Kinetic and Mechanistic Analysis of the Hydrodeoxygenation of Propanoic Acid on Pt/SiO₂

This manuscript summarizes observations made during the conversion of propanoic acid over Pt/SiO2 under H2-rich environments. Under these conditions, Pt is active for hydrogenation and hydrodeoxygenation, which leads to the formation of propionaldehyde, 1-propanol, and propane. Pt also facilitates decarbonylation of propanoic acid and propionaldehyde, which forms ethane and CO. The accumulation of CO with increasing residence time poisons the Pt catalyst and makes it difficult to achieve high propanoic acid conversions on practical time scales. During the conversion of propanoic acid on Pt/SiO2, sequential reactions play a critical role in determining product distributions. We resolve their contributions through analysis of rates and selectivities during the conversion of propanoic acid, propionaldehyde, 1-propanol, CO, and CO2 in various environments. We conclude that the main challenge facing the selective production of partial hydrodeoxygenation products-namely propionaldehyde and 1-propanol-is that one must facilitate dehydroxylation of propanoic acid while avoiding thermodynamically favorable decarbonylation and alcohol hydrogenolysis pathways. Because these side reactions are effectively irreversible under hydrodeoxygenation conditions, this can only be accomplished by suppressing rates of decarbonylation and 1-propanol hydrogenolysis. We reconcile macroscopic trends with a reaction mechanism that includes parallel and sequential reactions occurring during the hydrodeoxygenation of propanoic acid over Pt/SiO2.