Estudo experimental e teórico de zeólitas H-BETA e H-ZSM-5 na produção de ésteres alquílicos

Abstract

The application of zeolites in catalytic processes is of great interest in several chemical reactions that involves transformation of biomass into higher value-added products. In a similar sense, conversion of fatty acids into esters catalyzed by Hzeolites have sprouted a widely-varied academic and industrial attention due to it providing a sustainable route inside the Green Chemistry for the production of chemicals and energy, of interest in biodiesel synthesis. Thermogravimetric analysis (TGA-IR), FTIR and FTIR-ATR spectroscopy, and theoretical studies [DFT method at M06-2X/6-31G(D) level] as support of experimental data are employed to investigate the adsorption of methanol and acetic acid as reaction models of fatty acids on the surface of H-Beta and H-ZSM-5 zeolites. The results obtained by TGA-IR for adsorption of acetic acid and methanol in both zeolites show different processes of mass loss that correlated with the adsorption mode (physical and chemical adsorption). The spectra obtained by FTIR-ATR show that acetic acid is molecularly adsorbed on H-Beta through the carbonyl group on the Brønsted site and, by the hydroxyl group on the Lewis site, the experimental evidence is supported by the theoretical vibrational analysis. The acetic acid adsorption by C=O on the Brønsted site is more stable than other proposals of adsorption found in the literature. For HZSM- 5 zeolite, FTIR-ATR spectrum reported changes in the structural composition of the material, therefore it was not possible to compare theoretical and experimental results. Moreover, different models of adsorption/co-adsorption of acetic acid and methanol on the surface of H-zeolites have been studied by theoretical calculations in order to provide information that could be helpful in the interpretation of the first step of the mechanism of acetic acid esterification with methanol on H-Beta and HZSM- 5 zeolites.