Quantum chemical prediction of hydrocarbon cracking reactions

Ab initio methods are used to study the transition state structures and activation energies of ethane cracking, hydrogen exchange, and dehydrogenation reactions catalyzed by zeolites. The reactant and transition state structures are optimized by HF and MP2 methods and the final energies are calculated using a Complete Basis Set composite energy method. The computed activation barriers are 71.39 kcal/mole for cracking, 31.39 kcal/mole for hydrogen exchange and 75.95 kcal/mole for dehydrogenation using geometries optimized with the MP2 method. The barrier was the highest among all three ethane conversion reactions, indicating it is the most difficult reaction to take place. The relationship between activation barriers and zeolite deprotonation energies for each reaction are proposed so that accurate activation energies can be obtained when using different zeolites as catalysts. This is an abstract of a paper presented at the AIChE Annual Meeting (Austin, TX 11/7-12/2004).