Sustainable oxidative depolymerization of lignin over a ternary [VimAm]Br@POM@ACF catalyst: synergistic catalysis, solvent recyclability and life cycle assessment

Abstract

Lignin conversion into high-value aromatic monomers is severely hindered by undesired condensation, radical recombination, and poor product selectivity, restricting the sustainable substitution of fossil-derived aromatics in circular biorefineries. To overcome these challenges, a heterogeneous ternary catalyst ([VimAm] Br@POM@ACF) was fabricated by immobilizing ionic liquid-functionalized Keggin-type polyoxometalate onto KOH-treated carbon fiber cloth (ACF), integrating efficient catalytic oxidation with facile catalyst recyclability. In the oxidative depolymerization of a β-O-4 lignin model compound using H₂O₂ as a green oxidant, the optimized catalyst achieved 72.73% substrate conversion and 19.94% total aromatic monomer yield, with high selectivity to methyl mandelate (67.65%) and methyl benzoate (29.94%). The catalyst exhibited exceptional recyclability over five cycles, with minimal activity loss and ultra-low metal leaching. Compared with homogeneous catalysts, the ternary catalyst exhibited negligible metal leaching, and the methanol/water solvent system achieved >80% recovery after three cycles, reducing the process E-factor from 8.5 to 5.2. A cradle-to-gate life cycle assessment (LCA) identified the synthesis of POM and ionic liquid as the primary environmental hotspots, contributing significantly to global warming potential (526.87 kg CO₂ eq kg⁻¹ catalyst) and primary energy demand (6930 MJ kg⁻¹ catalyst). To our knowledge, this is the first report of a macroscopic ACF-supported IL-POM ternary catalyst for oxidative lignin depolymerization, and the first to couple catalytic performance with solvent recyclability, E-factor analysis, and LCA for this material class. This study not only presents a highly efficient and recyclable catalyst for lignin depolymerization but also provides a quantitative environmental profile and a roadmap for green optimization, advancing the sustainable upgrading of lignin toward a circular bioeconomy.