The suitability of elastic layered metal-organic frameworks (ELM) for the removal of carbon dioxide from methane-rich gas mixtures was evaluated using molecular modeling and experimental measurements for two adsorbents, ELM-11 [Cu(BF4)2(bpy)2] and ELM-12 [Cu(OTf)2(bpy)2]. These materials are promising absorbents for CO2 separation from biogas and natural gas mixtures, due to their reversible gated isotherm features and their high selectivity adsorption of CO2 over CH4 (S = 50-100 at 298 K) from gas mixtures with CH4 and CO2 compositions representative of biogas / natural gas extraction streams.
Grand canonical Monte Carlo (GCMC) simulation was used to calculate the framework adsorption capacity and CO2 selectivity from binary CH4 / CO2 mixtures. Simulated CO2 capacities above the gating pressure and CO2 isosteric heats of adsorption agree well with ELM-11 and ELM-12 experimental data. Ideal adsorbed solution theory (IAST) estimates of CO2 selectivity are generally in good agreement with the GCMC simulations, although modifications of the IAST methodology are needed in order to take into account the co-adsorption condition for CH4 uptake by CO2 gate-opening. Density functional theory (DFT) calculations are also presented for CO2 and CH4 interactions with the ELM-11 and ELM-12 framework structures. Upon the adsorption of CO2 in the ELM framework, the metal-anion bond of the framework was extended, whereas the same bond length is virtually unchanged upon CH4 adsorption. DFT analysis of the adsorption binding energies of other trace components in natural gas and biogas production (e.g. H2S, H2O, N2) and CH4 will also be presented and discussed.
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