Design Of Polyoxometalates, Redox Properties And Catalytic Studies

Polyoxometalates (abbreviated to POM) are "metal-oxygen molecules", and have been proven to be useful catalytic materials in fundamental studies as well as in practical synthetic processes. POM were suitable materials for both the catalyst design of practical processes and for fundamental researches regarding heterogeneous catalysis, owing to that the relationships among the following four levels of information can be established on the molecular basis, namely, (i) catalytic performance; (ii) chemical and physical properties; (iii) molecular and bulk composition and structure; and (iv) method of synthesis of catalysts. These relationships are most useful for the molecular design of the solid materials and as model system for fundamental research, providing opportunities for mechanistic studies on the molecular level. At the same time, polyoxometalates compounds become increasingly important for applied catalysis. However, the practical catalyst prepared by molecular design had hitherto not been realized. This may be because the understanding and study of their relationship between the composition, structure, chemical properties, physical function and the manipulation in synthesis were not quite enough for the practical and academic requirement.Therefore, in this work, a series of the multicomponent heteroply silicates with Keggin structure [SiMo_11-nW_nM(H₂O)O₃₉]^m- has been synthesized by a controllable method. The correlation between the redox properties and composition of the POM were investigated by cyclic voltammetric method in a systematic way. On the other hand, we also perform a comparative study on the electrochemical behaviors of [SiMo_11-nW_nM(H₂O)O₃₉]^m- in different media (aqueous solution, organic solvent and ionic liquids) by using cyclic voltammetry.The main contents are as follows:(1) According to the literature, a series of [SiMo_11-nW_nM(H₂O)O₃₉]^m- with the same structure have been synthesized. The POM were characterized by elemental analysis, ICP, XPS, IR, NH₃-TPD, Py-IR etc.. The results show that the experimental data are consistent with the theoretical values.(2) When increasing the number of the molybdenum atoms containing in anion, the first reduction wave potential of the tungstomolybdosilic heteropoly anion moves toward more positive potential, which exhibits the first sharply increasing and then slowly decreasing tendency. This phenomenon may be ascribed to the more reducible character of molybdenum and the synergistic effect of the molybdenum and the tungsten in the tungstomolybdosilic anions.(3) Incorporation of the transition metal ion depressed the oxidizing ability of the tungstomolybdosilic anion, owing to increasing the negative charge of the whole heteropolyanions. For a given oxidation state of transition metals, the oxidizing ability of the transition metal-substituted tungstomoJybdosilic anion increases with increasing electro negativity of the transition metal element.(4) In ionic liquids, the potentials for the reduction processes are more negative than that found in aqueous solution, but much more positive than those in DMF solvent. It was proposed that the potentials for the reduction process may be correlated with the solvent polarity. The tungstomolybdosilicates are most difficult to reduce in the least polar solvent medium DMF and are easiest to reduce in the most polar medium H₂O. Therefore, room-temperature ionic liquids, which could dissolve inorganic and organic compounds, are the best candidate for ideal replacement for volatile organic solvent in electrochemistry.(5) Though cyclic voltammograms of POM did exhibit remarkable difference in the medium change from one to another, it can be found that the change value of the first reduced peak potential for these heteropoly compounds almost were similar. This fact testifies that the electric charge and the solid geometry of heteropoly anions play main role in the interaction of the heteropoly anions and solvent molecules, except for the action of the solvent molecules.(6) Decomposition of isopropanol was performed to clarity the properties of POM as bifunctinoal catalysts, and the only products detected were propene and acetone. The experimental results show that the relationship between the composition of the POM and selectivity of oxidation product is not consistent with that obtained from electrochemical investigation. However, the process is still under investigation.