Two mechanisms for acetic acid synthesis from ethanol and water

Abstract

Two catalysts, Cu/ZnO/Al2O3 and Cu/ZrO2/Al2O3, were employed for acetic acid synthesis from ethanol and water. They were characterized by X-ray fluorescence, N2 physisorption, N2O titration, in situ X-ray diffraction refined by the Rietveld method, in situ X-ray photoelectron spectroscopy, temperature-programmed reduction followed by X-ray absorption near-edge structure, and temperature-programmed desorption of ethanol, H2O, CO2, and NH3. The Zn-based catalyst is composed of ZnO nanoparticles, Cu0, CuZn alloy, and Al2O3, whereas the Cu/ZrO2/Al2O3 is composed of Cu0, ZrO2, and Al2O3. Both catalysts show the same Cu0 metallic surface area. On one hand, the spectra of the H2O temperature-programmed desorption show that the ZnO nanoparticles of Cu/ZnO/Al2O3 promote water dissociation and, consequently, the redox properties of this catalyst. On the other hand, the Zr-based catalyst shows low activity for H2O dissociation and higher acidity and basicity than for the Zn-based catalyst. These different properties lead to different mechanisms for acetic acid synthesis from ethanol. Taking the redox mechanism of the WGS reaction into account, the steps of the acetic acid synthesis promoted by the Zn-based catalyst can be described as follows: first, ethanol is dehydrogenated alloy to acetaldehyde on Cu0 and CuZn; then, this aldehyde is oxidized on the ZnO–Cu0 interface to acetate, which desorbs, forming acetic acid; finally, H2O dissociates on the O vacancies of the ZnO–Cu0 interface and reoxidizes the oxide. The ZnO–CuZn alloy interface should also be considered in this mechanism. The second mechanism, which occurs mainly on the Zr-based catalyst, is related to the acetaldehyde and ethanol condensation forming ethyl acetate. This ester is hydrolyzed, synthesizing acetic acid and ethanol.