Special issue:
Carbon Capture and Storage Workshop, Texas A&M University in Qatar, April 2012
Guest editor: Howard JM Hanley
Metal-organic frameworks and porous
polymer networks for carbon capture
Julian Patrick Sculley, Jian-Rong Li, Jinhee Park, Weigang Lu, Hong-Cai Joe
Zhou
Chemistry Department, Texas A&M University, College Station, TX
DOI: http://dx.doi.org/10.5339/stsp.2012.ccs.16
ISSN: 2220-2765
Article type: Review article
Cite this article as: Sculley JP, Li JR, Park J, Lu W, Zhou HCJ. Metal-organic
frameworks and porous polymer networks for carbon capture, Sustainable
Technologies, Systems and Policies 2012 Carbon Capture and Storage Workshop:16
http://dx.doi.org/10.5339/stsp.2012.ccs.16
Running head: Sculley, Li, Park, Lu, Zhou, STSP 2012.CCS.16
Copyright: 2012 Sculley, Li, Park, Lu, Zhou, licensee Bloomsbury Qatar Foundation
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Metal-organic frameworks and porous polymer
networks for carbon capture
Julian Patrick Sculley, Jian-Rong Li, Jinhee Park, Weigang Lu, Hong-Cai Joe
Zhou1
1
Chemistry Department, Texas A&M University, College Station, TX
Zhou@chem.tamu.edu
Abstract
The ability to rationally design materials for specific applications and synthesize
materials to these exact specifications at the molecular level makes it possible to make a
huge impact in carbon dioxide capture applications. Recently, advanced porous
materials, in particular metal-organic frameworks (MOFs) and porous polymer
networks (PPNs) have shown tremendous potential for this and related applications
because they have high adsorption selectivities and record breaking gas uptake
capacities. By appending chemical functional groups to the surface of these materials it
is possible to tune gas molecule specific interactions. The results presented herein are a
summary of the fundamentals of synthesizing several MOF and PPN series through
applying structure property relationships.
Keywords: porous materials; metal-organic frameworks; carbon dioxide capture; gas
separation
1. Introduction
Carbon capture and sequestration (CCS) is a topic that has received a
considerable amount of attention in the last few years because of the importance
of reducing anthropogenic carbon dioxide emissions.1-3 The scientific
investigation of porous solid materials has traditionally involved a broad
spectrum ranging from chemists to materials scientists to engineers in large part
due to the industrial importance of these materials for separations.4 Over the last
three decades there has been resurgence in the chemical arena with the
development of metal-organic frameworks (MOF) as advanced porous
materials.4-6 These materials self-assemble from inorganic metal ions or clusters
and organic bridging ligands. Due to the modular construction using molecular
building blocks, it is possible to design a MOF to have precise structural
characteristics and physical properties. Through the power of organic synthesis
it is also possible to add functional groups to the organic linker without
changing the final framework topology.7
The current, amine-scrubbing technology survives because it drastically reduces
CO2 output from coal fired power plants, but it crushes the economic pillar
because of the enormous parasitic power demands imposed during the
solution’s regeneration process. MEA solutions (approximately 30 wt%
monoethanolamine (MEA) in water) work by chemically bonding CO2 to form
carbamates, but to regenerate them, the water solutions must be heated up,
which can account for about 30 percent of a power plant’s energy output.8, 9 In
order to reduce the energy penalty that coal or natural gas fired power plants
incur during the process of scrubbing CO2 from flue gas, new methods and
materials must be investigated. Some of the major technical challenges are that
flue gas is usually plagued with contaminants (SOx, NOx, and fine particulates),
it is hot (typically 40-60°C) and only contains approximately 14-16% CO2
(translates to a partial pressure in the gas stream of about 0.15 bar).10
Physisorptive materials such as MOFs work in regions of much lower sorption
enthalpies than chemical absorption and can therefore be used to mitigate
energy costs because regenerations are energetically favorable. Another aspect
of MEA scrubbing is the highly corrosive property of aqueous amine solutions.
To deal with this problem specially designed tanks are needed and the solutions
must be handled cautiously.10
2. Metal-organic frameworks
2.1. MOP construction
As the building units of MOFs, the design and construction of the metal-organic
polyhedra (MOPs) is a prerequisite. A great example of the modularity in
design is shown in Figure 1. By choosing a simple secondary building unit such
as the dicopper paddlewheel (in the center of the figure), and mixing it in
solution with a variety of linkers, numerous MOPs can be synthesized.11 Each
MOP has different physical properties that are related to the bridging linker.
Structure property relationships can be established between the functional group
and a desired physical property, such as high CO2 uptake at low pressure, which
can be used to increase selectivity of CO2 over N2 or CH4 (natural gas
purification). The metal cluster can also be replaced with other metals such as
molybdenum to generate isostructural MOPs with different physical
properties.12 These polyhedra can be bridged into three dimensional structures,
using ditopic moieties such as 4,4’-bipyridine,13 or by designing ligands which
will form polyhedral pores that are covalently linked such as the PCN-6x series
developed by us.14, 15
2.2. Polyhedra-based frameworks
A hierarchical approach can be used to connect these polyhedra into 3D
networks, by carefully designing linkers with topologies that will generate
polyhedra-based networks (Figure 2). All of the PCN-6x series MOFs have the
same topology with identical cuboctahedral cages, but increasingly larger
mesocavities by increasing the ligand length. This leads to increasing BET
surface areas of 3350, 4000 and 5109 m2/g for PCN-61, -66, and -68
respectively. An important trend that is observed for this series is the
proportional increase in CO2 uptake capacities at 35 bar, from 23.5 to 26.3 to
30.4 mmol/g. In terms of volumetric uptake this translates to a storage capacity
that is between 7.3 to 8.2 times the amount of CO2 stores in an empty container.
Similar trends are observed for other gases stored at high pressure for energy
applications, primarily H2 and CH4.15
2.3. Stimuli-responsive MOFs
MOFs can further be modified with pendant functional groups. These functional
groups can be attached to the periphery of MOPs (as shown in Figure 1) or to
the interior of channels creating gate type environments as with the meshadjustable molecular sieves (MAMS) shown in Figure 3a.Error! Reference
source not found.16 The MAMS structure has 1D channels with gated
chambers, which are controlled by temperature responsive functional groups.
By increasing or decreasing the temperature the pore size is slightly altered,
leading to temperature dependent selective gas adsorption. This idea of smart
materials can be further extended to include other stimuli responsive materials,
where light responsive groups control gate opening. For carbon capture
applications, one of the main concerns is to reduce the amount of energy
required to capture CO2. By switching from temperature controlled materials to
optically sensitive materials, one can easily imagine simply opening the
material up to sunlight to regenerate the sorbent. In the first example of this new
class of materials, shining UV light onto the sample reduced the CO2 uptake
capacity at 1 bar from 22.9 cm3/g to 10.5 cm3/g. Because this process is entirely
reversible it can be used to regenerate the material to readsorb more CO2 in
purification applications.17
3. Porous polymer networks
The benefits of stability lead Zhou’s group to investigate Porous Polymer
Networks (PPNs).18, 19 These hypercrosslinked polymers add additional merits
to the adsorbents family due to their low cost, ease of processing, and high
thermal and chemical stability. Initial publications showed that these covalently
bonded materials were indeed very stable and had similar surface areas and pore
volumes (both of primary importance to high pressure gas storage application)
compared to MOFs. The silane network (PPN-4) has the highest BET surface
area of any material with 6461 m2/g and a remarkable pore volume of 3.04
cm3/g. This tremendous porosity is directly related to the amount of gas stored
at high pressures (50 bar) of H2 (140 mg/g), CH4 (360 mg/g) and CO2 (2121
mg/g). To increase low pressure CO2 adsorption however, we introduced polar
functional groups (SO3H and SO3Li) as can be seen in Figure 4. By raising CO2
specific interactions in these materials, it is possible to tune gas-framework
interactions thereby creating highly selective materials. As an example, PPN-6
was synthesized by an optimized Yamamoto homo-coupling reaction19 using
tetrakis(4-bromophenyl)methane and has a BET surface area of 4023 m2/g. The
first attempt at functionalizing a PPN for this targeted application was by
stirring it in chlorosulfonic acid (followed by lithium hydroxide), as shown in
Figure 4. Both modified PPNs have exceptionally high CO2 uptake capacities at
pressures relevant to flue gas separations. Using the approximate partial
pressures of CO2 and N2 in flue gas (0.15 and 0.85 bar respectively) Ideal
Adsorption Solution Theory (IAST) selectivities were calculated for each
material using experimental pure gas isotherms. PPN-6-SO3Li has a selectivity
that compares favorably to NaX zeolite at about 150. The higher CO2 adsorption
capacity and lower N2 capacity of PPN-6-SO3H lead to a record high IAST
selectivity of 414. Additionally, these materials are thermally stable to above
400 °C, can be stirred in boiling water as well as strong acids and bases without
losing any of their adsorptive properties. This stability is important for real
industrial applications to ensure that the material will not degrade under the
conditions it is exposed to and be useful over a long period of time.20
4. Conclusions
Porous materials such as MOFs and PPNs can have a tremendous impact in
carbon dioxide capture technologies because we can rationally design smarter
materials to achieve the properties necessary while lowering the overall energy
consumption. We have demonstrated that these materials can achieve recordbreaking selectivities and storage capacities. Additionally new, stimuli
responsive materials will significantly reduce energy consumption during the
regeneration step.
Figure 1. Synthetic scheme of metal-organic polyhedra (MOPs).
Figure 2. (a) Ligands used in the construction of PCN-6x series; (b) Simplified
structure showing the cuboctahedral cages; (c) hierarchical assembly of cages; (d)
simplified network topology.
Figure 3. (a) Schematic illustration of Mesh-adjustable molecular sieves schematic and
selective gas adsorption; (b) Optically and thermally responsive MOF and CO2
adsorption showing reversibility.
Figure 4. Synthesis and postsynthetic modification of PPN-6 and gas adsorption
properties showing highly selective adsorption of CO2 over N2.
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