Syntheses, Structures And Performances Of Polyoxoniobates

Metal-Oxogen Clustes or polyoxometalate is a kind of inorganic oxygen anion clusters with unique functions.In general,the isopolyacid form of the same oxygen acid,and the heteropolyacid form of two or more oxygen acid.Because its surface has the rich more negative oxygen ions and unique structure,metal-oxygen cluster has attracted more and more attentions of scientists.Research and application of polyoxometalate has been developed more than 200 years so far,polyoxometalates play their roles in different fields,especially in such aspects as catalysis,biology,analysis,which as a polyfunctional material.Although the research and synthesis of polyoxometalate have a history of one hundred years,but for a long time the synthesis of polyoxometalate is mainly concentrated in tungsten oxide clusters,molybdenum oxide clusters,and vanadium oxide clusters etc.As same main group element,tantalum and niobium oxygen clusters have some progress until recently 20 years.But compared with other oxygen clusters the quantity of niobium oxygen clusters are too less.The main causes of slow development for polyoxometalates of niobium and tantalum is their strongly inert element and the harshly synthetic condition.Because their ability of nuclear waste treatment,antiviral,and a variety of potential application value of catalytic function,it still attracted a lot of researchers involved in the research work of polyoxoniobates.In this paper we synthesis of 4 functionalized polyoxoniobates which use K₇HNb₆O₁₉·8H₂O as the synthetic precursor with hydrothermal.Based on their established structure,the compounds have been characterized by IR,ESR,and X-ray powder diffraction,etc.And we also make a series of research on the epoxidation catalytic performance for all compounds.1.Under hydrothermal conditions,we synthesized a isopolyoxoniobate which based on[Nb₆O₁₉]^8-as the oxygen anion clusters.K₆H₂Nb₆O₁₉·17H₂O?1?Through the X-ray single crystal diffraction measurement and analysis,compound 1 shows a Lindqvist type polyoxoniobate with the three dimensional structure.Its specific structure is similar with the classical precursor,but due to the different number and position of potassium ion and free water molecules lead to its final form a 3D structure which is different from with the classical precursor.The X-ray powder diffraction and infrared spectrum characterization showed that compound 1 has a high purity and obvious features which is belonged to polyoxoniobate.And we also studied the catalytic performance of compound 1 in the styrene oxidation experiments.The experimental results show that compound 1 has a relatively good catalytic effect for the styrene oxidation experiments.2.Using the hydrothermal synthesis reaction,we synthesized three Keggin heteropolyoxoniobate which take the heteroatomic Si as the center and two V as the haps by adding the heteroatomic,the metal cation and organic amine ligand based on the idea of design and assembly.[Cu?en?₂]₄[Cu?en?₂?H₂O?₂]₂[SiNb₁₂V₂O₄₂]·14H₂O?2?[Cu?en?₂]₂[Cu?en?₂?H₂O?]₄[SiNb₁₂V₂O₄₂]·4H₂O?3?[Cu?en?₂?H₂O?₂]₄[Cu?en?₂?H₂O?]₂[SiNb₁₂V₂O₄₂]·11H₂O?4?The X-ray Single-crystal diffraction analysis shows that the compounds 2-4 are all attributed to the Keggin type niobium cluster anion structure.The basic characteristic structure of the polyanion can be described as the center is occupied by silicon atom and its two vanadium hats are located in the opposite position of the 12niobium clusters.The TMC of compound 2-4 are all composed of copper ions and ethanediamine.However,due to the different coordination methods and patterns,the structure of compounds 2-4 exhibited a great difference.A series of characterization were performed such as X-ray powder diffraction,IR and ESR,etc.The styrene epoxidation experiment using compounds 2-4 as catalysts was also carried out.The experimental results showed that compound 2-4 were all excellent for the styrene selective oxidation as the heterogeneous catalyst.According to the recycling experiment,it was observed that the conversion of styrene did not decrease apparently after multiple cycles which showing that compounds 2-4 have good recyclability.After each of the cycle experiments,we did the X-ray powder diffraction patterns to compare whether the structure of compounds 2-4 have changed or not.we can see that the structure of compounds 2-4 almost does not change that indicating the sample has a good stability in catalytic experiments.Finally,the catalytic effect of Nb₂O₅ was also compared with the compounds 1-4,the results showed that the structural and functional design synthesis of polyoxoniobate can better improve the catalytic effect when it used as a catalyst.