Gas Cleanup Using Elastic Layered Metal-Organic Framework Adsorbents

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.