Obtenção de ácido acético a partir de etanol e água

Abstract

Brazil’s ethanol production has been growing since the 1980s mainly because of its use in the fuels sector. However, due to the technology development in the electric car sector, it is possible that the ethanol demand would be affected. In this context, an alternative would be redirect part of the ethanol production to the chemical industry, since this alcohol is a platform molecule, i. e., several products could be generated from it, including the products called “drop in” which directly substitute petrochemicals. Among them, the acetic acid stands out due to its use as an important intermediate in polymer production. There are different processes of acetic acid production. However, the acetic acid synthesis through ethanol and water seems more interesting because, besides being a “greener” alternative, it doesn’t have the limitations presented in the process of the acetic acid synthesis involving ethanol and air, where the mixture ethanol/O2 is explosive in some concentrations. In this context, several catalytic systems were studied in the acetic acid synthesis through ethanol and water: Cr and Mn catalysts supported on commercial Cu/ZnO/Al2O3; Co3O4, ZnO and Al2O3 based catalysts; Cu, ZnO, ZrO2 and Al2O3 based catalysts, in addition to catalysts prepared from the aurichalcite, containing the same species. The catalysts were analyzed by XRF, N2 physisorption, XRD refined by the Rietveld method, TPR accompanied by XANES, N2O titration, XPS, CO2-TPD, NH3-TPD, H2O-TPD and ethanol-TPD. The Cr catalysts didn´t promote the acetic acid synthesis, whereas the catalysts containing Mn promoted the acetaldehyde oxidation, but favoring the acetone synthesis, just like the Co3O4, ZnO and Al2O3 based catalysts. The study of the Cu/ZnO/Al2O3 and Cu/ZrO2/Al2O3 catalysts showed that the reaction of the acetic acid synthesis through ethanol and water could occur via two different mechanisms depending on the catalysts physical-chemical properties. The oxidative mechanism follows these steps: firstly, the ethanol is dehydrogenated to acetaldehyde; then, acetaldehyde is oxidized by the lattice oxygen of ZnO, generating acetic acid; finally, the water molecule is dissociated at the ZnO oxygen vacancies, and the resultant “O” occupies the vacancy, completing the catalytic cycle. The associative mechanism can be described by the following steps: firstly, ethanol is dehydrogenated to acetaldehyde; later, this aldehyde reacts with an ethanol molecule generating ethyl acetate over pairs of acid and basic sites; finally, the esther is hydrolyzed generating acetic acid. The study of the catalysts prepared from the aurichalcite showed that the oxygen vacancies have fundamental role not only in the acetaldehyde oxidation step, following the oxidative mechanism, but also in the ethanol dehydrogenation reaction. Beyond that, the insertion of acid sites in these systems promote the acetaldehyde adsorption, facilitating its subsequent oxidation.