Investigation On The Reaction Mechanism Of Polyoxotungstates Solution By Mass Spectrometry

Polyoxometalates have drawn much attention because of their varied structure and special properties on fluorescence,magnetism and catalysis.Electrospray ionization mass spectrometry（ESI-MS）,as a sensitive and rapid method of characterization,has been used to probe the solution reactions of polyoxometalates.In this work,the mechanism of self-assembly and catalysis of polyoxotungstates clusters has been systematically investigated using ESI-MS as the main characterization method.（1）Mass spectra of nearly neutral aquous solution of isopolyoxotungstates:Nearly neutral aqueous solutions of Na₂WO₄ were characterized by mass spectrometry.The species and transformation of isopolyoxotungstates in aqueous solutions and during the process of ionization were discussed.Aqueous solution of paratungstate（NH₄）₁₀H₂W₁₂O₄₂ was also studied by ESI-MS.In addition,the different cleavage products of[W₆O₁₉]^2-in aqueous solution were proposed to be generated from the overlapping part of the nucleo plasmic relation range of[W₆O₁₉·H₂O]^2-.It was further confirmed by CID experiments.This phenomenon proved that the ions with unstable structures were likely to be dissociated into stable species in the process of electrospray ionization.（2）Self-assembly mechanism of Weakley-type polyoxotungstates:{Mn₄（α-SiW₉）₂}was synthesized by reaction of the lacunary Keggin type polyoxometalate K₈[γ-SiW₁₀O₃₆]and transition metal ion Mn²⁺.During the real-time monitoring on the binary system by ESI-MS,{Mn₂（β-SiW₈）},{Mn（β-SiW₉）},{Mn₃（β-SiW₈）（β-SiW₉）}and some other intermediates were observed.On this basis,the ternary system of K₈[γ-SiW₁₀O₃₆],Mn²⁺and Ln³⁺（Ln=La、Nd、Gd、Dy、Er）were further studied,and the self-assembly mechanism of{Ln₂Mn₄（α-SiW₉）₂}structure was proposed.Moreover,different ionic radius of Ln³⁺will lead to certain differences on crystal structures(including Ln³⁺coordination number,and their connection mode with multi metal oxygen octahedron,etc.).（3）Mechanism of epoxidation reaction catalyzed by tungstates:Using Keggin type heterpolyoxotungstates H₃PW₁₂O₄₀ and mononuclear tungstate H₂WO₄ as catalysts,the reaction process of cyclooctene epoxidation was real-time monitored by ESI-MS.By observing the species changes of catalysts in reaction solutions and the combination of catalysts with substrate,the mechanism of catalytic reaction of polyoxometalate clusters were proposed on aggregation and dissociation.As H₃PW₁₂O₄₀ was used as catalyst,it was degraded into minor spicies[PW₄O_16-x（O₂）_x]^3-（x=0,2,4,6,8）which performed as the active center of the epoxidation of cyclooctene.At the same time,the catalyst and the cyclooctane epoxide formed[HW₂（O₂）₂O₅(C₈H₁₄O)₂]^-as a co-catalyst.When H₂WO₄ as catalyst,it reacted with H₂O₂ and generated mononuclear tungsten peroxo species[HWO₂（O₂）₂]^-and[HWO₂（O₂）₃]^-,which catalyzed the epoxidation of cyclooctene generating cyclooctane epoxide.Cyclooctane epoxide combined with the catalyst generating the same spicies[HW₂（O₂）₂O₅(C₈H₁₄O)₂]^-as the co-catalyst.