Hydrogen-free reductive catalytic fractionation of biomass via synergistic solvent-catalyst interactions: mechanistic and kinetic insights.

This study systematically investigates the synergistic effects of catalysts and solvents in the hydrogen-free reductive catalytic fractionation (RCF) of lignin, employing substrate tests and kinetic analyses. Under optimal conditions, Ni/C, Ru/C, and Pd/C can achieve ideal monomer yield (50-54%). But Pd/C sustains over 95% alkylphenol selectivity independent of catalyst-to-biomass ratios, while Ni/C and Ru/C require optimized catalyst loading to minimize alkene-phenol formation. Protic solvents serve as the primary hydrogen source, as unequivocally confirmed by deuterium labeling experiments, whereas acidic additives, as probed by kinetics, primarily promote Cβ-O ether cleavage. Xylose promotes the ether cleavage to a certain extent over Ni/C in aprotic solvent, but much less effective in other cases. Introduction of water significantly deactivates Ni/C (<15% monomer yield at 50% water content) and shifts product distributions from ethylphenols to propylphenols over Pd/C, suggesting solvent-dependent adsorption changes. Ru/C exhibits the lowest energy barrier (64.68 kJ/mol) in hydrogen-free RCF, leading to 54% monomer yield in 50% aqueous ethanol. All these findings suggest that the catalyst and solvent interact synergistically to affect the reaction pathway and lead to different product selectivities. Our study provides valuable mechanism insights into the hydrogen-free RCF and benefits fundamental research for lignin depolymerization.