Green solvent pretreatments for lignocellulosic biorefineries: A review

Green solvents have emerged as a promising alternative to efficiently fractionate lignocellulosic biomass, often reducing the environmental impact of the process. This review paper explores the potential of green solvents as sustainable alternatives for lignocellulosic biomass pretreatment in biorefineries. The study focused its discussion on four of the most prominent green solvent classes: deep eutectic solvents (DES), ionic liquids (IL), supercritical fluids (SFs), and biomass-derived solvents. It analyzes their efficiency in fractionating cellulose, hemicellulose, and lignin while emphasizing industrial scalability and environmental impact. Considering the body of work evaluated here, although viscosity and recyclability issues persist, DES demonstrated high tunability and low toxicity, achieving notable delignification and enzymatic hydrolysis enhancement. ILs exhibited superior selectivity and adaptability for lignocellulose dissolution but faced challenges of high synthesis costs and environmental concerns. SFs, particularly supercritical CO2 combined with co-solvents, proved effective for rapid biomass fractionation but require high energy input for extreme operating conditions. Biomass-derived solvents like γ-valerolactone (GVL) and Cyrene® showed promise for efficient lignin removal and cellulose preservation. However, their production processes need further optimization for sustainability. Overall, this review highlights advancements in green solvent technologies, identifying key challenges such as recyclability, cost reduction, and lifecycle environmental impacts. It calls for further research on optimizing solvent systems and exploring diverse biomass types to enhance industrial application and environmental viability.