Recent Advancements in the Hydrodeoxygenation of Lignin-Derived Phenols to Cyclohexane-Based Products Over Supported Ruthenium Catalysts

Lignin, being an underutilized component of cellulosic biomass has the potential to be converted into value-added chemicals and fuels. However, the recalcitrant nature of lignin to degradation has been a major hurdle in the valorization of lignin. The depolymerization (catalytic and pyrolytic) of lignin results in the formation of lignin-derived bio-oil comprising low-molecular-weight phenolic compounds (monomers, dimers, and oligomers) with much higher oxygen content than that required for the fuels. The catalytic hydrodeoxygenation (HDO) of lignin-derived phenolic compounds is one of the alluring opportunities for converting biomass materials to chemicals and fuels. The current article systematically reviews the advancement in the production of cyclohexane-based compounds from five primary lignin-derived monomers, phenol (PHE), guaiacol (GUA), cerulignol (CER), eugenol (EUG), and vanillin (VAN) using supported ruthenium (Ru) catalysts. The carbon-based zeolites and TiO₂-based materials with distinguished properties are explored as the support for Ru nanoparticles. Understanding the characteristics of metal and its supports, along with the catalytic mechanisms and structure-activity relationships is crucial in developing HDO catalysts to produce selective products from lignin-derived phenolic monomers for practical applications. This review particularly highlights the significance of the synergistic interactions between ruthenium and the support materials during HDO reactions.