Ab Initio Prediction of Adsorption Isotherms for Small Molecules in Metal–Organic Frameworks

• Published in: Journal of the American Chemical Society 2016-10, Vol.138 (42), p.14047-14056
• Main Authors: Kundu, Arpan, Piccini, GiovanniMaria, Sillar, Kaido, Sauer, Joachim
• Format: Article
• Language: English
• ISSN: 0002-7863; 1520-5126
• DOI: 10.1021/jacs.6b08646

For CO and N2 on Mg2+ sites of the metal–organic framework CPO-27-Mg (Mg-MOF-74), ab initio calculations of Gibbs free energies of adsorption have been performed. Combined with the Bragg-Williams/Langmuir model and taking into account the experimental site availability (76.5%), we obtained adsorption isotherms in close agreement with those in experiment. The remaining deviations in the Gibbs free energy (about 1 kJ/mol) are significantly smaller than the “chemical accuracy” limit of about 4 kJ/mol. The presented approach uses (i) a DFT dispersion method (PBE+D2) to optimize the structure and to calculate anharmonic frequencies for vibrational partition functions and (ii) a “hybrid MP2:(PBE+D2)+ΔCCSD­(T)” method to determine electronic energies. With the achieved accuracy (estimated uncertainty ±1.4 kJ/mol), the ab initio energies become useful benchmarks for assessing different DFT + dispersion methods (PBE+D2, B3LYP+D*, and vdW-D2), whereas the ab initio heats, entropies, and Gibbs free energies of adsorption are used to assess the reliability of experimental values derived from fitting isotherms or from variable-temperature IR studies.