Two Metal−Organic Frameworks Modified with Polyoxometalate for Efficient Visible-Light CO2 Reduction: From Covalent Nodes to Host−Guest Cages

The urgent need to mitigate climate change has driven interest in photocatalytic CO2 reduction, yet traditional materials suffer from poor efficiency due to limited light absorption and rapid charge recombination. Polyoxometalate-based metal-organic frameworks offer a promising solution by combining the redox activity of polyoxometalate with the structural advantages of metal-organic frameworks. In this paper, two polyoxometalate-based metal-organic frameworks were synthesized under hydrothermal conditions: [Co(C7H8N4)2(H2O)][Co(C7H8N4)2](HBW12O40)·5H2O (BW12-Co) and Co1.5(C7H8N4)5(HSiW12O40) (SiW12-Co). BW12-Co is a chemically bonded framework where Keggin-type polyoxometalates serve as connecting nodes, while SiW12-Co features a host-guest architecture with polyoxometalate encapsulated within a zeolitic imidazolate framework. Both polyoxometalate-based metal-organic frameworks demonstrated exceptional performance as heterogeneous catalysts for the photoreduction of CO2. SiW12-Co achieved a remarkable CO generation rate of 12,673.98 μmol g-1 h-1 with 86.7% selectivity, surpassing most of the similar catalysts while maintaining excellent stability over six cycles. Their outstanding solvent stability and catalytic efficiency highlight the potential of polyoxometalate-based metal-organic frameworks for advanced applications in CO2 reduction.