Molten salt hydrates: innovative and versatile solvent systems for lignocellulosic biomass processing and valorization

Abstract

Efficient utilization of lignocellulosic biomass is crucial for advancing a sustainable bioeconomy yet remains challenging due to the intrinsic recalcitrance and structural complexity of plant cell walls. Molten salt hydrates (MSHs)—highly concentrated salt–water complexes with tunable hydration structures—have recently emerged as powerful and versatile solvent systems capable of dissolving, fractionating, and selectively converting lignocellulose under comparatively mild conditions. This review provides a critical overview of recent progress in MSH-enabled biomass processing and valorization. We first examine cellulose behavior in MSHs, including dissolution and hydrolysis mechanisms, crystalline polymorph transitions, and regeneration-induced structural reorganization, emphasizing how ion coordination and water activity govern cellulose accessibility and reactivity. The dissolution and transformation of hemicellulose, together with lignin depolymerization, condensation, ether-bond cleavage, and demethylation, are systematically summarized. Integrated lignocellulosic processing strategies are highlighted, covering direct saccharification for sugar production, selective fractionation of biomass components, and the generation of key platform chemicals, such as fructose, furan derivatives, levulinic acid, and functional oligosaccharides, within MSH-mediated reaction environments. We also discuss the fabrication of value-added cellulose-based materials enabled by dissolution–regeneration pathways. Finally, environmental and economic considerations, including solvent recyclability, salt–water loop closure, product separation, residual salt management, and process scalability, highlight both the opportunities and challenges of MSHs as recyclable and multifunctional solvent platforms for future biorefineries and eco-manufacturing.