Ligand-Induced Polyoxometalate Supramolecular Hybrids for Efficient Tetracycline Photodegradation

The construction of efficient and durable photocatalysts for antibiotic elimination remains a formidable challenge. Herein, two novel polyoxometalate-based metal−organic complexes, [Zn₄(Hppb)₆(ppb)₂(H₂O)₂][PW₁₂O₄₀]₂·H₂O (1) and [Ni(Hppb)₂(H₂O)₂]₂[PW₁₂O₄₀]·COOH·4H₂O (2), were successfully synthesized via a hydrothermal method by incorporating Keggin-type [PW₁₂O₄₀]^3- polyanions and transition metal ions (Zn²⁺/Ni²⁺) with a rigid π-conjugated Hppb ligand (where Hppb = 2-(3-(pyridin-2-yl)-1H-pyrazol-1-yl)benzoate). Both compounds feature stable three-dimensional supramolecular architectures constructed through coordination bonds, C−H···O hydrogen bonds, and π−π interactions, which bridge the metal−organic units and polyoxoanions. The π-conjugated Hppb ligand facilitates light harvesting and charge transfer, while the synergistic cooperation between the metal−organic matrix and POM units endows the hybrids with excellent photocatalytic activity for tetracycline degradation under both visible and UV light. Under optimized visible light, 1 and 2 achieved high degradation efficiencies of 92.7% and 90.6% within 110 and 140 min, respectively; under UV light, the degradation proceeded more rapidly, with 1 and 2 attaining 93.8% and 89.7% degradation within just 80 and 100 min, respectively. Structure−activity and photoelectrochemical analyses reveal that 1 outperforms 2 due to its more favorable structural features and charge separation efficiency. EPR results, together with radical trapping experiments, identified ^•O₂⁻ as the dominant reactive species. Both catalysts exhibited good recyclability. This work provides a viable strategy for designing robust POM-based photocatalysts for antibiotic-contaminated water remediation.