Ionic Liquids for Sustainable Lignin Biorefinery: From Dissolution Mechanisms to Electrocatalytic Upgrading

Abstract

Lignin is the most abundant renewable source of aromatic carbon, yet it remains largely underutilized due to its intrinsic recalcitrance. This recalcitrance arises from a heterogeneous three-dimensional network characterized by stable C–C bonds and interunit linkages that are prone to condensation. Ionic liquids (ILs), with their designable structures, negligible vapor pressure, wide electrochemical windows, and inherent ionic conductivity, offer a versatile and tunable platform to overcome this challenge. This review provides an integrated framework that bridges IL-mediated dissolution mechanisms and electrocatalytic lignin upgrading. We first analyze the molecular origins of lignin recalcitrance and elucidate how rational IL design, through synergistic hydrogen bonding, π–π interactions, and electrostatic forces, enables efficient solubilization, integrating both experimental and computational insights. We then critically compare IL-based electrocatalytic systems with conventional aqueous and organic electrolytes to establish performance benchmarks and identify key limitations. Finally, we demonstrate how IL-mediated electrocatalysis uniquely reconciles the competing demands of substrate solubility and charge transport, enabling selective cleavage of both C–O and C–C bonds. Looking forward, future progress will depend on integrating flow electrolysis, efficient separation and IL recycling, feedstock-adaptive process design, and life-cycle assessment to support scalable and sustainable lignin valorization. By unifying dissolution chemistry with electrocatalytic conversion, this review positions ILs not merely as solvents, but as enabling media that open new pathways toward sustainable and electrified lignin biorefineries.