Oxygen vacancy assisted PMo@CeO₂/MWCNTs catalyst for selective oxidative depolymerization of pine lignin

Efficient oxidative depolymerization of lignin requires coordinated oxygen activation, interfacial electron transfer, and accessible redox sites, yet integrating these functions within a single heterogeneous polyoxometalate-based catalyst remains challenging. Herein, a PMo/CeO₂-MWCNTs composite catalyst was constructed by immobilizing phosphomolybdic acid on a CeO₂/MWCNTs hybrid support, thereby combining oxygen-vacancy-associated ceria sites with an open and conductive MWCNT framework. Raman, XRD, FT-IR, SEM-EDS, BET, and related analyses confirmed successful immobilization of PMo on the composite support while preserving the essential structural characteristics of the Keggin anion. Among the catalysts with different PMo loadings, PCM-40 exhibited the best catalytic performance, showing a volcano-type dependence on loading that reflected the balance between active-site density and mass-transfer accessibility. Under the optimized conditions, the total yield of aromatic monomers from pine lignin reached 16.3 wt%, with vanillin and methyl vanillate as the major products. EPR measurements showed stronger signals assigned to ¹O₂, ·OH, and ·O₂^- after PMo immobilization on the CeO₂/MWCNT support, while HSQC analysis demonstrated extensive cleavage of β-O-4 linkages together with pronounced transformation of lignin substructures. A β-O-4 dimer study further supported a reaction pathway involving oxidative Cα-Cβ cleavage followed by subsequent oxidation and esterification. After five reuse cycles, the catalyst retained a substantial fraction of its initial activity, although gradual deactivation was observed. These findings demonstrate the potential of integrating redox-active PMo with CeO₂ and an open MWCNT framework for heterogeneous lignin oxidation.