Study On Synthesis And Catalytic Performance Of Phosphomolybdate And Phosphotungstate Polyoxometalate

Phosphomolybdate and phosphotungstate polyoxometalates（POMs）play an important role in polyacid chemistry.The traditional Keggin and Dawson types are relatively more studied,and the types of compounds studied are more comprehensive.In contrast,the study of non-traditional basket-type{P₆Mo₁₈}is relatively rare,and it is a relatively undeveloped system.The mixed-valence basket compound is a typical heteropoly blue,showing a good mixed-valence delocalization.The presence of penta-valent molybdenum makes the basket-type POMs as an electrocatalytic material with dual catalytic function.In addition,the band gap width（2.7-3.2eV）is similar to that of TiO₂,showing good semiconductor properties,therefore basket-type POMs is good catalytic materials.On the other hand,the three-dimensional high-linked Dawson and Keggin phosphomolybdenum/tungsten compounds have good performance and wide application in electrochemistry,electrocatalysis and photocatalysis,and are one of the hotspots in polyacid chemistry research.In this paper,five kinds of basket-type phosphomolybdates and one Keggin-type phosphomolybdate were synthesized by the method of medium temperature one-step hydrothermal by using alkaline earth strontium as template and flexible ligand as inducer and reducing agent.Compound 1 is a basket supramolecular polymer modified by a protonated ligand;Compound 2 is a basket-type dimer cluster formed by{Fe（H₂O）₂}unit;Compound 3 is an infinite zigzag one-dimensional chain structure formed by{Cu（H₂O）₂}unit;Compounds 4 and 5 are unique isomeric double-layer basket structures formed by{Sr（H₂O）₄}and{Cu/Ni（H₂O）₂}linker;Compound 6 is a complex three-dimensional network structure formed by the Keggin polyacid cluster as a bridging unit connecting different orientations one-dimensional metal organic chains{Cu₃（bepy）₅（H₂O）}.Two new types of highly linked phosphotungstates were synthesized by using the precursor method through using rigid ligand as structure inhibitors and adjusting of the ratio of raw material,pH,transition metal species.In compound 7,Dawson clusters are connected by Na linker to form a 2-D layer,which is further bonded by{Cu₂（bim）₂}unit forming a 3-D network structure;Compound 8 is a complex 3D network based on tetra-nuclear nickel sandwiched cluster and two kinds of connection units{K₂（OH）₂（H₂O）₂}and{K（H₂O）}.After elemental analysis,TG analysis and X-ray single crystal diffraction analysis,the chemical formula was determined as follows:（H₃bth）₆[Sr?P₆Mo₁₈O₇₃]2·4H₂O（1）,（Htib）₆[{Fe（H₂O）₂}₂{Sr?P₆Mo₁₈O₇₃}₂]·8H₂O（2）,（H₂tib）₃[{Cu（H₂O）₂}{Sr?P₆Mo₁₈O₇₃}]·4H₂O（3）,（H₃bth）₄[{Sr_0.5（H₂O）_0.5（H₂O）}₂{Sr（H₂O）₄}₂{Cu_0.5（H₂O）}₂{Sr?P₆Mo₄^VMo₁₄^VIO₇₃}₂]·5H₂O（4）,（H₃bth）₄[{Sr_0.5（H₂O）_0.5（H₂O）}₂{Sr（H₂O）₃（H₂O）_0.5（H₂O）_0.25}₂{Ni_0.5（H₂O）}₂{Sr?P₆Mo₄^V Mo₁₄^VIO₇₃}₂]·3H₂O（5）,[{Cu（bepy）₂（H₂O）}{Cu（bepy）₃}{PMo₁₂O₄₀}]（6）,[Na₂{Cu₂（bim）₂}₂{P₂W₁₈O₆₂}]·2H₂O（7）,{[K（H₂O）]₂[K₂（OH）₂（H₂O）₂][Ni（H₂O）₅]₂[Ni₄（H₂O）₂(PW₉O₃₃)₂]}·6H₂O（8）.（bth=1,6-bis（imidazol）hexane,tib=1,3-bis[（1H-imidazol-1-yl）methyl]benzene,bepy=1,4-Benzodiazine,bim=2,2-biimidazol）.The structure was characterized via infrared（IR）spectroscopy,ultraviolet-visible light（UV-vis）spectroscopy,X-ray photoelectron spectroscopy（XPS）,and single-crystal X-ray diffraction（XRD）and other means.Cyclic voltammetry of compounds 1-8 have reversible oxidation-reduction peak corresponding to the redox process of molybdenum or tungsten in the phosphomolybdate and phosphotungstate anionic clusters.Electrocatalytic studies show that the basal phosphomolybdate（compound 1-5）has a bifunctional electrocatalytic activity that catalyzes the catalytic reduction of hydrogen peroxide,nitrite and other inorganic molecules and catalyzes the oxidation of organic molecules ascorbic acid.Compounds 6-8,reduction of nitrite and hydrogen peroxide showed a better catalytic effect.The phosphomolybdate and phosphotungstates have a relatively good catalytic effect on degradation of typical organic dyes such as methylene blue and rhodamine B,and the degradation efficiency is up to 99.17%.The catalytic effect of the basket-type phosphomolybdate is the most prominent,and the degradation time is shorter than that of other compounds.It is a kind of light catalytic materials.