Products of hydrothermal treatment of lignin and the importance of ortho-directed repolymerization reactions

Abstract Alkaline hydrothermal treatment (HTT) of Protobind 1000 lignin was studied to improve the understanding of lignin conversion paths. HTT was performed at 300 °C using NaOH as base catalyst. Product separation by acidification and solvent fractionation resulted in a liquid product of low molecular mass and narrow molecular mass distribution and two high molecular mass solid fractions. Twenty volatile compounds accounted for about 36 wt% of the liquid product, however, the analysis by liquid injection GC–MS was estimated to detect only about 12 wt% of the total products. Analytical pyrolysis was applied to investigate the pyrolyzable fraction of the liquid product and the high molecular mass fractions. The pyrolyzable mass accounted for 49 wt% of the total products. The liquid product was quantitatively pyrolyzed at 500 °C (91 ± 10 wt%) while less than half of the high molecular mass fractions were pyrolyzed. Several chemical features of the pyrolysates suggested that ortho -directed alkylation and condensation reactions of phenolate ions were important for the production of oligomeric and polymeric aromatic compounds during HTT. Increased abundancy of hydroxyphenyl and o -alkyl aromatic pyrolysates in the high molecular mass fractions suggested that the syringyl units of the raw lignin were less affected by repolymerization reactions than hydroxyphenyl and guaiacyl units. These results suggest that the monolignol composition of lignin is an important factor for the valorization of lignin by alkaline hydrothermal treatment.