Understanding the structural changes and depolymerization of Eucalyptus lignin under mild conditions in aqueous AlCl3

Abstract

Lignin is a unique renewable source of phenolic products for the potential replacement of fossil fuels. In this study, swollen residual enzyme lignin (SREL) was prepared and then depolymerized in aqueous AlCl₃ to produce phenolic compounds. The residual lignin after depolymerization was further characterized by FT-IR, advanced NMR (2D-HSQC, ¹³C, and ³¹P NMR spectra) and GPC techniques, and the degradation products were identified and quantified by GC-MS. The results showed that the depolymerization reaction became the leading reaction under these conditions. Meanwhile, the depolymerization process also resulted in a large amount of a volatile fraction, which was composed of phenols, guaiacols, and syringols. Among the volatile fraction, syringaldehyde accounted for about 31.70% of the bio-oil under optimum conditions (at 160 °C for 1 h). Furthermore, the mechanism for lignin depolymerization in aqueous AlCl₃ was discussed and an enhanced understanding of the effects of aqueous AlCl₃ pretreatment on lignin chemistry was provided, which will improve the pretreatment methodology to reduce biomass recalcitrance.