科研之友
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摘要
The polyoxometalates substituted with noble metals, Pd(II); Pt(Il) and Ru(III), K-12{[WZnPd2II(H2O)(2)](ZnW9O34)(2)}. 38H(2)O, K-12{[WZnPt2II(H2O)(2)](ZnW9O34)(2)}. 36H(2)O, and Na-11{[WZnRu2III(OH)(H2O)](ZnW9O34)(2)}. 42H(2)O, were prepared by exchange of labile zinc atoms with noble metal atoms from the isostructural starting material, Na-12{[WZn3(H2O)(2)]ZnW9O34)(2)]}. 46H(2)O. The X-ray crystal structure of the ruthenium compound shows a structure compatible with a sandwich-type structure type with a WRuZnRu (Ru and W, Zn at opposing sides) ring between two B-XW(9)O(34) units. Magnetic susceptibility studies as a function of temperature provide convincing evidence of two ruthenium (III) centers with no magnetic interaction between them. The EPR spectrum is supportive of this formulation showing an anisotropic spectrum of a ruthenium (III) atom (S = 1/2) in an octahedral field. The IR and UV-vis spectra of the ruthenium compound as well as of the diamagnetic palladium and platinum compounds are consistent with an isostructural series of compounds. The water soluble polyoxometalates may be extracted into an organic phase e.g. 1,2-dichloroethane by the addition of methyltricaprylammonium chloride to form their quaternary ammonium salts. The catalytic activity of these compounds was tested for the oxidation of alkenes and alkanes using aqueous 30% hydrogen peroxide and 70% tert-butyl hydroperoxide as oxidants. The alkene oxidation proceeded in high reactivity and moderate selectivity to the epoxide product using 30% H2O2 Kinetic profiles as well as W-vis and LR spectra before, during and after the reaction indicate that the catalysts are stable throughout the reaction. Formation of epoxides rather than ketonization in the reaction of terminal alkenes as well as low reactivity with iodosobenzene indicates that the reaction is tungsten centered and not noble metal centered. Oxidation of alkenes with tert-butyl hydroperoxide gave mostly allylic oxidation and/ or addition of tert-butyl alcohol to the double bond. Oxidation of cyclic alkanes such as cyclohexane and adamantane was successful with tert-butyl hydroperoxide with catalytic activity 10 times higher than previously found for transition metal substituted Keggin compounds. Ratios of hydroxylation of adamantane at tertiary vs secondary positions indicates different active species in the palladium-, platinum-, and ruthenium substituted-polyoxometalates.
- 出版日期1995-11-8
- 单位中国科学院长春应用化学研究所
