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 DFT methods and the Quantum Theory of Atoms in Molecules. Results show that the strength of the interactions related to adsorption and co-adsorption processes are higher in the catalyst with 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 co-adsorption and the formed intermediates dominate this stabilization. For the transition state, 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 TS the methyl cation is multi-coordinated forming the following H2O···CH3 +···CB concerted bonds. It is demonstrated that through the electron density analysis it can be defined the criteria to discriminate between interactions related to the confinement effects and the reaction itself (adsorption, co-adsorption, 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.