Valorization of rice straw-derived lignin into vanillin via oxidative depolymerization over Mo/ZSM-5 catalysts

Rice straw, an abundant agricultural residue in Indonesia, contains significant lignin content that can be valorized into high-value aromatic compounds. Herein, lignin isolated via alkaline delignification was subjected to oxidative depolymerization using molybdenum-based catalysts (MoO₃ and MoPO) supported on ZSM-5. Structural analyses confirmed that the guaiacyl-rich, condensed nature of rice straw lignin posed inherent recalcitrance issues. Nonetheless, FTIR and GC–MS analyses of the depolymerization products confirmed the formation of carbonyl- and hydroxyl-containing aromatics, predominantly vanillin and acetovanillone. Quantitative evaluation revealed that the MoO₃/ZSM-5 and MoPO/ZSM-5 catalysts achieved vanillin yields of 5.60 % and 5.64 %, respectively, substantially surpassing those of the unsupported systems. XPS analysis demonstrated that the coexistence of multiple Mo oxidation states (Mo⁴⁺/Mo⁵⁺/Mo⁶⁺) facilitated selective β–O–4 bond scission, while the zeolite supported enhanced dispersion and porosity, enabling efficient substrate–catalyst interactions. These findings establish an efficient catalytic platform for converting recalcitrant lignin into vanillin, underscoring the potential of Mo/ZSM-5 systems to advance lignin valorization within a sustainable waste-to-value framework.