Kinetic Analysis of the Hydrodeoxygenation of Aliphatic Volatilized Lignin Molecules on Bulk MoO₃: Elucidating the Formation of Alkenes and Alkanes

Universal oxygen removal of depolymerized lignin molecules is a desirable route to funnel complex mixtures of bioderived molecules to high-demand "drop-in" petrochemical intermediates (aromatics and alkenes). Molybdenum oxide-based catalysts are of great interest for this valorization strategy, as they have shown proficiency in removing a wide range of oxygen functionalities without excess hydrogen consumption or aromatic ring saturation. Despite these advantages, MoO3 has demonstrated a propensity to saturate aliphatic molecules to low-value alkanes during deoxygenation, decreasing the economic potential of the conversion process. To better inform mitigation strategies, detailed kinetic experiments were applied to various alcohol-, aldehyde-, and ketone-containing model compounds to infer potential reaction pathways and mechanisms for forming alkene and alkane products. Consistent differences in product distribution, reaction rates, oxygenate, and H-2 orders were observed to exist between carbonyls and their alcohol analogs, allowing for the determination that the hydrogenation of carbonyls to alcohols is required and kinetically limiting for HDO on MoO3. It is also proposed that carbonyls competitively adsorb at hydrogenation-active hydroxyl species, leading to aldol condensation reactions and a negative carbonyl order for the rate of HDO. Once adsorbed/formed on oxygen vacancies, MoO3 likely mediates hetero- and homolytic cleavage of alcohol C-O bonds to create a combination of carbocation and free radical intermediates. Both intermediates can then proceed through a dehydration or hydrogenolysis pathway to directly form alkenes or alkanes, respectively, at varying rates depending on the stabilization of the reactive carbon by electron-donating constituents. This effect led to primary oxygen functional groups yielding the lowest initial selectivity to alkenes (similar to 50%). Further, kinetic analysis of cyclohexene and 1-pentene feeds showed that alkenes weakly adsorb onto HDO-active oxygen vacancies to undergo a sequential hydrogenation reaction. This low adsorption strength of alkenes relative to those of oxygen-containing molecules prevents further conversion of the initial HDO products.