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Physisorption and Chemisorption of Hydrocarbons in H-FAU Using QM-Pot(MP2//B3LYP) Calculations

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journal contribution
posted on 07.08.2008, 00:00 by Bart A. De Moor, Marie-Françoise Reyniers, Marek Sierka, Joachim Sauer, Guy B. Marin
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/AlF = 95) and for physisorption of 1-pentene, n-pentane, 1-octene, and n-octane in all silica FAU. The new parametrization predicts more stable chemisorption complexes than physisorbed π 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+···alkane interaction was found to contribute at most 7 kJ/mol to the total physisorption energy of n-alkanes while the H+···π 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.