Reactivity of alkanes on zeolites: a computational study of propane conversion reactions

In this work, quantum chemical methods were used to study propane conversion reactions on zeolites; these reactions included protolytic cracking, primary hydrogen exchange, secondary hydrogen exchange, and dehydrogenation reactions. The reactants, products, and transition-state structures were optimized at the B3LYP/6-31G level and the energies were calculated with CBS-QB3, a complete basis set composite energy method. The computed activation barriers were 62.1 and 62.6 kcal/mol for protolytic cracking through two different transition states, 30.4 kcal/mol for primary hydrogen exchange, 29.8 kcal/mol for secondary hydrogen exchange, and 76.7 kcal/mol for dehydrogenation reactions. The effects of basis set for the geometry optimization and zeolite acidity on the reaction barriers were also investigated. Adding extra polarization and diffuse functions for the geometry optimization did not affect the activation barriers obtained with the composite energy method. The largest difference in calculated activation barriers is within 1 kcal/mol. Reaction activation barriers do change as zeolite acidity changes, however. Linear relationships were found between activation barriers and zeolite deprotonation energies. Analytical expressions for each reaction were proposed so that accurate activation barriers can be obtained when using different zeolites as catalysts, as long as the deprotonation energies are first acquired.