Coupling reactions of ethanol and an ethanol/acetaldehyde mixture over Mo/MgO-SiO2 catalysts

• Feeding acetaldehyde to ethanol opens up new reaction pathways. • C4 products are not only formed by the coupling of two acetaldehyde molecules. • The reaction of coadsorbed species is demonstrated by the product distribution. • Ethyl vinyl ether and crotonaldehyde products are only formed from acetaldehyde co-feed. • Molybdenum does not increase C4 yields by producing excess acetaldehyde. Acetaldehyde has been identified as a key intermediate in the catalytic conversion of ethanol into butadiene or butanol. A variety of methodologies have been implemented to enhance the yield, including the promotion of in situ catalytic generation of acetaldehyde and the direct addition of acetaldehyde to the ethanol reactant. In the present study, molybdenum-promoted, partially silica-coated magnesium oxide catalysts (Mo/MgO-SiO 2 ) were prepared and utilized. The initiation of an intermolecular redox reaction by molybdenum led to the conversion of ethanol to acetaldehyde and ethane. However, the Mo-promoted catalyst exhibited lower basicity and, consequently, reduced activity in aldol coupling when compared to the supporting oxide. The conversion of ethanol and the yield of C 4 were found to be higher in the presence of the Mo-promoted catalysts in comparison to the MgO-SiO 2 support, yet butadiene selectivity was observed to be lower. The conversion of an ethanol/acetaldehyde mixture resulted in a substantial butadiene yield, accompanied by notable crotonaldehyde and crotyl alcohol yields. It is noteworthy that the latter two products were not obtained from pure ethanol. At elevated space velocities, two additional condensation products, ethyl vinyl ether and ethyl acetate, were obtained from the mixture. This suggests that the coupled products are formed not only by the condensation of two acetaldehyde molecules, but also by other coupling pathways occurring between intermediate products that are adsorbed on the catalyst surface.