Proton exchange and degradation of alkanes in ZSM-5

Abstract

Quantum chemical methods were used to study the adsorption and the mechanism of proton exchange, dehydrogenation, and cracking reactions of C2-C4 alkanes, such as ethane, propane, iso-butane, and n-butane, which were catalyzed by Brønsted acid site of ZSM-5 zeolite. The cluster approach employing 5T, 20T, 28T, 38T, and 96T models (T referring to the tetrahedral unit of the zeolite structure) and the periodic model were used. All transition state structures and corresponding activation energies were determined using DFT with PBE functional and atomic basis set of the split valence polarization (SVP) embedded in the Turbomole program. The obtained results showed that using the periodic model computed adsorption energies are in good agreement with the experimental values. They are 7.1, 10.5, 8.0, and 11.5 kcal/mol for ethane, propane, iso-butane, and n-butane, respectively. The adsorption energies are functions of the cluster-size and proportionally increase with the number of carbon atoms in the hydrocarbon chain, i.e. n-butane > propane > ethane. The penta-coordinated carbonium ions have been observed for the transition state structures of the proton exchange and dehydrogenation reactions. The proton exchange seems to be the precursor for the dehydrogenation reaction. An extrapolation scheme for estimating the reaction barrier which takes into account effects of cluster-size, zero-point energy, electron correlation, and basis set has been proposed. Using the extrapolation scheme computed reaction barriers for proton exchange reactions are underestimated while they were overestimated for dehydrogenation reactions and shows good agreement with experiments for the cracking reaction. The underestimation for the proton exchange reaction is probably due to the dynamic nature of H atom, while the overestimation for dehydrogenation reaction suggests that there might exist another TS structures with lower energy. Inclusion of corrections such as ZPE, basis-set, electron correlation, and especially cluster-size deem to be very important and necessary.