Study on the influencing mechanisms of lignin molecular structure on demethylation activation

The unclear demethylation mechanism, particularly the unclear effects of lignin structure, significantly constrains the advancement of lignin research and application. To address this issue, diverse lignins with distinct molecular structural features were demethylated using AlCl₃ as the demethylation reagent, followed by comprehensive characterization. Subsequently, systematic evaluations, including measurements of Cr(VI) ions adsorption capacity and antioxidant activity, were carried out to establish feedback validation mechanisms for elucidating how structural features affect demethylation efficiency. Overall, all types of lignin can undergo demethylation activation. Nevertheless, the extent of the increase in phenolic hydroxyl content varies significantly. Among them, alkali-extracted demethylated pine lignin (DALP) exhibits the highest increase in phenolic hydroxyl content. In eucalyptus lignin, guaiacyl (G-type) units are more prone to react with AlCl₃ to undergo demethylation activation than syringyl (S-type) units. When p-hydroxyphenyl (H-type) units are present in straw lignin, the demethylation reactivity of S-type units is enhanced. Moreover, all four lignins undergo relatively severe degradation. In comparison with eucalyptus and straw lignin, both DAL-P and organic solvent-extracted pine lignin (DOLP), which consist solely of G-type units, exhibit relatively severe degradation and a low retention ratio of β-O-4 bonds. This can be attributed to their small steric hindrance. When used as an adsorbent and antioxidant, its performance enhancement strongly correlates with hydroxyl content variation, thereby validating the aforementioned conclusion. This study provides a theoretical foundation for regulating the lignin molecular structure and for research on demethylation.