Elastic layered metal-organic framework (ELM) adsorbents exhibit a flexible two-dimensional latent porous crystalline structure. ELMs undergo abrupt reversible gated sorption transitions from an empty collapsed structure to a filled expanded porous state through cooperative adsorption of guest molecules between layer planes. The ELM gating transition and associated hysteresis loop is different than the gas condensation phase transition commonly observed for other nanoporous adsorbents, in that it is induced by rearrangement of the MOF structure rather than by sorbent pore size distribution effects. Certain ELMs have very high selectivity for CO2 adsorption from gas mixtures, and are intriguing adsorbents for cost reductions in carbon capture and storage operations for CO2 removal from nitrogen and other flue gases in post-combustion separations or from hydrogen in pre-combustion separation (e.g. following coal gasification). Moreover, some ELMs have attractive sorption capacities, enthalpies, and isotherm features for efficient recovery of captured CO2 using temperature or pressure swing adsorption. ELMs thus merit further investigation as materials for sorbent-based capture of industrial CO2 emissions.
We report results from molecular simulations of ELM-11 [Cu(BF4)2(bpy)2 (bpy = bipyridine)] and ELM-12 [Cu(bpy)2(OTf)2 (OTf = trifluoromethanesulfonate)] for separation of CO2 from pre- and postcombustion gas mixtures. Adsorption capacity, CO2 selectivity, and isosteric heat of adsorption for pure CO2 and binary CO2/H2 and CO2/N2 mixtures were calculated via grand canonical Monte Carlo (GCMC) simulation at temperatures (273 to 400 K), pressures, and compositions (5 to 15% CO2) representative of power plant flue gas streams. GCMC simulations of CO2 capacity above the gate pressure using a rigid model of ELM-11 agreedwell with experimental isotherms at 273 K. CO2 sorption capacity (140 mg/g) and heat of adsorption on ELM-11 are competitive with commodity adsorbents at realistic process conditions. Simulations project a CO2/N2 selectivity of 700 at 300 K, a capture ratio significantly higher than common activated carbons and zeolites. The simulated CO2 selectivity is comparable to experimental results for separating CO2 from N2 and O2 by temperature swing adsorption from 268 K to 311 K using ELM-11. Desorption of captured CO2 of more than 99% purity was obtained from a ternary mixture of 40% CO2, 48% N2 and 12% O2 at 268 K. GCMC simulations on ELM-11 for CO2/H2 mixtures representing shifted synthesis gas were also carried out and CO2 selectivities of 1000 and 400 were obtained at 298 and 373 K respectively. These compare favorably with reported values for commercial adsorbents used for gas separations. To investigate framework flexibility effects on CO2 sorption in ELM-11, a hybrid Monte Carlo/molecular dynamics (MCMD) method was applied. CO2 adsorption isotherms showed good agreement with experiments at 273 K. Mixed-gas simulations yield CO2/N2 selectivities of 70 and 20 respectively at 300 K and 350 K, comparable or superior to other common adsorbents, but substantially lower than selectivities predicted from rigid model framework simulations. Isotherms for CO2 and N2 on ELM-12 are contrasted to those obtained for ELM-11. The bulkier ELM-12 counter-ion yields a structure with microporosity in addition to latent porosity as found in ELM-11. Unlike ELM-11, ELM-12 exhibits a two-step isotherm with micropore filling followed by interlayer clathrate formation at a narrowly defined gating pressure.
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