Theoretical study of protonation of butene isomers on acidic zeolite: the relative stability among primary, secondary and tertiary alkoxy intermediates

Abstract

A density functional theory (DFT) study of the protonation of but-1-ene, (E)-but-2-ene and isobutene over a cluster representing the zeolite acid site (HT₃) was carried out. At the B3LYP/6-31+G** level of calculation all the reactions were exothermic, with respect to the isolated reactants, in forming an alkoxy species. Formation of a π-complex involving the double bond and the acidic proton was the first step and shows a small dependence with the olefin structure. The proton transfer involves a transition state with carbenium ion like character, which is reflected in the calculated ΔH^‡, being higher for the but-1-ene (to afford the 1-butoxy intermediate) and lower for the isobutene (to afford the tert-butoxy intermediate). However, the stability of the alkoxy formed shows a different trend. The tert-butoxy was computed to be only 1.5 kcal mol⁻¹ lower in energy than the π-complex between isobutene and HT₃ at the B3LYP/6-31+G** level of calculation, but the reaction becomes endothermic by 2.5 kcal mol⁻¹ when computed at B3LYP/6-311++G**. The calculated order of stability among the alkoxy species was 2-butoxy > 1-butoxy > tert-butoxy. These results show that electronic effects dominate ΔH^‡, which is associated with the kinetics of the protonation process, while steric effects play a major role in the stability of the alkoxy, which in turn is related to the thermodynamics of protonation.