Ab Initio Calculation of Thermodynamic Functions for CO2 Adsorption in Metal–Organic Frameworks: Entropic Effects of Lateral Interactions

We use quantum chemistry combined with molecular statistics to calculate Gibbs free energies, enthalpies, and entropies of adsorption for carbon dioxide binding to open metal sites in the metal–organic framework Mg2(dobdc), also known as CPO-27-Mg and Mg–MOF-74. For gas–surface interaction energies, we combine periodic MP2 and DFT+D for the full crystal framework with CCSD(T) for the adsorption site. Thermodynamic functions are calculated from anharmonic vibrational frequencies for the isolated sites and for higher surface coverages where adsorbed CO2 molecules interact with neighboring molecules. The intermolecular lateral vibrations bring about an additional set of configurations that are not present in the case of molecules at isolated sites. Lateral adsorbate–adsorbate interaction contributions can be calculated separately and taking them into account results in a good agreement (±1 kJ/mol) between thermodynamic functions derived from experimental isotherms and ab initio calculations.