A Targeted Review on Lignin and Its Depolymerization Methodologies

The surge in demand for renewable energy and bio-based chemicals has heightened attention on lignocellulosic biomass as an alternative to fossil-derived resources. Lignin, a major component of lignocellulosic biomass, is not used to a sufficient extent despite its potential as a renewable resource for multiple important chemicals and biofuels. This survey examines extraction, characterisation, and decomposition of lignin, highlighting recent innovations and hurdles in lignin valorisation. Various lignin isolation techniques, such as kraft lignin, soda lignin, organosolv lignin, ionic liquids, hydrolysis lignin and deep eutectic solvents, are explored, emphasizing their impact on lignin purity and structural integrity. Analytical methods including NMR, FTIR, GPC, TGA, and DSC provide critical insights into lignin’s complex chemical structure and physicochemical properties. The review also investigates diverse depolymerisation strategies: acid-catalysed, base-catalysed, metal-catalysed, hydrogenolysis, electrocatalytic oxidative and reductive reaction pathways and microwave-assisted processes focusing on their efficiency, selectivity, and applicability for industrial-scale to convert lignin into value-added products. Despite technological advancements, problems such as lignin’s structural heterogeneity, high energy requirements for processing, low yields, and catalyst costs impede widespread commercialisation. Future research should address cost-effective, recyclable catalysts, process integration within biorefineries, and optimisation strategies to enhance economic feasibility. Advancing lignin valorisation technologies is vital for achieving a circular bioeconomy and reducing dependence on fossil-based resources.