Catalytic hydrogenolysis of lignin to phenols: Effect of operating conditions on product distribution

Abstract Lignin is a three-dimensional, amorphous, polyphenolic and aromatic-rich material linked by aryl ether, biphenyl and diaryl ether bonds, and is a primary recalcitrant molecule in lignocellulosic biomass. Unlocking valuable chemicals and fuel molecules from lignin requires the cleavage of typical inter-unit linkages like β─O─4, α─O─4 and 4─O─5 in a selective manner. Catalytic hydrogenolysis and hydrodeoxygenation are promising techniques to produce valuable chemicals and hydrocarbons in high yields and selectivities. Deriving phenolic derivatives such as guaiacol, alkyl guaiacol, guaiacyl alcohols, and syringyl alcohols is of great interest in lignin depolymerization. This perspective article will highlight the importance of catalyst structure, active metal, nature of support, solvent, and operating conditions to obtain phenolic compounds from a variety of lignins via catalytic hydrogenolysis. Tuning the acidity of the catalyst is shown to influence its activity, and to curb the unwanted repolymerization and char forming reactions. Finally, the stability and recyclability of the catalysts are also evaluated to develop a sustainable lignin valorization approach.