Oxalic acid-mediated oxidative depolymerization of kraft lignin for vanillin and vanillic acid production

In this work, kraft lignin (KL) was depolymerized in an oxalic acid-mediated aqueous solution to vanillin and vanillic acid. The phenolic compounds were assessed by high-performance liquid chromatography (HPLC). The 13C nuclear magnetic resonance (13C-NMR) and heteronuclear single quantum coherence-nuclear magnetic resonance (HSQC-NMR) of the extracted products showed the presence of aromatic moieties, confirming the abundance of phenolic compounds and the absence of the lignin inter-unit linkages. The proton nuclear magnetic resonance (1H-NMR) and HSQC-NMR of the partially depolymerized lignin confirmed the cleavage of β-O-4 linkages. The findings showed that the depolymerization of KL (5 wt.%) in the presence of 1.25 M oxalic acid and 50 psi oxygen pressure at 125 °C for 60 min generated the maximum vanillin and vanillic acid yields of 15.7 wt.% and 11.7 wt.%, respectively. It was also revealed that increasing the oxalic acid concentration increased the yields of vanillin and vanillic acid. The current study demonstrated that the niobium pentoxide catalyst increased the vanillin and vanillic acid yields by 49.5% and 53% in KL depolymerization, respectively, while significantly decreasing the yields in lignosulfonate depolymerization reaction. The one-step aqueous reaction, relatively high production yield, recovery (up to 78.6 wt%), and recyclability (45 wt%) of the oxalic acid could make this process industrially attractive for the vanillin production from KL.