Reactivity of isobutane on zeolites: a first principles study

In this work, ab initio and density functional theory methods are used to study isobutane protolytic cracking, primary hydrogen exchange, tertiary hydrogen exchange, and dehydrogenation reactions catalyzed by zeolites. The reactants, products, and transition-state structures are optimized at the B3LYP/6-31G* level, and the final energies are calculated using the CBS-QB3 composite energy method. The computed activation barriers are 52.3 kcal/mol for cracking, 29.4 kcal/mol for primary hydrogen exchange, 29.9 kcal/mol for tertiary hydrogen exchange, and 59.4 kcal/mol for dehydrogenation. The zeolite acidity effects on the reaction barriers are also investigated by changing the cluster terminal Si-H bond lengths. The analytical expressions between activation barriers and zeolite deprotonation energies for each reaction are proposed so that accurate activation barriers can be obtained when using different zeolites as catalysts.