived from
fast-growing plants. The utilisation of such saccharide-based products of
photosynthesis, for the production of new functional, nanoporous materials (e.g. cryo- and aerogels), is receiving increasing amounts of interest both
academically and commercially due to the range of economic/process/chemistry advantages offered by such synthetic approaches (e.g. in biomaterials/
medicine). From a sustainability standpoint, the synthesis of such sustainable materials can, if conducted correctly, potentially represent a holistic
approach to the production of novel, inexpensive and highly applicable
“soft” polymeric and carbonaceous materials. This chapter will introduce the
reader to the Starbon® concept, its history and the variety of interesting carbonaceous materials that are accessible via the development of porous polysaccharide-derived materials (PPDMs). The chapter intends to provide the
reader with an overview of the area and highlight the exciting opportunities
open to the materials chemist based on the discussed synthetic approaches.
2.2 Porous Polysaccharide-Derived Materials
Nature provides a wide range of biosynthetic sugar-based polymers – the
polysaccharides (Figure 2.1). These renewable resources are readily available,
inexpensive and functionally rich (e.g. –OH, –C(O)OH, –NH2).8 They are the
products of natural processes (e.g. photosynthesis) and perform a wide range
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From Polysaccharides to Starbons®
55
Figure 2.1 The chemical structures of some common polysaccharides; (A) Chitin
(deacetylation leads to Chitosan); (B) hemicellulose; (C) cellulose; and
(D) a basic α-polyglucopyranose structure forming the basis for the
branched amylopectin and the linear amylose.
of biological functions, including as membrane and cell-wall components,
storage of photonic energy and as sequestering agents for water, nutrients
and metals in the cell environment.8–12
From a materials point of view, polysaccharides are known to self-associate or order into particular structures, physical forms or shapes in nature
(e.g. the starch granule, plant cell structures, etc.).8,13 They are also known,
perhaps more significantly in the context of this chapter, to form aqueous
“expanded” gels, which if desired can be dried to yield a porous solid.2 This
“expanded” phase provides the opportunity to the materials chemist to
access a range of novel porous materials including, cryo-, xero-, and aerogels. In their native form, polysaccharides have a low surface area and little
developed porosity. The “expansion” of these compact (often semicrystalline) polymeric structures is therefore vital for the development of porous
materials (e.g. sustainable porous carbons) that are relevant in applications
where mass transport/diffusion (e.g. chromatography) and surface interactions (e.g. liquid-phase catalysis) are critical to function. In this respect, the
early work of Glenn et al. and Te Wierik et al. in the 1990s, based on starches
demonstrated the preparation of xerogels (SBET < 145 m2 g−1), prepared via
a sol-gel-like process involving the thermal gelation and recrystallisation
(often to referred to as “retrogradation”) of starch, followed by the careful replacement of pore-­entrapped H2O for a lower surface tension solvent
(e.g. CH3CH2OH) and eventually air (e.g. via supercritical extraction).14–16
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Chapter 2
This work was revisited in the 2000s at the Green Chemistry Centre of
Excellence, University of York, where Clark and coworkers demonstrated the
potential of corn starch (∼73% amylopectin) in the production of low-density,
high surface area starch xerogels (SBET ∼ 120 m2 g−1). The resulting porous
starches were employed in stationary media in normal phase chromatography separations,17 and in the preparation of solid acid catalysts (e.g. starchSO3H).18,19 This work was extended to the microwave-assisted preparation of
high surface area (SBET > 180 m2 g−1), highly mesoporous starch-derived materials (Vmeso > 0.6 cm3 g−1; > 95% mesoporosity).20 This research demonstrated
that the key to the formation of the porous polysaccharide form in starch was
the generation of a gel phase via investigations based on the aqueous-phase
adsorption of a methylene blue probe dye. The obtained aqueous gel upon
extraction of gel bound water via a solvent-exchange process normally for
a lower surface tension alcohol (e.g. ethanol) and drying, yields low-density(ρ < 0.3 cm3 g−1) porous polysaccharide xerogels. ScCO2 of the alcohol saturated polysaccharide gel produces the corresponding aerogel materials, with
greatly enhanced porous properties.21–23 Using CdS and OsO4 contrast agents
for TEM, it was observed that the porous starches were composed of a number of slit-shaped pores that arose from polysaccharide nanocrystallite associations (Figure 2.2).20 In the same study it was also demonstrated that the
key mesopore forming polysaccharide in starches is the linear poly-α-(1-4)
glucopyranose, amylose, as opposed to the other branched starch polysaccharide, amylopectin.20
This work has been complimented by the Smirnova group who have
demonstrated the production of a variety of starch- and pectin-based aerogels in recent years, whilst the noteworthy work of the Quignard group over
the last decade or so has led to the preparation of a variety of polysaccharide
and polysaccharide–metal hybrid aerogels (Figure 2.3).2,21–23 These studies
demonstrate the possibility to shape the polysaccharide gel prior to drying
Figure 2.2 (A) Osmium tetroxide stained MS (180 °C) mesoporous particle, showing
the position of amylopectin and amylose. (B) Mesoporous polysaccharide supported CdS quantum dot material (MS (130 °C)), demonstrating the mesoporous pore structure. Reproduced with permission from
ref. 20.
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From Polysaccharides to Starbons®
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to form, for example, polysaccharide aerogel beads that have found application in drug delivery and heterogeneous catalysis (Figure 2.3).2,3,5–7,21–25
The reassociation and reorganisation of the polysaccharide chains during
recrystallisation generates a porous “aqueous” gel state, presenting material morphology similar at the micro- and nanometer scale to conventional
polymer-based (e.g. resorcinol–formaldehyde) aerogel materials.
Ours and other recent work has taken this approach to essentially form
a general “porous polysaccharide” preparation scheme (Scheme 2.1), in
principle applicable to the majority of polysaccharides.1,26 Gelatinisation
temperature/heating mechanism (e.g. microwaves),20 polysaccharide type/
structure,2,6 additive,21 and drying technique,22,23,27 have all been utilised to
direct textural (e.g. micro- vs. meso- vs. macroporosity) and physicochemical
properties (e.g. solid base vs. acid).6 Such porous polysaccharides (e.g. porous
starches) have been subsequently used as soft, sacrificial templates for the
preparation of a variety of inorganic and hybrid materials.28
Whilst the porous aqueous gel phase produced from the gelatinisation/
recrystallisation process is relatively stable and can be stored without significant reduction in the porous properties, the corresponding dried products
Figure 2.3 Optical and SEM images of hydrogel (first column) and aerogel spheres
(second column) and of cross sections of aerogel spheres (third and
fourth columns) of Cu-alginate (row a), chitosan (row b) and carrageenan (row c). Reproduced with permission from ref. 2.
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Scheme 2.1 Basic flow sheet description of the preparation of porous polysaccharide-derived materials (e.g. starch aerogels).
(e.g. aerogels), as might be expected for such a high surface area, hydrogenbond-based network, the system can be described as “metastable” after
drying and the replacement of solvent in the pores with air. The porous polysaccharides are essentially soft polymeric “metastable” networks, stabilised
by dense hydrogen-bonding between associated chains (e.g. helices) and
localised domains of short-range order, which have organised in the presence of H2O to regain the entropy lost during the expansion step. However,
if these promising textural and porous properties are produced from these
sustainable gel precursors, the question therefore is how one can stabilise
this transient and ultimately useful expanded surface area and porosity, in a
manner that goes beyond simple chemical crosslinking procedures.
2.3 First-Generation Starbons® – from
Starch to Carbon
One of the main inhibitors to the development and application of PPDM
is generally speaking their low mechanical/chemical resistance and “metastable” porosity. The PPDMs textural properties, although possible to rejuvenate, degrade with extended storage and exposure to moisture, as the
hydrogen-bond network relaxes in an attempt to return to lower energy
configurations. One approach to circumvent these problems, developed at
the Green Chemistry Centre of Excellence, (University of York, UK), initially
focused on the transformation of porous starches into more stable porous
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Scheme 2.2 The synthesis of Starbons® based on thermal decomposition of a
porous polysaccharide precursor, depicting the variety of functional
groups that can be generated via carbonisation temperature selection
and the associated ability to tune certain physicochemical properties.
carbonaceous forms, in a process developed by Budarin et al.; a materials
technology now classified under the trademark “Starbons®”.1,26,29,30 As will
be shown, this approach has now been generalised to a range of polysaccharides, with materials presenting chemical properties that are intermediary
between the polysaccharide precursor (e.g. Star… from the 1st-generation
precursor starch) and conventional carbons (…bons) (Scheme 2.2).
Starbons® may in principle be prepared at any temperature in the range 150
to 1000 °C, with increasing carbonisation temperature (Tp) leading to the synthesis of robust predominantly mesoporous carbons with, as will be discussed
throughout, properties suitable for a wide range of technologically important
applications, including heterogeneous catalysis, water purification, and separation media.31–34 The stability of these PPDM-derived Starbons® at Tp < 1000 °C,
provides scope to tune surface, textural and morphological properties,
dependent on the carbonisation temperature employed and the polysaccharide used as the precursor. The distinctive feature of Starbon® materials
technology is the tuneability of chemical properties between those that are
similar to the original polysaccharides (e.g. starch; high oxygen surface content) and more classical carbon surfaces, such that Starbons® effectively unite
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Chapter 2
the surface accessibility of mesoporous carbons with the complex chemistry
of charcoals (Scheme 2.2).
As mentioned earlier, 1st-generation Starbons® are derived from porous
starches. Starch (a composite of amylose and amylopectin) is a neutral polysaccharide composite. Conversion of inherently nonporous starch into highly
mesoporous forms (the properties of which are strongly dictated by the amylose/amylopectin ratio) was discussed in Section 2.2. However, to convert this
porous polysaccharide into a porous carbonaceous form, an acid-catalysed
thermal decomposition of the porous starch is required if the promising porosity and texture are to be maintained into the final product. Thermal decomposition without the use of an acid catalyst results in melting of the polysaccharide
network, before the necessary dehydration-initiated mechanism (required to
maintain the porosity) occurs. The use of the acid catalyst (i.e. p-toluene-­sulfonic
acid) allows the decomposition mechanism to proceed at temperatures (e.g. <
150 °C) lower than the “network melt” and the successful conversion of the
porous starch phase into stable nanostructured, porous carbonaceous materials. Therefore Starbon® prodution comprises three generalised stages:
1. preparation of an aqueous starch gel via a gelatinisation/retrogradation
step;
2. removal of the impregnated water via solvent exchange (and if required
supercritical drying) to generate a predominantly mesoporous starch
with SBET = 180–200 m2 g−1 (e.g. for high amylose corn starch);21
3. neutral mesoporous starch is then doped with a catalytic amount of an
organic acid (e.g. p-toluenesulfonic acid) and heated under vacuum to the
desired temperature to yield the carbonised equivalent – the Starbon®.
The lack of a template importantly avoids wasteful processing steps and
harmful chemicals enabling materials to be prepared at a temperature of
choice (e.g. Tp = 150–1000 °C). This enables surface chemistry tuneability
amenable to facile postmodification strategies (Scheme 2.2). Furthermore,
as a consequence of preparation-dependent surface chemistry, the hydrophilicity vs. hydrophobicity properties may be moderated, generating the
possibility of designer material synthesis for specific applications.
The material morphology of the starch-derived Starbon® resembles the
nanoscale morphology of the parent porous starch, structurally composed of
primary particles that have aggregated to form a typical “gel” network, with
the primary particle size being ca. < 30 nm (Figure 2.4). TEM image analysis
of starch-derived Starbon® materials prepared at increasing Tp, indicated that
the decomposition/carbonisation of the polysaccharide initiates at the pore
wall or entrances – associated with the point of deposition of the catalyst.
As will be discussed later, this decomposition behaviour has consequences
in terms of material porosity. The macromorphology of starch-derived Starbons® is complicated from a colloidal/nucleation chemistry description due
to the presence of “granule ghosts”, which are the remnants of the starch
granule structure. These may act as nucleation points for the recrystallising
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Figure 2.4 TEM of 1st-generation starch-derived Starbons produced at increasing
Tp: (A) Mesoporous starch; (B) 100 °C; (C) 150 °C; (D) 220 °C; (E) 300 °C;
and (F) 450 °C. Reproduced with permission from ref. 29.
porous polysaccharide phase during gel preparation. One way to remove
such “ghosts” is to go higher gelatinisation temperatures (e.g. > 170 °C), and
use the recrystallisation of the amylopectin as the seed upon which a highly
mesoporous, amylose-rich phase is deposited, leading to the formation of
regularly sized (∼ 5 µm), mesoporous/porous starch spheres.20 These regular
particles can then be converted into the Starbon® equivalent.
Examination of 1st-generation starch-derived Starbons® prepared at
increasing Tp via a variety of analytical techniques revealed the gradual
change in chemical functionality (bulk and surface), surface area and porosity (Figure 2.5). Prior to the “starbonisation” process, the porous starch precursors presented a SBET of ca. 180 m2 g−1, negligible microporosity with an
average pore diameter of ca. 6 nm, reflected in the observed Type IV/H3 N2
sorption isotherm (Figures 2.5(A) and (B)). Conducting the carbonisation
process results in maintenance of the total pore volume whilst pore-size
distributions demonstrated a predominance of mesopores (Vmeso = 0.4–0.6
cm3 g−1/Pore diameters = 8–16 nm). Significantly, the pore diameters of these
1st-generation Starbons® is > 5 nm, enabling access to pore sizes greater
than those typically synthesised based on previously reported hard-templating approaches.35 A substantial increase in the contribution of microporosity at Tp = 300 °C was observed and was proposed to be due to noncatalytic
decomposition processes, although mesoporosity remains the predominant
contributor to porosity (Figure 2.5(B)). At Tp > 600 °C, the materials present
well-defined micropores (D = 0.5 nm) and the corresponding porous distributions become bimodal in character, whilst specific surface areas typically
exceed 500 m2 g−1.
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Figure 2.5 Textural and chemical properties of 1st-generation starch-derived Star-
bons® prepared at increasing carbonisation temperature (Tp) – (A) Nitrogen
sorption isotherms; (B) Pore characteristics and surface areas – Expanded
starch doped with acid and heated at the indicated temperature [PV = pore
volume, SA = surface area, PD = pore diameter]; (C) DRIFT spectra compared to that of expanded starch and commercial carbon Darco KB; and
(D) 13C CP MAS NMR spectra. Reproduced with permission from ref. 29.
Diffuse reflectance infrared spectroscopy (DRIFT) analysis revealed that in
the Tp interval up to 250 °C, there is a thermal/catalysed dehydration and loss
of the starch hydroxy group functionality (i.e. diminishing peak intensity of
bands at 3300 and 1000 cm−1), alongside the appearance of bands associated
with carbonyl groups conjugated with olefinic (i.e. 956 and 1715 cm−1) and
vinyl ethers groups (i.e. 950–1200 cm−1) (Figure 2.5(C)). Heating to higher Tps
(e.g. 200–600 °C) results in the formation of an increasingly stronger “aromatic” character as evidence by bands at 875, 817, and 750 cm−1 (i.e. aromatic
C–H out-of-plane bending vibrations), as complemented by solid-state 13C
NMR investigations that revealed the development of aromatic systems with
increasing Tp (i.e. a broad arene/aromatic related resonance at δ = 123–128
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ppm) (Figure 2.5(D)). Higher Tps (i.e. > 700 °C) resulted in the formation of
very condensed extended aromatic structures, as evidenced by a reduction in
the corresponding band intensities from DRIFT spectroscopy.
It is worthwhile noting at this point, that such micro- and spectroscopic
analysis demonstrates quite elegantly one of the significant benefits of the
Starbons® technology platform; that is the relatively straightforward manner in which both bulk and particularly surface chemistry can be directed
or “designed” using the simple control vector of carbonisation temperature. This approach allows the production of a wide range of functional carbonaceous materials, the properties of which range from the hydrophilic,
polymeric materials through to highly condensed carbon-like materials,
importantly applicable to a wide range of applications.
2.4 Second-Generation Starbons®
2.4.1 Pectin-Derived Starbons®
Pectin, commonly derived from commercial waste citrus peel, is considered
to be a promising precursor for a wide variety of platform chemicals and
materials.21,36,37 Pectin is a multifunctional polyuronide – an α(1→4) linked
poly-galacturonic acid with a percentage (depending on the natural source)
of galacturonic acid monomers bearing a methyl ester at the C-6 carboxylic
acid position, with the resulting polysaccharide presenting a surprisingly low
pKa of 2.9 to 4.8, (dependent on the degree of esterification).38,39 In the context of this chapter, it is known that pectin gels can either be formed via thermal dissolution and retrogradation or by lowering the system pH.40 Utilising
these different gelation routes (i.e. inducing different gel structures as a result
of the polysaccharide configuration), porous pectin aerogels with remarkably
different pore structuring and morphology were generated with SBET > 200 m2
g−1, total pore volumes typically > 1.0 cm3 g−1 and N2 sorption behaviour reflective of large volume mesoporous materials (i.e. Type IV/H3 hysteresis).41 It
was also observed that depending on the preparation route employed, either
powders or monolithic forms of porous pectin could be prepared (Figure 2.6).
Given that the prepared pectin aerogels were composed of inherently
acidic polysaccharides, it was postulated that these porous polysaccharides
could be directly “carbonised” to yield Starbon® materials without the addition of p-toluene sulfonic acid dehydration catalyst (i.e. as for neutral porous
starch precursors). This approach proved to be successful with the synthesis of pectin-derived carbonaceous materials – adding another route to Starbons® from PPDM precursors, significantly using a precursor here sourced
from waste and therefore not competing with food sources. It was also found
that gels formed via a lowering of the system pH, could be “set” into any
desired form that could then be transferred into the carbonised Starbon®,
although dimensional shrinkage with increasing Tp was observed (Figure
2.7). Impressively, high SBET (>280 m2 g−1) and very attractive mesoporous features (Vmeso > 1.2 cm3 g−1; > 20 nm average pore diameter) were reported for
these carbonised monolithic pectin-derived Starbon® aerogels.
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Figure 2.6 Preparation route(s) to porous pectin demonstrating effect of gela-
tion mechanism on material morphology and N2 sorption properties.
Reproduced with permission from ref. 41.
By contrast, pectin-derived Starbons® prepared via thermal gelation, presented
nanoporosity and material morphology very similar in appearance to material
prepared from porous starches, whilst pectin-derived Starbons® prepared via
acid gelation, presented dramatically different nanoscale morphology with
materials being composed of a continuous, tortuous, carbonaceous nanorods
(Figure 2.8). This example quite elegantly demonstrates that this subtle difference in preparation methodology (i.e. the gelation step), can allow in part direction of the carbon morphology (as well as textural properties). Here the use of
the acid in the gelation step was considered to be key, leading to a reduction
in the amount of methylated carboxylate groups (i.e. acid hydrolysis increased
the number of “free” –C(O)OH groups), which effected the overall charge of the
polysaccharide, its phase-separation behaviour and perhaps most significantly
the polysaccharide configurations (e.g. as a consequence of different torsion
angles around the glycosidic bond) and hence, the change in the material morphology with respect to thermally gelated pectin-derived Starbons®.
It is worth noting that the synthesis of pectin-derived Starbons® (as for
alginic acid-derived material; Section 2.3), have a number of benefits over
1st-generation starch-derived Starbons®:
1. They are more uniform.
2. Access to the more functionally rich materials prepared at low Tp is easier.
3. Mesoporosity (in terms of volume and pore size) is considerably larger.
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Figure 2.7 Photograph depicting a pectin-derived aerogel monolith and corresponding carbon monoliths prepared at increasing carbonisation temperature (Tp). Reproduced with permission from ref. 41.
Figure 2.8 SEM (A) and (B) and TEM (C) and (D) Images of pectin-derived carbonaceous aerogels at Tp = (A) 350 °C, (C) 450 °C, and 700 °C (B) and (D).
Reproduced with permission from ref. 1.
The ability to easily form monolithic carbonaceous bodies is also another
significant advantage in terms of catalysis and separation/remediation applications, further adding value to this otherwise large-scale commercial waste
polysaccharide.42 It is also worth noting that in nature there exists a wide
range of pectins presenting varying degrees of methylation, molecular weight
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and degrees of branching, which potentially opens up access to a broad range
of pectin-derived gels and ultimately their carbonaceous derivatives.
2.4.2 Chitosan-Derived Starbons®
Recently, the production of basic chitosan (poly-β(1 → 4)-D-glucosamine)-based
aerogels have been reported, exemplified by the seminal work of the Quignard
group.2,3,5–7 As an approach adapted from this early work, we have recently presented initial work regarding the synthesis of porous (SBET > 140 m2 g−1; Vpore >
1.0 cm3 g−1), fibrous, chitosan aerogels to be used as precursors for the preparation of nitrogen-doped Starbons®.43 In this context, the direct thermal conversion of the chitosan aerogel was found to be very sensitive to heating rate
and in terms of porosity/morphology also to the final end carbonisation temperature. At low carbonisation temperatures (i.e. Tp < 650 °C), carbonaceous
materials with high nitrogen contents (i.e. 7.0–11.0 wt%) were synthesised,
which unfortunately retained only a proportion of the advantageous porous
properties of chitosan aerogel precursor. The carbon nanostructure of these
N-doped Starbons® was characterised with SEM and TEM, being composed
of a particularly attractive, fibrous nature at both the macro- and nanometre
scale (Figure 2.9).
Figure 2.9 SEM (A) and (B) and TEM (C) and (D) images of (A) chitosan-derived
aerogel (CA); (B) CA-derived nitrogen-doped carbon prepared at 450 °C;
(C) at 750 °C; and (D) 900 °C. Reproduced with permission from ref. 43.
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Increasing the Tp to 750 and 900 °C resulted in a partial collapse of the
(meso)porous structure and reduction in the promising textural properties
presented by lower-temperature materials. Characterisation by XPS, TG-IR
and electron microscopy, revealed that these textural changes occurred concurrently with a transformation in the surface nitrogen state(s) (i.e. pyrrole to
pyridinic), a corresponding reduction in surface nitrogen content and a folding/twisting up/densification of the nitrogen-doped carbonaceous fibres,
with SEM and TEM demonstrating elegantly this morphological transition
(Figure 2.9). The nanostructures of these N-doped Starbons® present high
curvature and interconnected porosity – such features are considered promising in the context of heterogeneous base catalysts. However, it is important
to note that work is still continuing on the production of heteroatom-doped
Starbon® materials with the aim to ultimately optimise synthesis such that
maintenance of the promising porous/textural properties of the parent
PPDM is carried into the carbonised product in unison with the successful
introduction of heteroatoms (e.g. N, S, P) into the Starbon structure. The synthesis of such Starbon® variants will also add another analytical dimension
to the characterisation of these promising carbon-based materials.
2.4.3 Alginic Acid-derived Starbons®
Alginic acid is a complex algal- or seaweed-derived acidic polysaccharide.8
Similar to pectin, this polysaccharide is polyuronide block copolymer composed of linear segments of (1 → 4)-linked β-D-mannuronic acid (M), α-L-guluronic acid (G) residues, synthesised as homopolymeric units of either M or
G residues or heteropolymer segments of M + G sequences.44 Furthermore,
in the context of the direct conversion of PPDMs into Starbons®, alginic acid
is also known to have a very strongly acidic character (i.e. pKa ∼ 3.0–3.8).45,46
Based on this inherently strong Brønsted acidic polysaccharide, porous
forms of alginic acid (i.e. xero- or aerogels) were prepared based on a thermal gelation (at 90 °C), followed by recrystallisation and controlled drying
(Scheme 2.1). The resulting porous alginic acid presented SBET > 250 m2 g−1,
pore volumes in excess of 1 cm3 g−1 and pore diameters > 20 nm, with the
presence of Brønsted acid sites confirmed via pyridine adsorption/DRIFT
spectroscopy analysis.
Therefore, it was anticipated given the highly mesoporous nature,
increased pore size and volume, and different meso : microporous ratio of
porous alginic acid as compared to porous starch, Starbons® materials with
distinctly differing textural properties could be accessed from this alginic
acid precursor (i.e. as compared to the acid-doped starch-based synthesis).
Furthermore, thermal analysis of alginic acid demonstrated that this porous
polysaccharide aerogel had a very similar thermal stability to acid doped
starch (i.e. used in 1st-generation Starbon® synthesis), with the main decomposition event importantly proceeding at a temperature (ca. 181 °C) below the
main melting/hydrogen bond breakdown of the polysaccharide network.33
As for pectin aerogels, to convert these very porous forms of alginic acid to
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®
Starbons , a simple heating step was performed to the desired Tp (under Ar)
to yield high surface area, mesoporous carbonaceous materials (Vmeso > 1 cm3
g−1 at Tp = 1000 °C), and large mesopore size (PD ∼ 14 nm), in all cases presenting, independently of Tp, Type IV/H3 reversible N2 sorption profiles; typical of polysaccharide-derived mesoporous materials (Figure 2.10(A)).
FT-IR analysis of porous alginic acid-derived Starbons® demonstrated a
gradual loss in polysaccharide character upon heating to Tp = 250 °C, as evidenced by the diminishing intensity/resolution of bands in the 1160–960 cm−1
region relative to carboxylic bands (i.e., 1727 and 1615 cm−1), the result of the
loss of chemisorbed water and decarboxylation (Figure 2.10(B)). Increasing
Tp to 300 °C, resulted in the development of carbonyl/olefinic-type groups,
as demonstrated by absorption bands in the 1750–1550 cm−1 range, which
appeared in unison with a band centred at 1605 cm−1 (i.e. for conjugated (C=C)
groups). As the carbonisation temperature passed 300 °C, bands associated
with C=O containing groups transited from 1727 cm−1 (AS1), 1724 cm−1 (Tp = 200
°C) to 1694 cm−1 (Tp = 300 °C), ultimately disappearing at Tp = 500 °C, as other
C–O-containing groups (e.g. olefinic, vinyl ethers, or lactones) evolved as the
main polysaccharide backbone decomposed and the carbonisation process
proceeds. Analogously to the synthesis of starch-derived Starbons®, distinct
aromatic features appeared at a Tp as low as 250 °C and dominate the spectra
at a Tp = 500 °C, (i.e. out-of-plane bending modes at 878, 820, and 755 cm−1
aromatic C–H). Complementary to this FT-IR analysis, solid state 13C CP MAS
NMR analysis revealed similar functional-group decomposition and formation events in the materials and demonstrates succinctly the transformation
of the hydroxyl-rich polysaccharide precursor into aliphatic/alkene groups,
through to carbonaceous materials presenting increasingly strong aromatic
character as a function of Tp (Figure 2.10(C)).
TEM images of alginic-acid Starbons® (e.g. from the aerogel AS1) provided
clear evidence of the mesoporous nature of the resulting carbonaceous materials (Figure 2.11). A combination of pores with diameters > 50 nm were also
observed revealing a contribution from macropore domains and ultimately,
in unison with N2 sorption, a hierarchical pore structuring in these Starbon®
materials. The TEM microscopy images also revealed slit-shaped pore morphology that was typical for all alginic-acid derived Starbons® independent
of Tp. This type of pore structuring is also representative of the Type IV/H3
isotherms observed from N2 sorption analysis (Figure 2.10(A)).
TEM microscopy also indicated that the carbonaceous materials were
ordered in a very localised manner, proposed to be the product of locally
aligned, linear segments of the parent polysaccharide gel that are transferred into the carbon product as a consequence of the uniform decomposition of the material system during carbonisation. These local domains
were believed to be essentially “crosslinked” during carbonisation, rendering the material nanostructure with a tectonic rod-like structure (i.e.
lengths of 200 nm/10–20 nm thickness) perforated by a series of interconnected slit pores.
gel (AS1) and its carbonaceous (Starbon®) equivalents prepared at increasing carbonisation temperature. Reproduced and
adapted with permission from ref. 33.
Figure 2.10 (A) N2 sorption; (B) DRIFT spectroscopy; and (C) solid state 13C CP MAS NMR spectroscopy analysis of alginic acid aero-
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Figure 2.11 TEM images of alginic acid aerogel-derived carbonaceous materials
prepared at increasing Tp. (A) 200 °C; (B) 300 °C; (C,D) 500 °C; (E,F)
1000 °C.
As will be discussed further in Chapter 4, the use of high surface area,
highly mesoporous aerogels derived from alginic acid, as precursors for the
generation of Starbon® stationary phases for the analytical LC–MS separation of saccharides and disaccharide isomers has been reported by White
et al.33 This work has recently been extended by Clark et al. based on the synthesis of the synthesis of Ca-Alginate-derived Starbons® (denoted as AMCS)
which presented increased particle size regularity relatively to the original
Starbon® stationary phase of White et al., without sacrificing the pore structuring of the original porous alginic acid-derived precursor.34 This work will
be discussed in more detail in Chapter 4. As a complimentary work to the
report of Clark et al., Brydson et al. have examined the differences in microstructure and bonding of these two alginic acid-derived stationary phases as
compared to a commercially available carbon stationary phase (i.e. porous
graphitised carbon (PGC)).47 A combination of HRTEM (Figure 2.12), electron energy loss spectroscopy (EELS), N2 sorption and XPS analysis allowed
a fine description of the relative differences between the three carbon-based
stationary phase media, indicating that planar carbon sp2 content was very
similar to that of traditional nongraphitising carbons, although both the
alginic acid-derived materials showed a much greater fullerene character
(i.e. curved graphene sheets). HRTEM images demonstrate that the alginic
acid-derived Starbon-800 °C (A800) had a much less ordered nanostructure
as compared to PGC, with only minimal ordering of (002) graphitic planes,
(ca. 2–3 aligned layers) (Figure 2.12(A–D)).
For alginic acid-derived material prepared at 1000 °C, low-magnification HRTEM microscopy images demonstrated that there was no change
in the material micro- or pore structuring as a consequence of increasing
the Tp from 800 °C (Figure 2.12(E)). Increased magnification allowed the
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Figure 2.12 High-resolution TEM images of commercial PGC at (A) Low and (B)
High magnification; Alginic acid-derived Starbon® prepared at 800 °C
(A800) at (C) Low (Scale bar = 50 nm) and (D) High magnification (Scale
bar = 5 nm); A1000 (E) low and (F) High magnification; and AMCS-NW
(G) Low (Scale bar = 50 nm) and (H) High magnification (Scale bar =
5 nm) and AMCS-W (I) Low (Scale bar = 50 nm) and (J) High magnification (Scale bar = 5 nm). Reproduced with permission from ref. 47.
visualisation of the developing fullerene-like character with an increased graphitic layer thickness (ca. 3–5 layers), rendering the alginic acid-derived carbon with features more typical of a nongraphitising carbon (Figure 2.12(F)).
Contrastingly, AMCS prepared at 800 °C presented extensive fullerene-like
character, with the addition of Ca2+ to gel precursor synthesis ultimately
leading to the preparation of a Starbon® with a high degree of smooth curvature and a more open porous network, with the removal of the inorganic
component revealing slightly thicker stacking layers (4–5 graphene layers)
(Figure 2.12(G–J)). In the context of this chapter, it is important to note that
this work highlights the impact of polysaccharide conformation in the gel
phase on the final structure of the porous carbon derived from these naturally occurring polymers. This is extremely important to understand as this
will directly impact on the applicability of the final carbon for specific applications. As will be discussed in Chapter 4, the stacking and local ordering of
the graphitic sheets in these alginic acid-derived Starbons® is critical to the
manipulation of the material properties for specific chromatographic applications. The works discussed in this section have also laid the basis for the
exploitation of alginic acid/alginate-containing biomass in the preparation
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of porous materials. The preparation of expanded monolithic porous macroalgae could potentially lead to the preparation of carbonaceous shaped
xero- and aerogel forms of the marine polysaccharide source. Furthermore,
such macroalgae samples feature an array of other polysaccharides presenting nitrogen- and sulfur-containing groups that could be utilised to generate
heteroatom-doped Starbon® materials.
2.5 The Synthesis of Starbons – Mechanistic
Considerations
As discussed earlier (Section 2.2), ours and other previous work has provided the initial basis for a generalised approach to the synthesis of PPDMs
in principle applicable to the majority of polysaccharides (Scheme 2.1). As
mentioned, the elaboration of this approach can also potentially act to valorise otherwise low-value polysaccharides, particularly when sourced from
wastes (e.g. orange peel-derived pectins, macroalgae, etc.).37,41,48 However, if
this elaboration is to incorporate successfully the preparation of new, innovative carbonaceous, Starbon® materials, then a detailed and developed
understanding of the decomposition mechanisms that occur during “starbonisation” need to be analysed and discussed accordingly in the context
of different polysaccharides, functionality and porosity/nanostructuration.
In this regard, briefly resuming discussion of 1st-generation Starbons®,
highlighting specifically recent insights, the synthesis involves the introduction of an acid catalyst (i.e. p-toluene sulfonic acid) to the surface of neutral
porous starch, followed by heating to the desired Tp (normally > 150 °C) to
induce the decomposition reactions (e.g. dehydration) at temperatures lower
than the melt of the hydrogen-bonded polymer network. First-generation
Starbons® are based on the (e.g. decomposition) chemistry and structure of
starch and more specifically the helical forming poly-α-(1 → 4)-D-glucopyranose–amylose. Therefore, as demonstrated earlier, it was thought it may be
possible to extend the initial Starbon® method of making mesoporous carbonaceous materials to other helical-forming polysaccharides, (e.g. alginic
acid and pectin), which would provide further characterisation evidence for
a more general biomass/polysaccharide carbonisation process description.
The decomposition chemistry of acid-doped starch- and alginic acid-­
derived Starbons® (as well as normal starch) was described in the elegant
work of Shuttleworth et al. via the use of diffuse reflectance IR spectroscopy
in Fourier transform mode (DRIFT) spectroscopy, with the aim of developing
a general analytical description applicable for the other types of porous carbons produced via polysaccharide-rich biomass carbonisation (Figure 2.13).
In the work of Shuttleworth et al. material samples were prepared in the temperature range of 100–700 °C, with the resulting analysis demonstrating that
based on the hydroxyl-rich polysaccharide (e.g. starch) precursor, an acid-catalysed thermal decomposition resulted in the evolution of carbonyl and ether
moieties to increasingly aromatic-rich structural groupings (Figure 2.13).
Acid-free starch (nonporous); and (C) Alginic acid; prepared as a function of carbonisation temperature. Reproduced with
permission from ref. 49.
Figure 2.13 Composite DRIFT contour plot analysis for the decomposition of (A) Starbon® produced from acid-doped porous starch; (B)
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At low carbonisation temperatures (e.g. 100–180 °C), hydroxyl groups (i.e.
from starch; ν = 1000–1100 cm−1) bonded to sp3-hybridised carbon predominate in the spectra. The detailed FTIR analysis revealed two major
carbonisation/decomposition transitions. The first major decomposition/
carbonisation event starts in the range 180–300 °C, induced via intermolecular crosslinking/dehydration of hydroxyls leading to ethers (ν = 1209 cm−1)
and carbonyls (e.g. carbon in sp2 hybridisation, ν = 1700 cm−1 at 200 °C). Additionally, in this temperature range heterolytic scission of glucosidic linkages
was believed to yield levoglucosan and shortened “dehydrated” polysaccharide chains.50,51 The second major carbonisation event was reported to occur
in the 300–550 °C range. This event is proceeded by further intramolecular dehydration to induce the formation of C=C (olefinic) double bonds (ν =
1670 cm−1) conjugated with carbonyl group. These functional groups arise by
the condensation of sugars (i.e. glucose, levoglucosan) with their decomposition products such as furfural and hydroxymethylfurfural.52–54 This is followed at preparation temperatures > 550 °C, by the conversion of 1D linear
conjugated groupings to 2D (surface plane) increasingly more condensed,
aromatic structures (ca. ν = 700–950 cm−1 (C–Haromatic)). The last transition is
significant as it results in the formation of structures with increased longrange order and features that are increasingly more typical of classical carbon materials. Notably, the same functional-group transitions were observed
in essence for acid-doped starch and alginic acid, indicating a general polysaccharide/biomass carbonisation mechanism. Based on this information a
proposed mechanism of polysaccharide carbonisation has been summarised
(Scheme 2.3).
The key decomposition steps at 300 and 550 °C (Figure 2.14(A) – points
I and II), result in significant rearrangement of the material molecular structure, as a result of the transition from a flexible polysaccharide nanoparticulate to a planar aromatic conjugated system, and correspond to significant
alteration in surface energy values (Figure 2.14(B)) and textural properties.
In this regard, for starch-derived Starbons® the formation of wormhole-like
interpore connections were observed after heating to 300 °C, in which micropores developed via entrapped decomposition product/gas evolution, initiating
the formation of a predominantly sp2-carbon structure. Starch-derived Starbons® prepared at Tp < 300 °C presented little micropore content, being composed of interconnecting network structures, as opposed to large mesoporous
domains provided from the original starch structure. Heating to Tp > 300 °C,
led to an interconnection of these domains and the production of the wormhole-like mesoporous carbons; features that well may be beneficial in certain
applications (e.g. supercapacitors).55,56
Whilst the decomposition chemistry may be of a general nature for acidic
polysaccharides (e.g. alginic acid) and the acid-doped neutral polysaccharides
(e.g. starch), the material porosity of Starbons® (e.g. micro- vs. mesoporosity;
mesopore diameters, etc.) are very subtly effected by the choice of polysaccharide precursor – presumably related to phase separation behaviour (e.g.
as a result of surface charge) and polysaccharide self-associations (e.g. the
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From Polysaccharides to Starbons®
Scheme 2.3 Mechanism of key steps in the acid-catalysed decomposition of starch
(i.e. the preparation of 1st-generation Starbon® materials).
formation of stabilised single or double-helical domains) (Figure 2.15). In the
context of porosity, 1st-generation Starbons® typically present higher micropore content as a function of Tp as compared to 2nd-generation (e.g. alginic
acid-derived) Starbons®, presumably as a result of the differences in kinetics
between surface initiated/catalysed vs. bulk decomposition process. Starbons®
prepared from inherently anionic/acidic polysaccharides typically produce larger
mesopore volumes (e.g. > 0.8 cm3 g−1) and diameters (e.g. > 10 nm). More subtle influences on material porosity relating to polysaccharides self-association
and indeed charge – e.g. such as double-helical structures in the amylose
homopolymer vs. the polyuronide block alginic acid copolymer (composed of
(1→4)-linked-ß-D-mannuronic acid (M), and α-L-guluronic acid (G) residues as
homo- or heteropolymer segments),44,57 – require further investigation.
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Figure 2.14 (A) 2D contour plot of preparation temperature vs. N2 adsorption energy
derived from the nitrogen-sorption isotherms; and (B) Surface-energy
impact/contribution versus preparation temperature. Reproduced
with permission from ref. 49.
Figure 2.15 Textural properties of porous starch, alginic acid, and pectins (i.e. via
gelation route) and Starbon® materials derived therefrom (at preparation temperatures (Tp) indicated) as a function of the C : O ratio
vs. (A) Total pore volume; (B) BET specific surface area; and (C)%
mesoporosity.
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Table 2.1 Adsorption capacities for dye molecules and surface area of dye coverage
for starch- and alginic acid-derived Starbon® adsorbents in comparison
to commercially available Norit®.a
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Methylene Blue
Acid Blue 92
Adsorbent
Adsorbent
Capacity (mg g−1)
Surface Area
(m2 g−1)
Adsorbent
Capacity (mg g−1)
Surface Area
(m2 g−1)
S300
S800
A300
A800
Norit
36
52
186
97
42
72
104
373
195
83
27
39
82
108
49
41
59
124
164
74
a
Reproduced with permission from ref. 58.
The study of Shuttleworth et al. is significant as it provides the basis for a
general understanding of the carbonisation processes involved in the conversion of polysaccharides to Starbons® and the relationship between chemical
functionality, decomposition temperature, surface energy/hydrophobicity/
polarisability and structure – an understanding that will be essential for the
future design of porous carbonaceous materials with properties specifically
prepared for a given application. In this context, in Chapters 3 and 4, the
exploitation of the materials features afforded by the Starbons® platform will
be discussed in the context of heterogeneous catalysis and chromatographic
science, respectively. With regard to other topical applications, starch and
alginic acid-derived Starbons (prepared at Tp = 300 and 800 °C) have been
shown to have excellent potential in the context of reversible sorbents for
water purification.31,58 This work by Hunt et al. demonstrates the advantages
of tuning surface chemistry and porosity in tandem based on the Starbon®
approach to allow a relatively simple optimisation of carbon material chemistry and porosity for a given purification/separation scenario, with Starbons®
often outperforming classically used carbon sorbents (e.g. alginic acid-derived Starbon® had a four times higher capacity for the adsorption of methylene blue than commercial Norit carbon; Table 2.1).58
2.6 Outlook and Conclusions
The utilisation of polysaccharides in the preparation of porous materials and
in particular carbonaceous materials (i.e. Starbons®) represents an interesting alternative to the materials prepared via conventional polymerisation and
cocondensation techniques (e.g. CMK, FDU, RF aerogels, etc.). Furthermore,
the use of polysaccharides derived from biomass potentially adds value to
inexpensive and typically waste products from industries such as the food
and forestry sectors.
The advantage of Starbons® lies in the great flexibility of surface and bulk
chemistry that can be induced in a relatively simple manner (i.e. selection of Tp).
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Chapter 2
Scheme 2.4 Textural and physicochemical properties of Starbons® relative to other
types of carbon materials. Reproduced with permission from ref. 49.
Starbon® materials exhibit outstanding mesoporous textural properties, with
pore volumes and sizes equal to carbon materials prepared via the more classical
hard-template-based approaches routes (Scheme 2.4). The Starbon® platform
offers flexibility in terms of Tp, enabling the opportunity to tune surface chemistry, (e.g. a material feature not necessarily accessible via hard- or soft-templating
approaches, particularly regarding low-temperature materials), whilst the use
of polysaccharides in Starbon® preparation circumvents the limited chemistry
offered by classical resorcinol–formaldehyde-based aerogel precursors, thereby
opening up a wide range of functional materials and the potential of simpler
postprocessing surface functionality introduction. The lack of a template avoids
wasteful processing steps and harmful chemicals and allows materials to be
prepared at a temperature of choice (e.g. 150–1000 °C). As a consequence of
preparation-dependent surface chemistry, the hydrophilicity vs. hydrophobicity
properties of the Starbon® material can essentially be moderated. These points
coupled with ability to direct porous properties (and to a degree material macrophology) via selection of polysaccharide precursor choice, generates the exciting
possibility of a materials tool box from which one could begin to design material
synthesis and allow deeper, more subtle, structure–activity relationships in specific applications.
The use of highly acidic polysaccharides is, apart from offering access to
Starbons® of differing textural properties, also advantageous from a process
and final end use point of view. It eliminates process steps involving the
introduction of the p-toluene sulfonic acid decomposition catalyst (and any
other steps that requires its removal), allowing access to more functional
lower-temperature Starbons®. The presence of residual “sulfur” may also
be problematic if the Starbon® material is ultimately to be used as a support in catalysis where the active site is sensitive to poisoning. The preparation of porous forms of alginic acid, pectin, chitosan and carrageenan
(e.g. –OH, –C(O)OH, –NH2, –SO3H) as precursors for Starbon® synthesis is
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From Polysaccharides to Starbons®
79
also significant as this will extend interest beyond the conventional “neutral” starch-based materials, and with regard to carbon fabrication, offering
scope for easier production routes, heteroatom introduction (e.g. for metal-free catalysis), decomposition chemistry control and in turn direction of
physicochemical (e.g. electrochemical, polarisability, etc.) properties of the
final material in a relatively flexible manner. Interestingly, it may also be
possible to eliminate the polysaccharide extraction step (Scheme 2.1) and
utilise directly naturally occurring structures or forms (e.g. macroalgae48)
to directly prepare useful Starbon® materials. The ability to prepare heteroatom-doped Starbon® varieties is currently under active investigation
as the development of such materials may extend the application remit of
these promising porous polysaccharide-derived carbons into the fields of
electrochemistry and metal-free catalysis.
Acknowledgments
PS gratefully acknowledges the Ministerio de Ciencia e Innovacíon for the
concession of a Juan de la Cierva (JCI-2011-10836) contract.
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CHAPTER 3
Porous Carbonaceous Materials
in Catalytic Applications
RICK A. D. ARANCONa, DUNCAN MACQUARRIEb AND
RAFAEL LUQUE*a,c
a
Departamento de Quimica Organica, Edificio Marie Curie (C-3), Campus de
Rabanales, Ctra Nnal IV-A, Km 396, E14014, Cordoba, Spain; bGreen Chemistry Center of Excellence, The University of York, Heslington, YO10 5DD,
York, UK; cState Key Laboratory of Electroanalytical Chemistry, Changchun
Institute of Applied Chemistry, Chinese Academy of Sciences, 5625 Renmin
Street, Changchun, Jilin 130022, China
*E-mail: q62alsor@uco.es
3.1
Introduction
Early human civilizations practised the technology of catalysis and biotechnology without current fundamental understanding of reactions and mechanisms (i.e. fermentation to produce wine). The understanding of catalysis
came only at the 18th century and became one of the most important drivers of the industrial revolution.1 In fact, the production of many industrial
chemicals was mostly due to the discovery of various heterogeneous catalysts
including V2O5 (for H2SO4 synthesis), Pt (for NH3 synthesis) and ZnO/Cr2O3
(methanol synthesis). At the dawn of the 21st century, the necessity to adopt
cleaner industrial catalysis rapidly intensified with the introduction of the 12
principles of Green Chemistry.2,3
The development of novel catalysts for various processes has been a major
driver of academic (and occasionally industrial) research, with the general
RSC Green Chemistry No. 32
Porous Carbon Materials from Sustainable Precursors
Edited by Robin J White
© The Royal Society of Chemistry 2015
Published by the Royal Society of Chemistry, www.rsc.org
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aim of switching to environmentally sound protocols and materials featuring as sustainable components as possible, whilst providing comparable or
better efficiencies, activities and selectivities to the prior state-of-the-art. In
this regard, classical homogeneous acid/base catalysts have been gradually
replaced by metal–complex catalysts and heterogeneous catalysts or catalyst
supports, which as will be seen in this chapter can be from a variety of unconventional sources (e.g. biomass).
As has been and will be alluded to in the course of this book, apart from
being a major future carbon source with a significant potential for valorisation
in the context of chemical and fuel production, biomass can also be employed
in the preparation of carbon and carbonaceous materials suitable for a variety of applications. Different types of porous carbonaceous materials from
biomass have been derived from biomass/waste sources, offering a number
of possibilities for materials in catalytic applications. This chapter aims to
provide an overview of recent research regarding the development of porous
carbonaceous materials (e.g. derived from biomass precursors) with a particular focus on the application of Starbon® materials in catalytic applications.
3.2
Biomass-Derived
Porous Carbonaceous
Materials
As introduced in previous chapters, an innovative example of the conversion
of biomass to applicable porous carbonaceous materials is the so-called Starbon® approach.4,5 As discussed in more detail in Chapter 2, Starbons® are
highly porous materials with a tuneable surface physicochemistry derived
via the controlled, acid catalysed, thermal carbonisation of, in the case of
1st-generation materials, a high surface area and mesoporous starch material.4 These porous biomass-derived carbonaceous materials possess several
advantages as compared to traditional carbonaceous materials.6 The protocol for Starbons® synthesis allows a singular controllable design of surface
functionality. In particular, low-temperature materials, prepared at carbonisation temperatures (T) < 350 °C, still present surface chemistry similar with
the parent polysaccharide (e.g. hydrophilic nature), making them particularly
suitable for applications in the aqueous phase. This is a particularly relevant
point regarding the development of catalytic materials capable of addressing
the aqueous phase-based chemistry of future Biorefineries.7,8
Furthermore, given the rich material functionality, Starbons® can be
chemically functionalised comparatively easily (as compared to conventional carbon) to achieve highly active catalysts for various heterogeneous
catalysed processes. As will be discussed in this chapter, these include the
generation of solid catalysts suitable for acid-catalysed processes (e.g. esterifications, etherifications, amidations),9–11 redox chemistries (e.g. oxidations,
hydrogenations),11–13 as well, as more recently, photocatalytic protocols.14
Another advantage of the Starbon® surface relates to the presence of functional groups (e.g. hydroxyl, carbonyls, carboxylates) capable of binding and
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potentially participating in redox processes, which in tandem with the presence of typically large mesopores, renders Starbons® extremely versatile supports for catalytic species. The potential of Starbon® materials has also been
extended with the development of alginic acid- and pectin-derived examples
that provide the opportunity to produce carbonaceous materials with very
large mesopore volumes and beneficial macrophology (i.e. monoliths).15,16
3.3
Sulfonated
Starbons® and Carbonaceous
Materials as Solid Acids
Luque et al. have used Starbon® as a support for a variety of acidic functional groups.9 Starbons® prepared at different carbonisation temperatures
and subsequently either sulfonated to provide strong Brönsted-acid sites,
impregnated to produce physisorbed ZnCl2 and BF3, or further functionalised with carboxylic acid groups via silylation of the hydroxyl groups with
cyanoethyl(trimethoxy)silane followed by acidic hydrolysis of the nitrile to
give surface-bound carboxylic acid groups. Characterisation of the resulting
acidic sites was performed partially via the diffuse reflectance FTIR analysis
of chemisorbed pyridine and 2,6-dimethylpyridine. Interestingly, the sulfonated materials were found to be the most acidic with Starbon®-500 having more sites than Starbon®-300, indicating that the sulfonation process
induced a degree of Lewis acidity to the material structure. The supported
Lewis acid species (i.e. ZnCl2 and BF3) presented both Lewis and Brönsted
properties, while the carboxylic acid showed weak Brönsted acidity only.
These supported acids were investigated in the acetylation of 5-acetyl methyl
salicylate, which has been previously used as a test for Lewis acidity.17,18
Here, the Starbon® catalysts provided exclusively O-acetylation, in contrast
to previous studies where both O- and C-acetylation were noted (Scheme 3.1),
speculated to be the result of a lack of strong Lewis acid character in the Starbon® catalysts.
These Starbon-supported acids were also investigated in the alkylation of
phenol with cyclohexene, a reaction that has been reported to illustrate Brönsted acidity. Here, the sulfonated Starbons® were the most active and gave
excellent conversions after 48 h; similar to sulfated zirconia, but significantly
lower than those of β-zeolite. Interestingly, the O/C selectivity was quite different, with both sulfated zirconia and β-zeolite giving predominantly C-alkylation (typically 80–90% C) while the Starbon acids gave predominantly
O-alkylation. The extent of C-alkylation in these systems did, however, gradually increase with time, which was explained by parallel reaction kinetics, but
could also be evidence of a trans-alkylation reaction from O- to C-products
(Scheme 3.2).
One of the more promising applications thus reported for Starbon®-based
catalysts has been their ability to catalyse acidic esterification in high water
content environments.5,7,19 Using sulfonated Starbon® catalysts, succinic
acid in aqueous ethanol was efficiently converted to diethyl succinate, in
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Scheme 3.1 Acetylation
of 5-acetyl methyl salicylate with acetic anhydride.
Scheme 3.2 Alkylation
of phenol with cyclohexene.
Table 3.1 Catalytic
activity of Starbon materials in the esterification of succinic
acid in aqueous ethanol.a
Catalyst
Conversion (%)
Selectivity to diester (%)
Sulfonated Norit
Sulfonated Darco
Sulfonated Starbon®
Zeolite beta 25
Sulfated zirconia
Montmorillonite KSF
70
80
98
50
42
35
24
29
99
18
15
15
a
Reproduced with permission from ref.19.
which a range of other solids of similar acidity failed to produce more than
a few percent of the diester (with the monoester being the main product;
Schemes 3.1 and 3.3; Table 3.1). It has been proposed that the pore system
where the catalytic groups reside is hydrophobic and the environment where
the catalysis takes place therefore has a relatively low water content in spite
of the water-rich bulk environment.
Niño-Gómez et al. have recently reported on the detailed characterisation
of sulfonic acid-functionalised Starbon®-300.20 Different sulfonation routes
were compared, including sulfuric acid and mixtures of sulfuric acid and
chlorosulfonic acid, achieving up to 10 mmol g−1 of acidic sites. NMR analysis using triethyl phosphine oxide as an acid-site probe demonstrated the
presence of very strongly acidic Brönsted sites and weaker sites that are likely
to be carboxylic acids and phenols formed during the pyrolysis and sulfonation steps. Conversion of oleic acid to ethyl oleate was evaluated using these
catalysts (Scheme 3.4).
It was found that there exists a certain correlation between activity and
acid-site content, although this was partly obscured by alterations in system
porosity under some sulfonation conditions. Selectivity was also excellent
to the ester and there was no evidence of addition of ethanol to the double
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Chapter 3
Scheme 3.3 Esterification
of succinic acid in aqueous ethanol.
Scheme 3.4 Esterification
of oleic acid using sulfonated Starbon®-300.
Scheme 3.5 Synthetic
routes to sulfated/sulfonated carbonaceous materials.
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bond in the molecule. The results above are broadly consistent with those
achieved by Zhu et al. who prepared sulfonated carbons via a very different
route (Scheme 3.5), involving the carbonisation of a monolayer of sucrose
on alumina, followed by dissolution of the alumina and subsequent attachment of arene-sulfonic acid groups via diazonium coupling.21 Preparation of
methyl oleate proceeded well, and their catalysts outperformed sulfonated
activated carbon and Amberlyst-15 (TOF 109 h−1 vs. 44 h−1 vs. 15 h−1).
Alternative sulfonated mesoporous carbonaceous materials have also
recently been proposed including those of Hara et al.,22 which are formed by
the partial carbonisation of soluble sugars and pyrolysed cellulose with sulfuric acid23 to produce sulfonated polycyclic aromatic systems. Interestingly,
these materials have very low surface areas (and presumably essentially no
porosity when measured by the standard N2 sorption), but are believed to
expand in the presence of water, leading to increased availability of active
sites. Similarly, a sulfonated carbonised cellulose has also been prepared by
Qi et al.24 The authors used the unusual approach of functionalisation with
mercaptoacetic acid, followed by H2O2 oxidation to generate the sulfonic acid
Scheme 3.6 The
synthesis of functional materials from cellulose to enable
the hydrolysis of cellulose provides a good and sustainable cyclic
approach, particularly in waste valorisation. Reproduced with permission from ref. 24.
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sites. Catalyst preparation involved the hydrothermal carbonisation of cellulose, incorporation of a superparamagnetic Fe3O4 component and subsequent –SO3H functionalisation.
The resulting material was found to be active in hydrolysing cellulose,
producing up to 68.9% of reducing sugars in a ionic liquid medium (130 °C
for 3 h) and 51% in aqueous medium (180 °C for 9 h). This particular study
illustrates that active catalysts for biomass hydrolysis can be sourced from
biomass itself and that this technology can possibly generate functional
materials mainly derived within the sphere of renewable resources (Scheme
3.6). Interestingly, this further highlights the sustainability of this technology when applied particularly to waste feedstock valorisation studies.
3.4
Other
Routes to the Introduction of
Mesoporosity and Associated Applications
A limited number of articles have also reported the preparation of carbonaceous materials, whereby phosphoric acid was utilised to generate mesoporosity. Please note that other systems based on phenol–formaldehyde25
technology (but using naturally derived phenolics) have been discussed elsewhere26 and will not be included here as formaldehyde-based materials have
inherent toxicity risks.
Guan et al. have recently demonstrated the formation of a mesoporous carbonaceous material from waste Bahia pulp via phosphoric acid promoted
expansion and carbonisation.27 This material can be sulfonated using standard methods to give a catalytically active material. The most active versions
of this catalyst are prepared by expansion at 250 °C followed by sulfonation at
90 °C. Sulfonation gave just over 1 mmol g−1 as determined by neutralisation
titration. This is very close to the value recorded by Niño-Gómez et al.13 (for
300 °C activated Starbons® sulfonated using sulfuric acid), but significantly
lower than the values the same authors achieved using the more reactive sulfuric acid/chlorosulfonic acid system (around double the loading). The Bahia
pulp-derived materials showed good activity for the transesterification of
methyl acetate with butanol, out-performing Amberlyst and H-ZSM-5 under
the same conditions (Scheme 3.7). Materials prepared by direct sulfonation
of unexpanded fibre showed low activity, as did expanded materials without
sulfonation, indicating that any phosphoric acid incorporated in the material has little or no activity.
Scheme 3.7 Trans-esterification
of methyl acetate using sulfonated expanded
Bahia pulp.
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A similar effect of phosphoric acid treatment on olive stone waste has been
demonstrated by Guerrero-Pérez et al. where mesopores were developed in
olive stone waste calcined at 500 °C after phosphoric acid impregnation,
whereas without phosphoric acid, the resultant material was essentially
microporous.28 The authors also claimed a beneficial stabilising effect
towards the oxidation is provided by the phosphoric acid treatment. They
used this material as a support for vanadium-based catalyst preparation and
its subsequent application in the oxidation of propene.
Kraft lignin has also been successfully converted to a mesoporous carbon
by phosphoric acid activation (impregnation and drying followed by 500 °C
treatment) to give a mesoporous carbon.30 The resulting material was then
used to support catalytic Pd nanoparticles (Figure 3.1(A) and (B)). The resulting catalyst was shown to be a useful catalyst in the Suzuki-Miyaura reaction
and in hydrogenations, with the presence of phosphorus groups at the carbons surface, especially C3P groups (Figure 3.1(C)), reported to be beneficial as it avoided the use of triphenylphosphine as a ligand for a variety of
Suzuki–Miyaura reactions (Figure 3.1(F)).29
3.5
Ordered Porous Carbonaceous Materials
Most porous carbon materials mentioned are not ordered in terms of pore
structuring and often present broad pore-size distributions. Such a random
nature to the pore “order” may affect the uniformity of the generated catalytic sites or the mass transfer/diffusion properties to and from the active
site. To address this limitation, ordered porous carbon materials (OPCs)
have been developed with a view to increasing the uniformity or “order” of
material porosity. The synthesis of these types of porous materials is typically performed using either one of two common methods.
The first method, termed “nanocasting”, involves the use of a sacrificial
mesoporous silica template prepared via classical surfactant micelle templating that generates the ordered porous phase (Scheme 3.8). Surfactant molecules are then removed via typical techniques (e.g. extraction, thermolysis,
etc.). A suitable carbon precursor (e.g. sucrose) is then used to impregnate the
porous silica. Following a carbonisation step, the silica template is removed
to reveal an ordered mesoporous carbon material.32 Notably, impregnation
can proceed via partial or complete pore filling, resulting in the formation of
differing porous properties in the resulting carbons.
As an example of this approach to the synthesis of functional carbon-based
materials, ordered mesoporous carbon nitrides (C3N4) have also been
reported as promising materials for catalytic applications (e.g. Friedel–Crafts
reactions).34 The introduction of suitable chemical bodies or chemical functionalisation of these materials has also led to an extension of their purpose
for a variety of catalytic applications.35 Recently, gold nanoparticles have
been introduced to the structures of mesoporous carbon nitrides to generate
a catalyst suitable for the synthesis of propagylamine achieving up to 96%
conversion after 24 h.33 In this report, material mesopores served to limit
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Figure 3.1
(A)
and (B) TEM images of the catalyst LAC–Pd (bar length: 20 nm); (C)
P2p spectra of the supports and catalysts obtained from kraft lignin; (D)
Pd3d spectra of the LACT–Pd catalyst; (E) Evolution of 4-vinylcyclohexene conversion and of the different reaction product yields as a function of the reaction time (25 8C, 35 psi H2 pressure, EtOH/water 4 : 1)
and (F) Examples of Pd-carbon catalysed Suzuki crosscoupling with
arylbromides investigated in this study. Reproduced with permission
from ref. 29.
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Scheme 3.8 Methods
involved in synthesising ordered porous carbonaceous materials. Reproduced with permission from ref. 31.
the growth of the catalytically active Au nanoparticles (Figure 3.2). Carbon
nitride-based materials are at the time of writing receiving extensive attention and the reader is referred to recently published articles and reviews for
further details.36–38
The second method, often referred to as “direct synthesis”, involves the
self-assembly of organic precursors, typically phenolic resin, and block copolymer surfactants into 3D mesostructures followed by carbonisation and surfactant removal (Scheme 3.8).31,39 As with disordered porous carbons, OPCs
can also be sulfonated to produce an active catalyst for oxidation reactions.
The work of Feng et al. demonstrated that functionalisation of the OPC surface with sulfonates via the diazonium salt method, produces a suitable catalyst for bisphenol A synthesis based on phenol, acetone, and acid as starting
reagents (Scheme 3.9).40
For coupling applications, porous carbon supports have already been
extensively modified using a variety of techniques.31,39 An example is the use
of mesoporous silica–carbon composites as support for metal catalysts (e.g.
Pd) for the coupling of chlorobenzene (Scheme 3.10). Synthesised using a
surfactant-templating method, the hybrid mesoporous catalysts provided a
conversion as high as 60% for Ullman coupling reactions. The main advantage of this composite relates to its stability as compared to similar mesoporous systems, with catalytic activity due to the porous walls of the catalyst
support that provide highly active sites for chemical reactions.
As a consequence of their porous nature, OPCs are also ideal catalysts for
hydrogenation and similar reduction reactions. OPC nanorods have been
shown to produce molecular hydrogen directly from the catalytic decomposition of methane. Work by Ozalp et al. on ordered mesoporous carbon nanorods
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Figure 3.2
Chapter 3
(A)
FE-HRSEM and (B) HRTEM images of Au nanoparticle encapsulated
in mesoporous-carbon nitride (MCN); (C) the corresponding N2 sorption
isotherms for pure MCN (●) and Au-MCN (■); and (D) Catalytic activity
of Au nanoparticle encapsulated MCN in the three-component coupling
reaction of benzaldehyde, piperidine, and phenylacetylene for the synthesis of propargylamine. Reproduced with permission from ref. 33.
Scheme 3.9 (A)
A schematic of the sulfonated ordered porous carbon materials; (B)
The sulfonated OPCs as catalyst for bisphenol synthesis with selectivity towards p-p′ product. Adapted from ref. 40.
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93
Scheme 3.10 Ordered
Pd-containing mesoporous carbonaceous-silica hybrid used
for Ullman coupling processes. Reproduced with permission from
ref. 39.
Scheme 3.11 Reduction
of benzaldehyde involving an ordered porous carbon
doped with Ru. Adapted with permission from ref. 42.
(CMK-3) and ordered mesoporous carbide-derived carbon has shown these
materials to be promising catalysts for methane decomposition under inert
conditions by monitoring the catalyst weight changes through thermogravimetric analysis (TGA) when methane was subsequently introduced.41 Increasing mass of the catalyst would indicate carbon deposition on the surface.
This catalytic combination has the potential of degrading the greenhouse gas
methane to produce hydrogen as a promising fuel alternative.
For reduction applications, OPCs doped with Ru can be used as catalyst
for hydrogenation reactions to produce benzyl alcohol and aniline at room
temperature and 4 MPa H2 pressure (Scheme 3.11).42 The main advantage of
using OPCs as metal support is that they can become recoverable and recyclable catalysts for heterogeneously catalysed processes. Further, incorporating
typical catalysts on a mesoporous surface allows them to have more diverse
applications.
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Scheme 3.12 Summary
of the different synthetic schemes that can be used to pro-
duce mesoporous carbon catalysts: (A) Involves the direct synthesis
of functionalised nonordered mesoporous materials that have a variety of application in catalysis due to the simplicity in their preparation; (B) This path shows the nanocasting technique to produce
ordered mesoporous carbon, although this is an efficient technique,
this is less preferred because of the danger of using HF as an etching
agent; (C) the direct synthesis pathway for producing ordered carbon
materials.
The primary advantage of using ordered porous carbonaceous materials
for catalysis relates to their good mechanical strength that has been shown
to be significantly improved as compared to that of their nonordered counterparts. Secondly, their ordered orientation allows for more efficient conversion rates. However, like any other porous materials, their synthesis and
subsequent functionalisation is an energy- and resource-intensive process
(Scheme 3.12).
3.6
Application in Hydrogenation Reactions
The hydrogenation of biorefinery platform chemical (e.g. succinic acid
(SA)) is of serious interest if this new refinery concept is to satisfy our
future chemical production needs. In this context, Luque et al. have
reported on the hydrogenation of SA under mild reaction conditions
(i.e. 10 bar H2, 100 °C) in aqueous ethanol (Figure 3.3).12 In this initial
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Porous Carbonaceous Materials in Catalytic Applications
Figure 3.3
95
(a)
AC-TEM and (b) AC-HAADF-STEM images of the 5% Ru–S300 catalyst. (c) and (d) are high-resolution AC-TEM and AC-HAADF-STEM
images corresponding to the areas marked by squares in (a) and (b),
respectively. Reproduced with permission from ref. 12.
communication, starch-derived Starbon®-300 °C (S300) was utilised as the
functional carbonaceous support for Pt, Pd, Rh, and Ru nanoparticles (5
wt% loading). The authors comment that this Starbon® was selected as
a support due to its highly heterogeneous (e.g. oxygenated) functionality
and stability under aqueous conditions. Ru–S300 and Pt–S300 were found
to present the highest conversion, with aberration-corrected high-angle annular dark field STEM (AC-HAADF-STEM) analysis indicating the
observed high activity was a consequence of these noble-metal nanoparticle being of a smaller diameter and more evenly dispersed on the support
as compared to the other investigated materials (Figure 3.3(A)–(D)). Selectivity was also observed to be different for Ru–S300 (i.e. hydrogenation to
tetrahydrofuran – 60–82%) as compared to Pd, Pt, and Rh–S300 materials,
which demonstrated a greater selectivity to the 1,4-butanediol product.
Significantly, the hydrogenation reaction conditions could be selected
to maximise production of γ-butyrolactone from Pd–S300 or Ru–S300
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Table 3.2 Catalytic
activity of investigated catalysts in the liquid phase microwave-assisted hydrogenation of LA to MTHFa,b.
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Selectivity (mol%)
Catalyst
Conv. (mol%)
MTHF
GVLc
PDOd
Otherse
Blank (no cat.)
Ru–S300
Rh–S300
Pd–S300
Cu-MINT
5%Pd/C
–
< 30
69
64
> 90
78
–
< 50
90
88
75
92
–
20
–
–
–
–
–
–
–
–
25
–
–
> 30
10
12
–
a
Reproduced with permission from ref. 13.
Reaction conditions: 0.1 mL LA, 0.3 mL FA, 0.1 g catalyst, 150 °C, 30 min MW irradiation, 300 W
(maximum power output).
c
Gamma Valerolactone.
d
1,4-pentanediol.
e
Other products obtained in the reaction included angelica lactone (AL) from FA-catalysed dehydration of LA as well as pentanoic acid (PA) and 4-hydroxyvaleric acid (HVA) from hydrogenation of LA.
b
(e.g. ≥ 65% selectivity at 45% conversion/10 h/5% Pd–S300). Importantly,
the reported S300-supported catalysts were shown to maintain > 95% of
their initial activity after five reaction cycles. Using S300-supported Ru
metal nanoparticles, the hydrogenation of itaconic acid in aqueous/ethanolic solution (100 °C; 1 MPa H2; 5 wt% Ru) was found to afford a 95% yield
of 2-methylsuccinic acid.43
The application of these Starbon®/noble-metal hydrogenation catalysts
has recently been extended to the hydrogenation of levulinic acid (LA) to
produce 2-methyltetrahydrofuran (MTHF) – a potential biofuel or alternative green solvent - under microwave irradiation (Table 3.2).13 The authors
reported on a different hydrogenation pathway compared to that for
Cu-based catalysis (e.g. Cu-microwave-induced nanotubes (Cu-MINT)), for
the S300-supported noble-metal catalysts (e.g. Pd, Ru, Rh) (Figure 3.4). The
formation of γ-valerolactone (GVL) was observed when employing these
catalysts (e.g. Ru–S300) and was proposed to be the key intermediate in
the hydrogenation scheme via three competitive processes (Figure 3.4(A)).
Angelica lactone (AL) was produced (as observed for S300-based catalysts)
and subsequently hydrogenated to GVL.
Luque et al.12 reported that GVL production is also favoured via hydrogenation of the carbonyl group at the 4-position of LA to 4-hydroxyvaleric
acid (HVA) on noble-metal sites (with the in situ generated hydrogen from
formic acid decomposition), followed by dehydration/cyclisation to GVL.
The authors indicate that this lactone intermediate can then be hydrogenated to either pentanoic acid or 1,4-pentanediol (PDO), with short batch
reaction favouring PDO production (and then MTHF via dehydration), with
PA formed in increasing quantity with longer times of reaction under flow
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Porous Carbonaceous Materials in Catalytic Applications
Figure 3.4
97
(A)
Proposed reaction pathways for levulinic acid conversion to a variety of important compounds based on the use of noble-metal catalytic
systems [key steps (bold arrows)] and the favoured/promoted catalysed
chemistries; (B) S300-supported Pd nanoparticles (5%wt loading).
Adapted with permission from ref. 12.
conditions. TEM images of the prepared S300-supported noble metal catalysts (e.g. Pd–S300; Figure 3.4(B)) indicated very finely dispersed small
(<9 nm) metallic nanoparticles within the mesoporous carbonaceous support structure.
3.7
Biofuel Synthesis
In a recent report, Starbon®-based catalysts have been employed in the
production of biodiesel-like biofuels (i.e. fatty acid methyl esters (FAMEs))
(Figure 3.5).44 Waste oils containing high concentrations of free fatty
acids (FFA) are converted to biodiesel-like fuels employing S400–SO3H
acids using both conventional heating and microwave irradiation. The
Starbon® acid catalysts were found to be active in the esterification of the
FFA with methanol and also triglyceride (TG) transesterification found
in the waste oil to yield biodiesel and glycerol. S400–SO3H (loading = 0.4
mmol g) was shown to be an active catalyst for the production of biodiesel from waste oil with high FFA content (>10 wt%), with conversion
to FAMEs reported to be >95 mol% (e.g. reaction time = 15 h, T = 80 °C,
rapeseed waste frying oil containing 10 wt% FFA). The reaction followed
a simultaneous FFA esterification and TG trans-esterification pathway,
demonstrating recyclability, remarkably higher activity as compared to
similar sulfonated materials (i.e. DARCO-G60-SO3H and silica-SO3H) and
commercial solid acids (e.g. Amberlyst – 70% FAME conversion; Beta25
zeolite – 25% FAME conversion; Figure 3.5(B)). The rates of reaction could
be dramatically increased by employing microwave irradiation, reducing
reaction times from ≥12 h (using conventional heating) to <45 min, without a reduction in activity.
The Starbon®-based catalyst maintained high FAME conversions after five
recycle runs (>70 mol%), with the use of microwave heating proving to be
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Figure 3.5
(A)
Trans-esterification of triglycerides to produce fatty acid methyl
esters (FAME biodiesel); (B) Activity comparison of different solid
acids in the production of FAMES from waste oils under optimised
reaction conditions; and (C) A comparison of subsequent uses of Starbon® acid in the trans-esterification of rapeseed waste frying oil under
conventional heating and microwave irradiation. Reaction conditions conventional heating: waste oil (1 mL), methanol (3 mL), starbon acid
(0.1 g), T = 80 °C, t = 12 h; Microwave irradiation: waste oil (1 mL), methanol (3 mL), S400–SO3H (0.1 g), 300 W, Tmax = 108 °C/Taverage = 83 °C, t = 30
min. Adapted with permission from ref. 44.
beneficial in terms of prolonging the high activity of the catalyst over a high
number of catalytic runs (Figure 3.5(C)).
3.8
Photocatalysis
The interest and potential of Starbon® materials in general and specifically in
catalysis has resulted in the production of a raft of patents and an associated
spin-off company called “Starbon® Technologies Ltd”.45 Sigma-Aldrich has taken
a license to distribute S300 and S800 materials and are available for purchase
via their internet portal.6 This provides the opportunity for other researchers to
utilise these materials and applications perhaps not initially envisioned in the
material development. A recent example in this context, describes the development of new Starbon®-based photocatalysts.14 In this report by Colmenares et al.
TiO2 nanoparticles were supported on S800, with the average particle size found
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Porous Carbonaceous Materials in Catalytic Applications
Figure 3.6
99
Photocatalyst
activities in the aqueous-phase degradation of phenol
(reaction conditions: 150 mL of mother solution, 150 mg of photocatalyst, Cphenol = 50 ppm, temperature = 30 °C, pressure = 1 bar). P25 = commercial TiO2; Ti/GO = TiO2/graphene oxide; Ti/Norit = TiO2 supported
on commercial carbon, Norit; Ti/Starbon® = TiO2 supported on S800.
Reproduced with permission from ref. 14.
to be ca. 30 nm. The resulting composite was found to be a highly promising
photocatalysts for aqueous-phase total mineralisation of phenol.
TiO2/S800 was found to have improved activity as compared to TiO2/
Norit and TiO2/graphene oxide, with the resulting activity of the Starbon®-based photocatalyst proposed to be the result of reversible adsorption of phenol on the hydrophobic of the S800 support and the highly
dispersed TiO2, anchored to the mesoporous carbonaceous support via
the ultrasound-induced impregnation method (Figure 3.6). The authors
also suggest the observed enhanced activity may also be the consequence
of a pure anatase phase with high crystallinity that leads to a reduction of
the electron–hole recombination rate on the Starbon surface. The authors
indicate that the TiO2 nanoparticles are strongly anchored and have good
contact to the S800 structure (i.e. no leaching was observed after 240 min
of photocatalysis), which is believed to enhance the photoelectron conversion (better than Norit and graphene oxide supports) of TiO2 by reducing
the recombination of photogenerated electron–hole pairs. This report is
significant as it highlights that the potential of semiconductor/Starbon®
composites in photochemical application, the activity/properties of which
(e.g. charge-transfer properties) may in principle be modulated (e.g. via
selection of Starbon® preparation temperature), leading ultimately to
designer novel hybrid mesoporous photocatalysts.
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3.9
Conclusions and Prospects
This chapter has aimed to provide a short overview on the possibilities of
porous carbons, and particularly those based on the Starbon® approach, in
catalytic applications. Porous carbon materials are at the forefront of materials research nowadays because of their easily tunable properties. Aside
from catalysis, they also have potential in solar cells, and other photovoltaic devices. The recent discovery of graphene, as a highly conducting carbon allotrope has also triggered the research on carbon catalysts for various
devices. Although a relatively new technology, the design and understanding
of porous carbons synthesis in terms of structures, functionalities and applications has advanced, allowing it to progress rapidly in various other fields
including medicine46−48 and related possibilities. In this regard, a promising
trend recently reported relates to the design of porous nanohybrid carbons
featuring magnetic separation.49 In this context, so-called MAGBON materials (magnetically separable Starbons) can be prepared and functionalised in
a similar way to that of Starbon materials but with a featured magnetic separation that enhances their applications in catalysis (e.g. alcohol oxidation
and C5 sugars dehydration to furfural). In summary, carbonaceous materials
both derived from biomass as well as OPCs still continue to have remarkable prospects for further developments and advanced applications that will
surely continue to pave the way to the widespread utilisation of such materials in useful processes and technologies.
Acknowledgments
Rafael Luque gratefully acknowledges Spanish MICINN for financial support
via the concession of a RyC contract (ref: RYC-2009-04199) and funding under
project CTQ2011-28954-C02-02 (MEC). Consejeria de Ciencia e Innovacion,
Junta de Andalucia is also gratefully acknowledged for funding project P10FQM-6711. Rafael Luque is also indebted to the Chinese Academy of Sciences from China for the provision of a Visiting Professorship at the State
Key lab of Electroanalytical Chemistry of the Institute of Applied Chemistry
in ChangChun (China) in 2014.
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48.M. M. A. Vinu, V. Sivamurugan, T. Mori and K. Ariga, J. Mat. Chem., 2005,
15, 5122.
49.M. Ojeda, A. M. Balu, A. A. Romero, P. Esquizani, J. Ruokolainen, H. Sixta
and R. Luque, ChemCatChem, 2014, 6, 2847–2853.
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CHAPTER 4
Application of Carbonaceous
Materials in Separation Science
ANDREW S. MARRIOTT*a, CARLA ANTÓNIOb
AND JANE THOMAS-OATESc,d
a
Bristol-Myers Squibb, Reeds Lane, Moreton, CH46 1QW, UK; bPlant
Metabolomics Laboratory, Instituto de Tecnologia Química e Biológica
António Xavier-Universidade Nova de Lisboa (ITQB-UNL), Av. República,
2780-157, Oeiras, Portugal; cDepartment of Chemistry, University of York,
York, YO10 5DD, UK; dCentre of Excellence in Mass Spectrometry, University
of York, York, YO10 5DD, UK
*E-mail: andrew.marriott@bms.com
4.1
Introduction
Separation science refers to processes whereby a complex sample mixture
is converted into two or more distinct product mixtures in which individual
components of the sample are enriched. In certain cases, a sample mixture
can be fully separated into its individual components. Separation is achieved
based on chemical and/or physical property differences between the components of a mixture and is dependent on the process used. In this context,
porous carbons are increasingly being adopted for separation processes
owing to their unique retention characteristics, chemical stability and the
ability to control pore structure through templated synthetic strategies.1,2
RSC Green Chemistry No. 32
Porous Carbon Materials from Sustainable Precursors
Edited by Robin J White
© The Royal Society of Chemistry 2015
Published by the Royal Society of Chemistry, www.rsc.org
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Chapter 4
The types of components separated by a porous carbon are dependent on
pore size – i.e. microporous or meso/macroporous:
●● Microporous carbons (<2 nm) in the form of membranes are readily used
in gas separation, e.g. CO2, CH4/H2 or O2/N2 separation. Carbon membranes are split into two main categories; carbon molecular sieve (CMS)
membranes and carbon nanotube (CNT) membranes. Whilst CMSs
are the more established carbon membrane type, CNTs have greater
mechanical stability and offer faster gas transfer, although an established economical method of manufacture has yet to be found.3 This
work is covered in greater detail in several published review articles.3–6
●● Meso- (2–50 nm)/Macroporous (>50 nm) carbons, in the form of particles or monoliths are used as stationary phases or sorbent materials in
chromatographic or solid-phase extraction (SPE) processes. The larger
pore dimensions enable the efficient separation of large(r) analyte
classes, which can range in size from simple aromatics up to biological
macromolecules.7 These carbons can be prepared for single-use applications, for example in SPE (a sample cleaning/concentrating technique
analogous to column chromatography), or they can be manufactured
for long-term, high-value applications such as high-performance liquid chromatography (HPLC) for the separation of polar molecules and
structural isomers.7
The synthesis of these mesoporous/macroporous carbons needs to be
tightly controlled to ensure a uniform pore size and homogeneous surface
are formed – features required to prevent permanent retention of smaller
analytes and/or deviations in separation performance. The main synthetic
strategy used in the preparation of these carbon types is “hard templating”,
involving the use of a structured support around which a carbonising material is formed. Pyrolysis and removal of the (e.g. inorganic) template leads to
the final porous carbon.2,8 Although the structured nature of the templating
process produces carbons with narrow particle- and pore-size distributions,
the methods themselves generally rely on oil-based starting materials as carbon sources and high temperatures to close microporous regions (affecting
analyte retention) as well as the use of rather harsh chemicals (e.g. HF(aq)
or concentrated NaOH(aq)) in the template-removal step. Therefore, recent
attention has been paid to the development of porous carbons from sustainable precursors for use as chromatographic media. As well as using more
readily available biomass as carbon precursors, there have also been efforts
to produce mesoporous carbons without the need for structure-defining
templates and high pyrolysis temperatures, opening the door to “greener”
mesoporous carbons for use in chromatography.9,10 This chapter primarily
focuses on the role of porous carbons in HPLC and their applications, the
disadvantages of their manufacture and current efforts aimed at the development of alternative stationary phases from sustainable carbon precursors
and synthesis strategies.
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4.2
4.2.1
105
Background
to High-Performance Liquid
Chromatography (HPLC) and Introduction of
Porous Carbon Stationary Phases
Overview
The separation of mixtures using liquid chromatography is carried out using
a chromatographic column containing the stationary phase, over which a
mobile liquid phase flows. Separation depends on differences in the extent
of interaction of the components of the mixture with the stationary and
mobile phases; components interacting strongly with the stationary phase
pass through the column and elute more slowly than components with lower
affinities for the stationary phase, that spend more time in the mobile phase
and thus elute more rapidly.
4.2.2
Efficiency of Column Separation
Separation efficiencies of chromatographic columns are maximised by optimising the exchange of components between stationary and mobile phases
and minimising the degradation of the resulting separation by optimising the
uniformity of behaviour and passage of components along the length of the
column. Plate theory is commonly used to determine column efficiency and
is based on the concept of a plate, defined as a point of interaction between
an analyte and the stationary phase. The higher the plate number (N) for a
column, the more efficient the separation over that column. When the retention time and full peak width at half-maximum height (w1/2) are known, N
can be calculated using eqn (4.1):
2
⎛ tg ⎞
=
N 8ln(2) ⋅ ⎜
(4.1)
⎟
⎜ w1 ⎟
⎝ 2⎠
Peak efficiency can be measured by determining the plate height (H), defined
as the distance between two plates within a column. The smaller the plate
height value, the greater the number of plates (points of interaction) there are in
the column, and therefore the more efficient the separation that is realised. By
knowing the column length (L), plate height can be calculated using eqn (4.2):
H=
L
N
(4.2)
Plate height is affected by the linear velocity (µ) of the mobile phase and
in 1956 van Deemter et al. described an equation taking into account three
constants, A, B and C, which impact on plate height (eqn (4.3)).11
H =A +
B
μ
+ Cμ
(4.3)
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Chapter 4
where “A” relates to eddy diffusion, “B” describes longitudinal diffusion and
“C” relates to mass transfer:
Eddy Diffusion: This refers to the differing paths that molecules of the same
analyte can take past the packed particles, to pass through the column. Variations in these path lengths mean some molecules can take longer to pass
through the column than others, leading to band broadening. This effect is
independent of mobile-phase velocity and can be minimised by reducing
particle size.
Longitudinal Diffusion: Injection of a sample mixture onto a column ideally
involves applying analytes in a thin, concentrated band. Over time, the natural process of diffusion occurs as the analyte band passes through the column, with longitudinal diffusion leading to band broadening. Longitudinal
diffusion is dependent on mobile-phase velocity, so that a faster flow flushes
the analyte band through in a shorter time, minimising this effect.
Liquid–Solid Mass Transfer: There are two types of liquid–solid mass transfer that affect column efficiency, i) trans-particle mass transfer and ii) surface
diffusion.12 Trans-particle mass transfer refers to the diffusion of an analyte
through the mesoporous network of a stationary phase particle. Band broadening is affected by the time required for an analyte to diffuse through the
particle – therefore column efficiency can be improved by decreasing particle size or using solid-core particles. Surface diffusion refers to the interaction between the analyte with the stationary phase functional groups and
the time required for equilibrium to be established between the stationary
phase and mobile phase. In this case, column efficiency is dependent on
mobile-phase flow rate, where the equilibrium required for an analyte being
absorbed back into the bulk flow cannot be achieved at high flow rates, causing band broadening. Therefore, in order to reduce band broadening by this
effect, flow rates need to be reduced.
Plotting plate height against mobile phase velocity for each of the three
constants, a cumulative plot, referred to as the “van Deemter curve”, can be
generated; the minimum of this curve provides the flow rate that gives the
lowest plate height and therefore the optimum peak efficiency (Figure 4.1).
4.2.3
Requirements for the “Ideal” Stationary-Phase Material
For high-performance liquid chromatography (HPLC) columns packed with
stationary phase particles, the requirements of an “ideal” stationary phase
material have been identified, taking into account van Deemter theory.13,14
These requirements demand that the particles:
●● Are spherical and small to optimise uniformity of the path of the components along the column, and to maximise efficiency of mass transfer
between mobile and stationary phases.
●● Are porous: for optimal mass transfer, porosity > 50% is desirable, with
uniform pore-size distributions (available in the range 6–100 nm) and
ideally with pores no smaller than 6 nm.
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Application of Carbonaceous Materials in Separation Science
Figure 4.1
●●
●●
●●
●●
●●
4.3
107
Plot
of mobile-phase velocity against plate height H for the A (red), B
(blue), and C (green) terms of the van Deemter equation, and cumulative van Deemter curve (dashed line).
Have Brunauer-Emmett-Teller surface area (SBET) high enough to provide adequate chromatographic capacity (i.e. 30–300 m2 g−1).
Have sufficient mechanical strength to withstand the pressures applied
to the column for packing and chromatographic separation. Particles
should not swell or shrink in different solvents, pH conditions, or on
changing temperatures or pressures, to avoid disturbing the uniformity
of the column packing.
Have good surface homogeneity (including pore surfaces) to optimise
uniformity of interactions along the column for optimal chromatographic peak shape.
Have chemical stability but can be amenable to chemical modification to
make it possible to tune the type and strength of component interaction.
Be readily and reproducibly manufactured, using an approach that is
scalable to commercial demands.
Introduction
of Porous Carbon Stationary
Phases
In the 1970s, carbon was initially proposed as a potential alternative nonpolar HPLC stationary phase to chemically bonded silica, since it benefits from
several significant advantages. Modified silica-based stationary phases can
suffer from manufacturing inconsistencies and huge differences in performance and behaviour of surface chemistries that are supposedly equivalent.
In addition, the instability of silica-bound functional groups, combined with
the difficulty of capping residual silanol groups on the silica particle, limit the
useable pH range (i.e. pH = 2–8). This, in turn, restricts the chromatographic
mobile phases that can be employed.7 Carbon was recognised to offer significant improvements over modified silica in these respects, although whilst
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graphite was recognised as having real potential as a liquid chromatographic
stationary phase, the materials initially produced were fragile and therefore
were considered inappropriate as chromatographic stationary phases.
The main difficulty was in producing carbons that were both hard and yet free
of micropores. Carbon–clad silica particles, developed by Colin and Guiochon,
as well as graphitised thermal carbon black (GTCB), investigated by Ciccioli and
coworkers, were early notable efforts.15–17 However, carbon-clad silica delivered
poor column efficiency and broad chromatographic peaks, whilst GTCB was
considered too fragile for HPLC applications (although chromatographic performance was good). The breakthrough came when a templating process was developed for the production of “porous glassy carbon” (now termed porous graphitic
carbon (PGC)). This synthetic approach yielded regularly sized carbon spheres
with sufficient mechanical strength that could be produced with porosity and
surface properties adequate for use as an HPLC stationary phase; it was at the
time viewed as an alternative reversed-phase material.14,18–21 However, since that
time PGC, which was made commercially available in 1988 by Thermo Electron
Corporation under the trade name Hypercarb®, has proved to be a unique chromatographic stationary-phase material with some unexpected, very attractive
and extremely useful properties; these include the retention of polar and ionic
analytes, selectivity with respect to structural isomers, and compatibility with
mass spectrometry-friendly mobile phases. For these reasons, PGC has found
utility in a wide range of application areas and is finding ever-broader adoption.
Such separation capabilities are also worth noting in the context of future Biorefinery schemes, which will require the use of stationary phases capable of separating complex mixtures of polar analytes.
4.3.1
Porous Graphitic Carbon and its Application
4.3.1.1 Basis of Retention Mechanism
During its development PGC was considered to be the perfect reversed-phase
(RP) material, although it is now clear that retention of analytes on PGC
increases rather than decreases with increasing analyte polarity, due to the
initially unexpected affinity of the graphite surface for polar analytes. This
retention mechanism was described by Knox and Ross as the “polar retention effect on graphite” (PREG).21 The PREG retention mechanism is characterised by a balance of two main factors: (i) hydrophobic eluent–analyte
interactions, which occur between a hydrophilic eluent and any nonpolar
segments of the analyte, and (ii) electronic interactions of polarisable or
polarised functional groups in the analyte with the delocalised π-electrons of
the graphite surface (Figure 4.2).21
As a result, carboxylic acids are often added to the mobile phase to contribute anions for electronic interaction with the π-electrons of PGC, and thus
help elute the more strongly retained compounds.22–24 An interesting study
by Elfakir and Dreux investigated the elution strength of several carboxylic
acids as regards their ability to elute inorganic ions from a PGC column, and
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Figure 4.2
109
Charge-induced
dipole–dipole interactions between PGC’s graphite
surface and (A) a positive or (B) a negative charge. Adapted with permission from ref. 7.
found that the elution strength decreased in the following order: heptafluorobutyric acid > trifluoroacetic acid > formic acid > acetic acid.25
Due to its surface characteristics, PGC has been shown to have a very particular behaviour in terms of retaining highly polar compounds that typically
show minimal retention on silica-based RP media, and thus generally elute
close to the void volume with minimal chromatographic separation. Other
advantages of PGC over classical RP, normal-phase (NP), and high-performance anion exchange chromatography (HPAEC) columns include:
1. The possibility to use mass spectrometry-compatible mobile phases
without the need for ion-pairing reagents, allowing efficient online
coupling with electrospray ionisation (ESI); and
2. Phase stability over the entire pH range (0–14) allowing the separation
of acidic or basic analytes in their neutral form, and stability over a
wide temperature range (≥200 °C), allowing ultrahigh-temperature liquid chromatography (UHT-LC).26,27 Hypercarb® has demonstrated stability when used routinely at temperatures ≥200 °C under isothermal
or temperature gradient conditions, a feature that further extends the
versatility of the material.28
4.3.2
Chromatographic
Applications of Porous
Graphitic Carbon
The increasing popularity of PGC over the past decades has allowed its
application in a wide range of complicated polar separations and biological
matrices (for recent detailed reviews on PGC see ref. 7 and 29). For example, PGC has long been applied in biomedical analysis.22,30,31 The use of
PGC in biomedical applications has been extensively reviewed by Lim and,
more recently, by Michel and Buszewski who included environmental applications.32,33 In the early 1990s, Koizumi investigated the chromatographic
behaviour of mono-, di- and oligosaccharides using PGC, and in these studies, it was evident that the elution patterns of these compounds are based on
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Chapter 4
the size and the planarity of the molecule, namely position and configuration of linkage, which is consistent with the so-called PREG retention mechanism.34,35 PGC has been extensively applied to the analysis of a wide range
of analytes, including polar compounds,36,37 positional isomers,38 isomeric
glycolytic intermediates (e.g. glucose-6-phosphate and fructose-6-phosphate
(Glc6P/Fru6P), and 2-phosphoglycerate and 3-phosphoglycerate (2 PG/3 PG)
from Escherichia coli cell extracts),39 nucleosides and their mono-, di- and
triphosphates,40 conjugated estrogen isomers,41 endogenous underivatised
water-soluble oligosaccharides from Triticum aestivum stems,42 water-soluble sugars and sugar phosphates from Arabidopsis thaliana leaves,43 droughtstress osmolytes, including raffinose family oligosaccharides (RFOs) from
Lupinus albus stems,44 bacterial lipid-linked oligosaccharide intermediates
in Campylobacter jejuni,45 and complex mixtures of oligosaccharides in glycan samples.46–50
4.3.3
Synthesis and Drawbacks of Porous Graphitic Carbon
The synthesis of PGC is based on a hard-templating or nanocasting approach
originally developed in the late 1980s by Knox and coworkers.18–20 In their
approach the hard template is a presynthesised inorganic template (e.g. a
porous silica) into which an organic carbon precursor resin (typically oilbased starting materials e.g. formaldehyde, hexamine, etc.) is impregnated
and pyrolysed. The template is then dissolved to leave the (meso)porous
replica material, after which further pyrolysis can be used to close micropores
or direct surface functionality. The development of PGC has since spawned
numerous variations based on different silica scaffolds.2 The first, an
“ordered mesoporous carbon (OMC)” termed CMK-1 was synthesised based
on the templating of a mesoporous aluminosilicate (MCM-48) template, as
reported by Ryoo et al. in 1999.51 Together these works demonstrated the
scope for tuning of pore shape, structure and diameter in the resulting carbon product, based on the selection of a given inorganic template.52 The PGC
stationary phase Hypercarb® is synthesised via the impregnation of porous
silica beads with a phenolic resin carbon precursor. The filled template is
typically pyrolysed at 1000 °C to generate a “carbon black” intermediate. The
now-redundant template is then dissolved using a hot potash or hydrofluoric
acid solution and the intermediate porous carbon is then pyrolysed at >2000 °C
under Ar to close undesirable micropores. The presence of micropores in
the final stationary-phase material can lead to the irreversible adsorption of
small analytes within the narrow, high-energy pores, hence the need for their
removal if an efficient separation is to be conducted. The pyrolysis temperature is then reduced to around 1000 °C and hydrogen gas passed through the
material to deactivate free radicals formed at the surface, and leave the final
graphite-like product.7,20
The synthesis of Hypercarb® has changed little in the years since its initial
introduction to the chromatography community, probably due to the minimal research aimed at development in this area. A major hurdle to development and a major disadvantage of Hypercarb® synthesis, from a green
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analytical chemistry standpoint, is the energy-intensive pyrolysis process
and harsh chemicals (e.g. HF(aq)) required to remove microporosity and template, respectively. Whilst mesopores generated by the templating process do
allow analytes to pass through the material and increase the effective surface
area of the porous carbon particles available for analyte interaction, this rigid
structure also limits mass-transfer efficiency through the stationary phase
compared with “monolith-like” hierarchical structures. In addition, the inert
hydrophobic surfaces generated in the Hypercarb® synthesis are not easily
chemically functionalised. Therefore, to open up further chemical applications of such materials, additional difficult modification steps are required
which both increases the price of the final product and reduces material mesopore content.53
The development of a low-temperature, facile, and efficient synthesis
strategy to generate mesoporous carbons with tuneable surface/chemistry
properties suitable for separation applications would therefore be desirable
to deliver a “green” alternative to PGC. The ability to alter the textural properties of such carbon phases and generate a hierarchical pore system featuring
macroporosity would also be advantageous in order to enhance mass-transfer
efficiencies – critical to fast, well-resolved separations.54 Porous carbons prepared from biomass-derived precursors are ideal candidates to exploit this
development opportunity and offer several other advantages with respect to
practicality, costs, and sustainability over other (e.g. oil-based) starting materials generally favoured in templating synthetic approaches. The following
section highlights several studies aimed at developing porous carbons from
sustainable precursors for use as separation media.
4.4
4.4.1
Sustainable
Porous Carbons in Separation
Science
Starbon®
4.4.1.1 Development of Starbon®
In the mid-1990s, (as mentioned in Chapter 2) pioneering independent work
by Glenn et al. and Te Wierik et al. described how porous aerogels could be
produced based on the gelatinisation and subsequent controlled drying of
starch gels.55–57 Both groups described how different drying methods had
diverse effects on SBET and porosity characteristics of the product. In particular, Glenn et al. noted that the gel had to be washed in progressively lower
surface tension solvents to prevent gel collapse.55
At the University of York, extending the work by Glenn et al. has led
to the development of high surface area (>120 m2 g−1), mesoporous corn
starch-based materials.58 Budarin et al. applied this material to column
chromatography and showed how utilising the polar hydroxyl group-rich
surface of the native polysaccharide in a manner analogous to silica, NP
chromatographic separation of ferrocene compounds could be performed
(Figure 4.3).59
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Figure 4.3
SEM
and separation profiles of ferrocene, acetylferrocene and diacetylferrocene over (A) native starch, (B) expanded starch, (C) silica,
(D) degraded expanded starch and (E) expanded starch stored under
hexane. Reproduced with permission from ref. 59.
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As discussed in detail in Chapter 2, these corn starch-based mesoporous
materials have subsequently been converted via acid-catalysed pyrolysis
to yield first-generation (i.e. starch-derived) Starbon®. In this approach the
promising pore structure of the expanded starch precursor is maintained in
the carbonaceous product, whilst the surface chemistry can be tuned depending on the final pyrolysis temperature, exploiting physicochemical changes
that occur when converting the hydrophilic hydroxyl-rich glucose monomers
into increasingly hydrophobic, aromatic structures and eventually amorphous carbon-like materials at high temperatures (>600 °C) (see Chapter 2
for further details).60 Further functionalisation with sulfuric acid led to the
development of Starbon®-acids that have shown good potential in catalysis
applications (Chapter 3).61,62 More recently, White et al. have described how
the textural and ordering properties of the expanded starch could be tuned
depending on the relative amounts of amylose and amylopectin in the starch
precursor.63 The higher the level of amylose, the higher the surface area and
porosity in the resulting dried material. Subsequent extension of this work
has demonstrated the use of other linear polysaccharides in the synthesis
of second-generation Starbon® in order to open up a range of porous polysaccharide-derived carbon materials. Particular success was obtained with
alginic acid and pectin (negating the need for a carbonisation catalyst) with
the resulting porous carbon materials exhibiting greater mesopore content
and importantly very limited micropore content as compared to 1st-generation starch-based Starbon® materials.9,64
4.4.1.2 Application of Starbon® in Chromatographic Separations
The separation potential of alginic acid-derived Starbon® (prepared at 1000
°C) was demonstrated by White et al., with the liquid chromatographic separation of a mixture of carbohydrates (Figure 4.4).9 The pore diameter of the
prepared phase was large (14 nm), with high mesopore volume (>1 cm3 g−1),
and very low micropore content (<0.04 cm3 g−1) composed of a predominantly
amorphous condensed carbon material, presenting some short-range (turbostratic) ordering. This material was found to provide sufficiently polarisable
surfaces on which to perform challenging liquid-phase separations of polar
sugar analytes. The chromatographic performance of Starbon® was comparable to that of Hypercarb® PGC in terms of analyte retention and isomer separation, and given the synthetic approach, provides scope for the production
of “designer” stationary phases with varying degrees of aromatic/π-system
character, easily accessible and tuneable for a specific separation scenario.
The seminal work presented by White et al. demonstrated the possibilities
offered by Starbon® in chromatography. The presented results interestingly
demonstrated that accessible high-volume hierarchical pore structuring
composed of predominantly large mesopores, coupled with short-range “carbon” ordering, is potentially sufficient to generate carbonaceous surfaces
particularly suitable to efficiently perform difficult separation of polar analytes, importantly in a cost-effective and sustainable manner.
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4.4.1.3 Limitations of Starbon® Synthesis
In liquid chromatography, the column efficiency (i.e. the number of theoretical plates, where a “plate” is a point of interaction between the analyte and stationary phase) of a particle-packed column is influenced by
three factors: eddy diffusion; longitudinal diffusion; and mass transfer,
proposed by Van Deemter in the 1950s (Section 4.2.2).11 The development
of high-quality commercial columns arises from the ability to pack the
stationary-phase particles into a homogeneous, stable bed to give reproducible chromatographic behaviour. Particle size and morphology are key
parameters that need to be controlled in order to produce a high-performance stationary-phase material. One of the advantages of the alginic
acid-derived Starbon® synthesis over that of commercial PGC is the natural
formation of a mesoporous structure that does not rely on the use of a silica template.9 Whilst White et al. demonstrated the formation of regularly
sized porous particle from starch (using microwave-based synthesis),63
Starbon® derived from this base would have too high a micropore content to make them useful as a high-performance separation media. Whilst
an initial alginic acid-derived Starbon® stationary phase (AS1000) and its
application in LC–ESI–QIT–MS has proven to be very promising, the current Starbon® synthesis does not enable the preparation of carbonaceous
particles with desirable micrometre-sized, narrow size distribution and
spherical morphology required for high-performance ­stationary-phase
materials.
Figure 4.4
Extracted
ion chromatograms obtained on the AS1000-LC-ESI-QIT-MS
separation of (A) 50 mM standard solution of a mixture of glucose,
sucrose, raffinose, stachyose, and verbascose; and (B) 50 mM standard
solution of a mixture of the disaccharide isomers trehalose and maltose,
detected at m/z 387 as formylated molecules ([M + HCOO]−). HPLC conditions: AS1000 column 50 mm × 4.6 mm inner diameter (i.d.)), 400
µL min-1, 5 µL injection, gradient mobile phase of acetonitrile (0.1%
formic acid/0.1% aqueous formic acid). Reproduced with permission
from ref. 9.
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4.4.2
115
Alginate-Derived Mesoporous Carbon Spheres (AMCS)
It is well known that water-soluble sodium alginate forms a hydrogel when
cured in a solution of divalent ions, due to the greater affinity of alginate for
divalent ions than for sodium ions.65–68 Calcium alginate gel beads formed in
this way are used as gelatinisers in the food industry,69 and as drug delivery
vehicles.70,71 Quignard et al. have shown how aerogels can be manufactured as
1.5 mm spheres (by syringing sodium alginate drop-wise into calcium chloride
solution) with the resulting materials having high mesoporosity (Vmeso = 1.16
cm3 g−1, average pore diameter: 38 nm) and large surface areas (SBET: 570 m2
g−1),72 – properties similar to alginic acid aerogels used in Starbon® synthesis. In
this context, recent work by Marriott et al. describes the successful preparation
of alginate spheres at the low-micrometre scale, followed by pyrolysis to the corresponding carbon. Pyrolysis of the aerogel under an inert atmosphere yielded
mesoporous carbonaceous spheres prepared via a “natural” templating process
(Figure 4.5). By controlling the calcium content of the aerogel, the pore structure of the resulting carbon could be altered, with a large amount of calcium (ca.
1.3 wt%) required to retain the pore structure of the precursor aerogel.10
A standard HPLC column was packed with AMCS and applied in the separation of a carbohydrate mixture; the separation behaviour was similar
Figure 4.5
SEM
images of alginic acid-derived mesoporous carbon (Starbon®) (A)
×250 and (B) ×6000 magnification and AMCS (C) ×250 and (D) ×5000
magnification. Both samples were pyrolysed to 800 °C. Reproduced
with permission from ref. 10.
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Figure 4.6
Van
Deemter curves for a 50 µg mL−1 sucrose standard solution eluting
at various flow rates from AMCS-LC-MS (■), Starbon®-LC-MS (●) and
PGC-LC-MS (s) systems. Reproduced with permission from ref. 10.
to that reported by White et al. for alginic acid-derived Starbon®, although
the smaller carbohydrates (e.g. glucose and mannitol) were more strongly
retained on the AMCS column than on the earlier alginic acid-derived Starbon® column. In addition, the efficiency of the AMCS column was compared
with that of a Hypercarb® column and a column packed with alginic acid-derived Starbon® material, by generating Van Deemter curves for each column,
by plotting the calculated plate height of a 50 µg mL−1 sucrose standard
eluted at various flow rates (Figure 4.6). It can be seen that the AMCS-packed
column mimics the efficiency of Hypercarb® at low flow rates, suggesting
that these spherical stationary-phase particles are more homogeneously
distributed than the irregular particles of alginic acid-derived Starbon®. As
flow rate increases, the efficiency curve for the AMCS column more closely
resembles that of the Starbon® column, which is likely due to the much larger
particle size of both these materials (Starbon® average = 28.6 µm; AMCS average = 23.4 µm) when compared with Hypercarb® (5 µm). Overall, the chromatographic performance of AMCS was found to more closely resemble that
of Hypercarb® than that of Starbon®, and whilst further reduction of particle size is required, the work by White et al. and Marriott et al. represents a
very promising step forward in the development of a more environmentally
friendly, biomass-derived carbon stationary phases.
4.5
Does
a Sustainable Porous Carbon
Need to be Graphitic?
The mechanisms of retention for PGC are reported to rely on the interaction
of graphitic planes at the particle surface with polar or polarisable analytes.
However, to generate such planes, pyrolysis temperatures typically >2000
°C are required.73 The fact that HPLC columns packed with either alginic
acid-derived Starbon® or AMCS materials offered similar chromatographic
separation of a series of carbohydrates to that of a commercial PGC column
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is surprising, given the differences in the pyrolysis temperatures used to generate the biomass-derived porous carbons and PGC.
Using a combination of transmission electron microscopy, core loss electron
energy loss spectroscopy (EELS) and X-ray photoelectron spectroscopy of the
carbon microstructure of both Starbon® and AMCS (prepared at a maximum
pyrolysis temperature of 1000 °C) was recently analysed by Brydson et al. in
order to understand how this microstructure relates to that present in PGC.74
In this investigation, PGC was described as having a carbon surface similar
to that of a traditional nongraphitising carbon (i.e. a carbon which does not
form crystalline graphite after high-temperature heat treatment)75 with extensive graphitic stacking (3–6 layers deep, 5–15 nm wide), as well as smooth
and angular curvature present at the joint of two graphitic stacks. This result
mirrored that of Knox et al. for Hypercarb® who described this material as a
nongraphitising carbon comprised of two-dimensional “turbostratic” graphite.20 In contrast, although the graphitic stacks were less developed than in
the commercial material, the microstructure of both the alginic acid-derived
Starbon® and AMCS materials contained 50–70% fullerene character; this is
significantly higher than the fullerene levels present in previously examined
nongraphitising carbons pyrolysed at similar temperatures.73,74 The high
fullerene content was speculated to arise through the pyrolytic breakdown
of the hexuronic acid residues to furans and C5 intermediates, e.g. cyclopentenones,76 which in turn could act as nucleation points for fullerene growth.
In this study, the EELS analysis data for Starbon® and AMCS appear similar
to those obtained by Titirici and coworkers for carbons prepared from crude
cellulosic biomass material by hydrothermal carbonisation and these are
probably similar for most carbons prepared from carbohydrate precursors.77
That a sustainable porous carbon is found to be fullerene-like as opposed to
graphitic does not suggest that the sustainably sourced material should be
dismissed as a viable competitor to PGC for chromatographic applications,
as evidenced in this chapter. Indeed, previous work by Grate and coworkers
has shown how increasing aromaticity and polarity of an analyte can increase
its retention on a fullerene surface in a manner analogous to that on PGC.78
The obvious difference between fullerenes and PGC is the curved vs. planar
graphite-like surface; the former is expected to induce stronger affinity than
the latter for nonplanar molecules, which can better fit into the curvature of
the microstructure.79
4.6
4.6.1
Other Sustainable Carbons in Chromatography
“Chocolate” Hydrophilic Interaction Liquid
Chromatography (HILIC)
Although thus far this chapter has focused on alternatives to PGC, whereby the
particle is a porous carbon, there has also been ongoing research into developing saccharide-based carbons to coat porous silica particles for use in hydrophilic interaction liquid chromatography (HILIC). HILIC is another suitable
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80,81
stationary phase for the separation of very polar compounds.
HILIC differs from NP–LC, by replacing the nonaqueous solvent used in NP–LC (e.g.
hexane or chloroform) with a polar, mostly organic solvent (generally 5–40%
water in acetonitrile). There is a broad range of HILIC stationary phase types,
from classical bare silica and aminopropyl-bonded silica beads, to silica gels
modified with polar functional groups. Examples of bonded functionalities
include amides, diols, cyclodextrin and the zwitterionic group sulfoalkylbetaine, with columns of this type (manufactured by Merck SeQuant®) termed
ZIC-HILIC columns.82 The bound functionality is able to strongly retain water
at its surface, creating a static aqueous layer. The partitioning effect caused
by the affinity of an analyte for the water-enriched layer of the hydrophilic
stationary phase and the hydrophobic bulk eluent is considered the basis for
separation with HILIC.83 The rich variety of hydroxyl groups and other polar
functionalities present in saccharides (and materials-derived therefrom) in
principle makes them ideal ligands for HILIC chromatography.
Recent work by Schuster and Lindner described the development of a
“chocolate”-like HILIC stationary phase whereby reducing sugars, acting as
ligand primers, are bound to amino-modified porous silica before undergoing a “controlled” Maillard reaction to create the final HILIC material, see
Figure 4.7.84 The “chocolate” name is derived from the colour of the material following the Maillard reaction (i.e. a nonenzymatic browning) and the
initial carbonisation of the carbohydrate precursor (Figure 4.7). Schuster
and Lindner compared their chocolate HILIC stationary phases prepared
from a number of different carbohydrates with bare silica and a commercial
Phenomenex Luna HILIC column by analysing six test mixtures to understand the retention behaviour of the new phase. They found that their cellobiose-functionalised column offered the best peak performance among the
chocolate HILIC columns in terms of plate counts. It was also reported that
the cellobiose-functionalised chocolate HILIC column outperformed the
Luna HILIC column with regards to separation efficiency of a variety of compound classes including purine/pyrimidine bases and polyphenols. Schuster
and Lindner concluded that the chocolate HILIC column uses a mixed-modal
Figure 4.7
Proposed
reaction scheme for chocolate HILIC packings, exemplified
for glucose-modified G-Choc HILIC. The brown circles represent the
as yet structurally less defined chocolate ligands. Reproduced with permission from ref. 84.
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Application of Carbonaceous Materials in Separation Science
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retention mechanism when operating under HILIC conditions, with adsorption and partition phenomena responsible for the retention and selectivity
characteristics observed.84 Further work is required to understand the reaction cascade of the Maillard reaction in order to fully understand how this
material can be most effectively used in HILIC chromatography. However,
chocolate HILIC certainly provides a promising alternative to the current
diol-functionalised HILIC phases currently on the market.
4.6.2
Carbon Coating of Silica Particles
Li and coworkers have developed an alternative approach to carbon coating
HPLC silica particles using carbon nanoparticles derived from corn stalk soot,
a product of corn stalk burning (Figure 4.8).85 The use of carbon nanoparticles in chromatographic separations, particularly carbon nanotubes, has
literature precedent in both HPLC,86–88 and gas chromatography,79,89–93
demonstrating a unique analyte selectivity analogous to PGC in the separation of highly polar and hydrophilic compounds. Li and coworkers were able
to produce carbon nanoparticles (Diameter = 6–18 nm) by oxidising the corn
stalk soot in nitric acid (Figure 4.8(B)–(D)). The resulting stationary phase
obtained after neutralisation of the acidic medium and immobilisation of
the carbon on a porous silica gel was packed into a column and applied to the
separation of nucleosides, sulfur compounds and a safflower plant extract.
This corn stalk soot-derived stationary phase was found to have retention
characteristics similar to HILIC.85
Figure 4.8
(A)
The route to the synthesis of the new separation phase proposed by
Li et al.; (B) TEM image of the carbon nanoparticles (CNPs), (C) HRTEM
image of the CNPs, (D) TEM image after immobilisation of CNPs on
silica gel surfaces. Reproduced with permission from ref. 85.
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4.7
Future Perspectives
Chromatography as a potential area of application is suggested in a number of articles relating to the development of sustainable porous carbons, yet
this proposition, except in the rare cases discussed in this chapter, has not
significantly been put to the test. This could be due to the demands of the
carbon material for generating a good-quality chromatographic particulate
stationary phase outlined in the introduction of this chapter i.e. extensive
mesoporosity, little to no microporosity, particles of spherical morphology
and a narrow particle-size distribution in the low micrometre range. In
addition to continuing the work involving Starbon® and AMCS materials,
carbonaceous material synthesised via the HTC process are also a potentially viable candidate to investigate. The preparation of porous spheres has
already been demonstrated by combining the HTC of d-glucose with both
hard- and soft-templating strategies, the latter through the use of Pluronic®
amphiphilic block copolymers as structure-directing agents and d-fructose
as the carbon precursor.77,94 In both cases, however, the overall porosity of
the material is low and would need to be improved to be an effective chromatographic stationary phase.
The inherent chirality of the polysaccharide structure has led to the successful development of chiral stationary phases used for the enantioseparation of analyte isomers. Cyclodextrin and cellulose-based phases, where the
polysaccharide is bound to the surface of silica beads, can be used “as is”
or be functionalised to enhance the enantioseparation of a particular compound class.95 Both cyclodextrin and cellulose have proven to be versatile
stationary phases and are used in a wide range of separation techniques
including HPLC, GC, capillary electrochromatography and, more recently,
supercritical fluid chromatography.95–98 Chiral chromatography therefore
could offer another application for porous carbons from polysaccharide
precursors so long as after pyrolysis the chiral groups on the parent carbohydrate are retained.
Another approach to using sustainable porous carbons in chromatography is through the preparation of carbon monoliths. Carbon monoliths are
single rods of carbon with an interconnected hierarchical pore network. The
advantages of monoliths over particles are that they reduce rates of mass
transfer by combining macroporous regions to ensure fast equilibration
of even large analytes whilst retaining a mesopore network to ensure good
sample-loading capacities. In addition, monoliths are much easier and safer
to handle than fine particulates and have a reduced back pressure compared to particle-filled columns when run at similar flow rates.54 Recently,
White et al. and Fellinger et al. have described the preparation of carbonaceous monoliths (termed “carbogels”) based on the conversion of glucose/
ovalbumin and glucose/sodium borate mixtures, respectively, whereby the
protein and borax act as structure-directing agents and the preparation of
carbohydrate-derived carbonaceous monoliths.99 In both cases porosity can
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be tailored based on reaction time and the concentration of the respective
additives. Currently, the potential for this material as a chromatographic
stationary phase has not yet been examined. These materials are discussed
in more detail in Chapter 6.
Although this chapter has focused on chromatographic stationary phases,
recently, graphitised carbons have been successfully applied in solid phase
extraction (SPE) protocols for rapid and selective sample preparation. In
SPE, separation is achieved through the interaction of three components:
the sorbent, the analyte and the solvent. The sample solution is loaded onto
the SPE solid phase (sorbent), interfering components are washed away, and
then the analytes are eluted with another solvent (or solvent mixture) into
a collection tube. Reviews by Hennion give a comprehensive overview of
this method of sample preparation and, in particular, the use of graphitised
carbon as SPE sorbents.100,101 There are two types of commercially available
graphitised carbon sorbents for SPE applications (i) graphitised carbon
blacks (GCBs) and (ii) porous graphitic carbons (PGCs). The first commercially available GCBs for SPE applications are referred to as Carbopack-B®
or ENVI-Carb® (Supelco) and Carbograph® (Alltech). PGCs are commercially
available for SPE applications under the name Hypersep®–PGC, and contain
similar packing material to the LC-grade Hypercarb®. Of these examples,
the most widely used is GCBs, where compounds are retained as a result of
the presence of two types of adsorption sites, the most abundant sites being
provided by the nonpolar carbon atoms arranged in a graphite-like structure that interact with analytes via van der Waals forces.102 Due to their ability to trap very polar and water-soluble analytes, GCBs have been especially
used for the extraction of many classes of polar compounds from aqueous
samples, including phenolic compounds,103,104 pesticides,105–107 organic pollutants,108 and amphoteric aromatic amines from wastewater samples.109
SPE is possible using several formats, e.g. cartridges, discs and pipette tips
and are considered single-use consumables. As such, the demands on the
sorbent packed into these cartridges are much less strict than the requirements of an HPLC stationary phase although the production must be inexpensive to ensure cost effectiveness. With regards to porous carbons from
sustainable precursors, the naturally high abundance and low cost of biomass make it an excellent precursor for the preparation of SPE phases so
long as the method to generate the carbon is simple, relatively cheap on
a bulk scale, and batch-to-batch consistency is assured. Other important
properties of an SPE sorbent are good flow characteristics (this can include
larger particles and a less strict particle-size distribution), high surface area
(to enable high sample loading) whilst retaining a high percentage of mesoporosity to retain fast mass transfer through the material. These properties fit many of the porous carbons from sustainable precursors currently
described in the literature, and ongoing research at the Green Chemistry
Centre of Excellence, University of York, UK is looking into developing a
robust SPE sorbent using Starbon® technology.
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4.8
Conclusions
PGC is an important material for use in separation science, particularly in
HPLC and SPE, due to its unique retention mechanism which offers an alternative technique to separate polar analytes and structural isomers. Although
hard-templating offers many advantages in the generation of mesoporous
regularity, there have been few major alterations to the original synthetic
steps developed by Knox and coworkers in the 1980s. This is likely due to
a lack of competitors in this commercial environment. The synthesis suffers from the use of oil-based phenolic resin carbon precursors, high temperatures required to eliminate microporosity in the final carbon material
and the harsh chemicals used in the template removal step, making this a
nonsustainable and energy-intensive material synthesis. This chapter has
highlighted several recent efforts to develop greener porous carbon stationary phases for chromatography using sustainable precursors. Starbon® and
AMCSs show promise as alternative PGC-type stationary phases. However,
further development must look to reduce microporosity and improve particle size and shape in order to compete with the commercial PGC Hypercarb®.
Work by Schuster and Lindner on “chocolate” HILIC84 and by Li and coworkers on carbon coated silica,85 show how these sustainable porous carbons can
be effective in the separation of a broad range of analyte classes using HILIC
retention mechanisms. Likewise, the preparation of carbonaceous monoliths
via the hydrothermal carbonisation approach (as discussed in forthcoming
chapters in this book) could also be promising candidate materials for the
development of tuneable carbonaceous stationary phases. Analogously, the
development of a low-cost synthetic route to prepare porous carbons for
SPE applications would be an obvious short-term target with which to commercialise sustainable porous carbons due to the less stringent structural
demands of the product. There is work to be done to develop a truly high-performance stationary phase, chiefly ensuring narrow particle-size and -shape
distributions (or dimensionally stable monoliths) whilst maintaining mesoporosity, but there is also reason to be optimistic that sustainable porous
carbons could become viable separation media in the near future.
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88.C. Andre, T. Gharbi and Y. C. Guillaume, J. Sep. Sci., 2009, 32, 1757.
89.M. Karwa and S. Mitra, Anal. Chem., 2006, 78, 2064.
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and O. Bakajin, Anal. Chem., 2006, 78, 5639.
91.C. M. Hussain, C. Saridara and S. Mitra, Anal. Chem., 2010, 82, 5184.
92.A. Speltini, D. Merli, E. Quartarone and A. Profumo, J. Chromatogr. A,
2010, 1217, 2918.
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Chem. Mater., 2011, 23, 4882.
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97.N. M. Maier and W. Lindner, Methods Princ. Med. Chem., 2006, 33, 189.
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PART 2
HYDROTHERMAL CARBONISATION
(HTC)
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CHAPTER 5
Hydrothermal Carbonisation
(HTC): History, State-of-the-Art
and Chemistry
ADAM MARINOVIC†a, FILOKLIS D. PILEIDIS†a AND
MARIA-MAGDALENA TITIRICI*a
a
Queen Mary, University of London, School of Engineering and Materials
Science, London, UK
*E-mail: m.m.titirici@qmul.ac.uk
5.1 Introduction
Our modern society consumes on a daily basis huge amounts of fossil
fuel-derived energy. As a consequence, greenhouse gases, in particular CO2,
are generated and typically released to the atmosphere and given its lifetime
contributes to global warming and associated manmade climate change that
the planet is currently experiencing. Of the natural resources available, the
fossil fuels (e.g. natural gas, coal, oil) are being depleted at alarming rates
and indeed their sources, given their formation mechanism, will not be
replenished on timescales relevant to our society. Therefore, there is a clear
necessity to identify and find alternative sources of energy in the short and
long term. In order to do this in as sustainable manner as possible, it will
be concurrently necessary to identify and develop new sustainable material
systems, which can help generate and provide this hopefully nonfossil based
†
These authors have contributed equally to this book chapter.
RSC Green Chemistry No. 32
Porous Carbon Materials from Sustainable Precursors
Edited by Robin J White
© The Royal Society of Chemistry 2015
Published by the Royal Society of Chemistry, www.rsc.org
129
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energy supply in as efficient a manner as possible. In this context, researchers are constantly seeking to improve synthetic routes and design new pathways to new materials in as simple, and cost-effective ways as possible with
the ultimate aim to use renewable resources as precursors. It is therefore
hoped that the new materials and material systems, will prove more active
and efficient in specific roles that are essential to the development of new
energy and chemical economies – namely catalysis, energy storage, water
and air purification. In addressing the chemical and material challenges of
these areas, the development of carbon-based nanomaterials will be critical
to the sustainability of future energy generation and provision.
In the design of new materials, it is possible to find inspiration and a vast array
of opportunities for shaping functional nanomaterials in nature, for example
using raw materials including carbohydrates and proteins. Koopmans previously highlighted the importance of producing new materials from biomass.1
Biomass is the most abundant renewable resource on Earth where terrestrial
biomass growth amounts to 118 billion ton per year (dried).2 In agricultural
cycles, ca. 12 billion ton per year of biomass is discharged as waste. Therefore
there is clearly a significant biomass pool available to be used in different ways,
one of them being the production of new functional nanomaterials.
There are a number of well-established technologies to convert biomass
into biofuels (i.e. as a petroleum substitution)3,4 or various platform chemicals.5–7 Conversion of biomass and waste into valuable nanomaterials is still
comparatively rare, but is a growing research area as evidenced by this book.
In this context, among the materials that can be produced from biomass,
biopolymers,8 porous silica9 or naturally inspired carbon materials are of
particularly note.
As alluded to in previous chapters, biomass-derived carbon materials are
classically prepared via thermal pyrolysis, often in conjugation with chemical
activation. This approach is well established and leads to a class of microporous carbons commonly referred to as “activated carbon”. They are normally
produced using chemical (H3PO4, KOH, etc.) or physical (water, CO2) activation agents, which act as gas/small organic molecules releasers during thermal treatment, resulting in the creation of a large volume of micropores
(and in turn high surface areas). Several detailed reviews on the preparation,
characterisation and applications of chemically activated carbons have been
produced.10 The main drawback of activated carbons is the lack of control
over their porosity, surface functionality and chemical structure. Therefore,
new alternative materials to the classical biomass-derived activated carbon
are necessary, whereby the porosity, morphology and surface functionalities
can be more precisely controlled. New carbon materials produced using low
energy consumption methods, inexpensive and renewable resources, with
flexible functionalities, morphologies and porosities are seen as ideal candidates for many applications related to environmental, renewable energy,
clean water, etc. and their development will be critical to address future
energy and chemical provision schemes (e.g. the aqueous phase chemistry
of the Biorefinery).
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Hydrothermal Carbonisation (HTC): History, State-of-the-Art and Chemistry
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Over the last decade or so, an approach termed hydrothermal carbonisation
(denoted herein as HTC) has re-emerged as a useful synthetic approach to
produce carbon materials from biomass precursors using water as a carbonisation medium, performed at low temperatures and under self-generated
pressures. It can be described as mimicking the natural processes involved
in coal formation from biomass in the synthetic laboratory. Contrary to the
millions of years required by nature to produce coal, HTC produces carbon
from biomass in only a few hours. The significant advantages of the HTC
approach as an alternative to other known carbon synthesis platforms are:
●●
●●
●●
●●
●●
●●
●●
●●
During the HTC process, carbonisation temperatures are low (130–250 °C).
Carbonisation takes place in water under self-generated pressures.11
The morphology of the resulting carbon materials can be precisely
controlled.12
Using natural templates,13 nanocasting procedures,14 or thermal treatment procedures,15 controlled porosity can be easily introduced.
The resulting carbon particles have polar (oxygenated) groups at the
surface that can be used in postfunctionalisation strategies.16
The obtained carbonaceous materials can be combined with other components, like inorganic nanoparticles, to form composites with special
physical and chemical properties.17
Additional thermal treatment allows control of surface chemistry and
electronic properties, while porosity and morphology are unaltered.
HTC synthesis can be described as “carbon neutral” (and potentially
carbon negative) if the CO2 fixed by the starting plant precursor are efficiently converted to a high-yielding carbon material.12
In this chapter a brief history and overview of the state-of-the-art of HTC
synthesised materials will be provided, with discussion regarding the formation mechanism of HTC carbon materials from various biomass precursors.
A very brief overview of the potential applications of these HTC materials
will also be given to provide an introduction to the following chapters in this
section.
5.2 State-of-the-Art
In 1911, the Nobel laureate Friedrich Bergius was fascinated by the idea of
discovering alternative fuels. Bergius’ main idea was that it should be possible to produce H2(g) from the addition of water to carbon-based materials if
the right pressure and temperature conditions are present (eqn (5.1)),18
C + 2H2 O → CO2 + 2H2
(5.1)
Essentially, what Bergius was trying to achieve was to inhibit the formation of carbon monoxide. Bergius managed to oxidise coal with liquid water
after a reaction at 200 bar; hence he was producing CO2 and H2. During his
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experiments on H2 generation without CO production Bergius made another
very important observation: when he used peat instead of coal, a carbonaceous residue was formed inside the high-pressure vessel, which had the
same elemental composition of natural coal. This discovery led Bergius to
investigate the high-temperature and high-pressure decomposition of various plant-based compounds into coal-like materials. He discovered that if
the biomass precursor was in contact with the liquid water, and at mild temperatures (200 °C) in the high-pressure vessel, it could no longer decompose
into gases. During his studies, Bergius used temperatures between 200 and
330 °C in the presence of liquid water at pressures of ca. 200 bar. In these
experiments, Bergius produced the very first HTC material. After this initial
discovery, Bergius and his assistant Hugo Specht did numerous studies on
the hydrogenation of this artificial coal.19 In 1931 Bergius was awarded the
Nobel Prize for these studies on the production of synthetic coal and hydrogenation studies. Unfortunately, after these early experiments, HTC was
largely forgotten, as were many other biomass-based chemical processes,
mainly the result of the discovery of huge fossil resources at the start of the
20th century, essentially rendering either H2 or carbon material production
from biomass not cost competitive.
A new era of HTC experiments emerged at the beginning of this century
with reports on the low-temperature synthesis (200 °C) of carbon spheres
using glucose or other sugars as precursors.20 Sun and Li in 2004 presented
carbon-­encapsulated metal nanoparticles (MNPs) and carbon microspheres
decorated with MNPs based on the hydrothermal treatment of glucose
solutions.21 Yu and Antonietti performed the HTC of starch in the presence of noble-metal salts under mild conditions (200 °C) to also produce
metal–­carbon hybrid nanostructures. In this report it was also noted that
the presence of metal ions accelerates the HTC process.22 Following on
from these initial experiments, the group of Titirici at the Max Planck Institute for Colloids and Interfaces (MPIKG, Golm, German) began to elaborate
more deeply research pertaining to the topic of HTC in the production of
nanostructured carbon materials. A number of reviews and book chapters
have recently highlighted this work.23–27 Today, in part due to the contributions from several leading research groups in the area, HTC is now a
well-recognised technology to produce carbon materials from sustainable
biomass precursors and is receiving increasingly more attention, as evidenced by the ever-increasing number of publications in the area (Figure 5.1)
and the number of HTC-associated “small-to-medium” enterprises that are
attempting to commericalise this material synthesis platform (see Chapter
12 for more details on this latter point).
The HTC approach is increasingly becoming an important topic in Asia,
in particular in China. The groups of Sun and Yu have both made significant
contributions to the area, pioneering its development in this region. Both
groups focused work initially on the production of inorganic–HTC composites.26,28 Thus, Sun et al. developed the one-step production of various metal/
metal oxide core–shell nanoparticles using a one-pot synthesis: glucose was
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Hydrothermal Carbonisation (HTC): History, State-of-the-Art and Chemistry
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Figure 5.1 (a) Exponential increase of papers published containing the key words
“Hydrothermal Carbonisation” and (b) Increased number of citations
in hydrothermal carbonisation.
Figure 5.2 (A) The schematic illustration of the formation mechanism of Ag@C
core–shell structured nanospheres. (B) Schematic illustration of Ag@C
nanospheres. (C) IR spectrum recorded on Ag@C nanospheres. Reproduced with permission from ref. 29.
used as the reducing agent to react with Ag+ ions and also served as the carbon precursor. The effects of hydrothermal temperature, time, and relative
reagent concentrations on formation of the final nanostructures were systematically studied (Figure 5.2).29
The same authors expanded this methodology to the production of metal
oxides@C core–shell particles. Thus, coupled synthesis of Sb8O11Cl2@C,
BiOCl@C, Sb6O13@C, SnO2@C, and MnCO3@C nanoparticles could be
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Figure 5.3 TEM micrographs of HTC-coated Ag nanowires. Reproduced with permission from ref. 22.
successfully produced and used for Li storage applications.30 Simultaneously, Yu et al. also reported that AgNO2 can be in situ reduced upon hydrothermal treatment in the presence of starch, resulting in the formation of
carbon coated Ag nanowires (see Figure 5.3).22
Yu’s group at Hefei National Laboratory for Physical Sciences at Microscale,
University of Science and Technology of China has since been extremely
active in the synthesis and characterisation of a wide variety of inorganic
carbon HTC materials. An overview of the group activities can be found
in various review papers and in more details regarding the preparation of
bacterial cellulose-derived HTC materials in Chapters 6 and 12.26,28,31–33 A
particularly interesting example from the Yu group is the use of glucose to
synthesise uniform core–shell Te@carbon-rich composite nanocables with
thin and long Te nanowires as the core and carbonaceous matter as the shell
components (see Figure 5.4).34
In this approach the diameter of the Te@carbon-rich composite nanocables could be controlled by adjusting the HTC time or the ratio of glucose
and Te precursors. The shell thickness of the core–shell nanocables could be
varied between 2 and 25 nm, whilst removal of the Te nanowire core led to
the production of functional, ultralong carbonaceous nanofibres. It was also
found that Te@carbon-rich composite nanocables had a strong photoluminescence in the blue-violet solar spectrum region. These nanofibres/nanocables also presented a number of polar, oxygen-containing functional groups
and were easily dispersed in ethanol or water, which opens up their use in a
vast array of possible applications.
Another interesting material developed by the Yu group is the production of
carbon aerogels using bacterial cellulose as a precursor.35–37 Here, supercritical drying of bacterial cellulose led to the production polysaccharide-based
aerogels precursors that were subsequently hydrothermally treated to produce high-quality carbonaceous aerogels. These carbon aerogels were subsequently functionalised with inorganic nanoparticles and their use explored
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Figure 5.4 SEM and TEM images of the samples obtained by reaction of 0.1 mmol
Te nanowires and 1.5 g of glucose: (a) and (b) with hydrothermal carbonisation at 180 °C for 12 h, (c) and (d) with hydrothermal carbonisation at 200 °C for 12 h. Reproduced with permission from ref. 34.
as electrode materials in either supercapacitors or Li-ion batteries.35–37 The
work of the Yu group will be touched upon in more detail in Chapter 6. It is
also important to note that there are many more groups in China performing
active research in the field of HTC.
In Japan, the research activities in HTC are far less pronounced. Prof. Endo
Morinobu has recently published a review entitled “Carbonisation under Pressure” in which he discusses carbonisation under pressure as being classified
into three routes:38
1. carbonisation of the precursors under pressure of their decomposition
gases;
2. carbonisation under hydrothermal conditions;
3. carbonisation and reduction of pressurised CO.
In this review, the formation conditions of carbon spheres in pure
and an individually separated state are discussed by focusing on the
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temperature–pressure conditions and the chemical composition of the precursors used. Additionally, Seiichi et al. have investigated the HTC conversion
of cellulose and characterised the resulting carbon material with FTIR,39 as
well as the conversion of empty fruit branches, which is an important problem in the palm oil production industry. The produced HTC materials of Seiichi et al. were subsequently compared with material prepared via traditional
thermal carbonisation under dry nitrogen. The hydrothermal conditions
at 573 K and dry conditions at 873 K had similar charcoal yields of 31.4%
and 27.8%, respectively. However, the calculated heating value of charcoal
obtained from the hydrothermal conditions at 573 K was high (25.8 MJ kg−1)
relative to that obtained from the dry conditions at 873 K (22.0 MJ kg−1).40
Furthermore, in the context of waste valorisation, the same group has also
investigated the HTC conversion of food wastes.41
HTC is also attracting significant interest in the field of municipal waste
treatment and remediation. In this context, Yoshikawa et al. have used
the HTC approach to recycle energy from sewage sludge by producing a
solid biofuel.42 The effect of the HTC temperature and holding time on
the biofuel recovering ratio, calorific value and energy recovery rate was
investigated. This evaluation fully considered the effect of the HTC conditions, mechanical dewatering, thermal drying, and biofuel recovery ratio.
Moreover, the energy consumption of sludge thermal drying was introduced to illustrate the economic efficiency of the HTC biofuel production
process more intuitively. The results demonstrated that the HTC biofuel
production process was more cost effective than the conventional thermal
drying. The HTC temperature was found to be the most important parameter to affecting the biofuel properties. The carbon content of solid biofuel increased both with HTC temperature and holding time, resulting in
an increase in the calorific value of biofuel; whereas, the biofuel recovery
ratio, defined as the mass ratio of solid biofuel to raw sludge, concurrently
dropped causing a reduction in the energy recovery rate. When the HTC
temperature was above 200 °C, the energy recovery rate was around 40%.
A moderate condition HTC temperature of 200 °C and holding time of 30
min was suggested to produce solid biofuel from sewage sludge with an
energy recovery rate of 50%.42
In the context of nanostructured carbon synthesis based on the HTC
platform, Kubo et al. (as will discussed in more detail in Chapter 6) have
recently published an article describing a dual-templating approach, combining amphiphilic block copolymers and latex nanoparticles for the synthesis of hierarchically porous carbonaceous monoliths based on the HTC
conversion of fructose.43 Elsewhere in Asia, HTC research in Singapore is
focused on the production of nanostructured carbon materials for renewable energy applications,44,45 fluorescent carbon quantum dots,46,47 or the
conversion of biomass/biowaste into solid char for energy purposes.48–51 In
India, Thanikaivelan et al. have published an interesting paper describing
the use of HTC to convert leather waste residues into interesting nanocarbons, which could be used as electrodes in secondary batteries.52
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In Australia, recent efforts are being currently directed towards the fundamental understanding of the formation mechanism of the nitrogen-doped
hydrothermal carbon spheres in highly acid, basic or neutral conditions.53
Donne et al. recently studied the effect of highly acidic (0.2 M H2SO4), neutral
(H2O), and basic (0.2 M NaOH) solutions with and without the addition of
0.2 M (NH4)2SO4 as nitrogen source on the chemical and structural morphologies of hydrothermally formed carbon spheres from sucrose at 200 °C for
4 h. The hydrolysis product yield, without the addition of (NH4)2SO4, varied
considerably (11.34 wt% H2SO4, 47.81 wt% H2O, and 3.54 wt% NaOH), as did
the spherical size (3.34, 4.57, and 6.63 nm for H2SO4, H2O, and NaOH, respectively). The addition of (NH4)2SO4 increased product yields considerably in
acidic and basic conditions (27.76 wt% H2SO4 and 14.73 wt% NaOH). Chemically, the hydrochars had a carbon content between 60 and 70 wt% and oxygen content between 22% and 29% with alcohol groups (12.29, 15.44, 11.26
atom% for H2SO4, H2O and NaOH, respectively); the main oxygen functionality, although carbonyls, carboxylic acids, and ketones were also present.
These oxygen functionalities fluctuated with the presence of (NH4)2SO4, with
reductions in alcohols (1−3 atom%) and ketones (1−3 atom%), and increases
in carboxylic acids. Nitrogen was located in pyridinic, pyrrolyic, and quaternary groups (6.24, 3.22, and 9.41 atom% for H2SO4, H2O, and NaOH, respectively). GC-MS revealed that levulinic acid was the predominate byproduct.53
In the USA, there are several excellent groups working on the engineering aspects of HTC, including process parameters and products optimisation, with an overall aim to derive a fundamental understanding of the
HTC process. The group of Heillmann and Steven from the University of
Minnesota have reported on a number of several careful studies on the
HTC of microalgae,54,55 distillers grains,56 low cellulosic biomass,57 as well
as proposing an “Industrial Symbiosis Concept” combining corn ethanol
fermentation with HTC and anaerobic digestion.58 Hoekmann et al. are
also investigating the HTC of lignocellulosic biomass. They have studied
the reaction kinetics of HTC of loblolly pine,59 the chemical, structural and
combustion characteristics of carbonaceous products obtained by HTC of
palm empty fruit branches,60 as well as the optimal conditions for the production of solid biochar fuel from waste biomass.61 Hoekman et al. have
converted lignocellulosic biomass (mixed wood feedstock) using HTC to
examine the effects of reaction conditions on the final product composition
and yield.62 In this study, the HTC reaction temperature was varied over the
range of 215–295 °C and the reaction hold time over the range of 5–60 min.
The authors observed that on increasing the reaction temperature and time
the amount of produced HTC char decreased, while the amounts of gaseous
products and produced water increased. Also, the energy density of the char
increased when the HTC reaction severity increased (higher temperature
and longer time). At 255 °C there was a 39% increase in energy density and
this rose to 45% at 295 °C (Figure 5.5). Higher-temperature conditions have
a higher process pressure as a consequence. This study demonstrated that
the HTC char, having the desired energy density, can be most effectively
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Figure 5.5 Effects of the reaction temperature and hold time on mass recovery
and energy content of HTC char from Tahoe Mix (biomass feedstock).
Hold time = 30 min, except where otherwise indicated. The mass is represented by squares, and the energy content is represented by circles.
Reproduced with permission from ref. 62.
produced at temperatures near 255 °C and pressures near 5 MPa (the resulting char had coal-like properties). During the HTC process, CO2 was the
dominant gaseous product representing 8% of the starting feedstock mass
treated at 255 °C. It was also observed that the aqueous solutions from the
HTC process at lower temperatures (215–235 °C) contained significant levels of sugars, but at higher temperatures (255–295 °C) the concentrations of
sugars were hugely reduced, while concentrations of acetic acid increased.
Another renowned group from USA working on HTC is the Berge group.
Their work has pioneered the use of HTC as an alternative to process and
recycle municipal wastes instead of ending up in a landfill. Berge et al. have
investigated the HTC of municipal waste streams (i.e. paper, food waste, mixed
municipal solid waste and anaerobic digestion waste) to evaluate the physical,
chemical and thermal properties of the produced hydrochar, to determine carbonisation energetics and to evaluate the environmental implications associated with the carbonisation of municipal waste streams.63 In this study it was
observed that 49–75% of the initially present carbon is retained in the char,
while 20–37% of the carbon is transferred to the liquid phase and 2–11% is
transferred to the gas phase. NMR results confirmed that both decarboxylation
(disappearance of C(O)O band) and dehydration (increase of nonprotonated
aromatics) occurs. HTC chars contained dominantly 30.2–49.2% alkyls and
29.1–39.5% aromatics. Process energetics suggested that feedstock carbonisation was exothermic reaction. More recently, Berge et al. have investigated
the influence of the residence time, temperature,64 biomass type,65 and water
quality66 on the resulting characteristics of the HTC products. Interestingly,
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Figure 5.6 FESEM and TEM images of carbon spheres prepared by hydrothermal
processing of fructose (a) and (b) at 135 °C and glucose (c) and (d) at
160 °C. Inset in each figure shows high-resolution TEM image. Reproduced with permission from ref. 69.
using 13C solid-state NMR studies, it was possible to demonstrate that in the
case of the HTC of cellulose, one can isolate the polyfuranic structure when
working at moderate temperatures and reaction times.
Another US group that is working in close collaboration with the Berge
group and is focusing on the application of the HTC materials from biomass
as soil component but looking also at their impact on the air and water quality, is the group of Ro. The work of this group is generally focused on the characterisation of the HTC chars from various biomass sources (i.e. manures)
using 13C solid state NMR spectroscopy.67,68 Related to the solid-state NMR
understanding of the HTC process, the Exarhos group have published work
comparing the dehydration and carbonisation process of glucose and fructose.69 The group has concluded that aqueous glucose solutions require relatively high temperature (>160 °C) to transform the sugar into porous carbon
sphere dispersions, whilst fructose dehydrates in water under 3–4 atm at
somewhat lower temperatures (120 °C) due to the presence of a more reactive furanose unit; in contrast to glucose, where a pyranose group is present,
with these subtle differences in HTC temperature and mechanism leading to
differences in particulate structure (Figure 5.6).
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It was speculated that glucose loses water first through an intermolecular
condensation reaction as a result of its stable pyranose structure when heated
under pressure. However, fructose initially forms 5-hydroxymethylfurfuraldehyde (HMF) through an intramolecular dehydration process followed by subsequent water loss to form carbon. Further intermolecular dehydration then
generates surface roughness (raspberry structure) during carbon-sphere
formation. While it is clear that fructose will easily form HMF according to
many years of literature, and at a lower temperature then glucose, no other
group working on HTC has observed thus far such a raspberry-like porous
structure in the case of fructose-derived HTC carbon materials.
Patil et al. at the Department of Chemical and Biological Engineering, University at Buffalo have also looked in detail at the structure of HTC materials
and also tried to understand their chemical structure.70,71 Here, FT-IR was
used to study the chemical structure of hydrothermal carbons formed during
the acid-catalysed conversion of glucose, fructose, and HMF. The spectra
were quite similar except for three groups of features that can be attributed
to furan rings and carbonyl groups conjugated with C=C bonds. It was
assumed that the IR spectra were consistent with a model where each of the
three reactants must first be converted to 2,5-dioxo-6-hydroxyhexanal (DHH)
before HTC materials can form via subsequent aldol addition and condensation. The differences in the IR spectral features can then be explained by variations in the concentrations of other aldehydes and ketones that can react
with DHH.70–72 A more detailed description of the possible HTC mechanisms
when various biomass precursors are converted as analysed by GC-MS, HPLC
and 13C solid-state NMR studies will follow later in this chapter.
In Europe, research on HTC, as mentioned earlier started at the MPIKG
in Golm, with the work of the Antonietti and Titirici groups contributing
to substantial progress regarding understanding the chemical structure of
HTC materials,12,73–75 introducing porosity,76 controlling the surface functionality,77,78 as well as applying the synthesised materials in a variety of
energy-­related applications,79–81 water purification,82 CO2 capture,83 and catalysis.84,85 In parallel, Sevilla and Fuertes have also contributed significantly to
the development of high-value HTC materials. They have published a paper
on chemical and structural properties of carbonaceous products obtained
by HTC of saccharides and cellulose.86,87 These papers were the first ones to
propose a model of the HTC material, suggesting a rather complex chemical
structure. These two initial studies have been further supplemented by additional 13C solid-state NMR studies performed by Baccile et al.12,73–75 Sevilla
et al. has also embarked on proving the application of HTC materials in
various applications including supercapacitors,88 and hydrogen storage.89
Work by the Marken group is also highlighting the use of core–shell HTC
carbon electrodes for sensing applications.90–92 Further discussion of the
development of the chemical structure of HTC materials will be made later
in this chapter (Section 5.5), whilst discussion regarding nanostructured
HTC materials and their associated applications will be given further in
Chapters 6–8.
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5.3 Humins and Associated Materials
As mentioned earlier, many researchers have attempted to generate a fundamental understanding of the structure and the formation mechanism
of HTC materials. As will be described in Section 5.5, during the HTC of
biomass, the formed solid residues are also commonly referred to as
“humins” and often considered an unwanted byproduct of biomass-to-fuels or biomass-to-­platform chemical production schemes.93 Therefore, many
researchers working on the topic of biomass conversion to chemicals are
interested in understanding the formation mechanism of these materials
and their potential for producing biofuels. Recently, Weckhuysen et al. investigated the formation mechanism of “humins”, demonstrating that their formation involves a number of reactions other than aldol condensations.94 A
molecular structure study using elemental analysis, IR, solid-state 13C NMR
spectra and pyrolysis-GC-MS revealed a furanic structure with alcohol, acid,
ketone and aldehyde functional groups, which is formed via a dehydration
pathway. Based on this information a model for the molecular structure for
­glucose-derived humins was proposed. It was also found in another study
that xylose-derived humins possess a more conjugated molecular structure.95
Heeres et al. also investigated routes through which humin byproducts are
formed along with a determination of their molecular structure, based on
an extensive multiple-technique-based study as a function of sugar feed, the
presence of additives (e.g. 1,2,4-trihydroxybenzene), and the applied processing conditions.96
Elemental analysis indicated that humins are formed through a dehydration pathway, with humin formation and levulinic acid yields strongly
depending on the processing parameters (Figure 5.7). The addition of
implied intermediates to the feedstocks showed that furan and phenol
compounds formed during the acid-catalysed dehydration of sugars are
indeed included in the humin structure. IR spectra, sheared sum projections of solid-state 2DPASS (CNMR)-C-13 spectra, and pyrolysis GC-MS data
indicated that humins consist of a furan-rich polymer network containing
different oxygen functional groups. The structure is, furthermore, found
to strongly depend on the type of feedstock. A similar study and model for
the molecular structure of humins has also been proposed by Ptasinski et
al.97 Heeres et al. also performed a thorough characterisation of the HTC
structure obtained from glucose and fructose involving 13C solid-state studies. The 13C-NMR spectra and interpretation were found to be very similar
to our own results (Section 5.5). In addition, the authors also performed
pyrolysis product distribution studies on these HTC materials. From these
results it was concluded that furanics were the main liquid products at low
temperatures while at higher temperatures phenolics and benzofuranes are
observed. This is a very interesting study as it opens up new perspectives for
the HTC materials to be used also as platform materials for the production
of interesting biochemicals, which may serve as building blocks for renewable polymers, biofuels and biofuel additives.98
the catalytic route.) Reproduced with permission from ref. 96.
142
Figure 5.7 Proposed pathway for the catalytic hydrotreatment of d-glucose. (Th. represents mainly the thermal route and Cat. represents
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Figure 5.8 Demonstration of the advantages of biochar addition on soil coloura-
tion and crop yield; (left – untreated solid; right – biochar-treated soil).
Source: www.biochar-international.org/biochar/soils.
Aside from the chemical aspects of the HTC platform, the contribution of
chemical engineers to the field is also worth mentioning, particularly with regard
to research concerning the optimisation of the process parameters and technology upscaling. Here, the work of the Kruse group at the Karlsruhe Institute of
Technology (KIT) is highlighted and their research has focused on the influence
of the lignin content in the initial precursor as well as conversion kinetics.99,100
Kruse and coworkers have also investigated the degradability of the HTC products along with their effect in soil and plants nutrients uptake.29,30,101,102 Analogously, Ziegeler and Funke have also investigated various process parameters
affecting the HTC process.22,26 Recently, Funke et al. have also highlighted the
differences between hydrothermal and vapothermal carbonisation.103
Regarding agricultural uses there is strong interest in using HTC materials
as biochars within the concept of “Terra Preta”, whereby the conversion of
biomass to solid carbon, the CO2 is trapped into the final material.104 The
addition of this “biochar” product to soil can bring beneficial properties to
future crop productions (Figure 5.8). Any biochar application to soils aimed
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at achieving soil carbon sequestration can only claim success if beneficial
effects on plant yield, soil water availability, soil fertility or other positive
amelioration effects can be shown.
However, there are a lot of questions that need to be answered before considering the agricultural use of biochar in particular in relation to carbon
sequestration. These questions refer to stability, positive effects on plants
growth, soil fertility, toxicity and nutrients retention. There are also several
groups doing high-quality research in biochar and HTC biochar in Germany.
These groups are trying to answer the aforementioned issues of biochar soil
use. A number of references are highlighted for the readers interest, namely
from the Rilling group,105–110 the Kammann group,65,111–114 and the Glaser
group.111,115–117
5.4 Societal and Commercial Aspects
The development of HTC technology aside from the development of nanostructured, porous materials, can also have potential impacts in terms of
energy and material balances in our daily processes. In this regard, researchers at Loughborough University, UK, lead by Professor M. Sohail have taken
the HTC concept to a new level: HTC is used to convert human waste into
carbonised material to provide heat, minerals for soil conditioning, and
water for flushing and hand washing. The Sohail group have termed this
approach as “Continuous Thermal Hydrocarbonisation” and has the advantage that is kills all pathogens to create safe-to-handle, valuable material and
uses power from exothermic heat generated during processing. Significantly,
these researchers won the second prize for the Reinvent the Toilet Challenge
awarded by Bill Gates.118
Besides the academic interest, HTC is also generating a significant degree
of commercial interest. Many companies have embarked on the design
of large-scale HTC reactors for the conversion of biomass into HTC at the
industrial scale. The main applications of these HTC products are in the field
of solid fuel. By converting biomass into HTC material there is a significant
increase in the energetic value and the resulting material can be used as CO2
neutral coal, as long as the same amount of biomass used to generate the
HTC is cultivated back. However, things are not that straightforward and
life-cycle assessment needs to be implemented to understand the full picture.
Some companies are interested in the concept of biochar as explained above,
but the full risks associated with this concept have not yet been completely
understood. Other companies are looking to commercially HTC materials as
carbon-based adsorbents for water purification. To mention a few examples
briefly, HTC companies in Europe are: Germany: Ava-CO2, Artec, Suncoal,
Carbon Solutions, Terra Nova Energy, UK: Anatco, Spain: Ingelia, and many
others around the world. Further discussion regarding the commercialisation of HTC materials (and indeed other approaches to sustainable carbons)
will be given in Chapter 12.
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5.5 Chemistry behind the formation of HTC
Materials
The hydrothermal treatment of saccharides essentially involves three main steps:
dehydration, polymerisation and finally carbonisation/aromatisation; (NB: additional hydrolysis step for polysaccharides) (Figure 5.9), It is now well accepted in
the literature that prior to the dehydration of glucose, it first isomerises to fructose via the Lobry de Bruyn–Alberda van Ekenstein isomerisation. As fructose
is formed, this intermediate then dehydrates to ultimately release three water
molecules and forms 5-(hydroxymethyl)furfural (HMF), which acts as the main
“reactive monomer” involved in the formation of HTC materials. During the
dehydration of glucose, other low molecular weight compounds may also form,
including levulinic and formic acid (Figure 5.10). They result via the reaction of
HMF with water (Figure 5.12(d)). These small molecules also contribute to the
HTC carbon network formation either via aldol reactions (Figure 5.12(b) – i.e.
between HMF and levulinic acid) or by simple physisorption. The small molecular weight acids also have a catalytic role as they lower the pH and thus promote
further dehydration reactions. When a pentose is used (i.e. a 5 C carbohydrate),
HTC formation proceeds via the formation of furfural instead of HMF resulting
ultimately in a more condensed HTC material structure.
Regarding HTC material formation, it is very important to establish a relationship between the type of carbohydrate used, its molecular and chemical complexity and the final HTC carbon structure. Preliminary solid-state
13
C NMR investigations on HTC carbons derived from different mono- and
polysaccharides (i.e. fructose, glucose, starch, xylose) highlighted that the
main factor affecting the chemical nature of the HTC product was the structure of the parent sugar (Figure 5.11).12 Pentose-derived (e.g. xylose) HTC
carbons possess a stronger aromatic character than material derived from
hexoses (e.g. glucose). Such a difference is demonstrated by a more intense
peak at δ = 125–129 ppm in the solid-state 13C CP MAS NMR spectrum in
the former case, which is characteristic of aromatic carbons belonging to
graphitic or long-range conjugated double-bond structures.
The different chemistry of the HTC reaction intermediates (HMF vs. furfural) may provide a reason for the observed increased in material aromatisation. Further gas-chromatography coupled to mass spectrometry (GCMS)
and solution 13C NMR experiments on the glucose system confirmed that the
major intermediate in the reaction mixture is 5-HMF. This finding, coupled
to the evidence that the solid-state 13C NMR spectrum of HMF-derived HTC
carbon is very similar to all the HTC spectra obtained from different types of
mono- and poly-hexoses, leads to the conclusion that polymerisation/condensation reactions involving HMF are the route of formation of HTC carbon.
Furthermore, by a simple comparison of 13C CP MAS NMR spectra of various
HTC materials, it was also observed that the degree of initial polymerisation
of the hexose-based saccharides (i.e. mono-, di- or poly-­saccharides) does not
influence the final structure, since all the 13C spectra of HTC carbons derived
Figure 5.9 Simplified schematic representation of the hydrothermal carbonisation mechanism. Reproduced with permission from ref. 73.
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Figure 5.10 Glucose dehydration to HMF via fructose isomerisation; further rehydration of HMF into levulinic and formic acid.
Figure 5.11 Solid-state 13C CP MAS NMR spectra (tCP = 3 ms) of HTC materials (C-)
produced from C-starch, C-amylopectin, C-sucrose and C-maltose,
C-HMF, C-glucose, C-xylose and C-furfural. Reproduced with permission
from ref. 12.
from hexose-based saccharides are characterised by identical resonances.12
These experimental findings by Baccile et al. have since been corroborated by
other research groups.69–71,95,97
However, how do the HTC materials form from HMF and what are the associated chemical reactions involved? A clear and straightforward answer cannot
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Figure 5.12 Possible chemical reactions of HMF during the hydrothermal carbonisation process.
be provided at this stage due to the complexity and multitude of reactions
occurring simultaneously under hydrothermal conditions. In this context, in
situ monitoring with FT-IR or LC-MS could provide some answers; however, to
the best of our knowledge such experiments have not been performed as yet. At
this stage, some potential reactions can be postulated based on the chemistry of
the reactive monomer, HMF, and the reaction conditions (Figure 5.12). HMF is
a highly reactive and can react further either via its substituents (hydroxyl and
carbonyl) or via the furan ring. The hydroxyl group can be involved in nucleophilic substitution reactions (Figure 5.12(a)). The aldehyde group of HMF can
undergo aldol condensations with aldehydes or α-ketones (Figure 5.12(b)). In
the presence of alcohols, the same aldehyde group can form hemiacetals (Figure 5.12(c)). The furan ring can react with water, resulting in ring opening and
formation of levulinic and formic acid as described previously (Figure 5.10; Figure 5.12(d)). Diels–Alder reactions between the conjugated furan ring (Figure
5.12(e)) or the conjugated aldehyde substituent on the furan ring (Figure 5.12(f))
and a double bond resulting during HTC by fractionation of biomass could also
lead to structural aromatisation.
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Figure 5.13 13C CP MAS NMR experiments (tc = 3 ms) recorded on pure C13-­glucose-
derived carbons (HC glu) and several nitrogen-containing hydrothermal carbon materials: (a) sample HC glu, (b) HC glu-albumin, (c) HC
glu-N15-glucosamine, and (d) HC glu-C13-N15-gly. In a, glucose is used
alone, but it can coreact with (b) ovalbumin16 and (c) 15N-glucosamine.
In d, 13C-glucose reacts in the presence of 15N-glycine. Note that the
better signal-to-noise ratio of HC glu and HC glu-C13-N15-gly derives
from 13C enrichment. Reproduced with permission from ref. 74.
There is no doubt that indeed the aldol reaction between levulinic acid-­derived
alpha hydrogen diketones plays a very important role in the formation of HTC,
however, this is not the only reaction happening inside the autoclave. In this
context, as introduced earlier, Weckhuysen et al. and others demonstrated that
“humin” formation involves reactions other than aldol condensations.94,95 Obviously, the precise chemistry of the system is difficult to ultimately determine
especially as the HTC structure is strongly influenced by the process parameters such as precursor type and concentration, pH, residence time, pressure,
and temperature. The situation is complicated still further with the introduction of heteroatom (e.g. N)-containing precursors to the synthesis. However, it is
clear that the addition of nitrogen to the growing HTC material system results
in the generation of carbonaceous materials with greater aromatic character (δ =
125–129 ppm) at lower temperatures than normal (Figure 5.13).74 In this regard,
Chapter 10 will briefly introduce how the use of solid-state NMR can help to elucidate chemical bonding and surface properties in carbon materials.
5.6 Outlook and Conclusions
Within this chapter we aimed to offer the reader a flavour of what the HTC
process is and how it can be used to convert sustainable precursors into
functional carbonaceous materials. We have also tried to explain in as a simple a manner as possible, the potential chemical reactions responsible for
the formation of these types of carbonaceous materials. The many possible
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applications, porous structures, hybrid systems and functionalities have also
been highlighted while describing the state-of-the-art. It is important to note
that the fact that HTC materials are prepared initially at low temperatures
confers a serious advantage in terms of carbon-material synthesis. This
bottom-up approach therefore allows access to highly functional materials
and the ability to utilise this feature in postsynthesis chemical functionalisation,119 morphology and porosity control,119,76 hybridisation with inorganic
nanoparticles,79 and heteroatom doping (conferring further useful material
functionality and properties).120 All these are very difficult tasks when it comes
to traditional carbon-material synthesis. With the HTC platform, tuning of
material porosity and morphology is possible by using the right combination
of processing/templating procedures.76 Further tuning of the surface functionality as well as of electrochemical properties are readily possible by further
heat treatment.121,122 Thus, depending on the postcalcination temperature the
functionalities can be tuned to the desired state (e.g. carboxylic groups at
550 °C) or completely removed rendering the materials electronically conductive (e.g. >750 °C). Heteroatom doping is also possible, again taking advantage
of the low-temperature reactions (e.g. Maillard, Strecker reactions), allowing
the chemical incorporation of various sustainable heteroatom containing
substances (e.g. amino acids, proteins). Particularly interesting here are the
N-doped carbon materials taking place via Millard chemistry.123 All these
aspects of HTC technology will be described in depth in later chapters of this
book.
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CHAPTER 6
Porous Hydrothermal Carbon
Materials, Nanoparticles,
Hybrids and Composites
NICOLAS BRUN*a,b, SHU-HONG YUc AND ROBIN J. WHITE*d
a
Department of Chemistry, Graduate School of Science, Kyoto University,
Kitashirakawa, Sakyo-ku, Kyoto 606-8502, Japan; bICGM, Institut
Charles-Gerhardt Montpellier, UMR 5253 CNRS-ENSCM-UM2-UM1,
8 rue de l’Ecole Normale, F-34296 Montpellier, France; cDepartment
of Chemistry, University of Science and Technology of China, Hefei
National Laboratory for Physical Sciences at Microscale, Jinzhai Road
96, Hefei 230026, PR China; dUniversität Freiburg, FMF - Freiburger
Materialforschungszentrum, Stefan-Meier-Straße 21, 79104 Freiburg
im Breisgau and Institut für Anorganische und Analytische Chemie,
Albertstrasse 21, 79104 Freiburg, Germany
*E-mail: nicolas.brun@enscm.fr; robin.white@fmf.uni-freiburg.de
6.1
Introduction
Within the field of sustainable carbon materials, the hydrothermal carbonisation
(HTC) of biomass and derivatives has emerged this last decade as a prevalent
approach.1,2 First proposed by Bergius in the early 1910’s, and more systematically investigated by Berl and Schmidt in the 1930’s, the hydrothermal approach
is based on the degradation/dehydration of (poly)saccharides into furan-based
intermediates at relatively mild conditions (typically temperatures between
130 and 250 °C; Figure 6.1). Typically, monosaccharides (hexoses or pentoses)
degrade into furan-based intermediates (5-hydroxymethylfurfural (HMF) or
RSC Green Chemistry No. 32
Porous Carbon Materials from Sustainable Precursors
Edited by Robin J White
© The Royal Society of Chemistry 2015
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Porous Hydrothermal Carbon Materials, Nanoparticles, Hybrids and Composites
Figure 6.1
157
Simplified
HTC chemical pathway based on the conversion of the
model hexose saccharide glucose indicating reactions condensation,
the potential polymerisation/resinification reactions that can occur to
lead to the formation of carbonaceous nuclei which eventually ripen
and precipitate after 16 h of reaction time.
furfural, respectively), before nucleation/growth to carbon-like colloid spheres
(Figure 6.2(a)), termed HTC materials or hydrothermal carbons. It has been shown
that the polymerisation occurs via intermolecular and aldol condensation reactions involving both furan species and dehydrated glucose-based products, such
as levulinic acid. For more information about the HTC mechanism(s), the readers are invited to refer to Chapter 5 and articles cited therein and specifically
Baccile et al.3–5 and Fuertes et al.6 In terms of sustainability and suitability of
technological and economic aspects, the hydrothermal approach presents many
advantages. On the one hand, HTC proceeds in water or hydroalcoholic mixtures
at less extreme conditions, and could allow condensation, shaping and structuration in one step. This aspect gives HTC the designation of “chimie douce” of
carbon,7 largely inspired by the inorganic sol-gel process. Regarding this aspect,
an analogy can be drawn between Stöber silica-based particles and the microscopic spheres made via the hydrothermal treatment of monosaccharides in
water, both processes taking place via the LaMer model.8 On the other hand,
even though most of the reports propose the use of dry pure saccharides (for
better repeatability and reliability), in a practical way HTC can be easily extended
to wet starting products and raw biomass. As a direct consequence, complicated
drying schemes and costly isolation procedures, in principle, can be avoided.
The use of wet impure precursors also allows the direct upgrading of agricultural
biowastes,9,10 and untapped byproducts generated during degradation of lignocellulose in paper mills,11 and biorefineries12. Moreover, the plethora of available
natural precursors, such as chitin,13 d-glucosamine,14,15 chitosan,16 aminated
tannin,17 proteins,18 or even microalgae,19 could in fine lead to a large variety
of functionalities. Last but not least, it has been shown that under appropriate
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158
Figure 6.2
Chapter 6
SEM
micrographs of the hydrothermal carbon from glucose at 240 °C:
(a) Before chemical activation and (b) After chemical activation. (c) N2
adsorption isotherms, (d) N2 adsorption QSDFT PSD of glucose-derived
(G for Glucose) HTC-activated carbons depending on the hydrothermal
temperature: G-180 °C, G-240 °C and G-280 °C. Reproduced with permission from ref. 22.
conditions, the majority of the carbohydrate precursors ends up fixated in the
final carbonaceous material, leading therefore to an interesting CO2 efficiency
(or sequestration).20
As mentioned in Chapter 5, HTC typically yields microscopic nonporous
spheres with limited control over morphology and texture. In this context, a variety of synthetic methodologies have been reported describing
the design of tuneable, nanostructured porous HTC materials. The aim of
this chapter is to introduce recent advances in this rapidly developing field.
The first section covers activated hydrothermal carbons with recent examples based on the biomass-derived hydrothermal powders discussed. The
second and third sections focus on the use of templating, from sacrificial
inorganic hard templates, (e.g. silica-based premade moulds), to organic
soft templates (e.g. block copolymers). The fourth section introduces more
recent reports describing the template-free synthesis of hydrothermal gels
and related aerogels. Finally, the design of porous carbons from direct
hydrothermal treatment of natural systems will be presented as will a few
words on biomass-derived HTC nanodots and nanocomposites.
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6.2
159
Activated Hydrothermal Carbons
Activation processes have been widely used in industry and academic
research to obtain inexpensive and highly porous carbon materials from
various organic precursors (mainly lignocellulosic materials or synthetic
polymers). For an exhaustive review about “activated carbons”, the reader
is invited to refer to previous work by Marsh and Rodriguez-Reinoso.21 Two
different types of activated carbons can be discriminated, depending on the
treatment applied to the precursors: (i) Physically (or thermally) activated
carbons, obtained via the selective gasification of individual carbon atoms
using carbon dioxide or water vapour at 800–900 °C; and (ii) Chemically activated carbons, involving the incorporation of reagents in the organic precursor before further carbonisation. Some reagents, such as ZnCl2 and H3PO4,
act as dehydrating agents at temperatures <500 °C, simultaneously inhibiting structural shrinkage and related porosity loss generated during pyrolysis.
Conversely, alkali hydroxides, such as KOH, have no dehydrating effect and
react at temperatures >700 °C via oxidative processes to produce CO or CO2.
Both processes, physical and chemical lead to the preparation of activated
carbons with surface area ≤3000 m2 g−1. However, chemical activation has
been largely favoured as material mass yield is similar or even superior to
that obtained without any further treatment, i.e. better than 25 wt.%, while it
is below 10 wt.% for physical activation.
Widely applied to raw lignocellulosic biomass, KOH activation has been
recently extended to hydrothermal carbons by Sevilla et al.23–25 and Titirici
et al.12,22,26 Highly porous carbons could be obtained from a two-step HTC and
subsequent chemical activation of monosaccharides (i.e. glucose, glucoseamine26), polysaccharides (i.e. starch, cellulose) and raw biomass (i.e. eucalyptus
wood sawdust, rye straw)22,24 Typically, hydrothermal carbons are activated in
the range 700–800 °C for 1–2 h under a nitrogen flow, using hydrothermal
carbon/KOH weight ratios between 1 : 2 and 1 : 4. Sevilla et al.24 studied the
impact of the chemical activation parameters and biomass-derived precursors
on the final pore-size distribution and volume. More recently, Falco et al.22
reported on the influence of the HTC temperature on the porosity of chemically activated hydrothermal carbons, highlighting HTC as a unique tool for
tailoring the porosity of activated carbons (Figure 6.2). Typically, SBET values >
2400 m2 g−1 are reached, while further reactivation of activated cellulose-based
hydrothermal carbons,24 leads to slightly higher surface area and pore volume, (i.e. 2700 m2 g−1 and 1.2 cm3 g−1, respectively). Interestingly, relatively
high mass yields, >33 wt.% and ≤48 wt.%, were systematically obtained.24 By
comparison, the maximum yield for KOH-activated carbons prepared from
raw lignocellulosic biomass was reported as 25 wt.%, with materials presenting similar porosities.21 This significant improvement may be due to the more
“coal-like” structure of HTC materials, and might prompt the emergence of a
new class of activated carbon materials for industrial applications. As will be
discussed in Chapters 7 and 8, their use as promising adsorbents for hydrogen storage24 and carbon dioxide capture23 or as high-performance electrodes
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12,25
27
for supercapacitors
or more recently for Li–S batteries opened up prospects in a broad field of applications. Nevertheless, activated hydrothermal
carbons are mainly microporous, displaying predominantly super-micropores (i.e. 0.7–2 nm). Whilst they are a desirable feature in gas adsorbents or
supercapacitor electrodes, micropores are not appropriate for applications
involving the diffusion of bigger molecules or the immobilisation of large
catalytic bodies (e.g. metal nanoparticles or enzymes). Moreover, the initial
morphology of hydrothermal carbons is rapidly altered by the activation treatment (Figure 6.2(b)), making the design of finely macro-/mesostructured and/
or monolithic materials a significant challenge.
6.3
Porous
HTC via Hard Templating: Premade
Sacrificial Inorganic Moulds
Traditionally, one of the most used approaches to design porous materials has been based on the nanocasting or impregnation of premade sacrificial hard-templates. A hard-template could be defined as follows: a rigid/
semirigid solid (e.g. monoliths or films) or colloidal sol, bearing an organic
(e.g. polymers or carbon), inorganic (e.g. silica or alumina) or hybrid three-­
dimensional crosslinked framework. Hard templates suppose strong bonds
(e.g. metallic, covalent, ionic or iono-covalent bonds) and, most of the time,
difficult elimination. On the contrary, so-called soft templates imply weaker
bonds (i.e. van der Waals forces, hydrogen bonds, etc.), including molecular
solids (e.g. ice crystals), supramolecular assemblies (e.g. micelles), latex dispersions, liquids (i.e. emulsions) or gases (i.e. foams).
In the case of carbon materials, sacrificial hard templates are usually made
of thermally stable inorganic solids or colloidal sols. Typically, after coating
or impregnation of the template and polymerisation/carbonisation of the
organic precursors, the inorganic part is removed via chemical etchings.
Relying on a plethora of possible moulds, the hard-templating approach
offers a large versatility and a fine structural and morphological control of
the carbon materials obtained in fine.28
6.3.1
Silica-Based Hard Templates
The first examples of hard-templated hydrothermal carbons were reported in
2007 by Titirici et al.29,30 and Ikeda et al.31 Both used mesoporous (e.g. SBA-15
type30) or nonporous silica particles as sacrificial moulds and monosaccharides (or derivatives) as carbon precursors. However, different synthetic pathways were followed: (i) Titirici et al. proposed a control over the hydrophobicity
of silica microspheres through re-/dehydroxylation (via thermal treatment
at 800 °C) and/or trimethyl-group grafting (using trimethylchlorosilane).
Macroporous casts, mesoporous hollow spheres, mesoporous microspheres
and even nanoparticles could be selectively obtained. Using silica hard templates, Demir-Cakan et al. could also synthesise functional mesoporous HTC
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containing imidazole groups by simple HTC of glucose in the presence of
vinylimidazole.32 They were successfully used as heterogeneous catalysts for
various transesterification, Knoevenagel and Aldol reactions;32 (ii) Ikeda et al.
reported on the shifting of the isoelectric point of silica-based nanoparticles
from pH 2 to pH 5 via the postgrafting of amino groups. Such modification
could favor electrostatic attraction forces between positively charged NH2-silica and negatively charged saccharide-based nuclei. In particular, using silica
nanoparticles with porous shell and depending on the glucose concentration,
both the thickness (from 27 to 49 nm) and porosity (from 200 to 1600 m2 g−1)
of the final carbon shell could be finely tuned. Another approach developed
by Wan et al.33 proposed to use poly(vinylpyrrolidone) (PVP) as an additive to
promote the adsorption and subsequent hydrothermal coating of glucose on
a premade colloidal suspension of silica microspheres.
More recently, Ikeda’s procedure was extended by Han et al.34 and Brun
et al.35 to hollow nanospheres depicting carbon shell thicknesses from 5 up
to 10 nm, depending on the precursor in use (i.e. glucose or xylose) and the
pyrolysis temperature (Figure 6.3). Such nanostructured materials could be
successfully used as electrode materials for supercapacitors, displaying a
high specific capacitance of up to 270 F g−1 in KOH aqueous electrolyte,35 or
in high-power lithium–sulfur batteries.34 As for the former application, Brun
et al.34 reported an initial discharge capacity of 1000 mAh g−1 together with
a coulombic efficiency of almost 100% until the 20th cycle. Extending this
procedure to cocontinuous silica monoliths, namely Chromolith®, hierarchically porous HTC materials could also be prepared from monosaccharides
(i.e. glucose and xylose) and even polysaccharides (i.e. sucrose and starch).36
Such macroporous monoliths showed micro-/mesopore volumes and BET
surface areas of up to 3.1 cm3 g−1 and 1400 cm2 g−1, respectively.
Figure 6.3
(a)
Schematic representation of the silica-based hard-templating
approach developed by Ikeda et al. (b) and (c) TEM micrographs of
NH2-Silica-based Stöber nanoparticles. (d) and (e) Associated hard-templated HTC hollow spheres. Reproduced with permission from ref. 34.
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6.3.2
Nonsilica-Based Hard Templates
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6.3.2.1 Anodic Alumina Membranes
Kubo et al.37 reported on the use of anodic alumina membranes (AAO) as templates and furfural as carbon precursor for the HTC synthesis of open-ended
carbonaceous tubular nanostructures (Figure 6.4(a) and (b)). After removal
of the AAO template by chemical etching (i.e. concentrated phosphoric acid
washing at 65 °C), HTC tubes displaying a hollow internal diameter of about
125 nm and a wall thickness of about 40 nm could be obtained. Increasing
the temperature of the further pyrolysis allowed modulation of the surface
chemistry from rather hydrophilic to rather hydrophobic, while increasing
the SBET up to 700 m2 g−1 (Figure 6.4(c) and (d)). Due to the high degree of
surface functionalities (e.g. hydroxyl or carboxylic groups) provided by hydrothermal carbon materials, such HTC tubes could be easily modified via the
Figure 6.4
(a)
SEM and (b) TEM micrographs of synthesised tubular carbons after
further pyrolysis at 750 °C. Nitrogen sorption isotherms of synthesised
tubular carbons after further pyrolysis at (c) 550 °C and (d) 750 °C.
Reproduced with permission from ref. 37.
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covalent attachment of 3-aminopropyltriethoxysilane and the subsequent
covalent grafting of carboxyl modified poly-N-isopropylacrylamide (PNIPAAm)
through amide bond formation. The as-synthesised HTC-PNIPAAm materials
depicted a reversible thermo-responsive behaviour, associated with a slightly
higher LCST (i.e. lower critical solution temperature) as compared with free
PNIPAAm aqueous solution (ca. 35 °C instead of 32 °C). As mentioned by the
authors, this feature potentially enables applications as drugs nanostructured
containers bearing switchable encapsulation release of bioactive molecules.
More recently, Hu et al.38 developed a similar approach using glucose instead
of furfural as carbon precursor and reported on the nucleation/growth of WS2
nanoparticles (NPs) confined on the inner walls of the as-­synthesised carbon-based tubes (CTs). To ensure that the WS2-NPs did not grow on the outer
walls of the tubes, the impregnation of the (NH4)2WS4 precursor in aqueous solution was performed before removal of the hard template. CTs not only allowed
the carbothermal reduction of WS 24− anions, but also provided a confined
nanoenvironment to the WS2-NPs growth. Such WS2-NPs@CTs showed promising performances as electrode materials for electrochemical supercapacitors,
delivering a high rate capability (i.e. a specific capacitance of up to 337 F g−1 at
a high current density of 10 A g−1) together with reasonable stability over 500
cycles.
6.3.2.2 Tellurium Nanowires
The first report about the use of tellurium nanowires as hard templates for
the synthesis of bioresourced hydrothermal carbons was published by Yu
et al.39 in 2006. Premade flexible tellurium nanowires, ca. 4–9 nm in diameter
and hundreds of micrometers in length, could be easily and homogeneously
coated with glucose-derived HTC carbon shells (Figure 6.5(a) and (b)). The
thickness of the carbon coating could be nicely tuned from 2 up to hundreds
of nanometers by simply increasing the hydrothermal reaction time from 4 to
12 h at 160 °C39 or decreasing the amount of tellurium nanowire templates
(while the concentration of glucose was kept unchanged).40 After removal of
Te cores via chemical etching in H2O2/HCl mixtures, HTC nanofibres could
be obtained (Figure 6.5(c)). By applying an evaporation-induced self-assembly
process in ethanol to such nanofibre suspensions, centimetric free-standing
flexible membranes, of about 10–50 µm in thickness, were fabricated.40,41
Such hydrophilic porous membranes depict excellent size-selective rejection
properties, better than most of the electrospun polymer nanofibres mats
reported in literature, and could be used for the effective filtration and separation of metallic (i.e. Au, Ag) and silica nanoparticles.40
HTC nanofibres membranes were also used for water purification with high
adsorption rate and capacity for cationic dyes (i.e. methylene blue) and metal
cations (i.e. Pb(ii) and Cr(iv)).41 Due to excellent mass-transport properties, the
authors reported on the complete adsorption of methylene blue at a flux as
high as 1580 L m−2 h−1, which is one to two orders of magnitude higher than
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Chapter 6
Figure 6.5 TEM
images of (a) and (b) typical Te@HTC core–shell nanocables and (c)
a corresponding carbon nanofibre obtained by removal of the tellurium
core by chemical etching. Reproduced with permission from ref. 39. (d)
Schematic illustration of the synthetic steps involved in the synthesis of
compressible HTC nanofibre hydrogels. (e) Photograph and (f) SEM images
of an as-synthesised monolithic wet gel. The inset in the right image of (f)
shows a photograph of a cryogel. Reproduced with permission from ref. 45.
for commercial membranes. After modification of the HTC nanofibres with
beta-cyclodextrins at 60 °C, the nanofibre membranes became an ideal molecular filter for capturing organics through complexation with cyclodextrin molecules.42 Moreover, the HTC nanofibres prepared from glucose at a relative-low
temperature (160 °C) were highly functionalised with abundant hydroxylic and
carboxylic groups, which enabled these fibres to serve as ideal supports for
loading inorganic nanoparticles (e.g. Fe3O4, TiO2, and noble metals) with various functions.43,44 Through a simple evaporation process, similar to the pristine HTC nanofibres, the formed hybrid nanofibres could also be assembled
into free-standing membranes with interesting multifunctional properties.44
More recently, Yu et al. extended this approach to the preparation of compressible HTC nanofibre hydrogels and so-called aerogels (which, stricto
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sensus, should in fact be considered as cryogels since they were obtained
via freeze drying; Figure 6.5(d)–(f)).45 These dried gels could be employed as
competitive adsorbents for a range of organic solvents and oils, as well as 3D
scaffolds for the nucleation/growth of metal nanoparticles.45 Interestingly,
Yu’s group also proposed the synthesis of nitrogen-doped porous carbon
nanofibres, via the coating, polymerisation and subsequent carbonisation of
polypyrrole on Te-templated HTC nanofibres.46 These materials showed high
electrochemical capacitance of 202 F g−1 at a current density of 1.0 A g−1, and
can be seen as promising building blocks for the design of N-doped 2D or 3D
electrodes, such as in the oxygen-reduction reaction.
6.3.2.3 Nanostructured Hard Templates Formed
in situ During HTC
Besides the pre-existing hard-templates as discussed above, the nanostructured inorganic materials that were generated in situ during the HTC process
could also serve as efficient hard-templates to form porous HTC materials,
although it is difficult to predict the morphologies of final HTC products
with this method. Yu et al. first prepared Ag@C nanocables in 2004 by onestep HTC of starch and silver salt at 160 °C.47 It was believed that the soluble
starch could reduce efficiently silver ions into Ag NPs that aggregated gradually through the oriented attachment mechanism and induced the formation
of 1D nanocables. Later, such a method was extended successfully by Yu et al.
to prepare a series of other metal@carbon (or polymer) coaxial nanocables,
such as Cu@C,48 Ag@poly(vinyl alcohol) (PVA),49 and Cu@PVA.50
Selenium@hydrothermal carbon core–shell composite particles were synthesised through the one-pot microwave-induced hydrothermal treatment
of starch in the presence of selenous acid (Figure 6.6).51 Interesting for the
design of composite nanostructures, such a synthetic pathway could also
offer the possibility to obtain HTC hollow spheres after easy removal of the
selenium core particles by thermal evaporation in vacuum at ∼250 °C for 10
min (Figure 6.6(c)). Nevertheless, as far as toxicity to human health and environment is concerned, the use of selenium does not encompass in principle
the “green chemistry” precept recommending less hazardous chemical syntheses. This element also involves a supply risk.
To conclude on the hard-templating approach applied to the HTC process,
the few examples presented herein showed its large potential in terms of
versatility and structural/morphological control of the as-synthesised hydrothermal carbons. Nevertheless, as hard-templating approaches rely most of
the time upon harsh chemical etching (involving potentially toxic or highly
corrosive reagents and/or products) of sacrificial inorganic constituents and
laborious multistep synthetic pathways, few major questions may arise: are
such approaches friendly with so-called “sustainable” or “green” procedures?
And, to some extent, are they compatible with industrial scale-up? Although
these aspects are highly dependent on the nature of both the sacrificial
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Figure 6.6
(a)
Schematic representation of the in situ hydrothermal generation of
selenium@hydrochar core–shell nanohybrids. TEM micrographs of (b)
The as-synthesised selenium@hydrochar core–shell nanohybrids and
(c) The corresponding hydrothermal carbon hollow spheres obtained
after thermal evaporation. Reproduced with permission from ref. 51.
template and the chemical etching (e.g. NaOH solutions should be preferred
to fluoride-based acidic solutions in the case of silica-based templates), one
may assume that milder alternatives would be preferable to fully encompass
the “green chemistry” precepts. In this context, the use of easily extractable
and/or noncarbonisable organic templates, namely soft-templates (Figure
6.7), has arisen as an attractive and versatile alternative.
6.4
6.4.1
Porous HTC via Soft Templating
Supramolecular
Self-Assemblies: From OMCs to Hybrid
Hollow Spheres
6.4.1.1 Micellar Self-Assemblies
First developed on silica-based materials in the 1990s via the successive
discoveries of MCM and SBA series,52–56 the direct synthesis of ordered
mesostructures through supramolecular self-assemblies (Figure 6.7(a)) has
recently aroused great research interest in the field of carbon materials.57 The
synthesis of such highly ordered mesoporous carbons (commonly named
OMCs) was first proposed in 2004 by Dai et al.58,59 and Zhao et al.60,61 via the
evaporation-induced organic–organic self-assembly (named EISA) of di-/triblock copolymers and resorcinol–formaldehyde carbon precursors. Due to
the weak noncovalent interactions of carbon precursors with block copolymers, a large variety of mesostructures could be reached.57 More recently,
using Pluronic®/phenolic-formaldehyde systems, hydrothermal autoclaving
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Figure 6.7
167
Examples
of soft-templating approaches developed for the design of porous
HTC. (a) Schematic representation of micellar self-assemblies. Adapted
from ref. 60. (b) Mesostructured hydrothermal carbon. Reproduced with
permission from ref. 64. (c) Schematic representation of vesicular self-assemblies and (d) Associated carbon hollow spheres. Reproduced with
permission from ref. 67. (e) Polystyrene latex dispersion and (f) Associated carbon hollow spheres. Reproduced with permission from ref. 68. (g)
Schematic representation of a diluted macroemulsion and (h) Associated
HTC hollow spheres. Reproduced with permission from ref. 69.
processes could also lead to OMCs, using relatively mild conditions (i.e. at
100 °C maximum).62,63 Nevertheless, supramolecular self-assemblies applied
to the HTC of biomass derivatives were not reported until recently. To elude
the use of relatively high temperatures and allow access to stable micellar
self-assemblies, Kubo et al.64 proposed to use fructose as a sustainable precursor (Figure 6.7(b)). As compared to other hexoses, fructose could dehydrate and polymerise at a temperature as low as 130 °C. In the presence of
Pluronic® F127 together with an appropriate swelling agent (i.e. trimethylbenzene; TMB), and after thermal template removal (at 550 °C under N2),
hydrothermal sustainable OMCs could be obtained. Due to the solubilisation of oil (i.e. TMB; the term “oil” referring to any water-insoluble liquid)
within the micellar self-assemblies, such thermodynamically stable swollen micelles are often referred to as microemulsions (not to be mistaken for
the so-called ordinary emulsions; see Section 6.5). Although the presence of
TMB was not critical to yield ordered porous structures, it allows the mesopore volume proportion to increase from about 20% up to about 60% of
the total pore volume. Nevertheless, SBET and mesopore volume remained
relatively poor and could not exceed 120 m2 g−1 and 0.06 cm3 g−1, respectively
(while values of about 260 m2 g−1 and 0.03 cm3 g−1 could be reached without
TMB).64 Surprisingly, using exactly the same procedure as Kubo et al. without
TMB, Liu et al.65 reported nanoporous carbons displaying SBET ≤ 1100 m2 g−1.
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The authors could tune both the pore-size distribution and the surface area
via a simple adjustment of the hydrothermal synthesis temperature from 130
to 200 °C. Such nanoporous carbons showed competitive electrochemical
capacitances of up to 290 F g−1 (in KOH aqueous electrolyte; at a scan rate
of 1 mV s−1)65 and promising performances in removal of heavy metals from
industrial waste waters66 (after postmodification with amine groups).
6.4.1.2 Vesicular Self-Assemblies
First proposed as sustainable HTC precursors by Shin et al. in 2008,70 cyclodextrins (CDs) were recently used as elegant building blocks for the design
of hydrothermal soft-templated porous nanoparticles.67 These natural cyclic
oligosaccharides, composed of d(+)-glucose units and depicting hydrophobic inner cavities of 4.5–8.5 Å, could be seen as “sweet nanorings”. They are
known to form inclusion complexes and have inspired interesting developments of original supramolecular architectures (e.g. polyrotaxanes) for more
than two decades.71,72 In particular, α-cyclodextrins (α-CDs) have been shown
to interact with ethylene oxide units of various poly(ethylene oxide)–poly(propylene oxide)-based (PEO–PPO) copolymers, giving polypseudorotaxanes.
Drawing their inspiration from these studies, Yang et al.67 could use the self-­
assembling of α-CDs and PEOx–PPOy–PEOx triblock copolymers (i.e. Pluronic®
F127) to obtain large and stable vesicle structures in water (Figure 6.7(c)).
After formation and self-assembly of the inclusion complexes, and hydrothermal treatment at 200 °C, hollow carbon-based particles could be easily
obtained (Figure 6.7(d)). The authors could tune both the inner pore diameter (from 100 up to 220 nm) and the average wall thickness (from 50 up to 80
nm) by just varying the initial concentration of F127. After pyrolysis at 900 °C
under argon, both average diameters and wall thicknesses slightly decreased
while further micro-/mesopores could be generated, leading to BET surface
areas of up to 430 m2 g−1. Such porous carbon hollow spheres were employed
as anodes in lithium-ion batteries depicting interesting specific charge capacities of up to 450 mAh g−1. Recent developments proposed by Yang et al.73
allowed the synthesis of double-layer hybrid structures consisting of hollow
hydrothermal nanocarbons and MnO2 nanocrystallites, through the diffu−
sion and subsequent redox reaction of MnO4 ions within carbon walls. Such
hybrid nanoarchitectures demonstrated promising performances as electrodes materials for electrochemical double-layer supercapacitors.73
6.5
Oil-in-Water
Macroemulsions: From Hybrid
Hollow Spheres to Carbo-HIPEs
Emulsions, widely present in cosmetic, pharmaceutical and even the food
industry (mayonnaise, milk, ouzo, etc.), have been shown as a flexible alternative to burnable polymeric templates for the design of hollow spheres
and nano-/micro-/or even macrocellular porous structures.74 As mentioned
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previously, one can discriminate microemulsions (or nanometer length scale
swollen micelles) and ordinary (macro-) emulsions (micrometer length scale).
Briefly, ordinary emulsions are metastable thermodynamic systems where
two immiscible liquids (basically, an oily phase and an aqueous phase) are
mixed together through mechanical energy supply processes, in such a way
that an internal phase made of micrometric droplets is dispersed within a
continuous phase. To minimise the water/oil interfacial tension, and consequently enhance the thermodynamic stability, the use of surfactant molecules (or solid-state nanoparticles in the special case of the so-called Pickering
emulsions) is a necessary condition to emulsions formulation. As biomass
precursors and derivatives are generally hydrosoluble, oil-in-water emulsions
are needed to design cellular structures. In such systems, oil is the dispersed
phase, playing the role of a washable soft template, while water is the continuous polymerisable phase.
6.5.1
Diluted Macroemulsions
Recently, oil-in-water emulsions (also termed direct emulsions) have been
applied to HTC materials. In particular, Jia et al.69 reported on the synthesis
of micrometer-sized copper@carbon hybrid hollow spheres in diluted trioctylamine (TOA)-in-water emulsions. Such spheres were obtained via the interfacial complexation (by amine groups of TOA) and subsequent reduction of
Cu2+ cations and concomitant hydrothermal carbonisation of ascorbic acid
(VC) (Figure 6.7(g)). The overall mechanism could be seen as a synergistic
cooperation between VC and CuCl2 since: (i) ascorbic acid is not only used
as a carbon source but also as reducing agents to form in fine embedded Cu
nanoparticles; and (ii) Cu2+ cations catalyse the hydrothermal carbonisation of VC, allowing the formation of carbonaceous matrices. These hollow
spheres display an average diameter of about 3.5 µm with copper@carbon
shells of about 70 nm thickness (Figure 6.7(h)). The authors proposed their
use as imaging contrast agents, but various potential applications could also
be envisaged as electrocatalysts, electrode materials or even templates for
inorganic nanomaterials.69
6.5.2
Concentrated Macroemulsions
Moving from diluted to concentrated macroemulsions, microcellular (cells
diameters from 1 to 50 µm) and/or macrocellular (cells diameters superior to
50 µm) continuous structures can be obtained. Typically, concentrated emulsions or high internal phase emulsions (HIPEs) are characterised by an internal
phase volume fraction equal or superior to 0.74, reaching as a result the critical value of the most compact arrangement of uniform spherical droplets.74
Polymerisation of the continuous phase and removal of the droplets of the
dispersed one yields macroporous foams, referred as to organic, inorganic or
even hybrid poly-HIPEs, depending on the nature of the monomers.74–77 Lately,
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Figure 6.8
Chapter 6
(a)
Schematic representation of the carbo-HIPE synthetic pathway: (A)
Emulsification (oil-in-water emulsion); (B) Hydrothermal treatment;
and (C) Soxhlet extraction, drying and further thermal treatment at
950 °C under an inert atmosphere. (b) SEM and (c) TEM micrographs
of a typical carbo-HIPE after further pyrolysis at 950 °C under an inert
atmosphere. Reproduced with permission from refs. 78, 79.
taking the poly-HIPE approach one step further, Brun et al.78,79 developed the
design of bioresourced micro-/macrocellular carbon monoliths via the prepolymerisation and subsequent hydrothermal carbonisation of saccharide
derivatives (i.e. furfural or 5-hydroxymethyl-2-furaldehyde) and phloroglucinol within the continuous phase of an oil-in-water emulsion (Figure 6.8(a)).
After oil removal by simple soxhlet extraction, typical macrostructures made
of 10–50 µm diameter aggregated hollow spheres could be obtained (Figure
6.8(b)). Because of the close packing of oil droplets together with the presence
of nonpolymerised interstices remaining in the continuous phase, narrower
macropores with diameters below 10 µm could also be generated, ensuring a
highly interconnected open macrostructure (Figure 6.8(b)). The use of FeCl3
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as Lewis acid catalysts for both the prepolymerisation and HTC stages, led to
the presence of residual iron species within the hydrothermal carbon framework after extraction and drying. During pyrolysis at 950 °C under an inert
atmosphere, such species could generate additional mesopores while increasing the overall graphitisation degree of the final conductive materials, termed
carbo-HIPEs. According to the authors, these concomitant phenomena were
both due to the generation of graphitic hollow rings (Figure 6.8(c)) induced
by the catalytic effect of iron species towards the graphitisation of amorphous
carbons at rather low temperatures.78,79 Recently, using nitrogen-containing
biomass derivatives (i.e. N-acetylglucosamine), N-doped hydrothermal carbon foams could also be synthesised.79 Promising preliminary results were
obtained for their use as intrinsic electrocatalysts for the oxygen-reduction
reaction (ORR), both as powdered and monolithic electrodes.
Interestingly, because of large pore diameters and macropore volumes, carbo-HIPEs could be used as monolithic biocatalyst supports for the enzymatic oxidation of glucose.78 In the same vein, microbial macrocellular bioreactors could be
designed via the inoculation and in situ growth of bacteria within carbo-HIPEs.
These bioreactors could be successfully applied to the continuous-flow anaerobic oxidation of acetate in artificial wastewater.80 Both studies could clearly
highlight the potential of bioresourced carbo-HIPEs within enzymatic and microbial bioelectrochemical systems, including for the production of biofuels (i.e.
biorefineries) and the generation of “green” electricity (i.e. biofuel cells).
6.6
Polystyrene
Latex Dispersions: From Hollow
Spheres to Coral-Like Structures
As an elegant substitute to silica-based Stöber particles, White et al.68 proposed to use polystyrene (PS) latex dispersions as soft templates for the
adsorption and subsequent hydrothermal coating of glucose-derived carbonaceous layers (Figure 6.7(e) and (f)). PS latex dispersions are often considered as hard templates or even emulsions. However, nonvulcanised rubber
microparticles are stricto sensus neither rigid solids nor liquids, and therefore PS lattices can be seen as a singular family of soft templates. Unlike inorganic hard-templates introduced earlier in this chapter, PS lattices’ removal
does not involve any harsh chemical etching. The random jumble of polystyrene chains directed by multiple intermacromolecular Van der Waals forces
could be readily removed by further pyrolysis at 550 °C under an inert atmosphere. Concomitantly, the hydrothermal carbon coating could be turned
into well-defined and monodisperse carbonaceous hollow nanospheres with
shell thicknesses of about 12 nm and SBET ≤ 460 m2 g−1. A facile postmodification with amino groups yielded stable aqueous dispersions under acidic
conditions, presaging potential applications as heterogeneous catalysts,
encapsulation agents (e.g. in drug-delivery systems) or sorbents.
To yield partially graphitised carbon shells, PS-latex@hydrochar composites were pyrolysed at 1000 °C under an inert atmosphere. Such conductive
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81
nanospheres could be applied as anode materials for lithium-based, and sodium-­
based batteries.82 As for the former application, one of the best rate performance
ever measured for a sodium-ion battery anode material could be reached,82 allowing a capacity of about 50 mAh g−1 at a current density as high as 10 A g−1 to be
maintained. According to the authors, these promising results are mainly due
to the mass transport “booster effect” associated with the large surface area and
short diffusion distance provided by this unique hollow nanospheres structure.
Recently, Kubo et al.83 described a sophisticated dual-templating approach
combining the HTC of glucose in the presence of polystyrene latex nanoparticles and Pluronic® F127 block copolymer self-assemblies (Figure 6.9(a)–(d)).
This approach could lead to so-called coral-like carbon nanoarchitectures,
bearing trimodal hierarchical porous structures (Figure 6.9(e) and (f)).
According to the authors, the formation of such nanoarchitectures is based
on the triple role played by the block copolymers, which can be seen as the
keystone of the as-synthesised structures. First, at the microscopic level
(Figure 6.9(e)), their intrinsic supramolecular self-assembly could generate
ordered microporous carbon structures, in the manner of the HTC materials
introduced Section 6.4.1.1. Secondly, via a destabilisation of the PS-latex dispersion during the HTC process (Figures 6.9(b) and (c)), close packing could
be induced, leading in fine to inverse opal structures depicting 50–60 nm
diameter monodisperse spherical pores (Figures 6.9(e) and (f)). Last but not
least, macroscopic cocontinuous structures could be obtained via a fine control of the degree of spinodal phase separation, directed by the block copolymer and providing micrometer-sized interconnected void spaces (Figure
6.9(f)). These coral-like carbonaceous materials are expected to be promising
candidates for applications as catalysts, biocatalysts and/or electrocatalysts
supports as well as sorbents in separation media.
Drawing its initial inspiration from the inorganic sol-gel chemistry, the
soft-templating approach has been successfully extended in recent years to the
hydrothermal carbonisation of biomass derivatives. As shown via these recent
examples, elegant synthetic pathways could be developed leading to discrete
hollow spheres,67–69 ordered mesoporous powders64 or even hierarchically
porous monoliths.78,79,83 The last example reported by Kubo et al.83 might be a
stimulus to creativeness in a near future for the design of sustainable hierarchical hydrothermal carbons by combining two or more compatible soft templates
or even hard and soft templates.84 With this aim, the use of Pickering emulsions stabilised by latex or silica-based sacrificial particles for the formulation
of hierarchical macrocellular HTC materials could be an elegant approach.
6.7
Template-Free
Hydrothermal Carbon Hydrogels
and Related Dried Gels
In the context of sustainable fabrication of advanced materials, not only the
sustainable aspect of the precursors is important, but also the coherence of
the synthetic pathway itself towards green chemistry precepts. As mentioned
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Figure 6.9
(a)–(d)
Schematic representation of the synthetic strategy for the design
of coral-like HTC structures; Insets in (b), (c), and (d) correspond to
photographs of synthesis Solution, carbon monolith before template
removal and after template removal, respectively. (e) TEM and (f) SEM
micrographs of a typical coral-like carbon monolith obtained at a HTC
temperature of 130 °C, after template removal via pyrolysis at 550 °C.
Reproduced with permission from ref. 83.
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previously in this chapter, the choice of the template is determinant as it
could dramatically affect the ecofriendly feature and the potential industrial
scale-up of the overall synthesis. In this way, being rid of any kind of sacrificial templates for the design of highly porous materials could appreciably
enhance the whole sustainable aspect. Supposedly, template-free (i.e. without
sacrificial templates) approaches would decrease the number of synthetic
steps and/or reagents, reduce the amount of wastes and avoid harsh chemical
etchings. Both the efficiency and cost of such pathways would be optimised.
The design of template-free porous networks was first developed for silica-based materials, via a fine control over both the sol-gel transition and
the drying of the as-synthesised wet hydrogel. On the one hand, an understanding of the gelation process could allow minimisation of the particle size
and generating narrow interstitial void spaces. On the other hand, a careful
drying could allow withstanding capillary forces induced by evaporation,
avoiding any structural collapse and preserving the wet hydrogel structure.
Depending on the drying process applied, three different families of dried
gels can be differentiated: (i) the xerogels, dried by simple evaporation; (ii) the
cryogels, dried via freeze drying (i.e. sublimation); and (iii) the aerogels, dried
under supercritical conditions (mostly using supercritical CO2). In fine, such
dried gels, especially aerogels, depict high surface areas and pores volumes.
For the last decade, syntheses of biomass-derived gels have been widely
developed, especially through the hydrothermal carbonisation of carbohydrates. In this section, recent examples of hydrothermal carbon dried gels
are presented. Stricto sensus, most of the materials presented herein cannot
be seen as aerogels, since they were not systematically dried under supercritical conditions. Nevertheless, due to similar airy porous structures, low density and high porosity, xerogels and cryogels are often mistakenly considered
as aerogels.
6.7.1
Salt-Mediated Hydrothermal Gelation Approaches
6.7.1.1 Borax-Mediated Gelation
Recent approaches reported on the use of salts as additives for the control over the hydrothermal gelation of carbohydrate-derived monomers.
Fellinger et al.85 first proposed the addition of sodium borate (i.e. borax; Figure 6.10). In this study, borax acts both as HTC catalyst and structure directing agent. While the final HTC mass yield was still relatively poor (∼44 wt.%
at 200 °C for 19 h), the use of borax could result in higher carbon yields,
up to 73% with respect to glucose (against 42% without salt). This catalytic
effect of borax towards monosaccharide’s dehydration is well-known and
was previously reported by Riisager et al.86,87 According to the authors, borax
salts accelerate the isomerisation of glucose to fructose,86,87 while partially
avoiding the yield-lowering acetalisation with HMF (Figures 6.10(a) and
(b)).88 The borax-mediated approach could also offer a fine control over the
final morphology of the carbogels: an increase in the amount of borax was
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Figure 6.10
175
Scheme
representing the side reactions, (a) Acetalisation; (b) Borate–
diol complexation, within the hydrothermal carbonisation of sugar
(herein simplified as dioles) in the presence of borax. (c) Photographs
and (d–g) TEM micrographs of typical borax-mediated carbogels
depending on the initial concentration of borax (written [Borax] on
the scheme). Borax concentration increases from left (Carbogel-100) to
right (Carbogel-750). Reproduced with permission from ref. 85.
found to significantly decrease the primary particles diameter up to 8 nm
(Figures 6.10(d)–(g)). Concomitantly, the generation of narrower interstitial
void spaces between the aggregated nanoparticles could induce an increase
in the SBET (ca. 230 m2 g−1). After further thermal treatment at 900 °C under a
flowing nitrogen atmosphere, conductive carbogels depicting electrical conductivities as high as 290 S m−1 and SBET between 200 and 600 m2 g−1 could be
obtained. With a view to applying such borax-mediated carbogels as intrinsic
electrocatalysts, Wohlgemuth et al.89 have reported on the addition of a nitrogen-source (i.e. 2-pyrrol-carboxaldehyde, PCA) to the initial borax/glucose
mixture. After further thermal treatment, the resulting conductive carbogels
displayed high nitrogen contents, SBET and electrical conductivities, up to 6
wt.%, 300 m2 g−1 and 900 S m−1, respectively. Such nitrogen-doped carbogels
could be used in the oxygen-reduction reaction in basic media.
Recently, Wang et al.90 developed a modified borax-mediated HTC method
for the synthesis of Pd@N-doped carbogels. In this study, poly(ionic liquid)
s (e.g. poly(1-vinyl-3-ethylimidazolium bromide) could be used both as nitrogen-containing additives and surface stabilising/pore generating agents. In
fine, N-doped carbogels with nitrogen contents of ∼5 wt.% and SBET = 424 m2
g−1 were synthesised and used as catalysts supports: using an ultrasonic-assisted/NaBH4 deposition/reduction method, Pd nanoparticles with a mean
size of 6 nm could be successfully immobilised. Such Pd@N-doped carbogels
showed highly competitive catalytic performances towards the solvent-free
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Chapter 6
aerobic oxidation of hydrocarbons (turnover frequencies, TOFs, up to 860
h−1) and alcohols (TOFs up to 210 000 h−1). However, the preparation of ionic
liquids requires a larger number of synthetic steps and higher energy consumption as compared with biomass-derived precursors. In this way, despite
a number of interesting properties and features largely highlighted by Wang
et al.90 in their study, the use of poly(ionic liquids) as a precursor for the synthesis of sustainable carbonaceous materials has to be questioned.
6.7.1.2 Beyond Borax: Molten and Melting
Salt-Mediated Gelation
Apart from borax salts in aqueous medium, recent approaches developed
the design of biomass-derived porous carbons in highly concentrated salin
media. Xie et al.91 reported on the ionothermal carbonisation of various
carbohydrates (i.e. glucose, fructose, xylose and starch). Instead of water, a
room-temperature molten salt (i.e. 1-butyl-3-methylimidazolium tetrachloroferrate(III); [Bmim][FeCl4]) was used both as reusable solvent and catalyst.
The generation of sponge-like nanomorphologies made of interconnected
nanoparticles (ca. 50–100 nm diameter) could be promoted by the further
surface stabilising role of [Bmim][FeCl4]. After ionothermal treatment at
180 °C for 24 h, washing, filtration (in order to recover/recycle the ionic liquid) and simple vacuum drying, carbon solids depicting SBET of up to 155 m2
g−1 together with bulk density as low as 0.1 g cm−3 were obtained. As mentioned earlier (see Section 6.7.1.1), the use of ionic liquids for the synthesis
of sustainable carbonaceous materials has to be questioned. However, in the
synthetic pathway developed by Xie et al.,91 ionic liquids are not used as precursors but as catalysts/structure-directing agents, and could be easily recovered and recycled.
Recently, Fechler et al.92 developed a so-called salt-templating pathway. This
approach proposes the hydrothermal carbonisation of glucose under hypersaline conditions. The authors proposed to use different binary systems based
on zinc chlorides (i.e. ZnCl2) mixed with lithium (i.e. LiCl), sodium (i.e. NaCl)
or potassium (i.e. KCl) salts, forming low-melting eutectics. In the manner of
the chemical activation mentioned previously (see Section 6.1), ZnCl2 acts as
a Lewis-acid dehydrating catalyst and as a template ( justifying the so-called
salt-templating approach) during the HTC of glucose. Moreover, as observed
in this study, the hypersaline conditions seem to provide an efficient surface
stabilisation leading to aerogel-like structures made of 10 nm aggregated
nanoparticles. Interestingly, the salt templates could be easily removed by
washing the samples overnight in water, even though previous grinding of
the monolithic materials was necessary. Surprisingly, the preservation of the
porous scaffolds did not require the use of freeze drying nor supercritical drying. SBET as high as 673 m2 g−1 could be reached via simple vacuum evaporation
and without any solvent exchange (i.e. directly from the samples washed with
water). The authors also claimed the potential reuse of the salt templates.
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Overall, the salt-mediated approaches presented herein have many advantages. Highly porous carbon materials could be synthesised without sacrificial templates or costly drying processes. However, although potentially
recycled, such approaches involve the use of catalysts, expensive and/or
unsustainable additives. In particular, the use of ionic liquids or zinc chloride (as zinc is known to involve supply risks) in so-called “sustainable processes” is debatable.
6.7.2
Ovalbumin-Derived Gelation Approach
Baccile et al.93 first proposed the hydrothermal treatment of glycoproteins
(e.g. ovalbumin) in the presence of glucose. Nitrogen-doped carbonaceous
nanoparticles or even continuous nanosponges with relatively high specific
surface areas (e.g. SBET > 200 m2 g−1) could be produced. Recently, White et al.18
reported on the optimisation of the hydrothermal reaction parameters (i.e.
5.5 h at 180 °C) and the use of an appropriate drying technique (e.g. supercritical CO2 extraction) to design low density (<0.1 g cm−3) nitrogen-doped
carbogels with SBET ∼ 270 m2 g−1 (Figure 6.11). According to the authors, the
glycoproteins play a key role in the formation of the evolving hydrothermal
carbon network. The proteins act as: (i) surface stabilising/structure directing agents; (ii) initiating gelators through Maillard- or Strecker-type reactions between amino groups and glucose; and (iii) nitrogen sources allowing
nitrogen contents as high as 8 wt.% to be reached. Interestingly, the chemical nature (Figure 6.11(d)) and porosity could be easily tuned via thermal
annealing under an inert atmosphere (from 350 to 900 °C), while the 3D
cocontinuous structure was maintained. Recently, Wohlgemuth et al.94 proposed to introduce thienyl-based comonomers (i.e. S-(2-thienyl)-l-cysteine
Figure 6.11 (a)
SEM and (b) and (c) HR-TEM micrographs of typical ovalbumin-­
derived aerogels after pyrolysis. (d) Relative composition determined by
XPS of different N-containing groups at the aerogel surface as a function
of the temperature of pyrolysis. Reproduced with permission from ref. 18.
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178
and 2-thienyl carboxaldehyde) to the initial protein/saccharide system. Thus,
dual-doped carbonaceous aerogels could be synthesised. While the nitrogen
content and the porous structure of the final aerogels were both preserved,
the S/N weight ratio could be modulated from 0 to 0.24. Interestingly, synergistic or consecutive mechanisms between sulfur and nitrogen functionalities could allow enhancement of the electrocatalytic activities towards the
oxygen-reduction reaction (ORR) in basic medium. The application of these
materials in electrocatalysis will be discussed further in Chapter 8. As compared with salt approaches, such ovalbumin/glucose-derived hydrothermal
carbogels are fully based on sustainable and cheap starting products, and
can therefore be seen as promising “green” alternatives to traditional carbon aerogels. However, the final HTC mass yield, though higher than the one
obtained without glycoproteins, remained relatively poor (<50 wt.% of the
added biomass was recovered as hydrothermal carbon).18,94
6.7.3
Phenolic-Derived Gelation Approaches
Drawing their inspiration from the “phenolic-furfural” approach developed
by Pekala et al.95 in the 1990s, Ryu et al. demonstrated the use of phenolic compound additives in the hydrothermal carbonisation of monosaccharides.96 Following this procedure, they could largely enhance the yield of the
typical HTC microspheres, up to 20-fold. Phenolic compounds, and particularly phloroglucinol, were shown to react with dehydrated sugars, i.e.
HMF or furfural, acting as crosslinkers via intermolecular dehydrations and/
or electrophilic aromatic substitutions.97,98 Taking this “phenolic–sugar”
hydrothermal approach a step further, Brun et al.99 could synthesise hydrothermal hydrogels from phloroglucinol–monosaccharide mixtures without
any catalyst, see Figure 6.12. Importantly, a final HTC mass yield as high
as 68 wt.% could be reached, which is 50% higher than the yields reported
for the borax-mediated approach (see Section 6.7.1.1). Moreover, phloroglucinol can be seen as a biomass-derived precursor, making this synthetic
approach more sustainable.100–102 After supercritical drying, airy continuous
interlinked macromorphologies made of small aggregated particles of about
15 nm were obtained. Such organic carbogels could display SBET as high
as ∼1100 m2 g−1, mainly related to a surprisingly high microporous contribution, ca. 650 m2 g−1. After further thermal treatment at 950 °C under an
inert atmosphere, SBET and Smeso of ca. 650 and 150 m2 g−1 were characterised respectively. Lately, nitrogen-containing biomass derivatives (i.e. N-acetyl-d-glucosamine or d-glucosamine hydrochloride) could be incorporated
to the “phenolic–sugar” system, leading to N-doped carbogels with SBET and
nitrogen contents ca. 450 m2 g−1 and 5 wt.%, respectively. The electrocatalytic
properties towards the ORR in basic media were attractively tuned depending on both the chemistry and morphology of the final gels.15 The main drawback of this procedure lies in the difficult graphitisation (associated with the
use of phenolic precursors), resulting in a relatively high amorphous character and low electrical conductivity (∼10 S m−1).
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Porous Hydrothermal Carbon Materials, Nanoparticles, Hybrids and Composites
Figure 6.12
Possible
reaction pathways between dehydrated sugars, herein 5-HMF,
and phloroglucinol: (A) electrophilic aromatic substitution and (B)
Intermolecular dehydration. (b) Photograph of a typical wet hydrogel
obtained via the “phloroglucinol-sugar” pathway. (c) Photograph, (d)
SEM micrograph, (e) TEM micrograph and (f) nitrogen sorption isotherm of a typical “phloroglucinol–sugar” aerogel. Reproduced with
permission from ref. 99.
Recently, Celzard et al. extended their tannin-derived gelation approach
to the HTC process.103,104 In this approach, the synthesis of nitrogen-doped
carbon microspheres through the hydrothermal treatment of nitrogenated
tannins was possible.17 Tannins are readily available and cheap natural flavonoids, and display reactivity towards dehydrated sugars similar to resorcinol or even phloroglucinol. Consequently, the hydrothermal “tannin–sugar”
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approach shall appear in the near future as an elegant pathway for the design
of sustainable carbogels. Further details regarding these materials will be
provided in Chapter 12.
6.7.4
Carbon Nanotubes-Assisted Structure Formation
Zhang et al. proposed to use acid-treated multiwalled carbon nanotubes,
noticed t-MWCNTs, as heterogeneous catalysts, structure-directing agents
and colloidal building-blocks for the synthesis of glucose-derived hydrothermal carbogels (Figure 6.13).105 After freeze drying, t-MWCNTs–hydrothermal
carbon flexible composites could be produced. The random jumble of interconnected hydrothermal carbon-coated nanotubes could generate low-density porous networks (ca. 0.02 g cm−3). The hydrophobic carbogels obtained
after further thermal treatment were successfully applied to the adsorption
of oils and organic solvents. Although not mentioned by the authors, one
may presume that such t-MWCNTs/hydrothermal carbon composites would
provide enhanced electrical conductivity associated with the singular properties of carbon nanotubes. In this case, these materials could be promising
as electrode materials or electrocatalyst supports.
Figure 6.13
(a)
Schematic representation of the carbon nanotubes-assisted
hydrothermal gelation. (b) and (c) TEM micrographs of typical
as-synthesised cryogels. Reproduced with permission from ref. 105.
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6.8
181
Porous
Carbons from Direct Hydrothermal
Treatment of Natural Systems
The HTC of naturally occurring materials has also been shown to lead to
porous hydrothermal carbons without any additive but water.13,106–109 As compared with the bottom-up approaches presented thus far in this chapter, the
HTC of natural systems is to be viewed as a top-down synthetic methodology.
Though limited in terms of structural control (e.g. dependent on the original
natural structure) such approaches represent real advantages as far as sustainability and cost are concerned.
6.8.1
Natural Scaffolds with in situ Hard Templates
6.8.1.1 Crustacean Exoskeletons
One of the first inspiring work on the use of natural scaffold with in situ hard
template for the design of porous hydrothermal carbons was proposed by
White et al.13 – were used as a hydrothermal carbon precursor (Figure 6.14).
After HTC, a secondary thermal pyrolysis step at 750 °C and subsequent
CaCO3 removal with acetic acid washing, porous nitrogen-doped carbons
with SBET, pore volume and nitrogen content as high as 328 m2 g−1, 0.87 cm3 g−1
Figure 6.14
TEM
images of prawn shell-derived carbon materials prepared via a
hydrothermal carbonisation step (at 180 °C) and a thermal carbonisation step (at 750 °C): (A) and (B) Before; and (C) to (F) After acid washing to remove the inorganic component. Reproduced with permission
from ref. 13.
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and 5.8 wt.%, respectively could be obtained. This approach has since been
extended to the use of more complex lobster shells with resulting nitrogen-doped carbons finding excellent use as supports for Pt2+ catalysts in the
direct oxidation of CH4 to CH3OH based on the Periana system.110
6.8.1.2 Rice Husks
Recently Wang et al. reported on the use of rice husks as agricultural wastes
precursors (100 million tons of rice husks are generated every year) for the
production of porous hydrothermal carbons and carbons.108 Rice husks are
complex structures made of cellulose (ca. 38 wt.%), lignin (ca. 22 wt.%),
hemicellulose (ca. 18 wt.%) and silica. To degrade the hemicellulose and
lignin components before HTC, they were initially treated with formic acid.
Subsequently, the resulting cellulose/silica composites were hydrothermally treated at 230 °C for 48 h (as cellulose cannot be easily converted
at lower temperatures3). The as-synthesised hydrothermal carbons present
BET surface area and pore volume of 52 m2 g−1 and 0.12 cm3 g−1, respectively. After further pyrolysis at 900 °C in an inert atmosphere and subsequent silica removal by chemical etching, fibrous carbon networks could
be produced and SBET and pore volume could be increased up to 243 m2 g−1
and 0.41 cm3 g−1, respectively (Figure 6.15). Such materials could be used
as anode materials in lithium-ion batteries with high rate performance, i.e.
displaying a reversible capacity of 137 mAh g−1 at 10 C.
The use of raw agricultural or food wastes with in situ hard templates
for the production of porous carbons seems really promising and could be
compatible with both the overall food-processing chain and the associated
bioethanol generation. As far as sustainability is concerned, the main drawback of this pathway lies in the use of harsh chemical etching for inorganic
component removal.
6.8.2
Natural Scaffolds without in situ Hard Templates
6.8.2.1 Unicellular Fungus
Ni et al. have produced micrometric carbonaceous hollow spheres from the
direct hydrothermal treatment of yeast cells, Saccharomyces cerevisiae.107
These elliptical unicellular fungus are made of sturdy cell walls mainly consisting of a polysaccharide layer constructed from coiled β-1,3-glucan chains
(Figure 6.16(a)). Interestingly, such hollow spheres (Figure 6.16(b) and (c))
were found to display amphiphilic properties, allowing their stable dispersion in various solvents of opposite polarity. According to the authors, this
feature is due to the fact that both carbonisation and hydrolysis reactions
occurred during the hydrothermal process (Figure 6.16(a)). A spontaneous
phase-transfer phenomenon could also be observed upon addition of only
several drops of water to fungus-derived hollow spheres initially dispersed
in nonpolar solvents (i.e. toluene or chloroform; Figures 6.16(d) and (e)).
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Porous Hydrothermal Carbon Materials, Nanoparticles, Hybrids and Composites
Figure 6.15
183
SEM
micrographs of (a) Pristine rice husk, (b) Rice husk after formic
acid treatment and HTC (RH-Acid-HTC), and (c) Rice husk after formic
acid treatment, HTC, further pyrolysis and silica removal by chemical etching (RH-Acid-HTC-900-HF). (d) Nitrogen sorption isotherms
of rice husk-derived hydrothermal materials at different stages of the
synthesis. RH-Acid-HTC-900 corresponds to rice husk after formic
acid treatment, HTC and further pyrolysis. Reproduced with permission from ref. 108.
These materials may be promising in catalysis, adsorption, as well as for
drug delivery.
6.8.2.2 Watermelon Soft Tissues
Sponge-like carbogels were recently developed by Wu et al. via a simple onepot approach directly from the hydrothermal treatment of the soft tissue of
watermelon.109 Cutting directly the native watermelon pieces could offer an
easy shaping, from 10 mL up to 1 L hydrogels. After freeze drying, low-density (ca. 0.58 g cm−3), flexible monoliths could be obtained. Fe3O4 nanoparticles were subsequently embedded into the gel, via a facile in situ solution
approach, leading, after further calcination under an inert atmosphere to
magnetite–carbon hybrid aerogels. Besides the interesting magnetic properties inherent to the supported Fe3O4 nanoparticles, the as-synthesised
materials showed excellent specific capacitances when used as electrodes
in supercapacitors, up to 330 F g−1 over 1000 cycles at a current density of
1 A g−1. In terms of sustainability, even if it could be a way to promote food
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184
Figure 6.16
(a)
Schematic representation of the pore-formation mechanism and
the chemical structure transformation of the fungus-derived HTC
hollow spheres. (b) and (c) SEM micrographs of the as-synthesised
hollow spheres. (d) and (e) Photographs of fungus-derived hollow
spheres in different solvent systems. Reproduced with permission
from ref. 107.
wastes’ upgrading, some issues related to the competition with the food
chain may arise.
6.9
Biomass-Derived
HTC Nanodots and
Nanocomposites
Beyond the structuration of porous hydrothermal carbons via templating or
gelation approaches, a specific interest has recently emerged in the synthesis of hydrothermal carbon nanodots.111–113 Sun et al.111–113 have developed
a simple strategy to prepare nitrogen-doped photoluminescent carbon dots
(CDs) through a shortened hydrothermal treatment (i.e. 3 h at 180–200 °C)
of raw biomass such as willow bark,111 grass,112 or pomelo peel.113 After further dialysis and centrifugation, CDs of less than 5 nm could be separated.
Yang et al. also reported on the synthesis of amino-functionalised fluorescent CDs by hydrothermal treatment of chitosan.114 Such carbon dots could
be successfully used for the detection of glucose111 and metal ions (Cu2+ and
Hg2+).112,113 Because of low cytotoxicity, they have great application potential
in biological labeling and as biosensors.
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6.10
185
Summary and Outlook
It is hoped that this chapter has provided a substantial overview for the
reader of the possibilities regarding porous carbon materials synthesis
from sustainable precursors based on hydrothermal carbonisation. A range
of synthetic routes (e.g. hard and soft templating, controlled phase separation/gelation, activation, etc.) to the preparation of micro-, meso- and
macroporous carbonaceous materials has been presented. As will be seen
in forthcoming chapters, these materials are suitable for a variety of applications relevant to modern and future industrial practices (e.g. the Biorefinery). The bottom-up synthesis of the HTC platform provides access to a
wide range of texturally and morphologically different materials often with
high surface areas and hierarchical porosity. The advantage of the HTC
platform is further extended when one considers the variety of chemistry
that is afforded by the low synthesis temperature of HTC. In this context,
material can be structured using a suitable template or phase-separation
behaviour based on a low-temperature synthesis, whilst material chemistry
can then be tuned by selection of either synthesis temperature or a secondary thermal carbonisation step – taking these materials beyond conventional organic aerogel-based carbons and indeed traditional porous silica
preparation.
The HTC synthesis approach enables materials to be treated (e.g. thermally)
to render structures porous and importantly, analogously to Starbon® materials (Chapter 2), enables physicochemistry and functionality to be essentially
dictated, which may be achieved in a very subtle manner. Given the range
of synthetic “tricks” currently available in traditional sol-gel chemistry (e.g.
from porous silica synthesis; ordered porosity, monoliths, etc.), these can
be in principle transferred to this relatively simple carbonaceous material
preparation. The use of simple, soluble, sustainable sugar-based molecular
precursors is attractive, as is the dehydration, polymerisation and condensation chemistry, to form nanostructured gels, whilst structuration, specific
surface area, and hierarchical transport structure can be controlled based on
an understanding of phase separation, demixing, and crosslinking kinetics
and if desired the addition of suitable additives. Therefore, the synthesis of
sustainable porous carbonaceous materials via HTC can be described going
forward as a sol-gel chemistry of carbon. The tools of the sol-gel chemist can
therefore be extended to the synthesis of carbonaceous materials based on
sustainable precursors, generating a materials platform suitable to address
the challenges of future energy and chemical provision (e.g. the Methanol
Economy, Biorefinery, etc.) where current carbon materials will not be suitable (e.g. hydrophobic, microporous). Therefore, the synthetic approaches
discussed in this chapter will provide access to sustainable future materials
chemistry with a tool box to subtly design carbon materials and allow structure–activity relationships to be developed for real material optimisation in
a specific application.
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CHAPTER 7
Hydrothermal Carbon
Materials for Heterogeneous
Catalysis
LI ZHAO*, PEI-WEN XIAO AND BAO-HANG HAN
National Center for Nanoscience and Technology, Beijing 100190, China
*E-mail: zhaol@nanoctr.cn
7.1
Introduction
As discussed throughout this book, carbon-based materials have received
extensive attention and increasing interests with the development of carbon nanotubes and graphene, due to their unique characteristics as well as
their possible applications in industry, the environment and daily life.1–3 As
introduced in Chapter 3, one of the most significant applications of carbon
materials is in the area of catalysis. Catalytic application of carbon materials is an old topic, probably as old as the discipline of physical chemistry,
or even older. Carbon materials are particularly suitable in heterogeneous
catalysis due to their resistance to acid/basic media, their potential to control porosity and surface chemistry, as well as environment aspects. Furthermore, carbons can act as catalysts in their own right due to their inherent
structural properties,4 enabling for example (as discussed further in Chapter
8), the activation of oxygen and chlorine for selective oxidation, chlorination,
and dechlorination reactions. The surface chemistry of carbon also plays an
important role in their application behaviour, since a huge variety of surface
functional groups are known to exist, including those based on the incorporation of heteroatoms (i.e. oxygen, nitrogen, sulfur, halogens, hydrogen,
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Porous Carbon Materials from Sustainable Precursors
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© The Royal Society of Chemistry 2015
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etc.). Furthermore, carbon-based materials are also known to be prominent
catalyst supports, allowing the anchoring of metal or metal oxide (nano)particles, which in turn, can be applied in desired catalytic applications.
Over the past century, various research groups have conducted extensive
studies on the fundamental understanding of carbon materials as catalysts
or catalysts support. Many articles and reviews have been published relating to this subject.6 Of the material types thus investigated, activated carbon fibres (CFs), nanotubes (CNTs), and nanofibres (CNFs) have traditionally
acted as catalyst support.7,8 These carbon materials have many advantages as
compared to traditional supports, including chemical inertness, high-temperature/oxidation resistance, potential pore-structure control and macroscopic shaping, surface modification options and simple end-of-life material
recovery.3,9
The four main properties affecting a carbon-based material’s efficacy as a
catalyst support are surface chemical groups, surface area, pore structure,
and chemical activity. High surface area and well-developed pore structure
play important roles in achieving homogeneous and high dispersion of
metal or metal oxide (nano)particles on the surface of porous carbon support. Surface chemical groups, such as amino, hydroxyl, sulfonic, and other
functional groups, can be obtained via chemical modification with heteroatoms (N, S, P, B, etc.). The presence of these surface functional groups can
dictate the acidity or basicity and hydrophobic or hydrophilic characters
of the catalyst support. These properties in turn will influence the catalytic
behaviour of carbon-supported catalysts.10,11 Furthermore, the presence of
surface functional groups also influences the chemical activity of porous carbon as determined by the influence of the binding forces between the support surface and catalyst.
More recently, the concepts of “sustainable development” and “green
chemistry”, which both aim to use renewable resources, green solvents, and
safe reactions to minimise wasteful byproducts, to produce valuable chemical materials by simpler, less energy-intensive and more efficient processes,
are attracting increasing attention from the catalytic research community.12,13
As introduced in Chapter 5, hydrothermal carbonisation (denoted herein
as HTC) is a promising sustainable technique for the synthesis of applicable carbon materials derived from sustainable precursors (i.e. biomass or
biomass derivatives). HTC synthesis is ∼100 year old technique originating
from investigations into “charcoal formation”.14 It was first described in 1913
when Bergius carried out the hydrothermal transformation of cellulose into
coal-like materials.15 Afterwards, more systematic investigations were performed by Berl and Schmidt in 1932 varying biomass source and treatment
temperature between 150 °C and 350 °C.16 As indicated earlier in this book,
the HTC process has received somewhat of a recent renaissance, with initial reports relating to the conversion of glucose into uniform carbonaceous
particles.17,18 Different carbohydrate sources including xylose,19 fructose,20
sucrose,21 cellulose,22 cyclodextrins,23 starch, raw biomass,24 etc. have also
been investigated. The HTC process has a number of significant advantages
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Hydrothermal Carbon Materials for Heterogeneous Catalysis
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including the low toxicological impact of materials and processes, the use
of renewable resources, whilst removing the need for organic solvents, catalysts or surfactants. Furthermore, the technique is facile, since it requires
low-technology instruments as well as high energy and atom economy.18,19,25
The HTC process is particularly appropriate to produce functional carbonaceous materials. For example, one-step HTC process of glucose in
the presence of ovalbumin,26 vinylimadozol,27 or acrylic acid,28 resulted in
­nanometer-sized carbonaceous materials with unique morphologies, porous
structures and increased surface functionality. HTC has also been extensively used for the production of various functionalised hybrid materials via
the addition of metal salts (or metal nanoparticles (NP)) to the HTC recipe.
In 2004, Li et al. reported on the decoration of hydrothermal carbon spheres
from glucose with noble metal (Pd, Ag, Au) NPs.17 Yu et al. reported the onestep preparation of carbon/Ag hybrid nanocables via the HTC of starch in
the presence of AgNO3.29 Other metal–carbon hybrids, including SnO2@C,27
Pd@C,30 Se/C,31 Fe3O4@C32 and Si@SiOx/C,33 have also been reported. Furthermore, hybrid materials with complex structures and specific properties,
including Fe3O4@TiO2,34 SiO2@SnO2@C,35 Ag@MFe2O4 (M = Ni, Co, Mg,
Zn),36 were also produced in order to meet different applications demand.
In this context it is the intention of this chapter to provide the reader with
an overview of the application of HTC-based materials in catalysis, with a
particular focus on the use of functional carbonaceous materials as well as
carbon–metal (metal oxide) nanocomposites or hybrids. The HTC synthesis
platform allows the preparation of a wide range of materials with unique
morphologies, structure and chemical functionalities necessary to meet the
different demands of topical heterogeneous catalysis.
7.2
Heteroatom
Functionalised HTC Materials in
Heterogeneous Catalysis
For specific applications, the chemical functionalisation of a carbon material
may be required to introduce appropriate functional surface group/bulk dopants (e.g. to introduce surface acid sites).37,38 In this context, doping of the
carbon material structure with heteroatoms (e.g. N, S, B, P) is a popular route
to tune both the structural and physicochemical properties. In this subsection, the synthesis, characterisation as well as the catalysis application of
heteroatom (N, S, B, P, etc.) will be discussed.
7.3
Nitrogen-Containing Carbons in Catalysis
N-doped carbons, as will be discussed in further detail in Chapter 8, are now
playing an important role in cutting-edge innovations for energy conversion and storage technologies such as supercapacitors and proton-exchange
membrane fuel cells, adsorption and CO2 capture. Such heteroatom-doped
carbons are also important in heterogeneous catalysis. Nitrogen can be
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Figure 7.1
Chapter 7
Common
“nitrogen”-containing organic precursors used in nitrogendoped carbons synthesis. Reproduced with permission from ref. 52.
introduced into carbon material structure predominantly via two methods.39,40 One way is the treatment of presynthesised carbon materials at
high temperatures with nitrogen-containing gases or liquid, such as NH3,
HCN, NC–CN or nitrogen-containing ionic liquid.41–44 Alternatively, the
carbonisation of nitrogen-containing organic compounds or mixture of
nitrogen-­containing precursors with nitrogen-free materials, such as poly(acrylonitrile),45 poly(aniline),46 phthalocyanines,47 vinylpyridine,48 melamine,49
acetonitrile,50 and N-heterocycles (e.g. pyridine) (Figure 7.1).51
By using these two different approaches to synthesis, carbon materials with
different nitrogen-containing functionalities can be obtained, with former
typically leading to surface “doping”, with the latter resulting in bulk heteroatom doping. This can impact on the N-containing functionalities on or
within the carbon materials that in turn determine material acidity or basicity,
conductivity, oxidation stability and thus the choice of route will dramatically
affect the activity of the material in a given application (Figure 7.2).53
N-doped CNTs and CNFs have received extensive attention in the context of
heterogeneous catalysis.54,55 However, their production methods rely on chemically harsh and multistep processes, typically involving high-­temperature
treatment and often leading to the generation of significant quantities
of waste, generally inert surface chemistry (e.g. making further chemical
modification difficult), whilst in some cases the overall carbon yield of the
process is rather limited. Furthermore, conventional nitrogen-containing
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Figure 7.2
Types
of nitrogen-containing functionalities on the carbon materials:
(a) Pyrrole-like group; (b) Nitrile; (c) Secondary amine; (d) Nitro group;
(e) Nitroso group; (f) Tertiary amine (g) Primary amine; (h) Pyridine-like
group; (i) Imine; ( j) Amide; (k) Lactam; (l) Pyridone; and (m) Quaternary
amine. Reproduced with permission from ref. 53.
Figure 7.3
Biomass-based
precursors used for nitrogen-doped carbon synthesis.
precursors are not necessarily sustainable and relatively expensive as compared to ­biomass-derived precursors, such as the carbohydrates or amino
acids ­(Figure 7.3). In this context, biomass or biomass derivatives are qualified carbon raw materials for the synthesis of valuable carbon materials.
Recently, as discussed in Chapter 5, the HTC process can be described in
terms of carbohydrate dehydration, condensation, polymerisation, and aromatisation.19,25 When organic monomers are added to the HTC system, the
controlled dehydration products of carbohydrates are partially replaced by
the monomers, thus a new type of carbonaceous latex material with a high
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Chapter 7
degree of functionalities can be produced. In this way, N-doped hydrothermal
carbon could be obtained by adding selected nitrogen-containing monomers
into the system, for example, the one-step HTC of glucose in the presence of
glycine,56 ovalbumin,26,57 or vinyl imadozole.27 N-doped carbons could also
be obtained by using nitrogen-containing carbohydrates or raw biomass as
precursors, including chitosan, glucosamine,58 crustacean exoskeletons (e.g.
lobster shells)59 leading to the production of nanometer-sized carbonaceous
materials with unique morphologies, porous structures and high surface
area, presenting carbonisation temperature-dependent nitrogen-containing
surface functionality. These materials had relatively high nitrogen content
(e.g. >7 wt%), and the nitrogen-containing functionalities, as will be alluded
later in this chapter, allowed them to be successfully employed in heterogeneous catalysis.
The one-step HTC of glucose in the presence of vinyl imidazole (VI)
resulted in porous carbonaceous materials presenting imidazole groups,
as reported by Demir-Cakan et al. (Figure 7.4).27 The obtained materials
(denoted as HC-xIm; where x = amount of VI used in the synthesis) could be
further functionalised to yield surface imidazolium bromide groups.27 The
imidazole groups were successfully incorporated into the resulting materials, as confirmed by FT-IR spectrum and zeta-potential measurements. The
successful alkylation of surface imidazole moieties were determined by
­energy-dispersive X-ray spectroscopy (EDX). The nitrogen content was found
to increase with increasing amount of VI monomer, with the nitrogen content
Figure 7.4
Electron
micrographs of the HC-10Im sample: (a) and (b) SEM; (c) and
(d) TEM. Reproduced with permission from ref. 27.
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of HC-10Im being >8 wt%. Zeta-potential measurements demonstrated the
basic character of the materials, with imidazole-containing materials presenting positively charged surfaces at acidic pH due to the protonation of the
nitrogen atom linked to the carbon.
The catalytic activity of the functionalised materials (HC-10Bu2ImBr) was
investigated for three test reactions, which were previously reported to be
promoted by imidazolium halides: (i) the aromatisation of unsaturated six
rings (especially Diels–Alder condensation products), (ii) Knoevenagel and
Aldol condensations, and (iii) transesterifications (Figure 7.5). The nitrogencontaining carbon materials synthesised by Demir-Cakan et al. showed high
yields under mild conditions in the Diels–Alder condensation of naphthquinone and cyclohexadiene, the Knoevenagel condensation of benzaldehyde
with malononitrile as well as the Aldol condensation of benzaldehyde with
acetophenone.
N-doped carbons could also be obtained by using nitrogen-containing
biomass-derived carbohydrates including glucosamine (GA) and chitosan
(CH) as carbon sources. N-doped carbons derived from these precursors,
denoted as HC–GA and HC–CH, respectively, were obtained via a one-step HTC
process.58 The morphology of the samples showed a continuous network
Figure 7.5
Reported
reactions catalysed by HC-10Bu2ImBr: (A) the aromatisation
of Diels–Alder condensates; (B) Knoevenagel and Aldol condensations;
(C) transesterification. Reproduced with permission from ref. 27.
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Chapter 7
formed of agglomerated intercalated spheres (Figure 7.6). These materials
contained a significant amount of nitrogen in their structure (≥9 wt%), with
the corresponding functionalities/bonding motifs characterised by 13C and
15
N solid-state NMR and XPS, indicating the existence of protonated C–NH–C
groups, either in the pyrrole-like or amide forms, and pyridine-like, C–N=C
aromatic networks. The relative proportions of these N-containing functionalities determined the basic character of the material, as confirmed by
zeta-potential analysis, presenting positive zeta-potentials below pH ∼5 with
the existence of protonated N species at the surface with values up to 30 mV
at pH 2.5 (Figure 7.7).
More recent work by the authors has demonstrated that the easily synthesised basic HC–GA and HC–CH materials are also capable of catalysing C–C
coupling reactions including Knoevenagel condensation (benaldehyde with
Figure 7.6
Scanning
electron micrographs of the nitrogen-doped carbons obtained
upon hydrothermal carbonisation of (a) Chitosan (HC–CH); (b) Glucosamine (HC–GA). Reproduced with permission from ref. 58.
Figure 7.7
Zeta-potential
experiments of HC–G, HC–CH and HC–GA. Reproduced
with permission from ref. 58.
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Hydrothermal Carbon Materials for Heterogeneous Catalysis
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malononitrile) and cycloaddition reaction (benaldehyde with malononitrile)
(Figure 7.8).
The above-mentioned nitrogen-doped carbons also show improved electronic
performance with respect to non-nitrogen-containing materials.58 N-doped
CNTs, graphitic carbon or graphene, have already been used in electrochemistry
due to their excellent electrocatalytic activity, long-term stability and excellent
resistance to crossover effects,60–65 Recently, studies have shown the applicability of N-doped carbons derived from sustainable precursors or related N-doped
carbon/metal composites in a variety of energy-related applications including the oxygen-reduction reaction (ORR) (please refer to Chapter 8 for further
details),66,67 the electro-oxidation of glucose,68 and the direct oxidation of methane.69 Referring to the latter example, White et al., have demonstrated the use
of N-doped carbons derived from biomass precursors as support media for the
heterogenisaiton of Pt2+ complexes.69 In this report, the hydrothermal carbonisation and further thermal treatment of chitin-based precursors (i.e. the crustacean exoskeleton of lobsters), followed by an acid wash to remove the inorganic
component of the shell, was found to yield highly porous N-doped carbons
with a high surface area (>400 m2 g−1) and tuneable chemical properties. The
presented materials had a high nitrogen content (>5% N) and high pore volume
(Vpore > 0.6 cm3 g−1) material, with Pt loadings of ca. 6 wt%. (Scheme 7.1).
The catalytic performance of these sustainable Pt@N-doped carbons was
tested in the direct oxidation of methane to ultimately yield methanol – a
potentially important fuel of the future. The initial activity of the N-doped
carbon-based catalyst was found to be superior to the molecular benchmark
originally described by Periana et al. and significantly better than that of the
previously reported solid catalysts (Table 7.1).70
Figure 7.8
Catalytic
activities of the carbons on (a) Knoevenagel reaction of benaldehyde with malononitrile, and (b) Coupling reaction of CO2 and
propylene oxide. (unpublished results); (a Reaction condition: 0.5 g
catalyst, 20 mmol benaldehyde, 20 mmol malononitrile, 8 ml toluene,
room temperature, 72 h; b Reaction condition: 0.5 g catalyst, 70 mmol
propylene oxide, 2.0 MPa CO2, 120 °C, 4 h).
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200
Scheme 7.1 Preparation
of coordinatively modified Pt@NDC (nitrogen-doped
carbon) materials derived from crustacean exoskeleton of lobsters
(ExLOB). Reproduced with permission from ref. 69.
Table 7.1 Conversions,
yields, selectivities to methanol (methylbisulfate) and catalytic activities (TOFs) achieved under given reaction conditionsa.
Entry
Catalyst
X (%)
1
2
3
4
5
Pt@CTF
7.0
Pt(bpym)Cl2
17.9
Pt@ExLOB-900 (1st run) 33.8
Pt@ExLOB-900 (2nd run) 18.5
Pt@ExLO B-900 (3rd run) 6.0
Y (%)
S (%)
TOF b (h−1)
TOF c (h−1)
6.0
17.2
31.7
17.6
5.6
85.4
96.3
94.0
95.2
91.6
174
912
2074
1938
1826
233
779
1227
1516
1802
a
Reproduced with permission from ref. 69.
Determined from a pressure drop from 69 to 67.5 bar (ESI).
c
Determined from the amount of methanol produced within 30 minutes.
b
Whilst further discussion will be made in Chapter 8, the use of HTC-based
material in electrocatalytic applications will be briefly introduced here. In
order to improve the sustainability aspects of fuel cell electrocatalysts, Sevilla
et al. have reported on the development of metal-free N-doped mesoporous
carbons containing small quantities of graphitised carbon as ORR catalysts.66 In this report, polypyrrole was used as the N-doped carbon precursor
and SBA-15 or silica xerogel as sacrificial hard templates. The resulting materials combined a high surface area (SBET ∼ 1000–1500 m2 g−1), an accessible
mesoporous structure as well as a high nitrogen content (3.6–5.5 wt%). The
mesoporous structure was proposed to reduce mass-transport limitations,
whilst the incorporation of nitrogen was believed to play an important role
in ORR activity. For this material, as determined from the XPS N1s photoelectron envelope, the main contribution corresponds to quaternary-N, followed by pyrrolic/pyridonic and pyridinic N. As a result, the N-doped porous
carbons exhibit an enhanced intrinsic electrocatalytic activity towards the
ORR. Furthermore, unlike commercial Pt catalysts, they are unaffected by
the methanol crossover effect.
Moving to more sustainable nitrogen-containing carbon precursors, Brun
et al. have reported on the synthesis of a range of N-doped carbon aerogels
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Hydrothermal Carbon Materials for Heterogeneous Catalysis
Figure 7.9
201
SEM
(a) and TEM (b) images of the obtained nitrogen-doped mesoporous carbon (950-G1AG1). Reproduced with permission from ref. 71.
based on the hydrothermal conversion of mixtures of glucose, D-glucosamine and N-acetyl-D-glucosamine and phenolic compounds (i.e. phloroglucinol and cyanuric acid).71 This approach led to the synthesis of monolithic
and highly porous N-doped aerogels, with high specific surface areas (SBET
= 600–700 m2 g−1) and pore structuring featuring both micro- and mesopores (Figure 7.9). The nitrogen content was characterised between 2.5 and
8 wt%, with XPS results indicating that all the synthesised N-doped carbon
aerogels contained mainly quaternary nitrogen species (i.e. binding energies of 401.2–401.4 eV).
The catalytic activities of these materials were tested in the ORR. Cyclic
voltammetry (CV) and polarisation measurements were performed (Figure
7.10). These sustainably sourced N-doped carbon aerogels performed very
well with the combination of a micro-/mesoporous network and appropriate
“N” bonding motifs influencing the electrocatalytic activity. Tuning the pyrrolic/pyridinic content ratio was found to enable a degree of control over the
electron process selectivity (i.e. 2 vs. 4 electron processes), with the higher
pyrrolic/pyridinic content ratios generating the best performance.
Utilising raw biomass-fermented rice as starting materials, Gao et al.
demonstrated a facile and scalable approach to produce large-scale porous
N-doped carbon spheres.72 The obtained materials showed homogeneous
spheres with inner porous structures (Figure 7.11). The materials presented high specific surface areas (2105 m2 g−1) and porosity (1.14 cm3 g−1),
as well as a relatively high nitrogen content (6 wt%), composed of both pyridine-like (398.5 eV) and quaternary (401.5 eV) N atoms. For this material,
it was proposed that a higher quaternary nitrogen content led to a higher
ORR activity and wettability, whilst the existence of a C–N bond rendered
materials extremely resistant to crossover effects and CO poisoning, as
compared to the commercial Pt/C catalyst comparison. (Figure 7.12)
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Chapter 7
Figure 7.10
(a)
Cyclic voltammogram measured for the nitrogen-doped carbon
aerogel 950-G1AG1 in N2-saturated 0.1 M KOH solution (dashed line)
and in O2-saturated 0.1 M KOH solution (solid line). (b) Cyclic voltammograms of: 950-G1GA1; 950-G1AG1; 950-AG2; and 950-AG21CA2
measured in O2-saturated 0.1 M KOH solution (solid lines). Scan rate =
100 mV s−1. (c) Polarisation curves measured in N2-saturated 0.1 M KOH
solution (dashed line) and in O2-saturated 0.1 M KOH solution (solid
lines) for the: 950-G1GA1; 950-G1AG1; 950-AG2; and 950-AG21CA2
measured in O2-saturated 0.1 M KOH solution. (d) Polarisation curves
measured in O2-saturated 0.1 M KOH solution (solid lines) comparing
950-G1AG1 with: 900-glucose, 950-G2, 20 wt% Pt@C (dotted grey line).
Scan rate = 10 mV s−1; rotation rate = 1600 rpm. Reproduced with permission from ref. 71.
Figure 7.11
(A)
SEM, (B) TEM, images of N-CSs. Scale bars: (A) 5 mm, (B) 50 nm.
Reproduced with permission from ref. 72.
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Hydrothermal Carbon Materials for Heterogeneous Catalysis
Figure 7.12
(A)
CV curves of (a) Pt/C, (b) N-CSs, (c) The directly carbonised products
of fermented rice, (d) The directly carbonised products of unfermented
rice, and (e) Bare-GCE electrode in N2-saturated and O2-saturated 0.1
M KOH solution at a scan rate of 10 mV s−1. (B) RDE curves for N-CSs in
O2-saturated 0.1 M KOH at different rotation speeds. Scan rate: 10 mV s−1.
The inset shows the partial K–L plots derived from the RDE measurements of the sample. (C) and (D) CV curves of N-CSs and 20 wt% Pt/C
in O2-saturated 0.1 M KOH solution with and without 10 vol% methanol at a scan rate of 10 mV s−1. (E) The percentage of current density
( j) vs. time chronoamperometric responses obtained at the 20 wt% Pt
and N-CSs electrodes at 0.30 V in O2-saturated 0.1 M KOH. The arrow
indicates the introduction of N2 or CO into the electrolyte. (F) ADT
measurements of N-CSs as determined after 1500 continuous CVs in
O2-saturated 0.1 M KOH. Scan rate: 10 mV s−1. Reproduced with permission from ref. 72.
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7.4
Sulfur-Doped Carbons for Catalysis
Complementing nitrogen as a dopant, sulfur is receiving increasing attention in current carbon materials research. Due to its large size, easily polarisable lone pairs, its incorporation into carbon-based materials can lead to a
range of versatile functional materials with a wide range of potential applications, including heterogeneous catalysis,73 sorption,74 as well as in the areas
of energy conversion and storage.75,76 As for nitrogen-doped carbons, the
traditional synthesis methods for the sulfur-containing materials generally
involves the pyrolysis of sulfur-containing polymer based carbons,77–79 or the
arc vaporisation in the presence of sulfur-containing compounds (e.g. thiophenes).80 By using these methods, a variety of modes of “S” incorporation
into a carbon network can be obtained (Figure 7.13).81
Wohlgemuth et al. have recently employed amino acids (and derivatives) as
sustainable sulfur-(and nitrogen-) containing carbon precursors (Figure 7.14).82
Hydrothermal treatment of glucose and l-Cysteine or S-(2-thienyl)-l-­cysteine
led to the synthesis of discretely sized carbonaceous microspheres with
nitrogen and contents of ca. 5 wt% and 3–12 wt%, respectively (Figure 7.15).
Incorporation of the amino acid and its degradation products occurred presumably via a combination of Maillard and Strecker reactions. The addition
of cysteine gives rise to pending sulfur functionalities, while addition of thienyl-cysteine resulted in structurally bound sulfur within the carbonaceous
framework. The pending surface thiol groups were effective for the adsorption of metal NPs, whereas structurally bound sulfur and nitrogen are known
to alter electronic properties such as conductivity.80 Pyrolysis experiments by
Wohlgemuth et al. at 900 °C resulted in the production of materials with an
almost three times higher specific conductivity than that recorded for the
corresponding undoped carbon material, as well as an increased interlayer
distance of the heteroatom-doped carbon sheets. However, significantly in
terms of application these materials presented rather limited surface area
or porosity.
Structurally bound sulfur-doped ordered mesoporous carbons (denoted as
OMC-S-X) (X = 1, 2 and 3) have also been synthesised for use as metal-free
ORR electrocatalysts (Scheme 7.2).83 In this approach, SBA-15 was used as
a sacrificial hard template to generate mesopores in the resulting carbon
replicas, where sucrose and benzyl disulfide were employed as carbon and
sulfur precursors, with the material’s sulfur content dictated by the sucrose/
benzyl disulfide ratio employed in the synthesis. Characterisation of the synthesis materials revealed two sulfur-bonding motifs: sulfide groups (C–S–C)
and oxidised sulfur groups (C–SOx–C). The authors here proposed that the
two different modes of sulfur incorporation had a large influence over the
resulting ORR activity, with OMC–S providing higher electrocatalytic ability
as compared to the corresponding OMC, demonstrating the necessity for the
sulfur incorporation to enhance the electrocatalytic performance. In addition, it can be clearly seen from the catalytic results, and by considering the
XPS analysis, that the C–S–C plays the conclusive role in promoting the ORR,
in agreement with earlier reports.84–86
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Hydrothermal Carbon Materials for Heterogeneous Catalysis
205
Figure 7.13 Types
of sulfur-containing groups on carbon materials (two different
types: inplane and out-of-plane), sulfone, sulfoxide, sulfonic acid, thiol,
disulfide and sulfide bridges). Reproduced with permission from ref. 81.
Figure 7.14
Biomass-derived
sulfur-containing precursors employed in the synthesis of N/S-doped HTC carbons by Wohlgemuth et al.82
Figure 7.15
SEM
images of solid product after HTC of (a) pure glucose, (b) Glucose
with cysteine; and (c) Glucose with thienyl-cysteine. Reproduced with
permission from ref. 82.
The incorporation of sulfur (e.g. at the surface) of carbon materials is also
of significant interest in the context of thermochemical catalysis. In this
context, Xiao et al. have synthesised carbon-based strong acids through the
one-step hydrothermal carbonisation of furaldehyde and p-toluene sulfonic
acid in aqueous solution (Scheme 7.3).87 The novel carbon-based solid acid
possessed high acidity, and the catalytic activities were investigated by esterification and oxathioketalisation.
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206
Scheme 7.2 Illustration
of the preparation of OMC-S as metal-free catalyst for
ORR. Reproduced with permission from ref. 83.
Scheme 7.3 The
synthetic route of the C–SO3H. Reproduced with permission from
ref. 87.
The catalytic activity of C–SO3H was investigated in two traditional acid-­
catalysed reactions; 1) the esterification of acetic acid and butanol as well
as 2) the oxathioketalisation of cyclohexanone and mercaptoethanol. The
C–SO3H catalyst exhibited remarkably high activities, much higher than that
of the conventional solid acids, including zeolite-HY, Amberlyst-15, and the
carbonaceous materials from only furaldehyde (carbon), and comparable
to the homogeneous sulfuric acid equivalent, importantly demonstrating
acceptable recyclability (Figure 7.16).
7.5
Other Heteroatom-Doped Carbons in Catalysis
As well as N and S, other heteroatoms are of interest in the context of carbon
material doping including B, O, P, and Se. B-doped carbons as a consequence
of size, have reduced interlayer spacing and enhanced stability at high temperature.88 Boron has one electron less than a carbon atom, and it replaces
carbon in localised states below the Fermi level.89 These states are caused by
the presence of holes in the structure, so that carbon could be considered
as a p-type conductor and is more likely to react with donor-type molecules.
While P atoms, though larger than carbon atoms, can also be incorporated
within the carbon nanotube lattice, the phosphorus-­containing groups
behave as an n-type donor, and thereby modify the electronic properties.81,90
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Hydrothermal Carbon Materials for Heterogeneous Catalysis
Figure 7.16
207
Catalytic
activities for esterification (reaction conditions: butanol,
20 mmol; acetic acid, 24 mmol; catalyst, 50 mg, 25 °C, 7 h.) and oxathioketalisation (reaction conditions: cyclohexanone, 20 mmol;
2-mercaptoethanol, 24 mmol; catalyst, 50 mg, 25 °C, 2.5 h). Reproduced with permission from ref. 87.
Dual doping of carbon structures has also been investigated including N–B
doping, N–S doping, and S–B doping.91,92 While N–P doping remains somewhat less well documented and calls for future research.93,94 Guo et al. have
synthesised ordered mesoporous B-doped carbons via coimpregnation and
HTC of sucrose and 4-hydroxyphenylboronic acid precursors after impregnation of a SBA-15 template.91 The catalytic performance of the obtained
material was tested for ORR under alkaline conditions, exhibiting excellent
ORR activity with higher selectivity and better long-term stability than commercial Pt/C catalysts. It is also important to note that for the application of
the heteroatom-doped carbons in catalysis, the interplay between material
functionality and nanostructuration plays an important role in the catalytic
behaviour.68
7.6
HTC-Supported
Metal Complexes or
Nanoparticle-Based Catalysis
Since the HTC approach provides a convenient and potentially energy-saving route to porous carbon synthesis, the preparation of carbon-supported
catalysts has also attracted a lot of interest. The physical and chemical surface properties, together with the intrinsically attractive properties, including
large surface area and tuneable pore-size distribution, make porous carbon
an excellent candidate as catalysts support. Moreover, with careful postprocessing, functional groups can be modified at the porous carbon surface, thus
the performance of porous carbon as catalyst support has the potential to be
improved. Impregnation, adsorption, and deposition precipitation are the
most used preparation methods in this regard. Two different methods can be
employed in the impregnation process: incipient-wetness impregnation and
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208
Chapter 7
excess-solution impregnation. The former method can precisely control the
amount of catalyst precursors by wetting the carbon support in a controlled
drop-wise manner. The latter method allows the deposition of well-­distributed
catalyst precursors on the carbon support. The adsorption method for preparing carbon support catalyst has a close relationship with pore size, pore structure and functional groups on the carbon support. Deposition precipitation
is often conducted in an excess of solution considering the pore volume of
the support. Surface carboxylic acid groups of the porous carbon contribute
significantly to obtaining well-dispersed catalyst through chemical inter-reactions between metal precursors and the carboxylate groups.
The catalytic performance of hydrothermal/solvothermal carbon-­supported
metal complexes (denoted as HTC-MC/STC–MC) has been shown to be strongly
influenced by the physicochemical properties of the support media. Previous
reports have demonstrated the use of HTC as a support for Pt or Pd NPs for
application as electrocatalysts in the oxidation of methanol and ethanol,95,96
and applications particularly relevant in the development of low-temperature,
high power density direct alcohol fuel cells.97–101
Since Pd is 50 times more abundant than Pt, the use of Pd in electrocatalytic processes is of significant interest. Yuan et al. synthesised a novel carbon
material with a coin-like hollow (CHC) structure via a green solvothermal
route without the use of toxic reagents.100 The obtained CHC was prepared
with ethanol as carbon source using Mg/NiCl2 as a carbonisation catalytic
system. The resulting carbon was employed as a support for Pd NPs for application as electrocatalyst in the oxidation of methanol in alkaline media. The
pores and channels in the catalyst layers, formed with the help of micrometresized CHC, were proposed to play an important part in increasing oxidation activity (i.e. 2930 A g−1 compared to 870 A g−1 Pb on Pd/C electrocatalyst
obtained from the CV at scan rate of 5 mV s−1). Similarly, Lv et al. prepared
HTC carbon microspheres using dextrose as carbon precursor.101 Pd NPs
was loaded on to the HTC carbon microspheres through wetness impregnation and the resulting materials investigated as catalysts for the oxidation
of formic acid in 0.5 M H2SO4 electrolyte/0.5 M HCOOH system. This catalyst presented a better catalytic performance than the Pd/CNT and Pd/XC-72
comparisons, proposed to be related to the high dispersion and exposed
active crystal planes of Pd NPs supported on HTC carbon microspheres. Xu
et al. have also reported on the preparation of carbon microspheres based on
sucrose through HTC.102 In this report, H2PtCl4 or PdCl2 was used as noblemetal precursors for the corresponding Pt or Pd NPs, which were loaded
onto the prepared carbon support via the room-temperature chemical reduction using NaBH4. The electrocatalytic performance of these two different
materials was assessed in alkaline media, demonstrating that the prepared
electrocatalysts provided a better performance than the corresponding NPs
supported on the comparison carbon black support. A performance comparison of the prepared electrocatalysts in the oxidation of methanol and
ethanol (Pt or Pd loadings of 0.10 mg cm−2; Table 7.2), demonstrated that
the HTC-supported Pd NPs showed higher activity and better steady-state
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209
Table 7.2 Comparison
of electrochemical performance of methanol and ethanol
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oxidation on Pt/C, Pt/CMS, Pd/C and Pd/CMS electrode with Pt or Pd
loading 0.10 mg cm−2.a
Es(V)
jp (mA cm−2)
Ep(V)
j at −0.3 V (mA
cm−2)
Electrode Methanol Ethanol Methanol Ethanol Methanol Ethanol Methanol Ethanol
Pt/C
Pt/CMS
Pd/C
Pd/CMS
−0.52
−0.51
−0.38
−0.49
−0.51
−0.51
−0.52
−0.58
−0.13
−0.12
−0.08
−0.09
−0.18
−0.18
−0.14
−0.14
39
61
24
50
7
12
27
65
9.4
11.4
0.2
2.5
4.0
6.8
5.6
17.0
a
Reproduced with permission from ref. 102.
Figure 7.17
Schematic
illustration of the synthetic procedure of Pt@C/MC. Reproduced with permission from ref. 103.
electrolysis than Pt for ethanol electro-oxidation in alkaline media, demonstrating the great potential of this material in direct ethanol fuel cells.
Wen et al. have reported on the in situ entrapment of well-distributed
Pt/C nanoparticles within mesoporous carbon via a template route.103 In
this approach, SBA-15 was impregnated with glucose and H2PtCl6, followed
by hydrothermal treatment, to yield Pt NPs encapsulated in hydrothermal
carbon as a core–shell structure. After a secondary thermal carbonisation
step and the removal of the SBA-15 template, core–shell Pt/C NPs within
the nanochannels of mesoporous carbon (Pt@C/MC) with micropores were
formed (Figure 7.17). The linear scanning voltammetry revealed that the
as-synthesised Pt@C/MC catalysts had a high methanol tolerance during
the ORR – an important consideration in the development of direct methanol
fuel cells.
Wang et al. have reported on the synthesis of carbon nanosphere supported
Pt NPs and their application in methanol and ethanol electro-oxidation in
alkaline media.104 Carbon nanospheres were synthesised via a combined
hydrothermal/composite−molten−salt (CMS) approach, with the resulting
materials demonstrating improved catalytic performance as compared to a
conventional carbon standard, proposed to be the result of a combination of
porous structure and the high degree of material carbonisation.
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210
Chapter 7
A hydrothermal carbon supported Pt–Ru NP alloy has also been synthesised using furfural and Pt and Ru acetylacetonate as precursors.105 Via
thermal treatment under different conditions, Pt–Ru/C or Pt–RuO2 was synthesised as electrocatalysts. Both catalysts showed high catalytic activity for
dry methane reforming but low durability. PtRu–C materials with different
Pt : Ru atomic ratios were also investigated, with a Pt : Ru atomic ratio of
50 : 50 generating the best catalytic performance for methanol electro-oxidation.106 Tusi et al. prepared PtRu/C materials through hydrothermal carbonisation using starch as the carbon source and reducing agent.107 By adjusting
the pH using tetrapropylammonium chloride, potassium hydroxide or tetrapropylammonium hydroxide, materials with different particle size and
pore structure were prepared. From CV of PtRu–carbon hybrid materials in
0.5 mol L−1 H2SO4, the materials with mesopores and small particle size were
found to have better electrocatalytic performance.
Wang et al. have synthesised carbon-riveted PtPu/C catalysts via a combined microwave-assisted polyol and HTC method.108 The durability of carbon-reverted PtRu/C and mass activity was higher than observed for RtRu/
Vulcan XC, which was proposed to be related to the high surface ratio of Pt
and Ru and the anchoring effect of the carbon nanolayer formed during the
in situ HTC of glucose.
Ni/C hybrids have been prepared as supports for PtRu NPs Electrocatalysts
via a combined HTC and thermal carbonisation approach.109 The incorporation
of Ni into the carbon material network was proposed to improve the electroconductivity/graphitisation of the support and in turn improve the activity of
the catalyst in methanol electro-oxidation as a result of a higher open-circuit
voltage and maximum power density than the PtRu/Vulcan XC72 comparison.
Besides Ru NPs, CeO2 was also introduced to the Pt/C system to investigate the catalytic activity for methanol oxidation. Zhao et al. have synthesised
hollow carbon (HCS) spheres using silica particles as the hard template via
HTC.110 The materials were used as supports for PtRu/HCS or Pt/CeO2/HCS
catalysts using a wet impregnation method. The activities of the catalysts
were measured by CV and CO stripping voltammetry. The CV measurement
was performed using a three−electrode cell in a 0.5 M H2SO4 solution containing 1.0 M methanol with the potential cycled between 0 and 1.0 V and
a scan rate of 50 mV s−1. The maximum peak current density for methanol
oxidation was found for the 20Pt/35CeO2/HCSs catalyst and performed much
better than a range of commercial carbon-based catalysts (Figure 7.18).
Except for acting as methanol or ethanol electro-oxidation catalysts, carbon-supported metal NPs have also found application in water electrolysis
(e.g. tin-doped carbon hollow spheres111). Hollow carbon spheres were synthesised by treating glucose under hydrothermal and intermittent microwave conditions in the presence of sacrificial polystyrene templates. During
tin loading over the carbon spheres, rutile SnO2 was formed, considered the
favoured electrocatalytic phase for hydrogen and oxygen evolution during
water electrolysis under acidic conditions, with a 30 wt% Sn loading providing the best electrocatalytic activity.
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Hydrothermal Carbon Materials for Heterogeneous Catalysis
Figure 7.18
211
(a)
Cyclic voltammograms of methanol electro-oxidation for Pt/CeO2/
hollow carbon spheres; and (b) Cyclic voltammograms for 20Pt/
35CeO2/hollow carbon spheres and a range of other electrocatalysts.
Reproduced with permission from ref. 110.
As mentioned above, it is important to note that the form of the porous
carbon support plays an important part in an electrocatalytic process. A large
surface area, adequate pore size, large pore volume, well-dispersed metal
species on the hydrothermal carbon, and the interaction force between the
metal and hydrothermal carbon surface are the predominant factors influencing catalytic performance. More detailed discussion regarding the development of HTC-based electrocatalysts will be provided in Chapter 8.
7.7
HTC in Photocatalysis
TiO2 is an inexpensive, nontoxic, and useful photocatalyst and as such has
been extensively studied and applied in the degradation of organic pollutants, air purification, water splitting, and as a demister.112–114 However, the
application of pure TiO2 is rather limited as a consequence of its bandgap of
3.2 eV, which corresponds to a small UV fraction of solar light. Theoretically,
the main processes in semiconductor photocatalysis proceed via photon
adsorption and electron–hole generation, charge separation and migration, to surface reaction sites or recombination sites, and surface chemical reaction at active sites. Therefore, over the past few years, considerable
effort has been applied to improve the photocatalytic efficiency of TiO2 in
the visible-light region.115–117 Numerous reports on TiO2 doped with B,114 F,118
N,119 C,120 S,121 or I,122 have demonstrated a significant improvement of the
visible-light photocatalytic efficiency. In this context, given the convenient
synthesis of porous functional carbon materials through hydrothermal or
solvothermal methods discussed throughout this book, including the capability to introduce heteroatom dopants during synthesis, a series of C-doped
photocatalysts have been reported.
Zhong et al. have synthesised carbon-deposited TiO2 NPs through a one−
pot hydrothermal method.123 TiCl4 was added to the glucose solution with
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Chapter 7
vigorous stirring, followed by hydrothermal treatment, to produce carbon-deposited TiO2. A series of materials were synthesised at different glucose solution concentrations and their activity investigated in the photocatalysed
decomposition of aqueous solutions of 2, 4-dichlorophenol (DCP) and acid
orange 7 (AO7). The photocatalyst with the highest photocatalytic activity for
the degradation of AO7 was G15-TiO2@C, while that for the degradation of
DCP was G5-TiO2@C. Two kinds of sensitisation processes, carbon sensitisation and dye sensitisation, are responsible for the visible-light-induced photocatalysis of TiO2@C. Carbon sensitisation reached its optimal condition in
G5, while dye sensitisation occurred in its maximum efficiency in G15. Later,
Zhao et al. reported on the synthesis of C-doped TiO2 photocatalyst through a
one-step solvothermal method.124 By dissolving furfural and titanium(iv) isopropoxide in ethanol, followed by solvothermal treatment and calcination,
C@TiO2 material was prepared. The product had a high surface area and
anatase crystalline phase that contributed a lot to absorbing a high amount
of photoenergy. Moreover, the coupling of the inorganic and organic components as a “dyade” was proposed to generate the special properties of the
structurally entangled hybrid (Figure 7.19). The C@TiO2 materials showed
higher visible-light adsorption and better methyl orange (MO) degradation
activity than pure TiO2 and other materials (i.e. P25 or N–P25).
A method of synthesising C-modified TiO2 nanotube arrays was developed by Yang et al.125 TiO2 nanotubes were obtained by anodising Ti foils,
which were then hydrothermally treated in the presence of glucose. After
further thermal treatment, C-modified TiO2 nanotubes were obtained. Since
the materials had improved electron and hole separation properties, the
obtained nanotube arrays showed enhanced photocatalytic activity under
visible light as compared to pure TiO2.
Raw biomass has also been employed as a carbon source in the synthesis
of photocatalysts. Zhang et al. used raw rice as a carbon source and tetrabutyl titanate as the Ti source to synthesise carbon-modified mesoporous
TiO2 though a supercritical ethanol/solvothermal method.126 The carbon
was doped in the TiO2 lattice or at the surface of the TiO2. The bimodal carbon modification played an important role in improving the catalytic performance. Phenolic resin has also been employed as a carbon source in the
synthesis of graphite-like surface modified TiO2 photocatalyst through a
hydrothermal process (Figure 7.20).127 Since the phenolic resin is adverse to
photocatalysis, the obtained materials were washed with THF to remove the
soluble phenolic resin to enhance the photocatalytic performance which gave
a MO degradation rate of 100% in 4 h. Additionally, carbon-modified TiO2
composite materials were synthesised hydrothermally followed by pyrolytic
treatment to induce an enhanced photocatalytic water-splitting activity.128
According to the authors, mechanistically methanol was oxidised in two ways
by the photogenerated holes involving direct oxidation and the formation of
hydroxyl radicals. In this example, the carbon facilitated the electron transfer
and minimised the electron−hole recombination, thus the production of H2
was increased.
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Hydrothermal Carbon Materials for Heterogeneous Catalysis
Figure 7.19
213
(A)
The photocatalytic mechanism and (B) photocatalytic degradation
of methyl orange under visible light (at λ > 420 nm) over a C@TiO2
dyade hybrid material reported by Zhao et al. Reproduced with permission from ref. 124.
Since exposed {011} facets of TiO2 facilitate the photocatalytic process, different methods have been adopted to enhance this crystalline feature. Carbon and lanthanum codoped TiO2 nanocrystals with exposed {001} facets
were synthesised though a one-step hydrothermal method to produce catalyst with high UV and visible light activity.129 Glucose was used as the crystal
growth directing agent and carbon source (Figure 7.21). Although it is a simple mixing and hydrothermal treatment, materials that had smaller crystal
particles and better photocatalytic activity than pure TiO2 were obtained.
A TiO2-based photocatalyst with exposed {001} facets was obtained by mixing glucose and Ti(OBu)4 in the presence of HF (aq) with vigorous stirring
and hydrothermal treatment.130 As well as being the carbon source, glucose
also acted to aid the assembly of TiO2 together with HF, contributing to an
improvement in photocatalytic activity.
The fabrication of hollow structures is expected not only to enhance diffusion during the photocatalytic process, but is also expected to improve
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Chapter 7
Figure 7.20
Synthetic
process and radial distance-dependent organic covering of
PF/TiO2. Reproduced with permission from ref. 127.
Figure 7.21
Glucose-mediated
and -induced transformation pathway for the fabrication of TiO2 nanocrystals with {011} facets. Reproduced with permission from ref. 129.
active-site accessibility to the reactants, resulting in photocatalytic performance improvements. Thus, hierarchically porous hybrid C–TiO2–C composites of a hollow sphere macromorphology, were studied as photocatalyst in
the visible-light photo-oxidation of rhodamine B (RhB).131 In the presence of
dodecylamine, the carbon source furfural and Ti-alkoxide assembled on the
self-conglobated template during the condensation and hydrolysis process,
after calcination, hollow-sphere photocatalysts with enhanced performance
were obtained. Using SiO2 as the hard template, hollow TiO2@C composite
spheres with mesoporous structure were synthesised.132 Since H2O served
as the etching agent in the high-pressure environment, the SiO2 dissolved
during the hydrothermal process, which avoided the harsh condition for
removing SiO2 with HF (aq) (Figure 7.22). The as-prepared material had
enhanced photocatalytic activity in rhodamine B degradation.
Core–shell nanofibres of TiO2@C featuring well-dispersed Ag NPs in the
carbon layer have been reported.133 The TiO2 nanofibres were first synthesised, then dispersed in glucose solution followed by a 4 h hydrothermal
treatment step, leading to the C-doped TiO2. After activation with SnCl2, the
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Hydrothermal Carbon Materials for Heterogeneous Catalysis
215
Figure 7.22
Schematic
illustration of synthetic procedure used for fabrication of
TiO2@C hollow composite structure. Reproduced with permission
from ref. 132.
Figure 7.23
Possible
mechanism of the visible-light-induced photodegradation of RB
with TiO2@C/Ag nanofibres. Reproduced with permission from ref. 133.
C-doped TiO2 was added to a Ag(NH3)2OH solution with vigorous stirring
to produce the Ag NPs and carbon-codoped TiO2 nanofibres. The resulting
materials had better photocatalytic performance under visible light than
pure TiO2 in MO degradation. In this report, it was postulated that the Ag
NPs trapped electrons on the conduction band and inhibited recombination
of electron–hole pairs (Figure 7.23). Due to the one-dimensional structure
property of this material, it is easily recycled with little decrease of photocatalytic performance which would promote their industrial application.
As discussed before, the bandgap can be narrowed by doping with nonmetal elements, (e.g. N), which can create a midgap state acting as an electron donor or acceptor in the bandgap of TiO2. In such a system, the bandgap
is lowered and the optical adsorption region shifts into the visible light. In
this context the synthesis of N-doped and carbon-modified TiO2 was conducted by adding TiCl4 into an l-lysine aqueous solution in an ice bath with
vigorous stirring followed by hydrothermal treatment.134 Here, l-lysine acted
as a ligand to control nanocrystal growth and as a source of nitrogen and
carbon. The N-doped lattice resulted in a narrower bandgap, whilst carbon
species at the surface of the photocatalyst enhanced the visible light harvesting and the separation of protons and electrons, resulting in an improved
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216
photocatalytic activity (Scheme 7.4). Similarly, chitosan has been employed
as carbon and nitrogen source, leading to the synthesis of N/C-codoped TiO2
nanostructures prepared through a solvothermal method.135
Other hydrothermal carbon-based approaches to metal-oxide photocatalysts have also been reported to show good photocatalytic properties. Carbon-supported CuO–BiVO4 was obtained by a hydrothermal process and
impregnation synthesis.136 Carbon spheres were synthesised hydrothermally
from sucrose, following deposition of the BiVO4 and CuO components via an
in situ impregnation process. The final photocatalyst was obtained with calcination. Since the synergistic effect of CuO–BiVO4 heterojunction and carbon
spheres, the photocatalyst showed better catalytic performance than pure
BiVO4. The carbon spheres here were proposed to hinder crystal growth and
acted as a photosensitiser to transfer electrons to CuO–BiVO4 heterojunction
that narrows the bandgap of BiVO4 and suppresses electron−hole recombination (Scheme 7.5).
Scheme 7.4 The
proposed photocatalytic mechanism over the N–TiO2/C nanocomposites. Reproduced with permission from ref. 134.
Scheme 7.5 A
proposed visible-light photodegradation mechanism of organic
compounds over CuO-BVO@C photocatalyst. Reproduced with permission from ref. 136.
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Hydrothermal Carbon Materials for Heterogeneous Catalysis
217
Carbon-coated SnO2 photocatalysts have also been reported, synthesised
via a microwave hydrothermal method using sucrose as a carbon source.137
The hydroxyl radicals were found to be the main active species in the oxidation of RhB. Since the carbon layer increased the adsorption capacity of
organic dyes and hindered the recombination of photogenerated charges,
the photocatalyst showed enhanced catalytic performance.
In this section, carbon-supported photocatalysts were introduced. From
the result discussed above, the surface area and pore structure of the photocatalysts affected the photocatalysis process by providing accessible active
sites and diffusion pathways for organic pollutants through which to proceed. By doping with heteroatoms, the TiO2 gap can be altered to improve
photocatalytic efficiency. Moreover, other kinds of metal oxide also exhibited
good photocatalytic activity.
7.8
Other Catalysis
Since the noble metals at the nanoscale have become the subject of intense
interest in various fields as a result of their outstanding properties, these
metals had been widely used as catalysts in, for example, hydrogenation, oxidation, and crosscoupling reactions.138–142 Functional hydrothermal carbon
with high surface areas and large pore volumes can act as a good support for
noble-metal catalysts.
In this regard, Makowski et al. employed furfural as a carbon source and
reducing agent for reduction of Pd(acac)2 to produce a Pd@HTC material.143
The Pd salt was effectively reduced by furfural to yield in situ formed Pd NPs
in the early stages of particle formation, such that the Pd NPs sit in the centre of the forming HTC carbon sphere (Figure 7.24). The resulting materials
showed relatively high selectivity for the hydrogenation of phenol to cyclohexanone (Table 7.3).
Structure and morphology controllable Ag/C nanocables and carbon hollow spheres encapsulating Ag NPs in their cores were synthesised through
Figure 7.24
(a)
SEM image and (b) TEM image of Pd@hydrophilic-C. Reproduced
with permission from ref. 143.
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218
Table 7.3 Catalytic
activity of differently supported Pd for the hydrogenation of
phenol.a,b
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Selectivity (%)
Catalyst
Time/h
Conversion (%)
Cyclohexanol
Cyclohexanone
Pd@hydrophilic-C
Pd@hydrophilic-C
Pd@hydrophilic-C
Pd@hydrophilic-Cc
10% Pd@C
10% Pd@C
10% Pd@Al2o3
10
20
72
20
20
1
20
60
>99
>99
45
100
100
100
—
5
50
30
100
100
100
>99
95
50
70
0
0
0
a
Reproduced with permission from ref. 143.
Ina typical reaction, 50 mg of catalyst were added to 100 mg of phenol and the mixture was
heated to 100 °C under 1 MPa of hydrogen pressure.
c
Reference test in cyclohexane.
b
a hydrothermal process.144 In the experiment, imidazolium ionic liquid was
used as a soft-template to prepare the hybrid. By altering the concentration
of the soft template, the synthesis of Ag/C hybrids was controllable. Meanwhile, the obtained catalysis showed excellent performance in the oxidation
of 1-butanol by H2O2. Yu et al. have reported on the synthesis of FexOy@C
spheres via a one-pot hydrothermal co-hydrolysis–carbonisation approach
using glucose and iron nitrate as precursors.145 The HTC of glucose in this
context is accelerated by the iron nitrate catalyst, so the hydrothermal process was conducted at the low temperature of 80 °C. FexOy@C spheres were
obtained through a layer-by-layer growth mode. The obtained catalysts have
been employed in Fischer−Tropsch synthesis.
Hybrid NiAl-layered double hydroxide/carbon (LDH/C) composites were
successfully assembled by crystallisation of the LDH in combination with
the HTC of glucose.146 The resulting materials have excellent catalytic performance for the growth of multiwalled CNTs. By adjusting the carbon content
of the catalysts, the structural ordering of the resulting CNTs can be tuned.
Moreover, the method can be applied to a series of systems for controllable
assembly of LDH/C composites.
7.9
Conclusions
In summary, functional carbonaceous materials as well as carbon/metal or
metal-oxide hybrids prepared via HTC have been discussed. To meet the
needs of different topical applications in heterogeneous catalysis, the HTC
carbon could be modified with heteroatoms or metal/metal oxides, whilst
the relationship between the surface- or bulk-doped functionalities and the
performance in different catalytic reactions were carefully discussed. The
different experimental conditions, carbon precursors, the unique morphologies, structure and chemical functionalities of the resulting carbonaceous
materials have also been introduced.
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Hydrothermal Carbon Materials for Heterogeneous Catalysis
219
Heteroatom-doped HTC materials (especially nitrogen- and sulfur-doped
carbons) with special porous structure, are of particular interest, confer special
electronic properties (as will be discussed further in Chapter 8), as well as
an acid or base character to the materials, which makes them applicable in
typical acid- or base-catalysed reaction as well as in electrocatalysis. The HTC
carbon materials can also be used as supports for metal or metal-oxide NPs
and also in the preparation of HTC nanocomposites or hybrids to meet the
needs of applications including electrocatalysis, photocatalysis and a variety
of metal-catalysed reactions. It was also found that the properties of the HTC
materials (surface area, porous structure, and pore volume), the functionali­
ties, dispersion of metal species on the hydrothermal carbon, as well as the
interaction force between the metal and hydrothermal carbon are the main
factors that affect the catalytic performance.
Although the HTC-based materials have been successfully used in many
heterogeneous reactions, there are still some questions and challenges,
such as: how to use real biomass waste and alternative metal salts (more
abundant, e.g., Fe) as precursors to synthesis functionalised HTC for heterogeneous catalysis? Can we produce the HTC-based catalysts at large scale?
How can we make even effective catalysis in green and mild conditions (even
shorter reaction time, lower temperature, etc.)? It is the authors’ opinion
that the flexibility offered by the HTC synthesis platform has a bright future
and wide remit in the field of heterogeneous catalysis and with innovative
thinking, the aforementioned problems and challenges in time will be duly
addressed.
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CHAPTER 8
HTC-Derived Materials in
Energy and Sequestration
Applications
REZAN DEMIR-CAKAN*a AND MARTA SEVILLA*b
a
Gebze Technical University, Block R, 203, P.K: 141, 41400 Gebze-Kocaeli,
Turkey; bInstituto Nacional del Carbón (CSIC), C/Fco. Pintado Fe, 26 Oviedo –
33011, Spain
*E-mail: demir-cakan@gyte.edu.tr; martasev@incar.csic.es
8.1
Introduction – Energy Storage
We are living in a society that is almost fully dependent on fossil fuels. Our
addiction to fossil fuels caused a rapid societal development but the consequences on the environment in terms of CO2 emissions and global warming
are devastating, which is expected to result in a 4 to 6 °C temperature raise by
2100.1 Moreover, it is estimated that oil reserves will be used within the next
40 years, while coal and natural gas may last at most for another 150 years.
Therefore, novel solutions are required to allow the exploitation of renewable energy sources (e.g. wind, hydro or solar) in the most efficient manner
without causing any further ecological disasters. However, as most renewable energy sources are discontinuous, energy storage is of upmost importance for current and future societal needs.2 In this regard, technologies that
can stimulate economic growth and CO2 emission-free transportation modes
(e.g. replacing internal combustion engines with electric traction) should be
highlighted. The fundamental principles of energy storage and conversion
dealing with photovoltaic, batteries, supercapacitors or fuel cells, are well
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Porous Carbon Materials from Sustainable Precursors
Edited by Robin J White
© The Royal Society of Chemistry 2015
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known and understood. Now the challenge is to find means to convert and
store energy on a large scale in a sustainable way with high efficiency and low
cost. While the search for high-performance energy storage and conversion
devices remains the main target, the cost of these devices, which is dependent on material abundance and production processes, is becoming another
important factor. Sustainability, renewability, and green chemistry must be
taken into consideration when selecting electrode materials for the next generation of conversion or storage devices, especially when high-volume applications (e.g. automotive vehicles or grid storage) are considered. It becomes
increasingly clear that meeting this challenge calls for innovations at the
materials levels since these technologies are intrinsically limited by material availability. As a whole, new compounds must continue to be developed
and done so in the context of Green and Sustainable Chemistry principles,
relying on environmentally friendly/benign chemicals and processing. This
chapter will provide an overview on the use of hydrothermal carbonisation
(HTC) as a green synthesis technique for the production of novel materials
for energy storage-related applications, including their use as electrodes in
supercapacitors, fuel cells or rechargeable batteries.
8.2
Electrodes in Supercapacitors
Electrochemical double-layer capacitors (EDLCs), also known as supercapacitors, rely on charge separation at electrode/electrolyte interfaces to store
energy. The reduction of the charge-separation distance to dimensions similar to those of the ions within the electrolyte (i.e. <3 nm), coupled with the
large surface area of the porous electrodes, makes them capable of storing
much larger amounts of energy (ca. 3–5 Wh kg−1) compared to conventional
capacitors. Significantly, the electrostatic nature of the storage process gives
rise to higher power densities, excellent reversibility and longer cycle lifetimes as compared to batteries. Another kind of electrochemical capacitor or supercapacitor, is the pseudocapacitor. In this case, energy is stored
through fast and reversible Faradaic charge-transfer reactions between the
electrolyte and electroactive species on the electrode surface. Reactions
that result in Faradaic charge transfer are mainly of the redox type where
changes in material oxidation state occur – analogous to battery systems.
The amount of energy stored is therefore higher than in EDLCs (i.e. 5–30 Wh
kg−1), although cycling performance and power density is normally lower.
Owing to these energy–power characteristics of supercapacitors, they occupy
the niche between conventional capacitors and batteries, as demonstrated
in the Ragone plot (specific power vs. specific energy; Figure 8.1). As a consequence, supercapacitors are ideal devices for short-term power applications,
including uninterruptible power supply systems (e.g. back-up supplies to
protect against power disruption), regenerative storage, load levelling, quickcharge applications or start-ups.
The use of high-capacitance materials (high surface area or highly electroactive species) is a key factor to ensure high energy density in such devices.
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Currently, research efforts are focused on the design of electrode materials
with tailored pore-size distributions, as well as tailor-made electrolytes, with
the aim to increase energy density without compromising power capability
so as to fulfil the requirements of more demanding emergent applications.
The utilisation of electroactive species (e.g. metal oxides – MnO2, NiO2, V2O5,
etc.) is also receiving increasing attention as a route to further increasing
energy stored. Additionally, new configurations are being considered so that
the next generation of supercapacitors, will be known as “hybrid supercapacitors”, where one of the electrodes is an EDLC-type electrode and the other is
a pseudocapacitance-type electrode or a battery-type electrode.
Porous carbon materials are considered the main candidate for supercapacitors as a result of cost, availability, typically large surface areas, versatility
with regards to porosity development/surface chemistry, good conductivity
and a lack of negative environmental impact. They behave mainly as EDLCs
with their large surface area providing high capacitance values. Nevertheless, many carbons (e.g. activated carbons) possess surface functional
Figure 8.1
Ragone
plot for various electrical energy storage systems. Times shown
are the time constants of the devices, obtained by dividing the energy
density by the power. Reprinted by permission from ref. 3 © 2008, Macmillan Publishers Ltd: Nature Materials.
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Chapter 8
groups, which give rise to an additional pseudocapacitance contribution.
N- and O-containing functionalities are known to give rise to faradaic redox
reactions, increasing the capacitance values and thus the energy density of
the supercapacitor.4–8 An additional advantage of the presence of O and/or
N-containing moieties is that they improve electrode wettability and in the
case of N, material electroconductivity may also improve. Therefore, the
common methods for increasing capacitance of carbon electrode materials
focus on the preparation of high surface area carbons with appropriate pore
size and surface functionalities.
HTC carbons are O-rich by nature, with the O content being tuned through
the operating conditions (e.g. temperature, time, precursor and concentration).9–11 Furthermore, via appropriate selection of the initial HTC carbon
precursor, operating conditions or additional additives, other functionalities
can be introduced in the final carbonaceous material. Thus, as mentioned
throughout this book, a variety of N-rich carbons have been synthesised by
performing the HTC in the presence of ovalbumin,12 or N-containing (poly)
ionic liquids,13 N-containing organic compounds,14 and by using N-containing
carbohydrates,15–17 or nitrogen-rich biomass.18,19 Likewise, S-doped carbons
have also been produced via the HTC process by adding S-containing amino
acids to the synthesis.20 As mentioned in this book, HTC carbons generally
possess limited open porosity, with heat treatment at elevated temperatures
leading only to a moderate increase in surface area,12,20,21 hampering application in surface-area-sensitive applications (e.g. supercapacitors). However,
as described below, this limitation has been recently circumvented via the
chemical activation of HTC carbons.
Wei et al. were the first to explore the supercapacitor performance of HTCbased activated carbons (AC).22 These activated carbons were prepared by
chemical activation with KOH (KOH/sample weight ratio = 4 and T = 700–800 °C)
of hydrothermally carbonised cellulose (C), starch (S) and sawdust (W). The
synthesised porous carbons possessed SBETs in the 2100–3000 m2 g−1 range
and pore volumes (Vp) ≥ 1.4 cm3 g−1. These carbons had a relatively narrow PSD in the supermicropore range, with virtually no pores of diameters
>3 nm. The performance of these materials in an organic electrolyte (1 M
TEABF4 in acetonitrile (AN)) was spectacular, recording the highest capacitance ever reported for porous carbons in a symmetric two-electrode configuration using this electrolyte; i.e. 236 F g−1 (100 F cm−3) at 1 mV s−1 (Figure
8.2(a)). AC-W, activated at 800 °C, exceeded the specific capacitance of commercial activated carbons optimised for EDLC applications, (e.g. YP-17D), by
100%. Furthermore, these HTC carbon-derived ACs were capable of retaining
64–85% of the capacitance as current density was increased from 0.6 to 20 A
g−1 (Figure 8.2(b)). Activation at 800 °C produced the largest volume of small
mesopores in the range 2–3 nm, resulting in improved capacitance retention
at high sweep rates in the CV measurements or high current densities in
the charging/discharging tests (Figure 8.2(b)). The small reduction-oxidation
peaks visible in the CV at around 0 and 2 V at the slowest sweep rate are
believed to originate from oxygen-containing functional groups remaining
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Figure 8.2
229
Electrochemical
characterisation of activated carbons derived from
hydrothermally synthesised carbon materials in 1 M TEABF4 solution
in acetonitrile at room temperature: (a) cyclic voltammograms (CV)
of the activated carbon obtained from sawdust at 800 °C with KOH/
sample = 4 (AC-W800) and (b) capacitance retention with current density in comparison with that of commercially available YP-17D activated
carbon. Reproduced with permission from ref. 22 © 2011, Wiley-VCH
Verlag GmbH & Co. KGaA, Weinheim.
in the carbon samples (Figure 8.2(a)). These peaks completely disappear at a
rate of 10 mV s−1, suggesting relatively slow redox reaction kinetics and that
pure EDLC capacitance (without pseudocapacitance contribution) in these
materials exceeds 193 F g−1. This combination of very high specific and volumetric capacitance and good rate capability of the HTC carbon-derived ACs
is unmatched by state-of-the-art activated carbons and other nanocarbons
(e.g. nanotubes, onions and graphene).
Wang et al. have also applied a chemical-activation process to hydrothermal
carbons, in this case using phosphoric acid as activating agent and rice husk
as carbon precursor.23 The activation temperature was varied between 300
and 700 °C and the weight ratio of phosphoric acid to HTC carbon between 1
and 6. In this way, the obtained porous materials ranged from supermicroporous to mesoporous, with SBET in the 700–2700 m2 g−1 range and Vp ≥ 2 cm3 g−1.
Although no PSDs were presented in this report, based on the shape of the
respective isotherms, it can be envisaged that they are broader than those of
the ACs obtained by Wei et al. It should be noted that several authors have
pointed out that KOH allows the development of narrower PSDs in comparison with other activating agents.24–26 The specific capacitance of these ACs
reached 130 F g−1 (measured at 2 mV s−1 in a three-electrode cell configuration) in 6 M KOH (aq), a value quite below that measured ACs with similar SBET
but with narrower PSD in the micro–supermicropore range (235–286 F g−1).27
This is probably due to the fact that, as shown by Chmiola et al.,28 a good
match between the electrode pore size and the dimensions of the electrolyte
ions is critical for an optimal performance of supercapacitors. Falco et al.29
and Wang et al.30 have, respectively, shown the successful exploitation of
byproducts from ethanol production and paper manufacturing as precursors
for the synthesis of high-performance electrode materials. In both cases,
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Chapter 8
chemical activation with KOH was employed, generating SBET in the 2200–
2300 m2 g−1 range for spruce and corncob hydrolysis products, and ≥3000 m2
g−1 for paper pulp mill sludge. In this way, Falco et al. achieved capacitances
in the 270–300 F g−1 range in 0.5 M H2SO4 (aq) (measured at 0.25 A g−1, symmetric two-electrode cell), whereas Wang et al. recorded a maximum capacitance of 162 F g−1 (0.1 A g−1, symmetric two-electrode cell) in 1.5 TEABF4/AN,
and 163 F g–1in ionic liquids (0.1 A g−1, symmetric two-electrode cell).
Other authors have opted for somewhat more exotic HTC precursors
including pollen,31 fungi32 and hemp.33 The KOH-activation of HTC carbons derived from various pollens led to highly mesoporous materials with
ultralarge SBET, in the 2700–3000 m2 g−1 range, whereas KOH-activation of
hemp fibre-derived material resulted in highly interconnected carbon
nanosheets (thickness < 100 nm; Figure 8.3(a)) with SBET in the 1500–2300
m2 g−1 range and a mesopores content of 40–60%. The lotus pollen-derived
carbon had a high capacitance of 185 F g−1 in 1 M TEABF4/AN and 207 F
g−1 in neat 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIMBF4) electrolytes at a current density of 1 A g−1 (symmetric two-electrode cell). The
corresponding gravimetric energy densities are as high as 46 Wh kg−1 in
organic electrolyte and 88 Wh kg−1 in ionic liquid electrolyte, outperforming
the commercial AC RP20. Hemp fibre-derived carbon nanosheets showed
impressive rate performance in the ionic liquid electrolyte 1-butyl-1-methyl­
pyrrolidinium bis-(trifluoromethylsulfony)imide (BMPY TFSI), with the
cyclic voltammograms of a quasirectangular form at a high scan rate of 500
mV s−1, demonstrating excellent ion transport behaviour even in the viscous
ionic liquid (Figure 8.3(b)). At 20 °C and 100 A g−1, materials activated at 750
and 800 °C retained >70% of their capacitance at 1 A g−1 (Figure 8.3(c)). This
impressive capacitance retention is ascribed to the high mesopore volume and
nanoscale diffusion distances normal to the nanosheets thickness, allowing
rapid ion transport.
Figure 8.3
(a)
High-resolution TEM micrograph highlighting the porous and partially ordered structure of the hemp-derived carbon nanosheets prepared at 800 °C (CNS-800), (b) CV curves of CNS-800 for three different
scan rates (tested at 20 °C) and (c) Capacitance retention in charge/
discharge experiments (tested at 20 °C) for the carbon nanosheets,
baseline commercial activated carbon (AC) and baseline graphene
nanoplatelets (CG). Reproduced with permission from ref. 33. © 2013,
American Chemical Society.
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N-doped HTC carbons have also been used as electrodes in supercapacitors after a chemical activation step. Zhao et al. activated a D-glucosaminederived HTC carbon with KOH, using a weight ratio of KOH to carbon = 1–4,
and T = 600 °C.34 Although KOH was used, a high degree of activation was not
achieved (SBET < 600 m2 g−1), possibly the result of the low activation temperature employed (i.e. 600 °C) in order to preserve a high N-dopant content in carbon product. Indeed, N-containing functionalities are preferentially oxidised
during the activation process.35–37 In spite of this, these materials exhibit
excellent electrochemical performance in 6 M KOH (aq) and 1 M H2SO4 (aq),
achieving specific capacitances of 220 and 300 F g−1 at a current density of
0.1 A g−1 in basic and acidic electrolytes, respectively, (in a three-electrode cell
system). This superior capacitance is due to the combination of EDLC capacitance and pseudocapacitance arising from redox reactions of N-containing
functionalities, as proved by CV. Additionally, good capacitance retention at
high current density (4 A g−1) was observed, demonstrating good conductivity
and quick charge propagation in both acid and base electrolytes.
Even though the heat treatment at elevated temperatures of HTC carbons
leads to only a moderate increase in surface area, as mentioned before, this
option was adopted by Xia et al.38 In this report, HTC carbons obtained from
β-cyclodextrin, sucrose and glucose precursors were pyrolysed under Ar at
900 °C, generating SBET in the 400–500 m2 g−1 range and average micropore
sizes between 1.9 and 2.1 nm. The electrochemical analysis of these HTC carbon spherules was performed in 30 wt% KOH (aq) electrolyte, recording specific capacitances > 162 F g−1 at 1 mA cm−2 (symmetric two-electrode cell) and
good capacitance retention, in the 63–82% range when the current density
was increased to 20 mA cm−2. The specific capacitance values are remarkable
taking into account the SBET of the spherules (specific capacitance per surface
area – 0.32–0.38 F m−2). The nonrectangular shape of the voltammograms,
as well as the nonconstant slope of the charge/discharge cycles, indicated a
pseudocapacitance contribution, which would explain such high values of
specific capacitance per surface area.
As mentioned earlier, the use of metal oxides may lead to pseudocapacitance, increasing thereby the energy stored in supercapacitors. However,
metal oxides normally exhibit limited conductivity, which is detrimental for
the power performance of the devices.39 Additionally, cycling performance
is also normally worse than for EDLCs. One investigated route to overcome
these limitations, has been the synthesis of metal-oxide–carbon composites,
where the carbon component acts to stabilise the metal oxide and provide the
necessary electronic conductivity.3,40,41 Zhang et al. prepared HTC carbon@
MnO2 rattle-type hollow spheres under mild conditions.42 The as-prepared
materials showed a mesoporous MnO2 shell and a carbonaceous sphere core,
with the composition and shell thickness controllable experimentally. The
capacitive performance of these rattle-type hollow spheres was evaluated
using both CV and charge/discharge methods in a 0.5 M Na2SO4 electrolyte
in a three-electrode cell system. A specific capacitance as high as 184 F g−1
at a current density of 0.125 A g−1 was reported, with the good capacitive
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performance a result of the mesoporous structure and high surface area of
the MnO2-based hollow spheres.
Liu et al. coated Fe3O4 nanorods with carbon using the HTC process.43 As
a comparison, they also prepared bare Fe3O4 nanorods. The electrochemical
properties as measured by CV, galvanostatic charge–discharge cycling and
electrochemical impedance spectroscopy in a three-electrode cell configuration, demonstrated that HTC carbon-coated Fe3O4 nanorods presented
improved electrochemical performance as a result of the carbon layer. A specific capacitance of 276 F g−1 is achieved at a current density of 0.5 A g−1 in
1 M Na2SO3 (aq) for the carbon-coated Fe3O4 nanorods in comparison to that
of 209 F g−1 for bare Fe3O4 nanorods. Furthermore, after 500 cycles, 81% of
the specific capacitance remains for the carbon-coated Fe3O4 nanorods, in
comparison to 73% for bare Fe3O4 nanorods. In this regard the carbon layer
acts to (i) improve electronic conductivity of the network, (ii) suppress inner
nanocrystal aggregation and preserve the nanocharacteristics of the active
material during cycling and (iii) buffer the volume change during the charge/
discharge process.
HTC spheres have also been used as sacrificial templates for the synthesis
of hollow spheres of Co3O4, with resulting materials possessing good supercapacitor performance.44 The thickness of the hollow Co3O4 sphere shell
was ca. 130 nm and composed of numerous small nanocrystals. It should
be noted that even though HTC was performed at 140 °C, carbonaceous
microspheres were formed, as a result of catalytic nature of cobalt ions in the
hydrothermal carbonisation of sugars. The hollow Co3O4 spheres exhibited
excellent cycling performance and good rate capacity when used as supercapacitor electrodes, attributed to the small particle size and the sufficient
space available to interact with the electrolyte.
The results of the studies described above demonstrates the versatility of the
HTC process for the synthesis of high-performance electrodes for supercapacitors, such that (i) facile N-doping can be achieved using N-containing precursor
or N-containing additives, (ii) exploitation of byproducts of industrial processes
is possible, (iii) highly porous materials with tuneable pore sizes can be synthesised via postsynthesis activation, (iv) composites with pseudocapacitive
materials can be easily synthesised and (v) metal-oxide nanostructures can be
obtained by using hydrothermal carbons as sacrificial template.
8.3
Electrocatalysts in Fuel Cells
Fuel cells (FC) are extremely promising alternative sustainable electrochemical energy-conversion systems. These devices convert chemical energy (i.e.
a fuel) into electrical energy continuously as energy-storage compounds (e.g.
H2) constantly flow into the cell. They consist of an anode, a cathode and an
electrolyte (Figure 8.4). At the anode, using dihydrogen as example, the fuel
is oxidised to produce electrons (e.g. H2 = 2H+ + 2e– or O2− + H2 = H2O + 2 e–),
which travel along an external circuit to the cathode creating an electrical
current, and protons, passing through the electrolyte to the cathode, where
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Figure 8.4
233
Schematic
illustration of the different types of fuel cells (FC) (SO, solid
oxide; MC, molten carbonate; PA, phosphoric acid; A, alkaline; PEM,
polymer electrolyte membrane; DM, direct methanol).
the oxidant combines with the protons and electrons to produce water as
only byproduct (e.g. 1/2O2 + 2H+ + 2e– = H2O or 1/2O2 + 2e– = O2−).
As fuels, H2, alcohols or hydrocarbons (including carbonaceous materials)
can be used, and as oxidant, normally oxygen/air is used. At both the cathode
and anode, catalysts are necessary for the electrochemical reactions to proceed
at acceptable rates and low temperatures. PEMFC are being considered for
transportation due to their low weight, and high energy and power density,
whereas DMFC, owing to their lower power density but higher fuel density,
are the suitable choice for mobile and other portable devices. The catalyst
used at both the anode and cathode of commercialised PEMFC vehicles is
composed of Pt nanoparticles (NP) dispersed on carbon black, with a typical
Pt content per vehicle of 80 g.45 Both cost and durability of these catalysts,
however, needs to be improved for the large-scale commercialisation of such
vehicles. Therefore, the development of novel and alternative catalysts with
a lower production cost, better activity and durability than traditional catalysts is the main research driving force in this area. Here, both PEMFC and
DMFC will be discussed, focusing on the development of sustainable HTC
carbon-based catalysts either as supports or as catalysts with intrinsic properties for fuel electro-oxidation at the anode as well as oxygen electroreduction
(ORR) at the cathode.
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8.3.1
Chapter 8
Anode Catalyst Supports in Direct Methanol Fuel Cells
Catalysis in electrochemical systems (e.g. FC) takes place at the interface
between reactant, catalyst and electrolyte; commonly referred to as the
­triple-phase boundary (TPB). As a consequence, the support should ideally
possess a highly accessible porosity, facilitating contact between these three
phases and also have good electrical conductivity. The key properties of a
carbon electrocatalyst support are therefore: (i) high crystallinity (good electric conductivity), (ii) relatively high surface area for good catalyst dispersion,
(iii) an open, accessible porosity and (iv) resistance against corrosion. HTC
carbons, owing to their low synthesis temperature, have a poor electronic
conductivity, which can be increased by heat treatment at high temperature
or via catalytic graphitisation. These processes also generate certain porosity
that can be useful for the deposition of catalyst NP. Additionally, the high
concentration of O-containing functionality may be useful to anchor catalytic species (e.g. clusters or NP), inhibiting catalyst agglomeration. These
considerations have driven several authors to study the performance of
HTC carbon as electrocatalyst support. The first to explore the use of a HTC
­carbon-based material as an electrocatalyst support for the electro-oxidation
of methanol were Yang et al.46 In this report, 10 wt% Pt NP were deposited on
carbon spherules obtained by HTC of sucrose at 190 °C and post-treatment at
1000 °C using two different methods – a polyol method and chemical reduction with Na2S2O4. The carbon spherules heat-treated at 1000 °C possessed a
SBET = 400 m2 g−1, arising from the presence of micropores of 0.6–1.6 nm, and
the amorphous nature of the carbon structure. Pt NP deposited through the
polyol method had diameter (D) = 5 nm, whereas those deposited in aqueous
solution tended to agglomerate and exhibit a broader particle-size distribution, from 6 to 40 nm. The prepared catalysts exhibited lower electrochemically active surface areas of Pt (18.2 and 54 m2 g−1 for the polyol and chemical
reduction methods, respectively) than commercial Pt/Vulcan XC-72 (61.4 m2
g−1; D = 3.7 nm) due to the larger particle size. However, the Pt utilisation
in the catalyst prepared through a polyol method (90.5%) was higher than
that of Pt/Vulcan XC-72 (81.0%), attributed to better contact of the electrolyte with the Pt NP on the monodisperse spherules. The low Pt utilisation
in the catalyst prepared via chemical reduction in aqueous solution (34.4%)
was attributed to Pt NP agglomeration. With regards to the electro-oxidation
of methanol, the catalyst prepared through the polyol method exhibited the
highest current.
Kim et al. analysed the performance of a HTC carbon-derived graphitic
carbon made in the presence of Fe and post treated at 900 °C, as support
for PtRu NP.47 The presence of iron in the HTC process, followed by the heat
treatment at 900 °C, led to the generation of a graphitic material (denoted as
SC-g; d002 = 0.341 nm and Lc = 4.4 nm), as well as the development of porosity
(SBET = 252 m2 g−1). Conducting the HTC process in the absence of Fe and
under static or dynamic conditions (denoted as SC-1 and SC-2), generated
amorphous carbons and with SBET values of 112 and 383 m2 g−1. The Pt/Ru
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nanoparticles (60 wt%) were deposited by a NaBH4-reduction method. The
NP size, determined by XRD analysis, was 3.5, 2.6 and 2.7 nm for SC-1, SC-2
and SC-g, respectively. The larger NP size for the SC-1 support is due to its
smaller surface area, as demonstrated by TEM images, indicating the formation of more agglomerates than in the other two material examples. The
performance of these catalysts towards methanol electro-oxidation was analysed by CV at room temperature in 0.5 M H2SO4 (aq) containing 2 M CH3OH
using a PtRu/Vulcan catalyst as the benchmark comparison. All the supported PtRu catalysts show an anodic peak current at 0.45–0.50 V, attributed
to methanol electro-oxidation. When comparing their anodic peak current
densities, only PtRu/SC-g possessed a higher value (21.3 mA cm−2) than
PtRu/Vulcan (16.4 mA cm−2), as a result of its graphitic structure enhancing
electrical conductivity. PtRu/SC-2 exhibited a higher catalytic activity (12.8
mA cm−2) than PtRu/SC-1 (9.9 mA cm−2) due to a better metal dispersion on
SC-2. However, both catalysts exhibited lower catalytic activity than PtRu/
Vulcan owing to the higher graphitic ordering in Vulcan.
A different approach to the generation of graphitic structures from
HTC carbon was reported by Sevilla et al.48,49 Here, a two-step process was
employed in which the synthesised HTC carbon was impregnated with nickel
nitrate and subjected to a heat treatment at 900 °C. As a result, carbon nanocoils were formed of a highly crystalline nature (Figure 8.5(a)), as revealed
by well-defined (002) lattice fringes (Figure 8.5(b)). These carbon nanocoils
exhibited a relatively high SBET, of 114–134 m2 g−1, exclusively ascribed to
the external surface of the NP – i.e. they do not contain framework confined
porosity. These structures potentially can reduce mass transfer resistances of
reactant/products involved in the electro-oxidation of methanol. These nanocoils thus gather the key properties of electrocatalyst supports; i.e. a relatively
high and easily accessible surface area combined with high crystallinity.
Figure 8.5
(a)
TEM image of a carbon nanocoil obtained from hydrothermally carbonised sucrose and (b) HRTEM image of a carbon nanocoil obtained
from hydrothermally carbonised sucrose with deposited catalyst
nanoparticles (dark points) (Inset, detail of a PtRu nanoparticle showing the cubic structure). Reproduced with permission from ref. 48, 49
© 2007 and 2009, Elsevier.
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49
Pt/Ru NP, and Pt NP were deposited on the graphitic carbon nanocoils (GCN) synthesised by Sevilla et al. and their activity assessed in the
­electro-oxidation of methanol, using the widely used electrocatalyst support
Vulcan XC-72R carbon as a benchmark comparison. A high dispersion of NP
(black dots in Figure 8.5(b)) was obtained on the nanocoils (Pt NP D = 3–3.3
nm) though their surface area is half that of Vulcan (Pt NP D = 2.6 nm). For
both Pt/Ru and Pt NP, the electrocatalysts prepared on the GCN exhibited
a higher activity than Vulcan-supported equivalents; a consequence of the
GCN combining good electrical conductivity and accessible surface, reducing the diffusional resistances of reactants/products. Additionally, the crystalline structure confers high oxidation resistance, suggesting that these
electrocatalytics will have, under an oxidative environment (typical of fuel
cell electrodes), a longer durability.
Joo et al. have reported on the use of a combined templating/catalytic
graphitisation in the HTC-based synthesis of graphitic porous carbons.50
Here, uniform silica NP (D = 100 nm) were used as sacrificial template,
sucrose as carbon precursor and Fe as graphitisation catalyst. The HTC process was followed by heat treatment at 900 °C, to produce a carbon material
composed of large spherical pores of 100 nm (a replica of the silica template),
exhibiting a high surface area (425 m2 g−1) and large Vp (0.42 cm3 g−1). Additionally, this material possesses a graphitic nature, as probed by HRTEM,
XRD and Raman spectroscopy. The preparation of the Pt catalyst was carried
out by formaldehyde reduction method. This resulting catalyst exhibited a
higher methanol electro-oxidation current density than the commercial Pt
catalyst (ETEK) and a catalyst supported over porous amorphous carbon synthesised by carbonisation of sucrose (i.e. without hydrothermal treatment).
This high activity is closely related to the unique properties of graphitic carbon together with the porous characteristics of the material, which favour
rapid mass transfer.
Wen et al. combined hard templating with the HTC process, with resulting
materials employed as supported for Pt catalysts.51,52 In one of the studies, an
anodic macroporous aluminium oxide (AAO) was employed as template and
glucose as carbon precursor, and the HTC process was followed by heat treatment at 900 °C.51 After removal of the template, open-ended CNT-type structures were obtained with a diameter of ca. 200 nm (wall thickness = 10 nm),
close to the pore size of the AAO template. Deposition of Pt NP (20 wt%)
was performed on the AAO/CNT composites using a H2PtCl6 precursor and
NaBH4 reduction. The Pt–CNT–Pt hybrid composites were then liberated by
dissolving the AAO template with HF (aq). Via this synthetic approach, Pt NP
(D = 3.5 nm) could be deposited on both the inner and outer surfaces of the
CNTs. For comparison, Vulcan-XC72 was also used for the preparation of a
Pt NP catalyst (ca. 16.7 wt% Pt; D = 3.7 nm). The electrochemically active surface area, measured in 0.5 M H2SO4 (aq) was 39 m2 g−1 for Pt–CNT–Pt and 25
m2 g−1 for Pt/Vulcan XC-72. The larger electrochemically active surface area
may be attributed to the improved dispersion of Pt NP on the CNT. When
subjected to CV in 0.5 M H2SO4 (aq) containing 0.5 M CH3OH, the Pt–CNT–Pt
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catalyst exhibited a higher mass peak current density (25.3 mA g ) as compared to Pt/Vulcan (14.7 mA g−1), indicating a higher catalytic activity for the
CNTs-supported catalyst. Additionally, the Pt–CNT–Pt catalyst possessed a
higher tolerance than Pt/Vulcan to incompletely oxidised species accumulated on the electrode surface. This superior catalytic performance of the Pt–
CNT–Pt catalyst in the electrochemical oxidation of methanol was proposed
to be related to the aligned nanochannels of the catalyst structure facilitating
electrolyte and methanol diffusion, enhancing contact between them and Pt
NP and, therefore, an increase in the number of TPBs.
In another study, an SBA-15 template was used, and introduced directly
with the Pt precursor to the autoclave, so that HTC and deposition of the Pt
nanoparticles on the HTC carbon took place concurrently.38 The resulting
composites were then carbonised at 750 °C and the template was removed
to produce a “Pt@C/mesoporous carbon (MC)” catalyst. For comparison reasons, Pt/CMK-3 and Pt/Vulcan XC72 catalysts were also prepared. As shown by
TEM and HRTEM, a uniform dispersion of well-separated Pt NP (D = 3–5 nm)
was synthesised. However, replication of the SBA-15 porous structure was not
achieved, as confirmed by XRD, attributed to the fragility of the thin carbon
film (at the SBA-15 pore-wall surface), which would partially collapse during
template removal. Furthermore, Pt NP were found to be covered by a thin
carbon layer. The SBET of Pt@C/MC was 633 m2 g−1, with a Vp = 0.55 cm3 g−1,
with the corresponding PSD centred in the mesopore range, although
some microporosity was also present, as well as textural mesoporosity. This
catalyst exhibited no catalytic activity towards methanol oxidation and, in
fact, a capacity to tolerate high concentrations of methanol. However, it possessed an admirable activity for ORR due to the large surface are of the MC
support as well as the well-distributed Pt NP. Conversely, the activity for ORR
was greatly impaired for electrodes prepared from Pt/CMK-3 and Pt/Vulcan
XC-72 due to methanol oxidation. The authors proposed that the unique
Pt@C/MC composite nanostructure endowed a high catalytic activity for
methanol tolerant ORR. Since the Pt NP were coated by a partially microporous thin carbon, oxygen could presumably diffuse through the film to
access the Pt NP, whilst methanol is hindered from doing likewise. Evaluation of the electrocatalyst durability through repeated CV cycles in an O2-saturated electrolyte consisting of 0.5 M methanol was also conducted. A 4%
variation in current density was observed after 40 cycles, indicating that the
Pt@C/MC electrode had a considerable stable electrocatalytic activity for
ORR despite the existence of the well-known “poisonous” methanol in the
electrolyte. Furthermore, the loss of electrochemically active surface area of
Pt would be greatly alleviated as a result of the carbon film on the surface of
the nanoparticles.
In an earlier study, Wen et al. deposited Pt NP through chemical reduction with NaBH4 over hollow carbon spheres and hemispheres (HCSs).53
These HCSs were synthesised by HTC of glucose in the presence of sodium
dodecyl sulfate (SDS) at 170 °C followed by heat treatment at 900 °C. A good
dispersion of Pt NP (D = 5.7 nm) at the inner and outer surface of the HCSs
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Chapter 8
was observed, with the resulting catalyst exhibiting a higher activity towards
methanol oxidation than those prepared using Vulcan XC-72 and glucose-­
derived HTC microspheres. This enhanced activity was attributed to the
higher SBET, improved Pt NP dispersion, high conductivity and the reduction
of the liquid sealing effect.
Pt–CeO2/CNT catalysts have been prepared by Zhou et al.54 by introducing
some modifications to the procedure developed by Wen et al.51 In this report,
a solution containing CeCl3 and H2PtCl6 was used and heat treatment was
performed at 450 °C under N2. A uniform dispersion of Pt–CeO2 nanoparticles was achieved over the resulting CNT-like structures. The catalyst with
the highest electrocatalytic activity in the electro-oxidation of methanol was
found to be a Pt : CeO2 catalyst with molar ratio of 2 : 1, as demonstrated by
CV and chronopotentiometric characterisation. The higher electrocatalytic
activity of Pt–CeO2/CNT in comparison to Pt/CNT was attributed to simultaneous catalysis by CeO2 and Pt. CeO2 acts in the same manner as Ru in Pt–
Ru/C catalysts, such that the formation of OHads species on CeO2 at a lower
potential can transform the CO-like poison (COads) on Pt to CO2, thereby
releasing Pt active sites for further electrochemical reaction.
The one-pot synthesis of a HTC carbon-supported Pt/Ru catalyst has been
achieved by introducing Pt and Ru salts into the aqueous solution of starch
(pH = 11).55 The HTC synthesis step was followed by a heat treatment at 900 °C
to activate the samples for methanol electro-oxidation. The electrochemical oxidation of methanol was evaluated in 1 M methanol in 0.5 M H2SO4
(aq) at room temperature. Electro-oxidation of methanol commenced at
0.45–0.55 V vs. RHE; a value typical for carbon-supported Pt/Ru catalysts
(vs. 0.7–0.8 V for carbon-supported Pt catalysts),56 and an increase of current
values was observed with an increase in Ru content. More recently, the influence of different catalyst synthesis conditions on the electrochemical activity was investigated,57 based on precursor solution pH selection (i.e. alkaline
– KOH and tetrapropylammonium hydroxide (TPAOH) – and acid medium)
and the addition of tetrapropylammonium ion (TPA+). They observed that
the addition of TPA+ promotes an increase in SBET and total Vp, whilst alkaline
medium favoured smaller NP sizes. The best catalyst in methanol electro-­
oxidation (studied by chronoamperometry at 0.5 V) was prepared using
TPAOH, found to be composed of NPs with D = 12 nm and the second highest
SBET and Vp, after that synthesised in the presence of TPA+. Thus, a compromise between metal particle size and pore structure is essential to obtain a
good catalytic activity for methanol electro-oxidation.
The studies described so far carried out the HTC process at a low temperature, i.e. 180–200 °C, so that an additional step was necessary to increase
material conductivity (e.g. graphitisation process or heat treatment at a higher
temperature, 900–1000 °C). This additional step was avoided by Xu et al. by
performing the HTC process at 600 °C.58 As a result, carbon microspheres
of D = 1.5–2.0 µm were synthesised, which in spite of the high temperature
used, exhibited abundant hydroxyl groups, as characterised by FTIR. These
carbon microspheres were subsequently employed as supports for Pd and Pt
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NPs, deposited through chemical reduction using NaBH4. Well-dispersed Pt
and Pd NPs were found at the outer surface of the carbon microspheres, with
NP size being smaller than when using the carbon-black-supported reference. The low surface areas typically exhibited by HTC microspheres would
suggest that metal NPs supported on such materials would easily coalesce.
However, this was found not to be the case in this report, a fact attributed to
NP stabilisation by strong bonding interactions with surface oxygen groups.
As a result, the Pt and Pd NP catalysts supported on these HTC carbon
microspheres exhibited larger electroactive surface areas (double that of the
carbon-black comparison) and higher activity towards methanol/ethanol
oxidation in alkaline media than the carbon black support comparison. The
observed higher activity was attributed to improved metal NP accessibility
at external surface of the carbon microspheres. Additionally, the µm-sized
carbon spheres may act as structure units to form pores and channels that
significantly reduce the liquid-sealing effect.
Complementarily to the HTC process, a solvothermal method using
­ethanol as carbon source and solvent has allowed the synthesis of coin-like
hollow carbon (CHC),59 and graphitised lace-like carbon (GLC),60 which were
then used as supports for Pd and Pt NP, respectively. CHCs were synthesised
using Mg/NiCl2 as catalyst, presented D = 1–3 nm and thicknesses < 154 nm
and a disordered graphitic structure with an interlayer distance of 0.34 nm,
as determined by HRTEM and corroborated by FT-Raman, and SAED/XRD
patterns.59 The authors found that CHC formation took place at synthesis
temperatures ≥550 °C with the morphology maintained <700 °C. In spite of
the high synthesis temperatures, the CHC contained 12% oxygen, present as
hydroxyl and carbonyl groups, as characterised by FTIR analysis. Prior to the
deposition of Pd NP (by direct chemical reduction of PdCl2 with tannic acid),
CHCs were treated with 6 M HNO3 (aq) at 30 °C for 4 h. As a result, material
SBET increased to 400 m2 g−1, 20 times higher than without the treatment. For
comparison, Vulcan XC-72 was employed as a benchmark support material.
TEM and XRD analysis revealed Pd NP deposited CHCs had a D = 7.4 nm,
with the electrochemical active surface area calculated as three times higher
than that of Pd/Vulcan comparison, implying a larger TPB for the reaction, as
a consequence of the high CHC surface area and unique morphology. Thus,
µm-sized CHCs act as structure units to form pores and channels in the catalyst layer that can significantly reduce the liquid sealing effect, as observed
for carbon microspheres. As a consequence, the Pd/CHC catalyst exhibited
an activity towards methanol oxidation three times higher than Pd/Vulcan
(Figure 8.6(a)).
Regarding GLC materials, Mg was used as reducing agent.60 The optimised
conditions for GLC synthesis were found to be 12–16 h at 600–650 °C. XRD
analysis indicated that the GLC possessed a high crystalline order. Prior
to the deposition of Pt NP, GLC was activated in molten KOH. As a result,
the graphitic content of the GLC increased after activation, as revealed
by Raman and XRD analysis, further reflected by an increase in the SBET
from 26 to 1710 m2 g−1 and electrical conductivity from 186 to 236 S cm−1.
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Figure 8.6
(a)
Linear potential sweep curve of Pd/CHC and Pd/Vulcan in 1 M KOH +
1 M methanol (sweep rate = 5 mV s−1) and (b) cyclic voltammograms of Pt/
GLC and Pt/Vulcan electrocatalysts in1 M CH3OH + 0.5 M H2SO4 solution
at 30 °C (scan rate = 5 mV s−1). (Extracted with permission from ref. 59.
© 2008, Elsevier).
Pt nanoparticles were deposited on this material, as well as Vulcan XC-72,
by the direct chemical reduction of chloroplatinic acid at 130 °C using ethylene glycol as a reducing agent. Compared to Vulcan XC-72, a better dispersion of Pt NP was observed on the GLC support with a narrower particle-size
distribution centred at D = 5.2 nm – a value consistent with XRD analysis
(D = 5.6 nm). The Pt/activated-GLC catalyst had a 2.6 times higher catalytic
activity towards methanol oxidation (Figure 8.6(b)) and a 2.5 times higher
electrochemically active surface area compared to Pt/Vulcan XC-72. Considering that both electrocatalysts have similar NP sizes, the results imply that
a larger TPB was formed on the Pt/GLC catalyst due to the large surface area
and unique morphology of the support. Such carbon structuration would
make it easier for the liquid electrolyte and methanol to diffuse into the
electrocatalyst layer, leading in turn to a better utilisation of Pt and reduced
concentration polarisation.
More recently, Lian et al. have shown for the first time the synthesis of carbon microspheres by the solvothermal method using much lower temperatures (as low as 160 °C) using iodine as a carbonisation catalyst.61 Ethanol
acted as solvent, reducing agent and carbon source, whilst FeCl3 was used
as the oxidant, being reduced during synthesis to FeCl2, and, as mentioned,
iodine was used as catalyst. Microspheres of 2–3 µm diameter were synthesised at 160 °C when the concentration of FeCl3 was 0.5–1.0 M (concentration
of I2 = 0.04 M). It is worth mentioning that when the concentration of FeCl3
was 2.0 M or higher, uniform carbon fibres with diameters of ca. 8 µm were
obtained. The following redox reactions for the formation of carbon under
solvothermal conditions were proposed:
2I2 + CH3CH2 OH = 4I − + 2C + H2 O + 4H+
2Fe3+ + 2I −= 2Fe2+ + I2
4Fe3+ + CH3CH2 OH= 4Fe2 + + 2C + H2 O + 4H+
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This material was used as support of Pt NP, which were electrodeposited
from a solution of 0.5 M H2SO4 and 3 mM H2PtCl6. The electrochemical
surface area of such a catalyst was 9.80 m2 g−1 in contrast to 7.51 m2 g−1 for
commercial ETEK (20 wt% Pt). Also, its catalytic activity towards methanol
electro-oxidation was higher than that of ETEK, i.e. 89.5 mA mg−1 Pt vs.
78.5 mA mg−1 Pt (measured at 0.7 V vs. SCE).
8.3.2
Catalysts for the Oxygen-Reduction Reaction (ORR)
The large Pt load used in the current FC vehicles is mainly due to the
­sluggish ORR taking place at the cathode, which normally requires
≥0.4 mg(Pt) cm−2.62,63 Therefore, significant efforts have been devoted
in recent years to find more economic and abundant alternatives to Pt.
Since Jasinski’s first report on the ORR catalytic activity of cobalt phtalocyanines,64 non-noble ORR catalysts based on transition-metal and N-containing functionalities have drawn increasing attention. In particular,
N-doped carbons (with or without metal) are attracting interest as they
have proven to be more active and stable than macrocyclic N4–metal complexes in acidic media. What is more, they are able to catalyse the ORR in
the absence of a metal catalyst.
Thus far, the best results with metal-free N-doped carbons have been
obtained by Gong et al., who showed that vertically aligned N-containing
CNTs (VA-NCNTs) can outperform Pt as ORR in alkaline FCs in terms of electrocatalytic activity, long-term operation stability, and tolerance to crossover
effect.65 The integration of electron-accepting “N” in the conjugated CNT
plane is proposed to impart a relatively high positive charge density on adjacent carbon atoms. The charge-density distribution, coupled with aligning
the NCNTs, facilitates a four-electron reduction process and excellent performance (a steady-state output potential of −80 mV and a current density of
4.1 mA cm−2 at −0.22 V, compared with −85 mV and 1.1 mA cm−2 at −0.20 V
for a Pt/C electrode). The difficulty in the reduction of oxygen to water originates from the strong double bond of the O2 molecule, and as such the catalytic mechanism aims to dissociate the O=O bond. The catalyst’s ability to
interact with O2 and the degree of this interaction are crucial for catalytic
activity toward ORR. In this regard, Gong et al., based on the experimental
observations and quantum mechanical calculations by B3LYP hybrid density functional theory,65 attributed the improved catalytic performance of
VA-NCNTs to the electron-accepting ability of the “N” atoms, which creates
a net positive charge on adjacent carbon atoms in the CNT plane, attracting
electrons from the anode, facilitating the ORR. The N-induced charge delocalisation can also change the chemisorption mode of O2 from the usual
end-on adsorption (Pauling model) at the N-free CNT surface to a side-on
adsorption (Yeager model) onto the N-doped CNT electrode. The N-induced
charge transfer from adjacent carbon atoms could lower the ORR potential,
while the parallel diatomic adsorption could effectively weakens the O–O
bond, facilitating ORR at the VA-NCNT electrode.
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However, there is still much controversy about the exact catalytic role in
the ORR of different N-containing functionalities (e.g. pyrrolic, pyridinic and
graphitic) present in N-doped carbons. Matter et al. suggest that pyridinic “N”
may not be the active site for the ORR, but may be a marker for edge-plane
exposure, which facilitates oxygen chemisorption.66 Some reports also suggest that pyridinic “N” may not be an effective promoter of the four-electron
ORR process. Thus, Luo et al. synthesised purely pyridinic N-doped graphenes
and found them to be selective for a two-electron reduction pathway,67 as
commonly observed for glassy carbon and pyrolytic graphite.68 Conversely,
Lui et al. recently proposed that graphitic nitrogen leads to good catalytic
activity, demonstrating that nitrogen content does not directly correlate
with catalyst performance.69 Materials (i.e. N-doped mesoporous graphitic
arrays) with higher nitrogen content showed lower selectivity and activity. In
another report, Strelko et al., applied the semiempirical quantum chemical
method AMI and identified pyrrolic “N” as the binding state that gives rise
to the smallest bandgap and thus the best electron transfer capabilities.70
A combined experimental and theoretical approach is therefore essential to
elucidate the role of the different types of N-moieties in N-doped carbons and
thereby carry out a rational design of high-performance ORR catalysts.
While HTC-derived carbons have been extensively used as catalyst supports
for the anode electro-oxidation reaction, reports for the ORR at the cathode
are just starting to appear. The first report on the ORR activity of HTC-derived carbon catalysts corresponds to N- and dual N/S-doped carbon aerogels
(termed carbogels) prepared via one-pot HTC of glucose, ovalbumin and cysteine or cysteine derivatives.20 The as-synthesised aerogels possess N and S
contents of 5.5 wt% and 1–4 wt%, respectively, and SBET > 190 m2 g−1. A secondary pyrolysis step (900 °C) was used to further tune the carbon aerogel
conductivity and heteroatom binding states. This pyrolysis process lead to
a homogenisation of bulk and surface “S” content, presumably via the loss
of pendant, weakly bound S-containing species. In this way, these carbogels exhibited S contents of 0.7–1 wt% (thiophene-like S), whilst N content
remained at 4–5 wt% (as pyridinic-N and quaternary-N). Furthermore, the
aerogel surface areas increased upon pyrolysis to 220–320 m2 g−1, due to the
removal of micropore-bound decomposition products, accompanied by an
improvement in conductivity to the 500–660 S m−1 range. A comparison of the
ORR catalytic activity of dual N/S-doped carbogel with that of a N-doped carbogel (prepared in the absence of the S-dopant), demonstrated the positive
effect of “S” in unison with N doping on activity in the ORR in both acidic and
basic media. The catalytic activity in basic medium was higher than in acidic
medium, as is typical for carbon materials. In acidic conditions all doped carbogels showed very good stability compared to a Pt-based catalyst, as well as
an activity that is still much better than ordinary carbon supports, but as yet
not competitive to the noble-metal systems. Koutecky–Levich plots showed
that both 2- and 4-electron processes take place for all the carbogels tested.
It was observed that S doping improved the selectivity towards a 4 electron process in 0.1 M KOH, and towards a 2-electron process in 0.1 M HClO4.
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Whereas the 4-electron mechanism is important for ORR in FCs owing to
H2O2 cell poisoning, selective 2 electron processes are of great interest for
the electrochemical synthesis of H2O2.71 The authors proposed a synergistic
mechanism between N and S dopants, whereby N directly or indirectly (via the
adjacent carbon atom) aids O2 dissociation and S facilitates proton transfer.
Adopting the borax-assisted preparation approach, N-doped carbogels were
prepared via one-pot HTC of glucose and 2-pyrrol-carboxaldehyde (i.e. the
N dopant).14 Similarly to parent approach (Chapter 6), particle size and hence
surface area was tuned from 75 nm down to 15 nm by varying the amount of
borax added (SBET = 40–400 m2 g−1). Material N content was adjusted in the
range 2.0–5.5 wt% by varying the amount of N dopant added to the precursor
mixture. As in the earlier work, a secondary pyrolysis step (900 °C) was used to
increase conductivity and material stability, leading to an increase in carbon
content to 90%, whilst “N” content increased slightly (3–6 wt%) due to the
relative changes in weight after the loss of less stable (e.g. O-containing) functionalities. The resulting carbogels exhibited SBET in the range 30–300 m2 g−1
(generated from interparticle voids in the mesopores range) and conductivities in the 400–900 S m−1 range. The ORR activity focused on investigating
the effect of surface area and “N” content of the prepare carbogels. Featureless cyclic voltammograms are observed for all the samples in a N2-saturated
KOH solution (dashed lines), whereas a strong cathodic peak is visible upon
saturating the solution with O2 (solid lines), showing the catalytic effect of
the N-doped carbogels towards ORR (Figure 8.7). The analysis of the polarisation curves revealed that the activity increases with surface area and nitrogen content. What is more, while a commercial platinum catalyst used for
comparison is poisoned after methanol addition, the N-doped carbogels are
virtually unaffected. This methanol tolerance is a widely recognised advantage of N-doped materials.69,72 Koutecky–Levich analysis of the best catalyst
indicated that the selectivity of the N-doped carbogels tends towards an ideal
Figure 8.7
Cyclic
voltammograms obtained in N2-saturated (dashed lines) and
O2-saturated (solid lines) 0.1 M KOH for (a) borax concentration series
(N content ∼3 wt%), (b) 2-pyrrol-carboxaldehyde concentration series
and N3B3_900 (sample with large N content and surface area). (c) Methanol crossover for N3B3_900 and commercial Pt@C. Reproduced with
permission from ref. 14. © 2013, Royal Society of Chemistry.
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4-electron process. It is worth mentioning that no trace metals arising from
residual borax or impurities in the glucose precursor were detected (only 35
ppm of Fe), so that the high performance of these materials can be exclusively attributed to their high nitrogen content and surface area.
The synthesis of highly porous N-doped carbogels from ­carbohydrate-based
derivatives (i.e. glucose, d-glucosamine and N-acetyl-d-glucosamine), and
phenolic compounds (i.e. phloroglucinol and cyanuric acid) has also recently
been reported.17 The possibility of using more complex water-soluble biomass derivatives, such as gum arabic, was also demonstrated. The as-prepared materials are composed of small and interlinked nanoparticles of
D = 15–20 nm, exhibiting SBET = 140 to 600 m2 g−1, and predominantly mesoporous. The N content of these materials varied from 2.6 to 7.7 wt% depending on the dopant used. After further thermal treatment at 950 °C to induce
electrical conductivity (10 S m−1), the SBET slightly altered (i.e. 130 to 650
m2g−1) and the N content increased to 2.9–5.0 wt%. Their electrocatalytic
activity in the ORR was analysed through CV and linear sweep voltammetry
using a rotating-disk electrode (RDE) in 0.1 M KOH. A well-defined cathodic
peak was observed during CV experiments in an O2-saturated solution at −0.3
to −0.4 V vs. Ag/AgCl. The analysis of the polarisation curves evidenced that
the N-doped carbogels with the higher SBET and mesoporous surface areas
displayed the lowest over-potential, while the sample with a low mesoporous
surface area exhibited the largest onset potential of all samples tested.
Unfortunately, these carbogels were not found to be very selective for either
a 2- or 4-electron process, with a ∼2.8-electron process observed at −0.4 V,
shifting to a ∼3.7 electron process at −0.8 V vs. Ag/AgCl. This catalyst, with
the best electron-transfer capabilities and tending to favour the 4 electron
pathway at lower potentials, i.e. -0.8 V vs. Ag/AgCl, is the one with the largest
pyrrolic-/pyridinic-N ratio. This result agrees with the findings of Lou et al.67
and Strelko et al.,70 who showed, respectively, that purely pyridinic N-doped
graphenes are selective for a two-electron reduction pathway and pyrrolic-N is the binding state that confers the best electron-transfer capabilities.
Concerning its stability, methanol tolerance is lower than other Carbogels
described above, although it is enhanced as compared to Pt-based catalysts
previously reported in the literature.
The studies carried out so far demonstrate the powerful potential of
N-doped and dual N-/S-doped carbons based on HTC synthesis routes, as
sustainable metal-free catalysts for the sluggish ORR at the FC cathode.
Apart from sustainability, economics are also in favour of these materials
(given the typically low cost precursors involved), making them ideal for
future clean-energy solutions.
8.4
Electrodes in Rechargeable Batteries
Rechargeable batteries, also known as secondary batteries, are electrochemical units that convert the chemical energy stored into electrical energy. A
reversible battery comprises one or more electrochemical cells connected
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in series or parallel. However, the term battery has evolved to indicate the
product powering a device, regardless of the fact that it contains one or more
cells.73 Each cell consists of a reductant (anode/negative) and an oxidant
­(cathode/positive) electrode (both sources of chemical reactions) separated by
an electrolyte solution containing dissociated salts, which enable ion transfer
between the two electrodes. The development of rechargeable batteries dates
back to 1859 with two systems, Pb-acid and Ni–Cd. Later, Zn/MnO2, Ni-MH,
Na–S, Li-ion (with liquid or polymeric electrolytes) have come to dominate
the market, while many other are still under research and development, (e.g.
Li–S, Li-air, Na-ion, Mg-ion and others). In the following sections, focus will
be given on three main rechargeable battery technologies, one commercially
available system (i.e. Li-ion batteries) and two maturing systems (i.e. Na-ion
and Li–S batteries). In this context, discussion will be limited to the literature
concerning materials produced via the HTC synthesis platform.
8.4.1
Li-Ion Batteries
Over the past 25 years, Li-ion batteries (LIBs) have played a crucial role in the
development of energy-storage technologies. LIBs are currently used in 90%
of rechargeable portable electronic devices. Whilst great improvements have
been accomplished, and active research continues, current LIB technologies
currently only provide a gravimetric energy density of 140 Wh/kg for a full
system.74 The first attempt to construct a rechargeable LIB was by Whittingham in the mid-1970s.75 The motivation for using a battery technology based
on Li metal as anode relies on the fact that Li is the most electropositive
(−3.04 V vs. SHE) as well as the lightest metal (0.53 g cm−3). However, there
are a number of safety considerations, including dendrite formation leading
to short circuiting, limiting the use of Li metal in secondary batteries. Therefore, metallic Li is now typically substituted for Li-based compounds. A typical LIB consists of a negative electrode (generally graphite), and a positive
electrode (generally a lithium metal oxide), separated by a Li-ion conducting
electrolyte (e.g. LiPF6/ethylene carbonate-diethylcarbonate solution; Figure
8.8).76,77 When a battery is cycled, Li+ exchange between the positive and negative electrodes. During discharge, the positive electrode becomes reduced
(as Li+ is inserted), and the negative electrode is oxidised (Li+ is extracted).
The converse occurs during charging. The electrochemical binding energy
difference for Li between the two host lattices drives the electron transport
through the external circuit and hence does the useful work.78 The theoretical Li storage capacity of a graphite anode in LIBs is 372 mA h g−1, forming
the compound of LiC6. The charge–discharge total reactions on based on Li
intercalation and de-intercalation are shown as follows:
Positive electrode : LiMO2 ⇔ Li1− x MO2 + xLi + + xe −
Negative electrode : y C + xLi + + xe − ⇔ Li x C y
Overall : LiMO2 + yC ⇔ Li1− x MO2 + Li x C y
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Figure 8.8
8.4.2
Schematic
representation of a lithium-ion battery. Negative electrode
(graphite), positive electrode (LiCoO2), separated by a nonaqueous liquid electrolyte. Reprinted with permission from ref. 77 © 2013, American Chemical Society.
Anode Materials
8.4.2.1 Carbon-Based Anode Materials
Regarding LIBs, carbon-based intercalation compounds are the mostly used
anode active materials. The concept of carbon-based intercalation compounds dates back to 1840.79,80 Although the use of a carbon-based rechargeable cells was suggested earlier, only recently has Li-intercalated graphite
been proposed as an anode material by Armand and Touzain.81 Later, Sony
developed a rocking-chair battery in which Li+ is transferred from one intercalation compound to another of very different Li-potential. Among the carbon-based materials, graphite is particularly important since it has a high
electronic conductivity as a result of the delocalised π-bonds. In graphite, the
Li+ intercalates with every six carbon atoms. The LiC6 stoichiometry results a
storage capacity of 372 mAh g−1. Whilst graphite has a relatively low storage
capacity, only a small volumetric change of ca. 10% occurs, allowing for at
least 500 cycles, depending on the current rate used. Disordered carbons (the
so-called hard carbons) have been shown to store more Li storage than the
theoretical capacity.82 This phenomenon is still difficult to explain via graphite intercalation compound science, and new schemes are currently being
established. One such mechanism suggests that the Li ion can be stored
in nanoscopic cavities of the nongraphitic carbon materials and not only
between graphene layers. A more detailed discussion regarding Li storage
in carbon nanostructures can be found elsewhere.83 Also, in terms of materials development, low-temperature forms of carbons (e.g. more amorphous)
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would be preferable in order to decrease the amount of electrical energy used
in anode production.
There are numerous reports on the use of pure carbon materials as negative electrodes in Li-ion batteries as well as a number of reviews summarising
these findings.83,84 The following section describes progress made by the use
of HTC method. In 2002, Huang et al. used HTC to develop LIB anode material.85 Initially spherical hard carbon was prepared from the HTC of aqueous
sugar for 5 h at 190 °C, followed by heat treatment of the recovered product at
1000 °C (under Ar). Whilst the hard carbon products were shown to have a Li
capacity ≥ 430 mAh g−1, only a couple of cycles were reported. The higher than
graphite capacity was explained by the disordered character of carbon structure. Additionally, these materials had relatively high surface area and a high
number of micropores, known not to be beneficial for Li+ storage.86 These
unfavourable results from carbohydrate-based carbon materials as anode
materials remained untouched for 10 years. Recently, Tang and White et al.
demonstrated the use of glucose-derived hollow carbon nanospheres (HCNs)
prepared as discussed in Chapter 6, the introduction of polystyrene latex
nanoparticle templates to the HTC synthesis led to the hollow-sphere morphology.87 The diameter of the HCNs was ca. 100 nm with a shell thickness of
ca. 12 nm. The results demonstrated the two advantages of the HCN structure
over Huang’s original work.85 First, the structure ensured continuous electron transport and secondly, the very thin shell (< 12 nm) guaranteed a very
short Li diffusion distance and a well-defined and large electrode/electrolyte
contact area.88 The electrochemical behaviour of the HCNs was characterised
by CV at a scan rate of 0.1 mV s−1 (Figure 8.9(a)). The voltammogram showed
two cathodic peaks, at ca. 1.3 and 0.8 V, that appear only in the first cycle. It
was assumed that the peak at higher potential is a product of the reaction
between lithium and functional group(s) at the carbon surface, whereas the
peak at 0.8 V is related to the formation of a solid electrolyte interphase (SEI).
SEI formation is a common phenomenon linked with electrolyte decomposition on the carbon surface. Galvanostatic discharge–charge curves of hollow
carbon nanospheres at a rate of 1 C showed two plateaus at 1.4 and 0.8 V in
the first discharge curve, corresponding to the additional peaks in CV curves
(Figure 8.9(b)). A large irreversible capacity (ca. 700 mAh g−1) was explained
by irreversible Li+ insertion into potentially unique positions, such as cavities
or sites in the vicinity of residual hydrogen atoms in the carbon material. The
steep charge–discharge slope was similar to typical disordered carbon materials with a similarly low degree of graphitisation. The charge curves showed
three different potential regions attributed to the following different mechanisms: (1) a slope from 0 to 1 V, corresponding to the Li+ deintercalation from
disordered graphene layers; (2) a slope plateau from 1 to 1.5 V, indicated in
the CV curve as a broad peak at 1.4 V, related to trapping at hydrogen-terminated dangling bonds; and (3) at > 1.5 V, another slope region, ascribed to
extraction from some defect sites with higher energies, such as vacancies.89,90
The reversible capacity of the HCNs, at a rate of 1 C [one Li+ per six formula
units (LiC6) in 1 h], reached values up to 370 mAh g−1 (Figure 8.9(c)), which
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Figure 8.9
Chapter 8
(a)
Cyclic voltammetry of hollow carbon nanospheres. The graph shows
the first two cycles between 3 and 0 V at a scan rate of 0.1 mV s−1. (b)
Galvanostatic discharge–charge curves of hollow carbon nanospheres
at a rate of 1 C. (c) Cycle performance of hollow carbon nanospheres
cycled at a rate of 1 C. (d) Rate performance of hollow carbon nanospheres. Reproduced with permission from ref. 88 © 2012, Wiley-VCH
­Verlag GmbH & Co. KGaA, Weinheim.
is much higher than graphite. Rate performance of the HCNs was also performed showing their high rate capability (Figure 8.9(d)).
Titirici et al. recently demonstrated the use of rice-husk cellulose as a precursor for HTC material preparation.91 Prior to HTC, the lignin and hemicellulose fractions of the rice husks were removed using the formic acid
method.92 Concentrated formic acid (i.e. 88 wt%) degraded 85% of hemicelluloses and 70% of lignin at 60 °C for 6 h,92 with the acid depolymerising
hemicelluloses and lignin at 60–130 °C within 8 h. The authors noted that
direct hydrothermal carbonisation in a high concentration of formic acid
(95 wt%) should be possible but it will corrode the autoclaves at 230 °C
(therefore pretreatment was preferred). Moreover, the SiO2 component was
used as an “in situ” hard sacrificial template to introduce porosity into the
resulting carbon materials. After further heat treatment and SiO2 removal,
the electrochemical properties of the rice husk-derived carbon was investigated as LIB anode material. After the HTC process at 230 °C for 48 h,
the continuous cellulosic network was not disrupted. In order to increase
the level of structural order and the electronic conductivity of the material,
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carbonisation at 900 °C under an inert atmosphere was performed. To
demonstrate the necessity for the HTC step, a direct pyrolysis of rice husk
after the formic acid treatment was also conducted. The morphology was
compact and it did not exhibit the fibrous porous network observed for the
HTC carbon material. Thus, it was concluded that the HTC step induces
important morphological transformations of the cellulose fraction via
partial hydrolysis into glucose/hydroxymethylfurfural and conversion
into an HTC carbon network. Treatment with HF (aq) to remove silica
was performed and the resulting materials possessed a structure made of
interdigitated thin carbon fibres. Testing as anode materials for LIBs via galvanostatic charge–discharge curves at a current density of 75 mA g−1 (namely
0.2 C, 1 C corresponds to 372 mAh g−1) demonstrated that whilst the typical
charge–discharge profile for graphite at an operation voltage of 0.0 V was
observed, the rice-husk derived carbon presented a hysteresis at 1.0 V for
lithium insertion, a behaviour similar to petroleum pitch-derived carbons.
The initial discharge capacity was 789 mAh g−1, which was higher than the
theoretical capacity of the graphite. The irreversible capacity during lithium desertion was 396 mAh g−1. As explained above, this is rather common
for carbon based materials and it is partly associated with the decomposition of carbonate-based electrolyte EC-DMC, which can easily have reduction reaction below 1.0 V (versus Li+) and SEI formation on the electrode. At
137 mA g−1 at 10 C, namely charged in 6 min, the rice husk-derived carbon
delivered superior electrochemical performance.
Uner et al. have reported on the HTC conversion of abundant agricultural
biomass (i.e. hazelnut shells) to carbon nanostructures suitable for LIB applications.93 KOH activation, MgO templating, and thermal treatment under an
inert atmosphere were utilised to improve textural and morphological properties, control surface chemistry and enhance thermal stability. The HC-600,
obtained by successive hydrothermal and thermal treatments of ­hazelnut-shell
precursor without additional chemical agents presented minimal surface functionality, maximum aromaticity and structural order, optimum
SBET, and a well-developed micro- and mesoporous network. The electrochemical performance of HC-600 after 100 cycles, demonstrated a discharge
capacity of 291.54 mAh g−1 at 1 C. The high first cycle discharge (1331.19
mAh g−1) and charge (607.53 mAh g−1) capacities of HC-600 were accompanied
by the highest first cycle coulombic efficiency (CE) recorded of all the samples investigated (45.64% CE). Xia et al. have also prepared one-­dimensional
hierarchical porous hydrothermal carbon fibres from alginic acid.94 The carbon fibres consisted of a three-dimensional network of nanosized carbon
with a good rate capability and capacity retention compared with commercial graphite. The main problem associated with these carbon materials
is that the values for higher capacities are obtained when the potential is
close to 0 V versus Li/Li+, which is not safe, especially for high-power applications such as electrical vehicles. In such a low operation voltage region, the
electrolyte is prone to decompose and form the SEI on the anode surface.
Concurrent with the electrolyte decomposition, gases are released and build
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up pressure in the cell. This situation will endanger the safety of the battery
system as gases accumulate with increasing cycling time.
In parallel, on-going research efforts focus on the development of carbon
alternatives in the hope of finding materials with both larger capacities and
slightly more positive intercalation voltages compared to Li/Li+, to minimise
any risks of high surface area Li plating at the end of fast recharge, which
are associated with safety problems.74 In this context, it is important to note
that apart from carbon-based materials used as the LIB anode, hybrid HTC
carbon composites are also used in the field of rechargeable batteries. To
obtain HTC-derived carbon hybrid composites two approaches are reported;
i) a one-step process in which metal salts are directly added in an aqueous
carbohydrate solution. During HTC in the presence of low redox potential
metal salt precursors, the metal ions are dissolved and predominantly positioned near in the hydrophilic shell of the resulting particles. This method is
versatile and generalised to any type of low redox oxidation potential metal
salts and proven with several metal salts (e.g. FeCl3, NiCl2, CoCl2, RuCl2,
etc.),95 and ii) HTC is performed in the presence of preformed NPs. Carbohydrate-derived monomers formed in situ during HTC are then converted
to form a “coating” on the preformed NPs. One particular example is the
coating of Si NPs.96 The HTC technique has also allowed the uniform incorporation of preformed NPs into the carbonaceous matrix.97 These composites are particularly interesting since a carbon coating can further eliminate
problematic volume expansions during cycling.
8.4.2.2 Metal Oxide-Based Anode Materials
The insertion of Li+ into TiO2 layers is well documented.98–100 Independently
of the TiO2 polymorph in use (i.e. rutile, anatase, brookite), the insertion
reaction of Li+ into TiO2 can be expressed as:
TiO2 + xLi + + xe − =
Li x TiO2
In this redox reaction, the insertion of positively charged Li+ is balanced
with the uptake of electrons to compensate Ti(iii) cations in the Ti(v) sublattice, which usually results in a sequential phase transformation of the original TiO2 as a function of Li+ content.98 The theoretically calculated capacity of
TiO2 is 330 mA h g−1, which is a little lower than that of graphite.101 However,
the volume change of TiO2 as a consequence of Li+ insertion is <4%, rendering TiO2 electrodes with outstanding structure stability and thus extremely
long cycling life in LIB applications. Another advantage of TiO2 is the safer,
higher operation voltage (1.5–1.8 V versus Li/Li+) thus eliminating electrolyte
decomposition and gaseous product release. However, the disadvantage is
that like for any inorganic material, TiO2 is not electrically conductive (ca.
1 × 10−12 S m−1) resulting in low energy/power densities. Its conductivity can
be improved by combining it with carbon in small amounts. In this respect,
carbon-coated TiO2 nanotubes were prepared by performing the HTC of
glucose in the presence of the inorganic nanotubes. A thin carbon coating
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was obtained that effectively suppressed the aggregation of TiO2 nanotubes
during a postcalcination step. Consequently the HTC-carbon-coated TiO2
nanotubes delivered a remarkable Li-ion intercalation/deintercalation performance with reversible capacities of 286 and 150 mAh g−1 at 250 and 750
mAh g−1, respectively.102
Among numerous new candidates for anode materials, SnO2 is considered a promising substitution for commercial graphite owing to its high theoretical Li+ capacity (782 mAh g−1), low cost and safe working potential (a
few hundred millivolts higher than Li+/Li).97,103 Unfortunately, its practical
application is greatly limited by undesirable rapid capacity fading, low initial
CE and poor rate performance; products of large volume variation (300%)
during the long-term charge–discharge process,96 the irreversible reaction in
the first cycle, and the low electronic conductivity of SnO2. The large volume
variation leads to the pulverisation of the electrode and the loss of electrical
contact between the active Sn and conductive additives or the current collector. In addition, previous studies have revealed that the SnO2 anode may
electrochemically react with Li+, leading to the formation of small and active
Sn particles that are easy to aggregate into larger and inactive clusters during
lithium alloying and dealloying.104–106 All these factors result in a deterioration of the electrode structure and decay of electrochemical properties.
There are several reports concerning the preparation of HTC-derived
SnO2 composites and their use as LIB anode materials. Demir-Cakan et al.
reported a simple and surfactant-free method to prepare mesoporous SnO2
microspheres by using the HTC conversion of furfural in the presence of a
SnO2 NP sol.97 During HTC, the SnO2 NPs are uniformly distributed within
the resulting hydrophilic carbon spheres (Figure 8.10(a)–(c). The size of the
microspheres could be tuned depending on the concentration of furfural
and HTC reaction time and temperature. The discharge (Li insertion)/charge
(Li extraction) curves of the mesoporous SnO2 electrode were obtained in
1 M LiPF6 EC/DMC electrolyte solution at a current density of 100 mA g−1
(Figure 8.10(d) and (e)). As regards to the first cycle, the mesoporous SnO2
microspheres showed much higher reversible capacity (960 mAh g−1) in the
voltage range of 0.02–2.5 V as compared to nonporous SnO2 (670 mAh g−1).
Furthermore, the mesoporous SnO2 microspheres exhibited an initial CE of
ca. 53%, remarkably higher than the nonporous SnO2 (i.e. 39%). The reaction
of SnO2 with Li+ follows two general reaction equations; the first represents
the conversion of SnO2 to Sn and Li2O nanocomposite. Such conversion reaction materials are highly used to increase the capacity of anodes in LIBs.107
Lithium can thus be stored reversibly in a transition-metal oxide through
a heterogeneous conversion reaction, followed by a 2-step alloying reaction
between Sn and Li+.
SnO2 + 4Li + + 4e − → Sn + 2Li2 O
Sn + xLi + + xe − ↔ Li x Sn(0 ≤ x ≤ 4.4)
The mesoporous SnO2 microspheres allowed for discharging–charging at
higher current densities, with stable reversible capacities ca. 370 and 200
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−1
−1
mAh g at current densities of 1 and 2 A g , respectively (Figure 8.10(d)
and (e)). The significant improvement in electrochemical performance was
attributed to the unique mesoporous structure of the SnO2 microspheres
with a variety of favourable properties: the 10 nm mesopores allow liquid
electrolyte diffusion into the electrode bulk and hence also fast transport
of the conductive ions (e.g. solvated Li+); the mesopores may be expected to
buffer against local volume change, with the nanosized building blocks (D =
3–7 nm) providing a low absolute volume change during the Li–Sn alloying–
dealloying reactions; the short diffusion length for Li insertion are beneficial
in retaining the structural stability as well as leading to a good cycling performance and high rate capability.
Complementarily to the work of Demir-Cakan et al.,97 Archer et al. have
used a glucose-mediated HTC synthesis for the gram-scale production of
nearly monodisperse hybrid SnO2 nanoparticles.108 In this synthesis, glucose
plays a dual role; facilitation of rapid precipitation of polycrystalline SnO2
nanocolloids and the creation of a uniform carbon coating on the SnO2 cores.
The thickness of the coated layer could be easily manipulated by variation of
Figure 8.10
(a)
SEM micrograph of the SnO2 composed of agglomerated nanoparticles; (b) and (c) TEM microtomed micrographs of the SnO2 (d)
discharge–charge profiles for the mesoporous SnO2 sample cycled
between voltage limits of 0.05 and 1 V at current densities of 1 and 2
A g−1; (e) variation in discharge–charge capacity versus cycle number
for the mesoporous SnO2 sample cycled at current densities of 1 and
2 A g−1. Reproduced with permission from ref. 97 © 2008, American
Chemical Society.
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the glucose concentration in the synthesis. The resulting HTC carbon-coated
SnO2 colloids exhibited significantly enhanced cycling performance for lithium storage. By reduction with hydrogen, a simple route to carbon-coated tin
nanospheres was also demonstrated. Lithium storage properties of the latter
materials are also reported, suggesting that large irreversible losses in these
materials are caused not only by the initial irreversible reduction of SnO2 as
generally perceived, but also by the formation of the SEI.108
Following on from the aforementioned works, several SnO2 composites
were studied in the literature. Huang et al.103 reported the synthesis of a nanocluster composite assembled by interconnected ultrafine SnO2–C core–shell
(SnO2@C) nanospheres via a simple one-pot HTC method and subsequent
carbonisation. As an anode material for LIBs, the obtained nanoconstruction
could provide a 3D transport access for fast electrons and Li+ transfer. With
the mixture of sodium carboxyl methyl cellulose and styrene butadiene rubber as a binder, the SnO2@C nanocluster anode exhibited superior cycling
stability and rate capability due to a stable electrode structure. Discharge
capacity reached to 1215 mAh g−1 after 200 cycles at a current density of 100
mAh g−1. In a similar fashion, Lu et al. studied a C/SnO2/C core–shell–shell
nanofibres prepared via single-spinneret electrospinning followed by carbonisation and hydrothermal treatment.109 The designed hybrid nanofibres
exhibited excellent electrochemical properties, including high reversible
capacity with high CE and impressive rate capacity in LIBs due to the morphological stability and reduced diffusion resistance, which are induced by
both the carbon core and deposited carbon skin. Yang et al. reported the
synthesis of carbon-coated SnO2 (SnO2–C) nanotubes through a simple HTC
conversion of glucose and subsequent carbonisation approach by using Sn
nanorods as sacrificial templates.110 The hollow nanostructure, together
with the carbon matrix that has good buffering effect and high electronic
conductivity, could be responsible for the improved cyclic performance.
Apart from SnO2, several other metal oxides have been investigated in the
field of LIBs. For instance, Tu et al. prepared a spherical NiO/C composite by
dispersing NiO in glucose solution and subsequent HTC at 180 °C.111 Electrochemical tests showed that the NiO/C composite exhibited higher initial
CE (66.6%) than the pure NiO (56.4%) and better cycling performance. The
improvement of these properties was attributed to the carbon, as it could
reduce the SBET and enhance conductivity. A layered Co–Fe double-hydroxide
nanowall array has also been reported, with the array grown directly from
a flexible alloy substrate added to the HTC of glucose.112 After annealing
under Ar, the carbon-coated CoFe mixed-oxide nanowalls were found to have
improved electrical conductivity and superior electrochemical performance in terms of specific capacity and cyclability for LIBs as compared to a
­carbon-free sample and a sample made by a previous carbon-coating method.
N-doped HTC carbons have also been investigated in the field of LIBs. Li
et al. have reported the synthesis of a CoSnO3 composite (i.e. CoSnO3@N–C)
and its use as a high-capacity anode for LIBs.113 The CoSnO3@N–C was synthesised via the HTC of carboxylated chitosan and subsequent carbonisation
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Chapter 8
approach. The CoSnO3@N–C material exhibited enhanced lithium storage
capacity and cyclic performance, delivering a reversible capacity of 650 mAh
g−1 after 50 cycles at 100 mA g−1, compared to bare CoSnO3. Yang et al. have also
described the synthesis of Co2SnO4@C core–shell nanostructures through a
similar HTC based route.114 The resulting materials were employed as LIB
anodes exhibiting improved cyclic performance compared to pure Co2SnO4
nanocrystals. The carbon matrix provided a good volume-buffering effect
and high electronic conductivity, which may be responsible for the improved
cyclic performance. He et al. have reported on the HTC synthesis of small,
uniform Fe3O4 nanocrystals (D = 9 nm) encapsulated in interconnected carbon nanospheres (D = 60 nm).115 Following annealing under Ar, the resulting
hybrid Fe3O4@C nanospheres were found to be high-rate LIB anode materials capable of a high reversible lithium storage capacity (784 mAh g−1 at 1 °C
after 50 cycles), high CE (99%), excellent cycling stability, and superior rate
capability (568 mAh g−1 at 5 °C and 379 mAh g−1 at 10 °C).
Carbon-coated MoO2 nanobelts were synthesised by Wu et al. via HTC followed by carbonisation under an inert atmosphere, using α-MoO3 ­nanobelts
as precursor/self-template, ethanol as the reducing agent and glucose as
the carbon source.116 Under the protection of polymer resulting from glucose dehydration/HMF polymerisaton, the 1D morphology could be retained
during the reduction and carbonisation processes. Tested as LIB anodes, the
carbon-coated MoO2 nanobelts exhibited a reversible capacity of 769.3 mA h
g−1, at a current density of 100 mA h g−1 in the first cycle, retaining 80.2% of
the capacity after 30 cycles.
8.4.2.3 Silicon-Based Anode Materials
Various anode materials with improved storage capacity and thermal stability
have been proposed for LIBs in the last decade. Among these, Si has attracted
great interest as a candidate to replace commercial graphite owing to its
numerous appealing features. It has the highest theoretical capacity (Li4.4Si =
4200 mA h g−1) of all known materials. Furthermore, it is highly abundant, low
cost, and safe (lithiated silicon is more stable in typical electrolytes than lithiated graphite).117–120 However, practical use of Si powders as a negative LIBs
electrode is still hindered by two major problems: the low intrinsic electric
conductivity and severe volume changes during Li insertion/extraction, leading to poor cycling performance.121–123 Tremendous efforts have been made
to overcome these problems by decreasing the particle size, using Si-based
thin films and Si–metal alloys, dispersing Si in an inactive/active matrix, and
coating with carbon, as well as using different electrolyte systems. In these
approaches a variety of composites of active and inactive materials have been
widely exploited in which the inactive component plays a structural buffering
role to minimise the mechanical stress induced by the huge volume change
of active Si, thus preventing the deterioration of the electrode integrity.
The Si/C composites synthesised by HTC combine the advantageous properties of carbon (long cycle life) and silicon (high lithium-storage capacity)
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to improve the overall electrochemical performance of the LIB anode. In
contrast to high-temperature processes, HTC has been applied towards
the simultaneous coating of preformed Si NPs (D = 20–50 nm) in a one-step
procedure with a thin layer of SiOx and C (from glucose) to form Si@SiOx/C
nanocomposites (Figure 8.11(a) and (b)).96 In this report, pure Si NPs and
Si@SiOx/C nanocomposite electrodes were cycled at a current density of 150
mA g−1 between the voltage limits of 0.05–1 V in vinylene carbonate (VC)free and VC-containing 1M LiPF6 in ethylene carbonate/dimethyl carbonate
(EC/DMC) solution. It was observed that pure Si NPs deliver very high discharge and charge capacities of ca. 3200 and 1800 mA h g−1, respectively, in
the first cycle. However, after several cycles, the capacity rapidly decays to 20
mA h g−1 (Figure 8.11(c)). When VC was added to the electrolyte, the Si NPs
Figure 8.11
(a),
(b) HRTEM image of the silicon nanoparticles coated with SiOx and
carbon (c) galvanostatic discharge–charge curves (Li insertion, voltage decreases; Li extraction, voltage increases, respectively) of pure Si
nanoparticles (I, II) and Si@SiOx/C nanocomposite (III, IV) electrodes
cycled at a current density of 150 mA g−1 between voltage limits of
0.05–1 V in VC-free (I, III) and VC-containing (II, IV) 1M LiPF6 in EC/
DMC solutions. (d) Cycling and rate performance of pure Si nanoparticles and Si@SiOx/C nanocomposite electrodes cycled in VC-free and
VC-containing 1M LiPF6 in EC/DMC solutions (solid symbols: charge;
empty symbols: discharge.) Reproduced with permission from ref. 96.
© 2008, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.
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produced a slightly better cycling performance. The pure, nonpassivated Si@
SiOx/C nanocomposite also showed better cycling performance than the pure
Si NPs, but still exhibited a rapid capacity decay in the VC-free electrolyte,
suggesting that the SiOx/C coating on Si nanoparticles was still not sufficient
for achieving good cycling performance. Finally, when the Si@SiOx/C nanocomposite electrode was cycled in the VC-containing electrolyte, an excellent
cycling performance was achieved. The reversible capacity is as high as 1100
mA h g−1 at a current density of 150 mA g−1, with no further decay of capacity
even after 60 cycles. A large irreversible capacity was observed in the first
discharge and charge process; however, after the initial cycles, the CE was
> 99%. Rate performance of Si@SiOx/C nanocomposite electrodes cycled in
VC-containing were performed also high rate capability (Figure 8.11(d)).
Apart from the coating of Si, such NPs can also be inserted into the carbon
spheres as reported by Demir-Cakan et al.124 Here, HTC was used to produce
C–Si nanocomposites using glucose as the carbon precursor and Si NPs (D =
20–50 nm). In order to improve the stability and electrical conductivity of the
nanocomposite, after the HTC step, the resulting composites were further
carbonised at 900 °C for 2 h under N2. In contrast to other reports,96 it was
found that a higher carbon precursor ratio could be used so that a composite
material composed mainly of carbon and a low amount of silicon could be
obtained without any significant reduction in anode performance. Similar
C–Si composites have also been prepared by performing the HTC of glucose with a Pluronic F127 soft-template/pore-forming agent in the presence
of Si NPs, and a subsequent thermal carbonisation step.125 In the resulting
composite, the Si NPs were individually and separately coated with a porous
carbon shell of a thickness = 15–20 nm and a pore size of D = 3–5 nm. The
composite electrode exhibited excellent cycling stability and rate capability, delivering a stable capacity of 1607 mAh g−1 at a current density of 0.4
A g−1 after 100 cycles, and a reversible capacity of 1050 mAh g−1 even at a
high current density of 10 A g−1. Detailed analysis by CV and electrochemical impedance spectroscopy revealed that the composite showed favourable
electrochemical kinetics due to the nanosized porous carbon shell, which
facilitated the formation of a SEI film and the transportation of Li ions
and electrons, decreasing the charge-transfer resistance, thus significantly
improving the electrochemical performance compared with the bare nano-Si
electrode.
8.4.3
Cathode Materials
After the first assembly reported by Yoshino and Nakijima with the configuration of the Li1−xCoO2/C cell,126 there has been substantial research effort to
establish improved cathode materials. The most studied positive electrodes
for practical rechargeable LIB have been devoted to transition-metal oxides
such as LixMO2 (M = Co, Ni, Mn), LixMn2O4, LixV2O5 or LixV3O8.127 These
oxides are reasonably good ionic and electronic conductors and Li+ insertion/extraction proceeds in 4.0–5.0 V range (vs. Li+/Li). Safety concerns and
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cost are also encouraging the development of new positive electrode materials
based on 3D frameworks composed of transition metals and polyanions
(XO4)n. Despite the weight drawback, which results in smaller theoretical
gravimetric capacities, the inductive effect from the polyanions, arising from
the presence of groups such as (PO4)3–, (SiO4)4–, (SO4)2–, give rise to higher
redox potentials compared to those in more simple oxides.128,129 The presence
of polyanions (XO4)y− with strong X–O covalent bonds increases the potential
further as a result of the strong polarisation of oxygen ions toward the X
cation, which lowers the covalency of the M−O bond. Since the discovery of
the olivine-based lithium iron phosphate by Padhi et al.,128 LiFePO4 has been
highlighted as one of the most promising cathode materials for large-scale
LIBs owing to its high stability, environmental safety, high power, and low
cost. In LiFePO4, lithium is extracted reversibly from the triphylite LiFePO4 at
ca. 3.5 V (vs. Li+/Li). The triphylite LiFePO4 adopts the olivine structure type
and has a gravimetric capacity of 170 mA h g−1, generating a high energy density in the cell. Importantly, LiFePO4 is stable against overcharge or discharge
and is compatible with most electrolyte systems.
The main drawback of LiFePO4 as an electrode material lies in its low intrinsic electronic conductivity (1 × 10−9 S cm−1 at RT), which cannot be solved by
simple mixing of active particles with a sufficient amount of electron conductor additives.130 However, LiFePO4 becomes conductive in the presence of
small amounts of carbon,131,132 as well as when doped with various cations,
forming compounds of the type Li1–xMz+xFePO4 (z ≥ 2) with exceptional conductivities, due to charge compensation (10 × > than pure LiFePO4).133 To
overcome the low conductivity of LiFePO4, Titirici et al. have reported a onestep, template-free, low-temperature solvothermal synthesis of hierarchical
LiFePO4 mesocrystals coated with N-doped carbon, where N-acetylglucosamine acts as the carbon precursor and N source.134 Due to an increased conductivity of the N-doped carbon following thermal treatment of the initial
HTC product, the coated mesocrystal exhibited a superior performance compared with the pure LiFePO4. In a similar fashion, Paranthaman et al. modified the surface of rod-like LiFePO4 with a conductive N-doped carbon layer
using HTC followed by postannealing in the presence of an ionic liquid.135
The conductive surface-modified rod-like LiFePO4 exhibited good capacity
retention and high rate capability as the N-doped carbon layer improves conductivity and prevents aggregation of the rods during cycling.
Besides the significant amount of research and hope placed on olivine
LiFePO4 as a LIB cathode material for electric vehicle applications, alternative compositions and structures with similar theoretical capacities have also
been synthesised using the HTC synthesis platform. For example, the silicates are receiving increasing attention as a product of cell safety, as well as
the possibility of extracting > 1 Li+ ion per unit formula, and therefore a high
theoretical capacity.136 Additionally, the orthosilicate group material renders
excellent thermal stability through strong Si–O bonding. However, these compounds suffer from the same disadvantage as related olivine phosphates –
very poor electronic conductivity. Using HTC synthesis, Aravindan et al.
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258
prepared in one step, carbon-coated Li2MnSiO4 with a flower-like morpho­
logy and good electrochemical performance (100 mA h g−1).137 The improvement in electronic conductivity due to the carbon coating was validated
through electrochemical impedance spectroscopy.
Li-ion battery technology is at the focus of intensive R&D efforts by prominent research groups and industries throughout the world. After 25 years of
Li-ion batteries that power most commonly used portable electronic devices,
it seems that this technology has reached a high level of maturity that
enables it to be pushed towards more demanding applications. Opportunities exist for the chemist to bring together oxide and polymer or carbon (e.g.
­grapheme) chemistry in imaginative morphologies. Here, the main research
directions taken for improving the performance of the electrode materials
in LIBs has been reviewed with a focus on the HTC synthesis platform. As
such LIB technology relies on a rich and versatile chemistry, leading to a wide
range of attractive electrode materials for both positive (LiCoO2, LiMn2O4,
LiFePO4) and negative electrodes (C, Sn, Si, etc.).
8.4.4
Na-Ion Batteries
Wide-scale implementation of renewable energy will require growth in production of inexpensive, efficient energy-storage systems. In this context, LIBs
have played and will continue to perform a crucial role in the development
of energy-storage technologies due to the high energy and power, long cycle
life as well as operating at high temperature. Great improvements have been
accomplished in which current LIBs can provide ≥ 140 Wh kg−1 gravimetric energy density. However, as the use of large format LIBs becomes widespread, the cost of Li has roughly doubled from the first application in 1991
to today.138 Based on the wide availability and low cost of Na, sodium-ion
batteries (NIBs) have the potential to assist in meeting the large-scale grid
energy storage demands of the future (particularly with regards to mobility),
due to its natural abundance as well as the similar chemistry to LIBs.139,140
Since NIBs are an emerging technology, the discovery of new materials to
enable Na electrochemistry and fundamental mechanistic description is still
developing.
In LIBs, during charge and discharge, Li+ as a charge carrier diffuses into
electrode materials via intercalation, alloying or conversion reactions discussed earlier. As Na is located just below Li in the s block; similar chemistry
can be expected. However, due to the larger size (D = 1.02 Ǻ) of Na+ and the
higher redox potential (−2.71 V vs. SHE) of Na/Na+ compared to Li analogues
(−3.04 V vs. SHE), the different interactions between Na+ and host structures
can influence the kinetics and thermodynamic properties of NIBs. Unfortunately, one of the challenges of NIBs is the limited number of anode materials. Graphite, for example, has proven to be unfavourable in NIBs, unlike
the successful anodic application of lithiated graphite in LIBs (>95% of the
commercial LIBs use graphite).141 However, when different electrodes (e.g.
hard carbons, amorphous carbons, etc.) are used as intercalation media, the
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Na storage mechanism is different. Thus, several types of nongraphitic carbons have been tested, and capacities between 100 and 300 mA h g−1 under
differing conditions have been reported.142,143 Although these capacities
are promising, cyclability was insufficient for application in NIBs; the cells
were only cycled a few times and more importantly, could only be obtained
at extremely low currents or elevated temperatures, suggesting very slow
kinetics for Na+ storage. Therefore, alternative carbon materials are needed
in order to achieve satisfactory performance at room temperature and at
higher currents. A recent and very important report in the field describes
clear improvements with faster kinetics and higher capacity by introducing
nanoporosity and a hierarchical pore system into the carbon anode. High
capacities can be achieved at room temperature at a C rate of C/5, while also
exhibiting long cycling stability. The outstanding performance of this templated carbon is illustrated via comparison with several commercial porous
carbons and nonporous graphite as ref. 143. Since it was proven that Na+
can be inserted in disordered amorphous carbons, carbons produced via the
HTC platform can be considered potentially suitable as anode materials in
NIBs. In this respect, White, Maier et al. have recently reported on the use
HCNs as a negative electrode materials for NIBs.144
HTC-derived HCNs were prepared as described previously,88 followed by
thermal treatment at 1000 °C, to remove the latex template and increase the
conductivity of carbon (Figure 8.12(a) and (b)). The cycling stability during
Na+ insertion/extraction in the HCN was investigated at a current density of
50 mA g−1 for the first 10 cycles and then 100 mA g−1 for subsequent cycles.
CV and galvanostatic discharge/charge cycling (Figure 8.12(c) and (d)) were
performed to characterise the Na+ ion insertion/extraction properties. In the
first CV cycle, pronounced cathodic peaks were observed at 1.39 V, 0.36 V and
near 0 V (Figure 8.12(c)). The peak at 1.39 V was attributed to the reaction of
Na+ with functional group(s) at the carbon surface. The peak at 0.36 V was
assumed to be the result of electrolyte decomposition, leading to the formation of an SEI. The clear Na+ insertion peak observed at lower voltages near
0.0 V is similar for Li+ insertion in carbonaceous materials. Furthermore, a
clear peak at 0.11 V in the reverse cycle was observed, a feature attributed
to Na+ extraction from shell nanopores. The observed capacity loss over the
initial cycling steps stems from SEI film stabilisation and irreversible Na-ion
insertion. After 100 cycles, a reversible capacity of ca. 160 mA h g−1 was stably
maintained (Figure 8.12(e)). The CE approached to 94% after several cycles,
whilst the observed irreversible capacity during each cycle was attributed to
the incomplete stabilisation of the SEI for the presented NIB system. The
electrochemical impedance spectra of HCN electrode was measured with
the corresponding Nyquist plots consisting of a depressed semicircle in the
high- and middle-frequency regions, and a straight line in the low-frequency
region (Figure 8.12(f)). The semicircle could be attributed to the SEI film
and contact resistance at high frequencies, and a charge-transfer process in
the middle frequency, while the linear increase in the low-frequency range
may reflect Warburg impedance associated with Na+ diffusion in the carbon
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Figure 8.12
(a)
TEM image, (b) HR-TEM image of hollow carbon nanospheres,
(c) CV of hollow carbon nanospheres showing the first three cycles
between 3 V and 0 V at a scan rate of 0.1 mV s−1, (d) Galvanostatic
charge/discharge curves at a current rate of 50 mA g−1, (e) Cycle performance of hollow carbon nanospheres, f) Impedance spectra of
hollow carbon nanospheres electrode after 10th, 30th, 50th, and
80th cycle. The inset is the enlarged spectra. Reproduced with permission from ref. 144. © 2012, Wiley-VCH Verlag GmbH & Co. KGaA,
Weinheim.
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electrode. The rate performance of these materials was also excellent and
clearly superior to nonporous HTC-synthesised carbon spheres,85 proving
the important role played by the hollow-sphere material morphology.
8.4.5
Li–S Batteries
Rechargeable lithium–sulfur batteries (LSBs) have interested the battery community for many years and this technology is proposed to play a very important
role in next-generation sustainable, electromobility devices.145 LSBs offer a
five-fold increase in energy density compared with present LIBs, whilst the
LSB configuration allows operation at room temperature and provides a
low equivalent weight, high capacity (1672 mA h g−1), low cost (ca. $150 per
ton) and environmentally benign factors. All these characteristics cannot be
accomplished with current LIB technology. In an LSB cell the overall redox
couple, described by the reaction S8+16Li↔8Li2S lies at an average voltage of
ca. 2.2 V vs. Li. However, LSB technology faces several drawbacks leading to a
poor life cycle that prevent its practical realisation. Each cell compartment,
namely the sulfur cathode, the metallic lithium anode and the electrolyte are
equally responsible for the limited performance of LSB cells; (i) the insulating
nature of sulfur always requires close contact with conductive additives, (ii)
the soluble polysulfide species (Li2Sx, x > 2) generated during the battery
operation diffuse throughout the separator and deposit on the Li electrode
resulting in a loss of active material, (iii) the use of Li metal anode adds a
number of safety issues (e.g. liquid electrolyte decomposition), (iv) formation
of the nonsoluble Li2S product that is a highly insulating material and results
in unstable electrochemical contact within sulfur electrodes.146,147
To address these issues a number of innovations have been reported. For
instance, modifying electrolyte formulation via additives in order to form a
protective surface film on the Li electrode,146 or using polymers rather than
liquid-type electrolytes was attempted,148 with the aim to restrict polysulfide solubility. Additionally, several approaches regarding the preparation
of highly electronic conducting, porous C/S composites have been reported,
in order to capture dissolved polysulfide species within the electrode configuration. Perhaps the most elegant of these approaches is that proposed
by Nazar et al.149 Here, an ordered mesoporous carbon composite was produced to provide both an electronic percolation path through the electrode
and an adequately controlled porosity to retain part of the electrochemically
generated polysulfides species. Following this initial study, many different
type of carbon have since been employed to confine sulfur and its reduced
species.150–153
The first HTC materials tested as cathodes in LSB were HCNs similar to
those discussed earlier in the context of LIB88 and NIB144 electrodes. However,
in this later report, the HCNs were prepared using silica templates instead of
latex.154 Three different HTC/S composites were compared. The first two were
hollow spheres; the first was prepared via diffusing molten sulfur into the
solid and the second by simple hand milling. The third tested material was
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a nonporous HTC carbon infiltrated with sulfur by simple physical mixing.
Stable cycling properties could be obtained when using the HCN materials,
indicating that the highly porous shells act as an absorbent for soluble polysulfides, especially since a very small amount of silica remaining from the
template was also present in this composite, which was previously proved to
be beneficial for polysulfide adsorption.150 A HTC-based nanocasting method
has also recently been reported based on hierarchically meso-/macroporous
silica monolith templates for the synthesis of hierarchically porous carbon
monoliths for use in LSBs.155 The selection of post-HTC-synthesis thermal
carbonisation temperature allowed control of porosity at different length
scales, functionality and conductivity, affording the opportunity to adapt the
synthetic parameters for LSB application. A large amount of sulfur was subsequently infiltrated within the carbonaceous scaffolds reaching an initial
discharge capacity of 1305 mA h g−1 at a current density of 167.5 mA g−1. More
recent work from the same group has also demonstrated the combined good
cycle ability and rate performance of HCNs with the high specific capacity
at first discharge of N-doped carbogels. The beneficial contribution of each
constituent material, i.e., N-doped carbogel and HCNs, led to a promising
synergistic effect, with a high specific capacity of >700 mA h g−1 reported with
limited fading over 25 cycles.156
8.5
CO2 Capture
The mitigation of CO2 emissions is a crucial issue as this gas is the main
anthropogenic contributor to climate change. Among the possible strategies for CO2 abatement, “carbon capture and storage (CCS)” strategies have
attracted significant interest. In this regard, the use of solid sorbents to capture CO2 in pressure, temperature or vacuum swing adsorption systems constitutes a promising alternative.157 To accomplish this objective, the sorbent
needs to satisfy important conditions: (i) low cost and high availability, (ii)
large CO2 uptake, (iii) high sorption rate, (iv) good selectivity between CO2
and other competing gases (i.e. N2); and (v) easy (ideally low energy) regeneration. However, the development of a solid sorbent that satisfies all these
conditions has proved so far to be complex. Taking into account the potential scale involved in the production of porous carbons for CO2 capture, the
use of renewable sources for fabricating these materials would seem highly
desirable. In this respect, low-cost, sustainable porous carbons such as those
derived from HTC materials would constitute a good alternative, particularly
if the carbon precursor is derived from waste or low-value biomass.
The CO2-capture performance of HTC-derived activated carbons was
investigated for the first time by Sevilla and Fuertes utilising porous carbons
obtained via the chemical activation with KOH of several HTC carbons prepared from starch, cellulose and eucalyptus sawdust.158 They observed that
the textural properties of the activated carbons derived from the different precursors are similar, which demonstrates that an inexpensive and widely available biomass subproduct such as sawdust constitutes an excellent precursor
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for the preparation of chemically activated carbons via hydrothermal carbons. These porous materials exhibited SBET of 1260 to 2850 m2 g−1, and large
Vps, in the 0.62 to 1.35 cm3 g−1 range. Both SBET and Vp are mostly associated to
micropores. The CO2 adsorption uptake of some of these materials is listed
in Table 8.1 for a pressure of 1 bar and three adsorption temperatures (i.e. 0,
25 and 50 °C). The CO2 capture capacities of the reported carbons prepared
from the different precursors are quite substantial and indeed similar (ca.
5.5–5.8 mmol(CO2) g−1), presumably the product of analogous pore characteristics of these materials. All the prepared materials exhibited CO2 capture
capacities of between, 5.2–6.6 mmol(CO2) g−1 at 0 °C, 2.9–4.8 mmol(CO2) g−1
at 25 °C and 1.8–3.6 mmol(CO2) g−1 at 50 °C. It should be noted that activated
carbons prepared from sawdust-derived HTC carbon under mild activation
conditions (KOH/HTC carbon = 2) induced improved CO2 capture capacities
compared to higher activation ratios (i.e. KOH/HTC carbon = 4). Thus, at 25
°C, CO2 adsorption capacities ≥6.6 mmol(CO2) g−1 and 4.8 mmol(CO2) g−1 are
obtained for samples prepared with KOH/HTC carbon mass ratio of 2 and
reaction temperatures of 700 °C and 600 °C, respectively. These outstanding
CO2 adsorption uptakes are ascribed to the fact that a large fraction of the
porosity of the mildly activated HTC samples corresponds to narrow micropores, which have strong adsorption potentials that enhance their filling by
the CO2 molecules.159
For practical applications, in addition to a high CO2 adsorption capacity,
sorbents need to show fast adsorption kinetics, a high selectivity towards
CO2 and they must also be easy to regenerate. Sevilla and Fuertes examined
the performance of the activated carbons prepared from sawdust-based HTC
carbon in relation to these prerequisites.158 CO2 adsorption is very fast, ca.
95% of CO2 uptake occurring in a span of 2 min (Figure 8.13). By contrast, N2
adsorption is slower, ca. 60 min required for maximum adsorption uptake,
whilst the [CO2/N2] selectivity measured under equilibrium conditions is 5.4.
Table 8.1 CO
2 capture capacities of hydrothermal carbon-based activated carbons
at different adsorption temperatures and 1 atm (equilibrium measurements unless otherwise stated)
CO2 uptake, mmol g−1 (mg⋅g−1)
Chemical activation
HTC precursor
T (oC)
KOH/HTC
0 °C
25 °C
50 °C
Ref.
Starch
Cellulose
Eucalyptus
sawdust
700
700
600
650
700
650
700
600
750
4
4
2
5.6 (247)
5.8 (256)
6.1 (270)
6.0 (262)
6.6 (288)
7.0 (306)
7.4 (325)
2.4 (105)a
–
3.5 (152)
3.5 (155)
4.8 (212)
4.7 (206)
4.3 (190)
4.4 (192)
4.5 (198)
1.4 (61.4)a
2.8 (123)
2.2 (196)
1.8 (79)
3.6 (158)
3.3 (145)
2.6 (116)
2.8 (125)
2.8 (123)
0.41 (18.2)a
–
158
158
158
Algae + glucose
E. prolifera
Rye straw
a
2
1
3
Dynamic uptake of CO2 using 15% CO2 (v/v) in N2.
158
160
161
162
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Figure 8.13
Chapter 8
(a)
Adsorption kinetics of CO2 and N2 at 25 °C and (b) CO2 ­adsorption–
desorption cycles obtained at 25 °C (CO2 concentration: 100%). The
carbon sample used in this experiment was prepared by activation
of a sawdust-based hydrothermal at 600 °C with KOH/hydrothermal
carbon = 2. Reproduced with permission from ref. 158. © 2011, Royal
Society of Chemistry.
Easy regeneration is another critical property that must be considered when
designing CO2 sorbents. In this respect, for the HTC-based carbons > 95% of
CO2 is desorbed within 3 min under these conditions, as demonstrated by
the adsorption–desorption cycles (Figure 8.8(b)). These cycles were repeated
seven times with no noticeable changes observed in desorption kinetics or
CO2 uptake.
Sevilla et al. have also recently analysed the CO2 capture capacity of highly
microporous N-doped carbons obtained by chemical activation with KOH of
HTC carbons obtained from mixtures of algae and glucose.160 These materials exhibit apparent SBET in the 1300–2400 m2 g−1 range and Vp ≥ 1.2 cm3
g−1, composed of uniform micropores, most of which with D < 1 nm. N-content was characterised in the range of 1.1–4.7 wt%, as mainly pyridone-type
structures (with minor contributions from pyridinic-N and quaternary-N).
These microporous carbons presented unprecedented capture capacities, ≥
7.4 mmol(CO2) g−1 (1 bar, 0 °C; Table 8.1). A good correlation between the
CO2 capture capacity at subatmospheric pressure and the volume of narrow
micropores was observed. The obtained results also suggested that pyridinic-N, pyridonic/pyrrolic-N and quaternary-N do not contribute significantly
to the CO2 adsorption capacity, owing probably to their low basicity in comparison with amines. This has been further confirmed very recently by Sevilla
and Fuertes through the analysis of N-free and N-doped porous carbons with
analogous SBET, Vp and PSDs.159
Zhang et al. also analysed N-containing porous carbons obtained via HTC +
KOH chemical activation as CO2 sorbents.161 Here, the ocean pollutant,
Enteromorpha prolifera was used as a carbon precursor, and the resulting
materials contained ≥ 2.6 wt%(N). The inorganic minerals contained in the
carbon matrix contributed to the development of mesoporosity and macroporosity, functioning as an in situ hard template, leading to high CO2 capacity
and facile regeneration at room temperature. The CO2 sorption performance
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was investigated in the range of 0–75 °C. The dynamic uptake of CO2 is 61.4
and 105 mg g−1 (1.4 mmol g−1 and 2.4 mmol g−1 respectively) at 25 and 0 °C,
respectively, using 15% CO2 (v/v) in N2 (Table 8.1). Meanwhile, regeneration
under Ar at 25 °C allowed 89% recovery of the carbon’s initial uptake after
eight cycles. More recently, Falco et al. have also employed a HTC + KOH
chemical activation approach for the synthesis of highly porous carbons
from glucose, cellulose and rye straw.162 The influence of the precursor and
HTC temperature on the porous properties of the resulting activated carbons
was investigated, with HTC temperature proving to be an extremely influential parameter on porosity development and micropore PSD. Thus, the use
of higher HTC temperatures (i.e. 280 °C) led to lower porosity development,
but a narrower PSD composed mostly of micropores. KOH chemical activation of the HTC carbons synthesised at lower temperatures (i.e. 180–240 °C),
produced activated carbons with higher total Vp and broader PSDs. The CO2
capture performance of these materials was analysed at 0 and 25 °C up to 30
and 40 bar, respectively, recording values ca. 25 mmol g−1 at 30 bar at 0 °C,
comparable to capacities obtained with superactivated carbons prepared by
KOH activation of anthracites with similar micropore volume.163 In the case
of adsorption at room temperature (25 °C), these materials adsorb up to 2.80
mmol(CO2) g−1 at 1 bar, and almost 20 mmol(CO2) g−1 at 40 bar.
All the studies described so far are based on KOH chemical activation to
introduce porosity in HTC carbons and enhance CO2 capture capacity. An
alternative and interesting approach for the use of HTC carbons for the CO2
capture is based on their functionalisation with amine groups that exhibit a
high affinity to CO2. In this regard, CO2 capture by means of an amine-rich
HTC carbon has been reported.164 This material was prepared by a two-step
process: a) HTC of glucose in the presence of small amounts of acrylic acid
and b) functionalisation of the carboxylic-rich HTC carbons with triethylamine. This aminated HTC carbon had a high CO2 capture capacity (≥ 4.3
mmol(CO2) g−1 at −20 °C). More importantly, these materials exhibited a very
high [CO2/N2] selectivity at low (−20 °C) and high (70 °C) temperatures, up to
110 at 70 °C. As extension of this work, Yang et al. have fabricated N-enriched
carbonaceous materials with hierarchical micro-/mesopore structures.165
In this report, porous carboxyl-rich carbons were prepared via the HTC of
glucose in the presence of acrylic acid and Brij 32 – a nonionic surfactant
structure-directing agent – leading to the development of a hierarchical pore
structure. Nitrogen was introduced to the surface of the materials via an acylation–amidation route using tetraethylenepentamine. The resulting materials possessed SBETs of 640–660 m2 g−1 and a micro-/mesoporous structure,
with the mesopores size distribution centred at D = 3.3–3.4 nm. The “N” content was found to be ≥ 11.6 wt%, existing at the material surface as amides,
imines, and primary amines. The CO2 capture capacity of the material with
the highest “N” content was 3.2 mmol g−1 at 25 °C, 2.4 mmol g−1 at 50 °C
and 1.7 mmol g−1 at 75 °C (at 1 bar). At high capture temperatures, i.e. 75 °C,
the capacity of this material doubled to values equivalent to traditional activated carbons due to CO2 chemisorption on the amine-rich material surface.
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Furthermore, CO2/N2 selectivity is 46 at 25 °C – a value superior to that
often observed in physisorption-based sorbents. The material also showed
good reversibility for CO2 adsorption and could be regenerated under mild
conditions.
8.6
Conclusion
This chapter presented the use of the HTC synthesis pathway for the preparation of nanostructured carbonaceous materials in the context of energy
storage and conversion as well as equally important gas (i.e. CO2) sorption
applications. As a stark contrast to the harsh, high-cost and energyconsuming technologies normally employed for the production of typical
carbonaceous materials, HTC represents a green and sustainable alternative, wherein the precursors are biomass-based and the reaction takes place
in pure water at mild temperatures without employing any hazardous surfactants or catalysts. Furthermore, the resulting materials are highly porous
solids and their surfaces are decorated with polar functional groups, thus
making them hydrophilic and functional. As was described, the functionality can be utilised for the introduction of useful chemical moieties for specific applications (e.g. amines for CO2 capture). The HTC approach offers
opportunities for the production of new and exciting low-cost carbon-based
materials, with potentially tuneable chemical functionality, crystallinity/conductivity and activity, with applications in different multidisciplinary fields
of increasing significance (i.e. batteries, fuel cells and CO2 capture). These
points are serious advantages in terms of materials production particularly
as compared to conventional synthesis routes as it offers a wide scope for
the development of designer materials for specific processes (e.g. in electrochemical applications – 2- vs. 4-electron electrocatalysis). It is hoped that
this chapter raises the potential impact of sustainable materials in the mind
of the reader in the context of energy and environmental applications. The
significance of the development of energy or capture materials via the HTC
platform is raised further if the carbon precursors employed in the synthesis are low-value compounds or biomass wastes, which would otherwise be
degraded in the biosphere (e.g. to release CO2 or other greenhouse gases to
the atmosphere). In this context, there is now a necessity to consider lifecycle analysis to demonstrate the benefits of these materials in the broader
sense of sustainability and of course costs. However, these HTC materials
and carbonaceous materials in general have a real potential in future energystorage technologies.
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149.X. Ji, K. T. Lee and L. F. Nazar, Nat. Mater., 2009, 8, 500–506.
150.R. Demir-Cakan, M. Morcrette, F. Nouar, C. Davoisne, T. Devic, D. Gonbeau,
R. Dominko, C. Serre, G. Ferey and J.-M. Tarascon, J. Am. Chem. Soc., 2011,
133, 16154–16160.
151.N. Jayaprakash, J. Shen, S. S. Moganty, A. Corona and L. A. Archer,
Angew. Chem., Int. Ed., 2011, 50, 5904–5908.
152.C. Liang, N. J. Dudney and J. Y. Howe, Chem. Mater., 2009, 21, 4724–4730.
153.H. Wang, Y. Yang, Y. Liang, J. T. Robinson, Y. Li, A. Jackson, Y. Cui and
H. Dai, Nano Lett., 2011, 11, 2644–2647.
154.N. Brun, K. Sakaushi, L. Yu, L. Giebeler, J. Eckert and M. M. Titirici,
Phys. Chem. Chem. Phys., 2013, 15, 6080–6087.
155.T. Y. Ma, L. Liu and Z. Y. Yuan, Chem. Soc. Rev., 2013, 42, 3977–4003.
156.N. Brun, K. Sakaushi, J. Eckert and M. M. Titirici, ACS Sustainable Chem.
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157.N. Hedin, L. Chen and A. Laaksonen, Nanoscale, 2010, 2, 1819–1841.
158.M. Sevilla and A. B. Fuertes, Energy Environ. Sci., 2011, 4, 1765–1771.
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PART 3
CHARACTERISATION OF POROUS
CARBONACEOUS SOLIDS
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CHAPTER 9
Porosity Characterisation
of Carbon Materials
JENS WEBER
Hochschule Zittau/Görlitz – University of Applied Science, Fachgruppe
­Chemie, Theodor-Körner-Allee 16, 02763 Zittau, Germany
E-mail: j.weber@hszg.de
9.1 Introduction and Definitions
A material is defined as porous if it contains void spaces, which go beyond plain
surface roughness, i.e. the voids need to be deeper than they are wide. Ideally,
the porosity of a material is distributed throughout the whole body, but for
certain applications, anisotropic porosity distribution might even be favourable. Porosity is accompanied always with an increased material surface area
compared to the nonporous equivalent. Surface area is an important factor for
many processes, ranging from adsorption to catalysis and many more. Hence,
there has always been a great interest in porous materials and porous carbons
are an example of the oldest (several thousand years) materials that mankind
has used for technological purposes, e.g. for adsorption.
As highlighted throughout this book, porous carbons still play an enormously important role in modern technology, ranging from well-established
adsorption and purification processes to more recent and emerging technologies such as supercapacitors or fuel-cell electrodes. The fine description of
a material’s porosity is important for any application and it comes as no surprise that the characterisation of porous carbons has always been and still is
a very active field. This chapter intends to provide an overview on the most
RSC Green Chemistry No. 32
Porous Carbon Materials from Sustainable Precursors
Edited by Robin J White
© The Royal Society of Chemistry 2015
Published by the Royal Society of Chemistry, www.rsc.org
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Chapter 9
important and widespread methods of characterisation in this context with
a specific reference to the description of carbonaceous materials. A focus is
also set on more recent developments and questions and it is hoped that
the chapter can provide both: (1) a basic introduction for any scientist interested to porosity characterisation and (2) also a summary of the state-of-theart regarding recent advances and open questions. The chapter is organised
as follows: The introduction will provide the necessary information on the
nomenclature and physical quantities needed for the description of carbon materials. The central component of the chapter will introduce typical
methods for porosity characterisation including gas adsorption, scattering
and diffraction methods and microscopy as well as a few less common techniques. The final section of the chapter will provide a brief introduction to
other characterisation techniques (e.g. nuclear magnetic resonance (NMR)
spectroscopy), which will be covered in more detail in Chapters 10 and 11.
9.2 Definitions
Porous carbons can – as any porous material – be subdivided based on porosity details. It is possible to distinguish between open and closed porosity, i.e.
based on whether pores of the material are accessible from the outside or
not. To determine whether the pore space is accessible or not is highly importance for many applications, but it is certainly not trivial in its determination.
Generally, a combination of methods are employed to resolve this question.
The main classification of porous materials is, however, based on the size
of the permanent pores in the dry state. According to the definitions provided by the International Union of Pure and Applied Chemistry IUPAC,1
three size regimes are distinguished:
●● Micropores have the smallest pore sizes of diameters (D) or widths
smaller than 2 nm.
●● Mesopores have sizes between 2 and 50 nm.
●● Macropores have pore diameters >50 nm without any upper limit.
This classification of pore size can be understood based on the forces and
potentials that are created as a consequence of pore dimensions. Generally,
any interface provides an attractive potential to other molecules as a consequence of intermolecular forces (van-der-Waals/dispersion forces). In the case
of microporous materials, the attractive potentials overlap within the given
size range, resulting in – simply speaking – increased interactions compared to
planar or weakly curved surfaces. As a consequence, micropores are believed
to be filled by a different process compared to larger pores and are discussed
separately. Mesopores (i.e. D = 2–50 nm) also have special features compared to
very large pores. Fluids confined in such pores experience a strong curvature,
which couples back to their thermodynamic behaviour via the Young–Laplace
equation. This generates a drastically lowered melting point for the confined
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Porosity Characterisation of Carbon Materials
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fluid and reduces the condensation pressure of gases, i.e. gases condense at
pressures much below their bulk saturation pressure. These effects are utilised
in the characterisation of porous materials by gas adsorption or cryoporometry. Curvature effects become strongly reduced as the (pore) radius increases
and can be neglected for very large pores. This is the basis of the classification of macropores where neither overlapping potentials nor strong curvature
effects are active. It is worth mentioning that the surface area at constant pore
volume is highest for micropores and lowest for macropores.
Material porosity is usually characterised by a set of parameters; the most
important of these area: a) specific surface area S (or sometimes SA or SSA),
in m2 g−1; b) specific pore volume Vpore, in cm3 g−1 and its distribution with
regard to the pore radius rpore; and c) Porosity (dimensionless) as the volume fraction of the pores. The determination of porosity usually requires the
knowledge of the density of the material, a parameter that cannot be defined
easily for a porous body. Two different densities can be distinguished: the
skeletal (sometimes also called framework density or true density) ρsk, and
the apparent density ρapp (macroscopic density). The skeletal density is the
density of the pore-wall material (e.g. 2.1 for amorphous silica). It is usually
measured by pycnometry, typically by gas pycnometry under the assumption
that all pores are accessible. If some pores are not accessible, the material
would show a lower skeletal density than expected. If a macroscopic body of
the material (e.g. an activated carbon pellet) exists that can be weighed and
whose dimensions (volume) can be measured, its apparent density ρapp can
be calculated based on the quotient of its mass and volume. The apparent
density of a porous body is lower than its skeletal density if at least a few
pores are accessible from the outside. In the case that all pores are not accessible, the skeletal density should be equal to the apparent density.
The defined porosity descriptors above can be measured by different methods and depending on the choice of methods there might be differences in
the obtained values. For example, the pore volume of all pore size ranges
from nano- to micrometer scale cannot be measured by a single method due
to resolution issues. Hence, it is advisable to clearly state the method used
when reporting porosity parameters. The advantages and limitation of the
different methods will be discussed in the next section.
9.3 Methods
9.3.1 Gas Adsorption Techniques
9.3.1.1 Overview
The adsorption of gases on surfaces has long been known and its use for the analysis of porous materials is one of the most commonly employed and reported.
It is mainly used for the analysis of micro- and mesoporous materials, where it
gives access to S, Vpore and even pore size/volume distributions. Macroporous
carbons can only be accurately analysed with regard to their specific surface area.
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Two processes can be distinguished based on the strength of the gas–surface interactions:
●● Physisorption – involves only weak interactions (mainly van-der-Waalstype, adsorption heat: 5–50 kJ mol−1).
●● Chemisorption – involves the formation of strong ionic or covalent
bonds between the surface and the gas (adsorption heat typically 100–
200 kJ mol−1).
Most methods used for the characterisation of porous carbons make use
of physisorption processes – N2 sorption at 77.4 K being the most prominent
example. Chemisorption on the other hand is especially useful for the determination of catalytically active sites. More detailed information on the use
of chemisorption for the analysis of catalytically active carbons can be found
in the literature.2,3
The next subsections will discuss the use of gas physisorption in porosity characterisation. Different modes of interaction between the gas and
the surface can be observed depending on the thermodynamic state of
the adsorbate (Scheme 9.1). For macroporous substrates, the formation of
mono- and multilayers of adsorbate can be observed, allowing the extraction
of information on the accessible specific surface area. Pore filling is rarely
observed in macropores under standard operation conditions. In contrast,
gases condense and hence fill micro- and mesopores principally at subcritical temperatures, while no condensation can be expected at supercritical
conditions. Condensation can occur at subambient pressures or upon application of high pressures, depending on the saturation pressure of the gas at
the given temperature.
Scheme 9.1 (A) Formation of mono- and multilayers by adsorption from the gas
phase, valid for all pore sizes and outer surface areas materials except
micropores; (B) The attractive potentials in micropores overlap, resulting in micropore filling rather than formation of mono- and multilayers. (C) Capillary condensation is observed in mesopores after the
formation of multilayers of adsorbate.
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The different interaction modes allow the distinction of different experiments, which will be discussed. The “classic” N2 or Ar sorption experiments,
which are conducted for extraction of S and micro- and mesopore volume
and associated size distributions are conducted at the condensation temperature of the gas at ambient pressure, that is at 77.4 K for N2 and 87.3
K for argon. The experiment hence runs at subcritical conditions (reduced
temperature: T* = 0.58 to 0.61) and subambient pressures. In contrast, H2
sorption at 77.4 K is performed at supercritical temperatures (T* = 2.35). As a
consequence, material micro- or mesopores can only be filled upon application of high pressures of the order of several MPa. Finally, the widespread use
of CO2 sorption at 273.15 K will be discussed. In this case, the temperature is
already close to the critical temperature (T* = 0.9 K) and the saturation pressure is high. As a consequence, only micropores can be filled (and hence analysed with regard to their size and volume) at subambient pressures, while
the filling of mesopores requires the application of higher pressures.
A sorption isotherm (taking the adsorption step as an example) is measured in all the above-mentioned experiments, i.e. the adsorbed amount of
gas (expressed as specific molar amount or specific volume uptake) vs. the
applied pressure (given either as absolute pressure or as relative pressure
p/p0, where p0 is the saturation pressure of the gas at the given temperature).
Experiments involving subambient pressures are usually performed using
so-called “volumetric” machines. These might be better termed “manometric”, as the determination of the adsorbed gas volume is based on measurements of pressure changes inside the measurement cell. These occur as a
consequence of the adsorption process. Conversely, high-pressure measurements are performed mostly on a gravimetric basis, i.e. the adsorbed amount
is indeed determined by weighing of the sample using special balances. More
information on the experimental details can be obtained from manufacturers of the machines or relevant literature.4–6
9.3.1.2 Cryogenic N2/Ar Sorption – The Classical Approach
N2 sorption experiments at 77.4 K (as well as Ar sorption at 87.3 K) can give
access to parameters including specific surface area, pore-size distribution
and pore volumes. The experiment is usually conducted using automated,
manometric machines. The uptake of gas is usually given in terms of excess
adsorption amount (vs. He) as a function of relative pressure p/p0, where p0 is
the respective saturation pressure of the gas at the chosen temperature. The
p/p0 range is usually chosen between 0.01 and 0.995. Dedicated analysis of
microporous materials requires the measurement down to relative pressures
of 10−5 to 10−7. The corresponding sorption isotherms can be differentiated
into several types, depending on the porous nature of the material (Figure
9.1).1 This classification is based on an IUPAC report from 1985. An update
on this report is currently being prepared by a IUPAC working group. The
most important isotherm types, typically observed for N2 sorption, are types
I, II and IV. Type I isotherms are commonly found for material presenting
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Chapter 9
Figure 9.1 Types of adsorption/desorption isotherms according to ref. 1, Repro-
duced with permission from ref. 1. (Please note that the y-axes are not
scaled, allowing for no direct comparison on the adsorbed amount.)
micropores and very small mesopores (D ∼ 2–4 nm). Micropores are filled
by adsorbate already at very low relative pressures, so a plateau is reached
after all of them have been filled. Type II (absence of a hysteresis) is common
for macroporous materials or small particles, where multilayer formation
(and in some cases adsorbate condensation at very high relative pressures)
is observed. Type IV is very commonly observed for mesoporous materials,
with the hysteresis profile characteristic of mesoporous domains with the isotherm shape reflective of differing mesopore organisation/geometries. Generally speaking, the observation of parallel adsorption/desorption branches
in the hysteresis region can be understood by the existence of well-connected
mesopores.
Specific surface area is usually determined by application of the Brunauer-­
Emmett-Teller (BET) model, which is an extension of the well-known Langmuir model. The basis of both models is the formation of a physically bonded
adsorbate monolayer on the adsorbent surface under the assumption that
all adsorption sites are energetically homogeneous. The BET model does (in
contrast to the Langmuir model) take the formation of multilayers of adsorbate into account. This adds a further level of complexity to the underlying
formulas compared to the simpler Langmuir model. Further details pertaining to the mathematical derivation of these models are found in the relevant literature. Briefly and importantly in the context of this chapter, the
BET model gives access to the monolayer volume Vmono that is linked to the
specific surface area (eqn (9.1)) of a material under the assumption that N
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adsorbate molecules of known cross section σ and molar volume Vmolar form
a monolayer with a corresponding volume Vmono.
SBET =
N ⋅ σ ⋅ Vmono
Vmolar ⋅ mAdsorbent
(9.1)
Generally, the BET model can be applied to any adsorption isotherm,
regardless if the material is micro-/meso- or macroporous. The BET model
was, however, initially developed to describe flat surfaces and is hence best
applied to meso- and macroporous materials, i.e. materials with relatively
large pores. It is important to note that the basis of this model, namely the
formation of multilayers, is no longer valid when applied to microporous
materials. In such materials, there is an overlap of the attractive potentials of
the pore walls, which lead to the volume filling of micropores. Still, the BET
model remains one of the most widespread methods for analysing the specific surface areas of microporous materials and provides a common basis
for comparison of materials. There is indeed some backup for this (formally
not correct) procedure, which was obtained from a theoretical and computational assessment.7,8 In the case of disordered microporous materials (e.g.
microporous polymers) it has been observed, however, that the BET model
gives results that do strongly deviate from geometric surface areas.9 This fact
was mostly attributed to morphological issues and pore topology and is in
conceptual agreement with other studies on the problem of micropore accessibility.10 The problems are most severe in the case of the so-called low-pressure hysteresis. This effect manifests itself during desorption such that this
component of the isotherm does not reunite with the adsorption branch
even at very low pressure and long equilibration times,11 is related to pore
connectivity and accessibility issues (Figure 9.2). These can dominate especially in the case of very small and disordered micropores and the existence
of such hysteresis behaviour might hence give information on pore connectivity. If one takes the mentioned issues into account, the BET model will
continue as an important description of material surface analysis, although
it might need future assistance from other models (i.e. density functional
theory (DFT)-based analysis) or methods (CO2 adsorption, scattering, etc.;
see below) if very detailed information is needed.
The extraction of pore-size and volume information from N2 or Ar sorption
isotherms is possible via a number of different approaches, depending on the
predominant pore size. Micropores have been analysed by (semi)empirical
methods including the Horvath–Kawazoe (HK),12 or Dubinin–Radushkevich
(DR) methods.13,14 These approaches have been long-used and are still used,
but might be considered outdated since the rapid development of DFT-based
methods. The interested reader is referred to a variety of authoritative literature on these approaches,4,6,14,15 while the DFT-based models will be discussed later (Section 9.3.1.3).
The extraction of pore-size information of mesoporous materials by classical approaches will be discussed shortly, as the thermodynamic foundations
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284
Figure 9.2 On the existence of low-pressure hysteresis. (A) case of insufficient
degassing or equilibration conditions: N2 adsorption–desorption isotherms at 77.4 K on activated carbons LMA-232 (red) and LMA-233
(Blue), measured using different equilibration times: closed symbols:
80 s and open symbols: 300 s; Reproduced with permission from ref. 11,
(B) existence of pronounced low-pressure hysteresis (N2 adsorption
at 77.4 K on a microporous polymer material), hysteresis stays present even after long equilibration times; Reproduced with permission
from ref. 10.
are the same as for other methods, namely cryoporometry and mercury
intrusion methods. Fluids that experience a strong curvature (e.g. via meniscus formation of only a few nanometers) generally respond via a change in
their thermodynamic properties (in comparison to the bulk state). The wellknown Young–Laplace equation (eqn (9.2)) relates in its simplest form the
pressure difference to the radius of the meniscus r and the surface tension
of the liquid γ:
Δp =
2γ
r
(9.2)
Based on this equation, it can be postulated (in a simplistic fashion) that
gases will condense at lower pressure than their respective bulk saturation
pressure if confined to small pores. The relation between the radius of the
respective meniscus and the relative pressure (p/p0) is given by the Kelvin
equation (eqn (9.3)), where Rgas is the universal gas constant and T the absolute temperature.
2γ Vmolar
p
ln ⎛⎜ ⎞⎟ =
⎝ p0 ⎠ r ⋅ Rgas T
(9.3)
Under the assumption that mesopore walls are already covered with an
adsorbate layer of a thickness t before the onset of fluid condensation, the
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pore size of a cylindrical pore can be estimated from rpore = rmeniscus + t. This
formula is the basis of the classical BJH mesopore analysis protocol, which is
named after the inventors Barrett, Joyner and Halenda. The method assumes
that the total mesopore volume can be subdivided into various fractions of
pores with increasing radius, resulting in a methodology for the analysis of
mesopore distributions. The steeper the volume increase upon condensation (adsorption branch), the more homogenous is the pore-size distribution. It should, however, be noted that the relation between p/p0 and rpore is
not linear; the relation between steepness and pore-size dispersity is hence
different at different p/p0.
A characteristic feature of adsorption/desorption in mesopores is the
observation of hysteresis behaviour between the adsorption and desorption
branch after a certain pore size is surpassed, resulting in Type IV isotherms.
This effect is closely associated to the thermodynamic properties of the confined fluid. If the pores are large enough, metastable states can exist, which
result in the observed separation of adsorption/desorption branches and
a “hysteresis”. For pores smaller than this threshold value, no hysteresis is
observed. Generally the desorption branch represents equilibrium and most
methods evaluate the desorption branch for calculation of pore-size distributions. This is often correct, but there are systems that cannot be analysed
in this way.16 The emptying of mesopores can be significantly hindered, if
the pores are connected to the outside through small necks (so-called inkbottle-type pores). Those pores are mostly characterised by adsorption and
desorption branches, which are not parallel to each other. In this case the
emptying of the mesopores occurs either after the throat has been emptied
(desorption branch analysis provides data regarding pore neck size) or after
the condensed and confined fluid has reached thermodynamic instability.
The second case is known as cavitation (usually observed if the neck size is
smaller ∼ 5 nm), and the pore is emptied by diffusion after the formation of
a gas bubble, while the pore entrance stays filled with condensed fluid. In
this case no information on the size of the pores or the pore entrance can
be obtained from analysis of the desorption branch (Figure 9.3). Finally, it
should be noted that the issue of cavitation is still under investigation and
more information on this important topic can be obtained from the cited
literature.16–19
Another problem associated with the classical BJH method is the basis of
this model on cylindrical pore shapes. Modern carbon materials, derived by
the use of templates, might have very well defined pore shapes that are very
much different from cylinders (e.g. spheres, gyroidal, etc.). The BJH model
is known to severely underestimate the pore size in such cases.20 The use of
DFT methods that take pore geometry into account is a potential solution to
this problem.
It is hence advisable to use methods for pore-size distribution extraction
that take the aforementioned issues into account (e.g. DFT). It is also important to choose the correct branch for analysis, irrespective of the method. As
the scaling between condensation pressure and pore radius is not linear, it
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Figure 9.3 Schematic illustration of pore-blocking and cavitation phenomena.
Reproduced with permission from ref. 16.
is also advisable to measure as many points as possible in the high-pressure
region if large mesopores will be analysed.
9.3.1.3 Density Functional Theory-Based Approaches
DFT-based models have received a high degree of interest in the last few
decades. DFT-based methods provide a potential bridge between different length scales (i.e. micro- to mesopores) within a single model. Various reviews are available that provide a deeper insight into the underlying
basics, and therefore this chapter will focus on only the main concepts of
this approach.6,21–23
DFT-based methods can be classified as microscopic models that take fluid–
wall interactions explicitly into account. These manifest themselves by local
ordering effects of adsorbed fluids near the pore wall, which have an impact
on the fluid-density profile. Fluids confined in small pores hence do not show
a homogenous density profile across the pore. Early work by Tarazona, Evans
et al. has shown that nonlocal density functionals (NLDFT, also known as
smoothed density approximation, SDA),24,25 are capable of taking short-range
correlation more realistically into account and therefore provide an improved
description of the equilibrium states of confined adsorbate. The local shortrange ordering near the pore wall leads to characteristic oscillations of the fluid
density near the wall. This effect is especially important for small mesopores
and micropores, where the pore size is only of a few molecular diameters.
Based on this starting point, it is possible to calculate the density profile
ρ(r) of a given adsorbate–adsorbent system (e.g. N2–carbon, based on known
intermolecular interaction parameters using Lennard-Jones approaches)
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for any idealised pore of given pore width and shape.
ρ(r) does show
a pressure dependence, and it is hence possible to calculate the adsorption
isotherm per unit pore volume (eqn (9.4)). Likewise, the adsorption isotherm
per unit pore area can be accessed by integration:
R
p
2
=
N v ⎛⎜ ⎞⎟
r dr ( ρ ( r ) − ρ g )
2 ∫
⎝ p0 ⎠ R 0
(9.4)
where Nv is the adsorbed amount and ρg is the equilibrium gas density.
Figure 9.4 shows exemplarily model isotherms of H2 and CO2 on microporous carbon possessing slit-like pores of different widths. Pores of the
smallest size (one to two molecular diameters) result in isotherms that show
a very steep increase at very low p/p0. This is also observed for other gases
and allows the extraction of the first information straight from the isotherm
shape. Simulation methods, such as grand-canonical Monte-Carlo (GCMC)
methods can then be used to validate that the NLDFT approach reproduce
fairly the local density profiles.29
The above procedure provides a way of calculating model isotherms of
fixed pore size and shape. Derivation of pore-size distribution and associated
information such as pore volume and specific surface area is based on a fit of
the experimental isotherm by a set of model isotherms (the so-called kernel,
i.e. fixed pore shape and interaction parameters) according to eqn (9.5):
p
N ⎛⎜
⎝ p0
⎞=
⎟
⎠
∫
Wmax
Wmin
⎛ p
⎞
N ⎜ , W ⎟ f (W ) dW
⎝ p0
⎠
(9.5)
Figure 9.4 (A) Selected NLDFT isotherms of H2 at 77 K in carbon micropores of
different sizes. Reprinted with permission from ref. 22; Adsorption isotherms of CO2 in individual carbon slit pores at 273.2 K – (B) GCMC
and (C) NLDFT isotherms generating pore widths of 3.65 Å and 6.27 Å,
respectively. Reprinted with permission from ref. 28.
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where N(p/p0) is the experimental data, f(W) the pore-size distribution function, and N(p/p0,W) the isotherm on a single pore of width W.
NLDFT methodology has an upper pore size limit of some tens of nanometres, depending of the exact model. Hence, NLDFT can span the pore
size range between micro- and mesopores. DFT methods are nowadays
implemented in most commercial gas sorption machines. Various models are available, giving possibilities to analyse various pore morphologies
and even combinations thereof (e.g. cylindrical, slit-like, and spherical) and
pore-wall chemistry (e.g. carbon, zeolites or silica). Modern kernels have also
been developed for the analysis of adsorption branches next to desorption
branches, in accordance with the arguments given on the choice of the “correct” branch in the previous section.
The latest developments in the DFT methodology are related to the understanding of defects,19 and the investigation of adsorption/desorption-induced
deformations (i.e. which have an impact on carbon capture and storage technologies, see below).30–33 Quenched-solid DFT (QSDFT) was introduced as a
new method recently and was originally developed to describe the adsorption/desorption of N2 or Ar on carbon surfaces.34,35 This methodology takes
surface roughness and adsorbent heterogeneity into account and might be
an attractive choice for the analysis of materials with highly heterogeneous
surfaces – including in the context of this book, functional (e.g. heteroatom-doped) carbonaceous materials that lack a high-temperature polishing
or “carbon” homogenisation (Figure 9.5). As can be seen, QSDFT shows a
pore-size resolution that cannot be achieved by other methods – the BJH cannot resolve micropores whilst the artificial minimum at 1 to 2 nm pore diameters in NLDFT analysis is also avoided within the PSD.
Research and optimisation of DFT models is still a highly active area and
hence there cannot be a single suggestion of which is the best model to use.21
Currently, QSDFT methodology seems to be the state-of-the-art method for
the analysis of micro/mesoporous amorphous matter (e.g. carbonaceous
Figure 9.5 (A) N2 adsorption/desorption isotherm and (B) respective QSDFT pore-
size distribution of as synthesised HTC-based micro-/mesoporous
ordered carbon C-MPG1-meso. Reproduced with permission from ref. 36.
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materials) by N2 or Ar adsorption. Finally, it should be noted that there are
other useful analyte gases beside N2 and DFT models have been developed
for Ar, H2 and CO2 as well.22,27,28 H2 (at 77.4 K) is a potential probe molecule
for the detection/characterisation of ultramicropores that are not accessible to N2.37–40 Although the H2 adsorption capacity at 77.4 K is measured for
many samples nowadays as a consequence of the search for H2-storage systems, the use of H2 adsorption as an analytical tool has not found widespread
application thus far. Finally, the use of CO2 sorption at ambient temperatures
provides many advantages for the analysis of ultramicroporosity that will be
discussed in the next section.
9.3.1.4 CO2 Adsorption as an Analytical Tool
CO2 sorption has become of increasing interest within the context of a sustainable energy supply. The capture of CO2 (e.g. from power-plant flue gas)
could reduce emissions produced from fossil-fuel combustion. Microporous
materials provide some promise to selectively adsorb CO2 over other gases,
which makes them interesting from a technological point of view.41–43 Whilst
this topic is currently of high interest to the research community, the use
of CO2 sorption as an analytical tool is less widespread, though its use has
been especially well known within the carbon community for many years.
CO2 sorption has some advantages over cryogenic N2 or Ar adsorption when
it comes to the analysis of carbons with very small and narrow micropores
that are not accessible (and hence not analysable) to N2 at 77.4 K at all.22,44–49
Adsorption experiments are typically conducted at 273 K or 298 K, i.e. at
much higher temperatures compared to cryogenic N2 adsorption experiments. Due to the high saturation pressure at ambient temperatures (≈26 140
mmHg at 273 K), CO2 adsorption at p ≤ 1 atm (relating to relative pressure p/p0
∼ 0.03) provides information about very narrow micropores (<1.5 nm) only.
Larger pore sizes become accessible if high-pressure adsorption is used. CO2
adsorption experiments on ultramicropores are typically faster compared to
N2 adsorption experiments. The instrumental demands, which are necessary
to reach very low relative pressures, are also minimised. This is a consequence
of the high saturation pressure; hence no turbomolecular pumps are necessary. Most commercial gas sorption machines can be run using CO2 as the
adsorbate without any problems. NLDFT or GCMC models are also available
to analyse the adsorption data with regard to specific surface area and PSD.
Besides these models, classical (semi)empiric models are also applicable.
Thus far, only low-pressure CO2 adsorption was discussed, and this approach
can be complemented by the use of high-pressure CO2 adsorption that can
provide access to BET surface areas and total loadings. BET surface areas
obtained from high-pressure adsorption do generally agree well with predictions from NLDFT or GCMC if there are no mesopores or significant outer surface area present.50 In short, CO2 sorption analysis is a very useful tool, when
it comes to the analysis of microporous carbons and some examples of its
use have been referenced. Given the current importance of carbon materials
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for modern technology, a few important effects regarding application, namely
elastic deformations and adsorption heat will now be discussed.
Adsorption events in micropores (and also to a lesser extent in mesopores)
are usually accompanied be elastic deformations, which relate to the existence of a solvation pressure originating from the confined fluid. The related
“swelling” of charcoal has been known for roughly 100 years, and is expected
to have consequences for applications of carbon as packed column absorbers
or for the targeted storage of carbon in soil or coal deposits. The problem was
hence revisited in recent years by various groups using different experimental
and/or theoretical methods.30–32,51–54 Deformations may be of a contractive or
expansive nature, depending on the finer details of the carbon (Figure 9.6).33
If CO2 is slightly too large to fit within very narrow slit-like pores, an expansion will be observed. This can be understood in a simplistic way by assuming
that the net gain of adsorption energy is greater than the cost of deformation.
Likewise, contraction can be expected in cases were the pores are slightly too
large to accompany CO2 in a dense packing. Contraction will hence increase
the “contact” area and hence the adsorption energy.
The above scenario is true for perfectly flat surfaces. Neimark et al. have
shown that the pore-surface heterogeneity/roughness will once more influence the picture and provided a theoretic treatment on a QSDFT-like basis.55
The deformation topic is currently still under investigation and more knowledge will certainly be gained in the next years, given the technological importance of adsorbent deformations.
Finally, the interaction strength of CO2 with microporous materials shall
be investigated from the viewpoint of their potential in CO2 capture and
storage. An important parameter, which is usually investigated in order to
access the potential usefulness of new adsorbents, is the isosteric heat of
adsorption qst. The isosteric heat of adsorption qst can be obtained from CO2
adsorption measurements at different, but nearby temperatures, often from
isotherms at 273 K and 283 K or 298 and 303 K. Various calculation protocols have been suggested and discussed in the literature.41,56 Typically, the
Clausius–Clapeyron approach (eqn (9.6)) is used, which provides the relation
between pressure and temperature at constant loading. The slope of the plot
of ln p vs. 1/T gives access to the heat of adsorption:
q 1
(ln p)V =
− st + D
R T
(9.6)
where R is the universal gas constant and D is a constant.
Modern software environments provided by the manufacturers of gas sorption machines often provide the option to determine qst as a function of the
gas loading. The results obtained therefrom may be crosschecked with calculations by “hand”, following accepted guidelines, especially, if unusually high
heat of adsorptions are produced.41 Krishna, Long et al. have developed a
precise analytical methodology to determine the isosteric heat of adsorption
from single- or dual-site Langmuir fits of experimental data.56 This approach
identifies the adsorption enthalpy associated with supposed adsorption sites
slit-shaped carbon pores of different effective pore widths, Heff = H − 0.34 nm at 333 K. The solid blue line on the upper panel
corresponds to bulk carbon dioxide at 333 K; (C) Solvation pressures CO2 at 333 K vs. pore size computed for bulk pressures
from 0.03 to 27 MPa. Maximum solvation pressures correspond to slit-shaped carbon pore width of 0.23 nm, and the minimum is found in pores of pore width of 0.36 nm. The predicted CO2 configuration within the pore sizes belonging to maxima/
minima of the solvation pressure is shown above, configuration 2 denotes the imperfect packing, leading to contraction.
Reproduced with permission from ref. 33.
Figure 9.6 (A) Absolute value of adsorption (upper panel) and (B) corresponding solvation pressure (bottom panel) of CO2 in a series of
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and can also clearly identify their quantity. The method is rather simple
to implement and demanding only a very good quality adsorption data fit.
Contrary to MOFs and zeolites, which may contain various very well defined
adsorption sites, the situation in amorphous microporous materials is different. Here, a rather continuous distribution of adsorption sites, which differ
only slightly from each other, may exist. Still, the dual-site adsorption model
usually provides good results in such cases and its use together with the stringent approach of Krishna, Long et al. should be followed.56 It is important to
note that some reports have shown an unusual temperature dependence of
the isosteric heat of adsorption.57,58 This issue (and its potential impact on
the determination and reliability of the qst data) has to the best of the authors
knowledge, however, not yet been deeply discussed.
Various studies on the tuneability of the isosteric heat of CO2 adsorption in
microporous carbons have been published in recent years within the context
of Carbon Capture & Sequestration (CCS) technology. Often N, S or other heteroatoms are believed to have significant influence on the CO2 capture properties. Indeed, the heteroatoms provide some polarity on the adsorbent surface,
whose impact should, however, not be overinterpreted. Importantly, qst is not
only influenced by the presence of functional groups or heteroatoms (mostly
studied for MOFs and silicas),41,42,59–64 but also by the pore width and heterogeneity. Without going into detail on the demands for successful CCS materials (the interested reader is referred to recent reviews on the topic),41–43,65 the
effect of pore size will only be discussed briefly here. Based on studies on narrow pore size carbon materials, it was shown that small pore sizes can indeed
lead to high heats of adsorption (in the range 28–34 kJ mol−1) even in the
absence of functional groups.48 Common values for larger pore size activated
carbon are in the range of 20 to 25 kJ mol−1, i.e. significantly smaller. This was
even more clearly demonstrated by Sevilla et al., who conducted a study on
the influence of the presence or absence of “N”-containing functionalities in
microporous carbons on the CO2 uptake properties.66 The important information obtained from this study is that the micropore-size distribution can be
more important than the presence of functional groups. Small pores (ultramicropores < 0.8 nm) get filled (volume filling) at low pressures, while larger
pores are simply surface covered by the adsorbate. Calculation of the isosteric
heat of adsorption by the author based on the isotherm provided within this
study, found indeed an increase of the adsorption heat with decreasing pore
size, irrespective of the presence of heteroatoms. The overlay of the attractive potential exhibited by the opposite pore walls could potentially explain
the higher heats of adsorption associated with the micropore-filling process.
Those findings might be important for the assessment and calculation of heat
management needed for industrial-scale adsorber units (which are already at
the pilot scale).67
In summary, CO2 sorption is an extremely versatile tool for the analysis
of microporous carbons (less experimental demands, assessment of specific
surface areas and PSD) and will continue to be an important method. There
are a few ongoing research issues (deformation/interaction strength), which
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have been outlined briefly in this section, highlighting their potential industrial importance in CO2 adsorption, separation and storage.
9.3.2 Scattering and Diffraction Methods
Scattering methods, especially small-angle X-ray but in principle also neutron
scattering (SAXS and SANS) are well known, versatile methods for the analysis of porous materials.68–71 They allow the analysis of either solvent-filled or
desiccated networks, closed porosity and also the monitoring of processes
(e.g. adsorption events) by in situ techniques.72,73 Next to small-angle experiments, which can give information on the nm scale (Figure 9.7), wide-angle
X-ray scattering or diffraction (WAXD, WAXS), also known as X-ray diffraction (XRD), can give information at the atomic (Å) scale. This can in some
cases allow the extraction of additional information regarding material (e.g.
micro-) porosity.
The analysis of porous materials by small-angle scattering (SAS) has been
reviewed several times,68–71 and it has been demonstrated that SAS experiments can provide information regarding pore size, specific surface area and
also pore wall size, even in disordered systems. In the case of ordered pores,
information concerning morphology and crystal lattice dimensions can also
be obtained by analysis of the Bragg peaks, following guidelines established
by classical X-ray crystallography.74
The signal in SAS depends on the different scattering power of the two
phases of a porous material (pore, called α-phase and pore wall, called
β-phase). For X-rays, the scattering power of the phases is related to the electron density ρ(r) and hence to the atomic number of the elements constituting the phase. For neutrons, there is no clear correlation of the scattering
power of each element to the atomic number and the scattering power is
usually given in terms of scattering length density. Importantly, the different
isotopes of an element can have totally different scattering power (the most
prominent example is certainly H and D), which can be used for contrast
enhancement or matching.
Figure 9.7 (A) Schematic setup of diffraction/scattering experiments; (B) Sche-
matic overview on the different length scales probed by wide-angle
and small-angle scattering; and (C) Schematic drawing of an unordered porous materials (pores: α-phase, pore wall: β-phase) and chords
through the two phases. Adapted with permission from ref. 75.
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The principles of SAS will be described in the following, but for a more
detailed overview, classical textbooks on SAS are recommend.69,76,77 A very
basic description of coherent scattering would start from the primary beam
hitting (and thereby interacting with) the sample (Figure 9.7). This leads to
the creation of a scattered beam of photons or neutrons, which is deflected
from the primary beam by a scattering angle 2θ. In the following, X-ray scattering is particularly highlighted but the principles are analogous in SANS.
Typically, scattering vectors are used in SAS instead of scattering angles and
the relation between θ and the scattering vector s is (eqn (9.7)).
s
=
G 2sinθ
s=
(9.7)
λ
The scattering vector s is related to the (also very often used) scattering
vector q by: q = 2πs. The intensity of the scattered light or neutrons I(s) is
measured using suitable (2D) detectors. It is related to the square of the electron density difference Δρ(r), which itself is given by the difference between
the electron densities of phase α and β to the average electron density of the
illuminated volume:
G
G 2
G
GG
I ( s ) ∝ ( Δp ( r ) ) ∫ Θ ( r ) exp 2π i s r dV
(9.8)
(
)
V
where Θ(r) = 1 (α-phase) and Θ(r) = 0 (β-phase).
In the case of empty pores, the scattering contrast should be large enough to
allow for a structure analysis. If the pores are, however, filled with a liquid that
has the same scattering power as the pore wall, a contrast match is achieved and
no scattering signal can be detected any longer. Using this principle it is possible to analyse whether a certain liquid can enter a pore or not. Additionally,
this enables the analysis of adsorption processes within pores and might contribute to better understanding of pore connectivity, accessibility, etc.72,73,78,79
Depending on the structure of the porous material under examination, different information can be obtained from a scattering pattern. If an ordered
structure is present, it is possible to extract information on the unit-cell size
and the symmetry (e.g. Figures 9.8 and 9.9). The SAXS pattern of a HTC-derived
ordered mesoporous carbon presents clear diffraction peaks due to the cubic
Im3m symmetry and a unit cell size of 18.9 nm are visible and in this case, the
SAS pattern can be analysed just as any XRD pattern (Figure 9.8).36
A powerful demonstration of how such an analysis might look has been
provided by Solovyov et al.,80 who analysed the SAXS/XRD patterns of CMK-1
materials, prepared by replication of MCM-48. MCM-48 itself presents a
gyroid structure (space-group: Ia 3 d), which gives two independent, but
interwoven carbon subframeworks upon replication. These are displaced as
a consequence of attractive interactions between them, resulting in a loss of
symmetry. This can be clearly followed and even modelled from the XRD data
(Figure 9.9). Much less intense reflections are also observed upon replication.
If no displacement would have happened, the replicated material should have
shown the same scattering pattern as the starting material. More information
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Figure 9.8 Synchrotron SAXS pattern of as-synthesised C-MPG1-meso. Reprinted
with permission from ref. 36.
on the analysis of highly ordered porous carbon materials via diffraction and
simulation can be obtained from the literature mentioned above as well as
from other sources.81,82 It should not be forgotten, however, that the use of
electron diffraction/crystallography is also a very versatile and complementary tool for the analysis of ordered porous carbons.83 Many porous carbons
are, however, not ordered at all or show only relatively short-range order. The
next examples discussed are hence disordered (or only short-range ordered
pores) with a well-defined shape, such as cylinders or spheres.
Short-range ordering can give rise to a diffraction peak. The corresponding
d-spacing (d = s−1) should, however, not be directly attributed to a pore size.
The peak represents a correlation length and can for instance be analysed
using Percus–Yevick approaches, which ultimately enables the calculation of
pore sizes.72,73,84,85 In the absence of any short-range order the scattering pattern can still be highly defined due to form factors of well-shaped pores in the
case of sufficiently low polydispersity. The scattering patterns of geometric
shapes (e.g. spheres, cylinders and disks) can be calculated and show some
distinct features, which allow, in principle, identification of pore shapes
from the measured scattering pattern. Cylinders show a dependence of I(s)
on s that follows a power law of I(s) ∼ s−1 in the intermediate s range. Disks
show a power law of I(s) ∼ s−2 in the intermediate s-range, while spheres give
rise to a characteristic pattern that shows undulations. All form factors yield
a power law of I(s) ∼ s−4 in the high s range. This behaviour is known as Porod
behaviour. It was shown that this feature is present in all two-phase systems
(porous materials), even if there is no well-defined pore shape (e.g. Figure
9.7(C)). Porod behaviour allows an analysis (of such disordered systems)
through extraction of the Porod length lp (which is itself some kind of average
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Figure 9.9 Models of the structure unit cell and XRD powder patterns (root-square
weighted) for the mesoporous silica MCM-48 material (a), a modelled single
enantiomeric carbon subframework (b), and the replicated carbon CMK-1
material (c). Experimental profiles are shown by symbols; calculated profiles are given as a solid line. Reproduced with permission from ref. 80.
chord length; e.g. Figure 9.7(C)). Other parameters such as number-average
pore sizes <lα>, the surface area and the chord length distributions (which
are related to pore-size distributions; Figure 9.10) can be calculated from lp
using the following equations:68,69,86,87
l p 4φ (1 − φ )
=
S
V
I
1
=
l p (1 − φ ) lα
(9.9)
(9.10)
At this stage, no explicit explanation of the necessary calculations is
given; these can be found in the cited references. The Porod decay does
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Figure 9.10 (A) SAXS curves and (B) chord-length distributions of two microporous activated carbons with different degree of activation, and (C)
Representation of r.g(r), illustrating the changes upon activation more
clearly. Reproduced with permission from ref. 86.
sometimes not show as a clear I(s) ∼ s−4 decay and other exponents are
observed. This has sometimes been attributed to surface fractals,69 but
these arguments are questioned by others and may also be explained by 1D
density fluctuations.86
In the following, three studies regarding the detailed SAXS analysis of activated carbons are highlighted in order to illustrate the wide possibilities of
SAS. The first example is based on the so-called chord-length distribution
(CLD) analysis. This can be performed on SAS patterns of microporous carbons, if they exhibit a clear Porod behaviour. An example is shown in Figure
9.10, where the scattering patterns and the chord-length distribution g(r)
of two activated carbon with different degree of activation (C1: highly activated, high specific surface area; C2: lower degree of activation, lower surface
area) are shown.86 The two materials showed a counterintuitive behaviour
when employed in the adsorption of medium-sized organic compounds. C2,
which has a somewhat lower surface area and porosity as determined by N2
adsorption, showed a higher uptake of organics compared to C1. Analysis
of the finer details of the SAXS patterns shows that C1 has a higher fraction
of very small micropores compared to C2 (Figure 9.10(C)). This explains the
low capacity for larger organic molecules, while having a high capacity for
gases. Additionally, it could be shown that both samples contain also a large
amount of closed porosity not accessible to N2.
These SAXS patterns can be considered as typical for activated carbons
and other methods of their analysis and interpretation have also been suggested (Figure 9.10). A recent study has shown the potential of SAXS for the
analysis of impregnated activated carbons,88 where special interest was spent
on the analysis of the distribution of the impregnated chemicals/functionality within the carbon. Activated carbons need to be chemically modified for
certain purposes and special adsorption applications and their final performance will largely depend on the distribution of the introduced material (e.g.
functional groups) within the carbon. The introduction of guest molecules
will change the electron density profile (and thus the shape of the measured
SAXS patterns; Figure 9.11) and it is possible to extract the wanted information on the degree of micropore filling and the volume of grains originating
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Figure 9.11 (A) experimental (dots) and fitted (lines) SAXS patterns of activated
carbon material with increasing amount of chemical impregnate
(sodium benzoate), please note the changes of the hump (less pronounced with increasing filling) between q = 0.1–0.3 Å, which originates from micropores; (B) Exemplary plot of the parameters/sample
information, which can be deduced from SAXS data of impregnated
carbons: micropore filling fraction and total volume of the impregnant micrograins. Reproduced with permission from ref. 88.
from the impregnate from the patterns. For more detailed information on
this analytical model, ref. 88 is recommended.
Finally, modern SAXS techniques, such as µ-spot beams can also provide
insight into the distribution of porosity within carbon fibres. Lozano-Castelló
et al. analysed the distribution of porosity within chemically activated carbon
fibres of different origins.79 The spot size of the used X-ray beams is usually
much smaller than the species of interest (here: 0.5 to 2 µm spot size vs. 5 to
20 µm fibre diameter) and can be focused with high precision, allowing data
collection form different regions of the fibres. The technique could prove
that there are differences between activated carbon fibres prepared either
from pitch or from poly(acrylonitrile) (PAN). The PAN fibres themselves are
anisotropic, having a denser core and a looser outer shell. Whilst this structure is kept after chemical activation (resulting in anisotropic activation),
there are, however, differences regarding the total porosity depending on the
used activation agent. On the contrary, an isotropic porosity distribution was
found in the case of pitch-based carbon fibres.
The strength of scattering techniques is based on the possibility to analyse also disordered materials down to the subnanometre level, a task that
cannot be achieved easily with microscopy. The use of modern techniques,
such as µ-spots (as shown in the last example) can provide spatially resolved
information, which cannot be provided by e.g. gas adsorption techniques.
All this, combined with the possibility to perform in situ measurements of
pore-filling processes or even electrochemical processes in microporous carbon electrodes,89 makes scattering a very powerful (though experimentally
demanding) technique for the analysis of carbon materials.
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9.3.3 Microscopy
The use of electron microscopy (EM) and force microscopy are extremely
important in the characterisation of porous material and have found widespread use for the visualisation of all pore sizes. These techniques are especially advantageous for the analysis of ordered porous materials, as these
allow the easiest interpretation of the obtained pictures, but can also be
applied to disordered materials. The details of the most widely used methods: scanning and transmission electron microscopy (SEM and TEM, respectively) and details of the experimental procedures and special requirements
for the analysis of carbon, will be discussed in further detail in Chapter 11.
Latest developments including holography and tomography have also been
summarised recently by various authors and will also be introduced in Chapter 11, these references do also refer to more classic and fundamental articles on electron microscopy.90–96 Regarding other techniques, atomic force
microscopy (AFM) and scanning tunnelling microscopy (STM) have also
been used for the analysis of carbons, these techniques are, however, mostly
applied to planar carbons such as graphite or graphene. STM is particularly
useful as it also can provide electronic information of the surface, of interest
for the analysis of catalytically active (heteroatom-doped) carbons.97
EM is mainly used to acquire initial information regarding specific carbon
material morphology and key structural features (e.g. fibre diameters, particle
size, pore size). SEM is widespread in many laboratories and especially useful
for the determination of macropore size in a given material, but it could be also
used to estimate the thickness and surface homogeneity (e.g. of carbon molecular sieve membrane).98 The combined use of an energy-dispersive X-ray (EDX)
detector with SEM may also be beneficial to map and determine the elemental composition of heteroatom-doped carbons. As will be discussed in more
detail (Chapter 11), TEM and especially high-resolution techniques (HRTEM)
can be used to identify mesopore sizes, large micropores and indeed local
ordering (e.g. stacking/graphitisation) of the carbon phase. Electron diffraction can often be combined with TEM and is a source of additional crystallographic information. More advanced techniques including freeze fracturing
or ultrathin sample cutting/microtome and other information sources associated with EM (e.g. electron energy loss spectroscopy, EELS) may help to
gain further insight into the basic building units of carbon materials and
the reader is referred to the above-mentioned references for more information. To illustrate the power and potential of EM, two exemplary images of
a so-called carbogel are provided (Figure 9.12).99 At the macroscopic scale,
the carbogel appears foam-like, composed of large meso- and macroporous
domains based on SEM image analysis (Figure 9.12(A)). The HRTEM image
in turn provides clearly information regarding the basic structural units
of this material, composed of a branched continuous fibrous structure with
branch diameters of 10–20 nm (Figure 9.12(B)).
As another example, demonstrating the power of EM in the context of
ordered porous carbon characterisation, the combination of HRTEM, in this
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Figure 9.12 left-hand side, SEM picture of an HTC derived carbogel; right-hand
side: TEM picture of the HTC-derived carbogel. Reprinted with permission from ref. 99.
Figure 9.13 TEM images (A) of CMK-1 viewed from the [111] and respective TEM
image simulations based on the models of (B) undisplaced and (C)
displaced subframeworks. Reprinted with permission from ref. 80.
case of CMK-1, and associated selected-area electron diffraction (SAED) pattern with the TEM image simulations, allows the description of the two main
possibilities of the replication process (Figure 9.13).80,100 CMK-1 is basically
built from two individual carbon subframeworks that arise as a consequence
of pore-structure replication of the parent MCM-48. Two possibilities could
be imagined – undisplaced and displaced subframeworks and a comparison
of TEM images and simulations clearly indicates that the subframeworks are
displaced, but not significantly distorted.
AFM is also useful to provide insight into the structuration of micropores. The microporous structure of activated carbon fibres was analysed
in a detailed STM study by Daley et al. 20 years ago.101 Cross sections and
fibre surfaces were found to be composed of ellipsoidal-shaped micropores
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Figure 9.14 Representative STM image of a Kevlar-derived activated carbon fibre
revealing slit-shaped microporosity. Reproduced with permission
from ref. 102.
coexisting with elongated and ellipsoidal mesopores. The average pore size
determined from the STM analysis was in good correlation with gas adsorption data, with respect to the trend in pore size of the different fibres. STM
was also used for the real-space analysis of microporous carbons, prepared
by the templating of zeolites and demonstrating the structural order of the
original zeolite could be translated only partly into the resulting microporous carbon.102 Figure 9.14 shows finally a representative STM image of a
Kevlar derived activated carbon fibre, showing the characteristic elongated
micropores of 1 to 2 nm width.
Tomography can be regarded as an emerging subclass of imaging methods. Here, we just wish to highlight some recent advances in electron and
X-ray tomography to demonstrate the power of these methods. A detailed discussion is not possible and we would like to refer the reader to the relevant
literature. As will discussed further in Chapter 11, electron tomography has
already been transferred to the fourth dimension and can already provide
time-resolved imaging, which enables additional information regarding system dynamics, e.g. upon thermal excitation.103 Figure 9.15 shows the exemplary tomograms of multiwalled CNTs at different time scales.
X-ray tomography is another well-known method (µ-CT), which has recently
been advanced towards increasingly higher-resolution capabilities (i.e.
< 50 nm – so-called ptychographic X-ray computed tomography – PXCT). This
technique has recently been applied to the analysis of polyacrylonitrile and lignin-based carbon fibres.104 The technique does not only gives information on
the pore structure, but also provides information regarding the degree of material carbonisation based on the determination of spatial density distribution.
It should be mentioned that microscopy should always be done on different parts of one (or ideally various) sample areas of a material to gain as
accurate an insight (e.g. regarding sample homogeneity) on the statistics and
to exclude the interpretation of images that might represent only a minority
fraction of the overall material morphology. In summary, imaging methods
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302
Figure 9.15 Tomograms and images resolved in time. (A) Representative time frames
of 3D volume images taken at t = 50 ps and 25 ns for the MWNT specimen. The bracelet shape (radius 620 nm) and the detailed tubular morphology are displayed; from the 3D volume models, the length of the
ring (L) was measured to be 4.4 mm. (B) Cross section of tomographic
images. Shown are 4.6 nm thick 2D slices in the xy-plane, perpendicular
to the optical axis, at t = 50 ps (top) and 25 ns (bottom). (C) TEM image
of a MWNT specimen. A typical projection image at a = 0° is given with
the dimensions indicated. Reproduced with permission from ref. 103.
have seen a dramatic increase of their power in recent years and allow nowadays unprecedented insight into carbon material structuration. Given the
current lines of development of hardware and computing technique, it can
be expected that more is to come.
9.3.4 Other Methods
In unison with the methods discussed thus far in this chapter, there are a
huge variety of other methods (e.g. cryoporometry, NMR methods and positron annihilation lifetime spectroscopy) available for the characterisation
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Porosity Characterisation of Carbon Materials
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of porous materials. Among them are methods suitable for the analysis of
meso- or micropores, although some of them have, to the best of the author’s
knowledge, not been yet applied to carbons.
The use of Hg intrusion methods are quite common place in the characterisation of (e.g. macroporous) carbons but will not be discussed deeply in this
chapter. Such intrusion methods are based on the Young–Laplace equation
and pore sizes (more correctly the pore neck sizes) are basically related to
the pressure necessary to force mercury into the pore space, and hence the
pore volume is measured by the total Hg uptake. Drawbacks are the use of
mercury (which should be banned, wherever possible according to recent UN
statements) and the destructive nature of the analysis. It can be envisaged
that Hg intrusion will be exchanged by some more advanced methods in the
future and the reader is referred to related literature on this topic.105–107
Cryoporometry, also known as thermoporometry, is another feasible
technique to study meso- or macroporosity in the solvent-filled state. Its
use for the analysis of porous materials is, however, not as widespread as
N2 sorption for example. Nevertheless, a variety of fundamental research
throughout the last years has shown its reliability.108–110 Furthermore, basic
experiments can be performed by differential scanning calorimetry (DSC).
A short overview about the technique is given here, despite the fact that
cryoporometry has not been heavily applied for the analysis of carbons yet.
It could, however, be a useful method for the characterisation of as-synthesised carbonaceous materials (e.g. HTC materials often have water-filled
pores). It might also be possible to analyse the impact of drying protocols
(which could lead to partial pore closure) in such materials. This technique
is also based on a thermodynamic relationship derived from the Young–
Laplace equation, i.e. the Gibbs–Thompson equation. The phase-transition temperature (i.e. the freezing/melting temperature) is sensitive to the
presence of curvature or more generally to the pore radius R. Melting of
solvent confined in mesopores or small macropores takes place at significantly lower temperatures compared to the bulk state. Comparable to N2
adsorption, it is possible to calculate pore sizes and PSDs and studies have
been performed to check the reliability of cryoporometry data for inorganic
materials.108,110–112
The freezing-point depression of solvent confined in polymer networks
was already discussed more than 50 years ago by Kuhn and coworkers. The
effect was attributed to the confinement of solvent within the mesh formed
by the crosslinked polymer chains and mesh sizes of ca. 50 nm were suggested.113 The usage of cryoporometry for the analysis of the solvent-state
of porous polymer resins was brought forward by Brun et al.114–116 It allowed
the analysis of resins in the state of use (solvent-swollen), which can naturally provide more relevant information compared to an analysis of the dried
materials. Following this line, there have been various reports on the use of
cryoporometry for hydrogels.117,118 For example, Wang et al. have shown that
the extraction of the PSD can help significantly to understand the transport
of active molecules through a hydrogel, while the knowledge of the mean
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Chapter 9
pore size alone cannot provide this level of understanding. As stated in the
beginning, the method has (to the best of the authors knowledge) not been
applied yet to carbon or carbonaceous materials, but it could be in principle
very useful (e.g. for the analysis of carbonaceous aerogels before and after
drying).99,119,120
As will discussed in Chapter 10, nuclear magnetic resonance (NMR) spectroscopy can be used in a number of ways.121 Additionally to the characterisation of pore-wall chemical structure generated from magic-angle spinning
(MAS) solid-state NMR, there are also techniques to analyse adsorbates.
Pulse-field gradient (PFG) NMR can, for instance, be used to analyse molecular transport within pores.121,122 Another NMR technique, which can be used
for the analysis of micropores is 129Xe-NMR, as this noble gas is rather sensitive to the chemical environment and its chemical shift has been shown to
be related to pore size.123–128 Usually, the chemical shift needs to be measured
as a product of Xe pressure in order to remove the influence of Xe–Xe interactions by extrapolation to zero pressure. 129Xe-NMR was used as an independent probe to verify the pore sizes of microporous polymers, whose analysis
by gas adsorption was not trivial.129–131 Hence, it can be expected that it could
also be a very useful technique for the analysis of carbon-based materials.
Positron-annihilation-lifetime spectroscopy (PALS) is another often employed
technique in the characterisation of microporosity.132–134 It is based on the
formation and annihilation of positronium particles, which can be formed
by the reaction of positrons and electrons. Positrons (p+), which are the antiparticles of electrons are created in a p+ source and allowed to interact with
the sample to be analysed. In rather infrequent events, they can react with
electrons to form a positronium particle (Ps), which can differ in spin, giving
rise to ortho-Ps and para-Ps. The Ps particles can interact with matter and
annihilate under the emission of radiation, which allows the measurement
of the o-Ps and p-Ps lifetimes. The lifetime depends on the presence and size
of voids, which enables the determination of pore sizes. An advantage of the
technique is its sensitivity also to closed porosity, which can otherwise only
be accessed by scattering methods. PALS has been used often for the analysis
of microporous materials, but its use can also be expanded on the analysis of mesoporous materials.135 Although the technique is rather exotic to
chemists, it can give valuable insights into a porous material’s structure and
was already used for the analysis of activated carbon.136 The results obtained
by PALS were consistent with those obtained by SAXS and can hence deal
as some kind of verification for the more widespread SAXS analysis. PALS is
rather widespread in the field of microporous polymers and has hence been
transferred to carbon molecular sieve type materials prepared from polymer
precursors.137
Next to the methods mentioned, the interested reader is referred to a
variety of specialised textbooks regarding porous materials, including the
“Handbook of Porous Solids” or proceedings of conference series such as
“Characterisation of Porous Materials” or “International Symposium on the
Characterisation of Porous Solids”.
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9.4 Conclusion
Within this chapter, a relatively brief, concise overview of the most important
methods (including basics, examples and open questions) employed in the
characterisation of porous materials, setting a focus on porous carbons, was
provided with the intention to provide the initial inspiration and guidance
to any researcher looking for new or additional methods for the characterisation of new carbons and materials in general. As the development of new
materials proceeds, there is always a need for the analytical sciences to keep
pace and to provide new or increasingly refined methods, which can give
answers to the newly arising questions. In this regard, it is expected that in the
near future, the “standard” gas adsorption analysis protocols will continue
to evolve, with perhaps the DFT analysis of adsorption isotherms ultimately
replacing the classical semiempirical methods. The use of CO2 adsorption
as an analytical tool will also become an increasingly more accessible and
accepted method. Major advances are also expected regarding microscopy
and tomography. With increasing computation power and new methods available, tomography will become a highly versatile tool, generating important
information on pore connectivity, a topic that was not discussed within this
chapter. Analogously, NMR or fluorescence microscopy methods may also
increasingly help to analyse diffusion processes within pores. Furthermore,
combining such approaches with e.g. scattering methods using in situ methods might be envisaged in the future. An understanding of these processes
on a microscopic scale could boost applications in separation technology. In
summary, though it seems a mature research topic, porosity characterisation
will remain a critical technique in the development and application optimisation of new materials including the development and challenges presented
by complex, highly functional carbonaceous materials.
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CHAPTER 10
Bulk and Surface Analysis of
Carbonaceous Materials
PETER S. SHUTTLEWORTH*a, NIKI BACCILEb, ROBIN J.
WHITE*c, ERIC NECTOUXd AND VITALIY L. BUDARINd
a
Departamento de Física de Polímeros, Elastómeros y Aplicaciones
Energéticas, Instituto de Ciencia y Tecnología de Polímeros, CSIC, c/
Juan de la Cierva, 3, 28006, Madrid, Spain; bLaboratoire de Chimie de la
Matière Condensée de Paris, Collège de France, 11, Place M. Betrthelot,
75005, Paris, France; cUniversität Freiburg, FMF - Freiburger Materialforschungszentrum, Stefan-Meier-Straße 21, 79104 Freiburg im Breisgau and
Institut für Anorganische und Analytische Chemie, Albertstrasse 21, 79104
Freiburg, Germany; dGreen Chemistry Centre of Excellence, University of
York, Heslington, York, Yorkshire, YO10 5DD, UK
*E-mail: peter@ictp.csic.es; robin.white@fmf.uni-freiburg.de
10.1
Introduction
As mentioned earlier in this book, the classical biomass-derived carbons
are the “activated carbons” (ACs). This class of porous carbon covers a wide
range of amorphous materials, typically produced from renewable resources
such as wood, peat moss, coconut shell and fruit pits but also from fossil
fuel-derived precursors.1,2 ACs have large surface areas due to their very complex porous structures that often contain a combination of micro (D < 2 nm),
meso (D = 2 to 50 nm) and macropores (D > 50 nm) (Figure 10.1).1 As mentioned in Chapter 9, these porous features are also found in a variety of carbonaceous materials (e.g. Starbons®, hydrothermal carbons (HTC), etc.)
RSC Green Chemistry No. 32
Porous Carbon Materials from Sustainable Precursors
Edited by Robin J White
© The Royal Society of Chemistry 2015
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312
Figure 10.1
Chapter 10
Schematic
representation of structure of (a) activated carbon; (b) activated carbon granule. Reproduced with permission from ref. 1.
Figure 10.2 Scheme
of the different oxygenated structures that can be present on
the surface of a carbon: (a) carboxylic acid groups, (b) phenolic groups,
(c) carboxylic acid anhydrides, (d) lactols, (e) carbonyl groups, (f) ether
groups, (g) lactone groups and (h) quinone groups. Adapted from ref. 6.
In addition to the complicated and varying texture of ACs and indeed the
majority of carbonaceous materials produced from sustainable precursors,
the analysis of their properties is further complicated by a wide variety of
complex surface chemistry. It is known that besides the multiple-bond systems (e.g., alkenes, alkynes, conjugated and aromatic π-systems), different
functional groups (in particular, those containing oxygen), can be present
on the AC surface and similar groupings have been characterised in carbonaceous materials highlighted throughout this book (e.g. hydrothermal carbons derived from a CnH2nOn base) (Figure 10.2).3–5
The highly developed hierarchical (predominantly microporous) pore structure combined with variety of functional groups present at the surface of carbons derived from sustainable precursors, has made ACs classically applicable
as adsorbents in filtration, purification, separation and catalysis, whilst the
development of new, less condensed (e.g. more hydrophilic, functional, mesoporous) carbon-based materials are also demonstrating their advantages in
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Bulk and Surface Analysis of Carbonaceous Materials
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the mentioned fields. In both cases, the complexity of both bulk and surface
chemistry requires complex analytical characterisation. Techniques, including N2 sorption porosimetry (as discussed in Chapter 9), X-ray photoelectron
spectroscopy (XPS), thermogravimetric analysis (TGA), titration, bromination
and solid-state nuclear magnetic resonance (ssNMR) spectroscopy have been
shown to provide in general a good and accurate account of both physical,
bulk and surface chemistry properties. In this context, the following chapter,
with a particularly focus on the incorporation if oxygen (e.g. via chemisorption,
exposure to air or controlled pyrolysis) into such carbon materials, will examine the use of these characterisation techniques (i.e. TGA, XPS, FTIR and chemical titration methods) initially on the analysis of commercial carbon materials
NORIT® CASP, DARCO®, R008 (NORIT®) and VULCAN® although it is important
to note that these techniques are applicable to the majority of carbons including those discussed throughout this book. The chapter will conclude with a
dedicated discussion regarding the use of a variety of ssNMR techniques in the
characterisation of carbons with a detailed review of the chemical structure of
saccharide-derived hydrothermal carbons.
10.2
10.2.1
Thermal Gravimetric Analysis
Introduction
Thermal gravimetric analysis (TGA) is an analytical techniques used to examine both the physical and chemical properties of a material as a function
of increasing temperature (e.g. at a constant heating rate), or as a function
of time (with constant temperature and/or constant mass loss). Variables
include, heating rate: dynamic (5 °C or 10 °C or 20 °C min−1, etc.) or isothermal and atmosphere (typically nitrogen or air). The recorded trace therefore
represents the percentage mass change in a sample as a function of the loss
or gain of a certain molecule or a change in the state of the given material.
Under a nonoxidising atmosphere, the mass loss observed has the potential
to provide an estimation of the oxygen content of an AC carbon and can also
yield information on the nature of the oxygenated functional groups present
on the surface. Indeed, the temperature at which certain functional groups
leave the surface of the carbon has been widely studied in the literature (Table
10.1).7 However, classification is not completely representative and the use of
complimentary analytical techniques is often required for a fair description.
10.2.2
Results and Discussion
As a general example of the behaviour of ACs, the thermal desorption of
“O”-containing functional groups releases gases (typically CO2 and/or CO
and/or H2O) at a temperature that is generally specific to the nature of the
functional groups/groupings. In the following discussion, the study of the
evolved gases has been performed using both gas phase IR (only for the CO2
evaporation associated with carboxylic groups as the gas cell could only be
heated up to 250 °C) and TG-IR (Figure 10.3).
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Table 10.1 Common
group types found at the surface of ACs materials and their
corresponding decomposition temperatures. Adapted from ref. 7, 8.
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Type of group
Figure 10.3
Name of the oxygenated
functional groups
Species released
Peak temperature (°C)
Carboxylic acid
CO2
240
Lactone
CO2
670
Phenol
CO
630
Carbonyl
CO
710
Anhydride
CO + CO2
550
Ether
CO
700
Quinone
CO
810
Typical
CO and CO2 evaporation profile a carbon using TG-IR, and the
results obtained from IR gas cell analysis at the specified temperatures.
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Bulk and Surface Analysis of Carbonaceous Materials
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As examples, four different commercial carbons were analysed by TG-IR,
with the TGA performed under an inert N2 atmosphere using a Netzsch 409
STA. The mass change was recorded over the 30 to 1200 °C temperature
range, heating at 10 °C min−1. All the samples had been dried in a vacuum
oven at 100 °C for 24 h and kept in a desiccator before analysis. The red curve
represents the derivative of the mass loss, which means that each peak is
associated with desorption of a compound. In the temperature interval 100
to 150 °C, only the removal of physisorbed water is observed. Regarding the
rest of the spectrum, depending on the desorbing oxygen species, CO, CO2 or
a combination are released, with, apart from water, only CO2 being released
up to 200 °C (Figure 10.3). Depending on the species released, the oxygen
percentage is equal to the mass loss found by TG multiplied by the ratio
of oxygen that is contained in the leaving gas (Table 10.2), with the results
shown in Table 10.3.
It can be seen that the carbon samples can be classified as follows, from
higher to lower oxygen content: CASP > Darco > ROO8 > Vulcan. The surface chemistry quantity of these carbon samples also appeared to be different (Figure 10.4). Whereas CASP is a high oxygen containing carbon due to
its significant mass loss, Vulcan shows only a minor change as a result of
Table 10.2 Oxygen
content calculation derived from TGA results.a,b
From CO2 evaluation
From CO evaluation
( M ⋅ MWO )
(M·MWO)/MWCO
% Oxygen
2
MWCO2
a
M: % mass loss found by TG.
MW: molecular weight.
b
Table 10.3 Oxygen
content of the carbons derived from the TG analysis.
Darco
Oxygen content High: 5.1%
Figure 10.4
R008
CASP
Vulcan 72r (Infineum)
Medium: 2.8%
High: 9%
Low: 1.6%
Thermogravimetric
analysis of the four different carbons ((A) Mass
loss and (B) Derivative).
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316
Chapter 10
heating. These results will later be correlated with those from X-ray photoelectron spectroscopy.
Based on the corresponding derivative TGA plots and as a consequence
of their relatively low oxygen content, no particular peaks can be seen for
Vulcan or ROO8, whereas three peaks are observed for Darco and CASP
(Figure 10.4(b)). The first two “event” peaks, which are similar for both carbons, are located at ca. 100 °C and ca. 240 °C, associated with the removal
of physisorbed water (even if the samples were dried before analysis), and
desorption of CO2 (i.e. from carboxylic acid group decomposition), respectively. The third peak, which represents the highest mass loss event, differs
for the Darco and CASP carbons. For CASP, it is centred at 700 °C, whereas it
is situated at 800 °C for Darco. It is difficult to assign those particular peaks,
as a number of groups may be responsible for mass loss at those temperatures. The peaks are broad (particularly for CASP), which make their assignments even more problematic. It is for this reason that other characterisation
techniques are needed to differentiate oxygen functionality, such as phenol,
ether or carbonyl that can be responsible for the behaviour of these samples.
Given the comparatively condensed nature and typically low loading of
“O”-containing functionality, the characterisation of ACs by TG/FT-IR analysis can be relatively straightforward particularly when compared to the more
complex, multifunctional bulk and surface chemistry of carbonaceous materials. In this context, Titirici et al. have investigated the decomposition of
HTC materials derived from glucose (prepared at a hydrothermal treatment
temperature of 180 °C) by TG/FT-IR, before and after a solvent-extraction
step (Figure 10.5).5 Based on the corresponding TG/dTG traces, the thermal
decomposition of glucose-derived HTC material (denoted as HTC-G) was
reported to be composed of two relative broad decompositions over the temperature ranges 1) 160–270 °C and 2) 350–600 °C) (Figure 10.5(a)); with the
former corresponding to the thermal evolution of levulinic acid (LA) (boiling
point range: 245−246 °C) proposed to be physisorbed or “embedded” within
the highly crosslinked polymeric HTC bead during the material synthesis.
To demonstrate this further, Titirici et al. compared this TG/dTG analysis
with that produced from the decomposition of the same materials after washing/soxhlet extraction with ethanol, hexane, and THF. This first demonstrated
the successful removal of the majority of the embedded LA, resulting in the
observation of one relative broad decomposition event (350–600 °C) (Figure 10.5(b)). GC-MS analysis of the extracted ethanol fraction demonstrates
the presence of LA (Figure 10.5(c) and (d)). The main decomposition over the
range T = 350−600 °C observed in this report was proposed to represent the
restructuring/charring of the structural carbon motifs and a corresponding
loss of volatile species (Figure 10.5(e)). Commencing at 400 °C, FTIR analysis
of the resulting gas phase products demonstrated that the strongest band
intensities were the result of CO2 and CO removal, followed by CH4 starting
at ca. 450 °C, with a corresponding increase in band intensity up to ca. 550
°C. Continued heating of the HTC-G sample above this temperature corresponded with a reduction in intensity of all the detected signals at different
Figure 10.5
(a)
Thermogravimetric analysis of glucose-derived HTC carbon at 180 °C before extraction and (b) after three consecutive
extractions with ethanol, hexane and THF. (c) Gas chromatograph of the extract obtained after Soxhlet extraction with ethanol of glucose-derived HTC carbon at 180 °C. (d) Mass spectrum of the peak indicated in (a) (red, literature data; black,
experimental data). (e) Extracted traces from TG-IR analysis at selected wavenumbers (cm–1) for glucose-derived HTC carbon
at 180 °C before extraction. The legend shows either the name of the gas the trace can be attributed to, or the wave number
the trace is detected at. Reproduced with permission from ref. 5.
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rates. The chemical events observed in this study were proposed to be related
to associated furan ring-opening chemistry/decomposition (e.g. methylene
bridge loss, crosslinking, ring opening, aromatisation, etc.).9–12
10.3
10.3.1
X-Ray Photoelectron Spectroscopy (XPS)
Introduction13
In X-ray photoelectron spectroscopy (also referred to as electron spectroscopy for chemical analysis, ESCA), core electrons are excited by X-ray irradiation to leave the atom, meaning that all elements apart from hydrogen can be
detected. The binding energy (BE) of these core electrons can be derived from
their measured kinetic energies. Furthermore, XPS is a surface-sensitive analytical technique since the escape depth of the photoelectrons amounts to
only a few atomic layers. The BE depends on the atomic species but is also
affected by shielding of the nuclear charge that is lowered or raised by bonding of the atom to more electronegative or electropositive atoms, respectively.
The differences in BE for various binding states are quite small compared to
the line width, especially with electronegative elements (e.g. oxygen). Therefore, deconvolution of overlapping peaks is necessary. However, the results of
curve fitting are influenced to some extent by the somewhat arbitrary inputs
for the number, shape and width of the peaks. Although modern instruments
provide a sufficient resolution for O1s electrons, it is more convenient to measure the C1s signal. Carbon atoms differ in their BE depending on whether
they are linked to a heteroatom (e.g. O, N, etc.) by a single bond (e.g. phenols,
furans, pyrroles and ethers), a double bond (e.g. carbonyl, imine groups), or
two heteroatoms (e.g. carboxyl groups, lactones). The corresponding signals
appear as satellites on the high-BE side of the main C1s peak of the carbons.
10.3.2
Elemental Analysis
Elemental analysis (by XPS) reveals that the oxygen content for the different
carbons is in a similar range to those previously found using TGA (Table 10.4).
Apart from a trace of nitrogen, no other elements have been detected on the
surface of the samples. It is possible, as explained in the introduction, to characterise the type of bonding that exists between the carbon surface and the
oxygen detected during the elemental analysis by interpreting the high-resolution XPS spectra for carbon and oxygen.
10.3.3
High-Resolution C1s Spectra
For calibration purposes, the C1s electron binding energy corresponding to
graphitic carbon was referenced to 285.0 eV (Table 10.5; Figure 10.6(a)). After
the base line was subtracted, curve fitting was performed using a nonlinear
least squares algorithm assuming a Gaussian peak shape. This peak-fitting
procedure was repeated until an acceptable fit was obtained.
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Bulk and Surface Analysis of Carbonaceous Materials
319
Table 10.4 Elemental
analysis of standard AC materials and a selection of carbo-
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naceous materials (e.g. prepared via HTC) as determined by XPS survey scan quantification [*determined by combustion analysis]. Adapted
from ref. 14.
Elemental analysis:
Type of carbon
%C
%O
CASP
DARCO
ROO8 (Norit)
Vulcan
*HTC-G
*HTC-G-350 °C
*HTC-G-550 °C
*HTC-G-750 °C
HTC-G-900 °C
90.87
91.47
93.58
96.70
64.47
66.95
84.66
94.04
96.60
9.13
8.53
6.42
3.30
30.84
29.02
12.51
4.29
2.90
Table 10.5 Assignment
for the C1s envelope. Adapted from ref. 15.
Binding energy (eV)
Assignment
285.0
286.0
286.7
287.9
289.0
Nonfunctionalised carbons sp2, sp3
Carbons sp2 linked to nitrogen.
Carbons linked to oxygen by a single bond.
Carbons linked to oxygen by a double bond.
Carbons linked to two oxygen atoms by one single and
one double bond.
π → π* shake up satellites
ca. 291.0
Figure 10.6
Example
of (A) C1s envelope and (B) O1s enveloped for a commercial
Darco® carbon.
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The results obtained for the different ACs are similar with the corresponding high-resolution C1s spectra separable into three main characteristic
peaks (Figure 10.6):
●● 285.0 eV – typical major component, corresponding to nonfunctional
carbon; i.e. the contribution of carbon (sp2 and sp3) belonging to the
carbon skeleton and the contribution of the aliphatic sp3 that corresponds to hydrocarbons.
●● 286.7 eV – related to C linked to O by a single bond (e.g. phenols, alcohols or others functional groups).
●● 288.7 eV – associated with groups such as carboxylic acid, esters, lactones or anhydrides.
It is important to note that no peak at around 288.0 eV appeared after
deconvolution. This means that no or very few carbonyl groups are present
on the carbon surface, whereas acids, esters and lactones can be found. The
analysis of such highly condensed carbons (e.g. Darco) can be contrasted
with the analysis of carbonaceous materials. As an example, the high-resolution C1s photoelectron envelope for HTC-G can be broken down into three
main contributions; (1) 285.0 eV (C1, C–C and C−Hx), (2) 286.3/286.6 eV (C2,
C–O–H (hydroxyl), C–O–C (ether)) and (3) 287.9/288 eV (C3, C=O (carbonyl))
with a minor shoulder at 289.3/289.44 eV (C4, O=C–O (acid or ester)).
The high-resolution C1s envelope of HTC-G (i.e. at 180 °C) is composed of
three main contributions (analogous to the aforementioned ACs); (1) 285.0
eV (C–C and C–Hx), (2) 286.3 eV (C–O–H (hydroxyl), C–O–C (ether)) and (3)
287.9 eV (C=O (carbonyl)) with a minor shoulder at 289.3 eV (O=C–O (acid or
ester)) (Figure 10.7(a)).16 As might be expected peaks with binding energies of
286.3 and 287.9 eV, respectively, are of a relatively high intensity demonstrating the presence of considerable amounts of oxygenated functionalities (e.g.
furan and carbonyl moieties). Pyrolysis of this HTC-G precursor to 550 °C
results in a major reduction of “O”-related material content/functionalities
and a resultant increase in surface hydrophobicity (Figure 10.7(b)). A peak
at 286.4 eV can be attributed to isolated phenol-type groupings. Increasing
the pyrolysis temperature to 900 °C removes the majority of the remaining
“O”-related functional groups and simultaneous increase of the peak at 291.1
eV (i.e. π → π* shake up satellites) and extension of pregraphenic polyaromatic domains as the major building unit of the carbon scaffold, with the
material presenting very similar “C” chemistry (and elemental composition)
to the previously discussed Darco carbon (Figure 10.6 and Table 10.4).
10.3.4
High-Resolution O1s Spectra
Similar to Table 10.5 the binding energy constants for the O1s can be seen in
Table 10.6 with a representative spectrum for a Darco® sample (Figure 10.8(A)).
High-resolution O1s XPS analysis presents results similar for all typical AC
samples. Three peaks appear after deconvolution:
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Bulk and Surface Analysis of Carbonaceous Materials
321
Figure 10.7 C1s
photoelectron envelope of (a) HTC-G (i.e. at 180 °C) and postpyrolysed
at (b) 550 °C and (c) 900 °C. Reproduced with permission from ref. 14.
Table 10.6 Assignment
for the O1s envelope. Adapted from ref. 15.
Binding energy in eV
Assignment
531.6
532.3
533.1
534.4
Oxygen linked to a carbon by a double bond.
Ar–OH, oxygen in bridge.
R–OH and C–O–C.
Physisorbed water.
●●
●●
●●
ca. 531.4 eV – corresponding to oxygen linked to a carbon by a double
bond (mainly from carboxylic acid groups as no carbonyl groups were
detected in the C1s spectrum).
ca. 533.2 eV – associated with functional groups (e.g. phenolic or
C–O–C(ether)).
536.4 eV – due to the presence of physisorbed water.
The second peak is the most significant, meaning that the most numerous “C” and “O”-based functional groups for these nonmodified ACs are
in the form of alcohols and/or ether groups. In the case of more functional
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322
Figure 10.8
(A)
Example of a C1s envelope: Darco after being heated at 350 °C in
a nitrogen atmosphere; (B) Example of an O1s envelope: Darco after
being heated at 350 °C under a nitrogen atmosphere.
carbonaceous materials (e.g. HTC-G), the amount of O present in the sample can reach as high as 30 wt%, with the majority of this oxygen content
represented as furanic, carbonyls, primary and secondary alcohols as well
as a number of ether-based systems. Performing a postsynthesis pyrolysis
treatment on such carbonaceous materials results in a reduction of oxygen
content (i.e. from 30 to <3 wt% at 900 °C) with oxygen-based functionality
becoming increasingly similar to the previously mentioned AC surface chemistry with increasing treatment temperatures.
10.3.5
Discussion
From the results obtained by XPS (for both C1s and O1s) it can be concluded
that the major part of oxygen contained at the surface of the different ACs
is linked by a single bond (presence of phenol and/or ether groups), whilst
for carbonaceous materials, more complex structural units can be observed
(e.g. furan-based groups). TGA results demonstrate the presence of carboxylic acid groups on the surface of ACs: further confirmed by XPS analysis. As
the carboxylic acid groups are responsible for both peaks in the high-resolution O1s spectrum, an assumption that the rest of the oxygen at the surface
is linked to the carbon by a single bond can be made. It can be seen in the
O1s spectrum for most of the AC samples that the peak that represents C
connected to O via a single bond is as large as the one for the O linked to C
by a double bond. Thus, it can be concluded that the –C(O)OH groups are
responsible for the totality of the minor peak or that there is twice as much
O connected to C by a single bond than O linked to C by a double bond on
each surface. To prove this assumption, it is possible to heat the carbon in
order to remove the carboxylic groups (heat to 350 °C). Then, if the peak for
the oxygen linked to a carbon by a double bond remains, it means that other
groups containing this type of bond (e.g. lactones, carbonyl or anhydride) are
present (Table 10.7; Figure 10.8).
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analysis of the carbons after being heated at 350 °C in a
Table 10.7 Elemental
nitrogen atmosphere by XPS.
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Elemental analysis
Type of carbon
%C
%O
Difference in oxygen
content (%)
CASP
DARCO
ROO8 (Norit)
Vulcan
91.64
92.12
93.75
96.81
8.36
7.88
6.25
3.19
0.77
0.65
0.17
0.11
Figure 10.9
O1s
envelope for Norit-ROO8 (A) before and (B) after heating.
The O1s spectrum obtained for the heated sample of Darco is very similar
to that of the untreated material. Only a minor change in the ratio between
the first and second peak can be observed (the first peak increases in intensity,
which is the opposite of what was expected). This phenomenon can either be
due to a “reoxidation” of the sample (the same by which the oxygen functional
groups are created during “activation” of carbon black) or due to some rearrangements occurring as a result of heating. It can also be observed that the
heat treatment has a drastic effect on the Norit-ROO8 carbon (Figure 10.9),
with an interchange of two first peaks as a result of heating as the peak that
represents esters or alcohols loses intensity. This means that some of the
C–O–C or C–OH groups change into groups where the oxygen is linked to
the carbon by a double bond. In this case, the “reoxidation” of the carbon is a
more probable explanation as no rearrangements can produce this effect. This
is also confirmed with the carboxylic peak still being present after heating.
10.4
10.4.1
Infrared (IR) Spectroscopy
Introduction13
Infrared (IR) spectroscopy is an analytical technique that measures the infrared intensity vs. the wavelength (wave number) of light. IR spectroscopy
detects the vibration characteristics of functional groups in a sample. When
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a sample is submitted to a beam of infrared light, the chemical bonds within
this sample will stretch, contract and bend. Diffuse reflectance IR Fourier
transform spectroscopy (DRIFTS) is a variation of this technique typically
employed in the analysis of powdered solids. Diffuse reflectance occurs when
the IR beam enters the sample and is then partially reflected off the surface
of a particle or transmitted. Typically, the IR energy reflecting off the surface
is lost. The light that passed into the material maybe be absorbed or reflected
out again. These transmission–reflectance effects can occur many times in
the sample, which increases the length of the path. Hence, the radiation that
reflects from an absorbing material is composed of surface-reflected and
bulk re-emitted components, which when summed are the diffuse reflectance of the sample. In the following example, the use of DRIFTS in the characterisation of AC materials is discussed.
10.4.2
Experimental
The major problem associated with the use of IR spectroscopy in the characterisation of carbons is that such materials act as very effective blackbody
absorbers. This can result in an attenuation of the beam energy throughput. It is for these reasons that prior to characterisation, the carbons need to
be diluted with an appropriate transparent medium to allow sufficient signal intensity. In this study, all the carbons have been diluted with KBr with
a ratio of 1 mg of carbon to 300 mg of KBr (Figure 10.10). This technique
has received a lot of interest for the characterisation of surface chemistry
of carbon blacks with the resulting band assignment for the different types
of carbon functional groups (Table 10.8).17,18 These band assignments can
Figure 10.10
IR
spectrum for the different carbons diluted 300× using KBr.
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also be applied (and indeed with a higher degree of resolution) for carbonaceous materials.5 Another FTIR technique that is particularly suitable for the
characterisation of carbon-based solids is attenuated total reflectance (ATR)
FTIR, which relies on the principle of total internal reflectance to the characterisation of a material (and in fact typically eliminates the need to dilute the
sample prior to analysis).5,19–21
The results obtained are similar for all the ACs investigated in this study
and show the presence of broad bands making assignment of particular functional groups difficult. The major problem is the complicated nature of the
absorption bands in the 1750–1700 cm−1, 1650–1610 cm−1 and 1570–1520 cm−1
regions. Although a strong peak situated at ca. 1500 cm−1 can be associated with
–C(O)OH, other peaks between 1200 and 1700 cm−1 can also be due to groups
such as quinones, carboxylic acids or lactones groups. Ether C–O groups can
also be present as indicated by the peak at ca. 1000 cm−1. Diluting the sample
further using KBr (1 : 10 000) can help resolve the spectra (Figure 10.11).
For a typical AC materials (e.g. CASP), three major peaks at 1245, 1600 and
1720 cm−1 can be assigned (Figure 10.11):
●● 1245 cm−1: C–O in ethers.
●● 1600 cm−1: Quinones, carbonyl, C double bond aromatic stretching.
●● 1245 + 1720 cm−1: Lactones.
●● 1245 + 1600 + 1720 + 3060 (small peak) cm−1: Carboxylic acids.
However, it is still difficult to characterise the surface chemistry of a respective carbon sample after this refinement process and based on this technique alone due to overlapping of the peaks. In the case of more functional
carbonaceous materials the challenge is often greater given the range and
number of functional groups found at the surface. Taking HTC-G as a representative example, broad absorption bands between 3700 and 3100 cm−1 are
often observed for as-synthesised carbonaceous materials corresponding to
O–H (bonded) stretching vibration. A band at 2925 cm−1 is also commonly
Table 10.8
Generalised IR assignments of functional groups on carbon surfaces.
Adapted from ref. 7.
Group or functionality
Assignment regions in cm−1
C–O in ethers (stretching)
Alcohols
Phenolic groups: C–OH (stretching)
Phenolic groups: O–H
Carbonates, carboxyl carbonates
C double bond aromatic stretching
Quinones
Carboxylic acids
Lactones
Carboxylic anhydrides
C–H stretching
1000–1300
1049–1276 and 3200–3640
1000–1220
1160–1200 and 2500–3620
1100–1500 and 1590–1600
1585–1600
1550–1680
1120–1200 and 1665–1760 and 2500–3300
1160–1370 and 1675–1790
980–1300 and 1740–1880
2600–3000
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Figure 10.11
IR
spectrum for CASP diluted by 10 000 using KBr.
observed demonstrative of methylene-type groups (e.g. ν(C–H)stretch), whilst in
the region 1700–1600 cm−1 bands can be attributed to C=O and C=C stretching vibrations, respectively.
In conclusion, characterisation of a carbon-based material using IR can
provide the user with a greater insight into the surface chemistry, although
this can be a challenging process depending on the carbon in question and
the level of material condensation and heteroatom (e.g. oxygen) content.
10.5
10.5.1
Boehm Titration
Introduction
It is well known that carbon black possesses functional groups on its surface
that are due to a chemisorption of oxygen from air during cooling after the
activation process. Using the aforementioned characterisation techniques,
it can be difficult to differentiate groups such as carbonyls and phenols/lactones. However, the surface of a carbon is typically acidic in character,22 due
to the presence of carboxylic groups, lactones or lactols and hydroxyl groups
of phenolic character.23,24 These functional groups will have different acidity
depending on their chemical environment (e.g. size and shape of polyaromatic layer, presence and position of other substituents and charge of neighbouring dissociated groups). This results in a variation of acidity sufficiently
large to allow differentiation by a titration method. A titrimetric technique
developed from Boehm’s work,23 in which carbon is allowed to react with a
series of bases can allow quantification of the functional groups present on
the surface of the carbon. By using progressively stronger bases, it is possible to yield information on the total titrateable surface charge as well as
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24
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individual oxygen-bearing functions on the surface of the carbons. This
allows for the broad classification of titrateable functional groups at the carbon surface and provides an estimate of the relative amounts of the various
functional groups.25,26
10.5.2
Experimental
As a general example, this method involved suspending 0.2 g of carbon in
25 ml of a 0.1 M solution of either sodium ethoxide (NaOCH2CH3) in ethanol, or sodium hydroxide (NaOH) or sodium bicarbonate (NaHCO3) in
water, and stirring for 24 h. To provide a fair analysis, the carbon material
is typically ground and sieved to the same particle size, dried and kept in a
desiccator prior to analysis. Indeed, particle size is a factor likely to have an
impact on the result obtained because access of the reactant to the inner surface may be restricted due to diffusion limitations within larger particles.27
After filtration and solid recovery, a 10 ml aliquot added to 15 ml of 0.100 ±
0.0005 M HCl. The HCl neutralises the unreacted base and minimises further reactions between atmospheric CO2 and the various bases from occurring. The solution is then back-titrated with 0.1000 ± 0.0005 M NaOH using
a water-ethanol solution, and methyl red as the indicator. Sodium ethoxide
titration is not, strictly speaking, an acid–base titration where, for example,
OH− or HCO3− replaces H+ from acid functional groups of different ionisation
potential on the carbon surface. It is, instead, thought to be an addition reaction with formation of sodium salts of a hemi-acetal or ketal from lactones or
lactols. Sodium hydroxide will titrate phenols, lactones and carboxyls, while
sodium bicarbonate will give information concerning carboxylic acid groups
alone.34 Sodium hydroxide is a strong base in water and is able to neutralise
acids with strong or weak ionisation potential. Therefore, sodium hydroxide
titrations are true acid–base neutralisation reactions and measure surface
charge attributable to carboxylic acids, phenols, lactones and other groups
with acidic or ionisable hydrogen. By subtraction of any base from the next
more exclusive base it is possible to estimate the surface charge residing in a
variety of functional groups as mmol H+ equivalent/g carbon.
10.5.3
Results and Discussion
The Boehm titration results for the ACs investigated in this study are in
accordance with data obtained via previously discussed XPS and TG analysis
(Table 10.9). CASP was found to have the highest oxygen content followed by
Darco, Norit-ROO8 and Vulcan. From TGA and XPS, it was only possible to
differentiate one type of functional group (carboxylic acid). It was found that
around one third of the oxygen content of CASP was in the form of carboxylic acid, half for Darco and virtually zero for Vulcan and Norit-ROO8. Those
results are similar to the ones derived from the titration method. Moreover,
according to this analysis, the remaining oxygen is mainly in the form of
phenol/lactone groups.
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Table 10.9 Boehm
titration results for a general selection of activated carbon
materials.
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Amount of functional groups (mmol g−1)
Type of carbon
pKa < 6.37
“CO2H”
6.37 < pKa < 10.25
“Phenol + lactone”:
10.25 < pKa < 15.74
“carbonyl”
Total
CASP
Darco
ROO8
Vulcan
0.28
0.25
0.08
0.04
0.52
0.39
0.28
0.28
0.04
0.01
0a
0a
0.84
0.65
0.36
0.32
a
Below the detection limit.
In conclusion, it can be said that the Boehm titration is a useful technique
in the characterisation of a carbon material surface. However, as it is only
capable to detect acidic sites of a certain strength,28 it cannot be used in an
accurate, quantitative manner.29 On the other hand, this method provides
useful information about the kinds of groups present: groups that are difficult to characterise via other methods. Nevertheless, there are some limitations associated with this particular technique. Indeed, due to a variety of
inter- and intramolecular interactions (e.g. hydrogen bonding), the Brønsted
acidity of the different functional groups can overlap. This can result in “a
heterogeneity of the surface that can make difficult any attempt to delineate surface functionalities on carbon according to their acid base properties”.30 Also, it is
difficult to compare the behaviour of a particular functional group present on
a carbon surface with the same group of organic molecules due to the effect
of possible neighbouring surface group. It has been postulated by Laszlo et
al. that there are no two groups behaving exactly alike on a carbon solid.31
This means that the same functional group (e.g. –C(O)OH) will have different
properties depending on their chemical environment at the surface.
10.6
Bromination
10.6.1
Introduction
Bromination of a carbon material can be used to characterise the number of
C=C present at the material surface.31 The proposed mechanism for the bromination of an alkene follows the general two-step mechanism of addition; a
reaction that is possible due to the polarisability of the electrons in the C=C
bond (Figure 10.12). The first step is electrophilic attack by Br2 to the π bond
to give a cation intermediate – a bridging bromonium ion – that controls the
stereochemistry of the addition reaction.
The second step of the reaction involves the nucleophilic addition of the
negatively charged bromide anion. This anion attacks the carbon via an SN2
reaction pathway and opens the ring of the bromonium cation resulting in
the formation of the dibromide with a well-defined stereochemistry in an
overall antiaddition conformation (Figure 10.13).
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Figure 10.12
Synthesis
of the bromonium ion for a standard alkene (i.e. ethene).
Figure 10.13
Termination
of the bromination.
10.6.2
Experimental
The standard AC systems exemplified in this study were brominated using
solutions of Br2 at various concentrations at room temperature. The procedure involved was as follows:
●● An 8 mmol solution in tetrahydrofuran (THF) was prepared by mixing
2.53 g of Br2 solution with 80 ml of THF, (denoted as solution 1).
●● All the solutions were prepared using 1.0 g of AC. 20 ml solutions of Br2
(different concentrations obtained by dilution of solution 1 with THF)
were added to the AC.
●● Samples were stirred for 24 h at room temperature and then filtered and
washed three times using THF.
●● Samples were then dried under vacuum (e.g. Schlenk line) for 24 h at
room temperature, and then kept in a desiccator before analysis.
10.6.3
Results and Discussion
The TGA of the treated ACs (e.g. Darco is represented in Figure 10.14 with
presented data representative for all investigated ACs) shows that Br2 is
chemisorbed by the carbon and that the amount of Br2 rises with the concentration of the solution used.
As illustrated by the dTG traces (Figure 10.14(B)), the new “bromo” groups
adsorbed to the carbon surface fully decompose between 150 and 220 °C,
with two peaks at ca. 170 °C and 210 °C appearing when the material carries a
high bromine loading, whereas at a low bromine loading only one peak at ca.
170 °C is present. According to Gayathri et al. the first peak is associated with
physically adsorbed/trapped bromine and the second peak represents the
presence of chemisorbed bromine.32 From the presented results, it can be
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Chapter 10
Figure 10.14
Typical
TGA (A) and dTG (B) analysis for a representative Darco sample and Darco samples brominated at increasingly higher loadings.
Figure 10.15
Graphical
representation of the relationship between Br2 solution
concentration and Br2 loading (as determined by TGA mass loss
data) for four representative ACs.
concluded that the bromine first saturates the micropores and then begins
to attack the C=C bonds at the material surface. This is further confirmed by
comparing the amount of Br2 physisorbed between the different ACs, comparable to the micropore volume found during porosimetry analysis (results
not shown). In order to compare the different modified ACs, a graph of Br2
loading (represented by the mass loss found between 190 °C and 220 °C) vs.
the Br2 solution concentration is presented (Figure 10.15).
It can be seen that saturation is obtained for all the investigated ACs using a 2
mmol solution of Br2 (Figure 10.15). It is difficult to derive from the results the
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Bulk and Surface Analysis of Carbonaceous Materials
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exact number of C=C bonds present on the surface of the carbon. Indeed, it has
been found that the bromination of C60 using excess Br2 leads to the formation
of a maximum brominated product C60Br24.32 Unfortunately, it is not possible to
know the exact structure present on the surface (number of aromatics). Only a
comparison between carbons is possible. From the results obtained, it can be
seen that Norit-ROO8 contained the most significant number of C=C bonds at
its surface. The others can be classified as follows: Vulcan > CASP > Darco. A
study by Papirer et al. concluded that the Br surface coverage diminishes as the
surface area of the carbon sample increases.33 The results obtained here, apart
from CASP, are in accordance with this study. On the other hand, a study by
Gonzalez et al. on Cl2 adsorption concluded that the amount of Cl2 chemisorbed
is related to the availability of hydrogen atoms, nonconjugated double bonds
and carboxyl and hydroxyl groups due to five different reaction possibilities:34
1. Substitution reactions: C–H + Cl2 → C–Cl + H–Cl
2. Addition reactions at nonconjugated double bonds: C=C + Cl2 → ClC–CCl
3. Dehydrogenation reactions: HC–CH + Cl2 → C=C + 2HCl
4. Substitution reactions at the carboxyl group: 2COOH + Cl2 → 2COCl +
H2O + 1/2O2
5. Substitution reactions at the hydroxyl group: 2COH + Cl2 → 2CCl + H2O +
1/2O2
However, for the representative carbon materials presented in this study,
CASP should be able to chemisorb more Br2 than the others as it has the
higher oxygen content. This signifies that reactions (1) and (2) are predominant, as has been observed by Tobias et al.35
10.7
10.7.1
Solid-State Nuclear Magnetic Resonance (ssNMR)
Introduction
Nuclear magnetic resonance (NMR) spectroscopy is commonly associated
with the fields of organic chemistry and structural biology, where analysis is
generally performed in dilute liquid solutions. Contrastingly, NMR is a less
well known routine technique for the study of solid-state matter. In fact, the
absence of Brownian motion, which generally averages out specific magnetic
interactions in solution (e.g. dipolar coupling), allows their observation in the
solid state, leading to a significant loss of signal intensity and resolution, if
well-adapted technical countermeasures are not employed. Two of the most
relevant interactions to deal with in the solid state are: chemical shift anisotropy (CSA) – directly related to the chemical environment of the nuclei; and
dipolar coupling – a through space interaction and directly related to internuclear distance. If these interactions have a dramatic, detrimental effect on
spectral resolution, a number of tools exist to overcome them to obtain a
well-resolved spectrum. In this regard, magic-angle spinning (MAS), strong
heteronuclear and homonuclear decoupling, and crosspolarisation (CP) are
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routinely employed to resolve ssNMR spectra. For a general overview on NMR
theory, ref. 36 and 37 are recommended, whilst for more specific insights into
ssNMR, ref. 38–40 are particularly noteworthy. A fair compromise between a
broader explanation and shorter reading of basic NMR principles and their
application in chemistry can also be found in ref. 41–43. The following section will focus on several well-known carbon-based systems for which 13C
MAS ssNMR has provided an extremely valuable contribution to their structural analysis. Examples focus on two forms of nanocarbons, fullerenes and
nanotubes, then lignin, cellulose, their chars obtained from pyrolysis, and,
finally, carbons obtained from the hydrothermal treatment of carbohydrates.
10.7.2
Fullerenes and Nanotubes
Fullerenes, discovered in 1985, are a family of carbon allotropes, molecules
composed entirely of carbon, in the form of a hollow sphere, ellipsoid, tube,
or plane.44 Spherical fullerenes are also called buckyballs, and cylindrical
ones are commonly referred to as carbon nanotubes (CNTs) or buckytubes.
Graphene is an example of a planar fullerene sheet. Fullerenes are similar in
structure to graphite, which is composed of stacked sheets of linked hexagonal rings, but may also contain pentagonal (or sometimes heptagonal) rings
that would prevent a sheet from being planar. Applications vary from medicinal use to heat-resistant devices and superconductivity. 13C ssNMR studies
have been performed on fullerenes since the early 1990s, and confirmed the
chemical homogeneity of the 60 carbon atoms for the C60 and the expected
inhomogeneity of the C70 equivalent.45 Initial structural studies (bond-length
calculation, molecular motion)46–49 including spin relaxation dynamics of C60
under different external conditions (e.g. pressure, temperature)50–53 were followed by more detailed investigations on the interactions between fullerene
and intercalation compounds, focusing on molecular mobility and van der
Waals interactions.54–59 Finally, recent studies directed more efforts to the
understanding of molecular entrapping within fullerene cages.60,61 Recent
review papers have shown some of these aspects already and for this reason
we are limited here to a short and broad description, for each category outlined above.62–65
Most structural studies have been performed using 13C NMR under both
static and MAS conditions. Due to the high molecular mobility of the fullerene C60 cage in the solid state under ambient conditions, static NMR is sufficient to provide evidence of the characteristic isotropic resonance at δ =
143 ppm. At low temperature, on the contrary, part of CSA is reintroduced,
as expected, but a small fraction of a mobile phase is kept at temperatures
as low as 100 K.48 Spin-lattice T1 relaxation times have been largely investigated under different conditions. The first study proposed by Tycko et al.
revealed discontinuous values of T1 as a function of temperature.50 This is
due to a phase transition from FCC (face-centred cubic) to SC (simple cubic)
phase, which was already seen from DSC and XRD experiments at ca. 250
K. Similar conclusions were drawn in a T1 study as a function of pressure.53
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Mechanisms of relaxation were mainly attributed to CSA in the low-temperature range while the interaction between nuclear spins and molecular
rotation was invoked to explain the T1 behaviour at high temperature values,
above 400 K.51
CNTs are categorised as single-walled (SWNTs) or multiwalled (MWNTs)
and are entirely composed of C-sp2 bonds, similar to those of graphite,
providing the molecules with their unique strength. Under high pressure,
nanotubes can merge together, trading some sp2 bonds for sp3 bonds, giving the possibility of producing strong, “unlimited-length” wires through
high-pressure nanotube linking. These cylindrical carbon molecules exhibit
extraordinary strength and unique electrical properties, and are efficient
heat conductors that make them potentially useful in many applications in
nanotechnology, electronics, optics and other fields of materials science, as
well as potential uses in structural materials. Until the work of Tang et al.66
ssNMR of CNTs proved to be very challenging due to some intrinsic problems
in the production process, allowing the incorporation of relatively small, polluted i.e. (with paramagnetic species from metal catalysts) in the sample.62
Initial reports focused on static and MAS ssNMR spectra acquisition with the
former highlighting nonisotropic and nonplanar behaviour of the chemical
shift tensor, while the latter showed a single, multicomposite, peak centred at
δ = 124 ppm suggesting a metallic and semiconducting character of the material. Confirmation for the existence of the electron-conducting behaviour is
also provided by the linear relationship between the spin-lattice T1 relaxation
time and temperature, as described by the Korringa relationship.67 After this
pioneering study, several others started to appear and focused their interest towards a better characterisation of the magnetic properties of CNTs as a
result of their metallic behaviour. 13C ssNMR both under static and MAS conditions and T1 analysis constitute the main tools for investigating the precise
nature of the metallic and semiconducting properties of CNTs. More details
on this topic have been already reviewed and can be found in ref. 68.
10.7.3
Lignin, Cellulose and Their Chars from Pyrolysis
Lignification is the polymerisation process in plant cell walls transforming
phenolic monomers into radicals, and coupling them with other monomer
radicals (only during initiation reactions), or more typically crosscoupling
them with the growing lignin polymer/oligomer, to build up a phenylpropanoid polymer.69–73 Even though extensive research efforts have been made
to elucidate the finer structural details of the highly complex polyaromatic
lignin, a definitive model does not yet exist (one of the available models can
be found in ref. 74). This task is further complicated by the wide natural
variation in lignin structure, with the main difficulties arising during characterisation due to the high level of chemical and structural heterogeneity of its
bonding patterns. Nonetheless, the polyphenolic nature of lignin has been
ascertained, and the most abundant constituent monomers characterised as
p-coumaryl, coniferyl and synapyl alcohols.75
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Chapter 10
Solution NMR spectroscopy is a proven tool in the analysis of lignin, but
most of the time the biopolymer and its derivatives must be decomposed
or fractionated into model compounds. In particular, two-dimensional (2D)
13
C–1H correlated (HSQC, HMQC) spectroscopy continues to be the method
of choice to identify unambiguously the different lignin units and the subunit bonding patterns.76,77 The entire lignin fraction can also be analysed
in the so-called “cellulolytic enzyme lignin,” in which large fractions of the
polysaccharides are removed by enzymatic (e.g. cellulase) digestion of crude
wood, for instance.78,79 Of course, isolation or fractionation may cause significant modification of the original structure yielding unrepresentative final
results. This is not the case if ssNMR spectroscopy is used: however, the analysis becomes definitely more challenging. Interesting 13C-enrichment techniques have been developed to allow a direct study of protolignin directly in
the cell walls.80,81 Selective 13C-enrichment can be obtained by using 13C-enriched compounds (e.g. monolignol glucosides, ferulic acid, phenylalanine)
in seedling cultures and tissue-cultured cells, for instance. The achievement of selective 13C-enrichment at a specific carbon has been confirmed by
ssNMR80,82 and structural studies were proposed, for instance, by Terashima
et al.83 They evaluated specific alkyl–alkyl and alkyl–aryl ether linkages on
enriched wheat straw via 13C CP MAS experiments. Evaluation of alkyl–aryl
ethers has also been a matter of debate both in lignin and lignin-derived
polymers. In lignins, their amount has evolved from 63% to 80% and eventually to 74%.83,84 Conversely, type III kerogens, which are lignin geoderived
coals, were shown not to contain significant amounts of alkyl–aryl ethers by
means of chemical shift analysis of the corresponding 13C ssNMR spectra
and DFT calculations.85,86 This information is extremely helpful in the study
of carbonaceous material structure evolution during the lignocellulosic
coalification process and further studies using CP MAS have contributed to
study the structure of natural coals and in particular the problem of signal
attribution to aromatic and aliphatic species.87
Cellulose is a polysaccharide consisting of a linear chain of β(1→4) linked
d-glucose units (which differs from the α(1→4)-glycosidic bonds in starch)
and is the structural component of the primary cell wall of green plants, many
forms of algae and the oomycetes as well as a secretion product of some species of bacteria (e.g. Gluconacetobacter xylinus). Cellulose is a straight-chain
polymer where no coiling or branching occurs, with extended and stiff rodlike conformation. The multiple hydroxyl groups on the glucose monomers
form hydrogen bonds with oxygen atoms on the same or on a neighbouring
chain, holding them firmly together side-by-side and forming microfibrils
with high tensile strength. From a structural point of view, native cellulose
is a semicrystalline solid with two allomorphs, Iα and Iß, where the former
is the metastable, low-density, form while the latter is the thermodynamically most stable, high-density, form. 13C CP MAS ssNMR has been crucial in
the discovery and identifications of Iα and Iß.88,89 In terms of relative abundance, Iα and Iß are generally found in differing mixtures and the proportion
depends on the origin of the cellulose biopolymer. For instance, Valonia and
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bacterial cellulose are rich in Iα, while animal cellulose contains more of Iß
structure.90,91
Upon pyrolysis, cellulose undergoes thermal decomposition leading to the
elimination of small volatile species (e.g. CO2, CO, CH4, H2O) and condensation reactions that produce a complex polyaromatic network, commonly
referred to as char. Early FT-IR studies can be traced back to the 1960s,37
whilst 13C ssNMR started to be employed much later.92 However, its use has
become rapidly widespread and actually necessary to complement FT-IR
data, so that up-to-date models of the early stages of the carbonisation process could be proposed.93 In 1994, Pastorova et al. presented one of the first
studies, where 13C NMR was combined with GC-MS and FT-IR to elucidate the
structure of char obtained from pyrolysing cellulose between 250 and 400
°C for 150 min.94 It was shown that cellulose keeps its initial structure up to
250 °C, while major chemical modifications occur at 270 °C; both phenolic
and furanyl groups were detected as volatile compounds. The idea that the
structure of char from cellulose or other biopolymers (e.g. pectins, wood) is
mainly constituted of an aromatic motif including furanoic compounds connected via aliphatic bridges is generally accepted.95,96 For instance, Zhang et
al. suggested the presence of furfuryl motifs in char obtained from pyrolysed
starch.95 Nevertheless, in most studies, probably due to the lack of a clearcut proof, the structures were rather interpreted as being composed of polyaromatic hydrocarbons,87,97–100 as in lignins or coal.92,101,102 The mechanism
of char formation and the fate of the polysaccharide network at medium/
high pyrolysis temperatures (<500 °C) has also attracted much attention
and the elucidation was mainly possible via a fine NMR study. Wooten et al.
have shown that, after 30 min at 300 °C, cellulose undergoes depolymerisation to form an “intermediate cellulose” product, which then transforms
into a “final carbohydrate” (FC) before aromatisation and that was associated
with large amount of oligo- and polysaccharides.93 In this context, the use
of ssNMR techniques to characterise the decomposition of porous polysaccharides (i.e. Starbon® synthesis) as discussed earlier in this book has also
proven to be very useful.19,20,103
10.7.4
Carbonaceous
Materials Prepared via Hydrothermal
Processing
HTC materials derived from the hydrothermal processing of carbohydrates
and their degree of structural complexity is very similar to the one found in
char, lignins and related derivatives. Elemental analysis indicates their predominantly carbonaceous nature (C > 60 wt%). In the context of their characterisation, FT-IR and XPS offer comparatively poor resolution whilst the
absence of well-resolved diffraction peaks in XRD indicates that the material is typically amorphous. A basic study using ssNMR in a similar way to
the studies on coal mentioned above (i.e. employing only MAS and CP), can
be sufficient to obtain a preliminary analysis of the HTC material chemical
structure. However, it does not provide a definitive model as in the case of
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Chapter 10
lignins, kerogens or any other biopolymer-derived char and more advanced
experiments must be, as discussed later, employed.
As discussed earlier in this book (Part 2), the formation of HTC carbons
takes place via the dehydration of the carbohydrate precursors to furanic
species (i.e. 5-HMF, furfural), the parent intermediates of HTC carbon. The
first, important challenge in terms of material characterisation is the establishment of a relationship between the type of carbohydrate used, their
complexity and the final HTC carbon structure. A preliminary 13C ssNMR
study on HTC carbons derived from different mono- and polysaccharides
(i.e. fructose, glucose, xylose and starch) highlighted that the main factor
affecting the chemical nature of the HTC product is the structure of the parent sugar.104,105 Pentose-derived (e.g. xylose) HTC carbons possess a more
marked aromatic character than hexoses (e.g. glucose). Such a difference is
demonstrated by a more intense peak at δ = 125–129 ppm in the 13C CP MAS
ssNMR spectrum in the former case, which is characteristic of aromatic
carbons belonging to graphitic or long-range conjugated double bonds
structures. In this context, pentoses are expected to be dehydrated to furfural under hydrothermal conditions. On the other hand, 5-hydroxymethylfurfural (5-HMF) is the main dehydration product of hexoses. As reported
in the literature, the reactivity of these two intermediates is different and
this is reflected in the chemical structure of the respective HTC carbons.
Furthermore, by a simple comparison of 13C CP MAS NMR spectra of the
various HTC materials in study, it was also observed that the degree of initial polymerisation of the hexose-based saccharides (mono, di- or polysaccharides) does not influence the final structure, since all the 13C spectra
of HTC carbons derived from hexose-based saccharides are characterised
by identical resonances. Preliminary experiments leading to structural resolution employ CP and CP-derived experiments and in particular a variation of the contact time can help select between mobile and rigid carbons,
between protonated and nonprotonated and even between CH3, CH2 and
CH groups using more specific inversion recovery crosspolarisation experiments (refer to ref. 105 for more details). Connectivity between carbon
groups can be obtained with 2D 13C homonuclear double quantum–single
quantum (DQ–SQ) experiments, one of the most powerful tools to deduce
the main structural units and the major bonding patterns of a carbonaceous framework structure. In particular, on-diagonal peaks indicate 13C
spin pairs, belonging to equal chemical environments, while off-diagonal
crosspeaks show the linkage between carbons that are present in different
functional groups (Figure 10.16).105 For a typical HTC material from glucose, the combination of these experiments suggests that furan moieties
are their major constituent.
All hexose-derived HTC materials show strong similarities in their 13C
NMR signature,104 but this is not the case for cellulose-derived HTC carbons obtained under the same conditions. Temperature-dependent 13C CP
MAS NMR experiments performed on cellulose-derived HTC carbons show
that at T = 180 °C, cellulose is still unaffected by hydrothermal treatment,
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Bulk and Surface Analysis of Carbonaceous Materials
Figure 10.16
337
13
Two-dimensional
C DQ-SQ MAS NMR correlation spectrum
recorded for HTC-G(180 °C) with τE = τR = 285.3 µs. Reproduced with
permission from ref. 105.
since its characteristic resonances (i.e. δ = 65, 72, 75, 84, 89 and 105 ppm)
are still present and well resolved. No resonances are observed in the aromatic region, indicating no relevant HTC carbon formation. At higher temperatures, dramatic differences are observed: all characteristic cellulose
resonances disappear, whilst, new resonances emerge. For instance, at T =
200 °C, a strong resonance in the δ = 120–130 ppm region is now observed
for the samples synthesised in the temperature interval T = 200–280 °C,
while the same peak is not observed for a pure glucose-derived HTC material synthesised below 200 °C.106 Contrarily, at higher temperatures, evolution of the relative intensities is similar for both systems. These findings
highlight that:
●● Cellulose-derived HTC carbons contain a higher amount of aromatic
arene-like groups than other hexose-derived carbons. This is probably
due to the higher temperature needed to degrade cellulose.
●● The identical peak evolution patterns for temperature values > 200 °C
indicates similar chemical transformations in both systems, thus indicating that the HTC processing of both precursors is characterised by
similar reaction pathways beyond this temperature threshold.
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13
C ssNMR analysis of cellulose-derived HTC carbons synthesised at different reaction times confirmed the findings previously highlighted.106 All HTC
samples obtained from cellulose are characterised by the presence of the
central resonance at δ = 125–129 ppm. This feature is present since the early
stages of the reaction contrary to what was observed for the treatment of
glucose as a function of time at the same temperature. This finding suggests
that the HTC of cellulose does not proceed solely through a furane-composed
intermediate (i.e. HMF), as observed in the case of the model monosaccharide
(i.e. glucose) and other hexoses. The major conversion mechanism is instead
thought to be the direct transformation of the cellulosic substrate into a final
carbonaceous material composed of polyaromatic arene-like networks, presumably involving reactions that are normally characteristic of the pyrolysis process. During cellulose pyrolysis, the char formation is attributed to a
manifold of reactions leading to cellulose intramolecular rearrangement and
formation of a cellulose-derived polymeric compound, referred to as intermediate cellulose. This reaction intermediate then converts to aromatic network
structures at extended reaction times.94,104,107,108 This mechanistic speculation is well supported by the similar 13C NMR profiles of cellulose-derived
HTC carbon and char obtained from lignocellulosic biomass pyrolysis.100,109
In conclusion, characterisation of carbon/carbonaceous materials structure has been and still is an intense field of study. A variety of analytical tools
(i.e. XRD, IR, Raman, XPS) are routinely exploited for such a task, with 13C
ssNMR having been demonstrated in the past years to play a major role, as
well, even if 13C low abundance, low sensitivity and long relaxations times can
be a significant problem. Important theoretical and technical developments
of this technique in the past 50 years have made the study of this nucleus easily accessible. In particular, the use of CP using protons as a source of magnetisation has largely contributed to study complex carbonaceous materials
without isotopic enrichment. As mentioned in this section, this technique
has been largely exploited in the study of carbons but more advanced pulse
schemes are enabling an increasingly refined description of carbon-atom
connectivity, something that is impossible to do at the moment with other
analytical techniques.
10.8
Linear
Solvation Energy Relationship Analysis
19
Using F MAS NMR Spectroscopic Probes
The majority of the physical methods (excluding ssNMR) applied for carbon
material (e.g. ACs) analysis provides information regarding either surface
chemical composition or physical structure of pore network within carbons.
However, a simultaneous effect of surface geometry and functionality plays
a vital role in many applications determined by the surface-adsorbate interaction (e.g. adsorption, water purification, catalysis).110 To explain and predict the peculiarities of behaviour of different carbon surfaces, a method that
takes into account all aspects of subtract-surface interactions is essential.
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Linear solvation energy relationships (LSER) can help to characterise the
solid surfaces more comprehensively. At the moment, the LSER method is
commonly used to describe properties of a solvent by taking into account
both its specific and nonspecific characteristics. The former includes properties such as hydrogen-bond accepting ( ∑ β 2H ) and donating ( ∑α 2H ) ability
whereas the latter characterises interactions such as dipolarity/polarisability
(π*). Abraham et al. have described an analogous expression, which can be
applied to characterise any given solid surface by probing it with absorbed
molecules.29 The fundamental approach investigates the free-energy-based
properties of the absorption process and describes the surface properties
in terms of five terms [ ∑α 2H , ∑ β 2H , π 2H, R, LogL] according to the following
equation:29
XYZ
= ( XYZ )0 + rR + sπ 2H + a ∑α 2H + b∑ β 2H + lLogL16
(10.1)
where, XYZ is the free-energy-based property, R2 is an excess molar refraction
term that reflects the ability of adsorbate to interact with adsorbent through π
and σ electron pairs, and L16 is the gas–liquid partition coefficient of the probe
molecule dissolved in n-hexadecane. L16 includes both an endoergic cavity
term and exoergic adsorbent-adsorbate general dispersion interaction. The
parameters r, s, a, b, l are linked to the chemical structure of the accessible
sites of the surface and the represent the ability of the adsorbent to interact
with probe molecules. Perturbations in UV-absorption frequency of solvatochromic dyes have been used to construct the LSER equation for siliceous
materials. However, nontransparency to visible, UV or IR radiation makes this
method inapplicable to carbon materials. It has been reported recently that
inverse gas chromatography (IGC) can be used to construct LSER equations
for different types of commercial coals and soots.111,112 Application of IGC is,
however, limited. At low temperatures there is only a narrow choice of probes,
which are gaseous. On the other hand, at high temperatures, the adsorption
of a probe molecule on an inhomogeneous surface occurs in the same way
as on a homogeneous one and the information about the specific surface is
lost.113 Moreover, classical IGC systems are not in true equilibrium during the
retention period and need extrapolation to infinite dilution of probe and zero
flow rate of the carrier gas to approximate true equilibrium parameters. Adsorbate–adsorbent interaction energy can only be calculated at ideal equilibrium
on a uniform surface in the absence of adsorbate–adsorbate interactions.114
As a consequence, these GC methods may be applicable for the characterisation of solid materials as adsorbents and stationary phases for chromatography, but they are not applicable for complete characterisation of the surface
necessary for catalytic applications that require information about the energy
and distribution of surface sites. Recently, a new method of constructing an
LSER equation has been proposed to study the energetic characteristics of
carbon surfaces at different temperatures in order to evaluate the distribution
of the sites with varying energy, by using a broad range of probe molecules
with different bonding characteristics via 19F MAS NMR spectroscopy.
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10.8.1
Validation
of 19F MAS NMR Spectroscopic
Probing Method
Of these spectroscopic techniques 19F NMR spectroscopy is ideally suited in
the context of probing-method development, given its high resolution and
sensitivity (concentrations of probe molecules as low as tens of µmol g−1
of studied solid), which allows the probing of very minute variations in
surface properties. A wide range of potential organofluorine probe molecules are readily available or easily synthesised and can be used for measurements.115,116 A 19F-NMR approach has been used by Fry et al. to quantify
hydroxyl groups on silica gel and fibres by reaction with a trifluoromethyl-containing silane, a method that was able to discriminate between surface and inpore sites.117 Changes in 19F and 13C NMR spectra have also been
reported when benzenoid compounds are adsorbed on surfaces using a
solid–solid ball-milling technique.118 In 2004 this approach was successfully
demonstrated by adsorbing trifluoroacetic acid on a range of carbons (and
other) surfaces.119 Trifluoroacetic acid (TFA), a small fluorine-containing
molecule, has been investigated as a probe for obtaining information about
surface properties via solid-state NMR. TFA is an appropriate probe due to its
high fluorine content, suitable boiling point and low volatility. In the establishment of this new probing method, several materials commonly used in
catalysis were selected: trimethylsilanised silica, two silicas, two aluminas,
titania and three carbon-type materials. The position of the chemical shift of
TFA (δobs) when adsorbed on these solids was found to be extremely sensitive,
not only to the nature of the surface but also to the surface concentration of
the probe itself, and therefore is potentially very useful for the characterisation of surfaces.
It was shown that the value of δobs corresponds to low surface concentration
and is characteristic of the specific sample. In contrast, at high concentrations (≥5 µmol m−2), δobs becomes sharper and equal for each material. It was
proposed that under these conditions TFA possesses liquid-like behaviour
(δobs = δliq). In this case the chemical shift (Δδ = δobs − δliq) can be used for
characterisation of the energy interaction of TFA and the sample surface. In
order to validate this methodology further, these findings were compared
with existing measures of surface properties, via a graphical comparison of
Δδ against the normalised energy of the π – π* transition (ETN) for Reichardt’s dye, known to provide a useful estimate of surface polarity,120 and also
against the Dubinin–Radushkevitch measure of surface energy (EDR), calculated from N2 adsorption methods (Figure 10.17). It can been seen that a
characteristic change in Δδ correlates with other estimates of surface energy
and therefore can be used as a method of estimation of energy interaction
of specific surface with a specific molecule. Furthermore it has been found
that the 19F MAS NMR spectroscopic probe molecules method can be used
for direct measurement of sample specific surface area (BET) based on the
dependency of normalised the chemical shift ΔδN (ΔδN = Δδ/Δδmax) from the
average distance between adsorbed molecules D (Figure 10.18).
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Bulk and Surface Analysis of Carbonaceous Materials
341
Figure 10.17
Correlation
of Δδmax with ETN and EDR measurements. Reproduced
with permission from ref. 119.
Figure 10.18
Plot
of variation in ΔδN with mean intermolecular distance for all
materials. F represents a fluorinated probe molecule. Reproduced
with permission from ref. 119.
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342
Three distinct domains are apparent:
i. A region of isolated molecules (low concentration region) in which the
chemical shift (ΔδN) remains unchanged with distance and is charac­
teristic of each surface studied.
ii. A region in which the chemical shift changes rapidly with distance. The
probe molecules can now “feel” one another, and this effect increases
with decreasing D.
iii. A high concentration region in which the chemical shift remains
unchanged with distance. This is a liquid-like or condensed domain;
further increase in concentration (i.e. decrease in D) produces no significant change in chemical shift. The chemical shift for the region is similar
for all surfaces studied so far. In fact, the molecules cannot actually get
nearer than close packing, but are now forming 3-dimensional clusters.
The distance at which the chemical shift is no longer changing (D0) should
correspond to the average distance between TFA molecules in the liquid
phase and using experimentally obtained value of D0 (3.8 Å) the surface area
of any activated carbon can be estimated.
10.8.2
Theoretical
Background of 19F MAS NMR
Probe Spectroscopy
To develop this LSER method based on 19F MAS NMR spectroscopy, the chemical shifts for a wide range of fluorinated reporter molecules with different
geometry and chemical functionality were recorded in the low-concentration
domain (i.e. region (i), Figure 10.18). The absolute chemical shift of the fluorine nucleus adsorbed on the surface (δmeasured) is determined only by the
electronic structure of probe molecule in the absence of any influence from
the surroundings (δ0) and term that is, in fact, a result of surface–probe molecule interaction. Assuming that gas-phase molecules behave in an ideal manner, δ0 may be approximated to the gas-phase chemical shift, δgas. As such,
the difference between δmeasured and gas-phase chemical shift is result of only
surface–molecule interaction and consists of electrostatic term (δelectrostatic), a
specific chemical bonding term (δchem) and (eqn (10.2)):
surface
⎤⎦ δ electrostatic + δ chem
Δδ gas
= ⎡⎣δ measured − δ gas=
(10.2)
The influence of the electrostatic field on the chemical shift of fluorine
probe molecule includes contributions from the nature of fluorine nuclear
electrostatic shielding (δshielding) and distortion of electromagnetic field of
adsorbed molecule (δpolarisation) – eqn (10.3):
surface
Δδ gas
=δ shielding + δ polarisation + δ chem
(10.3)
The polarisation of the electron density at the surface, Ps, induced by the
molecule is:
=
Ps E=
c1 μmα s
mα s
(10.4)
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Bulk and Surface Analysis of Carbonaceous Materials
343
where c1 is constant, Em is the electric field of the molecular dipole moment,
µm (permanent plus induced components) and αs is the “polarisability” of
the surface.121
The polarisability term describes the change in the distribution of surface electron density in response to the presence of the adsorbed reporters,
and could consist of a number of contributions from dispersive forces, π-interactions and others (later αs and πs are used to distinguish between the
contributions from dispersive and π-interactions). Although the magnitude
of the electric field is also dependent upon distance and orientation, it was
assumed that these are similar for any given reporter molecule, and are contained in c1. The additional polarisation of the atom within the reporter molecule, δpolarisation induced by the change in electric field from the perturbed
surface, Es, is:
δ polarisation
= E=
c2 Psα m
(10.5)
sα m
Combining eqn (10.4) and (10.5):
=
δ polarisation c=
catom μmα sα m
1 c2 μmα sα m
(10.6)
where αm is the effective polarisability of the fluorine atom within the
reporter molecule and c2 is a constant similar to c1. Combining eqn (10.3)
and (10.6):
surface
s
Δδ gas
=
δ shielding
+ catom μmα sα m + δ chem
(10.7)
For probe molecules that have no functional groups for specific chemical interactions, only the electromagnetic and electrostatic terms need be
considered:
surface
s
Δ=
δ gas
δ shielding
+ catom μmα sα m
(10.8)
It is not necessary to know αm, catom and µm for all atoms in all reporter
molecules under study. As long as the properties of a given surface are
measured relative to some other surface, a reference surface may instead
be used to calibrate the behaviour of the reporters. Subtracting the elecr
tromagnetic term for the reference surface, δ shielding , from the change in
reference
­chemical shift on adsorption to the reference surface, Δδ gas
, gives, from
eqn (10.9):
reference
r
Δδ gas
− δ shielding
=
catom μmα rα m ,
(10.9)
or:
reference
r
c=
( Δδ gas
− δ shielding
) / αr
atom μmα m
(10.10)
where αr is the polarisability of the reference surface. Δδ
is the chemical shift relative to the reference surface. Substituting eqn (10.10) into eqn
(10.8):
α
surface
s
reference
r
Δδ gas
=
+ s ( Δδ gas
− δ shielding
δ shielding
)
(10.11)
αr
reference
gas
α s ⎤ ⎡α s ⎤
⎡δ s
surface
r
reference
Δδ gas
=
⎢ shielding − δ shielding α ⎥ + ⎢ α ⎥ Δδ gas
r ⎦
⎣
⎣ r⎦
(10.12)
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surface
reference
Δδ gas
= a + b Δδ gas
,
(10.13)
where
αs ⎤
⎡δ s
⎡αs ⎤
r
a=
⎢ shielding − δ shielding α ⎥ and b =
⎢α ⎥
r ⎦
⎣
⎣ r⎦
(10.14)
surface
reference
should therefore give a straight line (eqn
A plot of Δδ gas against Δδ gas
(10.11)–(10.13)) were the gradient is the ratio of the surface polarisability
terms, αs/αr. Furthermore, the intercept (a) has to be linear dependent from
s
r
slope (b) helping to estimate δ shielding and δ shielding (eqn (10.14)). Widely available
octadecyl-functionalised chromatography silica was selected as a reference
surface in the study.122 In order to minimise the effects of unknown or unexpected specific interaction or reactions, a large number of fluorinated reporter
molecules with different functional groups were used (Figure 10.19). These
broadly fall into the categories of aromatic (1–20), aliphatic (18–30), hydrogen-bond donor (HBD, 31–34) and hydrogen-bond acceptor (HBA, 35–37).
10.8.3
Estimation of the Electromagnetic Term (δshielding)
The electromagnetic term (δshielding) can be found as the chemical shift of a
hypothetical atom (within the reporter) that experiences no specific chemical interactions, and has no susceptibility to be polarised by the surface, the
electrostatic term will become zero and so will only experience the change in
chemical shift caused by electromagnetic shielding from the surface. However, it would be very difficult to find such a probe nuclei. It was proposed
reference
surface
against Δδ gas
for a series of compounds that conthat by plotting Δδ gas
tain more than one fluorine atom on the set of investigated surfaces, it is
possible to extrapolate and identify the point for the reference surface where
the chemical shift values are independent of the reporter molecule used (Figure 10.20). It was confirmed that the obtained trends for majority of multinucleus compounds are linear and all of them intersect around one point
(Figure 10.20(B)).
One common point for all probe molecules proves that the electrostatic
term both for activated charcoal and the reference surface can be estimated
using eqn (10.14) from the linear trend between the intercept (parameter a)
and the gradient (parameter b) (Figure 10.21). It was found that in the case
of Norit, the intercept and slope are equal to −0.5 and −7.50 correspondently,
S( AC)
r
which according to eqn (10.14), yield values of δ shielding and δ shielding
equal to
−0.5 and 7.5 (Figure 10.20). It was found that the electrostatic term is very
dependent on the nature of the porous material and varies from −0.5 for Norit
to 8.4 for alumina (Figure 10.21(B) and (C)). The highest observed chemical
shifts relative to the reference surface was calculated for AC Norit (9.0), which
is 7 ppm lower than for any of the other investigated (Figure 10.21(C)).
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Bulk and Surface Analysis of Carbonaceous Materials
Figure 10.19
Fluorinated
reporter molecules. Reproduced with permission from
ref. 122.
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346
Figure 10.20
F-probe
analysis of activated charcoal Norit. (A) Chemical shifts for
fluorine nuclease of perfluorotoluene (P20) versus reference surface.
(B) Plots of chemical shifts for six different reporter molecules.
Figure 10.21
Estimation
of shielding parameters of different surfaces: (A) Norit;
(B) Silica (K100); (C) Summary of results.
10.8.4
Estimation
Adsorption of Aliphatic and Aromatic
Reporter Molecules
To estimate the influence of the probe molecule structure on the surface
reference
surface
and Δδ gas
, different classes
adsorption characteristics values of Δδ gas
(aliphatic and aromatic) of compound have to be calculated and plotted
with obtained lines fitted using a least-squares method (Figure 10.22).
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Bulk and Surface Analysis of Carbonaceous Materials
Figure 10.22
347
Plot
of ( Δδ gas against Δδ gas , bottom x-axis, and ( Δδ gas
− δr),
top x-axis, for silica-C8H17. ○ = Nuclei in aliphatic environments; ● =
Nuclei in aromatic environments (intercept = 5.6 ± 0.2 ppm; Gradient
= 0.64 ± 0.03; R = 0.962).
surface
surface
reference
reference
Examples of these plots are shown with two x-axis scales Δδ gas
(botr
silica − C8 H17
tom) and ( Δδ gas
− δ shielding ) (top), and with the usual NMR convention
of increasing nuclear shielding (i.e. decreasing chemical shift ppm value)
from left to right. For three of the studied surfaces, 19F nuclei in aromatic
environments showed different behaviour from those in aliphatic envir
ronments, and in these cases the value for δ shielding is especially helpful: if
r
reference
( Δδ gas
− δ shielding ) is used for the x-axis, lines from classes of compounds
with similar behaviour with a common intercept may be described by a single parameter, i.e. the gradient. Octyl-functionalised silica gel, end-capped
with methyl groups, should interact with adsorbed reporter molecules
through electrostatic interactions alone – there are no functional groups
silica − C H
reference
for specific chemical interactions. Plotting Δδ gas 8 17 against ( Δδ gas
−
δr) gives the expected straight line (Figure 10.22), However, the gradient
(0.64) is less than unity, indicating that the polarisability of the surface is
lower than that of the reference material, presumably due to a greater influence of the silica substructure on the surface layer of octyl chains than on
the reference octadecyl alkane groups.
AC is used extensively for the purification of solutions, liquids, and gases.
Their ability to bind strongly to organic compounds is demonstrated by
highly exothermic heats of adsorption, and high surface energies.123 For the
surface of AC Norit almost all of the 19F chemical shift measurements fall on
the predicted line, indicating that the strength of the adsorption is not the
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348
Figure 10.23
AC( Norit )
Plot
of Δδ gas
against Δδ gas
, bottom x-axis, and ( Δδ gas
− δr),
top x-axis, for Norit activated charcoal. ○ = Nuclei in aliphatic environments; ● = Nuclei in aromatic environments (intercept = −0.4 ±
0.2 ppm; gradient = 0.92 ± 0.08; R = 0.832).
reference
reference
result of specific binding interactions, and the charcoal has no preference
for any particular class of molecule (Figure 10.23). Not all data points could
be collected for compounds adsorbed on this AC due to a line-broadening
effect of the diamagnetic susceptibility and/or paramagnetic defects in the
charcoal structure, which reduced the signal-to-noise ratio below the limits
of detection. Whilst the gradient is close to unity, a value for δs of −0.4 ± 0.2
(was calculated, which is shifted by > 7 ppm to lower frequency than any of
the other surfaces; Figure 10.21(C)).
Large chemical shifts have been reported previously for compounds
adsorbed on ACs, and are not restricted to 19F NMR. In addition to the findings discussed in this chapter, Wagner et al. report a ca. 5.5 ppm shift to
lower frequency in the 1H spectra of chloroform and 2-chloroethylphenyl
sulfide adsorbed onto AC, compared to the liquid phase.124 They report a
kinetic effect in which sharp peaks were replaced by broader peaks at lower
frequency than the original resonances, which they propose indicated the
slow migration of the probe molecules into the micropores of the material.
The large shift was ascribed to the high diamagnetic susceptibility of graphitic planes within the micropores of the AC. Harris et al. observed shifts
to lower frequency of 6–8 ppm for phosphates (31P) and 2H2O (2H) adsorbed
on ACs, also attributed to the shielding effects of graphene planes inside
the pores.125
In conclusion, every NMR-active molecule can act as a reporter, giving information on its surroundings. The use of many reporter molecules reduces the
effects of orientation and distance from the surfaces on the chemical shift
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Bulk and Surface Analysis of Carbonaceous Materials
349
data, thus giving a more general indication of surface properties. By choosing groups of reporter molecules that are expected to undergo specific types
of bonding, it is possible to investigate the ability of a surface to become
involved in specific interactions with π-orbitals and H-bonding functions.
The change in chemical shift of a fluorine atom within a molecule and its
adsorption to a surface from the gas phase may be described by the empirical
relationship:
surface
surface
Δδ gas
= δ shielding
+
(α s + π s )
αr
( Δδ
reference
gas
references
− δ shielding
) + δ HBA + δ HDB (10.15)
where δs is the change in chemical shift induced by the electric and magnetic
fields of the surface, αs/αr is the relative surface polarisability, πs/αr is an additional contribution to the surface polarisability due to its ability to interact
with aromatic molecules, and δ HBA and δ HBD are measurements of the surface
hydrogen-bonding ability.
10.9
Conclusion
It is important to note that the full quantitative description and characterisation of the surface and bulk characterisation of complex materials such as
those discussed in this chapter represents a significant analytical challenge.
Only via appropriate combinations of different techniques, including TGA
supplemented by TG-IR, XPS, Boehm titration and bromination supported
by various ssNMR methodologies, will increasingly accurate descriptions of
carbon and carbonaceous material chemistry be possible. From the work
presented it can be seen that classical analysis techniques discussed during
the first half of the chapter (i.e. for the characterisation of ACs) still have a
place in the analysis of new “functional” carbonaceous materials and can
yield a plethora of information regarding both bulk and surface chemistry
properties: C=C bond character and chemical functionality (in particular,
those containing oxygen).
The latter part of this chapter was dedicated to the area of ssNMR, which
has proved to be a highly valuable tool in their structural characterisation
of carbon and carbonaceous materials with MAS, strong heteronuclear and
homonuclear decoupling, and CP operational modes becoming increasingly
routine and important for the efficient and appropriate resolution of ssNMR
spectra. Finally, the development of a new linear solvation energy relationships analysis based on 19F MAS NMR spectroscopic probes was introduced
and discussed in the context of surface analysis. In this section, a variety of
fluorinated probe molecules were investigated to obtain information about
their surface properties (probes were chosen based on high fluorine content,
a suitable boiling point and a low volatility). This technique has been proven
to yield otherwise difficult to assess information such as the determination
of the “polarisability” of the surface. This method may prove to be very useful
in the description of complex carbonaceous (e.g. heteroatom-doped) materials discussed in earlier chapters in this book.
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CHAPTER 11
Microscopy and Related
Techniques in the Analysis of
Porous Carbonaceous Materials
SHIORI KUBO* AND NORIKO YOSHIZAWA*
National Institute of Advanced Industrial Science and Technology, 16-1,
Onogawa, Tsukuba, Japan
*E-mail: kubo-shiori@aist.go.jp; noriko-yoshizawa@aist.go.jp
11.1
Introduction
This chapter intends to highlight the significance of transmission electron
microscopy (TEM) and introduces readers to its role in nanostructure determination of porous carbonaceous materials. As seen in the previous chapters,
many interesting carbon nanostructures are obtainable via synthesis protocols utilising sustainable precursors and sustainable carbonisation routes.
In unison with many analysis methods available (e.g., X-ray techniques, sorption techniques, etc.), microscopy serves as one of the most powerful and
direct methods allowing increasingly precise determination of such complex
carbonaceous nanostructures. Of particular importance is TEM. As a consequence of a spatial resolution of < 0.1 nm achievable by recent microscopes,
highly detailed nanostructural analyses of porous carbonaceous materials
are possible via TEM and many novel carbon nanoarchitectures including
both solid matrix and voids (e.g., pores) in the wide size regime of 10−1 to
103 nm have been unravelled. The use of the TEM technique not only helps
acquire a basic understanding of nanostructural parameters of synthesised
carbon nanostructures, but also allows one to unravel structure–function
RSC Green Chemistry No. 32
Porous Carbon Materials from Sustainable Precursors
Edited by Robin J White
© The Royal Society of Chemistry 2015
Published by the Royal Society of Chemistry, www.rsc.org
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356
relationships when those carbonaceous materials are applied in technologically important fields of energy and environmental applications. Indeed,
there are an increasing number of examples in which scientists attempt
to link the nanostructural information of porous carbonaceous materials
obtained by TEM with observed electrochemical/catalytic/adsorption/separation performance. Thus, in tandem with an increasing accessibility to TEM
techniques, this form of microscopy is playing an increasingly indispensable
role in the development of modern carbon nanotechnology. At the same time
for allowing precise structural determinations, special care must be taken
depending on the types of matrix of interest. Thus, a successful TEM analysis
also entails selection and optimisation both of observation technique and of
related peripheral techniques such as sample-preparation.
In this chapter, a tutorial overview of the TEM technique is introduced. The
principles of associated TEM techniques including imaging principles and
formation of contrast are described, whilst the special care required in the
analysis of porous carbonaceous materials synthesised from sustainable precursors is also addressed. Examples of TEM studies of carbon nanostructures
are also presented to demonstrate the high effectiveness of this analytical
technique in unravelling highly complex carbon nanoarchitectures. In this
context, porous carbons synthesised by a combined nanocasting/hydrothermal carbonisation method and porous carbon – metal composites are utilised
as demonstrative examples. As for practical observation techniques, the ultramicrotome technique and newly emerging electron tomography techniques
are specifically highlighted, demonstrating that selection and optimisation
of the technique employed leads to nanostructure determination with high
precision and with a large amount of structural information obtainable.
11.2
11.2.1
Tutorial Overview of a TEM Technique
TEM as a Visualisation Tool
11.2.1.1 Imaging Principles of TEM
In a conventional TEM setup, parallel electron beams illuminate a wide area of
the sample at the same time, allowing a number of imaging modes selectable by
changing a combination of electromagnetic lenses and apertures. Figure 11.1
shows a schematic diagram of representative TEM imaging techniques in relation to the position of the electron beam, objective aperture, and selected-area
aperture. Real-space images are obtained using setup (B), (C) and (D), while a
reciprocal image is obtained with set-up (A). As for mechanical and optic basics,
these are discussed in more detail elsewhere.1 Bright-field (BF) and dark-field
(DF) images are obtained in a complementary manner. That is, electron beam
penetrating a specimen without any interactions with the specimen is solely
used in BF imaging, while the diffraction beam selected with the objective aperture in the selected-area electron diffraction (SAED) image is used in DF imaging. Since BF imaging eliminates scattered electrons and uses only transmitted
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Microscopy and Related Techniques in the Analysis of Porous Carbonaceous Materials 357
Figure 11.1
Schematic
diagram of representative TEM imaging techniques in relation to the electron beam, the objective aperture, and the selected-­area
aperture. (A) selected-area diffraction image (SAED) (B) bright-field
image (BF) (C) dark-field image (DF) (D) lattice image.
electrons, the “scattering contrast” effect is dominant in BF images. As electrons
have the nature of being scattered in areas with larger thickness and composed
of atoms with higher atomic number, BF imaging is useful to examine structure
with the distribution of thickness, such as porous texture, and elemental analysis of the specimen. DF images are beneficial to selectively visualise structures
contributing to the generation of specific diffraction spots.
Lattice images are made by interference between the transmitted beam
and diffracted beam. Briefly, a series of lines or fringes corresponding to the
crystallographic lattice planes are produced in lattice images. Note that lattice planes can be seen only when they satisfy a geometrical relationship,
namely the Bragg diffraction condition, with incident electron beams. The
strength of contrast in such fringe images is called “phase contrast”. Lattice
images are often confused with high-resolution (HR) TEM images. There is
in fact no clear definition of a “HRTEM image” as a technical term, but it normally refers to TEM images observed at a resolution ≥ 0.1 nm. In the meantime, lattice images show a series of crystallographic lattice planes. SAED
images show crystallographic information in the selected-area aperture
(with the area of ca. 0.1 µm φ). Illumination is performed in a parallel mode,
whilst the size of area for diffraction is determined only by the aperture used.
Diffraction spots, arcs or rings corresponding to crystallite sizes and their
orientations are produced in SAED images.
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11.2.1.2 Scanning Transmission Electron Microscopy
Another very useful imaging mode is scanning transmission electron microscopy (STEM), performed using a probe beam that scans a sample and is
detected after transmission through the sample. The spatial resolution
attainable is determined by the size of a probe beam employed (normally
of 1–5 µm in diameter). As in TEM imaging, electron diffraction, BF/DF and
lattice images are also attainable with STEM. High-angle annular dark-field
(HAADF) – STEM imaging is extensively used in nanoscale characterisation,
as contrast is theoretically proportional to the square of the atomic number
(i.e., termed Z contrast) and is not influenced by diffraction effects, which
often makes imaging distribution of a specific element easier even in a
less-crystalline structure. Furthermore, using probes in STEM mode allows
for nanodiffraction, energy-dispersive X-ray spectroscopy (EDS) nanoanalysis, and high-energy resolution for electron energy loss spectroscopy (EELS)
analysis.
11.2.2
TEM
as a Tool for Analysing Nanostructure of Porous
Carbonaceous Materials
11.2.2.1 Optimising TEM Conditions and Techniques for the
Observation of Carbonaceous Materials Synthesised
from Sustainable Precursors
As described above, in order for one to generate as precise a nanostructural
description of porous carbonaceous materials as possible, understanding
the TEM operational principle as well as the precise selection and optimisation of analysis conditions depending on the type of nanostructure under
investigation are highly important. It has been demonstrated in the previous
chapters that carbonaceous materials synthesised from sustainable precursors often possess highly complex nanoarchitectures and chemistry. Furthermore, due to the highly “flexible” and “controllable” nature of the synthesis
pathways employed (e.g. hydrothermal carbonisation), fine tuning of nanostructuration is possible at the nanometre to micrometre length scales. In
this regard, TEM serves as a powerful tool allowing structural characterisation both at low magnification (preferably in SA mode to examine several nm
to micrometre-order textures) and at high magnification to examine sub-nm
or nm-order lattice images.
From the viewpoint of carbon as a matrix, sustainable carbonaceous
materials prepared at relatively low temperatures (i.e. at which little or no
graphitic structuration has developed) are typically amorphous or noncrystalline regarding microtexture. Although additional thermal treatment at
elevated temperatures drives aromatisation/graphitisation and a graphitic
nature can be generated,2 carbonaceous microtexture with such limited crystallinity is sensitive to electron or ion beam irradiation. Also, sustainable carbonaceous materials can contain substantial amounts of hydrogen, nitrogen
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and oxygen. Materials consisting of such “light” elements easily allow the
electron beam to pass through, resulting in a weak contrast in the resulting TEM image. It is therefore necessary to enhance the image contrast for
detailed analyses. To overcome these difficulties attributed to the structural
characteristics of carbonaceous materials, it is reasonable to minimise the
accelerating voltage. The kinetic energy of an electron is proportional to the
acceleration voltage, and thus electron beam irradiation in TEM at a high
accelerating voltage can lead to structural damage of the specimen. In many
cases, observation at an acceleration voltage of ≤ 100 kV can suppress structural damage to carbonaceous materials. Concerning image contrast, this is
related to the power of the incident electron to penetrate a specimen. Electrons accelerated with lower voltage have smaller kinetic energy, which raises
the possibility of scattering at the specimen surface during TEM analysis.
The ratio of scattered electrons to penetrating electrons can be accordingly
increased, resulting in an enhancement of image contrast.
11.2.2.2 Selection of Sample Preparation Technique
Another prerequisite to achieve increasingly precise TEM characterisation
relates to the specimen-preparation technique. In particular, dispersion and
thinning of the specimen are important to avoid possible overlay of different
nanostructural information in the depth direction and offers an ease with
which increasingly precise nanostructure determination is conducted. Furthermore, electron beam diffusion into the surrounding area of the specimen can also be avoided, allowing for the efficient and selective irradiation
of the area of interest. The most typical sample-preparation techniques available are presented below:
Crushing or Pulverisation. Crushing or pulverisation can be used for any
type of brittle material. Specimens thin enough to be characterised with their
high-resolution lattice images can be prepared by this technique. Although it
is necessary for one to pay attention to the cleavage behaviour of a sample, it
can be easily tried as a first attempt of sample preparation.
Ultramicrotomy. Ultramicrotomy is used to prepare soft to relatively hard
materials. The ultramicrotoming process of carbonaceous materials (and in
general) includes embedding a sample in epoxy resin, followed by cutting
the epoxy-embedded sample with a glass knife, and precise slicing with a
diamond knife. Possible artefacts derived from morphology changes, fractures, tearings, microfissures, etc., caused by shear force from a knife should
be fully taken care of for one to avoid misunderstanding the resultant TEM
images.
Ion Thinning (Milling). Ion thinning (milling) can be used for any type of
material except biological materials and soft polymers. This method often
requires a longer preparation time for mechanical prepolishing, but it is possible for one to obtain thin specimens for high-resolution observations even
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360
from hard materials like sintered ceramics. Focused ion beam (FIB) thinning
can be widely applied including to composite materials. Most FIB apparatus comes equipped with a microscope to view the secondary electron image
during sample preparation, allowing for sample cutting at a precise position.
However, in both cases, the ion beam can induce structure damage to the
specimen from the atomic to the micrometre-scale order as a consequence of
the high-energy beam. In this regard, carbonaceous materials are likely to be
damaged, often resulting in the formation of amorphous surface layers, by
these ion-thinning techniques. The use of a plasma cleaner (e.g. using argon,
argon/oxygen, etc.) may help remove any damaged surface layers before TEM
observations.
11.2.3
Electron Tomography – “3D-TEM”
An image taken with TEM is a two-dimensional (2D) projection, and therefore it is difficult to characterise three-dimensional (3D) structures, such as
the exact shape of a particle and overlap of specific texture. Likewise, it is
not straightforward for one to analyse the degree of metal dispersion on the
carbonaceous matrix. In this context, rotation of the specimen holder can
be used to obtain 3D information in TEM. However, conventional sample
holders have only been designed to be rotated over a small angle (e.g., <10°).
Recently, the development of specimen holders capable of rotation over a
much wider range (e.g. +75° to −75°) at small increments (typically 1–2°)
has been produced enabling the development of “electron tomography” or
the “3D-TEM” technique. The strongest advantage of electron tomography
lies in the possibility to distinguish phases of interest based on contrast
differences. For example, pores, support materials, and metal phases can
be identified and separately analysed. The application of electron tomography to nanostructural characterisation of carbonaceous materials is also
important from the practical point of view. Evaluation of pore connectivity in a particle attracts attention in the study of porous carbon materials
as electron tomography is one of the few techniques capable of visualising the inner structure of a carbon particle. A demonstrative example of
electron tomography analysis of carbon nanospheres is shown in Section
11.3.2. Electron tomography in the sense of 3D characterisation is also
extremely powerful in characterising the distribution and location of catalyst nanoparticles, for example dispersed on the outer/inner surface of a
carbon support (Figure 11.6).
The basic two steps that form the basis of electron tomography are: (1)
acquisition of a series of conventional 2D-projection images of the structure
under investigation, and (2) calculation of 3D reconstruction of the sample derived from the initial 2D projections. During Step (1), each image is
recorded with a CCD camera whilst the specimen is tilted over a wide angular
range in small angular increments. Step (2) includes the position alignment
of the data, followed by the reconstruction of the image. The alignment process is often carried out by using an appropriate position marker, such as Au
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nanocolloids dispersed on the microgrid. Once the alignment is achieved, the
reconstruction is computed using a calculation algorithm of choice. Here,
much more care is taken to avoid possible beam damage since in tomography a specimen is subjected to an electron beam for an extended period
of time. One of the representative images derived from the reconstruction
process is defined as the “slice image”. The slice image is often recognised
as a virtual cross-sectional view of the object cut with any height and angle.
The resolution of the slice image is still controversial in fact as it is intricately influenced by the experimental conditions (e.g. specimen thickness or
shape). Therefore, the resolution is often estimated in many cases based on
the size of the calibration objects dispersed on the TEM grid with the sample
particles (e.g., Au or Ag nanocolloids of 2–10 nm size).
11.3
Examples
of Microscopy Analyses of Porous
Carbonaceous Materials
11.3.1
Ultramicrotome
and TEM in the Analysis of
Nanostructured Porous Carbonaceous Materials
In this section, examples of the TEM analysis of porous carbonaceous materials as well as porous carbon – metal composites are discussed. Examples
regarding ultramicrotome preparation and accompanying TEM analysis are
presented with a focus on the analysis of porous carbonaceous materials synthesised from sustainable precursors. Examples of the application of tomography in the characterisation of nanostructured carbonaceous materials are
also discussed. The suitability of this technique for the direct visualisation of
three-dimensional carbon nanostructures as well as metal distribution on a
carbon matrix is also demonstrated.
A good example of the utilisation of ultramicrotome-TEM has been reported
by Titirici et al. In this report mesoporous carbonaceous materials were synthesised via a hard-templating method, where mesoporous silica particles
(Merck Si100 with the particles size of ∼8 µm and mesopore size of 5–15 nm)
were used as an inorganic sacrificial template.3 During the hydrothermal
treatment at 180 °C, the mesopores inside the template silica particles are
infiltrated with furfural (i.e. the carbon precursor derived from the dehydration of xylose), with subsequent silica etching producing nanostructured
carbonaceous replicas. Here, the prerequisite for the successful replication
was found to be the matching of surface polarity between the silica surface
and carbon precursor (i.e., the degree of wettability/impregnation of silica
template with precursor solution). This conclusion was effectively unravelled
via ultramicrotome-TEM studies of both carbon–silica composites and carbon replicas. The hydrophobicity of the Si100 template was moderated via
methylsilylation. A moderate degree of methylsilylation resulted in only half
of the silanol groups being removed; leading to the filling of pores located
close to the particle surface. This resulted in the production of hollow carbon sphere structures after hydrothermal carbonisation and silica removal.
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Figure 11.2
Chapter 11
(A)
(B) and (C) TEM images of hollow mesoporous carbon spheres
obtained after dissolution of silica taken at different magnifications.
TEM images of (D) composite of silica with moderately hydrophilic
surface and carbon and (E) mesoporous carbon sphere obtained after
dissolution of silica from (D). Adapted and reproduced with permission from ref. 3.
Ultramicrotome-TEM analysis of the epoxy-embedded and carbon microparticles revealed the formation of a carbon shell with the thickness of 1 ∼ 2 µm,
indicating a certain degree of but still finite precursor impregnation (Figures
11.2(A)–(C)). Conversely, perfect replication of silica nanopores and spherical particle morphology is achieved when a moderately hydrophilic is produced via dehyroxylation of the surface via condensation of silanol groups to
siloxane groups. Ultramicrotome-TEM analysis of the carbonaceous product
produced in this scenario demonstrates extended degree of pore filling (Figure 11.2(D)), producing porous spheres composed of a continuous carbonaceous network upon removal of the template (Figure 11.2(E)) with SBET = 200
m2 g−1 and Vpore = 0.32 cm3 g−1. This example illustrates that observation of
that texture can be obtained to a great extent via ultramicrotome-TEM. Furthermore, the use of this technique also helps to link the observed results
with a description of the material formation (replication) process(es) (i.e., via
analysing the degree of pore filling).
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Microscopy and Related Techniques in the Analysis of Porous Carbonaceous Materials 363
Figure 11.3
(A)
TEM micrograph of ultramicrotomed ordered carbon –block copolymer composite and (B) SAXS pattern of carbon–block copolymer
composite (denoted as C-MPG1-com). Adapted and reproduced with
permission from ref. 4.
Another important application of the TEM technique coupled with ultramicrotome sample preparation is demonstrated by White et al. In this report,
the technique was employed to demonstrate the generation of an ordered
pore system in synthesised carbonaceous microparticles and elucidate nanostructural properties of the synthesised ordered porous carbons such as pore
size/symmetry, wall thickness and pore uniformity (Figures 11.3 and 11.4).4
As discussed in Chapter 5, such ordered porosity in hydrothermal carbon
is obtained via the hydrothermal carbonisation of a D-fructose at 130 °C
in the presence of an amphiphilic triblock copolymer (i.e. Pluronic® F127,
EO107PO60EO107) template to yield a precipitate of a template–carbon composite (denoted as C-MPG1-com). The low HTC temperature (i.e., as compared to
that for other hexoses) is key for successful templating as it allows for stable
formation of block copolymer ordered phase. Hydrogen-bonding interactions orientate the growing carbonaceous moiety, whilst pore structuring is
controlled by polymeric template. TEM observation of the epoxy-embedded
and ultramicrotomed microparticles reveals the ordered nanostructural textures composed of the polymer template and hydrothermal carbon generated
via the formation of polymer lyotropic phase during hydrothermal treatment
(Figure 11.3(A)). The diameter of an F127 micelle and the carbon wall thickness were measured to be ca. 10 and 6 nm, respectively (i.e., with a poreto-pore distance of ca. 16 nm). This is found to be in good agreement with
the result obtained by a small-angle X-ray scattering (SAXS, Figure 11.3(B)),
which demonstrates a well-resolved pattern with a d-spacing value of the first
peak equivalent to 16.8 nm corresponding to a unit-cell parameter of 23.6
nm for a Im3m cubic pore symmetry.
Here, it is important to state that such detailed TEM observations of the
inner texture of ordered carbon microparticles requires fine optimisation of
specimen preparation (e.g. see Section 11.2). Too thick a specimen sample
(A)
SEM, (B) (C) TEM and (D) SAXS pattern of ordered porous carbon obtained after removal of block copolymer template
(denoted as C-MPG1-micro). (E) SEM, (F) (G) TEM, and (H) SAXS pattern of ordered porous carbon synthesised with the addition of TMB (denoted as C-MPG1-meso). Adapted and reproduced with permission from ref. 4.
364
Figure 11.4
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Microscopy and Related Techniques in the Analysis of Porous Carbonaceous Materials 365
can lead to either loss of information due to a decrease in electron transmittance or to misinterpretation of pore size/shape/connectivity due to possible
overlay of several, different ordered domains residing in the depth direction.
At the same time, too thin a specimen can lead to a decrease in attainable
contrast between the carbonaceous and polymer template moieties. This
may also lead to a misinterpretation of the formed ordered nanostructures.
Therefore, optimisation of the sample preparation in tandem with that of
TEM analysis conditions is critical for the accurate TEM characterisation
of inner porous domains, particularly in the case of such porous carbonaceous materials. Among the various slicing techniques introduced in Section
11.2.2, ultramicrotome is considered the most suitable for these porous carbonaceous materials, as the mechanical slicing generates less damage to carbon matrix, as compared to e.g., ion milling.
Thermal treatment (under N2) at 550 °C removed the template to yield aromatic, functional carbonaceous microparticles with a cuboctahedron-like
external morphology (Figure 11.4(A), denoted as C-MPG1-micro). TEM analysis reveals the presence of the long-range regularly ordered pore structure
(Figure 11.4(B) and (C)). SSAXS analysis of C-MPG1-micro indicates maintenance of the near-perfect cubic Im3m symmetry from the parent composite
(Figure 11.4(D)), whilst TEM indicates the generation of a very thick pore wall
features (i.e., of ∼ 7–10 nm). C-MPG1-micro exhibits microporous features in
N2 sorption with SBET = 257 m2 g−1 and Vpore = 0.14 cm3 g−1 and with a pore-size
distribution centred at ∼0.9 nm. The resulting pores are found to be smaller
than before template removal, believed to be in part due to structural shrinkage (e.g., network condensation) or partial cocarbonisation of the block
copolymer. Addition of a pore-swelling agent (i.e., trimethylbenzene (TMB))
to the synthesis resulted in a shift in the pore size of C-MPG1-micro to the
mesopore region, as TMB acts to swell the spatial volume of the hydrophobic regions of the block copolymer template, leading to materials denoted
as C-MPG1-meso, without altering the particle macro-morphology (Figure
11.4(E)). Examination of pore structuring in C-MPG1-meso via TEM images
indicated maintenance of the well-ordered pore structuring upon addition
of the pore-swelling agent, with a pore diameter and wall thickness of ∼5 nm
and ∼10 nm, respectively (Figure 11.4(F) and (G)). SSAXS patterns indicated
a Im3m symmetry with a first peak equivalent to ∼18.9 nm demonstrating
an increased structural dimension (Figure 11.4(H)). Complementarily, in N2
sorption analysis, a bimodal pore-size distribution is generated with a new
mesopore diameter maximum at ∼4.0 nm and a discrete micropore peak at
1.0 nm. In this context, the detailed TEM analyses coupled with SSAXS and
gas sorption measurements play a great role in unravelling the ordered carbon nanostructures synthesised by a novel HTC–soft-templating protocol.
Kubo et al. have further extended this block copolymer templating by combining it with latex nanoparticle templating (i.e., a dual-templating approach)
to produce hierarchically porous carbonaceous materials with tuneable porosities and with an inverse opal-type nanostructures.5 TEM analyses of both
normal (i.e., ground and nonultramicrotomed) and ultramicrotomed samples
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366
Figure 11.5
Chapter 11
(A)
(B) SEM and (C) (D) TEM images of the coral-like carbon nanoarchitectures. Left column corresponds to the materials synthesised at 130 °C
and right at 180 °C. Insert: TEM image of ultramicrotomed coral-like
carbon nanoarchitecture synthesised at 130 °C. Adapted and reproduced with permission from ref. 5.
revealed the pore-wall textures residing in the synthesised, highly complex,
coral-like carbon nanoarchitectures. Synthesis was carried out at hydrothermal temperatures of 130 °C and 180 °C respectively, using d-fructose as a carbon precursor, whilst polystyrene latex nanoparticles (PSL) and F127 block
copolymer were used as templates. The finer synthetic details are discussed in
Chapter 5. Briefly, the block copolymer acts to destabilise the PSL dispersion
during the hydrothermal process resulting in a close packing of the nanoparticles. The self-assembly of the block copolymer template allows the generation
of an ordered porous texture in the forming carbon walls, whilst phase-separation kinetics during synthesis introduces a 3D continuous macroporous
carbon monolith structure. These effects are controllable via the variation in
hydrothermal temperature (i.e., between 130 and 180 °C), leading to nanostructure tuneability at multiple length scales.
After a dual-template removal at 550 °C, the monolithic structure is maintained, with SEM analysis revealing the formation of coral-like structures
with a hierarchical nanoarchitecture composed of large 3D continuous carbonaceous branches, in which the inverse opal-type nanostructure is embedded (Figure 11.5(A) and (B)). The overall skeleton thickness is found to be
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Microscopy and Related Techniques in the Analysis of Porous Carbonaceous Materials 367
thinner for the material synthesised at 130 °C (0.5–1 µm) as compared to that
at 180 °C (2–4 µm), proving that the temperature variation contributes in controlling micrometer-scale morphology (i.e., spinodal-type phase separation).
TEM analysis revealed the presence of an ordered pore system in the walls
of this hierarchical porosity particularly for the material synthesised at 130
°C (Figure 11.5(C)). This material feature was more finely observed in the
ultramicrotomed sample, demonstrating the presence of micro/small mesopores. (Figure 11.5(C), insert). In contrast, no ordered pore system is observed
for the material synthesised at 180 °C (Figure 11.5(D)). The lack of an ordered
mesophase at the PSL template surface during synthesis at 180 °C, resulted
in a denser close packing of the PSL nanoparticles, and the formation of a
thinner carbon wall (ca. 5–10 nm) as compared that at 130 °C (ca. 20–30 nm).
In this report, the use of ultramicrotome-TEM coupled with additional TEM
analysis was shown to be extremely useful in describing the fine nanostructure residing inside these highly complex carbon nanoarchitectures.
Similarly, however, based on a resol/triblock copolymer (i.e., poly(isoprene)-block-poly(styrene)-block-poly(ethylene oxide)) synthesis, Wiesner et
al. have synthesised gyroidal ordered porous carbons and the application of
ultramicrotome-TEM enabled observation of the formed ordered pore systems.6 In this report, an additional staining step was applied to the ultramicrotomed specimens of as-synthesised gyroidal, ordered organic–organic
hybrid composed block copolymer and resol-derived polymer in order to
gain an enhanced contrast between the two polymeric matrixes. The selective reactive staining of the poly(isoprene) block using OsO4 allowed the
precise determination of the template location – an important parameter to
allow tuning of pore size, shape and interconnectedness.
The advantages of the ultramicrotome-TEM technique are of course not
limited to the characterisation of complex carbon nanoarchitectures. The
analysis approach can also be applied to the examination of metal nanoparticle-loaded (ordered mesoporous) carbon materials (e.g. bimetal (PtRu)
nanoparticles/carbons),7 whilst also being utilised for observations of carbon fibre cross sections.8,9 TEM analysis can also be viewed as critical in the
description of fuel-cell catalysts. Daza et al. have used ultramicrotome-TEM
analysis to examine Pt–carbon composite materials, allowing the observation of the Pt NP distribution supported on a commercial carbon black, as
applied as a cathode in proton-exchange membrane fuel cells.10
11.3.2
TEM
Tomography in the Analysis of Nanostructured
Carbonaceous Materials
When employing ultramicrotome-TEM, in order to avoid possible oversight
and to obtain averaged structural/textural information throughout the sample being analysed, serial cross sectioning is often necessary. However, this
is not only time consuming, but also requires extensive sample preparation
(e.g. multiple sample slicing steps) and following observation in a consecutive manner in the depth direction. As a consequence, this technique is
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Chapter 11
thus considered somewhat not universal and operator dependent. In this
regard, a possible misinterpretation of carbon nanostructure may occur
when observing cross-sectioned specimen. Terasaki et al. point out that,
upon the observation of ordered porous carbons synthesised via an evaporation-induced self-assembly process from a phenol–formaldehyde precursor,
a cross-sectional plane hardly passes through the centres of the pores.11 It is
further mentioned that redeposition of materials during the cross sectioning
to the pore edges can also occur, leading to possible misinterpretation of
pore structures. In this context, as introduced in the previous subchapter, the
technique of TEM tomography, an emerging microscopic technique, offers
an opportunity to achieve (near) complete 3D visualisation of a specimen in a
relatively simple and straightforward manner. Possible misinterpretations as
described above can therefore be avoided. Currently, this exciting technique
is extensively used for three-dimensionally visualising locations of loaded
metal nanoparticles on various porous supports. Visualisation of homogenously incorporated CuZn bimetallic nanoparticles in SBA-15 mesoporous
silica coupled with the corresponding quantitative analysis of CuZn dispersity,12 as well as of Pt distributed on zeolite Y catalyst,13 have been reported.
The technique has also found use in describing the degree of mesopore formation in zeolite Y catalyst (i.e., formed via a combination of acid and base
leaching).14
With regard to the tomographic analysis of carbonaceous materials, this
technique has been extensively utilised particularly for determination of
metal-catalyst locations incorporated in carbon matrix, such as Pt on carbon
nanofibres,15,16 mesoporous carbon,17 carbon black,18,19 commercial Pt-incorporated membrane electrode assembly,20 Cu/carbon fibre composite,21 Pd/
CMK-3 carbon,22 and multiwalled carbon nanotubes (CNT).23–25 These analyses are conducted majorly in light of the use of the materials in catalysis or
fuel-cell technologies.
Among these reports, the characterisation of catalyst nanoparticle–CNT
composites has been extensively studied. It is speculated that the catalytic
activities are influenced by the curvature of the nanotube (i.e., inner or outer
surface). Thus, it is important to develop a technique that allows for selective deposition of metals onto inner/outer surfaces of CNT. Tessonnier et al.
demonstrate the selective deposition of Ni nanoparticles onto either inner
or outer surfaces of CNT, which is elegantly revealed by TEM tomography
(Figure 11.6).24 A two-step, incipient wetness impregnation method was performed in solvents of differing polarity/tension to control the location of the
metal nanoparticles (i.e., ethanol vs. water; Please refer to ref. 24 for further
details). Whilst determination of Ni particle location was found to be difficult
from 2D-TEM images (Figure 11.6(A)), quantitative determination of the metal-nanoparticle distribution on the CNT was successfully obtained based on
3D reconstructed images from TEM tomography, demonstrating the selective deposition of near 75% of Ni particles at the CNT interior via a method
called “metal inside” approach and 85% of Ni particles at the CNT exterior via
a method called “metal outside” approach, respectively (Figure 11.6(B)–(E)).
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Figure 11.6
(A)
2D TEM image of the sample “metal outside”. Longitudinal section
through reconstructed volume of (B) “metal inside” and (C) “metal
outside”, respectively. (D) and (E) Modelling of the reconstructed volumes of “metal inside”; (pink) carbon nanotube, (red) Ni nanoparticles inside the tube and (blue) Ni nanoparticles on the external
surface. Adapted and reproduced with permission from ref. 24.
This work elegantly demonstrates the effectiveness of TEM tomography for
the description of 3D distributions of metal nanoparticles supported on carbon materials. Similar to the work of Tessonnier et al., Gontard and Ozkaya
et al. used HAADF-STEM tomography to study the distribution of Pt and PtCr
particles supported on carbon black, which are used as heterogeneous catalysts in proton-exchange membrane fuel cells in the reported work.18 The
optimal detector angle was obtained experimentally, allowing acquisition
of signals with favourable intensities both from the loaded metal and carbon support. Through characterisation in this manner, the loaded metal was
found to be located in the majority at the carbon black support surface.
Application of TEM tomography is not limited to the characterisation
of nanoparticle dispersion on a carbon matrix. It has also been proven
to be a powerful tool in the analysis of highly complex 3D carbonaceous
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Figure 11.7
Slice
images computed from the original tilt series of the carbon nanospheres and their 3D model with imaginary plane at different height.
Adapted and reproduced with permission from ref. 27.
nanoarchitectures. As discussed in Chapter 5, White et al. demonstrated
the synthesis of nitrogen-doped monolithic carbon aerogels (i.e., carbogels)
based on the hydrothermal carbonisation of glucose and ovalbumin.26 The
product in this synthesis was a 3D hyperbranched carbon nanoarchitecture,
formed as a result of a spinodal-type phase separation as revealed by TEM
tomography analysis (please see supplementary information of ref. 26 for a
rather beautiful tomographic movie of the reported carbogels).
TEM tomography can further be performed at much higher magnification
allowing observation of carbon microstructures (e.g., grain size/boundaries,
structural defects and arrangement of nanoparticles,27 or pore size/shape/
connectivity and wall thickness11) developed in carbonaceous particles at
from several tens to hundreds of nm length scales. Yoshizawa et al. investigated the microstructure of carbon nanospheres of 100–200 nm in diameter used as negative electrodes in Li-ion batteries.27 Reconstruction of TEM
tomography images and subsequent image examination reveals the spatial
arrangement of the aggregating carbon nanospheres (Figure 11.7). It is further illustrated that the centre core of each nanosphere is surrounded by
nested shells which are finally connected near the outside graphitic texture.
Combining this observation with HR-TEM analysis and crystallographic data
indicated the presence of some structural defects concentrated along the ridgelines of the external graphitic component. The authors speculate that the
defects serve as the paths of ion migration into the nanosphere interior in an
electrochemical evaluation of Li-ion battery performance, with such channels being beneficial for superior high-speed charge–discharge behaviour.
11.4 Conclusion
This chapter provides a brief introduction to the underlying principles of TEM
and this technique’s application in the analyses of nanostructured porous
carbonaceous materials. Several important examples of TEM analyses of
porous carbonaceous materials were presented, with a specific highlight on
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Microscopy and Related Techniques in the Analysis of Porous Carbonaceous Materials 371
ultramicrotome-TEM and TEM tomography. Observation examples of not only
pure carbon nanoarchitectures but also nanostructured carbon–metal composites were also introduced with the accurate TEM characterisation of such
materials being of critical importance from an application viewpoint (e.g. in
fuel-cell catalyst design and optimisation). Selection and optimisation of both
TEM operational mode and specimen preparation were shown to be crucial in
achieving the precise determination of nanostructural parameters (e.g., pore
size, pore symmetry, wall thickness/textures, pore interconnectedness, hierarchical architectures) of porous carbonaceous materials. It is also demonstrated that TEM not only allows for nanostructure determination of resulting
carbonaceous materials but also aids insight into the formation processes of
those nanostructures (e.g., via elucidating the degree of carbon precursor wetting). This will, in turn, offer an opportunity to further extend a respective synthetic approach towards increasingly fine tuning of carbon nanostructures.
As for tomography, applications of this technique in the characterisation
of porous carbonaceous materials produced from sustainable precursors
and via sustainable routes are currently relatively sparse. However, in light
of a variety of interesting nanoarchitectures in this rapidly developing materials field, in part due to the flexible characteristics of sustainable carbonisation protocols as well as emerging applications in separation, catalysis, or
electrochemistry, as described in the earlier chapters, TEM tomography will
become an increasing key analytical tool in nanostructural characterisation
of such materials. Apart from tomography, the employment of other operational protocols, as introduced in the first section (e.g., TEM-EELS, TEM-EDS,
SAED), will further bring about an extended degree of variation in TEM characterisation methods in the context of carbonaceous materials.
Looking to the future, the development of TEM techniques for analyses
of porous carbonaceous materials will include improvement in resolution
(mechanical improvement allowing for series of imaging at increasingly
small rotation intervals) or in accuracy/reliability (development of new calculation algorithm for increasingly accurate reconstruction) in tomography,
or further variation in observation mode (e.g., an in situ TEM observation
in a liquid environment).28–30 In this context, future TEM techniques will
offer an opportunity for near-complete visualisation of the “true” nanostructure of porous carbonaceous materials, whilst further details in formation
mechanism of carbon nanostructures will become accessible, allowing for
highly detailed synthetic control and ultimately an access to tailor-made and
increasingly fine carbon nanostructures.
References
1.D. B. Williams and C. B. Carter, Transmission Electron Microscopy: A textbook for Material Science, Springer, New York, 2nd edn., 2009.
2.L. Yu, C. Falco, J. Weber, R. J. White, J. Y. Howe and M.-M. Titirici, Langmuir, 2012, 28, 12373–12383.
3.M.-M. Titirici, A. Thomas and M. Antonietti, Adv. Funct. Mater., 2007, 17,
1010–1018.
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4.S. Kubo, R. J. White, M. Antonietti and M.-M. Titirici, Chem. Mater., 2011,
23, 4882–4885.
5.S. Kubo, R. J. White, K. Tauer and M.-M. Titirici, Chem. Mater., 2013, 25,
4781–4790.
6.J. G. Werner, T. N. Hoheisel and U. Wiesner, ACS Nano, 2014, 8, 731–743.
7.F. Li, K.-Y. Chan, H. Yung, C. Yang and S. W. Ting, Phys. Chem. Chem.
Phys., 2013, 15, 13570–13577.
8.H. Ogihara, S. Takenaka, I. Yamanaka, E. Tanabe, A. Genseki, J. Koki and
K. Otsuka, Chem. Lett., 2008, 37, 868–869.
9.J. D. Gitz Gerald, G. M. Pennock, and G. H. Taylor, Carbon, 29, , 2, 139–164.
10.P. Ferreira-Aparicio, B. Callardo-Lopez, A. M. Chaparro and L. Daza,
J. Power Sources, 2011, 196, 4242–4250.
11.M. Klingstedt, K. Miyasaka, K. Kimura, D. Gu, Y. Wan, D. Zhao and O.
Terasaki, J. Mater. Chem., 2011, 21, 13664–13671.
12.G. Prieto, J. Zecevic, H. Friedrich, K. P. de Jong and P. E. de Jongh, Nat.
Mater., 2013, 12, 34–39.
13.J. Zecevic, A. M. van der Eerden, H. Friedrich, P. E. de Jongh and K. P. de
Jong, ACS Nano, 2013, 7, 3698–3705.
14.K. P. de Jong, J. Zecevic, H. Friedrich, P. E. de Jongh, M. Bulut, S. van
Donk, R. Kenmogne, A. Finiels, V. Hulea and F. Fajula, Angew. Chem., Int.
Ed., 2010, 49, 10074–10078.
15.C. Zhou, Z. Liu, X. Du, D. R. G. Mitchell, Y.-W. Mai, Y. Yan and S. Ringer,
Nanoscale Res. Lett., 2012, 7, 165.
16.F. Winter, G. L. Bezemer, C. van der Spek, J. D. Meeldijk, A. J. van Dillen,
J. W. Geus and K. P. de Jong, Carbon, 2005, 43, 327–332.
17.K. Wikander, A. B. Hungria, P. A. Midgley, A. E. C. Palmqvist, K. Holmberg and J. M. Thomas, J. Colloid Interface Sci., 2007, 305, 204–208.
18.L. C. Gonatard, R. E. Dunin-Borkowski and D. Ozkaya, J. Microsc., 2008,
232, 248–259.
19.L. C. Gontard, R. E. Dunin-Borkowski, R. K. K. Chong, D. Ozkaya and P. A.
Midgley, J. Phys. Conf. Ser., 2006, 26, 203–206.
20.S. Theile, T. Fuerstenhaupt, D. Banham, T. Hutzenlaub, V. Birss, C. Ziegler
and R. Zengerle, J. Power Sources, 2013, 228, 185–192.
21.A. Shaikjee, P. J. Franklyn and N. J. Coville, Carbon, 2011, 49, 2950–
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22.B. Karimi, H. Behzadnia, M. Bostina and H. Vali, Chem. - Eur. J., 2012, 18,
8634–8640.
23.O. Ersen, J. Werckmann, M. Houlle, M.-J. Ledoux and C. Pham-Huu,
Nano Lett., 2007, 7, 1898–1907.
24.J-P. Tessonnier, O. Ersen, G. Weinberg, C. Pham-Huu, D. S. Su and R.
Schoegel, ACS Nano, 2009, 8, 2081–2089.
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Carbon, 2006, 44, 2558–2564.
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Lee, A. Zettl and A. P. Alivisatos, Science, 2012, 336, 61–64.
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Alivisatos, Nano Lett., 2013, 13, 4556–4561.
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PART 4
COMMERCIALISATION
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CHAPTER 12
Other Approaches and
the Commercialisation of
Sustainable Carbonaceous
Material Technology
ROBIN J. WHITE*a, VITALIY L. BUDARINb AND PETER S.
SHUTTLEWORTHc
a
Universität Freiburg, FMF - Freiburger Materialforschungszentrum,
Stefan-Meier-Straße 21, 79104 Freiburg im Breisgau and Institut für
Anorganische und Analytische Chemie, Albertstrasse 21, 79104 Freiburg,
Germany; bGreen Chemistry Centre of Excellence, University of York,
Department of Chemistry, Heslington, York, YO10 5DD, UK; cDepartamento
de Física de Polímeros, Elastómeros y Aplicaciones Energéticas, Instituto
de Ciencia y Tecnología de Polímeros, CSIC, c/ Juan de la Cierva, 3, 28006,
Madrid, Spain
*E-mail: robin.white@fmf.uni-freiburg.de
12.1 Introduction
Starbon® and hydrothermal carbonisation (HTC) approaches to functional
carbon material synthesis from sustainable precursors have been discussed
earlier in this book. The most developed approaches to the synthesis of
porous carbonaceous materials are typically based on the use of saccharide-based precursors (i.e. starch to Starbons®; glucose to HTC carbon). In
view of a holistic approach to biomass and sustainable precursor utilisation in materials synthesis, the aromatic components (e.g. lignin, cellulose)
of plant biomass should also be considered as they typically represent a
RSC Green Chemistry No. 32
Porous Carbon Materials from Sustainable Precursors
Edited by Robin J White
© The Royal Society of Chemistry 2015
Published by the Royal Society of Chemistry, www.rsc.org
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signifi­cant component of the nonedible component and as such do not compete with food biomass cycles. The following section will provide an overview
and highlight some of the most recent and promising approaches to achieving the aim of producing, functional, applicable sustainable carbon materials. Whilst the HTC approach has been shown to convert lignocellulosic
biomass directly into hydrothermal carbon material, the ability to control
porosity or material morphology is somewhat hindered by the existing and
often hydrothermally resistant components of “real” biomass. Therefore,
approaches are necessary to demonstrate the potential to transform such lignocellulosic biomass and indeed to utilise nonfood-market products in the
production of applicable, carbonaceous materials.
Furthermore, we believe that it is very important to discuss ability of the
approaches discussed in this book to be scaled to at least “small-to-medium
enterprise” (SME) level. Therefore, the final part of this chapter aims to introduce the reader to a number of commercial endeavours to bring porous carbons from sustainable precursors to the market place.
12.2 Other Innovative Approaches to Porous
Carbons from Sustainable Precursors
12.2.1 Bacterial Cellulose
In Chapters 2–4, the development of porous polysaccharides via a gelation
approach to form aerogels and xerogels, which in turn act as precursors for
the synthesis of carbonaceous derivatives, Starbons® was discussed.1–8 In the
context of polysaccharide utilisation, and in particularly nonfood sources, the
production of new materials (including carbonaceous varieties) from forms
of cellulose (i.e. nanocellulose, bacterial cellulose (BC)) have been receiving
increasing attention.9,10 Interest in these varieties of cellulose stems from,
apart from being inexpensive and sustainable, attractive physicochemical
properties including tuneable hydro/oleophilicity, optical transparency, and
in some reports extremely good mechanically stable films or aerogels.11–16
This interest has been taken further, as reported in the literature, via the
production of carbonaceous films, membranes and porous gels from cellulose-based materials.17–20 In this regard, the group of Yu et al. have described
extensively the preparation of BC aerogels (Figure 12.1(a)) and their subsequent pyrolytic conversion to carbon aerogels (Figure 12.1(b)).21 In this report,
the BC pellicle precursor could be cut and shaped to the desired dimensions
followed by purification and freeze drying to yield the porous cellulose form –
termed in the report as an “aerogel”, although technically speaking it should
be referred to as a “cryogel”. The dried porous cellulose gel was then subjected to a thermal pyrolysis step under an inert (Ar) atmosphere between 700
and 1300 °C, to yield a “carbon nanofibre” (CNF) aerogel. The carbonisation
step induces a spatial volume reduction of ca. 15% in the CNF aerogel relative
to the polysaccharide aerogel precursor, with a corresponding reduction in
aerogel density of 9–10 mg cm−3 to 4–6 mg cm−3 for the carbonised product.
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Figure 12.1 Pictures and SEM micrographs of the bacterial cellulose aerogel (a)
and the corresponding carbon nanofibre aerogel treated at 1300 °C
(b). (c) Uptake of different organic liquids by the carbon nanofibre
aerogels. Reproduced with permission from ref. 21.
The CNF aerogels of Yu et al. were found to be constructed of amorphous, turbostratic nanofibres of D = 10–20 nm diameter (Figure 12.1(b)),
whilst the generation of a carbon network resulted in a reasonable electrical conductivity (20.6 S m−1). In terms of mechanical properties, these
CNF aerogels showed surprisingly high degrees of flexibility – a significant
advantage over conventional low-density silica-based aerogels. The CNF
aerogels could also withstand a manual compression to more than 90%
volume reduction, followed by near-total recovery of the original volume
after force release, indicating that these materials possessed unusual compliant and elastic features. Furthermore, these materials were also shown
to have excellent fire-resistant properties and extremely high absorption
capacities (i.e. 106–312 times their own weight) for organic pollutants and
oils (Figure 12.1(c)).
In a similar approach, Salazar-Alvarez, Titirici et al. have also utilised BC
aerogels for the preparation of carbon aerogels.17 Again, materials were
referred to as aerogels, even though freeze drying was employed. The prepared BC aerogels presented a highly porous network composed of a regular
nanofibre network (Figure 12.2(a)–(c)). To induce conductivity in the material, the precursor was thermally pyrolysed at 900 °C, resulting in a reduction
in the nanofibre thickness from ca. 20–70 nm to ca. 20 nm, with the overall
morphology of the material remaining relatively intact (aside from a significant reduction in the overall aerogel dimension) (Figure 12.2(d)–(f)).
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Figure 12.2 Morphology of bacterial nanocellulose and carbon aerogels. (a) Pho-
tograph of a freeze-dried bacterial nanocellulose aerogel (FDBNC), (b
and c) SEM images of FD-BNC aerogels at different magnifications,
(d) photograph of a carbon aerogel obtained through pyrolysis of
freeze-dried bacterial nanocellulose at 900 °C (FD-BNC-900), (e and f)
SEM micrograph of FD-BNC-900 aerogels at different magnifications.
Reproduced with permission from ref. 17.
The parent BC aerogel presented a total pore volume of 0.23 cm3 g−1 and
a SBET = 109 m2 g−1 as determined by N2 sorption analysis. After the pyrolysis
step, the SBET and material pore volume was found to have increased to 670
m2 g−1 and 0.83 cm3 g−1, respectively, the consequence of the carbonisation
process and material contraction. The carbon product was then subsequently
investigated as a potential anode material for Li-ion batteries. As reported
in this work, compared with other mesoporous carbons, the material of
Salazar-Alvarez, Titirici et al. exhibited a lower capacity but better capacity
retention behaviour at the same current rate (0.2C). However, the BC-derived
carbon aerogels showed a superior electrochemical performance that was
attributed to the high surface area, open 3D network and the crosslinking
between the CNFs.
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Additionally, in the field of energy storage, Zhi et al. have reported on the
production of carbon aerogels from BC and their use as a anode in Li-ion batteries in conjunction with deposited SnO2 and Ge nanoparticles.22 Similarly,
Yu et al. have also employed BC-derived CNF@MnO2,23 and nitrogen-doped
CNFs as electrode materials in the development of an asymmetric or all-solid-state supercapacitors.24 In the latter report of Yu et al. BC was freeze dried
as reported previously, followed by carbonisation at 600 °C and activation
in CO2 at 800 °C. The activated CNF aerogel was then immersed in an aqueous solution of NH3 of a desired concentration (to dictate the N loading in
the resulting material). The recovered CNF aerogel was then hydrothermally
treated at 180 °C for 12 h to produce the nitrogen-doped CNF material. The
material was then tested as a flexible all-solid-state supercapacitor device
utilising these nitrogen-doped CNF aerogels derived BC as the electrode
material (Figure 12.3). The material was found to be a pliable, flexible electrode capable of reversibly delivery a maximum power density of 390.5 kW
kg−1 and a good cycling durability with ca. 96% specific capacitance retained
after 5000 cycles. These are excellent performance characteristics with these
nitrogen-doped CNF aerogel electrodes synthesised via a low-cost, ecofriendly, low-temperature, and scalable fabrication hydrothermal synthesis.
The use of BC in the preparation of new carbon-based material certainly
is becoming an attractive topic, particularly in the preparation of hybrid
and composite materials, including BC-derived carbon/multiwalled carbon
nanotube,19 BC-derived carbon/nitrogen-doped graphene,18 whilst BC itself
has been used in the production of ultralight nanocomposite aerogels with
graphene oxide,25 amongst other reports. However, it is worth noting that
whilst these BC-derived carbon materials present certainly interesting and
potentially unique (e.g. mechanical) properties, it is not clear exactly how the
morphology of the BC material is not lost upon carbonisation, given the generally perceived thermal decomposition chemistry of polysaccharides (see
earlier discussion relating to the synthesis of Starbons®). Another concern
here will be the low carbonisation yield resulting from such a methodology,
especially in the absence of a carbonisation catalyst, although the addition
of iron-based catalysts has recently been observed to dramatically increase
the carbon yield.17
12.2.2 Filamentous Fungi
In an innovative approach to the preparation of porous carbon materials
based on the conversion of sustainable precursors, Wang, Chai et al. have
demonstrated a micro-organism-based route for the synthesis of carbon-fibre
monoliths through the growth of filamentous fungi as a precursor (Scheme
12.1).26 In this approach, the fungus (i.e. Aspergillus aculeatus) was cultured
in solution over 3 days with the addition of biomass nutrients, resulting in a
fungus concentration in the highest case of 11 mg mL−1. Using a controlled
washing/drying and shaping approach, Wang, Chai et al. demonstrated the
preparation of fungus-based membranes and aerogels. Application of a
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Figure 12.3 (a) A schematic diagram of the all-solid-state supercapacitor illustrating that the gelled electrolyte can serve as both the electrolyte and separator. (b) A digital photograph of a bent bacterial cellulose-derived,
nitrogen-doped CNF aerogel with a N loading of 9.4 wt% (bulk)/8.1
at.% (surface) (denoted as A-p-BC–N-25) flexible supercapacitor device
(2.4 cm × 1.0 cm), showing its good flexibility. (c) CV curves collected
at a scan rate of 50 mV s−1 for the A-p-BC-N-25 flexible supercapacitor device under different bending angles. The inset is the schematic
showing the device under stress and defining the bending angle. (d)
Cyclic voltammetry (CV) curves of the A-p-BC–N-25 flexible supercapacitor at different scan rates. (e and f) Galvanostatic charge–discharge curves of the A-p-BC–N-25 flexible supercapacitor at different
current densities. Reproduced with permission from ref. 24.
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Scheme 12.1 Illustration of the synthesis of carbon monolith. (a) culture medium;
(b) fungi suspension; (c) SEM image of fungi pellet; (d)–(e) monolithic fungi as carbon monolith precursor; (f)–(g) corresponding
carbon monolith. From a macroscopic perspective (total step), biomass was transformed into fibrous carbon sources with the fungi as
a bridge. Reproduced with permission from ref. 26.
pyrolysis step to the dried products under an inert atmosphere resulted in
the production of an intact carbon-fibre monolith in membrane or aerogel
form and a conductivity of > 1 S cm−1. As a consequence of this biomass-based
approach, the resulting carbon-fibre aerogel and membrane synthesised at
800 °C was doped with N (∼2.4 at%) and O (∼1.3 at%), with a SBET of ∼305 and
∼20 m2 g–1, respectively. Sorption analysis revealed the presence of meso- and
macropores within the synthesised carbon structures. The application of the
carbonised materials as a conductive interlayer to improve cyclability of Li–S
batteries was also demonstrated in this report, with an improved performance in the charge/discharge process, with a capacity of 650 mAh g−1 even
after 100 cycles at 0.5 C. This approach is particularly interesting as it allows
scope to feed waste biomass to these fungi and depending on the conditions,
prepare aerogels or membranes that can subsequently be transformed into
carbonaceous equivalents using a simple thermal carbonisation step. The
properties of which can presumably be tuned based on the growth time or
the biomass feed type and perhaps the fungus type employed.
12.2.3 Gelatin
Gelatin is a sustainable polymer derived typically from animal wastes and is
composed of various proteins, and from a material-preparation perspective
typically carries an average nitrogen content of ca. 16%. Gelatin is produced
via the partial hydrolysis of collagen, which can be sourced from skin, boiled
crushed bones or the connective tissues of animals used in food production
(e.g. cattle, chicken, and pigs). It is normally considered a waste, is inexpensive, renewable, environmentally friendly and commercially available.
Therefore, its conversion into useful materials would be beneficial from a
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Figure 12.4 FESEM (a), TEM (b) and HR-TEM (c) images of gelatin-derived carbons
(i.e. GAC4 – NaOH/carbon = 2 : 1 (w/w); nitrogen (77 K) sorption isotherms (d) and DFT pore-size distribution (e) of the gelatin-based carbons. Reproduced with permission from ref. 27.
waste-valorisation standpoint and if the materials present beneficial chemistry, potentially from an application standpoint as well.
In this context, Xu, Wu et al. have reported on the preparation of high surface area, nitrogen-doped porous carbon using gelatin as carbon precursor.27
The synthesis of the materials in this report was based on the carbonisation of
gelatin under an inert atmosphere, follow by additional chemical activation
of the product using NaOH, with porosity and the chemistry of the product
being strongly dependent on the weight ratio of NaOH/gelatin-­derived carbon. Materials prepared at different ratios presented surface areas between
323 and 3012 m2 g−1, whilst the nitrogen content varied between 0.88 and
9.26 at%, respectively. The materials, as can be seen from the corresponding
N2 sorption isotherms and pore-size distributions, are predominantly microporous in character with the pore size becoming increasingly wider with
higher amounts of activation agent (Figure 12.4). The synthesised carbons of
Xu, Wu et al. were then tested as electrode materials for supercapacitors, with
the unique microstructure and nitrogen functionalities generating a carbon
electrode with a high capacitance of ≤ 385 F g−1 in 6 mol L−1 KOH aqueous
electrolyte, with the performance attributed to the cocontribution of double-layer capacitance and pseudocapacitance. Promisingly, the synthesised
carbons showed excellent rate capability (e.g. 235 F g−1 at 50 Ag−1) and cycle
durability. However, overall mass yields of carbons produced from the gelatin
carbonisation step were not clearly stated in this report. This is not necessarily a problem given the precursor is waste and provided the exhaust gases are
suitable sequestered or recycled.
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The use of gelatin in the preparation of doped nanostructured carbons
as noble-metal-free electrocatalysts for fuel-cell applications has been
reported by Schnepp, Zhang et al.28 Based on a one-pot synthesis starting
with metal nitrates and gelatin, multiphase C/Fe3C/MgO nanomaterials were
then synthesised, followed by chemical etching to produce active carbon
electrocatalysts featuring trimodal porosity. Materials were tested for the
oxygen-reduction reaction demonstrating activity comparable with commercial platinum-based catalysts but importantly the reported materials had
improved stability and reduced crossover effects. To improve the overall sustainability of this approach however, the recovery of the metal components
upon etching would be desirable.
12.2.4 Silk Cocoon
Wu et al. have reported on a facile and low-cost route to the preparation
of 1D porous carbon microfibres based on the direct carbonisation of electrospun natural silk cocoon.29 The authors reported that as the result of
this one-step carbonisation treatment, the electrospun cocoon microfibres
could be directly transformed into 1D carbon microfibre meshes of ca. 6
mm diameter presenting a unique 3D porous network structure composed
of interconnected carbon nanoparticles of 10–40 nm diameter (Figure 12.5).
After carbonisation at 900 °C, the carbon presented a N2 sorption isotherm
typical of a predominantly microporous material with a specific surface
area of 796 m2 g−1, whilst t-plot analysis gave a surface area for micropore
and external large-sized pores equal to 569 and 227 m2 g−1, respectively. TEM
image analysis clearly indicated the presence of pores with D > 50 nm, whilst
total pore volume was reported as 0.43 cm3 g−1. The authors also reported
that the as-prepared carbon product provides superior electrochemical
performance as binder-free electrodes for supercapacitors and also good
adsorption properties (e.g. of organic vapour). Whilst the precursor is certainly sustainable, it is unclear as to the scalability of this approach and also
the actual cost of the synthesis as compared to other electrospun carbon
materials.
12.2.5 Flavonoids and Tannin
Another potentially promising route to the preparation of porous carbons
from sustainable precursors is to substitute the phenolic compounds used
in traditional organic aerogel preparation (e.g. Pekala gels) with nontoxic flavonoids of natural origin. Tannins, which are widely used in the textile (e.g.
leather treatment), nutrition and resin industries, have been used as precursors for the synthesis of hydrogels based on the polycondensation of this precursor with other sustainable partners; furfural30,31 or formaldehyde.32,33
In this context, the work of the Celzard group is specifically highlighted.
One significant report of interest from this group concerns the polycondensation of mimosa tannins and formaldehyde. Synthesis in acidic (pH ≤ 3.3) or
basic (pH ≥ 8.3) hydroalcoholic media at 85 °C for 3 days, led to the formation
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Figure 12.5 (A) and (B) SEM and (C, D) TEM images of silk cocoon-derived 1D
porous carbon. (B) and (D) correspond to the area indicated by a
rectangle in (A) and (C), respectively. Reproduced with permission
from ref. 29.
of an organic gel.32 Organic aerogels were prepared via drying with supercritical acetone followed by thermal carbonisation at 900 °C under high purity
nitrogen to produce the corresponding carbon aerogels. In this approach
carbon aerogels with SBET in the region of 700 m2 g−1, were synthesised. Based
on a cost analysis of the synthesis, the authors claimed their materials were
five times less expensive than traditional resorcinol-formaldehyde-derived
aerogels, making them the cheapest carbon aerogels ever reported.32
More recently, Celzard et al. have investigated reducing the formaldehyde quantity used in the tannin gelation process, with the aim to produce
“greener” and increasingly cost-effective organic aerogels. Two different synthetic procedures were reported:
1. the partial substitution of tannin by even more abundant and challenging to use lignin, a biopolymer derived from wood;34
and
2. the use of denatured then formylated soy proteins as natural crosslinkers within wattle tannin-based hydrogels.35
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The first synthetic route was capable of eliminating formaldehyde content
to 9.4 wt.% (vs. 42.5 wt.% for tannin–formaldehyde aerogels32). Significantly,
this reduction in formaldehyde use did not have a significant impact on the
surface area and porous properties (e.g. SBET = 478 m2 g−1) for the organic
aerogel product.
The tannin gelation approach of the Celzard group has also been used to
produce poly-HIPEs, foams and hybrid poly-HIPE-foam materials, significantly utilising a sustainable dispersed oil phase (i.e. sunflower oil).36 Carbonisation of the tannin-based organic gel precursors at 900 °C, led to the
production of macrocellular carbon monoliths with > 97% total porosity.
The Celzard group has also reported on a dual-synthesis approach involving hydrothermal carbonisation and the tannin gelation approach to prepare
nitrogen-doped carbon microspheres.37 Extensive investigations by the Celzard group have demonstrated the potential to manipulate synthetic mixture
composition, processing conditions and the use of secondary additives (e.g.
proteins) for the synthesis of a wide range of promising carbonaceous (e.g.
xero- and aerogel) materials (Figure 12.6).38–40 This approach has significant
promise and represents a very interesting development in the field of porous
carbons derived from sustainable precursors.
12.2.6 Lignin
Lignin is the third most abundant naturally occurring polymer after cellulose
and chitin.41 Whilst the latter two polymers are saccharide-based, lignin is
based on a crosslinked poly(phenol)-like structure and is perhaps the most
abundant sustainable source of phenol and its derivatives. Lignin has been
classically employed in the preparation of activated carbons.42 However,
the availability of plentiful hydroxyl groups in lignin macromolecules and
degradation products and its generally inexpensive nature render it a potentially useful precursor in the synthesis of mesoporous carbons in a manner
akin to the resorcinol–formaldehyde-based approaches of Pekala et al.43 The
highly branched nature of the lignin macromolecule inhibits the preparation of highly controlled porosity. However, given its abundance, the use of
lignin in mesoporous carbon synthesis can potentially provide additional
side streams and high value products for future Biorefineries and existing
paper-manufacture sites.
In this regard, there are thus far only two known reports regarding the
synthesis of surfactant-templated mesoporous carbon from lignin.44,45 In
these reports, Kraft-processed hardwood lignin was employed as the precursor and a mesoporous lignin-based gel was prepared via the evaporation
induced self-assembly of the lignin macromolecules and the classical soft
template Pluronic® F127 at different mass ratios. In the first report of Naskar
et al. the recovered, dried organic gel was then subjected to carbonisation by
heating to 1000 °C under an inert atmosphere to produce the mesoporous
material.44 Based on N2 sorption analysis, the lignin-derived carbons of Naskar et al. clearly presented Type IV hysteresis isotherm shapes indicative of
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Figure 12.6 SEM images of carbon meringues prepared with different concentra-
tions of tannin, obtained with secondary (left) and backscattered electron (right) detectors. From (a) to (e) and from (f) to ( j): CM30 (top),
CM35, CM40, CM 45 and CM50 (bottom). Reproduced with permission
from ref. 40.
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Figure 12.7 (a) N2 sorption isotherms for LMC-1 through LMC-6 at 77 K. (b) Pore-
size distribution of LMC samples calculated via the NLDFT method
from N2 sorption plots. (c) Cumulative pore-size distributions of
LMC-1 through LMC-6 calculated via the NLDFT method from N2 sorption plots at 77 K. Reproduced with permission from ref. 44.
a mesoporous material, with some evidence of a plateau region as relative
pressure approached unity, reflective of some degree of pore ordering (or
indeed particle size regularity) (Figure 12.7(A)). The corresponding pore-size
and volume distributions demonstrated that there is a degree of templating
in the material as a result of the use of the block copolymer surfactant in the
synthesis, although the regularity was dramatically affected by the synthesis
conditions (Figure 12.7(B)) In the later report, the synthesised mesoporous
carbon product was subjected to a physical or chemical activation step with
CO2 or KOH with the intention to increase microporosity and material surface area.45 The reported lignin-derived mesoporous carbons were found
to be useful controlled drug-delivery media44 and supercapacitor electrode
material.45
12.2.7 Ionic Liquids as a Solvent in Hydrothermal
Carbonisation
As mentioned in the introduction to this book, ionic liquids (ILs) have been
receiving extensive coverage as new, nonvolatile solvents with tuneable solvation and acidic or basic properties, for a range of catalytic processes.46–51
These unusual salt-based liquids are also finding interest as the synthesis
media for a range of organic and inorganic materials.49,52 In this context,
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Chapter 12
Taubert, Titirici et al. have demonstrated the use of metal-containing ILs in
the preparation of hierarchically porous carbonaceous materials in process
termed “ionothermal carbonisation”.53 In this report, porous carbon materials
from a variety of carbohydrate precursors (i.e. d-glucose, d-fructose, d-xylose,
and starch) were prepared utilising 1-butyl-3-methylimidazolium tetrachloroferrate(III), [Bmim][FeCl4] as a recoverable solvent and catalyst, to produce materials presenting relatively high surface areas from 44 to 155 m2 g−1
after ionothermal processing. A secondary thermal carbonisation step (at
750 °C) resulted in a significant increase in this value to > 350 m2 g−1. A combined approach to porosity analysis using CO2, N2 sorption and Hg intrusion,
revealed a hierarchical pore structuring.
The authors proposed that [Bmim][FeCl4] played a triple role in the synthesis, acting as both a soft template to produce the observed materials nanostructuring, a solvent and as a catalyst generating higher overall carbon yields.
One of the main advantages cited by the authors for using ILs for biomass to
carbon conversion is their negligible vapour pressure, which enables synthesis to proceed essentially at ambient pressure, removing in part any safety
concerns related with the use of high-temperature, high-pressure water. The
authors also highlighted that the use of other metal-containing ILs or secondary additives is also of significance as it can potentially enable the synthesis
of other useful porous hybrid carbons (e.g. Fe3C@C). This approach has been
recently extended by Wang et al. who investigated the conversion of glucose,
cellulose and sugarcane bagasse in methyl-imidazolium-based ionic liquids
with bulky alkyl side chains or bis(trifluoromethylsulfonyl)imide anion, as
efficient and recyclable templates for porosity control, leading to exciting
nanoarchitectures.54 The resulting materials produced in this study were
found to have promising performance in the oxygen-reduction reaction. The
use of polyionic liquids (PILs) in combination with a saccharide precursor has
also found application with regard to the preparation of heteroatom-doped
carbons, with the resulting materials finding application in catalysis,55 with
the PIL reportedly improving the overall carbon material yield56 and the
fire-retardant properties.57 Whilst these reports are certainly interesting, it
remains to be seen the overall application, cost and indeed CO2-reduction
benefit the use of IL-based solvents or cocarbon precursors may have upon
the life cycle of the resulting carbon material.
12.3 Commercialisation of Sustainable
Carbon Materials
In the follow section examples of current commercial efforts to bring sustainable carbon materials to the market will be introduced. As will be noted,
at the laboratory scale, the production of some of these materials may appear
cost inhibitive, but with the correct scaling factors, investment and economies of scale, these new sustainable materials can potentially be more cost
effective than current state-of-the-art equivalents.
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12.3.1 Starbon® Technologies Ltd
Researchers at the Green Chemistry Centre of Excellence at the University of
York, as discussed in Part 1 of this book, have developed a novel approach to
the synthesis of mesoporous carbonaceous materials – utilising the complex
molecular architectures afforded by polysaccharides (e.g. gels).1,4,5,8,58–60 The
formation of a polysaccharide gel in water opens up the normally compact
and often crystalline structure allowing the polymer network to become more
accessible via the formation of this expanded phase. In the case of neutral
polysaccharides (e.g. starch), further treatment with an acid dehydration catalyst and thermal carbonisation to temperatures between 200 and 1400 °C
transforms the expanded polysaccharide state into stable nanostructured carbonaceous materials (Figure 12.8). The use of acidic polysaccharides in the
process reduces the number of process steps from seven to six. Variations in
the temperature of polysaccharide carbonisation help enable the production
of a family of novel nanostructured carbons with readily controllable surface
oxygen concentration, hydrophobicity and functionality that have been registered under the trademark Starbon® and a number of associated patents.61–64
The high mesoporosity and tuneable surface functionality of Starbons® make
Figure 12.8 Flow-diagram overview of the synthesis of Starbons® based on starch,
pectin and alginic acid.
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them, as discussed throughout Part 1, promising for a variety of applications
including chromatography, catalysis, environmental remediation, etc., whilst
the materials synthesis provides simpler and less-wasteful route to mesoporous carbons synthesis.
The preparation of Starbon® significantly removes the need for the use of
templating, utilising the natural ability of polysaccharide to form thermally
reversible expanded gel phases to facilitate the preparation of a high surface
area, volume and functional porous polymer (e.g. aerogel), which can then
be readily converted to the carbonaceous equivalent. The synthesis, whilst
a multistep approach, is somewhat simpler in comparison to the preparation of commercial mesoporous carbons (e.g. porous graphitised carbon
(PGC®) from ThermoFischer). The synthesis does not require any sacrificial
(e.g. inorganic) materials to act as templates, all the solvents employed are
sustainable solvents (e.g. H2O, CH3CH2OH, CO2) can be recovered and recycled/reused, whilst the electrical energy required during the process can in
principle be supplied from renewable sources. Furthermore, if the exhaust
gases (e.g. CO, CO2, H2, etc.) can also be valorised, the overall “sustainability”
potential of this approach to carbon material synthesis can be improved still
further. In overview Starbon® technology is:
●●
●●
●●
●●
Green: process avoids the use of harmful chemicals;
Sustainable: polysaccharides are renewable resources that are widely
available in many countries;
Simple: methodology comprises three main stages;
Environmentally benign: nonpersistent, nonbioaccumulative and
nontoxic.
All these features make Starbon® technology promising for large-scale
production of nanoscale materials. In this regard and based on materials
and application data discussed in Part 1 of this book, a substantial portfolio of patents and trademarks has been produced resulting ultimately
in the founding of the spin-off company, Starbon® Technologies Ltd in 2012
with the aim to commercialise this promising carbon material technology
(Figure 12.9).61–65
Funding from the Engineering & Physical Science Research Council
(EPSRC) UK, in collaboration with one of the UK’s top contract chemical specialists has enabled this sustainable precursor-derived mesoporous carbon
Figure 12.9 The official commercial logo of Starbon® Technologies Ltd. Source:
www.starbon-technologies.com.
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material to be produced at any scale, under economically favourable conditions. On the base of this research and development of various cost models
it was decided that all product manufacture would be contracted out, with
product quantities > 200 kg (minimum viable industrial quantity) contracted
to industrial chemical manufactures and quantities < 200 kg to the local
Biorefinery Development Centre (BDC) found in York, UK in 2011 (Figure
12.10). Starbon® Technologies are in constant contact with these different
types of contractor, and have been assured of available product quantities,
price and quality.
During the process-development stage of the material for industrial manufacture the whole process was redesigned in order to accommodate the readily available industrial setup, which subsequently led to a number of choices
regarding the solvents and polysaccharide types used with the main factors to
consider being its availability at scale, cost and simplicity to process/recover.
This overriding philosophy led to a product as mentioned that could be manufactured and proportionally cheaper at scale, with no detriment to the final
product. The reduction in cost with scale is a common theme and with this
product it could be related to the more efficient solvent recovery, the same
time needed on plant and the economics of bulk raw material purchase. The
raw materials were also sourced from local manufacturers, minimising transportation or import tax duties, etc. However, during the process development
various starch types and polysaccharides were tested that would be considered as locally grown in countries like Brazil, China and France, etc., with the
results showing that with similar processing the desired resultant product
could easily be formed. This bodes well for possible franchises of Starbon®
Technologies in other countries, using a similar model of subcontracting
Figure 12.10 A preliminary product cost/scale relationship for the large-scale
manufacture of starch-based Starbons®.
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out its manufacture, but being able to utilise the countries locally sourced
starch/polysaccharide materials, keeping costs to a minimum, whilst maintain the products’ “green” credentials.
As mentioned earlier in this book, Starbons® have tuneable surface functionality and as such are highly desirable for nearly all applications where
diffusion of the species within the pore network is essential (e.g. from the
external surface to pore wall active site). In this regard the relationship
between Starbon® material chemistry and a potential application area has
been identified (Figure 12.11).
Although the potential for Starbon® mesoporous carbons is vast due to
their easily adaptable surface properties and array of pore structures, key
application areas with the greatest chance of market success have been
identified in chromatography, aqueous phase – esterification catalysis and
adsorption:
●● Chromatography
Starbon® materials derived from alginic acid are particularly attractive
chromatographic stationary-phase materials, as they present minimal
micropore content; this avoids reduction in separation efficiency as
a consequence of irreversible high-energy analyte adsorption in sub2-nm pores. It was found that these stationary phases are particularly
efficient at separating the sugars glucose (mono−), sucrose (di−) and
raffinose (trisaccharide). The resultant ion chromatograms had excellent peak shape, and near baseline resolution.
Figure 12.11 Changing of Starbon® surface functionality with temperature preparation, and the materials’ respective application.
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Catalysis – esterification in water
Starbon® is an excellent support for heterogeneous catalysis where its
unique and tuneable surface characteristics are appropriate for many
reactions including unexpectedly esterification reactions conducted in
aqueous media. This is particularly important in biomass fermentation
reactions, which produce a range of organic acids that can be utilised
as platform molecules in applications such as the production of polymers and higher-value intermediates. Esterification is one of the key
upgrading steps for these acids. The fermentation process is carried out
in aqueous media and the resulting aqueous broths require resource-­
intensive separation steps before the acids can be upgraded. The use of
Starbon® catalysts have overcome this problem.
●● Adsorption
Low-cost, naturally derived adsorbents have great potential for use in both
developed and developing countries for applications such as water purification and pollution control (e.g. metal removal). The potential of porous
carbonaceous materials for water purification requires a methodology that
grants control over their surface chemistry, the distribution of pore sizes
and as such makes Starbons® ideal candidates for these applications.
As mentioned earlier, a number of polysaccharides have been tested for market suitability to become a Starbon® mesoporous carbon, and in this regard:
●● Starch and alginic acid have been investigated predominantly as these
are some of the most abundant land and marine base sources of
polysaccharides.
●● Starbon®-300 (hydrophilic) (Product code: 702110) and Starbon®-800
(hydrophobic) (Product code: 702102) derived from starch, were the first
products to be launched in 2008 through Sigma-Aldrich, and have seen
continued increase in demand with product recognition.
●● Starbon®-300 and Starbon®-800 are currently on sale (at the time of writing) at €163.50 and €163.00 per 5 g, respectively.
This equates to a price of Starbon® at 32 600–32 700 €/kg, but it is important to note that this price will account for any mark-ups of sale through
Sigma-Aldrich and for near-lab-scale production. For comparison, a PGC
Hypercarb analytical chromatography column (30 × 4.6 mm) costs at the
time of writing €790. This column contains approximately 1.1 g of PGC
material (based on ρ = 2.2 g cm−3). Therefore, PGC costs approximately (without accounting for column fabrication costs) €3590 per 5 g and €718 000
per kg, clearly demonstrating (on a very simplified economic comparison)
the potential of Starbon® materials for market penetration (e.g. in the field
of analytical chromatography phase preparation). Following on from these
initial assessments made by Starbon® technologies and their associates, a
preliminary product cost/scale relationship for the large-scale manufacture
of starch-based Starbons® shown in Figure 12.10 was calculated. The very
●●
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strong relationship between scale of manufacture and the production price
of Starbon® materials, suggests that the adoption of this technology could
potentially be very interesting to investors given the high-value application
and potentially large profit margins as a result.
12.3.2 Hydrothermal Carbonisation
Of the approaches to the commercialisation of carbons from sustainable
precursors, the hydrothermal carbonisation (HTC) is perhaps receiving the
most interest, predominantly as a result of its general technological simplicity and its applicability to all biomass-based precursors. A significant number of SMEs are now attempting to either enter the market or demonstrate a
suitably energetically and economically feasible approach to the conversion
of biomass (and biomass wastes) to carbon materials (Table 12.1). A number
of companies are already at the pilot-plant scale and are currently looking
to expand as a result of significant investments. However, it is noted that to
be a truly feasible and an economically worthwhile endeavour, HTC needs
to be performed in a continuous process and a number of companies are
developing technologies to allow the production of materials in this manner.
It is also worth noting that the majority of companies are also examining
the HTC process as a mechanism to remediate waste (e.g. sewage, municipal
waste) to energetic “biocoal”, which can then be burned as per conventional
coal. Thus with an efficient energy and mass balance, potentially CO2-neutral
fuels may be generated.66,67 The use of materials produced via the hydrothermal conversion of food wastes in the production of soil additives is commonly referred to as “biochar”.68,69 As far as the authors are aware there are
currently no companies looking to exploit the HTC platform for the synthesis
of high-value, porous carbon materials for the applications highlighted in
Part 2 of this book (e.g. battery, supercapacitor, fuel-cell electrodes.)
12.3.2.1 AVA-CO2
This company intends to generate solutions for the conversion of biomass
into a sustainable source of energy on a globally applicable scale. Based
on the HTC approach, AVA-CO2 is currently planning, implementing and
operating HTC plants on behalf of a number of international customers.
The company uses the HTC platform to convert plant residues efficiently
and profitably into high-grade biocoal or biochar, to be used as an energy
fuel or soil additive, respectively. AVA-CO2 is currently headquartered in
Zug, Switzerland with a subsidiary currently in operation in Karlsruhe,
Germany. AVA-CO2 is noted for being perhaps the first company to launch
the first industrial-sized HTC plant in October 2010. AVA-CO2 is promoting the conversion of vegetable wastes via the HTC approach as a mechanism to significantly lower greenhouse gas emissions, whilst concurrently
producing an ecofriendly and renewable energy source, biocoal – which
AVA-CO2 refers to as “Cleancoal”.
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Table 12.1 Selection of known companies currently investigating the feasibility of
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large-scale hydrothermal carbonisation of biomass and biomass-derived
compounds.
Company
Location
Ava-CO2
Zug, CH;
Karlsruhe,
DE
Artect Biotechnologie
GmbH
SunCoal
Carbon­
Solutions
TerraNova
Energy
Antaco
Ingelia
Scale/Comment
Plant residue to high-grade biochar or AVA Cleancoal; 8400
tonnes biomass conversion
per year at Karlsruhe site; 8000
tonnes of Cleancoal production
per year at site in Rezlow, DE
Bad
Biocoal production – From 1.8
Königshofen,
L batch to small continuous
DE
operation plants working
at the 15 L to 3000 L plant
volume.
Ludwigsfelde, Pilot Plant in operation 2008–
DE
2010; Since 2011 converted to
customer testing purposes.
Scale unknown
Teltow, DE
Prototype CS-HTC90™ plant
commissioned October 2010 (a
90 min, continuous countercurrent HTC reactor/process).Fully
approved waste treatment facility according to German legislation and is since providing the
basis for further development.
Close association with Prof.
Markus Antonietti (Max Planck
Institute for Colloids and Interfaces, Golm, DE); Investigating
feasibility of 10000 tonne per
year biocoal production
Düsseldorf,
Sewage-remediation conversion;
DE
Scale – minimum 300 tonnes
dry matter per year or 1200
tonnes per year at 25% dry
matter content; Use of carbonisation catalyst to reduce
HTC process to 4 h; Proposed
revenue stream of 400 EUR per
tonne of biocoal.
Guildford,
Scale unknown; Focus on bioUK
fuel production from waste;
Continuous operation, leading developer of this technology in the UK.
Valencia,
11 000 tonnes per year (biomass);
Spain
Plant in operation since July
2010, currently operating on
plant-based biomass conversion to produce biocoal with
GCV of ca. 24 MJ kg−1; inverted
flow reactor design.
Link
www.ava-co2.
com
www.artecbiotechnologie.com
www.suncoal.de
www.cs-carbonsolutions.de
www.terranovaenergy.com
www.antaco.
co.uk
www.ingeliahtc.
com
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With the AVA Cleancoal product, the company indicates that a highgrade, CO2-neutral energy solid can be produced, with the highly efficient,
exothermal HTC process transforming nearly 100% of the biomass-derived
“C” into the final energy coal, which can be provided as easily storable solid
pellets or in powder form. In this approach, the company claims that the
Cleancoal product is a CO2-neutral energy source, addressing successfully
the criteria required for CO2-neutral certification, whilst the solid is an outstanding energy coal-equivalent (20–30 MJ kg−1; superior to high-grade lignite) with highly efficient combustion leading to very small NOx emissions.
Furthermore, the resulting combustion ash has a very high melting point
of 1400 °C – an important feature for use with high-temperature furnaces.
The company also states that the product, as a result of its high energy
density, renders transportation to end use point efficient, whilst the use of
suitable biomass, renders the product with extremely low amounts of toxic
substances and heavy metals. The carbon content of the Cleancoal product
is 70%, whilst the overall fixed CO2 is indicated as 2.5 kg. The company
is currently operating at 8400 tonnes biomass conversion per year at their
Karlsruhe site and a newer 8000 tonnes of Cleancoal per year production
capacity at their Rezlow site (Figure 12.12).
The first industrial-size HTC plant in the world was commissioned by AVACO2’s subsidiary in Karlsruhe. This set-up is composed of mixing tank, reactor
and outlet buffer tank (Figure 12.12; right to left). The reactor has an overall
capacity of 14400 litres and an annual processing capacity of 8400 tonnes of
biomass. The reactor works at temperatures of around 220 °C and at a pressure of 22 bar. This base test plant set-up can in principle be installed a customer sites, with the concept of “numbering-up” reactors to meet demand
but will typically consist of 8–12 reactors resulting in an annual capacity of
65000 to 100 000 tonnes of biomass. One of the innovations developed by
Figure 12.12 The 8400 tonne biomass hydrothermal conversion plant of AVA-CO2
in Karlsruhe, DE. Source: http://www.ava-co2.com/web/pages/en/
downloads/photo-archive.php.
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AVA-CO2 is the incorporation of flash tanks to store the exothermic heat of a
previous run until it is needed to provide energy to heat up fresh biomass.
The second product, AVA biochar, is proposed to be an appropriate soil
improvement additive specifically for the sustainable improvement of arid,
humin-poor or depleted soils. In addition, AVA biochar binds CO2 in the soil
over long periods of time, thus contributing to a reduction in greenhouse
gases. In particular, humin and nutrient-poor soils are proposed to be the most
appropriate soil types for use with AVA biochar. The company indicates that
the high porosity of their biochar and other specific characteristics results in
improved nutrient and water storage in the soil. AVA-CO2 is currently working
with leading companies and research institutions in the research and further
development of AVA biochar for purposes of soil improvement and additional
fields of application in agriculture and commercial horticulture.
12.3.2.2 Artec Biotechnologie GmbH
In Bad Königshofen (near Göttingen, DE), Artec has constructed a HTC plant
with a capacity of 20 L per day, meaning up to 100 kg(biomass) can be processed and converted to “biocoal” or “biochar”. The company indicates that
they possess the technology to provide customers with reactors that can
operate from 1.8 L batch to small continuous operation plants working at
the 15 L to 3000 L plant volume.
12.3.2.3 SunCoal
SunCoal Industries is operating with the intention to develop, construct and
operate industrial facilities that refine organic waste into biocoal (denoted
under the trademark SunCoal®). The company has developed patented CarboREN® technology, which is underpinned by the HTC platform, to provide
industrial solutions to two specific customer segments, namely municipalities
and large-scale industrial customers. The company claims, based on massand energy-balance calculations, that their patented technology is the most
effective available for biomass-to-fuel production, requiring only a minimal
amount of energy use (> 70% of the input energy is carried into the synthesised biocoal) (Figure 12.13). The modular construction is also considered
advantageous and can be supplied accordingly depending on customer needs.
SunCoal Industries has founded a technology centre in Ludwigsfelde, Germany, which includes currently a HTC pilot plant and associated research
laboratory. From 2008 to 2010, the pilot plant analysed the HTC process underpinning the proprietary CarboREN® technology, leading to process optimisation and the production of biofuel/coal with a high energy yield, optimisation of
the energy management to increase energy efficiency and handling of the process water streams. In 2011 the pilot plant was converted for customer-testing
purposes, to allow concurrent development of the CarbonREN® processes for
particular customer biomass precursors and the synthesis of product samples
for further research and development projects with external partners.
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Figure 12.13 The (A) materials- and (B) energy-balance profiles for the produc-
tion of SunCoal® using the propriety CarbonREN® technology of
SunCoal Industries. Source: http://www.suncoal.de/en/technology/
material-balance-and-energy-efficiency.
12.3.2.4 CarbonSolutions
On performing the HTC process, biomass is exposed to high temperatures
of between 160 and 250 °C, any system water (added or evolved) must be
prevented from evaporating. Thus the HTC process is typically performed
under elevated pressures (autogenous or applied) of 10 to 60 bar, typically
over a reaction time of > 15 h. As a result of close collaboration with the
Max-Planck-Institute for Colloids and Interfaces, Golm, DE, CarbonSolutions were able to reduce the HTC synthesis time to 90 min, allowing the process to be performed in significantly smaller and more efficient equipment
than was previously possible. This also has significant investment and safety
benefits. Furthermore, CarbonSolutions have also developed proprietary
technology based on a countercurrent reactor design, which allows fully continuous operation of the HTC process. This reactor prototype, trademarked
as “CS-HTC90™” was commissioned as part of a pilot-plant inauguration
in October 2010. CarbonSolutions have achieved full accreditation as a
waste-treatment facility according to German legislation (BImSchG). Current
development at CarbonSolutions involves a feasibility study for the production of 10000 tonnes per year of biocoal for CO2-neutral fuel substitution for a
major German industrial company, development of a high-end nanoadditive
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Figure 12.14 Modular design of TerraNova plants integrated in containers includ-
ing a chemicals storage area, control and diagnosis (upper container). Source: http://www.terranova-energy.com.
for primary industry use, concept design for sewage sludge disposal of a
major Far-East metropolis and the application of biochar in soil improvement. CarbonSolutions is very active in research and development of HTC
materials in part due to the beneficial access to the advice of Prof. Markus
Antonietti, Director of the Colloids department, Max Planck Institute for Colloids and Interfaces, who is also exploiting the HTC synthesis approach for
the production of high-value carbonaceous nanomaterials.
12.3.2.5 TerraNova Energy
TerraNova Energy has developed HTC plants that are capable of operating at
the lowest scale of 300 tonnes of dry matter conversion per year (i.e. 1200 tonnes
original substance at 25% dry matter content), see Fig. 12.14. A clever combination of a high-pressure pump, heat exchanger (to recycle the exothermic heat of
reaction for prewarming), a continuously running stirred-tank reactor, exchange/
discharger, dewatering and drying equipment results in a high-efficiency production of HTC material. TerraNova Energy is interesting as it is one of the few
companies that openly admits to using catalysts and additives to accelerate the
HTC process, ultimately reducing processing time to ca. 4 h.
Based on information supplied from the TerraNova Energy website,
using a plant with an annual capacity of 1000 tonnes of dry matter (4000
tonnes biomass at 25% dry matter), the specific treatment costs will therefore be a maximum of €280 per tonne of biocoal or €220 per tonne of input
biomass. TerraNova Energy propose the use of this technology specifically
in the context of a sewage-treatment plant with ca. 35000 population equivalent capacity, which they propose could generate a return on asset rate
of > 10% on the basis of average disposal costs of €250 per tonne (dry matter) of sewage sludge. Furthermore, they also propose to generate further
income from the use of CO2 certification obtained from the substitution of
fossil coal for electrical power generation. This is also viewed as an additional income stream in accordance with EEG (German Renewable Energy
Law), guaranteeing income of ca. €400 EUR per tonne of biocoal used in
electricity production.
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TerraNova Energy currently have an industrial demonstration plant at the
central sewage treatment plant in Kaiserslautern, whilst also exploring the
use of the HTC platform for the manufacture of biochar soil conditioner or
the recovery of nutrients such as phosphorous from sewage sludge. Collaboration partners for this SME include the Institute for Applied Material Flow
Management, at the Fachhochschule Trier, the Dept. of Chemistry at the University of Kaiserslautern and the Institute for Urban Water Supply Management at the RWTH-University Aachen.
12.3.2.6 Antaco
Antaco is an SME currently based in Guildford in the UK. It has the
expressed aim of developing technology to convert any type of biomass
with any level of moisture content into a solid biocoal within 4 to 10 h in a
continuous and efficient manner. The company claims that their process is
energy efficient in that it uses only ca. 12–15% of the energy produced to
power itself. The company is receiving extensive exposure in the UK, having won a Climate-KIC Venture Competition in 2012, as well as receiving
€50000 funding from Climate-KIC to develop a bench-scale model of their
process. The company has also been a national finalist and winner in the
Shell Springboard competition (winning a ca. €50000 regional award) and
has also been awarded €1m from the Department for Energy and Climate
Change to design a prototype plant with a water company, turning sewage
from 700 homes into biocoal.
12.3.2.7 Ingelia
In 2006, Ingelia began operation and the development of innovative solutions for organic waste processing in Nàquera, Valencia, Spain. The company operates with the stated aim to develop sustainable exploitation of
local resources through the HTC of indigenous biomass. Following several
years of development and engineering, the Ingelia prototype plant was constructed in June 2010. The commissioning of the plant took place between
June 2010 and August 2010. Since July 2010, the company has been operating
an industrial HTC plant utilising vegetable biomass feedstocks from pruning, gardening, agricultural and forestry sources. The conversion of these
otherwise low-value wastes into higher-value products is the intention of the
company, with the current plant designed and constructed to process any
type of organic waste. The desired product, biocoal is claimed to possess a
gross calorific value of ca. 24 MJ kg−1, which can then be used in electricity
generation, whilst the HTC byproducts are claimed to have a fertilising effect.
Ingelia was selected in 2013 as “Best Innovation” in the category “Public Private Partnership” on the V edition of “Innovadores-2013” Awards organised by
the Spanish newspaper “El Mundo”. With regards to local collaborations, the
company maintains cooperation with the Instituto de Tecnología Química
of the Universidad Politecnica de Valencia, headed by Prof. Avelino Corma.
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Ingelia is also investigating the extraction of a range of terpene compounds
during the HTC process to add further value and potentially the production
of sustainable solvents (e.g. p-cymene).
12.4 Summary and Outlook
Complimentarily to the move that developed “Starbon®” and “hydrothermal
carbonisation” platforms for porous carbon material production from sustainable precursors, a number of newer, emerging approaches to the utilisation of biomass and biomass-derivatives in the preparation of functional,
porous materials were introduced. Of particular note are the use of bacterial
cellulose developed predominantly by the Hu group (Section 12.2.1) and tannin/flavonoid-based organic gels and the carbonaceous equivalents by the
Celzard group (Section 12.2.5). The former approach exploits the most abundant of the polysaccharides and is importantly not derived from a food source.
The latter is also interesting in the same regard as it lays the basis for the
exploitation of lignin fragments, paper-processing wastes and a range of aromatic compounds that can be acquired at low cost including potentially from
the wastes of large HTC processes. However, from a carbon materials point
of view, the question still remains as to how the morphology of the bacterial
cellulose is maintained in the porous carbon equivalent with a loss or “melt”
of the H-bonded structure. In the opinion of the authors, this is an important
point to address. Likewise, regarding the production of tannin- and indeed
lignin-based porous phases (e.g. aerogels), to extend their application, further
investigation regarding the carbonisation at high temperatures is needed to
examine the benefit of preparing the organic phase from an aromatic precursor, analogously to the preparation resorcinol–formaldehyde-based aerogels,
although the nanostructuration of such materials based on soft-templating
strategies seems a possible avenue to explore (Section 12.2.6).
With regards to the commercialisation of porous carbons from sustainable precursors, the growth and development of a number of “smallto-medium” enterprises (SMEs) were highlighted including the fledgling
“Starbon® Technologies Ltd”, which is pioneering the development and scale
up of functional, highly porous carbonaceous materials from polysaccharide
biomass. This product is already available through scientific suppliers (i.e.
Sigma-Aldrich) and further developments regarding the applications of these
materials (see Part 1 of this book) will only lead to further interest in these
exciting materials. Furthermore, an increasing number of SMEs are deve­
loping pilot- to industrial-scale facilities for the production of biocoal and
biochar via the hydrothermal carbonisation platform. This is a very interesting approach to the remediation and conversion of agricultural, food and
municipal wastes into a higher value and energetic materials for electricity
generation and indeed soil improvement. However, it is important to note
that the major challenge here remains the successful development of truly
continuous processes, suitable reactor design for the efficient manage­ment
of heat of reactions and therefore overall process efficiency and indeed CO2
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Chapter 12
savings (relative to conventional fuels). However, a number of companies
are pushing forward in the area and continued improvements are expected
over the next decade or so, particularly given the potential of the approach.
Furthermore, if such continuous processing can be established for the production of nanostructured, functional and highly porous versions of HTC
materials, suitable for high-value applications in energy storage and genera­
tion, this will ultimately generate further revenue streams and interest for
potential investors in this technology.
Acknowledgments
PS gratefully acknowledges the Ministerio de Ciencia e Innovacíon for the
concession of a Juan de la Cierva (JCI-2011-10836) contract.
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Subject Index
References to tables and charts are in bold type.
1-butyl-1-methylpyrrolidinium
(BMPY), 230
1-butyl-3-methylimidazolium tetrachloroferrate, 176, 390
1-ethyl-3-methylimidazolium
­tetrafluoroborate (EMIMBF),
230
13
C NMR experiments, 62, 69, 141,
145, 332, 335–6, 338, 340
19
F NMR, 338–40, 342, 347–9
2-chloroethylphenyl sulfide, 348
2-pyrrol-carboxaldehyde (PCA), 175,
243
2-thienyl carboxaldehyde, 178
4-hydroxyphenylboronic acid,
207
aberration-corrected (AC), 95
accelerating voltage, 359
acetalisation, 174, 175
acetate, 88, 171
acetic acid, 12, 109, 138, 181,
206, 207
mercaptoacetic acid, 87
trifluoroacetic acid, 109, 340,
342
acetic anhydride, 85
acetone, 15, 91, 386
acetonitrile, 114, 118, 194, 228,
229
acetophenone, 197
acetyl methyl salicylate, 84, 85
acetylacetonate, 210
acetylation, 84, 85
deacetylation, 55
acid hydrolysis, 64
acid orange, 212
activated carbons
carbon aerogels, 21
impregnation, 20
porous glassy carbon (PGC),
21
agriculture, 7, 130, 144, 157, 182,
249, 399, 402–3
air purification, 130, 211
aldol reactions, 140–1, 145, 148–9,
157, 161, 197
aldonic acid, 29
algae, 16, 263–4, 334
macroalgae, 72, 79, 137, 157
alginate, 57, 70–1, 115
alginic acid
anode materials, 249
biomass-derived, 84
commercialisation, 391,
394–5
Starbon synthesis, 73, 74–5,
76
Starbons second generation,
64, 67–8, 69, 70–2
sustainability and graphite,
116–7
sustainability in separation
science, 113–6
407
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alkylation, 84, 85, 196
alumina, 32, 87, 160, 162, 340, 344
aluminosilicate, 110
Amberlyst, 87–8, 97, 206
amination, 17
amino acids, 30, 33, 150, 195, 204,
228
aminopropyltriethoxysilane, 163
ammonia, 11, 29
ammonium dihydrogen fluoride,
33
ammonium salts, 33
amylopectin, 55, 56, 60–1, 113, 147
amylose, 6, 17, 35, 54–6, 60–1, 72,
75, 113
anaerobic conditions, 137–8, 171
anatase, 99, 212, 250
anchoring effect, 192, 210
angelica lactone, 96
aniline, 93, 194
anisotropic activation, 298
annealing, 25, 177, 253–4, 257
anode materials
alginic acid, 249
carbon nanotubes (CNT),
250–1, 253
cellulose, 248–9, 253
chitosan, 253
cyclic voltammetry (CV), 247,
256
ethanol, 254
formic acid, 248–9
furfural, 251
high-resolution transmission electron microscopy
(HRTEM), 255
hydroxymethylfurfural (HMF),
249, 254
lignin, 248
microporous, 249
microspheres, 251–2
pyrolysis, 249
scanning electron microscopy
(SEM), 252
transmission electron microscopy (TEM), 252
Subject Index
anodic alumina membranes (AAO),
162, 236
Anthropocene, 3, 11
Anthropogenic Carbon Cycle,
12, 39
aqueous phase chemistry, 37,
130
Arabidopsis thaliana, 110
aromatisation, 145, 148, 195, 197,
318, 335, 358
arylbromides, 90
ascorbic acid, 169
Aspergillus aculeatus, 381
asymmetric synthesis, 37
atomic force microscopy (AFM),
299–300
attenuated total reflectance (ATR),
325
Australia, 137
autoclaving, 166
automotive, 20, 226
aviation kerosene, 13
bacterial cellulose, 134, 335, 378–9,
382, 403
Bahia pulp, 88
ball milling, 340
Barrett-Joyner-Halenda (BJH)
­protocol, 285, 288
benzaldehyde, 92–3, 197
benzofuranes, 141
benzyl disulfide, 204
binding energy (BE), 318
biochar, 137, 143–4, 396, 397, 399,
401, 403
biodiesel, 15, 97, 98
bioethanol, 10, 182
biofuel recovery ratio, 136
biofuel synthesis
esterification, 97, 98
biofuels, 5, 7, 15, 96–7, 130, 136,
141, 171, 397, 399
biomass-derived
alginic acid, 84
hydrogenation, 83
starch, 83
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Subject Index
biomedical, 20, 109
bioreactors, 171
biorefinery, 5, 11–7, 35, 38–9, 94,
108, 130, 185, 393
Biorefinery Development Centre
(BDC), UK, 393
bisphenol, 91, 92
blue-violet solar spectrum region,
134
Boehm titration
carboxylic acid, 327
ethanol, 327
Norit carbon, 327
thermogravimetric analysis
(TGA), 327
x-ray photoelectron spectroscopy (XPS), 327
booster effect, 172
borate, 120, 174, 175
tetracyanoborate, 31
tetrafluoroborate, 230
borax, 120, 174–8, 243–4
Boron (B), 206
Bragg peaks, 293, 357
Brazil, 393
bromination
hydrogenation, 331
Norit carbon, 331
thermogravimetric analysis
(TGA), 329, 330
Brønsted acids, 67, 84–5, 328
brookite, 250
Brownian motion, 331
Brunauer-Emmett-Teller surface
area (BET), 76, 107, 161, 168, 182,
282–3, 289, 340
buckyballs
see spherical fullerenes
bulk saturation pressure, 279, 284
butanediol, 95
butanol, 15, 88, 206, 207, 218
butyrolactone, 95
calcination, 14, 183, 212, 214, 216
postcalcination, 150, 251
calcium alginate, 115
409
calcium chloride, 115
camphor, 28
Campylobacter jejuni, 110
capillary condensation, 18, 280
capillary electrochromatography,
120
capillary forces, 174
carbon molecular sieve (CMS), 104,
209
carbogels
fuel cells, 242–4
future perspectives, 120
gels, 174–5, 177–8, 180
Li-S batteries, 262
microscopy, 299, 300
microscopy examples, 370
natural systems, 183
Carbograph, 121
carbon aerogels, 25–7
activated carbons, 21
condensation, 25
fuel cells, 242
gels, 178
innovated approaches, 378–81,
386
lignin, 27
mesoporosity, 25–6
microporous, 25–6
microscopy examples, 370
nitrogen-containing, 200–1,
202
state-of-the-art, 134
transmission electron microscopy (TEM), 26
carbon black, 5, 108, 110, 121, 208,
233, 239, 323–6, 367–9
carbon capture and sequestration
(CCS), 292
carbon capture and storage (CCS),
262, 292
carbon capture and utilisation
(CCU), 7
carbon cryogel, 21
carbon dots, 184
carbon footprint, 4, 7, 11, 17
carbon monoxide, 131
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410
carbon nanotubes (CNT)
anode materials, 250–1, 253
chiral nematic phases, 35
fuel cells, 236–8, 241
future energy economics, 13–4
gels, 180
graphitic nanocarbons, 27–8,
29, 30
HTC and photocatalysis, 212
HTC metal complexes, 208
hydrogenated reactions, 96
innovated approaches, 381
microscopy, 301
microscopy examples, 368, 369
multi-walled carbon nanotubes (MWCNT), 180
nitrogen-containing, 194, 199
other catalysis, 218
other doped carbons, 206
ssNMR, 332–3
stationary phases, 119
supercapacitors, 229
carbon neutral, 3, 131
carbon sensitisation, 212
carbon-clad silica, 108
Carbonisation under Pressure, 135
Carbopack-B, 121
CarboREN, 399
carbothermal reduction, 163
carboxylic acid
Boehm titration, 327
eutectic solvents, 33
HTC metal complexes, 208
infrared (IR) spectroscopy, 325
Starbons second generation,
63
state-of-the-art, 137
stationary phases, 108
sulfonated Starbons, 84–5
thermal gravimetric analysis,
314, 316
x-ray spectroscopy, 320, 321,
322
carrageenan, 57, 78
cattle, 383
cavitation phenomena
Subject Index
cellobiose, 118
cellulase, 334
cellulose
anode materials, 248–9, 253
chiral nematic phases, 35
CO2 capture, 262, 263, 265
future energy economies, 15–7
future perspectives, 120
hydrothermal carbons, 159
innovated approaches, 378,
379, 382, 387, 390
natural systems, 182
polysaccharide-derived, 54–5
ssNMR, 332–8
state-of-the-art, 134, 136,
139–40
sulfonated Starbons, 87–8
supercapacitors, 228
cellulose nanocrystals (CNCs), 35,
36
ceramics, 360
Characterisation of Porous Materials,
304
chemical shift anisotropy (CSA),
331–3
chemical vapour deposition (CVD),
27–8, 37
chemisorption, 241–2, 265, 280, 313,
326
chicken, 28, 29, 383
chimie douce, 157
China, 132, 134–5, 393
chiral chromatography, 120
chiral nematic phases
carbon nanotubes (CNT), 35
cellulose, 35
etching, 35
transmission electron microscopy (TEM), 36
chitin, 16, 35, 54, 55, 157, 199, 387
chitosan
anode materials, 253
HTC and photocatalysis, 216
nanocomposites, 184
nitrogen-containing, 196–7,
198
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Subject Index
polysaccharide-derived, 55, 57
Starbons second generation,
66–7
chlorobenzene, 91
chloroform, 118, 182, 348
chloroplatinic acid, 240
chlorosulfonic acid, 85, 88
chocolate, 28
chocolate HILIC, 117–9, 122
choline chloride (ChCl), 33
chord-length distribution (CLD), 297
chromatographic applications
commercialisation, 394
future perspectives, 120
general aspects, 19
other sustainable carbons, 119
Starbon synthesis, 77
Starbons second generation,
71
stationary phases, 107–9
sustainability and graphite,
116–7
sustainability in separation
science, 111, 113–4
Chromolith, 161
chronoamperometry, 203, 238
chronopotentiometric, 238
Cinnamomum camphora, 28
circular dichroism spectroscopy
(CDS), 35–6
citrus peel, 63
Clausius-Clapeyron approach, 290
Cleancoal, 396, 397, 398
cleavage behavior, 359
Climate-KIC Venture Competition,
402
CO2 capture
cellulose, 262, 263, 265
mesoporosity, 264
microporous, 264–5
starch, 262, 263
coal-fired power stations, 4
coalification, 5, 7–8, 334
cobalt (Co), 232
cobalt phthalocyanine, 241
cockroach legs, 28
411
coconut, 20, 28, 311
cocoon, 385, 386
co impregnation, 207
coin-like hollow carbon (CHC), 208,
239, 240
coking, 13
commercialisation
alginic acid, 391, 394–5
chromatographic applications,
394
esterification, 394–5
food industry, 396
microporous, 391
sucrose, 394
composite-molten-salt (CMS), 104,
209
condensation
carbon aerogels, 25
eutectic solvents, 33–4
gas adsorption, 280–2, 284–5
general aspects, 18
HTC and photocatalysis, 214
HTC formation, 145, 148–9
humins, 141
infrared (IR) spectroscopy, 326
innovated approaches, 385
mesoporous carbons, 22
microscopy examples, 362, 365
nitrogen-containing, 195,
197–8
ssNMR, 335
Starbon synthesis, 74
state-of-the-art, 140
coniferyl alcohol, 333
Continuous Thermal Hydrocarbonisation approach, 144
cookies, 28
coral-like structures, 171–3, 173, 366
corn
cobs, 230
ethanol, 137
oil, 15
stalk, 119
starch, 56, 60, 111, 113
corrosion, 234
coumaryl alcohol, 333
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412
Subject Index
crosscoupling reactions, 90, 217, 333
crosslinking procedures, 22, 58, 74,
185, 318, 380
crosspolarisation experiments, 331,
336
crude oil, 15
crushing, 359
crustacean shells, 8, 182, 196,
199–200
cryogels, 53–5
cryoporometry, 279, 284, 302–3
crystal lattice strain, 13
cyanoethyl(trimethoxy)silane, 84
cyanuric acid, 201, 244
cyclic voltammetry (CV)
anode materials, 247, 256
fuel cells, 235–6, 238, 244, 247
HTC metal complexes, 208,
210
innovated approaches, 382
linear scanning voltammetry,
209
Na-Ion batteries, 259, 260
nitrogen-containing, 201, 203
supercapacitors, 228, 229,
230–2
cyclic voltammograms, 202, 211,
229, 230–1, 240, 243, 247
cycloaddition, 31, 199
cyclodextrin, 118, 120, 164, 168, 192,
231
cyclohexadiene, 197
cyclohexane, 218
cyclohexanone, 206, 207, 217, 218
cyclohexene, 84, 85, 90
cyclopentenones, 117
cysteine, 177, 204, 205, 242
cytotoxicity, 184
Department for Energy and Climate
Change, 402
Department of Chemical and Biological Engineering, Buffalo USA, 140
dextran, 29
diamagnetic, 348
diamond knife, 359
diazonium coupling, 87, 91
dichlorophenol (DCP), 212
dicyanamide, 31
Diels-Alder reactions, 148, 197
diesel, 12
see also biodiesel
diethyl succinate, 84
diethyl carbonate, 245
differential scanning calorimetry
(DSC), 303, 332
diffuse reflectance infrared spectroscopy (DRIFT), 62–3, 67, 69, 72, 73,
324
dimethyl carbonate, 255
dimethyl ether, 12
dimethylpyridine, 84
dioxo-6-hydroxyhexanal (DHH), 140
dip coating, 31
dipolar coupling, 331
direct emulsions, 169
direct methanol fuel cell (DMFC),
13, 209, 233–41
direct synthesis, 24, 30, 91, 94, 166
dog, 28
drug delivery, 57, 115, 171, 183, 389
dry nitrogen, 136
dual-templating approach, 136, 172,
365–6
Dubinin-Radushkevich (DR), 283,
340
dye sensitisation, 212
deacetylation, 55
deep eutectic solvents (DESs), 33–4
dehydrogenation, 6, 17, 331
dendrite formation, 245
density functional theory (DFT),
283, 285–6, 288–9, 305, 334,
384
eddy diffusion, 106, 114
egg protein, 14
elastic deformations, 290
electrocatalytic, 178, 199–201, 204,
208, 210–1, 236–8, 241, 244
electrochemical double-layer capacitors (EDLCs), 226–9, 231
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Subject Index
electrochemical impedance spectroscopy, 232, 256, 258
electroconductivity, 37, 210, 228
electrolysis, 12, 209–10
electromagnetic, 18, 342–4
electron beam irradiation, 359
electron energy loss spectroscopy
(EELS), 70, 117, 299, 358, 371
electron spectroscopy for chemical
analysis (ESCA)
see x-ray photoelectron spectroscopy (XPS)
electron-holes, 99, 211–2, 215–6
electrospinning, 31, 253
electrospray ionisation (ESI), 109,
114, 200
electrospraying, 31
electrostatic forces, 161, 226, 342–4,
347
enantiomers, 7, 296
end-of-life material recovery, 192
end-on adsorption. see Pauling model
Energiewende, 3
energy-dispersive x-ray spectroscopy
(EDX), 196, 299
Engineering & Physical Science
Research Council (EPSRC), UK, 392
ENVI-Carb, 121
Environment and Development
summit, 9
environmental footprint
see carbon footprint
Epiactis prolifera, 263
epoxy resin, 359
Escherichia coli, 110
esterification
biofuel synthesis, 97, 98
commercialisation, 394–5
future energy economics, 17
introduction to mesoporosity,
88
other doped carbons, 207
Starbons second generation, 63
sulfonated Starbons, 84, 85–6
transesterification, 88, 97, 161,
197
413
estrogen, 110
etching
chiral nematic phases, 35
gels, 174
HTC and photocatalysis, 214
innovated approaches, 385
microscopy examples, 361
natural systems, 182, 183
ordered materials, 94
polystyrene latex dispersions,
171
templating hard, 160, 162–3,
164, 165–6
ETEK catalyst, 236, 241
ethanol
anode materials, 254
Boehm titration, 327
fuel cells, 239–40
future energy economies, 15
HTC and photocatalysis, 212
HTC metal complexes,
208–10
hydrogenated reactions, 94
polysaccharide-derived, 56
state-of-the-art, 134, 137
sulfonated Starbons, 84–5, 86
supercapacitors, 229
TEM technique, 368
templating hard, 163
thermal gravimetric analysis,
316, 317
ethyl oleate, 85
ethylene, 12–3, 168, 240, 245, 255,
367
ethylene carbonate, 245, 255
ethylimidazolium, 175
eucalyptus, 28, 159, 262, 263
eutectic mixtures, 33, 34, 37, 176
eutectic solvents
carboxylic acid, 33
condensation, 33–4
furfural, 35
scanning electron microscopy
(SEM), 34, 35
transmission electron microscopy (TEM), 34
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evaporation, 35, 163–6, 174, 176,
313, 314, 368, 387
excess-solution impregnation, 208
face-centred cubic (FCC), 332
faeces, 28
Faradaic charge transfer, 226,
228
fatty acid methyl esters (FAMEs), 97,
98
feedstocks, 4, 9, 15–7, 88, 137–8,
141, 402
fermentation, 7, 17, 82, 137, 395
Fermi level, 206
ferrocene, 28, 111, 112
ferulic acid, 334
Fischer–Tropsch synthesis (FT), 12,
218
fixed pore shape, 287
flavonoids, 27, 179, 385–6, 403
flavour compounds, 17
flue gas, 289
fluorine, 340, 342–4, 346, 349
foams, 160, 169, 171, 299, 387
focused ion beam (FIB), 360
food industry
commercialisation, 396
future energy economics, 16
graphitic nanocarbons, 28
innovated approaches, 383
macroemulsions, 168
natural systems, 182–4
Starbons second generation,
43
state-of-the-art, 136, 138
sustainability in separation
science, 115
forestry industry, 77, 402
formaldehyde
gels, 34
phenol mixture, 22, 88, 166,
368
polycondensation, 33, 385
porous graphitic carbon (PGC),
110
reduction, 236
Subject Index
resorcinol mixture, 25, 57, 78,
166, 368–7, 403
synthesis, 12
tannin mixture, 387
formic acid
anode materials, 248–9
HTC formation, 145, 147, 148
HTC metal complexes, 208
hydrogenation reactions, 96
natural systems, 182, 183
stationary phases, 109
sustainability in separation
science, 114
fossil based industries, 4
fossil fuels, 3–4, 11, 129, 225, 289,
311
Fourier transform (FT), 68, 140, 148,
196, 239, 316, 335
fractures, 359
framework density
see skeletal density
France, 393
free fatty acids (FFA), 97
freeze-dried bacterial nanocellulose
aerogel (FDBNC), 380
Friedel-Crafts reactions, 89
fructosamine, 118
fructose
future perspectives, 120
gels, 174, 176
graphitic nanocarbons, 29
HTC formation, 145, 147
innovated approaches, 390
mesoporous carbons, 24
microscopy examples, 363,
366
ssNMR, 336
state-of-the-art, 136, 139–41
stationary phases, 110
templating soft, 167
fructose-6-phosphate, 110
fuel cells
carbogels, 242–4
carbon aerogels, 242
carbon nanotubes (CNT),
236–8, 241
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Subject Index
cyclic voltammetry (CV), 235–
6, 238, 244, 247
ethanol, 239–40
hard templating, 236
high-resolution transmission electron microscopy
(HRTEM), 235, 236–7, 239
lignin, 241
macroporous, 236
microporous, 237
microspheres, 238–40
ovalbumin, 242
oxygen-reduction reaction
(ORR), 233, 237, 241–4
pyrolysis, 242–3
starch, 238
sucrose, 234, 235, 236
transmission electron microscopy (TEM), 235, 237, 239
fullerenes, 5, 35, 70–1, 117, 332
fungi. see fungus
fungus, 182, 184, 230, 381, 383
Aspergillus aculeatus, 381
furanoic compounds, 335
furanose, 139
furfural
anode materials, 251
eutectic solvents, 35
gels, 178
HTC and photocatalysis, 212,
214
HTC formation, 145, 147
HTC metal complexes, 210
macroemulsions, 170
mesoporous carbons, 22–3
microscopy examples, 361
other catalysis, 217
ssNMR, 336
Starbon synthesis, 74
templating hard, 162–3
future energy economics
carbon nanotubes (CNT), 13–4
esterification, 17
food industry, 16
hydrogenation, 11–3, 15–7
impregnation, 14
415
future energy economies
cellulose, 15–7
ethanol, 15
lignin, 14–7
mesoporosity, 13–4
nitrogen-doped carbons
(NDC), 14
ovalbumin, 14
sucrose, 15
future perspectives
carbogels, 120
cellulose, 120
chromatographic applications,
120
fructose, 120
macroporous, 120
mesoporosity, 120–2
ovalbumin, 120
porous glassy carbon (PGC),
121–2
pyrolysis, 120
soft templating, 120
future proof, 7
galacturonic acid, 63
galvanostatic charge-discharge
cycling, 232
gas adsorption
condensation, 280–2, 284–5
macroporous, 279–80, 282–3
mesoporosity, 283
microporous, 279–81, 282–3,
284, 286–90
gas physisorption, 280
gas separation, 5, 104
gas-chromatography (GC), 120, 137,
140–1, 316, 335, 339
gasification, 7, 20, 159
Gaussian peak, 318
gelatin, 383–5
gelatinisation, 57, 60–1, 111
gelation, 55, 63–4, 67, 76, 174, 176–
80, 184–5, 378, 386–7
gels
carbogels, 174–5, 177–8, 180
carbon aerogels, 178
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416
gels (continued)
carbon nanotubes (CNT), 180
etching, 174
fructose, 174, 176
furfural, 178
hydroxymethylfurfural (HMF),
174, 178, 179
microporous, 178
microspheres, 178–9
nitrogen-doped carbons
(NDC), 175, 177, 179
ovalbumin, 177–8
oxygen-reduction reaction
(ORR), 178
pyrolysis, 173, 177
scanning electron microscopy
(SEM), 173, 177, 179
starch, 176
transmission electron microscopy (TEM), 173, 175, 177,
179–81
x-ray photoelectron spectroscopy (XPS), 177
general aspects
chromatographic applications,
19
condensation, 18
macroporous, 18
microporous, 18–20
geothermal, 4
German Renewable Energy Law, 401
Germany, 3, 132, 140, 143–4, 396,
397, 398–403
Gibbs-Thompson equation, 303
Ginkgo biloba, 30
global warming, 129, 225
Gluconacetobacter xylinus, 334
glucopyranose, 56, 72
polyglucopyranose, 55
glucosamine, 66, 149, 157, 178, 196–
8, 201, 231, 244
N-acetylglucosamine, 171, 257
glucosamine chloride, 195
glucosamine hydrochloride, 178
glucose-6-phosphate, 110
glycerol, 97, 142
Subject Index
glycine, 30, 149, 195, 196
glycosidic bonds, 17–8, 64, 334
gold (Au), 89, 91, 92, 163, 360–1
grains, 137, 297, 298, 370
grand-canonical Monte-Carlo
(GCMC), 287, 289
granule ghosts, 60
graphene oxides (GO), 27–30, 99, 381
see also reduced graphene
oxides
graphitic carbon nanocoils (GCN),
236
graphitic nanocarbons
carbon nanotubes (CNT), 27–8,
29, 30
food industry, 28
fructose, 29
high-resolution transmission electron microscopy
(HRTEM), 29
pyrolysis, 28
scanning electron microscopy
(SEM), 29
thermogravimetric analysis
(TGA), 29
graphitised lace-like carbon (GLC),
239–40
graphitised thermal carbon black
(GTCB), 108
grass, 28, 184
Green Chemistry Centre of Excellence, 56, 58, 121, 391
Green Chemistry Principles, 6–11,
24, 82, 165–6, 172, 192, 226
Green Chemistry: Theory & Practice, 8
greenhouse gases (GHG), 3, 7, 14,
39, 93, 129, 266, 396, 399
guluronic acid, 67, 75
gum arabic, 244
hand milling, 261
Handbook of Porous Solids, 304
hard templating, 160, 161, 165
fuel cells, 236
ionic liquids, 32
mesoporous carbons, 21, 22
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Subject Index
microscopy examples, 361
Starbons first generation, 61
stationary phases, 110
hazelnut shells, 249
heat of adsorption, 290, 292
Hefei National Laboratory, China, 144
hemicellulose, 15–7, 55, 182, 248
hemp, 230
heptafluorobutyric acid, 109
hexane, 112, 118, 316, 317
cyclohexane, 218
hibiscus flower, 28
high internal phase emulsions
(HIPEs), 168–71, 387
high-angle annular dark field
(HAADF), 95, 358, 369
high-performance anion exchange
chromatography (HPAEC), 109
high-performance liquid chromatography (HPLC), 22, 104–8, 114–6,
119–22, 140
high-resolution scanning electron
microscopy (HRSEM), 92
high-resolution transmission electron microscopy (HRTEM)
anode materials, 255
fuel cells, 235, 236–7, 239
graphitic nanocarbons, 29
microscopy, 299
ordered materials, 92
other sustainable carbons, 119
Starbons second generation, 70
TEM technique, 357
high-temperature synthesis, 38
histidine, 195
hollow carbon nanospheres (HCNs),
247–8, 259, 261–2
holography
honeycomb fashion, 27
Horvath-Kawazoe (HK), 283
HTC and photocatalysis
carbon nanotubes (CNT), 212
chitosan, 216
condensation, 214
etching, 214
ethanol, 212
417
furfural, 212, 214
sucrose, 216–7
x-ray photoelectron spectroscopy (XPS), 213
HTC formation
condensation, 145, 148–9
formic acid, 145, 147, 148
fructose, 145, 147
furfural, 145, 147
hydroxymethylfurfural (HMF),
145, 147–8
ovalbumin, 149
starch, 145, 147
sucrose, 147
HTC metal complexes
carbon nanotubes (CNT), 208
carboxylic acid, 208
cyclic voltammetry (CV), 208,
210
ethanol, 208–10
formic acid, 208
furfural, 210
impregnation, 210
microspheres, 208
oxygen-reduction reaction
(ORR), 209
starch, 210
sucrose, 208
humic acid, 75
humins
condensation, 141
lignin, 143
pyrolysis, 141
hydrofluoric acid, 110
hydrogen bond acceptor (HBA), 344,
349
hydrogen bond donor (HBD), 344,
349
hydrogen economy, 11
hydrogenated reactions
carbon nanotubes (CNT), 96
ethanol, 94
hydrogenation, 94–6
starch, 95
transmission electron microscopy (TEM), 95, 97
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hydrogenation
biomass-derived, 83
bromination, 331
future energy economics,
11–3, 15–7
hydrogenated reactions, 94–6
introduction to mesoporosity,
89
ordered materials, 91, 93
other catalysis, 217–8
state-of-the-art, 132
hydrogenation reactions
formic acid, 96
hydrophilic interaction liquid
­chromatography (HILIC),
117–9, 122
hydrothermal carbons
cellulose, 159
microporous, 160
pyrolysis, 159
starch, 159
hydrothermal gels, 158, 174, 180
hydroxymethylfurfural (HMF)
anode materials, 249, 254
gels, 174, 178, 179
HTC formation, 145, 147–8
hydroxymethylfurfuraldehyde,
140
ssNMR, 336, 338
Starbon synthesis, 74
state-of-the-art, 140
hydroxymethylfurfuraldehyde,
140
hydroxyvaleric acid (HVA), 96
Hypercarb, 108–11, 113, 116–7,
121–2, 395
hypersaline conditions, 176
Hypersep, 121
imidazole, 161, 196–7
vinylimidazole, 161
imidazolium, 31, 196–7, 218, 390
ethylimidazolium, 175
methylimidazolium, 176, 230,
390
imidazolium bromide, 169, 175
Subject Index
impregnation
activated carbons, 20
excess-solution impregnation,
208
future energy economics, 14
HTC and photocatalysis, 216
HTC metal complexes, 210
introduction to mesoporosity,
89
ionic liquids, 30, 32
mesoporous carbons, 22
microscopy examples, 361–2,
368
other doped carbons, 207
photocatalysis, 99
stationary phases, 110
templating hard, 160, 163
incipient-wetness impregnation, 207
Industrial Symbiosis Concept, 137
infrared (IR) spectroscopy
carboxylic acid, 325
condensation, 326
Innovadores competition, 402
innovated approaches
carbon aerogels, 378–81, 386
carbon nanotubes (CNT), 381
cellulose, 378, 379, 382, 387, 390
condensation, 385
cyclic voltammetry (CV), 382
etching, 385
food industry, 383
fructose, 390
lignin, 386–7, 389
mesoporosity, 383
microporous, 384–5
microspheres, 387
nitrogen-doped carbons
(NDC), 381, 382, 384, 387
porous glassy carbon (PGC),
392, 395
pyrolysis, 380, 383
scanning electron microscopy
(SEM), 379–80, 383, 386, 388
starch, 390–1, 393–5
transmission electron microscopy (TEM), 384, 385, 386
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Subject Index
Instituto de Tecnologia Quimica of
the Universidad Politecnica de
Valencia, Spain, 402
internal combustion engines, 12, 225
International Symposium on the Characteristics of Porous Solids, 304
International Union of Pure and
Applied Chemistry (IUPAC), 18,
25, 278, 281
interstitial spaces, 174–5
introduction to mesoporosity
esterification, 88
hydrogenation, 89
impregnation, 89
microporous, 89
inverse gas chromatography (IGC),
339
ion thinning, 359–60
ionic liquids
hard templating, 32
impregnation, 30, 32
microporous, 31
nitrogen-doped carbons
(NDC), 30–1
pyrolysis, 31
scanning electron microscopy
(SEM), 32
soft templating, 30
transmission electron microscopy (TEM), 32
ionothermal carbonisation, 176, 390
iron nitrate, 218
irradiation, 96–7, 98, 318, 358–9
isosorbides, 15
isotherm profile, 18
itaconic acid, 96
Japan, 135
Karlsruhe Institute of Technology
(KIT), Germany, 143
Kavli Prize, 5
Kelvin equation, 284
kernel, 287–8
kerogens, 334, 336
kerosene, 13
419
Kevlar, 301
Knoevenagel reactions, 161, 197–8,
199
Korringa relationship, 333
Koutecky-Levich plots, 242–3
Kraft-processed hardwood, 24, 89, 387
lactic acid, 15
polylactic acid, 7
lactones, 29, 68, 96, 312, 314, 318,
320, 322, 325–8
angelica lactone, 96
butyrolactone, 95
valerolactone, 96
LaMer model, 157
Langmuir model, 282, 290
latex nanoparticles, 136, 172, 247,
365–6
lattice fringes, 235
layered double hydroxide (LDH), 218
leather waste residue, 136, 385
Lennard-Jones approaches, 286–7
levoglucosan, 74
levulinic acid, 96–7, 141, 145, 147,
148–9, 157, 316
Lewis acids, 84, 171, 176
Li-ion batteries (LIBs), 245–51,
253–8, 261
Li-S batteries
carbogels, 262
macroporous, 262
mesoporosity, 262
lignification, 333
lignin
anode materials, 248
carbon aerogels, 27
fuel cells, 241
future energy economies, 14–7
humins, 143
innovated approaches, 386–7,
389
mesoporous carbons, 24
microscopy, 301
natural systems, 182
ordered materials, 89–90
ssNMR, 332–6
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lignocellulosic biomass, 5–6, 15–6,
137, 159, 338, 378
linear scanning voltammetry, 209
linear solvation energy relationships
(LSER), 339, 342
liquefaction, 7
liquid chromatography (LC), 70,
109, 114, 116, 118, 121, 148
liquid electrolyte decomposition, 261
liquid sealing effect, 238–9
liquid-solid mass transfer, 106
lithiated graphite, 254, 258
lithiated silicon, 254
lithium metal oxide, 245
lithium-sulfur batteries (LSBs), 161,
261–2
loblolly pine, 137
Lobry de Bruyn-Alberda van Ekenstein isomerisation, 145
lobster shells, 182, 196, 199–200
longitudinal diffusion, 106, 114, 369
lotus, 28, 230
Loughborough University, UK, 144
low-pressure hysteresis, 283, 284
lower critical solution temperature
(LCST), 163
ludox nanoparticles, 32
Lupinus albus, 110
lyotropic phases, 35, 363
lysine, 215
macroalgae, 72, 79, 137, 157
macroemulsions
food industry, 168
furfural, 170
macroporous, 169
microporous, 168, 170
oxygen-reduction reaction
(ORR), 171
pyrolysis, 170, 171
scanning electron microscopy
(SEM), 170
transmission electron microscopy (TEM), 170
macroporous
fuel cells, 236
future perspectives, 120
Subject Index
gas adsorption, 279–80, 282–3
general aspects, 18
Li-S batteries, 262
macroemulsions, 169
mesoporous carbons, 23
microscopy, 299
microscopy examples, 366
other methods, 303
templating hard, 160–1
macroporous casts, 160
magic-angle spinning (MAS), 62, 68,
69, 145, 147, 149, 304, 331–40,
342, 349
magnetic properties, 100, 183, 278,
304, 313, 331, 333, 349
diamagnetic, 348
electromagnetic, 18, 342–4
paramagnetic, 333, 348
superparamagnetic, 88
magnetically separable Starbons
(MAGBON), 100
Maillard reaction, 118–9, 150, 177,
204
malononitrile, 197, 199
maltose, 114, 147
mannitol, 116
mannuronic acid, 67, 75
manometric, 281
manures, 139
mass transfer, 13, 19, 24, 89, 111,
114, 120–1, 235–6
liquid-solid mass transfer, 106
trans-particle mass transfer,
106
materials gap, 37–8
Max Planck Institute for Colloids
and Interfaces (MPIKG), 132,
400–1
mayonnaise, 168
medicines, 17, 54, 100
see also pharmaceutical
melamine, 194
membrane fuel cells, 13, 193, 367,
369
meniscus, 284–5
Menschutkin reaction, 31
mercaptoacetic acid, 87
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Subject Index
mercaptoethanol, 206, 207
mercury (Hg), 284, 303, 390
mesoporosity, 88–9
carbon aerogels, 25–6
CO2 capture, 264
future energy economies, 13–4
future perspectives, 120–2
gas adsorption, 283
innovated approaches, 383
Li-S batteries, 262
mesoporous carbons, 21–2
microscopy, 299
other methods, 303
polysaccharide-derived, 56–7
Starbons first generation, 61
Starbons second generation,
64
Starbons synthesis, 74, 76
sustainability in separation
science, 115
mesoporous carbons
condensation, 22
fructose, 24
furfural, 22–3
hard templating, 21, 22
impregnation, 22
lignin, 24
macroporous, 23
mesoporosity, 21–2
microporous, 22
microspheres, 22
porous glassy carbon (PGC),
22
soft templating, 21, 23–5
sucrose, 22, 23
transmission electron microscopy (TEM), 23
mesoporous hollow spheres, 160
mesoporous microspheres, 160
mesoporous-carbon nitride (MCN),
92
metal nanoparticles (MNPs), 132
metastable, 58, 169, 285, 334
methane, 27, 91, 93, 199, 210
methanol crossover effect, 200
methanol cycling, 7
methanol economy, 11–5
421
methanol tolerance, 209, 243–4
methanol-to-gasoline (MtG), 12–3
methanol-to-olefins (MtO), 13
methyl oleate, 87
methyl orange, 212, 213
methylbisulfate, 200
methylene blue, 56, 77, 163
methylene bridge loss, 318
methylimidazolium, 176, 230, 390
methylpyrrolidinium, 230
methylsilylation, 361
methylsuccinic acid, 96
methyltetrahydrofuran (MTHF), 96
micelle templating, 89
microemulsions, 167, 169
microfissures, 359
microporous
anode materials, 249
carbon aerogels, 25–6
CO2 capture, 264–5
commercialisation, 391
fuel cells, 237
gas adsorption, 279–81, 283,
284, 286–90, 292
gels, 178
general aspects, 18–20
hydrothermal carbons, 160
innovated approaches, 384–5
introduction to mesoporosity,
89
ionic liquids, 31
macroemulsions, 168, 170
mesoporous carbons, 22
microscopy, 300–1
microscopy examples, 365
nitrogen-containing, 201
other methods, 304
polysaccharide-derived, 57
polystyrene latex dispersions,
172
scattering, 293, 297–8
Starbon synthesis, 74
Starbons first generation, 63
Starbons second generation,
67, 70
supercapacitors, 229
templating hard, 161
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422
microscopy
carbogels, 299, 300
carbon nanotubes (CNT), 301
high-resolution transmission electron microscopy
(HRTEM), 299
lignin, 301
macroporous, 299
mesoporosity, 299
microporous, 300–1
scanning electron microscopy
(SEM), 299, 300
transmission electron
­microscopy (TEM),
299–300, 302
microscopy examples
carbogels, 370
carbon aerogels, 370
carbon nanotubes (CNT), 368,
369
condensation, 362, 365
etching, 361
fructose, 363, 366
furfural, 361
hard templating, 361
impregnation, 361–2, 368
macroporous, 366
microporous, 365
nitrogen-doped carbons
(NDC), 370
ovalbumin, 370
scanning electron microscopy
(SEM), 364, 366
soft templating, 365
transmission electron microscopy (TEM), 361–3, 364, 365,
366, 367–70
microspheres
anode materials, 251–2
fuel cells, 238–40
gels, 178–9
HTC metal complexes, 208
innovated approaches, 387
mesoporous carbons, 22
state-of-the-art, 132
sulfur-doped carbons, 204
Subject Index
supercapacitors, 232
templating hard, 160–1
microtome, 252, 299
see also ultramicrotome
microwave, 56–7, 96–7, 98, 114, 165,
210, 217
microwave-induced nanotubes
(MINT), 96
milk, 168
milling, 261, 340, 359, 365
see also ion thinning
mimosa, 385
Mobil gas, 12–3
molten carbonate, 233
monolignol glucosides, 334
montmorillonite, 85
multi-walled carbon nanotubes
(MWCNT), 180, 302, 333
N-acetylglucosamine, 171, 257
Na-Ion batteries
cyclic voltammetry (CV), 259,
260
transmission electron microscopy (TEM), 260
nanoadditive, 400
nanoanalysis, 358
nanobelts, 254
nanocasting, 23, 30–2, 89, 94, 110,
131, 160, 262, 356
nanocellulose, 378, 380
nanochannels, 209, 237
nanocluster, 253
nanocoils, 235–6
nanocolloids, 252, 261
nanocomposites, 158, 193, 216, 219,
251, 255–6, 381
chitosan, 184
nitrogen-doped carbons
(NDC), 184
nanoconstruction, 253
nanocrystals, 35, 56, 168, 213–5,
232, 254
nanodiffraction, 358
nanometre, 57, 66, 163, 169, 193,
196, 284, 288, 358
subnanometre, 298
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Subject Index
nanopores, 259, 362
nanoporosity, 39, 64, 259
nanorods, 64, 91, 232, 253
nanoscopic cavities, 246
nanosponges, 177
nanostructural, 355–6, 358–60, 363,
371
nanostructuration, 72, 207, 358, 403
nanotechnology, 20, 333, 356
nanotubes
see carbon nanotubes (CNT)
nanowall, 253
naphthquinone, 197
natural gas, 129, 225
natural systems
carbogels, 183
cellulose, 182
etching, 182, 183
food industry, 182–4
formic acid, 182, 183
lignin, 182
nitrogen-doped carbons
(NDC), 181–2
pyrolysis, 181–2, 183
scanning electron microscopy
(SEM), 183–4
neem oil, 28
network melt, 60
Netzsch 409, 315
nickel, 28, 235
nickel nitrate, 235
nitric acid, 119
nitrogen-containing
carbon aerogels, 200–1, 202
carbon nanotubes (CNT), 194,
199
chitosan, 196–7, 198
condensation, 195, 197–8
cyclic voltammetry (CV), 201,
203
microporous, 201
nitrogen-doped carbons
(NDC), 195, 198–9, 200–2
ovalbumin, 196
oxygen-reduction reaction
(ORR), 199–201
423
scanning electron microscopy
(SEM), 198, 201–2
transmission electron microscopy (TEM), 196, 201–2
x-ray photoelectron spectroscopy (XPS), 198, 200–1
nitrogen-doped carbons (NDC)
future energy economies, 14
gels, 175, 177, 179
innovated approaches, 381,
382, 384, 387
ionic liquids, 30–1
microscopy examples, 370
nanocomposites, 184
natural systems, 181–2
nitrogen-containing, 195, 198–
9, 200–2
Starbons second generation,
66–7
state-of-the-art, 137
sulfur-doped carbons, 204
templating hard, 165
Nobel Prize, 5, 131–2, 254
nonlocal density functionals
(NLDFT), 286–8, 289
nontoxic, 29, 211, 385
nonvulcanised rubber, 171
Norit carbon
Boehm titration, 327
bromination, 331
photocatalysis, 99
solvation energy relationship,
344, 346, 347, 348
Starbon synthesis, 77
sulfonated Starbons, 85
x-ray photoelectron spectroscopy (XPS), 319, 323
nucleation chemistry, 60, 117, 157,
163, 165
nut shells, 19
Nyquist plots, 259
octane, 12–3
oil-in-water emulsions, 168–70
olefinic groups, 62, 68, 74
olefins, 12–3, 62, 68, 74
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424
oleic acid, 85, 86
oleophilicity, 378
olive stones, 19, 89
olivine, 257
one-pot, 132, 165, 211, 218, 238,
242–3, 253, 385
opal-like structures, 172, 365–6
open-circuit voltage, 210
orange peel, 72
ordered materials
etching, 94
high-resolution transmission electron microscopy
(HRTEM), 92
hydrogenation, 91, 93
lignin, 89–90
sucrose, 89
thermogravimetric analysis
(TGA), 93
transmission electron microscopy (TEM), 90
ordered mesoporous carbons
(OMC), 22, 110, 166–7, 204, 206
ordered porous carbon materials
(OPCs), 89, 91, 92, 93, 100
osmium tetroxide, 56
other catalysis
carbon nanotubes (CNT),
218
furfural, 217
hydrogenation, 217–8
scanning electron microscopy
(SEM), 217
transmission electron microscopy (TEM), 217
other doped carbons
carbon nanotubes (CNT), 206
esterification, 207
impregnation, 207
oxygen-reduction reaction
(ORR), 207
sucrose, 207
other methods
macroporous, 303
mesoporosity, 303
microporous, 304
Subject Index
other sustainable carbons
chromatographic applications,
119
high-resolution transmission electron microscopy
(HRTEM), 119
porous glassy carbon (PGC), 119
transmission electron microscopy (TEM), 119
out-of-plane bending vibrations, 62
ouzo, 168
ovalbumin
fuel cells, 242
future energy economies, 14
future perspectives, 120
gels, 177–8
HTC formation, 149
microscopy examples, 370
nitrogen-containing, 196
supercapacitors, 228
oxathioketalisation, 205, 206, 207
oxygen-reduction reaction (ORR)
fuel cells, 233, 237, 241–4
gels, 178
HTC metal complexes, 209
macroemulsions, 171
nitrogen-containing, 199–201
other doped carbons, 207
sulfur-doped carbons, 204, 206
templating hard, 165
palm, 28, 136–7
paper mills, 157
parallel reaction kinetics, 84
paramagnetic, 333, 348
Pauling model, 241
peak efficiency, 105–6
peat, 132, 311
Pekala gels, 385, 387
pentanediol (PDO), 96
pentanoic acid, 96
percolation path, 261
Percus-Yevick approaches, 295
perfluorotoluene, 346
Periana system, 182
pesticides, 121
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Subject Index
petrorefineries, 4
pharmaceutical, 5, 7, 168
see also medicines
phase-transfer phenomenon, 182
phenolic resin, 33, 91, 110, 122, 212
phenolic sugar, 178
Phenomenex Luna, 118
phenylacetylene, 92
phenylalanine, 334
phenylpropanoid polymer, 333
phloroglucinol, 170, 178–9, 201, 244
phosphoglycerate, 110
phosphonium salts, 33
phosphoric acid, 88–9, 162, 229, 233
photocatalysis
impregnation, 99
Norit carbon, 99
photocatalytic water-splitting, 11,
212
photoelectron envelope, 200, 320,
321
photoluminescence, 134
photonic energy, 4, 55
photosynthesis, 4–6, 8, 54
photovoltaic devices, 100, 225
phthalocyanines, 194
Pickering emulsions, 169, 172
pigs, 383
piperidine, 92
plasma cleaner, 360
plastics, 12, 17
plate height, 105–6, 107, 116
plate theory, 105
Pluronic F127, 24, 120, 166–8, 172,
256, 363, 387
polar analyte separation, 39
polar retention effect on graphite
(PREG), 108, 110
polarisation measurements, 201,
202, 240, 243–4, 257, 342–3
pollens, 230
pollution, 9, 395
poly(1-vinyl-3-ethylimidazolium bromide), 175
poly(acrylonitrile) (PAN), 194, 298
poly(aniline), 194
425
poly(ethylene oxide) (PEO), 168
poly(propylene oxide) (PPO), 168
poly(vinyl alcohol) (PVA), 165
poly(vinylpyrrolidone) (PVP), 161
poly-galacturonic acid, 63
poly-N-isopropylacrylamide (PNIPAAm), 163
polyglucopyranose, 55
polyionic liquids (PILs), 390
polylactic acid, 7
polymer electrolyte membrane, 13,
233
polypseudorotaxanes, 168
polyrotaxanes, 168
polysaccharide-based gels, 54, 134
polysaccharide-derived
cellulose, 54–5
chitosan, 55, 57
ethanol, 56
mesoporosity, 56–7
microporous, 57
scanning electron microscopy
(SEM), 57
starch, 54–7, 58
transmission electron microscopy (TEM), 56
polystyrene, 23, 28, 167, 171–2, 210,
247, 366
polystyrene latex dispersions
etching, 171
microporous, 172
soft templating, 172
polyuronide, 63, 67, 75
pomelo peel, 184
pore size distribution (PSD), 158,
228–9, 237, 264–5, 283, 289, 292,
303
pore-blocking
Porod behavior, 295–7
porous glassy carbon (PGC)
activated carbons, 21
future perspectives, 121–2
innovated approaches, 392, 395
mesoporous carbons, 22
other sustainable carbons,
119
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426
porous glassy carbon (PGC)
(continued)
Starbons second generation,
70, 71
stationary phases, 108–11
sustainability and graphite,
116–7
sustainability in separation
science, 113–4, 116
porous graphitised carbon (PGC),
21–2, 70–1, 108–11, 113–4, 116–7,
119, 121–2, 392, 395
porous polysaccharide-derived
materials (PPDMs), 54, 58–9, 63,
67, 72
positron annihilation lifetime spectroscopy, 302, 304
postcalcination, 150, 251
postfunctionalisation strategies, 131
potash, 110
precipitation, 207–8, 252
Principles of Green Chemistry, 6–8,
82
printing, 31
propagylamine, 89
propandiol, 142
propylene, 12–3, 168, 199
propylene oxide, 169, 199
protolignin, 334
pseudocapacitance, 226–9, 231–2, 384
pseudographitisation, 25
ptychographic x-ray computed
tomography (PXCT), 301
pulp, 88, 230
pulse-field gradient (PFG), 304
pulverisation, 251, 359
pycnometry, 279
pyridinium, 31
pyrolysis
anode materials, 249
fuel cells, 242–3
future perspectives, 120
gels, 173, 177
graphitic nanocarbons, 28
humins, 141
hydrothermal carbons, 159
Subject Index
innovated approaches, 380, 383
ionic liquids, 31
macroemulsions, 170, 171
natural systems, 181–2, 183
ssNMR, 332–5, 338
stationary phases, 110–1
sulfonated Starbons, 85
sulfur-doped carbons, 204
sustainability and graphite,
116–7
sustainability in separation
science, 113, 115
templating hard, 161–2
templating soft, 168
x-ray photoelectron spectroscopy (XPS), 320, 322
pyrrol-carboxaldehyde (PCA), 175
pyrrolidinium, 31
methylpyrrolidinium, 230
quadropole ion trap (QIT), 114
quantum dots, 56, 136
quenched-solid density function
theory (QSDFT), 158, 288, 290
quinones, 312, 314, 325
naphthquinone, 197
raffinose family oligosaccharides
(RFO), 110
Ragone plot, 226, 227
Raman spectroscopy, 236, 239, 338
raspberry structure, 140
rattle-type hollow spheres, 231
raw sludge, 136
redox reactions, 83–4, 168, 226,
228–9, 231, 240, 250, 257–8, 261
reduced graphene oxide (RGO), 30
reflux conditions, 30
resorcinol/formaldehyde (RF) mixtures, 25–7, 33, 34, 37, 57, 77–8,
166, 179, 386–7, 403
retrogradation, 55, 60, 63
Rheinisch-Westfälische Technische
Hochschule Aachen University
(RWTH), 402
rhodamine B (RhB), 214, 217
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Subject Index
rice, 182, 183, 201, 203, 212, 229,
248–9
ring opening, 148, 318
Rio Declaration, 9
rotating-disk electrode (RDE), 203,
244
rubber, 171, 253
nonvulcanised rubber, 171
styrene butadiene rubber, 253
rutile, 210, 250
rye straw, 159, 263, 265
Saccharomyces cerevisiae, 182
sacrificial templating, 37–8
safflower plant, 119
salt-templating, 176
sawdust, 159, 228, 229, 262–3, 264
scaffolds, 110, 165, 176, 181–2, 262,
320
scanning electron microscopy (SEM)
anode materials, 252
eutectic solvents, 34, 35
gels, 173, 177, 179
graphitic nanocarbons, 29
innovated approaches, 379–80,
383, 386, 388
ionic liquids, 32
macroemulsions, 170
microscopy, 299, 300
microscopy examples, 364, 366
natural systems, 183–4
nitrogen-containing, 198,
201–2
other catalysis, 217
polysaccharide-derived, 57
Starbons second generation,
65, 66–7
state-of-the-art, 135
sulfur-doped carbons, 205
sustainability in separation
science, 112, 115
templating hard, 162, 164
scanning transmission electron
microscopy (STEM), 95, 358, 369
scanning tunnelling microscopy
(STM), 299–301
427
scattering
microporous, 293, 297–8
scattering angles, 294
scattering contrast, 294, 357
scattering pattern, 294–5, 297
scattering power, 293–4
scattering signal, 294
scattering vector, 294
Schiff base, 118
Schlenk line, 329
scotch tape, 27
seaweed, 67
selected-area electron diffraction
(SAED), 239, 300, 356–7, 371
selenium (Se), 165, 166, 193, 206
selenous acid, 165
semiconductors, 99, 211
Shell Springboard competition, 402
short-range ordering, 58, 113, 286,
295
side-on adsorption. see Yeager model
silanol groups, 107, 361–2
silica-based premade moulds, 158
silk, 385, 386
silver (Ag), 133–4, 163, 165, 193,
214–5, 217–8, 244, 361, 384
silylation, 84
methylsilylation, 361
simple cubic (SC), 332
Singapore, 136
single-walled nanotubes (SWNT),
333
skeletal density, 279
slit pores, 68
small-angle neutron scattering
(SANS), 293–4
small-angle scattering (SAS), 292–4,
297
small-angle x-ray scattering (SAXS),
293–5, 297–8, 304, 363, 364
small-to-medium enterprise (SME),
378, 402
smoothed density approximation
(SDA)
see nonlocal density
functionals
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428
sodium alginate, 115
sodium benzoate, 298
sodium borate, 120, 174
sodium carboxyl methyl cellulose,
253
sodium dodecyl sulfate (SDS), 237
sodium ethoxide, 327
sodium-ion batteries (NIBs), 258–9,
261
soft templating, 166–8
future perspectives, 120
ionic liquids, 30
mesoporous carbons, 21, 23–5
microscopy examples, 365
polystyrene latex dispersions,
172
soil aspects, 5, 139, 143–4, 396, 399,
403
sol-gel chemistry, 39, 55, 157, 172,
174, 185
solar energies, 4, 12, 100, 134, 211,
225
solid electrolyte interphase (SEI),
247, 249, 253, 256, 259
solid-phase extraction (SPE), 104,
121–2
solvation energy relationship
Norit carbon, 344, 346, 347,
348
solvothermal method, 208, 211–2,
216, 239–40, 257
sorbitol, 142
Soxhelt extraction, 170, 316, 317
soy, 386
Spain, 144, 397, 402
spatial density distribution, 301
spherical fullerenes, 332
spinodal decomposition process, 34
spinodal-type phase separation, 367,
370
spruce, 230
ssNMR
carbon nanotubes (CNT),
332–3
cellulose, 332–8
condensation, 335
Subject Index
fructose, 336
furfural, 336
hydroxymethylfurfural (HMF),
336, 338
lignin, 332–6
pyrolysis, 332–5, 338
starch, 334–6
x-ray photoelectron spectroscopy (XPS), 335
stachyose, 114
Starbon synthesis
alginic acid, 73, 74–5, 76
chromatographic applications,
77
condensation, 74
furfural, 74
hydroxymethylfurfural (HMF),
74
microporous, 74
Norit carbon, 77
starch, 72, 73, 74, 75–6, 77
Starbons first generation
hard templating, 61
mesoporosity, 61
microporous, 63
starch, 58–63
transmission electron microscopy (TEM), 60, 61
x-ray photoelectron spectroscopy (XPS), 67, 70
Starbons second generation
alginic acid, 64, 67–8, 69,
70–2
carboxylic acid, 63
chitosan, 66–7
chromatographic applications,
71
esterification, 63
food industry, 43
high-resolution transmission electron microscopy
(HRTEM), 70
mesoporosity, 64
microporous, 67, 70
nitrogen-doped carbons
(NDC), 66–7
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Subject Index
porous glassy carbon (PGC),
70, 71
scanning electron microscopy
(SEM), 65, 66–7
starch, 64, 67–8
transmission electron
­microscopy (TEM), 65, 66–8,
70–1
Starbons synthesis
mesoporosity, 74, 76
starch
biomass-derived, 83
CO2 capture, 262, 263
fuel cells, 238
gels, 176
HTC formation, 145, 147
HTC metal complexes, 210
hydrogenated reactions, 95
hydrothermal carbons, 159
innovated approaches, 390–1,
393–5
polysaccharide-derived, 54–7,
58
ssNMR, 334–6
Starbon synthesis, 72, 73, 74,
75–6, 77
Starbons first generation,
58–63
Starbons second generation,
64, 67–8
state-of-the-art, 132, 134
sustainability in separation
science, 111, 112, 113–4
templating hard, 161, 165
state-of-the-art
carbon aerogels, 134
carboxylic acid, 137
cellulose, 134, 136, 139–40
condensation, 140
ethanol, 134, 137
food industry, 136, 138
fructose, 136, 139–41
hydrogenation, 132
hydroxymethylfurfural (HMF),
140
microspheres, 132
429
nitrogen-doped carbons
(NDC), 137
scanning electron microscopy
(SEM), 135
starch, 132, 134
sucrose, 137
transmission electron microscopy (TEM), 134–5, 139
stationary phases
carbon nanotubes (CNT), 119
carboxylic acid, 108
chromatographic applications,
107–9
formic acid, 109
fructose, 110
hard templating, 110
impregnation, 110
porous glassy carbon (PGC),
108–11
pyrolysis, 110–1
steam invigoration, 21
Stöber silica-based particles, 157,
161, 171
Strecker reactions, 150, 177, 204
stricto sensus, 171, 174
styrene butadiene rubber, 253
subnanometre, 298
succinic acid, 15, 17, 84, 85–6, 94, 96
methylsuccinic acid, 96
sucrose
commercialisation, 394
fuel cells, 234, 235, 236
future energy economies, 15
HTC and photocatalysis, 216–7
HTC formation, 147
HTC metal complexes, 208
mesoporous carbons, 22, 23
ordered materials, 89
other doped carbons, 207
state-of-the-art, 137
sulfonated Starbons, 87
sulfur-doped carbons, 204
supercapacitors, 231
sustainability in separation
science, 114, 116
templating hard, 161
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430
sugarcane, 390
sulfoalkylbetaine, 118
sulfonated Starbons
carboxylic acid, 84–5
cellulose, 87–8
esterification, 84, 85–6
ethanol, 84–5, 86
Norit carbon, 85
pyrolysis, 85
sucrose, 87
sulfonic acid, 60, 63, 72, 78, 85, 87,
192, 205
chlorosulfonic acid, 85, 88
sulfur-doped carbons
microspheres, 204
nitrogen-doped carbons
(NDC), 204
oxygen-reduction reaction
(ORR), 204, 206
pyrolysis, 204
scanning electron microscopy
(SEM), 205
sucrose, 204
x-ray photoelectron spectroscopy (XPS), 204
sulfuric acid, 23, 36, 85, 87–8, 113,
206
sun energy
see solar
sunflower oil, 387
supercapacitors
carbon nanotubes (CNT),
229
cellulose, 228
cyclic voltammetry (CV), 228,
229, 230–2
ethanol, 229
microporous, 229
microspheres, 232
ovalbumin, 228
sucrose, 231
transmission electron microscopy (TEM), 230
supercritical drying, 60, 134, 176,
178
supercritical extraction, 55
Subject Index
supercritical fluid chromatography,
120
superparamagnetic, 88
surfactants, 7, 22, 28, 89, 91, 169,
193, 251, 265–6, 387–9
sustainability and graphite
alginic acid, 116–7
chromatographic applications,
116–7
porous glassy carbon (PGC),
116–7
pyrolysis, 116–7
x-ray photoelectron spectroscopy (XPS), 117
sustainability in separation science
alginic acid, 113–6
chromatographic applications,
111, 113–4
food industry, 115
formic acid, 114
mesoporosity, 115
porous glassy carbon (PGC),
113–4, 116
pyrolysis, 113, 115
scanning electron microscopy
(SEM), 112, 115
starch, 111, 112, 113–4
sucrose, 114, 116
Suzuki crosscoupling, 90
Suzuki-Miyaura reaction, 89
sweet nanorings, 168
Switzerland, 396
synapyl alcohol, 333
tannins, 27, 35, 157, 179, 385–7, 388,
403
tearings, 359
tellurium (Te), 134, 135, 163–5
TEM technique
ethanol, 368
high-resolution transmission electron microscopy
(HRTEM), 357
transmission electron microscopy (TEM), 356–61
template-free, 158, 172, 174, 257
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Subject Index
templating hard
etching, 160, 162–3, 164,
165–6
ethanol, 163
furfural, 162–3
impregnation, 160, 163
macroporous, 160–1
microporous, 161
microspheres, 160–1
nitrogen-doped carbons
(NDC), 165
oxygen-reduction reaction
(ORR), 165
pyrolysis, 161–2
scanning electron microscopy
(SEM), 162, 164
starch, 161, 165
sucrose, 161
transmission electron microscopy (TEM), 161–2, 164, 166
templating soft
fructose, 167
pyrolysis, 168
terpene, 403
Terra Preta, 143
tetrabutyl titanate, 212
tetracyanoborate, 31
tetraethylenepentamine, 265
tetrafluoroborate, 230
tetrahydrofuran (THF), 15, 95, 212,
316, 317, 329
methyltetrahydrofuran
(MTHF), 96
tetrapropylammonium, 210, 238
tetrapropylammonium chloride,
210
tetrapropylammonium hydroxide
(TPAOH), 14, 210, 238
thermal decomposition, 28, 59, 60,
72, 316, 335, 381
thermal gravimetric analysis
carboxylic acid, 314, 316
ethanol, 316, 317
thermodynamic instability, 285
thermogravimetric (TG), 67, 313,
314, 315, 317, 327, 349
431
thermogravimetric analysis (TGA),
313–8
Boehm titration, 327
bromination, 329, 330
graphitic nanocarbons, 29
ordered materials, 93
x-ray photoelectron spectroscopy (XPS), 318, 322
thermoplastic starch, 15
thermoporometry, 303
thiophenes, 204, 242
tidal energy, 12
tin-doped carbon, 210
titanium (Ti), 99, 212–4, 250
TMOS, 36
toluene, 60, 63, 72, 78, 182, 199, 205
perfluorotoluene, 346
tomography, 299, 301, 305, 356,
360–1, 367–71
toxicity, 9, 28, 88, 144, 165, 183, 208,
398
cytotoxicity, 184
nontoxic, 29, 211, 385
trans-particle mass transfer, 106
transesterification, 88, 97, 161, 197
transmission electron microscopy
(TEM)
anode materials, 252
carbon aerogels, 26
chiral nematic pha