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.

Automated summary

In this study, the alpha-etherification of lignin in diol pretreatment was enhanced, resulting in efficient isolation of lignin and subsequent valorisation. The diol pretreatment removed over 90% of lignin from Eucalyptus, enabling a high conversion of cellulose. The catalytic transfer hydrogenolysis of the isolated lignin produced a significant monomer yield, which was five times higher than the yield from the technical ethanol process. The findings suggest that diol pretreatment can effectively isolate lignin for hydrogenolysis without compromising the yield of lignin and cellulose hydrolysis.