(19e) Selective Reduction of Carboxylic Acids to Aldehydes over Promoted MoO3 Catalyst

Selective activation of carboxylic acids to form alcohols and aldehydes with highly oxophilic catalysts could enable the formation of a variety of highly valuable renewable products. Selective reduction with highly oxophilic supports carries the advantage of activating the acid functional group without perturbing other functionalities. The use of traditional metal catalysts for this purpose generally favors C-O cleavage followed by rapid decarbonylation to yield alkenes. These alkenes are readily converted to less desirable alkanes in the presence of H₂ or often lead to coke and catalyst deactivation in the absence of co-fed H₂. This limits applicability to produce high value chemicals such as butadiene via these routes.

In this contribution, we report a new chemistry that has been explored with aldehydes and ketones, reverse mars van Krevelen chemistry over MoO_x, for the selective conversion of carboxylic acids. Our study focusses on the selective conversion of pentanoic acid (PA) to aldehyde over MoO₃ at different temperatures via hydrodeoxygenation (HDO) reaction. Here, we hypothesize that the selective C-O cleavage reactions are occurring over the defects on the sub stoichiometric oxide via reverse Mar-Vars Krevelen chemistry. Reducibility of MoO₃ is well known to be challenging at low temperatures, therefore in order to facilitate the reduction process, very low loadings of platinum (Pt) were incorporated on the catalyst (0.05%Pt/MoO₃). The net effect is the increase on the rate of conversion of PA at lower temperature over MoO₃ by hydrogen spillover to promote partial reduction of the MoO_x while minimizing side reactions that may occur on the metal directly. X-Ray photoelectron spectroscopy (XPS) was used to investigate effect of H₂ during the partial reduction on both MoO₃ and 0.05% Pt/MoO₃ catalysts. Kinetics analysis reveals that low loading of Pt on MoO₃ decrease the apparent activation barrier for acid conversion by over 33 kJ/mol.