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

Abstract

Adsorption of carbon dioxide in the metal–organic framework CPO-27-Mg (Mg-MOF-74) is examined. We use accurate quantum chemical ab initio methods (wave function-type electron correlation methods for cluster 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 Mg2+ sites are −46 and −9 kJ/mol, respectively; for linker sites these values are −30 and +5 kJ/mol, respectively. For full monolayer coverage lateral interactions 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)...