A review of lignin-first reductive catalytic fractionation of lignocellulose

Reductive catalytic fractionation (RCF) of lignocellulose based on lignin first is an emerging biorefinery strategy that combines biomass fractionation with lignin depolymerization to obtain low-Mw lignocellulosic oils (monomers and low molecular weight oligomers) and hydrocarbon pulp. Both product streams are considered valuable resources for the biochemical-based industry. In this work, the recent research progress of RCF was reviewed through the aspects of feedstocks, catalysts, solvents, and carbohydrate retention. Different feedstocks with different structures of lignin units (S, G, H units) and the distribution of C-O bonds directly lead to a significant difference in the yield and selectivity of the monomers. For catalysts, homogeneous catalysts have better contact with the lignocellulose, but the monomer yield of homogeneous catalysts alone is not substantial as compared to the heterogenous catalysts. Different metal elements, particle sizes, doped heteroatoms, and carrier pairings in heterogenous catalysts significantly have an impact on the RCF results. Currently, to obtain monophenol yields close to theoretical yields, high delignification, and high carbohydrate retention, many studies use a combination of homogeneous and heterogenous catalysts for RCF of lignocellulose. The solvent is also an essential factor as the solvent properties directly affect the efficiency of RCF and also has a significant impact on the composition and distribution of RCF products. This paper summarizes, discuss, and propose solutions for the challenges encountered during the RCF strategies based on the major factors mentioned above.