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Cite this: Chem. Commun., 2013,
49, 9612
Received 31st July 2013,
Accepted 23rd August 2013
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Unprecedented activation and CO2 capture properties
of an elastic single-molecule trap†
Mario Wriedt,*a Julian P. Sculley,b Wolfgang M. Verdegaal,b Andrey A. Yakovenkob
and Hong-Cai Zhoub
DOI: 10.1039/c3cc45866k
www.rsc.org/chemcomm
The activation and CO2 capture properties of a microporous metal–
organic framework with elastic single-molecule traps were systematically
investigated. This material shows a unique low-energy gas-purge activation capability, high CO2 adsorption selectivities over various gases and
optimized working capacities per energy of 2.9 mmol kJ
1
at 128 8C.
Anthropogenic CO2 emissions have been identified as having deleterious impacts on our climate, and technologies to help mitigate
these effects, such as carbon capture and sequestration (CCS), have
begun development.1 It is currently one of the most important
research challenges in material science and chemistry and faces
some very difficult uphill battles regarding policy and implementation.2 Much of the current research focus is on CCS at point sources,
such as large fossil fuel or biomass energy facilities, industries with
major CO2 emissions (e.g. cement), natural gas processing, synthetic
fuel plants and fossil fuel-based hydrogen production plants. This is
in part because it seems to be economically more viable in the short
term to deploy at these locations and in part because of the difficult
research challenges that must be overcome. For example, potential
materials must withstand the harsh conditions and operate over many
cycles without degrading. Furthermore, energy efficient synthesis,
activation and regeneration, the ability to collect large quantities of
CO2 per cycle (typically referred to as working capacity), and thermal
and chemical stability to ensure long material lifetimes are paramount
to ensure low-cost deployment and operational expenses.
Metal–organic frameworks (MOFs) have been intensely studied
over the past two decades because of the many potential applications
ranging from catalysis,3,4 to sensors,5,6 to gas and liquid separations7,8
and/or storage.9,10 One of the main reasons for this research interest
has been the crystallinity, providing great insights into binding
mechanisms of the host/guest interface. Moreover, the flexibility of
framework design by systematic use of different organic and inorganic
building blocks allows framework tunability to tailor pore sizes, forms,
and surfaces to specific needs.11 This diversity in the synthesis enables
the design of MOFs with unique features, compared to other porous
materials: flexible frameworks, also known as dynamic or breathing
MOFs, show stimuli-responsive adsorption properties.12 Some of these
features have led to a number of MOFs to be promising candidates
for CCS materials,13 however, most known MOFs with high CO2
adsorption capacities and selectivities lack the above mentioned
stability and/or cost-effectiveness requirements.
In our own search for such materials, we recently reported a
stable microporous MOF, [Cu(tzc)(dpp)0.5]n1.5H2O (PCN-200-syn,
tzc = tetrazolate-5-carboxylate, dpp = 1,3-di(4-pyridyl)propane,
Fig. 1, right), which shows record high CO2 over N2 selectivity for
materials based purely on a physisorption process, i.e. no strong
chemical bonding interactions are formed between CO2 and the
framework.14 Established by the flexibility of the dpp linker and
verified by in situ synchrotron-based powder diffraction studies, we
developed the elastic Single-Molecule Trap (eSMT) that imparts
PCN-200 with its unique adsorption behavior. Here we report an
extended systematic study of PCN-200 to further investigate its
physical properties and establish other parameters that are useful
in determining its potential as a candidate CCS material.
PCN-200-syn was synthesized in gram quantities (Fig. 1, left),
activated by heating to 80 1C for 10 min, and investigated for its
single-gas adsorption behavior of various gases by volumetric
measurements. As shown in Fig. 2, top, the activated PCN-200-ac
exhibits high CO2 uptake (1.71 mmol g 1) at 195 K and 0.15 bar,
a
Department of Chemistry & Biomolecular Science, Clarkson University, Potsdam,
NY, 13699-5810, USA. E-mail: mwriedt@clarkson.edu
b
Department of Chemistry, Texas A&M University, College Station, TX, 77843-3255,
USA. E-mail: zhou@chem.tamu.edu
† Electronic supplementary information (ESI) available: O2, Ar, N2 and H2
adsorption isotherms at various temperatures; experimental details of the
adsorption, breakthrough, and diffraction measurements; purge-gas activation
studies with He, N2, CO2 and air. See DOI: 10.1039/c3cc45866k
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Chem. Commun., 2013, 49, 9612--9614
Fig. 1
Polycrystalline blue bulk material of PCN-200 (left) and its crystal structure (right).
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whereas the quantity adsorbed for O2, Ar, N2 and H2 is very low.
Under ambient conditions no detectable adsorption can be observed
for all gases except CO2, which is especially beneficial for the removal
of small CO2 fractions from hydrogen for use in fuel cells (hydrogen
upgrading) (see Fig. S2–S5, ESI†); such niche applications could aid
in further developing this technology before full-scale deployment
and are currently being investigated.
In Fig. 2, bottom, the pure CO2 adsorption isotherm at 313 K is
shown (approximate process temperature for carbon capture from
flue gases in an industrial setting) in comparison with CO2 adsorption values obtained from CO2–N2 mixed-gas breakthrough measurements, which are in excellent agreement with the single-gas
adsorption isotherm (for experimental details see Fig. S6–S10, ESI†).
These matching adsorption values obtained from two different
experimental set-ups clearly show that N2 does not influence
the CO2 uptake in gas mixtures; this finding was predicted by
GCMC simulations14 and is now experimentally confirmed.
Removing CO2 from flue gases with PCN-200 was further investigated using a temperature swing adsorption process (TSA), as first
demonstrated by Long et al.15 In Fig. 3, top, the working capacity of
PCN-200 was experimentally determined to be 1.3% by CO2 adsorption from a dynamic 15% CO2 atmosphere at 40 1C and desorption
via a purge of pure CO2 at 150 1C. This approach yields high-purity
CO2, which is essential for sequestration and utilization. In this
context, Long reported an amine doped MOF, mmen-Mg2(dobpdc),
which showed an impressive working capacity of 7.8%,15 however,
energy intense activation16 and lack of water stability17 of this MOF
family might negatively affect working performance.
Fig. 2 Gas adsorption isotherms of PCN-200 from volumetric measurements at
195 K for various gases (top, note that the N2 isotherm is overlaid with the Ar
one, for further temperatures see Fig. S1–S5, ESI†); CO2 adsorption isotherms of
PCN-200 from volumetric measurements at 313 K in comparison with adsorption
values obtained from mixed gas breakthrough measurements at 313 K and 1 bar
with 15, 50% CO2 in N2, and 100% CO2 (bottom, for details see Fig. S6–S10, ESI†).
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Fig. 3 Temperature swing process of PCN-200 for CO2 by simulation of adsorption from flue gas simulant and desorption with pure CO2 using TGA (top). The
difference in CO2 adsorbed at 150 1C under a 100% CO2 atmosphere (point A)
and at 40 1C under flowing 15% CO2 in N2 (point D) yields a 1.3% working
capacity. Changes in sample mass are shown in green and temperatures in blue.
Points B and E arise from introducing 15% CO2 at 150 1C and 100% CO2 at 40 1C
respectively, and point C from a small temperature underswing; temperature dependence of working capacity and working capacity per energy (bottom). Calculations are
based on adsorption/capture conditions: 15% CO2 in N2, 1 bar, 40 1C, and recovery/
desorption conditions: 100% CO2, 1 bar, variable temperature.
A high-throughput analytical model18 was used to further quantify
the working capacity of the TSA process as shown in Fig. 3, bottom. The
temperature-dependence of the working capacity is plotted as black
circles. At regeneration temperatures >87 1C the equilibrium capacity
under adsorption conditions is higher than at desorption, the working
capacity therefore becomes positive at this temperature and continues
to rise at higher regeneration temperatures (180 1C, 0.34 mmol g 1).
However, the most important quantification value of a TSA process is
not the bare working capacity, but rather the amount of captured CO2
per applied regeneration energy (working capacity per energy, plotted as
blue squares in Fig. 3, bottom). This value does not show the highest
possible working capacity for the material, but the temperature to
which it makes sense to heat the material and recover pure CO2. At
increasingly higher temperatures, it is less beneficial to input more
energy as the return of CO2 comes at a higher price. The most influencing parameters of this value are the temperature-dependent working
capacity and the heat capacity. The former parameter can be easily
obtained from just a few single-gas adsorption isotherms at different
temperatures and the latter value from DSC measurements.18 Because
of PCN-200’s low CP of 0.8 J (g K) 1 at 40 1C,14 a reasonable maximum
working capacity per energy of 2.90 mmol kJ 1 is found at 128 1C. In
this context, 30% MEA solutions show only a value of 2.16 mmol kJ 1
and the organic polymer PPN-6-DETA a respectable value of
7.53 mmol kJ 1,18 but no other MOFs were analysed yet by this model.
In terms of the total CCS energy consumption from material synthesis, to adsorption and cyclic regeneration, the activation processes
Chem. Commun., 2013, 49, 9612--9614
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of porous materials are mostly underestimated when considering
moving from lab- to industrial-scale quantities. However, the most
commonly used activation routes for highly porous MOFs, solventexchange followed by extensive heating19 and/or vacuum routines
(or even super-critical CO2 exchange20), are energy-intensive methods.
Using conventional routes, PCN-200 needs only very little energy: no
solvent exchange, heating to only 80 1C or applying a weak vacuum
for only a few minutes. During our systematic investigation of the
activation and adsorption properties of PCN-200 we discovered a new,
unprecedented activation method, namely gas-purge activation. PCN200-syn can be activated by simply purging gases, such as He, N2,
CO2, dry air or flue gas through a packed bed of the material. As
presented in Fig. 4, top, in situ synchrotron-based powder diffraction
studies show the gas-purge activation process in a flue gas simulant
stream at 295 K within just a few minutes, and simultaneously, the
formation of the CO2 loaded form of PCN-200 (for further diffraction
experiments with other purge gases see Fig. S11, ESI†).
Additionally, the breakthrough method was used to further investigate the activation behavior and the influence of the purge gas on
the activation duration. As shown in Fig. 4, bottom, PCN-200-syn was
activated in dry gas flow at 40 1C. By raising the amount of CO2 in the
purge gas stream the activation time decreases from B30 h in He,
to B21 h in 15% CO2 to B12 h in 100% CO2. This decrease in the
activation time can be explained by the high CO2-framework affinity,
the H2O molecules in the pores are more likely replaced by CO2
molecules instead of an inert gas. It must be noted that the above
mentioned activation process times are on a relative scale and
depend on experimental parameters such as the flow rate,
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packed density of the sorbent, temperature gradients and should
therefore not be compared to other results on an absolute time scale.
Taking into account that above described working capacity
per energy does not include activation energies, but assuming it
would, this value would be even higher considering gas-purge
activation in comparison with other materials which need
activation by energy-intense conventional methods.
In summary, we have shown by systematic breakthrough experiments that the presence of N2 does not influence the CO2 adsorption capacities of PCN-200 in mixed gas streams. Furthermore, H2,
O2 and Ar show no detectable adsorption under ambient conditions. The working capacities per energy show a respectable value of
2.9 mmol kJ 1 at 128 1C. But more importantly, the unprecedented
gas-purge activation capability enables a very low-energy activation
with no energy-intense heating or solvent-exchange procedures.
Although PCN-200 does not show one of the highest pure
CO2 adsorption capacities, we believe that its easy and low-cost
synthesis, water stability, and the above-presented features make
PCN-200 a promising CCS material in an industrial setting.
M.W. acknowledges support from Clarkson University (start-up
fund) and the German Academic Exchange Service (DAAD, postdoc
fellowship), and H.-C.Z. from the U.S. Dept. of Energy (DOE, DESC0001015 and DE-AR0000073). Use of the Advanced Photon Source,
an Office of Science User Facility operated for the U.S. DOE Office of
Science by Argonne National Laboratory, was supported by the U.S.
DOE under Contract No. DE-AC02-06CH11357. In this context we
thank Gregory J. Halder for his onsite help. In addition we thank
Hae-Kwon Jeong from TAMU for using his breakthrough equipment.
Notes and references
Fig. 4 Gas-purge activation of PCN-200 monitored by in situ synchrotron-based
powder diffraction at 295 K (top) and the breakthrough method at 313 K (bottom).
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