A mild acidic oxidative process for lignin-derived functionalized monomers under catalyst, solvent, and pressure-free conditions

Abstract

Lignoboost® (LB) Kraft lignin was subjected to a mild acidic oxidative depolymerization process under solvent-, catalyst-, and pressure-free conditions, achieving functionalized monomers with proportions of up to 79.4 % (w/w). This environmentally friendly method operates under naturally acidic conditions using only H₂O and H₂O₂, ensuring a high concentration of LB in suspension (300 mg/mL) and taking advantage of its inherent protonated nature. Monomers were directly recovered through simple drying, eliminating the need for precipitation or purification steps. Comprehensive characterization was performed using ATR-FTIR, HP-SEC, TGA, EA, ¹H NMR, and ³¹P NMR techniques. The weight-average molecular weight ($\overline{Mw}$) and polydispersity index (PDI) decreased significantly from 1985 g/mol and 2.91 to as low as 300 g/mol and 1.79, respectively, after 7 h at 55 °C. Operational conditions were tuned to selectively enhance specific functional groups: phenolic OH groups increased up to 5.17 mmol/g under shorter reaction times (60 °C, 3 h), while moderate temperatures and longer reaction times favored carboxylic OH groups (4.99 mmol/g at 55 °C for 7 h). Additionally, alkene (C=C) groups reached up to 81.52 % of total hydrogen content under the highest temperature conditions (65 °C for 2 h). This process not only enhances our understanding of concurrent functionalization and depolymerization mechanisms but also establishes a scalable and green methodology for producing functionalized monomers as precursors for sustainable bio-based polymers. These findings contribute to the development of green technologies for lignin valorization.