Catalytic Hydrodeoxygenation of Lignin-Derived Oxygenates: Catalysis, Mechanism, and Effect of Process Conditions

The high oxygen content of pyrolysis bio-oil with many organic functional groups in it limits its direct application as a blendstock. The upgradation of biomass-derived oxygenates into renewable fuels and value-added chemicals via catalytic hydrodeoxygenation (HDO) has received considerable attention in recent years. This review focuses on HDO of key model compound oxygenates, which sets the ground to propose the overall reaction mechanism of HDO of bio-oils. Catalysts play a vital role in HDO, and its design poses many challenges because of different reactions involved such as hydrogenolysis, hydrogenation, decarbonylation, and dehydration occurring simultaneously at different catalyst-active sites. The main objective here is to present a comprehensive introduction to the reaction mechanism involved in the HDO of bio-oil model oxygenates. For this, a thorough discussion of different reaction pathways taking place during the HDO of five model oxygenates, viz., anisole, guaiacol, eugenol, vanillin, and dibenzofuran, is presented. The model compounds are selected to provide a good description of the HDO of lignin-derived compounds present in bio-oils. Particular emphasis is placed on the effect of the catalyst, temperature, hydrogen partial pressure, and solvent employed on the product distribution. This review will aid not just in understanding the interrelations between the nature of the catalyst, HDO mechanism, and product distribution but will also provide thoughtful directions for the applications of HDO in real bio-oil upgradation.