Vapor phase hydrodeoxygenation of phenolic compounds on group 10 metal-based catalysts: Reaction mechanism and product selectivity control

Catalytic hydrodeoxygenation (HDO) of phenolics is a crucial step to convert lignin-biomass to chemicals and fuels. Catalysts based on group 10 metals (Ni, Pd and Pt) are widely used in HDO of phenolics. However, achieving a high yield toward the desirable product remains a challenge since a number of competing reactions take place at the same time, resulting in a complex product distribution. Herein, we review the recent advances in understanding the reaction mechanism on group 10 metal based catalysts and the novel strategies for selective deoxygenation of phenolics to aromatics on these catalysts under mild conditions (intermediate temperature and atmospheric pressure). Typically, HDO on Ni, Pd and Pt involves three main reaction pathways: a) hydrogenation to cyclohexanones/cyclohexanols; b) deoxygenation to aromatics, and c) C - C hydrogenolysis to CH4. Among the possible mechanisms proposed for deoxygenation to aromatics, the partial hydrogenation mechanism shows the lowest barrier for C - O cleavage, and is therefore most likely to happen. Strategies such as increasing affinity to the O atom of phenols on the surface of Ni, Pd and Pt based catalysts by either reducing the particle sizes, alloying with an oxophilic metal, modifying the metal catalyst with a reducible oxide, or combining with an acidic zeolite have been reviewed. These strategies have been shown to improve the selectivity of HDO to aromatics over the hydrogenation and C - C hydrogenolysis products and are expected to provide useful guidance for rational design of the selective HDO catalysts.