Molecular Insights on the Role of (CTA⁺)(SiO⁻) Ion Pair into the Catalytic Activity of [CTA⁺]–Si–MCM-41

In this work, a theoretical study in conjunction with a spectroscopic analysis by FTIR were carried out in order to obtain molecular insights on the role of (CTA⁺)(≡SiO⁻) ion pair into the catalytic activity of [CTA⁺]–Si–MCM-41, the non-calcined form of Si–MCM-41. The reaction of transesterification of ethyl acetate with methanol was used as a model of transesterification of vegetal oils using the mesoporous heterogeneous catalyst. The theoretical study was performed in the framework of density functional theory (DFT) in order to provide new insights that could be helpful in the understanding of these chemical reactions on solid surfaces. It is found that the most favorable reaction mechanism is the dual site mechanism and that the presence of the (CTA⁺)(≡SiO⁻) ionic pair is fundamental for the catalytic activity. The basic center of [CTA⁺]–Si–MCM-41 catalyst polarizes the adsorbed methanol and the acid center polarizes the carbonyl of adsorbed ethyl acetate. Thus, this bifunctional catalytic site makes possible the concerted adsorption of both reactants at the pore mouth and generates a concerted transition state that gives rise to the beginning of the transesterification reaction and the formation of the unstable tetrahedral intermediate which will give rise to the products.