Mechanisms of Hydrogen Exchange of Methane with H-Zeolite Y: An ab Initio Embedded Cluster Study

摘要

We present ab initio embedded cluster studies on the mechaism of hydrogen exchange of methane with H-Zeolite Y. We found that inclusion of the Madelung 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-poing energy (approx 10 kJ/mol) and tunneling (1.6 kJ/mol) corrections, the activition energy is predicted to be 124 (+-) 5 and 137 (+-) 5 kJ/mol for hydrogen exchange from two different binding the sites. These predictoins agree well with the experimental estiamte of 122-130 kJ/mol. We also found that it is necessary to include the Madelung potential to find preferrential proton sitting at site O1 versus site O4, in agreement with experimental observation.