Synthesis And Catalytic Activities Of Transition Metal-Modified Molybdenum Phosphate Oxide Clusters

With nucleophiclic oxygen-enriched surfaces and abundant topologies, polyoxometalate hybrid materials have broad applications in many fields such as catalysis, magnetism and materials science. Currently, an important advance is to decorate polyoxoanions with transition metal ions and then to assemble into different dimensional polyoxometalate clusters with unique structures. In this dissertation, seven transition metal-modified molybdenum phosphate oxide clusters have been designed and synthesized by hydrothermal synthesis. These crystalline materials have been structurally characterized by elemental analyses, single-crystal X-ray diffraction, IR spectroscopy, XPS, XRD and UV-vis spectra. By using synthesis of aspirin as a probe reaction, catalytic properties of crystalline materials have been studied in detail. The results as follow:On the basis of summarizing the law of synthesis, the synthesis conditions for product have been determined: concentration of reactant is between 0.1 mol?L-1 and 0.5mol?L-1, pH=3.5-6.0, T=180℃, nMo:nMetal (mole ratio of atoms of reactants) should be between 6:5 and 6:1.By elemental analyses and single-crystal X-ray diffraction analyses, the chemical formulas of seven compounds have been determined. Chemical formulas are as follows: (C₂N₂H₁₀)₅[(P₄Mo₆O₂₅(OH)₆)₂Co] (1), (Co(H₂O)₆)₂(C₅NH₆)₆ [(P₄Mo₆ O₂₅(OH)₆)₂Co]·10H₂O (2), [H₂en]₃Na₄[Ni(H₂O)₃][H₃₀(MoV₁₆O₃₂)Ni14(PO4)₂₆O₂ (OH)₄(H₂O)₈]·8H₂O (3), (C₂N₂H₁₀)[HCo(H₂O)₂P₂MoO₁₀] (4), (C₃N₂H₁₂)₄[Co₃ [P₄Mo₆ O₂₆(OH)₅]₂]·5H₂O (5), (C₂N₂H₁₀)4{Mn3 [P₄Mo₆O₂₆ (OH)₅]2}·6H₂O (6), (C₂N₂H₁₀)4 {Cd3 [P₄Mo₆O₂₆ (OH)₅]2}·6H₂O (7).Single-crystal X-ray diffraction analyses reveal that both structures of compound 1 and 2 are zero-dimensional units consisting of sandwich-shaped dimers formed by [P₄Mo₆O31]12-and Co²⁺. Compound 3 represents a one-dimension chainlike structure which is constructed from unusual divacant wheel-type polyoxoanions [H30P₂6Ni14Mo16O138(OH)₄(H₂O)8]12- and the [Ni(H₂O)3]²⁺ linkers. The vacant sites of the wheels are occupied by the H₂en ligands via H-bonds. Both anions of compound 4 and 5 possess two-dimensional layered structure. The layered structure of compound 4 is composed of [CoMoP₂] cluster anions that consist of {MoO6} octahedra, {CoO6} octahedra and {PO4} tetrahedra through sharing vertices. Compound 5 represents two-dimensional structure constructed from sandwich-shaped anions Co [P₄Mo₆O₂₆(OH)₅]2 2- and [CoO4]6- linkers. Compound 6 and 7 are isomorphic and represent three-dimension framework based on sandwich-shaped [M(P₄Mo₆)₂] cluster anions linked with {MO6} octahedra (M=Mn or Cd). By comparative analysis of results, we found that the structural types of the polyoxoanions depend on types of organic ligands with nitrogen and electron configuration of transition metal ions.The thermal stabilities show that polyoxoanions of all compounds are stable below 200℃. Catalytic results show that all compounds have catalytic activities for preparation of aspirin, and these catalysts can be recovered, reactivated and reused several times. The best experimental condition is: For compound 1, catalyst dose is 0.09g (mass fraction 3%), temperature is 80℃, time is 25min; For compound 2, catalyst dose is 0.06g (mass fraction 2%), temperature is 80℃, time is 20min; For compound 3, catalyst dose is 0.12g (mass fraction 4%), temperature is 80℃, time is 20min; For compound 4, catalyst dose is 0.09g (mass fraction 3%), temperature is 80℃, time is 15min; For compound 5, catalyst dose is 0.12g (mass fraction 4%), temperature is 80℃, time is 25min; For compound 6, catalyst dose is 0.09g (mass fraction 3%), temperature is 80℃, time is 25min; For compound 7, catalyst dose is 0.09g (mass fraction 3%), temperature is 80℃, time is 25min, respectively. The chemical reaction kinetics was studied and the apparent activation energy was calculated. By comparative analysis of the results of experimentation, the relationships between structures and catalytic properties of the compounds have been summarized: The conversion of salicylic acid increases with the surface area of compound; High nuclear structure of high dimensional compounds with large surface area show high catalytic selectivities and high activities. Furthermore, the reaction mechanisms have been inferred. Surface proton migration of transition metal-modified molybdenum phosphate oxide cluster anions were crucial to the catalytic synthesis of asprin. Meanwhile, we found that larger negative charge density on the surface of the cluster anions could stabilize the carbocation intermediates and then accelerate the reaction.