Hydrogenative degradation of PEG-functionalized lignin

The valorization of lignin to value-added basic chemicals is one of the most important technologies for efficient carbon recycling. While many catalytic systems have been developed for cleavage of monolignol linkages, especially for β-O-4 structures, the low solubility of lignin, which originates from its complicated polymeric structure, often makes it difficult to apply these catalytic process to degradation of real lignin-derived materials. Here, we investigated the degradation of poly(ethylene glycol)-modified lignin with transition metal complexes. Monolignols (4-methyl, 4-ethyl- and 4-propyl-guaiacol) were obtained as the degradation products. Although low solubility after detachment of the PEG moiety hampered efficient degradation, the addition of PEG for in situ protection of the hydroxy group was effective in maintaining the lignin solubility and improving the monolignol yields. The hydrogenolysis of PEG-modified soluble lignin was investigated with series of transition metal complexes to afford alkyl guaiacols. Although the introduced PEG moiety was also susceptible against the hydrogenolysis, in situ modification in PEG solvent was found effective for maintaining the lignin soluble and improved degradation efficiency.