Theoretical investigation of benzene alkylation with ethene over H-ZSM-5

Benzene alkylation with ethene over zeolite H-ZSM-5 has been investigated using density functional theory. Three different reaction mechanisms-two one-step schemes and one two-step scheme-have been studied on three cluster models of increasing size representing parts of the H-ZSM-5 framework. In the one-step schemes ethene protonation and C-C bond formation occur simultaneously. The two-step scheme starts with the formation of a stable ethoxide intermediate which subsequently reacts with benzene to form the reaction product. Activation energies obtained from the DFT results have been improved by single-point MP2 calculations. The calculated intrinsic activation energies of the one-step schemes are similar to the activation energy of the alkylation step in the two-step scheme. Numerical values of the MP2 corrected activation energies are in good agreement with experimental data. The largest cluster (33 T-atoms) was found to stabilize protonated ethylbenzene as a stable intermediate. The results of this study show the importance of using relatively large clusters for investigations of hydrocarbon transformation occurring in zeolites.