One-pot mechanochemical synthesis was demonstrated to be an efficient strategy to synthesize host-guest POM⊂rho-ZIF complexes (POM = polyoxometalate; rho-ZIF = zeolitic imidazolate framework with rho topology) with high crystallinity. In this work, the metastable rho-ZIF with large interior cavities and windows was used as host matrix for encapsulating and immobilizing bulky guest molecules with high loading efficiency and chemical stability. As novel catalysts, POM⊂rho-ZIF complexes were found effective for the selective oxidation of a series of sulfides to sulfoxides. Moreover, the heterogeneity of these composite catalysts was confirmed by leaching tests, and they can be recycled at least four times without significant loss of activity.
The solution chemistry of a series of mono-, di- and trilacunary Keggin silicotungstates was investigated by electrospray ionization mass spectrometry (ESI-MS) and general electrospray features especially for lacunary POMs are summarized. The reactions of vanadium incorporation into the lacunary structures were successfully monitored in real-time by a combination of ESI-MS and differential pulse voltammetry (DPV). It was found that all the reactions took place instantaneously and that a subtle speciation change occurred at prolonged reaction times for the pair of reactants, monovacant silicotungstate and sodium metavanadate, suggesting a conversion of mono- to divanadium substituted derivatives. This was shown to result from a solution process, not an ESI-induced reaction, by DPV measurements. The relative stabilities of the V-substituted products were assessed in both solution and gas phases.
Developing a new strategy for preparing highly active and recyclable heterogeneous polyoxometalate (POM) catalysts has attracted much attention in the past decades.
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