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

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 pi-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 DeltaH(double dagger), 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 pi-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 DeltaH(double dagger), 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.