DFT Investigation of Alkoxide Formation from Olefins in H-ZSM-5

We present a density functional theory (DFT) study of propene, 1-hexene, and 3-hexene protonation over representative H-ZSM-5 clusters to give covalent alkoxide intermediates. The influence of cluster size, olefin carbon number, olefin conformation, proton siting, aluminum siting, and bonding configuration (primary vs secondary) of the alkoxide intermediate was analyzed. We found the formation of a physisorbed π-complex involving the olefin double bond and the acidic proton to be relatively independent of olefin structure and site geometry. However, we show that the proton-transfer process for formation of the covalent alkoxide intermediate involves a carbenium-ion-like transition state, with an activation energy that is (1) dependent on the protonation site of the olefin and (2) relatively independent of the carbon number and double bond location of the olefin. Accessibility of the alkoxide oxygen site in the cavity was observed to play a significant role in the stability of the alkoxy species. We find that the overall energy of adsorption for alkoxides depends strongly on the crystallographic Al site and the specific host oxygen for the Brønsted proton. For larger alkenes we find a dependence on alkoxide conformations and report a 5 kcal/mol difference in energies of formation for different rotational orientations of 3-hexene alkoxide intermediates. Finally, we report a novel reaction path for propene chemisorption, whereby the primary alkoxide is bonded to the Brønsted host oxygen rather than a neighboring oxygen.