Polyoxometalate@Metal–Organic Framework Composites as Effective Photocatalysts

In the recent years, polyoxometalate (POM) encapsulated metal–organic framework (MOF) composites have attracted much attention in photocatalysis. Both POMs and MOFs have been attracting immense attention in this area. Furthermore, in order to promote charge transfer and separation between POMs and MOFs, theoretical and experimental analysis can be applied to match their energy levels; however, their individual applications are hindered by several defects, such as poor visible-light utilization efficiency. The combination of MOFs and POMs can benefit from the virtues of both POMs and MOFs while avoiding the drawbacks of them. Notably, MOFs with high specific surface area and long-range ordered structure ensure a uniform distribution of POMs, which cannot only prevent the self-aggregation of POMs but also allow the exposure of more active sites for catalysis. POM@MOF composites have been identified as promising materials for photocatalysis because of their diverse unique advantages, such as ultrahigh porosity, large specific surface area, and excellent electron redox transformation. In this work, we present an overview of the developments in POM@MOF composite-based catalysts for visible light induced photocatalysis. The strategies employed for the preparation of POM@MOF composites are summarized and discussed with a particular focus on the stability of such materials. The representative works on photocatalytic water splitting, CO2 reduction, degradation of pollutants, and selective oxidation of organics are highlighted. Special attention is paid to the synergistic effects between the MOF and POM that result in an enhanced performance. Besides, the stability and reusability of these materials are also discussed. Also, the unsolved problems and development opportunities of POM@MOF composites in the field of photocatalysis are proposed.