The Study On Catalytic Oxidation Mechanism Of Transition Metal Substituted Keggin Polyoxometalates By ESI-MS

Polyoxometalates（multi metal oxide or multi metal oxide clusters,polyoxometalates,POMs）is a kind of skeleton structures which in rich of W,Mo,Ti,Nb,Ta,V and other transition elements of metal oxygen cluster compounds.Because of their special structures and properties,many of them have catalytic activity.The transition metal vanadium,titanium substituted Keggin-type polyoxometalates in catalytic oxidation has a good application.In the catalytic reaction process,the composition and structure of catalyst may change,the formation of the reaction intermediate is the important bridge to establish the link between catalyst and the substrate.Therefore,detecting the intermediates in catalytic reaction plays a vital role in the study of catalytic oxidation mechanism.In this thesis,transition metal vanadium,titanium substituted Keggin polyoxometalates used in this catalytic system,by electrospray ionization mass spectrometry（ESI-MS）as main means to study the solution reaction,ion stability in gas phase and solid structures,the composition changes of catalyst in the reaction process was monitored.Finally the reaction mechanism was explored and infered.Inthestudyofthedivanadiumsubstitutedpolyoxometalates TBA₄[γ-H₂SiW₁₀V₂O₄₀]as catalyst and H₂O₂ as oxidant catalytic oxidation of thioanisole reaction mechanism,in order to explore the possible intermediates in the reaction process,the three components（catalyst,oxidant,substrate）reaction system was divided into two steps by using ESI-MS for real-time monitoring of the reaction.In the first step of the catalyst and the oxidant reaction,TBA₄[γ-H₂SiW₁₀V₂O₄₀]partially degradated to{SiW₁₁V}and{SiW₁₂}by reacted with H₂O₂.At the same time a series of Keggin-type peroxo-polyoxometalates species with one or two peroxide groups were generated and their structures through the collision induced dissociation test were studied.In the second step reaction with the substrate,the peroxide intermediate species oxidized sulfide.The same reaction system for TBA₅PMo₁₀V₂O₄₀ as catalyst,after the reaction with H₂O₂,{PMo₁₀V₂}anion skeleton suffered a great fragmentation,generating[PMo₆VO₂₄]^2-and{PMo₄}with its peroxo species as dominating fragment ions.As the substrate added in,{PMo₄}and its peroxide species disappeared while a series of Keggin-type polyoxometalates anion framework were formed including{PMo₁₂}species which indicated a the removal of vanadium oxygen clusters from the Keggin-type structure.The component of the catalyst was changed before and after the reaction.The collision induced dissociation test proved the difference between the two types of catalyst anions’stability,just explained the different catalytic behavior of the two catalyst showed before.Using Keggin-type TBA₅Si W₁₁VO₄₀ and TBA₄PW₁₁VO₄₀ as catalysts,the reaction of catalyst and oxidant only appeared a very small part of the vanadium oxide clusters removed away from the anion skeleton and the different counter cations binding with anion skeletons’peak intensity changes,unlike the divanadium substituted Keggin-type polyoxometalates’large degradation or fragmentation.Neither the existence of the possible peroxo intermediate.The reason is that the momovanadium substituted polyoxometelates unable to produce peroxo intermediate or the survival time of intermediate is too short to be captured by ESI-MS.In the process of the oxidation of stilbene,the reaction of TBA₄[HPW₁₁TiO₄₀]and TBHP produced a polyoxoxmetalate anion containing a single chain only.Different peroxo anion structures could be generated with different oxidant.It was successfully detected by electrospray ionization mass spectrometry.In this thesis,the mechanism of using electrospray ionization mass spectrometry for real-time monitoring of different types of transition metal substituted Keggin-type polyoxometelates catalytic oxidation reaction is studied.A relatively complete research method is established which shows the change of the catalyst in the reaction process and opens up a new door for the study in catalytic mechanism of other types of polyoxometalates.