Synthesis And Application Of Novel, Green And Reusable Catalysts Based On Polyoxometalates

Polyoxometalates are used widely in the liquid oxidation reactions as the effective catalysts, such as olefin epoxidation, alcohol oxidation and pyridine oxidation. Since the byproduct after reaction is only water, hydrogen peroxide is used commonly as a safe and clear oxidant in the oxidation reactions. In recent years, many classic catalytic systems based on polyoxometalates and hydrogen peroxide have been developed. However, most of these systems are homogeneous catalytic systems, in which the catalyst can not be reused after the first reaction. In addition, the use of toxic and carcinogenic chlorocarbons solvents is also obvious disadvantages in these homogeneous catalytic systems. Some heterogeneous catalytic systems have been reported recently to overcome the difficulty of catalyst reuse. But in these heterogeneous systems, the catalytic results are usually unsatisfied or the reaction conditions are very hard (high temperature or long reaction time) because of the blocked heat and mass transfer. Our goal is to design and synthesize the effective, green, reusable and novel catalytic systems based on polyoxometalates. These systems should have the three following principles and objectives:(1) the catalytic systems are reusable; (2) less toxic solvents are used as reaction medium; (3) the catalyst used in the oxidation could show the most efficient catalytic activity. Based on the above consideration, this thesis work has focused on the following aspects:1. Mesoporous silica KIT-1 supported with active polytungstophosphate anion of PWxOy species was synthesized and successfully used to catalyze the olefin epoxidation in the environmentally benign solvent of ethyl acetate. The immobilized catalyst of KJT-N+-PWxOy was stable and robust for the epoxidation. The molecular sieve support of KIT-1 and the immobilized catalyst of KIT-N+-PWxOy were characterized by FT-IR, UV-vis, XRD,31P MAS NMR, SEM, N2 adsorption/desorption, ICP and XPS in detail.2. Synthesis and application of reaction-controlled phase transfer catalytic system:new reaction-controlled phase transfer catalysts ([π-CsHsN(CH2)15CH3]3[PW4O32] and [π-C5H5N(CH2)11CH3]3[PW4O32]) were synthesized and used for the oxidation of olefins. The reactions were conducted in a biphasic mixture of 30% aqueous H2O2/ethyl acetate with unique reaction-controlled phase transfer characters:The catalyst dissolved in the system during the reaction when the catalyst was reacted with hydrogen peroxide. After the deficient H2O2 was used up, the catalyst precipitated gradually and could be easily recovered and reused after reaction.3. Synthesis and application of temperature-controlled phase transfer catalytic system:the catalyst of [(C18H37)2(CH3)2N]7[PW11O39] has temperature-responsive characters. The catalyst was soluble during the reaction with heating. After the reaction finished with the temperature dropping, the catalyst precipitated gradually from the system. The catalyst was easily recovered and reused several times without discernible loss in activity. This catalytic system was successfully used in olefin epoxidation and alcohol oxidation, and good to excellent catalytic results were obtained.4. Synthesis and application of water-soluble catalysts:the polyoxometalates of K8[BW11O39H]·13H2O,Δ-Na8HPW9O34and K7 [FeW11ZnO39]-15H2O were used as the effective catalysts for the selective oxidation of alcohols in water. After reaction, the organic products were separated from the aqueous phase and the aqueous phase containing the catalyst could be recycled to do the next oxidation. Water is used as the solvent in this catalytic system, which is more environmentally friendly.5. Synthesis and application of'sandwich'type polyoxometalates:a "sandwich" type tungstophosphate, [Zn4(H2O2)(PW9O34)2]10-, was used as the efficient and reusable catalyst for alcohol oxidation. The catalyst of K10Zn4(H20)2(PW9034)2·20H20 and the catalyst of [(Ci8H37)2(CH3)2N]8K2[Zn4(H2O)2(PW9O34)2] were used to catalyze the oxidation of alcohols in water with hydrogen peroxide. High yields of the corresponding carbonyl compounds were obtained. The catalysts were recycled several times without loss in activity.6. Catalytic system for alcohol oxidation in emulsion:the temperature -responsive catalyst of [(C18H37)2(CH3)2N]10[SiW9O34] can be formed an emulsion in water during the oxidation of alcohols. High yields of the corresponding products were obtained in mild conditions (65℃). After the reaction, the catalyst was recovered from the mixture after the addition of diethyl ether. There was no discernable loss in activity after several reaction cycles.7. Synthesis of pyridine-N-oxides:(1) the temperature-controlled phase transfer catalytic system of [(C18H37)2(CH3)2N]7[PW11O39]/H2O2/1,4-dioxane was used to catalyze the oxidation of pyridines to heterocyclic N-oxides. During reaction, the catalyst dissolved in the system with hearting, and effectively catalyzed the oxidation of pyridines (A 99% yield of pyridine-N-oxide was obtained within 3 h at 65℃). The catalyst could be recovered and reused after the oxidation by temperature reduction. (2) the water-soluble polyoxometalate of K6[PW9V3040]·4H20 could catalyze the oxidation of pyridines in water at room temperature (A 91% yield of pyridine-N-oxide was obtained in 12 h at room temperature). After reaction, the organic product was separated from the aqueous phase by extraction. The catalyst was still remained in the aqueous solution after extraction, and the aqueous phase could be directly used for the next run.