Ab Initio Adsorption Isotherms for Molecules with Lateral Interactions: CO2 in Metal-Organic Frameworks

Adsorption of carbon dioxide in the metal-Organic framework CPO-27-Mg-(Mg-MOP-74) is examined. We use accurate quantum chemical ab initio methods (wave function-type electron correlation methods for chiSter models-combined with density functional theory for periodic systems) to calculate gas surface site and gas gas interactions. At 298 K, the "zero-coverage" enthalpy and Gibbs free,energy of CO2 adsorption on me sites are -46 and -9 kJ/mol respectively; for linker sites these values are -30 and +5 respectively. For full monolayer coverage' lateral "interactions l.from nearby molecules contribute 6 and 5 kJ/mol, to-the adsorption enthalpy for CO2, at Mg2+ and linker sites, respectively. The predicted heats of adsorption and free energies of adsorption agree within 2.6 and 0.8 kJ/mol, respectively, with experimental values well within chemical accuracy limits (4.2 kJ/mol). We use two different ways of calculating isotherms -from equilibrium, constants for individual sites and interaction energies: (i) a Langmuir model, augmented with the mean-field (MF), approximation for lateral interactions, and grand canonical Monte Carlo (GCMC) simulations on a lattice of sites, which agree very well with each,'other. We use GCMC data to examine how different isotherm models (Langmuir, dual-site Langmuir, Sips, Toth, and mean-field) fit them. We conclude that the MP model yields the best fit over a wide pressure range With physically meaningfulpararneteks, i.e, adsorption constants foindivicipal sites' and lateral interaction energies.