A Review on Lignin Valorization for Sustainable Resource Recovery: Current Microbial and Enzymatic Methods and the Roles of Ionic Liquids and Deep Eutectic Solvents

Abstract

Lignin, as the most abundant renewable aromatic polymer on earth, holds immense potential as a feedstock for value-added products. However, its recalcitrant and heterogeneous structure presents significant challenges to efficient valorization. While microbial and enzymatic bioconversion offers a sustainable and specific route for lignin depolymerization, industrial implementation is often hindered by limitations such as low enzymatic efficiency, poor operational stability, and restricted substrate accessibility. This review systematically summarizes the current state of lignin bioconversion, focusing on the capabilities of various fungi and bacteria and the ligninolytic enzymes they produce, notably laccases and peroxidases. A key emphasis is placed on the emerging roles of “green” solvents, specifically ionic liquids (ILs) and deep eutectic solvents (DESs), in overcoming these limitations. These solvents not only enhance lignin solubility but also can activate and stabilize ligninolytic enzymes, thereby enabling more efficient depolymerization reactions. This review examines the mechanisms, advantages, and current challenges of integrating ILs and DESs into biomass lignin upgrading strategies. Finally, it discusses future research directions and potential application prospects.