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J Am Chem Soc. 2016 Oct 26;138(42):14047-14056. doi: 10.1021/jacs.6b08646. Epub 2016 Oct 17.

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

Author information

1
Institut für Chemie, Humboldt-Universität zu Berlin , Unter den Linden 6, 10099 Berlin, Germany.
2
Institute of Chemistry, University of Tartu , Ravila 14 a, 50411 Tartu, Estonia.

Abstract

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.

PMID:
27748594
DOI:
10.1021/jacs.6b08646

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