Synthesis and functionality of transition metal-centered polyoxometalate-based crystalline framework materials

Abstract

Polyoxometalates, a class of discrete nanoscale molecular metal-oxo clusters, have excellent catalytic and photoelectric/electrochromic properties due to their excellent electronic storage and redox capacity. They possess of plenty of potential applications as efficient catalysts in the development of clean and green fuels. However, polyoxometalates are easy to agglomerate and deactivate in the catalytic process, and possess of low specific surface area. Therefore, it is extremely urgent to introduce polyoxometalates into crystalline porous materials with well-defined structure, modifiable channels and high porosity. The introduction of transition metal into polyoxometalates can not only regulate their charge number, but also optimize their band structure. As well known, polyoxometalates, as one kind of unique metal oxide cluster, have been considered as promising secondary building units for construction of porous frameworks because of their adjustable compositions, variable topologies and oxygen-rich surface as well as their diverse physical and chemical properties. Recently, more and more attention has been paid to introducing polyoxometalates into frameworks to functionalize porous frameworks materials. So, these transition metal-centered polyoxometalates should be ideal building units for the design and synthesis of porous crystalline framework materials. These crystalline materials are expected to be used in the fields of single-molecule magnets, photocatalytic water splitting, desulfurization and so on. This review is directed to cover the main advances on the development of transition metal-centered polyoxometalate-based framework materials. In the past several decades, researchers have synthesized 1:12 Keggin type and Silverton type polyoxometalate, Anderson type polyoxometalate Waugh type polyoxometalate, Evans-Showell type polyoxometalate and heteropolyvanadate-based framework materials by hydrothermal method and conventional aqueous solution method. The water oxidation and photocatalytic degradation of organic pollutants of the framework materials were detailly studied. For example, the Keggin anion [CoW12O40]6– turned into an efficient and robust electrocatalyst upon its confinement in the well-defined void space of metal-organic framework (ZIF-8). In the electrocatalytic process, the [H6CoW12O40]@ZIF-8 composite crystalline framework materials is very stable for water oxidation that could retain its initial activity even after 1000 catalytic cycles. The catalyst has a turnover frequency (TOF) of 10.8 mol O2 (mol Co) –1s–1, one of the highest TOFs for electrocatalytic oxygen evolution at neutral pH. Further, the POM-based crystalline framework materials were used for oxidative desulfurization and the decomposition of chemical warfare agent. In these fields, the porous frameworks with catalytic active manganese(IV)-containing heteropolyvanadate [MnV13O38]7- as nodes and rare earth ions as linking units have been successfully prepared. These multifunctional polyoxometalate-based porous framework materials exhibit selective adsorption behavior and remarkable catalytic activity as heterogeneous catalysts for the oxidation of sulfides. A novel manganese(IV)-containing symmetrical heteropolyvanadate K7 [ MnV13O38] · 18H2O to K4Li2[MnV14O40] · 21H2O was also prepared, which is an deficient catalyst for the oxidation of 2-chloroethyl ethyl sulfide with tert-butyl hydroperoxide oxidizing agent. In this catalytic process, a TON of 40 is achieved in 8 h, showing potential application toward air decontamination technology. The results show that the transition metal-centered polyoxometalate-based crystalline framework materials possess of many functionalities, especially as excellent catalysts. In this paper, the research progress of crystalline framework materials constructed from transition metal-centered polyoxometalates is systematically summarized, and the challenges and opportunities in this field are also presented.