Selective Reduction of Carboxylic Acids to Aldehydes with Promoted MoO3 Catalysts

Selective activation of renewable carboxylic acids on promoted molybdenum oxides to form alcohols and aldehydes is reported. A combination of reaction kinetics, temperature-programmed reduction (TPR), and X-ray photoelectron spectroscopy (XPS) reveals that the activity scales with the concentration of Mo5+ active sites and is a strong function of surface hydrogen coverage. The addition of a very small loading (0.05 wt %) of Pt drastically increases rates of selective deoxygenation at lower temperatures (<350 °C) but diminishes rates at elevated temperatures due to over-reduction of the support. Here, it is reported that the incorporation of Pt clusters on MoO3 decreases the apparent activation barrier for acid conversion by over 32 kJ/mol, which highlights the significant role of site regeneration facilitated by hydrogen splitting and spillover. Our findings suggest that the rate-determining step for converting pentanoic acid shifts upon introducing Pt clusters from formation/regeneration of oxygen vacancies to H addition to the carbonyl carbon.