A multi-scale computational study on the mechanism of protonation of isobutene and 2-butene over HY zeolite

To understand the alkylation mechanism of isobutane with butene, it is of great importance to elucidate the difference of protonation behaviours among butene isomers at molecular scale, because it is the starting point of this reaction. Herein, we have calculated the protonation processing of isobutene and 2-butene on a 120T HY cluster model, using an embedded QM/MM (quantum mechanical/molecular mechanical) method. The formation energy of p-complex from isobutene is higher than that of 2-butene owing to the effect of electronic interaction and steric hindrance between the C=C bond of butene and the H atom of the HY zeolite. Moreover, the energy of t-butyl alkoxide (TBA) produced via isobutene protonation is higher than that of s-butyl alkoxide (SBA) from 2-butene, as the C-O bond of TBA is longer. In addition, the transition state of isobutene protonation is a t-butyl carbonium ion, while the corresponding structure during 2-butene protonation is a s-butyl carbonium ion, which is less stable. In conclusion, isobutene is much easier to be activated by Bronsted acid site of HY zeolite than 2-butene.