Study of Confinement and Catalysis Effects of the Reaction of Methylation of Benzene by Methanol in H-Beta and H-ZSM-5 Zeolites by Topological Analysis of Electron Density

Abstract

In this work we studied the host-guest interactions between confined molecules and zeolites and their relationship with the energies involved in the reaction of methylation of benzene by methanol in H-ZSM-5 and H-Beta zeolites employing density functional theory (DFT) methods and the quantum theory of atoms in molecules. Results show that the strength of the interactions related to adsorption and coadsorption processes is higher in the catalyst with the larger cavity; however, the confinement effects are higher in the smaller zeolite, explaining, from an electronic viewpoint, the reason why the stabilization energy is higher in H-ZSM-5 than in H-Beta. The confinement effects of the catalyst on the confined species for methanol adsorption, benzene coadsorption, and the formed intermediates dominate this stabilization. For the transition state (TS), the stability of the TS is achieved due to the stabilizing effect of the surrounding zeolite framework on the formed carbocationic species (CH3+) which is higher in H-ZSM-5 than in H-Beta. In both TSs the methyl cation is multicoordinated forming the following H2O···CH3+···CB concerted bonds. It is demonstrated that, through the electron density analysis, the criteria can be defined to discriminate between interactions related to the confinement effects and the reaction itself (adsorption, coadsorption, and bond-breaking and bond-forming processes) and, thus, to discriminate the relative contributions of the degree of confinement to the reaction energies for two zeolite catalysts with different topologies.