Metal-Substituted Vanadium-Based Polyoxometalate Catalysts for Catalytic Oxidation of Lignin: Activity and Reaction Mechanism

Lignin is an abundant renewable aromatic polymer, yet its oxidative conversion is often hindered by coke formation and recondensation of reactive intermediates, which cause catalyst deactivation and low product yield. Polyoxometalates (POMs) offer strong oxidative capability and tunable acidity, but vanadium-based POMs are commonly homogeneous and difficult to recover. Herein, K + was introduced into H 5 PMo 10 V 2 O 40 (PMoV 2 ) to construct a series of K + -substituted heterogeneous catalysts, K n H 5-n PMoV 2 (n = 1–5). Comparative analyses reveal that increasing K + substitution decreases the H + content and tunes Brønsted acidity while slightly lowering the redox potential by modifying the charge-balance state of the substituted species, thereby regulating the overall redox capability. Among these catalysts, KH 4 PMoV 2 delivered the highest lignin conversion (82.6%) and the maximum total yield of aromatic products (11.1%), including 2.4% aldehydes and 8.7% acids/esters. Recycling tests demonstrated excellent stability: lignin conversion remained at 79.0% after ten consecutive cycles with the catalyst structure well preserved. These results provide mechanistic guidance for balancing acidity and redox properties via controlled cation exchange and support the practical implementation of lignin valorization.