A density functional study on the nature of the adsorption complex between isobutane and H-ZSM5 and its implication for the mechanism of activation of alkane molecules over H-ZSM5 zeolite

Theoretical calculations on the nature of the adsorption complex between isobutane and H-ZSM5 and on the dehydrogenation reactions of isobutane and methane in this zeolite have been performed, using the 20T cluster to model the acid site of the zeolite. We considered the adsorption process and the chemical exchange reactions to take place through the hydrogen atom bonded to either a primary or the tertiary C atom of isobutane. The calculations were performed using density functional theory (DFT) with the hybrid functional X3LYP and the 6-31G** basis set. NMR chemical shifts were calculated at the X3LYP/6-31G** optimized geometries using an extended basis set (6-311G**++), in order to investigate the nature of the adsorption complex between the reagent molecules and the H-ZSM5 acid site. The results of the adsorption studies showed that within the range of the experimental and theoretical uncertainties, it is not possible to distinguish if the complex formed involves the primary or the tertiary atom of isobutane. In fact, the similarity of the adsorption energies suggests that both complexes could be formed. The same conclusion can be drawn from the analysis of the chemical shifts results. The activation energies for the exchange reactions at the primary and tertiary centers are also very similar and both transition states can be stabilized by a weak hydrogen bond. Therefore, the preference for the H/D exchange at the primary center observed at room temperature cannot be attributed to that effect. The activation energy for the methane exchange reaction was found to be larger than that for the isobutane molecule, as expected. However, while the predicted difference is in the range of 5.3-7.8 kcal/mol, the most favorable difference that one obtains from the experimental results is of 14.2 kcal/mol, a result which is difficult to understand if one considers the nature of the transition states and the differences in bond energies and in the steric hindrance experienced by the molecules. In conclusion, the theoretical calculations strongly support the experimental observations that a specific complex is formed between the zeolite and the isobutane molecule and also that the zeolite has enough acid strength to promote direct protonation of alkanes. On the other hand, the fact that, at room temperature, no HID exchange was observed at the tertiary carbon atom, does not exclude the possibility that other reactions which occur at higher temperatures, such as dehydrogenation and cracking, will involve the tertiary center. (c) 2007 Elsevier B.V. All rights reserved.