Lignin Fractionation Using Ionic Liquids and Deep Eutectic Solvents: A Comprehensive Review of Mechanisms and Process Variables

The depletion of nonrenewable resources has driven the need for sustainable lignocellulosic biomass valorization. However, traditional delignification methods, such as the Kraft process, are energy-intensive and present significant environmental concerns. This review provides a comprehensive critical assessment of biomass fractionation utilizing ionic liquids (ILs) and deep eutectic solvents (DESs) as highly tunable, greener solvent alternatives. We systematically examine how solvent chemistry including the selection of anions, cations, hydrogen-bond donors, and overall acidity dictates delignification performance. Furthermore, the interplay of process variables such as time, temperature, solids loading, and particle size is evaluated to optimize extraction efficiencies while mitigating unwanted lignin recondensation. The underlying chemical mechanisms of lignin solubilization and depolymerization are explored across acidic, alkaline, and neutral solvent systems, with particular emphasis on the cleavage of β-O-4′ ether linkages and the pathways of intermediate structures. To bridge the gap between bench-scale research and industrial implementation, this review addresses critical techno-economic challenges, including solvent toxicity, synthesis costs, complex recovery operations, and equipment corrosion. Ultimately, we identify existing knowledge gaps such as the need for standardized experimental reporting and predictive computational models and outline future perspectives to advance IL and DES systems toward scalable and economically viable closed-loop biorefinery applications.