First-Principles Simulation of Active Site Selectivity for CO₂ and H₂ Adsorption on Mg(OH)⁺/Mg²⁺ and Ca(OH)⁺/Ca²⁺ Zeolites

One very effective strategy for addressing global warming and transitioning to sustainable energy sources is selective CO2 separation over H2. Porous materials, particularly zeolites, have demonstrated enormous potential for energy-efficient separation techniques combined with storage. By altering interactions at cation-binding sites, zeolite’s gas adsorption characteristics toward carbon capture can be improved. In this study, Mg and Ca are assessed as extra-framework cations in divalent (Ca2+, Mg2+) and monovalent states (Ca(OH)+, Mg(OH)+) in faujasite and chabazite zeolites for CO2 capture over H2. The study also explores the impact of mixed cations, viz., a combination of both Ca2+ and Mg2+ in the framework, on their selective adsorption potential. This study uses DFT with dispersion corrections to calculate adsorption energies, enthalpies, and Gibb’s free energies of adsorbed CO2 and H2 molecules. Among the dispersion parameters evaluated, viz., D4, TS/HI, and MBD, D4 approaches experimental accuracy. In general, the adsorption trend obtained for CO2 is Mg FAU > Ca FAU > Mg CHA > Ca CHA > Mg(OH)+ FAU > Ca(OH)+ FAU > Mg(OH)+ CHA > Ca(OH)+ CHA. The heats of adsorption using PBE+D4 for CO2 are −46 kJ/mol for Ca FAU and −10 kJ/mol for Ca(OH)+ FAU, respectively. These values closely align with the experimental results of −45 and −6 kJ/mol, respectively, within a chemical accuracy limit of ±4 kJ/mol. The relative adsorption energies suggest that for both FAU and CHA, there exists a minimum difference of 26 kJ/mol in adsorption energies between CO2 and H2. Hence, this piece of work highlights that FAU with Ca and Mg as extra-framework cations in a six-membered cage can be a viable substitute to replace, the current best candidate in literature, viz., Li+, for selective CO2 capture.