Toward a reliable computational description of hydrocarbon activation in zeolites: a study of cracking, dehydrogenation, and H/D Exchange of alkanes in H-ZSM-5.
Zygmunt, S. A. ; Bootz, B. L. ; Miller, A. W. ; Curtiss, L. A. ; Iton, L. E.
During the past decade, quantum-chemical calculations have been used to model hydrocarbon reactions in zeolite acid catalysts. In the interest of computational feasibility, the zeolite has often been represented by a very small cluster model, at times including only one tetrahedrally-coordinated atom (a 1T cluster). The results of such calculations have given important qualitative insights such as possible reaction pathways and transition state geometries, but the calculated activation energies for hydrocarbon reactions have usually been 50 percent or more higher than experimental values. In our recent work we developed a methodology of quantum-chemical techniques and corrections that allowed us to calculate a quantitatively accurate activation energy for protolytic cracking of ethane in H-ZSM-5 (1). In order to test the limits of our computational method, we have carried out a study of protolytic cracking, dehydrogenation, and H/D exchange of the n-alkanes ethane, propane, and butane using a cluster model of H-ZSM-5. Our goal is to study the dependence of the activation energy on the alkane chain length in these reactions and to determine whether this method can produce results in quantitative agreement with available experimental results (2-5).
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