Quantum chemical prediction of hydrocarbon cracking reactions

For many years, researchers have been developing theoretical methods of estimating reaction rates and energetics when experimental measurements are not available. Recent advances have led to composite energy methods with near chemical accuracy. The performance of these new methods for predicting activation energies and rate constants have not been evaluated for large hydrocarbon cracking reactions. In this work, 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/mol for cracking, 31.39 kcal/mol for hydrogen exchange and 75.95 kcal/mol for dehydrogenation using geometries optimized with the MP2 method. The cluster effect and acidity effect on the reaction barriers are also investigated. 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.