Insight into the reaction mechanism and charge transfer analysis for the alkylation of benzene with propylene over H-beta zeolite

Abstract

The reaction mechanism of alkylation of benzene with propylene to produce cumene over H-beta zeolite was investigated by density functional theory (DFT) using a 5T cluster model at the B3LYP/6-31G* level. The Mulliken population analysis was conducted to investigate the charge transfer analysis. Two possible reaction pathways, i.e., consecutive (stepwise) and associative (one-step) mechanism, were proposed. DFT simulation results revealed that the activation energy of associative mechanism (106 kJ/mol) is much lower than that of consecutive reaction pathway (294 kJ/mol). That is, the formation of cumene over H-beta zeolite is more preferred through associative mechanism, wherein the protonation and alkylation occur simultaneously without isopropoxide formation. Then, Mulliken charge transfer (CT) analysis for the two reaction mechanisms was conducted. Both reaction mechanism and CT analysis results proved that it is the H proton connected to benzene ring transferred back to zeolite framework during the formation of cumene from transition state in the alkylation step, not the original active center H atom of H-beta zeolite. Finally, it was found that the electric properties of zeolite framework play an important role in the alkylation of benzene with propylene.