Enhancing α-etherification of lignin in Eucalyptus diol pretreatment to improve lignin monomer production

In this work, alpha-etherification of lignin in diol pretreatment was selectively enhanced at mild temperature for lignin isolation and subsequent valorisation. More than 90 % of lignin was removed from Eucalyptus at 120 degrees C in diol (ethylene glycol and 1,4-butanediol) pretreatment, resulting in > 90 % cellulose conversion in 24 h at 7.5 FPU/g glucan cellulase loading. Subsequent catalytic transfer hydrogenolysis of the isolated lignins with Ru/C in ethanol gave 15 % monomer yield on native lignin basis, 5 times of that from the technical ethanol process (170 degrees C). HSQC NMR analysis revealed that diol pretreated lignin (120 degrees C) contained ~23 % alpha-etherified beta-O-4 interunit bonds, indicating that lignin degradation (i.e. cleavage of beta-O-4 bonds) was suppressed via etherification by grafting a hydroxyl group at the alpha position of lignin. This finding was consistent with the isolated lignin (120 degrees C) had less number of phenolic OH and higher molecular weight via P-31 NMR and GPC analysis. 31P NMR analysis also revealed that diol isolated lignin contained more number of aliphatic OH than ethanol-isolated lignin, which increased lignin solubility and maintained the high yield (> 80 %) of isolated lignin from Eucalyptus at 120 degrees C as expected. In summary, diol pretreatment of woody biomass can effectively isolate more lignin for hydrogenolysis to valued-added monomers without compromising the isolated yield of lignin and hydrolysis yield of remained cellulose.