Synthesis, Characterization Of Novel Multicomponent Substituted Polyoxmetalate Clusters And Their Chemical Behaviour Of Ionic Liquids

The polyoxometalate clusters constitute a large, distinct fundamental category of compounds, having high potential for theoretical contributions and for practical applications. Catalysis by polyoxometalate clusters is a field of increasing importance. These species have several advantages as catalysts which make them economically and environmentally attractive, and provide a good basis for the molecular design. In the last few years, many lacunary polyoxometalate ligands containing one kind of addendum atoms have been reported. In contrast, very little work has been carried out on the mulicomponent polyoxometalates with transition elements. Herein, we report the synthesis, properties, structure and catalysis of five series of novel tetrabasic tungstomolybdosilic compounds with transition elements. This offers a number of new experimental materials to elucidate the relationship between the catalytic activity of the polyoxometalates and their components and structure in a systematic way.These new compounds were obtained from the reaction FLtSiMou-nWnCUo with the transition metal nitrate at pH = 4.4-5.1. The general molecular formula was determined: K4H2[SiM(OH2)Mo11-nWnO39].xH2O andK2H3[SiFe(OH2)Moi1.nWnO39]-xH2O (abbreviated as MWn) with n=2, 4, 5, 7, 9; M=Mn 2+, Co2+, Ni2+, Cu2+, Zn2+, Cd2+ and x= 13-29. By means of IR-spectra, UV-spectra, XPS-spectra, thermal analysis, the properties and structure were systematically investigated. It can be drawn that the ion radius, electron number and outer shell electronic structure of transition elements may have an effect on the structure and characteristics of bonds of multicomponent polyoxometalates with transition elements.The crystal structure of four samples was identified by single crystal X-ray diffraction at 293K and which keeps basically the character of the Keggin structure belonging to tetragonal P4/mnc, Z=2 with little difference in the size of cell parameters. It was found that the ratio of Mo to W and the characterization of substitutional transition metals have no effect on the crystal structure except the space occupancy and the number of lattice water.Redox properties of the above-mentioed polyoxometalates in which Mo or W were substituted were studied by means of differential pulse and cyclic voltammetry. The reducibility and reversibility can be rationalized on the basis of electronic structure and ionic radius of the substitutional transition metals.Polyoxometalate clusters have showed remarkable stability, catalytic selectivity and activity, and resistance to oxidative degradation under catalytic reaction conditions both in heterogeneous and homogeneous systems. In order to gain a number of new experimented materials to elucidate the relationship between the catalytic activity of such polyoxometalates and their components, structure, and structural specification for the substrates and mechanism of catalysis in a systematic way, the oxidation of cyclohexene and the hydroxylation of phenol were investigated using environmentally benign reagent, hydrogen peroxide as oxidant, and the previous multicomponent polyoxometalates as catalysts. The results are summarized as below.1 . Same ligand and different transition metals can result in different catalytic activity to H2O2 decomposition: Cu > Co > Mn > Fe > Ni > Cd > Zn;2. Same transition metal and different ligands also lead to different catalytic activity to H2O2 decomposition: MW2> MW5 > MW7 > MW9;3. The cyclohexene oxidation catalyzed by the multicomponent polyoxometalates shows that the conversion of substrates and the selectivity of products depends on the composition of the catalysts, esp on the transition metals;4. The ratio of Mo to W and the characterization of transition metals greatlyinfluence the catalytic performance in hydroxylation of phenol.Recently, room temperature ionic liquids have received an attention for their interesting characteristics from the v...