Unveiling the unique promotional role of samarium in polyoxometalate catalysed selective oxidation of benzyl alcohol

Abstract

Achieving precise activity control of polyoxometalate (POM) catalysts through target atom introduction remains a significant challenge. Herein, we report the rational design of rare-earth (RE, RE = Sm, Ce) substituted selenotungstates (RE-POMs) through a ligand reorganization strategy and their divergent catalytic behaviors in H₂O₂-mediated selective oxidation of benzyl alcohol. Catalytic evaluation reveals a striking activity trend of Sm-POM > parent {Se₂W₁₈} ≫ Ce-POM, unequivocally establishing that RE introduction enhances (Sm³⁺) or attenuates (Ce³⁺) intrinsic activity relative to the parent POM. The improved activity of Sm-POM is elucidated by the electronic modulation of active W centers by Sm, which is confirmed by the increased binding energy (BE) of W 4f and the strengthened stretching vibration of WO_d bonds. Oxygen species scavenging and trapping experiments reveal that singlet oxygen (¹O₂) is the dominant reactive oxygen species (ROS) for Sm-POM and {Se₂W₁₈} catalysed systems. The electron depletion at active W sites in Sm-POM increases their electrophilicity and consequently boosts the generation of ¹O₂. In contrast, the introduction of redox-active Ce³⁺ into POM promotes the production of hydroxyl radicals (˙OH) and suppresses the generation of ¹O₂, resulting in attenuated activity. Meanwhile, H₂O₂ utilization efficiency experiments demonstrate that Ce-POM induces rapid non-productive decomposition of H₂O₂, while Sm-POM enables gradual and efficient consumption of H₂O₂ for selective oxidation, which is crucial for sustainable catalytic processes. This work establishes RE ions as efficient electronic modulators and discovers RE-specific effects that are dominated by the intrinsic redox properties of RE ions combined with electronic modulation effects, providing a new paradigm for the rational design of activity-tunable POM catalysts for selective oxidation processes.