Process-Oriented Reductive Catalytic Fractionation of Biomass: Linking Feedstock–Solvent–Catalyst Interactions with Reactor Configuration, Recycling, and Scale-Up
Reductive catalytic fractionation (RCF) has emerged as a representative lignin-first biorefining strategy for converting native lignin into aromatic monomers while preserving a carbohydrate-rich pulp for subsequent valorization. Although previous reviews have summarized the effects of feedstocks, solvents, and catalysts on monomer production, a process-oriented synthesis that connects reaction chemistry with reactor configuration, solvent recycling, catalyst recovery, and scale-up remains limited. This review, therefore, reassesses recent advances in RCF from an integrated process perspective. The comparative analysis indicates that monomer yield cannot be predicted by lignin content alone but is governed by the combined effects of β-O-4 abundance, lignin condensation, S/G ratio, lignin-carbohydrate connectivity, tissue structure, and solvent accessibility. Solvent systems are further evaluated not only by their ability to promote lignin extraction and intermediate stabilization, but also by their influence on carbohydrate retention, solvent recovery, and process sustainability. Catalyst development is discussed in relation to metal–support interactions, hydrogen-transfer pathways, catalyst placement, deactivation resistance, and compatibility with recycled solvents. Particular emphasis is placed on the transition from batch screening to semi-continuous, flow-through, and continuous-flow RCF, where spatial separation of lignin extraction and catalytic stabilization can improve residence-time control, mass transfer, catalyst recovery, and process scalability. The analysis further indicates that high-pressure batch reactors remain useful for laboratory screening, but their direct industrial implementation is restricted by equipment cost, safety requirements, energy input, high solvent-to-biomass ratios, and solvent recovery burden. Flow-through and continuous-flow configurations, coupled with solvent-loop optimization and catalyst recovery, are, therefore, more promising for scalable RCF. Finally, techno-economic analysis, life-cycle assessment, solvent-loop design, lignin-oil upgrading, and standardized reporting requirements are discussed as essential criteria for industrial implementation. Overall, this review highlights that scalable RCF should not be regarded simply as an optimized depolymerization reaction, but as a reactor- and recycle-aware lignin-first process architecture that integrates feedstock selection, solvent-catalyst chemistry, separation strategy, catalyst lifetime, solvent recovery, and downstream utilization.