Molecular-level Understanding of the Rate-determining Step in Esterification Reactions Catalyzed by H-ZSM-5 Zeolite. An Experimental and Theoretical Study

Abstract

Transformation of biomass into renewable energy products is currently one of the most promising technologies for dropping dependence on fossil fuels. Biodiesel production may improve with the use of heterogeneous catalysts, such as zeolites. In this work, computational calculations in conjunction with thermal analysis (TGA), evolved gases analysis (EGA) and IR spectroscopic studies were used in order to obtain a more detailed information on the adsorption mechanism involved in the rate-determining step of esterification reactions inside the H-ZSM-5 zeolite. ATR-FTIR spectra showed a molecular adsorption of MeOH and acetic acid (AcOH) on the H-ZSM-5 surface, IR spectrum of AcOH showed perturbations in the region of the bands relative to the carbonyl group C=O, indicating molecular interactions by the adsorption processes, as predicted by theoretical calculations. Adsorption of a single molecule of AcOH by C=O in a single adsorption step followed by coadsorption of MeOH is more stable than adsorption involving the zeolite proton sharing, however energies values suggested competitiveness between both mechanisms. Interactions between the adsorbed molecules and the lattice structure are crucial in controlling the adsorption observed experimentally.