Reductive Catalytic Fractionation of Lignocellulosic Biomass: A New Promissing Method for Its Complex Processing

This review focused to the discussion of the results from recent research in a promising area of the complex processing of lignocellulosic biomass: reductive catalytic fractionation (RCF). The effect catalysts, co-catalysts, solvents, sources of hydrogen, and the nature of lignocellulosic raw materials on the selectivity in the production of monomeric lignin products is considered. Heterogeneous catalysts are mainly used in RCF processes, which allows the reductive depolymerization of lignin to obtain low molecular weight compounds while maintaining the carbohydrate components of the biomass. Of the considered catalysts based on platinum group and transition metals, those containing Pd, Pt, Ru, and Ni have the highest activity. The nature of the metal also affects the composition of the resulting products. For example, ruthenium catalysts produce 4-propyl guaiacol as the main product, while ones based on Ni and Pd yield 4-propanol guaiacol. Catalysts containing Mo, due to their lower hydrogenation activity, give monolignols or their esterified derivatives of while preserving the carbohydrate components of lignocellulosic biomass. However, bifunctional catalysts that contain both acidic and metallic active sites are the most efficient in RCF processes. Acid sites contribute to the breaking of etheric β-O-4 bonds, while metal sites catalyze reduction of the resulting intermediate compounds. An important aspect of selecting suitable catalysts for the RCF process is their reusability. The use of a ferromagnetic catalyst or a basket for the catalyst solves the problem of separating it from products of the process.