Glycerol-derived ethers enable hydrogen-free reductive catalytic fractionation of softwood lignin into functionalized aromatic monomers†

Abstract

Catalytic reductive processes facilitate deconstruction of lignins into value-added aromatics. This study explores the novel use of glycerol-derived ethers (GDEs), specifically 1-3-dimethoxypropan-2-ol (DMP) and 1,3-diethoxypropan-2-ol (DEP), as hydrogen transfer solvents for reductive catalytic fractionation (RCF) of softwood biomass, marking a departure from conventional use of high-pressure molecular hydrogen and short-chain alcohols. The influence of process conditions, namely, batch holding time, temperature, catalyst species and dosage, solvent-to-biomass ratio, acidic medium (by acetic acid addition), and water volumes as a co-solvent on the yield of aromatic monomers and delignification were evaluated. Under optimal conditions, GDE-mediated RCF of softwood achieved aromatic monomer yields and delignification up to 24.9 wt% and 90.7 wt%, respectively. Aromatic monomers with unsaturated and oxygenated side chains were observed including value-added species, such as vanillin, isoeugenol, coniferaldehyde, eugenol, and vanillic acid. This observation contrasts with prior RCF studies applying ex situ hydrogen which yield monomers with saturated alkyl side chains (e.g., 4-propylguaiacol, 4-ethylguaiacol). Mass-based green chemistry metrics (e.g., solvent intensity, process mass intensity) demonstrate GDEs supported material-efficient, catalytic deconstruction of softwood lignins into value-added aromatic monomers. MALDI-TOF analyses of resultant lignin oils revealed the occurrence of sidechain dehydration and decarbonylation of oligomeric species. HSQC NMR of lignin oils indicated the absence of native linkages, especially β-O-4 bonds, post RCF treatment. Furanic monomers derived from carbohydrate fractions were identified and furan yields were higher under neat solvent conditions (∼8 wt%) than in the presence of redox catalyst (∼2 wt%). This study demonstrated successful and optimized utilization of GDEs as hydrogen transfer solvents for RCF of softwood biomass, resulting in competitive yields of functionalized aromatics within the confines of green chemistry.