A computational study of methane catalytic reactions on zeolites

The cluster approach method is used to study the transition state structures and the activation barriers of methane hydrogen exchange and dehydrogenation reactions catalyzed by zeolites. The reactant and transition state structures are optimized at the B3LYP/6-31g* level, and the energies are calculated using CBS-QB3, a complete basis set composite energy method. The computed activation barriers are 33.53 kcal/mol for the hydrogen exchange reaction and 90.08 kcal/mol for the dehydrogenation reaction. The effects of zeolite acidity on the reaction barriers are also investigated by changing the length of the terminal SiH bonds. Analytical expressions between activation barriers and zeolite deprotonation energies for each reaction are proposed so accurate activation barriers can be obtained when using different zeolites as catalysts. Additionally, transition state theory is applied to estimate the reaction rate constants of the hydrogen exchange and dehydrogenation reactions from calculated activation barriers, and vibrational, rotational and translational partition functions.