(363af) Selective Deoxygenation of Carboxylic Acid to Aldehydes over Metal Oxides: Kinetics and Mechanism

The selective activation of renewable carboxylic acids to yield alcohols and aldehydes has the potential to generate a diverse array of highly valuable renewable products, from surfactants to valuable dienes. Utilizing highly oxophilic supports for selective deoxygenation offers the advantage of activating the target functional group without inducing side reactions, such as hydrogenation of C=C bonds or cleavage of C-C bonds, thereby preserving other functionalities. The use of reducible metal oxides shows high activity and selectivity toward C-O bond cleavage via reverse Mars-Van Krevelen chemistry. This study presents the selective conversion of pentanoic acid (PA) to aldehydes using MoO₃ in a gas-phase reactor. Incorporating very low loadings of platinum (0.05%) enhances the rate of PA conversion at mild temperatures over MoO₃ by promoting hydrogen dissociation. These minimal metal loadings mitigate undesired side reactions that may occur directly on the metal. Insights into the mechanism of hydrodeoxygenation (HDO) and the kinetic relevance of various elementary steps are discussed for PA conversion over MoO₃, utilizing a micro pulse reactor, along with DFT calculations. Further exploration into the role of promotion versus the creation of new highly active sites is undertaken by spatially separating Pt from MoO₃ on a conductive carbon nanotube support. This approach facilitates the disentanglement of the contribution of oxygen vacancies and hydrogen coverage over Mo from the unique sites formed at the Pt-MoO₃ interface. Our findings suggest that Pt functions to maintain a high hydrogen concentration at the surface. This study demonstrates that a Langmuir–Hinshelwood-type mechanism can elucidate the conversion of pentanoic acid over both MoO₃ and 0.05 wt.% Pt/MoO₃, postulating two types of active sites: metal sites activating H₂ and oxygen vacancies enabling PA deoxygenation. The rate-determining step (RDS) of the reaction shifts with the partial pressure of acid and H₂ coverage, transitioning from hydrogen addition to adsorbed acid species to H₂ dissociation.

I am interested in applying my expertise in catalyst characterization (In-situ XPS, TEM, SEM, XRD, IR, BET and Raman), catalyst synthesis, kinetics, and reactions across various areas such as hydrogen production, carbon capture, biomass conversion, catalytic plastic decomposition, and the development of new sustainable chemicals and processes.