Selective conversion of alcohols to aldehydes over metal oxide catalysts

Abstract: Nowadays, transition metal oxide catalysts are employed to a large extent in many chemical processes for the selective production of chemicals and intermediates. Although metal oxides are mostly known as oxidation catalysts, they can successfully catalyze a wide range of reactions, such as dehydration. Two different processes have been studied in the present work: the selective oxidation of methanol over V-Mo-based catalysts and the dehydration of glycerol over supported Nb- and W-oxide catalysts. Since the iron molybdate catalyst commercially employed in the selective conversion of methanol to formaldehyde suffers from deactivation by Mo volatilization, alternative catalysts such as supported Fe-V-oxide and unsupported V-Mo-oxide spinel-type catalysts were studied. The supported catalysts were found to be less selective than the commercial one, their showing a far more severe volatilization due to the limited amount of active metal they contain. At the same time, a spinel-type structure containing V, Mo or both was found to be flexible and stable under both reducing and oxidizing conditions, enabling the cations to change valence while preserving the spinel-type structure. In addition, the spinel structure was found to be stable with regard to volatilization during methanol oxidation. Nevertheless, V-Mo-oxide spinel-type catalysts, despite their excellent structural and stability properties, were found to be less selective than the commercial one and to not be suitable for replacing it without further improvement. In the production of biodiesel large amounts of glycerol are formed as a major by-product. Because of its low market demand, glycerol can represent a sustainable feedstock for producing chemicals and intermediates, such as acrolein, which is presently a fossil derivative obtained commercially from propene. The gas-phase dehydration of glycerol to acrolein over Al2O3-, SiO2-, TiO2- and ZrO2-supported Nb- and W-oxide catalysts was investigated here, tests regarding it being carried out under both anaerobic and aerobic conditions. ZrO2- and TiO2-supported W-oxides were found to be the catalysts that performed best under anaerobic and aerobic conditions, respectively, in terms of both their activity and their acrolein selectivity. A clear dependence of the activity and acrolein selectivity on the total acid site and Brønsted acid site concentration, respectively, was also observed. All of the catalysts were found to undergo deactivation due to poisoning of the active sites by coke molecules and heavy compounds. The deactivation phenomenon was found to be reduced considerably in the presence of oxygen.