Influence of Acid Strength and Confinement Effect on the Ethylene Dimerization Reaction over Solid Acid Catalysts: A Theoretical Calculation Study

Abstract

The influence of both Brønsted acid strength and pore confinement effect on the ethylene dimerization reaction has been systematically studied by density functional theory (DFT) calculations. In the theoretical calculations, both stepwise and concerted reaction mechanisms are considered. It is demonstrated that the reactivity of the ethylene dimerization reaction can be significantly enhanced by increasing acid strength no matter which mechanism is included, while on the basis of activated barriers, the concerted mechanism is preferred on weak acids and two mechanisms are competitive when the acid strength increases to a medium–strong acid. Due to the pore confinement effect that can effectively stabilize the ionic transition states of the dimerization reaction, the activity of the dimerization reaction is considerably improved inside the zeolite pore. Compared with the reaction on the isolated acid sites, the transition states of the stepwise reaction are more effectively stabilized than those of the concerted reaction inside the zeolite confined pore, resulting in the former reaction being preferred when the dimerization reaction occurs inside the zeolite confinement spaces. Additionally, on the basis of the systematic investigations on the alkene dimerization reactions over zeolites with varying pore sizes (such as ZSM-22, ZSM-5, and SSZ-13), it is demonstrated that ZSM-22 and ZSM-5 zeolites are effective catalysts for the ethylene dimerization.