Comparison of Ethylation at External Surface and Internal Cavity of H-MCM-22 Zeolite from Theoretical Calculations

The Summary of main observation and conclusion Understanding and optimizing structure of active sites is of significance in zeolite catalysis. Benzene ethylation is an industrially important process catalyzed by H-MCM-22 zeolite; while the active sites still remain elusive. In this work, density functional theory (DFT) calculations were employed to investigate the benzene ethylation at two different types of Bronsted acid sites (BAS) in H-MCM-22 zeolite, namely the internal cavity (IC) acid site and the external surface (ES) acid site. Both the stepwise and concerted pathways were addressed. The comparison of the calculated energetics of two pathways indicates that the benzene ethylation reaction primarily proceeds via the concerted pathway at both the IC and ES acid sites of H-MCM-22. The calculated overall Gibbs free energies at reaction condition (473 K and 3.5 MPa) on the IC and ES acid sites are 90 and 86 kJ/mol, with the rate constants of 1.20 x 10(3) and 2.92 x 10(3) s(-1), respectively. It indicates that benzene ethylation could occur both on the IC and ES acid sites, with the catalytic activity of IC acid site being slightly lower than that of ES acid site. Furthermore, we theoretically reveal that the acid strength at the ES site is slightly weaker than that at the IC site via the frequency shift after the adsorption of CO. The differences in dispersion interaction between ES and IC sites are also quantified by the adsorption of base molecules with different sizes. The calculated results in this work demonstrate that the acid sites at the external surface of H-MCM-22 zeolites are suitable for benzene ethylation to produce ethylbenzene, providing theoretical implications for tailoring the distribution of active sites in H-MCM-22 zeolite.