Amphiphilic W-substituted Keggin-type POM@CTAB catalyst for oxidative depolymerization of Chinese herbal residue...

Chinese herbal residues (CHR) are currently mainly disposed of through extensive and low-value methods such as stockpiling, landfilling, and incineration, leading to severe biomass resource wastage and environmental pollution. To address this challenge, an amphiphilic heterogeneous catalyst (W-substituted Keggin-type polyoxometalates@cetyltrimethylammonium bromide, POM@CTAB) was developed for oxidative depolymerization of CHR lignin into high-value aromatics. The catalyst integrates two targeted designs for CHR lignin, one is W⁶⁺ substitution optimizes POMs redox activity and acid strength, enabling selective cleavage of lignin's β-O-4 ether bonds while avoiding the toxicity of vanadium-containing POMs; the other is CTAB modification endows amphiphilicity-hydrophobic alkyl chains enrich hydrophobic CHR lignin around active sites. Response surface methodology (RSM)-Box-Behnken design (BBD) optimized reaction conditions (170 °C, 12 h, 3 mL H₂O₂ as oxidant and 0.35 g catalyst), achieving 81.22% conversion and 18.4% monomer yield for lignin model compounds. For real CHR-derived Eucommia ulmoides lignin (E. ulmoides), 94.73% conversion and 15.60% aromatic product were achieved; direct depolymerization of raw E. ulmoides residues also achieved 89.3% lignin conversion and 13.2% aromatic yield-validating the catalyst's applicability to unprocessed CHR. Mechanistic studies via GC-MS and 2D-HSQC NMR confirmed selective β-O-4 bond cleavage and the formation of high-value products. Life cycle assessment (LCA) showed 1 kg POM@CTAB production has a 34.50 kg CO₂ eq global warming potential and 496 MJ primary energy demand, with POM raw materials and ethanol as main environmental burdens. This work provides a green and efficient catalytic pathway for CHR lignin utilization, reducing pollution from CHR disposal and contributing to biomass-based aromatic chemical production.