Study On The Selective Oxidation Of Benzyl Alcohol Catalyzed By Polyoxometalates

Keggin structure polyoxometalates are good catalyst for both acidic and redox properties,therefore,they are often used as dual-function type catalysts.On the other hand,Polyoxometalates show a better selectivity and activity than the conventional catalysts in many organic reactions.Keggin type polyoxometalates,synthesis is simple and they have good thermal stability,low corrosive to the equipment,mild reaction conditions.Therefore,they attract widespread attention in the field of catalytic chemistry in recent years.Benzaldehyde is a very important chemical raw materials and intermediates,it has a wide range of applications,The conventional synthetic methods of benzaldehyde have various drawbacks.For example,toluene chloride hydrolysis method contains toxic chlorine elements and has a long process,severe corrosion of the equipment;Precious metal catalysts have higher costs;Oxidation of benzyl alcohol by manganese,chromium compounds has serious environmental problems.Therefore,the development of an environmental friendly and efficient catalytic system is imperative.H2O2 as an environmental friendly oxidant,has been widely applied to the catalytic oxidation reaction.At the same time,the polyoxometalates combined with H2O2 has a good effect and a better selectivity on oxidation of organic substrates.So the thesis bases on selective oxidation of benzyl alcohol to benzaldehyde,explores a simple,efficient,environmentally clean,green catalytic system.We also studied the impact of various factors on the reaction and made a detailed study on this reaction mechanism.1.Synthesis and study of the polyoxometalatesThe polyoxometalates:[TEAH]nH3-nPW12O40(n=1,2,3),[TEAH]4SiW12O40,[TEAH]3PMo12O40,[TEAH]3+nPMo12-nVnO40(n=1,2,3)were synthesized by an organic amine(triethylamine)and the heteropoly acids of the Keggin structure as raw materials.The polyoxometalates were fully characterized By IR,UV-vis and 31P MR.The results showed that the polyoxometalates were bonded to each other by stable ionic bond of organic and inorganic components.The synthesis of such catalysts was simple,and the catalysts maintained the original parent acids of the Keggin structure.2.Catalytic properties of the polyoxometalatesThis chapter used the polyoxometalates synthesized by the second chapter as the catalysts.A green line of benzaldehyde’s synthesis was explored,namely,The direct oxidation of benzyl alcohol in 30%H2O2 synthesized benzaldehyde in one step under the condition of the water as the solvent.We studied the reactivity of different catalysts,the amount of catalyst,reaction temperature,reaction time and other factors;applied the gas chromatograph to study the conversion and selectivity and examined the repeated use of the catalysts,The results showed that:[TEAH]nH3-nPW12O40(n= 1,2,3)type catalysts in the reaction of selective oxidation of benzyl alcohol showed high catalytic activity,the conversion was up to 99.6%,and selectivity up to 100%.At the same time,after five times of repeated use,the activity of catalyst essentially unchanged.The most important feature of this catalytic reaction system was that you can effectively control the process of oxidation of benzyl alcohol.The oxidation of benzyl alcohol was controlled to the aldehyde,and no further oxidation to the acid,at the same time,we used 30%H2O2 as the oxidant,the reaction procedure was simple,the reaction conditions were mild,the catalyst were inexpensive and easy to manufacture,these advantages were laying the foundation for environmentally friendly industrial catalytic system.3.Catalytic mechanism of the polyoxometalatesThis chapter made a detailed catalytic reaction mechanism study about[TEAH]3PW12O40 series in the oxidation of benzyl alcohol.We applied IR,UV-vis,31P NMR,elemental analysis and gave a system study of the catalyst species changes with time in the reaction.Finally,we inferred the mechanism of the reaction.In the reaction,The results showed that[PW12O40]3-was firstly degraded and oxidized to small molecules peroxygen polyoxometalate {PO4[WO(O2)2]4}3-In the presence of hydrogen peroxide,{PO4[WO(O2)2]4}3-presented in the aqueous phase and oxidized benzyl alcohol to benzaldehyde in the aqueous layer,then it turned into SAR species.SAR can be polymerized the precursor state[PW12O40]3-to complete the catalytic cycle.The continuous dissolution and oxidation of benzyl alcohol and the continuous cycle of the catalyst species kept the reaction ongoing until H2O2 depleted.