Current approaches, emerging developments and functional prospects for lignin-based catalysts - a review

Biopolymer functionalization is perceived as an innovative approach to revolutionizing bio-catalyzed chemical transformations. Lignin, the chief natural source of polyphenols, has an aromatic structure with plenty of beneficial chemical groups. Accordingly, lignin can be functionalized, copolymerized, or recombined with different types of chemicals to produce highly efficient catalysts. Moreover, the stability and biodegradability of lignin make it an efficient bio-immobilizer for diverse metals (Pt, Pd, Ag, Cu, Co, Fe, etc.), which are known for their excellent catalytic performances. In addition, the solid nature of the lignin-derived catalysts has excellent advantages in terms of simple catalyst recovery and reuse over consecutive cycles with high stability. The present work compiles the newly adopted approaches to valorize lignin in the field of catalysis. Lignin-derived acid and base catalysts, lignin metal-free catalysts, and lignin-supported metal catalysts have demonstrated the full potential of lignin in hydrolysis, esterification, oxidation, condensation, hydration, and multicomponent reactions. The synthesis methods, properties, and catalytic capacity of lignin-based catalysts were assessed and contrasted with conventional catalysts. Attractively, the inclusion of lignin in catalysis appeared as a novel upgrading strategy that offers eco-friendly and profitable materials, leading to simple and green chemical synthesis, as well as a revolutionary shift in the industrialization of biomass-derived materials generally, and functionalized lignin-based catalysts specifically.