Mechanistic insights into the formation of acetaldehyde and diethyl ether from ethanol over supported VOx, MoOx, and WOx catalysts

Catalytic pathways are described for reactions of ethanol to acetaldehyde by oxidative dehydrogenation and of ethanol to diethyl ether by condensation over VOxsingle bondAl2O3, MoOxsingle bondAl2O3, and WOxsingle bondAl2O3. Isotopic labeling shows that acetaldehyde formation occurs via rate-determining Csingle bondH bond cleavage of the CH2 group in an adsorbed alkoxide followed by removal of surface oxygen in a Mars and van Krevelen redox mechanism (as confirmed by in situ X-ray absorption, diffuse reflectance infra-red Fourier transform spectroscopy and UV–visible spectroscopy); diethyl ether formation occurs in parallel via coupling and condensation of two adjacent ethoxy species. Using a combination of in situ spectroscopic and kinetic analysis, catalyst properties influencing the formation of acetaldehyde and ether from the common adsorbed ethoxy intermediate are elucidated. X-ray absorption analysis during anaerobic ethanol titration is used to preclude the involvement of terminal Mdouble bond; length as m-dashO bonds during the reaction. A study of the activity of catalysts with the same MoOx domain size on Al2O3, TiO2, and CeO2 supports and binary oxides of MoOx and WOx on Al2O3 are used to prove that the active redox oxygen for acetaldehyde formation is the oxygen atom linking the active metal oxide domain to the support oxide. Ether formation ability of the metal oxide is related to the electronegativity of the active metal atom.