New Insights into the Effects of Acid Strength on the Solid Acid-Catalyzed Reaction: Theoretical Calculation Study of Olefinic Hydrocarbon Protonation Reaction

The protonation reactions of propene, isobutene, styrene, and α-methylstyrene on 8T solid acid models with varying acid strengths are theoretically studied in order to reveal the effects of acid strength on the solid acid-catalyzed reactions. It is shown that with the increase of acid strength (evaluated by deprotonation energy) the ion pair intermediates are preferentially formed, and their stabilities relative to the π complex and alkoxy species intermediates are considerably increased. Some alkoxy species for the bulkier olefinic hydrocarbons (i.e., isobutene, styrene, and α-methylstyrene) cannot exist as stable intermediates on strong solid acid models. It is further found that with the increase of acid strength the extent of energy reduction generally follows the order of ion pair > transition states (TS, TS′) > (π complex, alkoxy species), which is in line with the ionic character of these species. The ionic species are much more sensitive to acid strength than the covalent species regardless of intermediates or transition states. This can explain the increase of the stability of ion pair intermediates on strong solid acid models. In addition, the results obtained here can be used to interpret the effects of acid strength on the catalytic activities for other solid acid-catalyzed reactions, such as alkanol dehydration, alkane isomerization, and Beckmann rearrangement.