Sustainable valorization of lignocellulosic biomass for integrated biofuel and biomaterials production within the circular economy framework : recent advances in pretreatment processes — a comprehensive review

Abstract

Lignocellulosic biomass is a promising renewable feedstock for the sustainable production of biofuels, biochemicals, and bioenergy, offering a viable alternative to fossil-based resources. Its efficient conversion is considered essential for developing resilient low-carbon energy systems. Integrated biorefineries, which combine diverse feedstocks with multiple conversion technologies, enable efficient valorization of all biomass fractions, thereby improving resource utilization, sustainability, and economic viability. However, lignin valorization remains a major challenge due to its complex and recalcitrant nature. This review critically examines recent advances in lignin-first valorization approaches, with particular emphasis on reductive catalytic fractionation (RCF) and hydrothermal liquefaction (HTL) for the production of value-added chemicals and fuels. Special attention is given to emerging hydrogen-free and self-hydrogen-supplied RCF strategies, including alcohol-mediated transfer hydrogenation systems, which reduce dependence on external hydrogen while improving process safety and economic feasibility. Recent developments in multifunctional and bimetallic catalysts are also discussed in relation to enhanced lignin depolymerization, selective bond cleavage, and suppression of lignin repolymerization. In addition, the review evaluates recent progress in pretreatment technologies and their roles in improving lignin accessibility, conversion efficiency, and overall biorefinery integration. The techno-economic potential of integrated biorefinery systems is critically assessed, focusing on process scalability, intensification strategies, and major cost drivers. Broader sustainability and socio-economic implications are also discussed. Finally, key challenges, including catalyst stability, hydrogen management, feedstock variability, and process integration, are identified, and future research directions are proposed to support economically viable, scalable, and environmentally sustainable lignin valorization pathways for next generation biorefineries.

HIGHLIGHTS • Lignin-first approach enables high-value products such as sustainable aviation fuel. • Reductive catalytic fractionation (RCF) is a leading lignin-first valorization strategy. • Noble metal single-atom catalysts excel in RCF for phenolic monomer production. • HTL and RCF convert lignin into high-energy fuels and aromatic compounds. • Integrated biorefineries convert multiple feedstocks into diverse high-value products.