Synthesis of Propene from Ethanol: A Mechanistic Study

Physical mixtures comprising AgCeO2 and t-ZrO2 or MgO were employed as catalysts for the generation of propene from ethanol in the presence of water. The catalysts were characterized by means of several techniques such as X-ray diffraction, N2 physical adsorption, isopropanol conversion and ethanol to acetone Meerwein–Ponndorf–Verley (MPV) model reactions, NH3 temperature-programmed desorption, CO2 temperature-programmed desorption, and ethanol temperature-programmed desorption followed by diffuse reflectance infrared Fourier transform spectroscopy and mass spectrometry. Acid and strong basic sites, redox properties, and the capacity to conduct both water dissociation and MPV reduction are important characteristics for this reaction. Water is the oxidant agent; however, water may also affect the acidic–basic sites of the oxides, as demonstrated when MgO was used. The physical mixture comprising t-ZrO2 and AgCeO2 is active in the conversion of ethanol to propene, presenting the following steps. First, ethanol is oxidized to acetaldehyde; after that, it is oxidized to acetate species, which condene producing acetone. This ketone reacts with ethanol (MPV) generating isopropanol and acetaldehyde, which is oxidized to acetate and undergoes the same sequence described above. Finally, isopropanol is dehydrated to propene. At low temperatures (∼300 °C), generation of acetone seems to be the rate-determining step of the reaction, while at higher temperatures (>450 °C), MPV becomes the determining step. At these high temperatures, dehydration predominates. Thus, there is an optimum temperature range for propene synthesis. The acetaldehyde synthesis via MPV and its participation in the principal reaction path make propene synthesis a special cascade reaction. This paper presents a broader perspective for the generation of propene from ethanol, shedding light on the mechanism of this rather complex process.