Reactivity of alkanes on zeolites: A theoretical ab initio study of the H/H exchange

Ab initio calculations were performed to study the H/H exchange between lig ht alkanes (methane, ethane, propane, and isobutane) in protonated zeolites . The Bronsted acid site of the zeolite was represented by a T3 cluster (T = Si, Al). The results of the calculations, at the B3LYP/6-31G** and MP2/6- 31G**//HF/6-31G** levels, indicated that the transition state resembles a p entacoordinated carbonium ion. The enthalpy of activation was similar, rega rdless of the alkane and the type of hydrogen being exchanged. The Delta H- double dagger values, at room temperature, ranged from 32.2 kcal/mol for me thane to 36.2 kcal/mol for the exchange of the tertiary hydrogen of isobuta ne, both at the B3LYP/6-31G** level. These results are not in complete agre ement with experiments, as it has been shown that for isobutane only the pr imary hydrogens exchange at temperatures near ambient. This disagreement ma y be attributed to the fact that the cluster model employed in the calculat ions neither includes the electrostatic effects of the zeolite cavity nor t akes into account steric repulsion associated with the framework.