Physisorption and chemisorption of hydrocarbons in H-FAU using QM-Pot(MP2//B3LYP) calculations

The DFT parametrized zeolite force field in the QM-Pot program is extended with carbon-carbon, carbon-hydrogen, and alkoxy bond describing parameters. The extended force field has been combined with B3LYP and with MP2 as the high-level quantum mechanical (QM) method to simulate the physisorption and chemisorption of ethene, isobutene, 1-butene, 1-pentene, and 1-octene in H-FAU (Si/Al-F = 95) and for physisorption of I-pentene, n-pentane, 1-octene, and n-octane in all silica FAU. The new parametrization predicts more stable chemisorption complexes than physisorbed pi complexes, but with smaller chemisorption energies which are more reliable as shown by comparison with experimental results and with accurate hybrid MP2:DFT calculations. An embedded cluster size study shows that, due to the importance of the stabilizing van der Waals part in the MM contribution of the cluster, QM-Pot(MP2//B3LYP) calculations yield more reliable physisorption and chemisorption energies of hydrocarbons in zeolites than QM-Pot(B3LYP). The QM-Pot(MP2//B3LYP) results are in good agreement with available experimental data. In H-FAU, the H+center dot center dot center dot alkane interaction was found to contribute at most 7 kJ/mol to the total physisorption energy of n-alkanes while the H+center dot center dot center dot pi interaction contributes 20-25 kJ/mol to the total physisorption energy of alkenes. For n-alkene physisorption in H-FAU, a linear increase of both the physisorption and chemisorption energies of 8.7 kJ/mol per C-atom is found. The protonation energy of n-alkenes in H-FAU was found to be independent of the C-number and amounts to -50 kJ/mol for the formation of secondary alkoxides. The formation of tertiary alkoxides in H-FAU suffers slightly from steric constraints imposed by the zeolite framework.