Stepwise or Concerted Mechanisms of Benzene Ethylation Catalyzed by Zeolites? Theoretical Analysis of Reaction Pathways

Benzene ethylation over zeolites has a long-standing controversy on two different possible reaction pathways, i.e., stepwise and concerted mechanisms. To solve this doubt and further find out the suitable zeolite to catalyze benzene ethylation with ethylene, we employed density functional theory (DFT) and molecular dynamic (MD) methods to insight the reaction mechanisms of benzene ethylation over five widely-applied zeolites, i.e., ZSM-5, ZSM-22, ZSM-12, Y and MOR. ZSM-5 is the optimal framework than the other zeolites in both stepwise and concerted mechanisms, and the host-guest interaction between the intersection pore and reacted species drives to the lower maximum free energy span (viz. 23.41 and 20.96 kcal/mol for stepwise and concerted mechanisms) thermodynamically, and stepwise mechanism was confirmed as the superior pathway to catalyze benzene ethylation kinetically. Moreover, the faster diffusion rate of ethylene (50 times) than that of benzene in ZSM-5 indicates that the ethylation is diffusion limited and ethylene could preferentially occupy the Bronsted acid site to form surface ethyoxyl. Furthermore, the quantitative Mulliken population analysis and qualitative reduced density gradient (RDG) analysis reveal that the pore confinement effect drives the priority of ZSM-5 to catalyze benzene ethylation.

Automated summary

The study investigated benzene ethylation over zeolites and found that ZSM-5 is the optimal catalyst due to its lower free energy span and faster reaction kinetics. The preferential diffusion of ethylene in ZSM-5 suggests a diffusion-limited ethylation process, and pore confinement effect plays a role in catalyzing benzene ethylation.