Syntheses And Properties Of Polyoxometalate-based Metal-Organic Framework Crystalline Materials

Polyoxometalates(POMs), as a class of inorganic metal oxide clusters,cover an enormous range in size and structure and possess the superior potential applications in catalysis, magnetism and electrochemistry. Metal-organic frameworks(MOFs) are an emerging class of porous materials constructed from metal-containing nodes and organic linkers. Compared with traditional inorganic materials,the MOFs possess larger specific surface area and the higher porosity.So they have widely applications in many fields such as gas adsorption, sensing and drug release. A new class of hybrid materials has emerged by introducing POMs into MOF systems, so-called POMOFs. The POMOFs can combine the advantages of both POMs and MOFs, and open up new possibilities in the pursuit of multifunctional materials with novel topological structures and excellent functional properties.In this paper, twenty-two POMOFs have been synthetised under hydrothermal methods by introducing POMs into porous MOFs. And they have been characterized by single crystal X-ray diffraction, powder X-ray diffraction,elemental analysis, XPS, IR, etc. The photoluminescence properties,electrochemical properties of parts of compounds have been studied. Meanwhile,we have discussed the influence of POM anions, secondary spacers, and pH on the structures of POMOFs, and the relationship between structures and properties have been explored.1. We choose two kinds of different reaction system(Cu+bimb, Cu+pzta),and five new POMOFs with distinct architectures have been synthesized, except for alternation of the polyoxoanions under the identical reaction conditions. By introducing different polyoxomolybdates(Mo8O264-, Mo8),(PMo12O403-, PMo12)and(SiMo12O404-, Si Mo12) into the Cu+bimb system, compounds 1-3 were synthesized, respectively. Compound 1 is a 2-fold interpenetrated framework.Compound 2 shows a highly opened 3D supramolecular framework, whilecompound 3 exhibits a three-dimensional interdigitated architecture. By introducing different polyoxomolybdates(Mo8 and SiMo12) into the Cu+bimb system, compounds 4 and 5 were synthesized, respectively. Compound 4 shows a POM encapsulated 3D MOF, while compound 5 is a 3D POM- pillared POMOF.The photoluminescence properties and electrocatalytic properties of compounds 1-3 are investigated. They may be good candidates for potential solid-state luminescent materials. Compared to compounds 2 and 3, compound 1has the best electrocatalytic efficiency towards the reduction of iodate. The photocatalysis properties of compounds 4 and 5 are investigated. Compared to the compound 5, compound 4 shows a better photocatalysis activity in the degradation of RhB dye.2. We choose two kinds of different reaction system(Mo8/PW12+Cu+pzta and V2O6+Co), and six new POMOFs with distinct architectures have been synthesized, except for introduction of the secondary ligands under the identical reaction conditions. By introducing different polyoxometalates(Mo8, PW12) into the Cu+bimb system, compounds 6 and 7 were synthesized, respectively.Compound 6 shows a POM encapsulated 3D MOF structure, and compound 7 is a POM-templated 2D layer structure. Furthermore, the linear rigid bipy ligand is introduced into the reaction system of 6 and 7, respectively, and 8 and 9 were obtained. Compound 8 shows a 2D sheet layer and 9 exhibits a 1D poly-pendant chain. Compound 10 is synthesized by choosing inorganic system(Co+V2O6),which possesses a pair of entanglement double helixes in a 3D inorganic framework. The V-shaped bimb ligand is introduced into the reaction system of10, Compound 11 was obtained, which shows a single of entangled double helixes in a 3D inorganic-organic network.The photoluminescence properties and electrocatalytic properties of compounds 6-9 are investigated. They may be good candidates for potential solid-state luminescent materials. Compound 6 possesses bifunctional electrocatalytic activities towards not only the reduction of normal inorganic molecules H2O2 but also the oxidation of biologic molecules AA. Meanwhile,electrocatalytic properties and the photocatalysis properties of compounds 10 and11 are investigated. Compound 11 possesses good electrocatalytic avtivity for oxidation of both AA and DA. In the photodecomposition of Methylene Blue(MB), the decline in the photodegradation ratio in the presence of 10 or 11 suggests that they restrain the photodegradation of MB.3 Three kinds of isopolymolybdate((NH4)6Mo7O24), and two kinds of heteropolytungstates([PW12]3- and [P2W18]6- anions) into an identical copper+bib reaction system except different pH, and six new POMOFs with distinct architectures have been synthesized. Compound 12 is simple supramolecular structure, and compound 13 shows a 2D POMOF. Compound 14 is simple supramolecular structure. Compound 15 shows a POM-pillared POMOF.Compound 16 is a 1D poly-pendant chain and compound 17 exhibits a complicated(1D+3D) metal-organic pseudo-rotaxane framework. It is worth mentioning that in 17 each of the molecular ‘‘loop A'' encircles two molecular“strings”. This entangled fashion is also rare in the previous reports on pseudo-rotaxane structures. The electrocatalytic properties of compounds 13, 15 and 17 have also been investigated toward reduction iodate(IO3-) and nitrite(NO2-). Compound 15 has a good electrocatalytic effect toward the reduction of NO2- and compound 17 has a good electrocatalytic effect toward the reduction of IO3-.4. Three POMOFs with polyoxometalates-encapsulated metal-organic nanotubes and two POMOFs with polyoxometalates-encapsulated metal-organic nanocages have been synthesized under 170 °C hydrothermal system. In the structure of 18, the eight-connected As2W18 clusters as guests occupy the channels and incorporate with the MOF to achieve 3D POMOF architecture.Compounds 19 and 20 are isostructural. There is a 3D honeycomb MOF with hexagonal metal-organic nanotubes(MONT). Meanwhile, Dawson-type polyanions as nine linkages were encapsulated in MONT. Compounds 19 and 20 are isostructural. There is a 3D porous MOF with metal-organic nanocages(MONC). The Keggin polyanions(PW12/SiW12) as twenty-four linkages were encapsulated in each of MONC.The electrocatalytic properties and photocatalysis properties of compound18 are investigated. The compound 18 has a good electrocatalytic effect toward the reduction of H2O2 and NO2-. Meanwhile, it has good photocatalysis activity for the photodecomposition of MB. The diffuse reflectance spectrum of 19 was measured, which illustrates the presence of an optical band gap and the nature ofsemiconductivity. The photocatalysis properties of compounds 21 and 22 are investigated. It is worth mentioning that there are good photocatalysis activity under visible irradiation at the presence of compounds 21 and 22. Therefore, they possess possible potential for visible-light photocatalytic activity.