Mechanisms of hydrogen exchange of methane with H-zeolite Y: An ab initio embedded cluster study

We present ab initio embedded cluster studies on the mechanism of hydrogen exchange of methane with H-Zeolite Y. We found that inclusion of the Madelu ng field stabilizes the formation of a carbonium-like transition state, and consequently reduces the reaction barrier by 17-23 kJ/mol, relative to the corresponding bare cluster predictions. Using the CCSD(T)/6-31G(d,p) level of theory, including zero-point energy (similar to 10 kJ/mol) and tunnelin g (1.6 kJ/mol) corrections, the activation energy is predicted to be 124 +/ - 5 and 137 +/- 5 kJ/mol for hydrogen exchange from two different binding s ites. These predictions agree well with the experimental estimate of 122-13 0 kJ/mol. We also found that it is necessary to include the Madelung potent ial to find preferrential proton siting at site O1 versus site O4, in agree ment with experimental observation.