Revealing the Mechanistic Details for the Selective Deoxygenation of Carboxylic Acids over Dynamic MoO₃ Catalysts

The selective activation of renewable carboxylic acidscould enablethe formation of a variety of highly valuable renewable products,including surfactants, valuable dienes, and renewable hydrogen carriers.A kinetic study is performed to enhance understanding of the selectivedeoxygenation of carboxylic acid on promoted MoO3 at mildtemperatures. Although several studies indicate that deoxygenationof oxygenated biomass-derived compounds on MoO3 occursvia a reverse Mars-van Krevelen mechanism, this study suggeststhat the deoxygenation of pentanoic acid (PA) on an oxygen vacancycan also be explained by a Langmuir-Hinshelwood mechanism.A detailed analysis of the experimental data indicates that the incorporationof Pt on MoO3 shifts the reaction order with respect tohydrogen from 1 to 0.5 at a low partial pressure of PA. We revealthat the rate-determining step (RDS) shifts upon the incorporationof Pt from H-2 dissociation to H addition to adsorbed acidmolecules. We further illustrate how the RDS can shift as a functionof PA coverage. The inhibition effect of PA and its possible causesare discussed for both MoO3 and 0.05 wt % Pt/MoO3 catalysts. Here, we decouple promoter effects from the creationof highly active sites located at the Pt/MoO3 interface.The nature of the active site involved upon Pt incorporation is alsostudied by separating Pt from MoO3 at a controlled distanceusing carbon nanotubes as hydrogen bridges, confirming that the kineticallyrelevant role of Pt is to serve as a promoter of the MoO3.