Symposium EE10 : Recent Advances in Materials for Carbon Capture

Mesoporous carbon materials with uniform channels have attracted considerable attention because of their integrated applications ranging from separation and catalysis to electrochemistry. Up to date, increasing efforts have been devoted to improve the physicochemical properties of carbon materials for diverse applications. Among them, the incorporation of heteroatoms, nitrogen, into the carbon lattice can efficiently change the electronic structure of the carbon, which in turn enhanced the electric conductivity and surface polarity of the carbon materials.¹ It is well known that in many applications like separation and catalysis, it’s the active sites exposed to the interface that greatly determine the bulk properties of the mesoporous materials. Meanwhile, the highly ordered channels of the mesoporous materials facilitate the transport of the molecules. Therefore, it would be of great interest and challenge to synthesize carbon materials with uniform channel structures as well as highly nitrogen content in the channel surface. Most of the previous works have focused on the incorporation of nitrogen into the bulk mesoporous carbon. Herein, we report a facile and effective strategy for the preparation of ordered mesoporous carbon materials with a highly nitrogen containing coating layer through the polymer-ionic liquid assembly. Using this strategy, a highly surface nitrogen content of up to 8.1 % was obtained with nitrile-functionalized ionic liquids as nitrogen precursors. The N-doped mesoporous carbon had uniform pore channels with an average pore size of 6 nm and a highly BET surface area of 506 m²/g. We also show how to control the nitrogen content of the carbon materials by simply changing the structure and the amount of ionic liquids. The prepared N-doped mesoporous carbon materials demonstrate excellent CO₂ adsorption capacity at ambient condition (2.29 mmol/g). We also tested the N-doped mesoporous carbon as the electrode of the supercapacitor. The specific capacitances of N-doped mesoporous carbon calculated from the charge-discharge curve enhanced greatly compared with the values of the none-doped carbon, suggesting the doping of electron rich nitrogen atoms greatly modified the electron property of carbon materials. Therefore, the increased CO₂ uptake ability and enhanced capacitance both suggest the successful preparation of ordered mesoporous carbon with highly nitrogen-containing pore surface. This strategy based on polymer-ionic liquid assembly present a novel method to prepare N-doped mesoporous carbon with highly ordered structure and also is helpful for the preparation of other ordered composite material.
References
1. Lee J. S.; Wang X. Q.; Luo H. M.; Dai S. Adv. Mater. 2010, 22, 1004-1007.
Acknowledgements
This research was supported by the U.S. Department of Energy’s Office of Basic Energy Science, Division of Materials Sciences and Engineering, the National Natural Science Foundation of China (21222601, 21476192, and 21436010).

Among porous solids, metal-organic frameworks (MOFs) and Porous-Organic Polymers (POPs) continue to receive considerable scientific interest due to their wide range of potential applications in current demanding technologies. Porous solids, in general, are amenable to bottom-up assembly from judiciously designed molecular building blocks (MBBs) into a desired framework expanding and/or decorating a specific blueprint network topology. POPs are an emerging class of polymeric materials accessible through covalent linkage of pre-selected MBBs. POPs commonly exhibit rigid structures, high thermal and chemical stabilities, low densities, and in certain cases permanent porosity with specific surface areas surpassing those of well-known zeolites and porous silicates. Recent reports outlined several synthetic pathways to construct POPs including, but not limited to: (i) boronic acid condensation, (ii) imine formation, (iii) Sonogashira-Hagihara cross-coupling, (iv) triazine synthesis via nitrile trimerization, and (v) cobalt-catalyzed acetylene trimerization. Common to the aforementioned bond-making reactions is irreversibility under the reaction conditions used (with exception of imines and boranes), rendering isolation of crystalline POPs challenging. On the other hand, solvothermal syntheses of MOFs utilize the reversibility of coordination bonds to access highly crystalline materials.
Herein, we report a facile approach towards construction of Poly-Functional Porous Organic Polymers (POPs). The functional groups employed were selected to span the range of Lewis-base, neutral, to Lewis-acid character. Our results underline the effect of chemical functionality on the observed Q_st for CO₂ adsorption inside the material, being largest for functional groups with electron donating O- and N-centered Lewis base sites. Our systematic investigation within a family of POPs revealed a wide range for CO₂ heat of adsorption(23.8~53.8 kJ/mol) that is clearly associated to the chemical nature of the functional groups present. In addition, post synthetic modification on POPs reported herein demonstrated a facile pathway to dramatically enhance carbon dioxide uptake energetics.

Air Liquide is developing a cost effective hybrid CO2 capture process based on sub-ambient temperature operation of a hollow fiber membrane in combination with cryogenic distillation. These membranes, when operated at temperatures below -20C, show two to four times increase in CO2/N2 selectivity with minimal CO2 permeance loss compared to ambient temperature values. Operation of these commercial Air Liquide membranes at low temperatures provides an attractive combination of CO2 permeability and selectivity. This technology has the potential to meet Department of Energy / National Energy Technology Laboratory targets of 90% CO2 capture from air-fired coal power plants at a CO2 capture cost of Long term bench-scale testing with CO₂/N₂ mixtures at sub-ambient conditions verified that the enhanced separation performance seen at lab scale translated well to commercial membrane modules. A field test at 0.3 MW scale equivalent has been initiated at DOE’s National Carbon Capture Center to validate module performance with actual flue gas.
The membrane performance at the application conditions depends on various factors: membrane morphology, intrinsic polymer properties and module characteristics. An understanding of the required polymer characteristics has led to further development with a novel polymer. As demonstrated by small permeator testing at sub-ambient conditions, the resulting new membrane has > 5x the permeance of the commercial version with similar selectivity. Work is ongoing to validate this promising membrane lead.

Reference: T Chaubey et al. http://www.netl.doe.gov/File%20Library/Events/2015/co2captureproceedings/T-Chaubey-Air-Liquide-Cold-Membrane-Operation.pdf

Nanostructured architectures and unconventional separation media hold the key to future chemical separations in gas storage and purification, environmental remediation, and many clean-up activities. Replacing energy-intensive separation methods by energy-efficient processes, such as membrane separation or adsorption, is an essential step toward substantial future energy savings. This talk will be focused on our recent investigation on development of porous carbons, ionic liquids, and their combination for CO₂ separation. Hierarchical synthesis methodologies for porous carbons and polymers based on self-assembly will be examined to generate controlled adsorption sites integrated within novel separation media. The methodology to control the multitude of interactions that determine separation efficacy within ionic liquids will be also discussed.

The vast majority of the world’s energy is presently derived from the burning of fossil fuels, which releases vast quantities of carbon dioxide (CO₂) into the environment and results in undesirable climate change. Practical and cost efficient methods of CO₂ separation and capture would thus solve one of the most challenging problems facing humanity today. This presentation summarizes our effort on the development of novel polymer membranes for high flux CO₂ separation and the investigation of the fundamental understanding in its molecular transport through the polymer membranes. Our strategy focuses on tuning solubility selectivity in addition to diffusion selectivity for achieving high permeability membranes combined with good selectivity. Various synthetic techniques including ROMP and post functionalization were used and the careful design permits to prepare well-defined novel high permeable polymers containing CO₂-philic groups. The structure-property relationships including CO₂ uptake, CO₂ and N₂ permeability, CO₂/N₂ selectivity to the polymer structure will be discussed.

Mixed matrix membranes are an interesting gas separation technology because of their ability to combine the high performance of inorganic filler particles such as metal organic frameworks (MOFs) with the film forming capability of polymers. At the National Energy Technology Laboratory (NETL), mixed matrix membranes are being studied for the separation of carbon dioxide from post-combustion flue gas in fossil fueled power generation. Several combination of materials have proved successful in experiments using both simulated flue gas and actual flue gas at the National Carbon Capture Center, including Matrimid with UiO-66 MOFs, polyphosphazene with SIFSIX MOFs, and surprisingly, cross-linked ionic polymeric materials with silica particles. The latter combination which consists of non-porous silica, brings up a key issue with mixed matrix membranes that is still not well understood: the interaction of gases with the matrix, the filler, and the interface region. While it is widely recognized that a filler particle must be well bound to the matrix material in order to prevent bypass of gases through interfacial voids, the behavior of gases through the interface of a non-defective mixed matrix membrane remains a mystery. From an experimental standpoint, it is difficult to probe the interaction of gas molecules in different regions of a composite membrane. We have attempted to better understand the role of the interface using computational methods, and show that the design of the interface may indeed be as important as the design of the matrix and the fillers.

Carbon capture is desired in various industrial applications to reduce the global warming effect. As one of the key steps in carbon capture, energy efficient CO₂ separation process is critical to reduce the overall energy input. Membrane gas separation is considered as the most energy efficient process because it does not have the energy intensive phase-transition that exists in both cryogenic distillation and adsorptive methods. The ultimate challenge of membrane gas separation is to achieve high flux and high selectivity simultaneously with the least energy input. Much success has been demonstrated by the supported 2D materials, including metal organic framework (MOF) nanosheets, graphene and graphene oxides (GOs) atomic sheets. However, the fabrication of such 2D materials generally requires multiple steps and is difficult to scale to large and flexible substrates. Here we report, a facile, one-step and scalable process, initiated plasma enhanced chemical vapor deposition (iPECVD), being employed to directly deposit ultra-thin (i.e. sub-100 nm) and flexible metal organic covalent network (MOCN) layers on a membrane support of 175 cm² area. Tetraphenylporphyrin building units (with or without centering metal ions) were employed to form covalently bonded ultra-thin films for the first time. Such films achieved CO₂/N₂ separation performances exceeding the 2008 empirical upper bound. In addition, the same films were also successfully applied to separate other industrially important gas pairs (e.g., H₂/CH₄, O₂/N₂, CO₂/CH₄ and H₂/N₂).

An emerging class of mixed-matrix thin films based on a continuous crystal phase of metal-organic-frameworks grown on polymeric electrospun nanofiber supports has recently shown promise in gas separation technology. The complementary roles and tunability of MOFs and nanofibers in strength and selectivity makes this technology an intriguing prospect for both membranes for continuous separation and films for cyclic adsorption processes. However, before this technology may become widely accepted, fundamental work needs to be performed on the synthesis-structure-property relationships when designing these membranes. The range of polymers and MOFs that can be used must be found, techniques to synthesize defect free films must be developed, and the interactions between MOF and nanofiber needs to be better understood and optimized. The intent of this talk is to bring awareness to the potential advantages of these materials, discuss the fundamental questions our lab has already answered, and address future questions that we intend to solve.

Alkali metal oxides have been extensitvely investigated as a promising dry sorbent to capture CO₂ from the anthropogenic point sources including thermoelectric power plant, gas well, and iron works. Although they have high theoretical CO₂ sorption capacity, alkali metal oxides, especially MgO, have failed to show outstanding performance in CO₂ capture so far. Recently, it was revealed that eutectic mixture can highly utilize MgO toward CO₂ sorption by dissociating Mg²⁺ and O^2-. Fundamentally, CO₂ capture on MgO starts with surface chemisorption and ends with MgCO₃ formation throughout the body of MgO. The role of eutectic mixture might be facilitatating adsorption-mediated carbonation due to its strong solvation effect. However, the CO₂ sorption mechanism on neither pure MgO nor eutectic mixture promoted MgO has not been clearly explained for formation of surface specific MgCO₃ (pure MgO) and bulk MgCO₃ (eutectic mixture promoted MgO). In this study, eutectic mixture promoted MgO (denoted as KNO₃-LiNO₃-MgO) was prepared by physical mixing of eutectic mixture (KNO₃-LiNO₃) and MgO obtained from oxidation of comercial MgCO₃, and then it was subjected to in-situ transmission electron microscopy (TEM) for the direct observation of fundamental CO₂ sorption under non-vaccum (CO₂ atmosphere) and high temperature (> 350 °C). Morphological and crytallogrphical changes at the interface of MgO and euctectic mixture were observed in nano-scale by using SAED and EDS. Above the eutectic temperature (> 120 °C), KNO₃-LiNO₃ migrates on the MgO surface, activating O₂^- for the generation of MgCO₃ nuclei. These nuclei could accelerate MgCO₃ formation at triple phase boundary where chemisorbed CO₂ is supplied. The detailed chemistry behind this CO₂ sorption mechanism is well supported by first-principle calculation (DFT) and will be discussed in detail. This work was supported by KCRC through the NRF funded by Ministry of Science, ICT, and Future Planning (NRF-2015M1A8A1048902).

CO₂, one of the main combustion gases, has been claimed to be the major contributor that causes global climate changes. Various absorbents such as solvent-based and solid-based materials are promising candidates for capturing a large amount of CO₂. Among these various absorbents, solid sorbents that contain metal oxides have good advantages over liquid adsorbents due to lower operating costs and lower energy penalties. Metal oxides such as MgO are good candidates for adsorbing CO₂ because they have a high theoretical capacity based on calculations and appropriate operating temperature (around 573K) in pre-combustion conditions. However, MgO has a low actual capacity based on measurement because of it has a low kinetic reactivity and agglomerates. In order to overcome these demerits, MgO is modified by substrates, such as Al₂O₃, TiO_2, and SiO₂ to improve thermal stability, mechanical strength, and extended surface. In this work, we investigated the detailed thermodynamic properties of CO₂ adsorption between MgO and substrates from first principles calculations based on density functional theory (DFT).

Acknowledgement
This work was supported by KCRC through the NRF funded by Ministry of Science, ICT, and Future Planning (NRF-2015M1A8A1048902).

Covalent Metal-Organic Networks (CMONs) are a relatively unexplored subset of Metal-Organic Frameworks (MOFs) that exhibit a strong, covalent interaction between the organic linker and the metal center. The formation of these highly stable bonds result in fast reaction rates, preventing the isolation of crystalline material. We have utilized a protecting-group approach to limit the rate of formation and isolate a number of crystalline CMONs. We have also incorporated Lewis base sites within the pores of these materials, leading to the selective adsorption of carbon dioxide over other atmospheric gases. The synthesis, structure, stability and gas sorption properties of a series of CMONs will be described.

The objective of this research was to develop a new aerogel based CO₂ adsorbent with high performance in low-concentration CO₂. Herein, amine hybrid resorcinol-formaldehyde/silica composite aerogels (AH-RFSAs) were developed through a facile sol-gel process, directly mixing all the reacnts and solvent in a container to form the hybrid gel. The amine content in AH-RFSA was controlled by the ratio of silica source (APTES, 3-aminopropyltriethoxysilane) in the reactants. CO₂ adsorption performances of AH-RFSAs in 1% CO₂ gas mixture (CO₂+N₂) were investigated. The CO₂ adsorption capacity of AH-RFSAs increases first and then decreases with APTES content. This could be explained by the negative effect of APTES on the specific surface area of AH-RFSA. It is understandable that high APTES content in the reactants results in rapid sol-gel reaction, which leads to poor porosity of aerogels. Similarly, the CO₂ adsorption capacities of an AH-RFSA prepared with different sol-gel parameters are different although the APTES content is constant, due to the effect of sol-gel parameter on pore structure of aerogel. An aerogel based adsorbent with high CO₂ capture performance should meet two primary principles, i. e. high amine content and high specific surface area. The CO₂ adsorption capacity achieved with the new adsorbent at 25 °C in dry 1% CO₂ mixture gas is as high as 2.15 mmol/g. The adsorbent preparation method is inspiring and its resulting adsorbent is exceptional in capacity and regenerability.

Computational research offers chemical insights and design principles for new separation media and for understanding the separation process. In this talk, we summarize our recent applications of simulation methods from ab initio and density functional theory to classical molecular dynamics and Grand canonical Monte Carlo in understanding ionic liquids and porous carbonaceous materials for CO₂ separation, especially the post-combustion CO₂/N₂ separation. We highlight design and simulation of the porous two-dimensional (2D) materials as the highly selective membranes for CO₂ separation. Simulated structure-property relationships for the materials will be discussed in connection to the corresponding chemisorption, physisorption, or membrane process. In chemisorption, the focus is on reducing the heat of reaction with CO₂; in physisorption, the key is to increase the binding strength via CO₂-phyllic groups; in membrane process, the key is to increase solubility for ionic-liquid membranes and to control pore size for 2D materials. Challenges and opportunities for simulating emerging materials will be also discussed.

Due to the rigid framework and D3h symmetry, triptycene is considered to be an important building block to construct porous covalent organic framework (COF). Several triptycence-derived COFs have been prepared in experiments and studied as potential materials for selective CO₂ absorption and separation. Adsorption isotherms of CO₂ at different temperatures and pressures in these COFs are reproduced by grand canonical Monte Carlo (GCMC) simulations. Furthermore, we assemble 2-dimensional COFs from triptycene derivatives as the building units, and predict their adsorption capacities of CO₂. The effects of pore size and heteroatoms in framework are explored, providing guide to design of new sorbents for carbon capture.

Metal-organic frameworks (MOFs) are porous crystalline materials formed by inorganic and organic molecular building blocks via self-assembly. The building-block synthesis approach makes it possible to create a vast number of MOFs and to tune their properties for desired applications, such as selectively removing CO₂ from other gases. We have used a range of modeling techniques, in close interaction with experiment, to design, screen, and evaluate MOFs for pre- and post-combustion carbon capture.

For post-combustion capture, we demonstrated, both experimentally and computationally, a new design strategy for capturing CO₂ in nanoporous adsorbents. The approach involves “complementary organic motifs” (COMs), which have a precise alignment of charge densities that is complementary to the CO₂ quadrupole. Atomistic modeling predicted high binding of CO₂ with the COMs, and two promising COMs were post-synthetically incorporated into a robust MOF material using solvent-assisted ligand incorporation. We demonstrated that these COM-functionalized MOFs exhibit high capacity and selectivity for CO₂ relative to other reported motifs.

For pre-combustion carbon capture, we used high-throughput computational screening to find top-performing MOF materials. Approximately 50,000 hypothetical MOFs were screened via grand canonical Monte Carlo simulations for their ability to separate CO₂ from hydrogen. Top candidates among the hypothetical MOFs were identified, and structure/property relationships were developed. The structure/property relationships were subsequently used to find synthetic targets from a large body of known MOFs. Guided by the computational work, we synthesized and tested an existing MOF that had not been previously examined for this application, as well as one of the hypothetical MOFs. Experimental pure-component CO₂ and H₂ isotherms were measured and used to estimate the selectivity and the CO₂ working capacity for this separation.

One difficulty in developing adsorbents for carbon capture is the lack of simple material metrics for comprehensively evaluating the materials. Metrics such as the selectivity and the working capacity are certainly important but are inadequate on their own. To overcome this problem, we have established a collaboration between material developers and process modeling researchers. In preliminary work for post-combustion CO₂ capture, we developed a macroscopic pressure/vacuum swing adsorption (P/VSA) model consisting of a system of partial-differential, algebraic equations incorporating mass and energy balances, competitive Langmuir adsorption isotherms, and the linear driving force model. Four potential adsorbents, zeolites 13X and 5A and the MOFs HKUST-1 and Ni-MOF-74, were investigated and compared for CO₂ capture both with and without the presence of humidity. We are now using the model to develop guidelines for material researchers to more quickly evaluate materials for carbon capture.

Metal-organic framework (MOF) materials in general, and MOF-74 in particular, have promising properties towards the capture of small molecules such as CO₂, CO, and NH₃ among others. However, this capacity is highly reduced after the MOF has been exposed to humid conditions, making its use for practical applications impossible. In this work, we show—for the first time—that the source of the decrease in the uptake capacity is due to the water dissociation reaction (H₂O → OH+H), taking place at the metal centers of the MOF. Furthermore, we show that the H₂O → OH+H reaction in the confined environment of MOF-74 channels can be precisely controlled by the addition of the noble gas He. Elucidating the entire reaction process with ab initio calculations, we prove that the interaction between water molecules is critical to the formation of water networks, which reduce the activation barrier by up to 37% and thus influence the reaction significantly. This knowledge allowed us to design an elegant experiment where time-resolved IR measurements confirm that the formation of these networks can be suppressed by introducing He gas, providing unprecedented control over water dissociation rates. As the water dissociation is related to the structural instability of MOF-74 and the reduction in the CO₂ uptake capacity, our findings lay the groundwork for the design of a water stable MOF-74.

This work was entirely supported by the Department of Energy grant No. DE-FG02-08ER46491.

The ever-growing dependence on the consumption of fossil fuel brings enormous environmental concerns on the level of atmospheric carbon dioxide. Due to their large capacities to adsorb CO₂ at low energy costs, metal-organic frameworks (MOFs) represent a promising class of materials for capture and storage of anthropogenic CO₂. Among the diverse MOF family, several stimuli responsive frameworks have been observed to be able to alter CO₂ uptake capabilities when exposed to external stimuli, which can potentially exploited for low-energy gas capture and release processes. However, the underlying mechanisms for gas adsorption mechanisms and structural characterization of stimuli responsive MOFs remain mysterious. In this work, we investigated two stimuli responsive MOFs, namely, PCN123 and Zn(AzDC)(4,4’-BPE)_0.5 through a combination of Grand Canonical Monte Carlo (GCMC) methods, Density Functional Theory (DFT) and Molecular Dynamic (MD) simulations. In PCN-123 structure, we found that the trans isomers have higher CO₂ uptake capabilities than cis isomers due to the extra host-guest interactions provided by the azobenzene groups. For Zn(AzDC)(4,4’-BPE)_0.5 framework, we dynamically simulated how the structures respond the external stimuli. We observed that the “trigger release” mechanism can be ascribed to the isomerization induced localized structural changes. These findings give us a comprehensive understanding of the gas adsorption behavior of stimuli responsive MOFs and suggest a possibility of using stimuli responsive MOFs for low energy cost CO₂ capture.


[1] Computational Approaches of Understanding the Reversible Alteration of CO2 Adsorption in Porous Materials, Runhong Huang, Ravichandar Babaorao, Nikhil Medhekar, in review.

[2] Molecular Dynamics Simulations of Metal-Organic Frameworks for CO₂ trigger release mechanism, Runhong Huang, Ravichandar Babaorao, Nikhil Medhekar, in review.

Flexible framework nanoporous materials are nanoporous solids
that exhibit a large change in pore volume upon sorption/desorption
of gases, making them intriguing candidates for applications
such as carbon dioxide capture. We use density functional theory (DFT) to
investigate the transition between the narrow-pore (np) and
large-pore (lp) configurations of the flexible framework material MIL-53 (Cr)
by calculating the free energy as a function of volume. The calculated enthalpy of
MIL-53 (Cr) shows two minima corresponding to the np and lp configurations.
The Gibbs free energy versus temperature, incorporating the DFT phonon entropy,
shows a transition from np to lp at about 375 K with a 21 kJ/mol free energy barrier.
Calculations for hydrated MIL-53 show that the np configuration becomes relatively
more stable, due to water molecule hydrogen bonds that bridge the pores.

Amine modified advanced porous materials, such as metal-organic frameworks (MOFs) and porous polymer networks (PPNs), are promising candidates for carbon capture. MOFs/PPNs functionalized with high density of amine groups on their surfaces have relatively strong affinity towards CO₂. Moreover, the low heat capacity of these materials significantly reduces their regeneration energy. Herein, we adopt two different strategies to integrate amine groups into MOFs and PPNs respectively for CO₂ capture.

First, we employ Cr-MIL-101-SO₃H as the pristine MOF to tether alkylamine for CO₂ capture. The amine tethering process is based upon the fast reaction between sulfonic acid group in the MOF and amine group in alkylamine. Several experimental factors, including amine structure, amine quantity, amine tethering time, solvent temperature and solvent, are involved in the tethering procedure, which might affect the result. Thus, we conducted a systematic exploration to optimize these experimental factors and tried to rationalize how they would influence amine tethering process and the CO₂ capture performance. Under the optimized condition, the amine modified Cr-MIL-101-SO₃H has CO₂ uptakes of 51 cm³/g at 150 mbar under 40 °C, and 25 cm³/g at 0.4 mbar under 20 °C. The recyclability test of this material indicated no apparent capacity loss after multiple absorption/desorption cycles.

We further expand the amine incorporation concept to PPNs with a new strategy to synthesize a highly crystalline amine-functionalized PPN via Schiff base reaction. As this reaction process is reversible, the synthetic process could be easily controlled to generate high crystalline products. After the formation of the framework, the C=N bonds are reduced to C-N single bonds, endowing the framework with not only enhanced chemical stability but also abundant secondary alkyl amines, which have a great potential for carbon capture.

In order to correlate a MOFs potential use in various applications, particularly for carbon capture and storage, with its structural properties, there is need to perform an accurate, full textural characterization of the material. We investigated a series of selected MOFs by Ar (87 K) adsorption and high resolution CO₂ adsorption performed systematically over a wide range of pressures and temperatures. Subcritical CO₂ adsorption isotherms (in the temperature range from 253-293 K) were measured up the corresponding saturation pressure. Supercritical CO₂ adsorption experiments at 333 K were performed up to 200 bar. Although CO₂ cannot be used for pore size analysis of MOFs due to its strong quadrupole moment, which leads to specific interactions with polar surface sites, it has the advantage that it can fill narrower micropores than can be accessed by Ar at 87 K (or N₂ at 77 K). Furthermore, an advanced analysis of the CO₂ data allows one to obtain important information about total pore volume, specific surface area (determined from the supercritical CO₂ adsorption isotherm, independent of the BET approach), and heats of adsorption. Our results and suggested procedure allow one to achieve a much clearer picture of the textural characteristics of MOFs, particularly with regard to application in the areas of gas storage and separation.

To be effective, advanced carbon capture materials must have both the ability to adsorb large quantities of carbon dioxide and the ability to adsorb CO₂ selectively in the presence of multiple gases or steam (i.e., moisture). The capacity to adsorb CO₂ under different pressure and / or temperature conditions can frequently be linked to changes in the sorbent structure, which must be understood if sorbents are to be improved and optimized by material development. Meanwhile, a lack of selectivity to adsorb only CO₂ will result in other gases or water molecules being adsorbed and occupying sites within the sorbent that cannot then be occupied by adsorbed CO₂ molecules. Selective adsorption cannot simply be established by measuring the adsorption / desorption isotherms for different gases; it is necessary to determine whether one gas being adsorbed (e.g., CO₂), also lets in a second gas by some collective gate-opening effect. Again, sorption selectivity can frequently be related to sorbent structure and to microstructural or structural changes during the adsorption / desorption cycle.

To elucidate these issues, we have carried out a range of in situ small-angle X-ray and neutron scattering (SAXS and SANS), X-ray and neutron diffraction studies on several sorbent systems during adsorption and desorption of CO₂ and CO₂ gas mixtures under realistic pressure conditions. Pressure conditions include both static and flowing gas environments, and carbon capture materials studied have included flexible metal organic frameworks, zeolites and shale systems. Results will be presented that elucidate the connection between features in the sorption isotherm curves and associated changes in microstructure and structure of the sorbent systems.

[1] K.L. Kauffman, J.T. Culp, A.J. Allen, L. Espinal, W. Wong-Ng, T.D. Brown, A. Goodman, M.P. Bernardo, R.J. Pancoast, D. Chirdon, C. Matranga; “Selective adsorption of CO₂ from light gas mixtures by using a structurally dynamic porous coordination polymer,” Angew. Chem. Int. Ed., 50, 10888-10892 (2011).
[2] L. Espinal, W. Wong-Ng, J.A. Kaduk, A.J. Allen, C.R. Snyder, C. Chiu, D.W. Siderius, L. Li, E. Cockayne, A.E. Espinal, S.L. Suib; “Time-dependent CO₂ sorption hysteresis in a one-dimensional microporous octahedral molecular sieve,” J. Am. Chem. Soc., 134, 7944-7951 (2012).
[3] W. Wong-Ng, J.T. Culp, Y.S. Chen, P. Zavalij, L. Espinal, D.W. Siderius, A.J. Allen, S. Scheins, C. Matranga; “Improved synthesis and crystal structure of the flexible pillared layer porous coordination polymer: Ni(1,2-bis(4-pyridyl)ethylene)[Ni(CN)₄],” CrystEngComm, 15, 4684-4693 (2013).
[4] A.J. Allen, L. Espinal, W. Wong-Ng, W.L. Queen, C.M. Brown, S.R. Kline, K.L. Kauffman, J.T. Culp, C. Matranga; “Flexible metal-organic framework compounds: In situ studies for selective CO₂ capture,” J. Alloys and Compounds, 647, 24-34 (2015).

Having markedly shorter diffusion path lengths for molecules than conventional zeolites, hierarchical zeolites have advantages in catalysis and gas separations, but that has not been the case in the literature especially for the latter applications. Herein we present controlled crystal growth of hierarchical NaX zeolites based on reactive emulsion templating of high-concentration sodium aluminosilicate precursor solutions and show the merits of highly crystalline hierarchical zeolites in CO₂ separation. The precursor solutions are often called geopolymer resin which is prepared with inexpensive precursor such as NaOH, sodium silicate and calcined kaolin and an ambient curing of the inorganic resin usually results in a dense dried gel consisting of amorphous 3D network structure of corner-sharing AlO₄/SiO₄ tetrahedra. By employing vegetable oil to control the alkalinity during the crystal formation/growth, we demonstrate that hierarchical NaX zeolites with a high crystallinity (>95% based on micropore properties) can be produced in a high yield (>0.5 kg/L). The mesopores in the materials exhibit a broad pore distribution with average mesopore widths (4V/A) of about 10 nm, mesopore volumes up to 0.2 cm³/g and external surface areas up to 120 m/g. Unlike previous reports with nanosized zeolites, the newly synthesized hierarchical zeolites show improved gas diffusion, high CO₂-over-N₂ and CO₂-over-CH₄ selectivities (>300% enhancement in comparison to conventional 13X) and negligible regeneration loss. Together with the advantage of the newly developed scalable sustainable synthetic method, the exceptional CO₂ sorption properties of the materials present a viable industrial potential of the materials in reduction of the greenhouse gas emission.

The history of ionic liquids (ILs) effectively started in 1914, when the physical properties of ethylammonium nitrate ([CH₃CH₂NH₃⁺][NO₃^-], m.p. 13-14 °C) were first reported. ILs are generally defined as salts composed of discrete cations and anions with melting points below 100 °C, and many are liquid at ambient temperature. IL research has grown rapidly over the past decade due to the realization that these materials have many unique properties such as negligible vapor pressure and outstanding solvation potential. Ionic liquids have been further emphasized by the fact that their physical and chemical properties can be finely tuned by varying both the cation and anion.

Our research has focused on accurately measuring vapor-liquid equilibria (VLE) and vapor-liquid-liquid equilibria (VLLE) and using thermodynamic models to understand the phase behavior of binary gas mixtures in ILs. This presentation will focus on the importance of characterizing the global phase behavior of gases in ionic liquids and how this can provide insight into new applications. Solubility measurements of several gases including CO₂ in ILs will be discussed and important experimental details regarding VLE measurements using a gravimetric microbalance and VLLE measurements using a mass-volume technique will be highlighted. VLE data have been successfully correlated with a modified Redlich-Kwong equation of state (EOS), and in certain cases (e.g. CO₂) the EOS predicts partial immiscibilities (LLE) with lower critical solution temperatures (LCSTs) in the CO₂-rich side solutions. We have also found that gases such as CO₂ can exhibit different solubility behaviors in ILs (i.e. physical and chemical absorption) and that these behaviors can be analyzed with the EOS using a simple association model and excess thermodynamic functions.

Knowledge of gas and IL phase behavior has led to several practical applications including removal of CO₂ from flue gas which will be highlighted in this presentation.

Having worked on several approaches to CO₂ capture since about 2005, I have explored a number of physical and chemical solvents as well as polymer membranes. Initially, a great deal of these of materials were based upon ionic liquids (ILs). Due to a number of challenges encountered in the practical application of ILs due to the demanding requirements in CO₂ separation processes, I started to reconsider how best to use ILs to address CO₂ capture. Ultimately, more promising and robust materials may not come from ILs themselves, but from retrosynthetic analysis and a reconsideration of which structural variables and properties are (and are not) important. Through hybridization of the constituent parts of ILs into entirely new, yet seemingly familiar, substances with improved properties and economics can be realized. This talk will highlight lessons learned from ILs and how these have influenced recent work from my group in the development of new amine solvents and polymer materials that better address the demanding process conditions and requirements of CO₂ capture and related separations.

Encapsulated liquid sorbents have been proposed as a possible solution to the drawbacks of current carbon dioxide capture methods. A liquid sorbent surrounded by a permeable polymer shell can reduce the corrosivity, evaporative losses, and fouling associated with the neat sorbents. The encapsulation also facilitates rapid carbon dioxide uptake rates by providing high surface area. However, typical devices have limited throughputs on the order of milliliters per hour. Here we report the generation of encapsulated carbon dioxide sorbents using a parallelized design for high throughput. We demonstrate a design incorporating 12 parallel microfluidic double emulsion generators in a modular setup capable of producing encapsulated sorbents at rates over 500g per day. The parallelized generation of encapsulated sorbents provides a route towards low-cost and energy-efficient carbon dioxide capture for large-scale operations.

The storage, sequestration and selective binding of fuel gases in a safe and compact form represents a significant current challenge. There is wide-ranging interest in the development of stable materials that can store and release hydrogen, carbon dioxide, sulphur dioxide, methane and hydrocarbons with fast kinetics and high reversibility over multiple cycles. Porous co-ordination framework compounds have enormous potential in this regard. We report the synthesis, structural characterisation and gas adsorption studies of a range of metal-organic materials derived from stable carboxylate-linked complexes that exhibit high porosity and surface area coupled with high storage capacities and selectivities. Of particular interest are defect structures, doped materials and in situ structural and dynamic studies of gas-loaded materials that define the binding of substrates within pores at a molecular level.

We describe the preparation of a series of doubly-interpenetrated materials that show structures in which part of the structure of the second net is not fully formed leading to the formation of slit pores and defect structures. The synthesis, structure and properties of a unique non-amine-containing porous solid NOTT-300(M) (M = Al, Bi, In V, Sc, Cr, Fe) and doped analogues in which hydroxyl group within the pores bind selectively to CO₂, SO₂, acetylene and ethylene are also described. NOTT-300 exhibits highly selective uptake of CO₂ versus N₂, CH₄, H₂, CO, O₂, Ar, and in situ powder X-ray diffraction (PXRD), neutron diffraction and inelastic neutron scattering (INS) studies, combined with density functional theory (DFT) modelling, reveal that these hydroxyl groups bind CO₂ and SO₂ via the formation of supramolecular hydrogen bonds. These are reinforced further by weak supramolecular interactions with C-H hydrogen atoms on the phenyl rings. This offers exciting potential for the application of new capture systems based on the soft binding of CO₂ and SO₂ via the use of an “easy-on“ and “easy-off“ model.

This work has been extended to other propus materials and to the study of the storage and binding of acetylene and ethylene which bind to the metal-organic host via hydrogen bonding between hydroxyl group and the π-electron density of the unsaturated guest. Excellent gas capacities and selectivities between alkane vs alkene vs alkyne are observed, representing a methodology for purification of ethylene.

Nanoporous polymers attracted¹ significant deal of attention in recent years due to their permanent porosity, chemical tunability, physicochemical stability and exceptional gas sorption properties. In particular, C-C bond formation reactions played a significant role in the synthesis of several nanoporous polymers. Incorporation of polycyclic aromatic hydrocarbons, e.g., graphene nanoribbons (GNRs) into nanoporous polymers, however, has been a significant challenge using these C-C polymerization reactions mainly due to their low solubility and high affinity to restack to form graphitic layers due to interlayer π-π stacking and van der Waals interactions. In order to prevent restacking, we propose to introduce a permanent ‘spacer’ such as porosity between GNRs/graphene layers² within nanoporous polymers. Herein, we present³ a new polymerization strategy, that is catalyst-free Diels-Alder cycloaddition polymerization and subsequent FeCl₃-catalyzed intramolecular cyclodehydrogenation reaction, to introduce graphene nanoribbons up to 2 nm in length and 1.1 nm in width into a graphene nanoribbon framework (GNF). The first graphene nanoribbon framework showed high thermal stability up to 400^oC in air with relatively narrow pore size distribution and exhibited BET surface area of 679 m² g^-1. GNF possesses high affinity for H₂ (Q_st 7.7 kJ mol^-1, 1.03 wt% at 77 K, 1 bar), CO₂ (Q_st = 28.7 kJ mol^-1, 94.6 mg g^-1 at 273 K, 1 bar), and CH₄ (Q_st = 24. 1 kJ mol^-1, 11.5 mg g^-1 at 273 K, 1 bar). The enhancement in gas affinities was attributed to the unique combination of large π-surface area arising from graphene nanoribbons and small pores (~5.8 Å) in GNF. The application of GNF was also be extended to natural gas purification process with exceptional CO₂/CH₄ (5:95) selectivity of 62.7, which is being the highest value reported to date at 298 K.

[[1]] Patel, H. A.; Hyun Je, S.; Park, J.; Chen, D. P.; Jung, Y.; Yavuz, C. T.; Coskun, A., Nat. Commun. 2013, 4, 1357.
[2] Byun, Y.; Coskun, A. Chem. Mater., 2015, , 27, 2576–2583.
[3] Song, K. S.; Coskun A.*, ChemPlusChem, 2015, 80, 1127–1132.

Moisture swing adsorption (MSA) shows promise as a technique to pull carbon dioxide out of the ambient air on the gigaton scale at a relatively low energy cost. However, a weakness to this technique is the adsorbant; advanced materials may be needed for this specific purpose for MSA of carbon dioxide to be realized. Highly tunable metal-organic-frameworks embedded on electrospun nanofibers to make thin, compact, nanoscale adsorbants may be able to solve this challenge. Challenges and advances in fabrication and characterization of these membranes will be presented along with the potential of UiO-66 and its derivatives for moisture swing adsorption, and a brief overview of moisture swing adsorption.

Rising demand for fossil-based energy sources has resulted in an increase in the emission of greenhouse gases such as CO₂, which have an adverse impact on the environment. The emerging reliance on shale oil and gas as energy resources and the extensive use of water for hydraulic fracturing are environmental concerns. Therefore, there is a need to explore alternative fracturing fluids such as CO₂ for sustainable energy extraction. There is a lack of fundamental understanding of the chemical and morphological changes that occur in shales in the presence of CO₂. Further, the variable compositions of clays, carbonates, quartz, and other phases in shales complicate our understanding of CO₂-shale interactions. In this study, shales with varying compositions of clays, carbonates, and quartz are investigated. In-situ and in-operando changes in the molecular structure, microstructure, and pore volume of shales during adsorption and desorption of CO₂ are investigated using Wide, Small, and Ultra-Small Angle X-Ray Scattering (WAXS/SAXS/USAXS). These results show that depending on the composition of clays and carbonates in the shale, the adsorption and desorption behaviors vary and in some cases, the changes in the microstructure after a shale is treated with CO₂ may be irreversible.

The importance of CaO based sorbent for CO₂ capture is well studied in the literature [1,2]. CaO based sorbent deactivate after the initial step due to sintering by the build up of layer of CaCO₃ which hinder the diffusion of CO₂ to the CaO reaction front. Hydration is considered as the most promising to reactivate the sintered CaO based sorbent. However,many researchers believe that the reactivation mechanism through hydration is still sketchy and not well defined up to date. Here a mechanism is proposed which start with the diffusion of water through the sintered layer of CaCO₃ to hydrate CaO and forms a reaction front of hydrated lime according [3]
CaO + H₂O ------ Ca(OH)₂ + Heat ----- Ca²⁺ + 2 OH^- {1}
At the reaction front the pH becomes above 9 [1]. This causes the following reaction to shift to the right [4]
CO₂(g) + H₂O ------- CO₂(aq) ---- CO₃^2- + 2H⁺ {2}
Thus we have,
CaO + CO₂(g) + H₂O ------- CaCO₃ + 2H₂O {3}
The reaction depend on the diffusion of water and carbonate ions through the sintered CaCO3 layer that formed in the intial stage. The formation of new hydrated lime reaction front ( reaction 1 )and subsequent reaction with carbonate ions according to reaction 3. Thus as CaCO₃ is formed by capturing CO₂ through reaction 2 above, more water is also formed and a new reaction front of Ca(OH)₂ is formed.
This agrees with the fact that sulphidation deactivate the CO2 capture efficiency because absorption or formation of acidic gases lower the pH at the interface during steam hydration and thus reaction 2 shift to the left depending on the change of pH. At pH less than 7 the predominant species becomes bicarbonate ions and at much lower pH the the predominant species becomes non ionic dissolved carbon dioxide ( CO₂₍aq)). This is also in line with the fact that durability of the sorbent derived from Ca(OH)₂ is better compared to sorbent obtained from CaCO_3. Furthermore such mechanism may explain the beneficial effect of using Ca-acetate pellets in piolt plant fluidized bed reactors, modification of the CaO sorbent with alkali metal salts and modification by MgO. Finally assuming that the rate of diffusion of carbonate ions to be the rate limiting step. Then the rate of carbonation measured experimentally should be equal to the theoritical estimation of the rate of diffusion of carbonate ions through the sintered CaCO₃ layer.
[1] Wang et al. Energy and Fuels 22, 2326, 2008
[2] Felix Donat et al. Environmetal Sci. Tech. 45, 1262, 2012
V.L.Snoyink and D.Jones , Water chemistry, J Wiley Publication, 1980
[3] Carbonate Chemistry on page 156.
[4] Calcium Carbonate on page 282.

Calcium carbonate and dolomite are relatively abundant minerals and show after calcination the possibility to pick up carbon dioxide again. This can be especially interesting for the CO₂ capture at power plants.
Quick tests for checking the suitability of such substances can be done by thermal analysis techniques, like TGA. Combined with evolved gas analysis techniques, like mass spectrometry, the information can be highly increased.
In this paper the CO₂ desorption/absorption of calcined dolomite and calcite were measured with a TGA-DSC-MS instrument. The quantification of the CO₂ can be done either by the mass loss steps or by the calibration of the mass spectrometer signal by the PulseTA method.