Postcombustion CO₂ Capture in Functionalized Porous Coordination Networks

Motivated by recent experimental reports of zirconium porous coordination networks (PCNs) [<i>J. Am. Chem. Soc.</i> <b>2012</b>, <i>134</i>, 14690–14693], which have demonstrated a good stability and CO₂ adsorption capacity, we investigate the influence of flue gas impurities and functional groups on the performance of PCN frameworks in selective CO₂ capture. Using a combination of grand canonical Monte Carlo (GCMC) simulations and first-principles calculations, we find that O₂ and SO₂ impurities in flue gas have a negligible influence on CO₂ selectivity in all PCN frameworks. However, because of strong electrostatic interaction between H₂O molecules and the framework, CO₂ selectivity decreases in all PCN structures in the presence of water impurities in the flue gas. Our studies suggest that the PCN-59 framework can be a good candidate for selective CO₂ separation from a predehydrated flue gas mixture

Microwave-Assisted Solvothermal Synthesis and Optical Properties of Tagged MIL-140A Metal–Organic Frameworks

A series of tagged MIL-140A-R frameworks have been synthesized using a microwave-assisted solvothermal method. Compared with their UiO-66-R polymorphs, the absorption energies in the MIL-140A-R series (R = NH₂, NO₂, Br, Cl, and F) are extended toward the visible region because of the spatial arrangement of the linkers

Nitrogen-Doped Mesoporous Carbon for Carbon Capture – A Molecular Simulation Study

Using molecular simulation, we investigate the effect of nitrogen doping on adsorption capacity and selectivity of CO₂ versus N₂ in model mesoporous carbon. We show that nitrogen doping greatly enhances CO₂ adsorption capacity; with a 7 wt % dopant concentration, the adsorption capacity at 1 bar and 298 K increases from 3 to 12 mmol/g (or 48% uptake by weight). This great enhancement is due to the preferred interaction between CO₂ and the electronegative nitrogen. The nitrogen doping coupled with the mesoporosity also leads to a much higher working capacity for adsorption of the CO₂/N₂ mixture in nitrogen-doped mesoporous carbon. In addition, the CO₂/N₂ selectivity is almost 5 times greater than in nondoped carbon at ambient conditions. This work indicates that nitrogen doping is a promising strategy to create mesoporous carbons for high-capacity, selective carbon capture

Methane Adsorption and Separation in Slipped and Functionalized Covalent Organic Frameworks

Understanding atomic-level mechanisms of methane adsorption in nanoporous materials is of great importance to increase their methane storage capacity targeting energy sources with low carbon emission. In this work, we considered layered covalent organic frameworks (COFs) with low density and revealed the effect of slipping and chemical functionalization on their methane adsorption and separation properties. We performed grand canonical Monte Carlo simulations studies of methane (CH₄) adsorption and carbon-dioxide:methane (CO₂:CH₄) separation in various slipped structures of TpPa1, TpBD, PI-COFs, and functionalized TpPa1 and TpBD COFs as well. We observed that the slipping improves the total CH₄ uptake by 1.1–1.5 times, while functionalization does not have a significant effect on CH₄ uptake. We also observed improvement in CO₂:CH₄ selectivity due to slipping, whereas functionalization results in decrease in the selectivity. In all considered COFs, we found the highest CH₄ delivery capacity of 141 cm³ (STP) cm^–3 at 65 bar and selectivity of ∼25 at 1 bar in 60-AB slipped structure of TpBD COF. We analyzed the molecular details of CH₄ adsorption using binding energy, heat of adsorption, pore characteristics, and expectation energy landscape. Our results show that COFs with increasing profile of heat of adsorption with pressure have the higher CH₄ delivery capacity. In these COFs, we found proximity (∼4–6 Å) of CH₄ binding sites, resulting in higher CH₄–CH₄ interactions and hence the increasing profile of CH₄ heat of adsorption

Computer-Aided Design of Interpenetrated Tetrahydrofuran-Functionalized 3D Covalent Organic Frameworks for CO₂ Capture

Using computer-aided design, several interpenetrated imine-linked 3D covalent organic frameworks with diamondoid structures were assembled from tetrakis-4-formylphenylsilane as the tetrahedral node, and 3<i>R</i>,4<i>R</i>-diaminotetrahydrofuran as the link. Subsequently, the adsorption capacity of CO₂ in each framework was predicted using grand canonical Monte Carlo simulations. At ambient conditions, the 4-fold interpenetrated framework, with disrotatory orientation of the tetrahedral nodes and diaxial conformation of the linker, displayed the highest adsorption capacity (∼4.6 mmol/g). At lower pressure, the more stable 5-fold interpenetrated framework showed higher uptake due to stronger interaction of CO₂ with the framework. The contribution of framework charges to CO₂ uptake was found to increase as the pore size decreases. The effect of functional group was further explored by replacing the ether oxygen with the CH₂ group. Although no change was observed in the 1-fold framework, the CO₂ capacity at 1 bar decreased by ∼32% in the 5-fold interpenetrated framework. This work highlights the need for a synergistic effect of a narrow pore size and a high density of ether-oxygen groups for high-capacity CO₂ adsorption

CO₂ Adsorption in Azobenzene Functionalized Stimuli Responsive Metal–Organic Frameworks

Recent reports of externally triggered, controlled adsorption of carbon dioxide (CO₂) have raised the prospects of using stimuli responsive metal–organic frameworks (MOFs) for energy efficient gas storage and release. Motivated by these reports, here we investigate CO₂ adsorption mechanisms in photoresponsive PCN-123 and azo-IRMOF-10 frameworks. Using a combination of grand canonical Monte Carlo and first-principles quantum mechanical simulations, we find that the CO₂ adsorption in both frameworks is substantially reduced upon light-induced isomerization of azobenzene, which is in agreement with the experimental measurements. We show that the observed behavior originates from inherently weaker interactions of CO₂ molecules with the frameworks when azobenzene groups are in cis state rather than due to any steric effects that dramatically alter the adsorption configurations. Our studies suggest that even small changes in local environment triggered by external stimuli can provide a control over the stimuli responsive gas adsorption and release in MOFs

Interpenetrated Zirconium–Organic Frameworks: Small Cavities versus Functionalization for CO₂ Capture

Porous interpenetrated zirconium–organic frameworks (PIZOFs) with various functional groups are explored for CO₂ capture using molecular simulation and experiment. Functionalization enhances the CO₂ uptake and selectivity over other gases, but small cavities play an even more important role. Particularly at low pressures, small cavities enhance the CO₂ adsorption density nearly 5 times greater than the functionalization. PIZOF-2 outperforms the other PIZOF structures for CO₂ separation from methane and nitrogen (related to raw natural gas and postcombustion of coal mixtures) due to the combination of small cavities around 5 Å in diameter and functionalized linkers with methoxy groups attached to the central ligand. The small cavities within the interpenetrated structures are crucial for achieving high selectivities, especially for cavities surrounded by a combination of 6 benzene rings, 2 metal clusters, and 4 methoxy groups that offer a tight overlapping potential energy field, ideal for “catching” CO₂

Interpenetrated Zirconium–Organic Frameworks: Small Cavities versus Functionalization for CO₂ Capture

Porous interpenetrated zirconium–organic frameworks (PIZOFs) with various functional groups are explored for CO₂ capture using molecular simulation and experiment. Functionalization enhances the CO₂ uptake and selectivity over other gases, but small cavities play an even more important role. Particularly at low pressures, small cavities enhance the CO₂ adsorption density nearly 5 times greater than the functionalization. PIZOF-2 outperforms the other PIZOF structures for CO₂ separation from methane and nitrogen (related to raw natural gas and postcombustion of coal mixtures) due to the combination of small cavities around 5 Å in diameter and functionalized linkers with methoxy groups attached to the central ligand. The small cavities within the interpenetrated structures are crucial for achieving high selectivities, especially for cavities surrounded by a combination of 6 benzene rings, 2 metal clusters, and 4 methoxy groups that offer a tight overlapping potential energy field, ideal for “catching” CO₂

Polar Pore Surface Guided Selective CO₂ Adsorption in a Prefunctionalized Metal–Organic Framework

Selective CO₂ adsorption over other small gases has been realized in an ultra-microporous metal–organic framework (MOF). In the quest of manifesting such selective carbon capture performance, the prefunctionalized linker strategy has been espoused. A new Zn­(II)-based three-dimensional, 3-fold interpenetrated metal–organic framework material [Zn­(PBDA)­(DPNI)]_<i>n</i>·<i>x</i>G (PBDA: 4,4′-((2-(<i>tert</i>-butyl)-1,4-phenylene)­bis­(oxy))­dibenzoic acid; DPNI: <i>N</i>,<i>N</i>′-di­(4-pyridyl)-1,4,5,8-naphthalenetetracarboxydiimide; <i>x</i>G: <i>x</i> number of guest species) with unusual rob topology is synthesized following a typical solvothermal synthesis protocol, which gleans a modest CO₂-selective adsorption trend over its congener gases (saturation CO₂ uptake capacity: 2.39 and 3.44 mmol g^–1, at 298 and 273 K; volumetric single component isotherm based separation ratios at 0.2 bar: 189.4 (CO₂/N₂, 256.5 (CO₂/H₂), 12.3 (CO₂/CH₄); at 1 bar: 6.8 (CO₂/N₂, 17.1 (CO₂/H₂), 7.1 (CO₂/CH₄)). The compound also exhibits selective benzene sorption over its aliphatic C₆-analogue cyclohexane. The structure–property correlation guided results supported by theoretical introspection further emphasize the omnipresent role of crystal engineering principles behind culmination of such targeted properties in the nanoporous MOF domain, to realize selective sorption facets

Post-synthetic Structural Processing in a Metal–Organic Framework Material as a Mechanism for Exceptional CO₂/N₂ Selectivity

Here we report the synthesis and ceramic-like processing of a new metal–organic framework (MOF) material, [Cu­(<b>bcppm</b>)­H₂O], that shows exceptionally selective separation for CO₂ over N₂ (ideal adsorbed solution theory, <i>S</i>_ads = 590). [Cu­(<b>bcppm</b>)­H₂O]·<i>xS</i> was synthesized in 82% yield by reaction of Cu­(NO₃)₂·2.5H₂O with the link bis­(4-(4-carboxy­phenyl)-1<i>H</i>-pyrazolyl)­methane (<b>H</b>_<b>2</b><b>bcppm</b>) and shown to have a two-dimensional 4⁴-connected structure with an eclipsed arrangement of the layers. Activation of [Cu­(<b>bcppm</b>)­H₂O] generates a pore-constricted version of the material through concomitant trellis-type pore narrowing (<i>b</i>-axis expansion and <i>c</i>-axis contraction) and a 2D-to-3D transformation (<i>a</i>-axis contraction) to give the adsorbing form, [Cu­(<b>bcppm</b>)­H₂O]-<i>ac</i>. The pore contraction process and 2D-to-3D transformation were probed by single-crystal and powder X-ray diffraction experiments. The 3D network and shorter hydrogen-bonding contacts do not allow [Cu­(<b>bcppm</b>)­H₂O]-<i>ac</i> to expand under gas loading across the pressure ranges examined or following re-solvation. This exceptional separation performance is associated with a moderate adsorption enthalpy and therefore an expected low energy cost for regeneration