Study On Selective Oxidation Of Sulfide By O₂

Sulfoxide is an oxidation product of sulfide and has been widely used in medicine,pesticides,functional materials,precious metal extractant organic synthesis,and oxidative desulfurization of fuel oil.Oxidation of sulfide is the most direct and most commonly used method for the synthesis of sulfoxide,but there is a problem of excessive oxidation of sulfoxide to sulfone.Therefore,how to control the target product in the sulfoxide stage is the most critical issue in the synthesis.Finding an efficient thioether oxidation system to obtain highly selective target products has become a research hotspot of thioether oxidation.At present,the methods of selective oxidation sulfides to sulfoxides are of paramount importance in organic chemistry,still many oxidation processes use non-environmentally friendly oxidants or catalysts.Molecular oxygen is considered as an ideal“green”oxidant,due to its cheap price,clean and easily accessible gas.In recent years,some functionalized catalysts,such as heteropoly acids or quaternary ammonium salt,have been widely used in different oxidation reactions,including the selective oxidation of thioethers.In this thesis,the research on molecular oxygen and non-catalytic molecular oxygen oxidation of vanadium-substituted Keggin polyoxometalates was carried out for selective oxidation of sulfoxide:(1)The performance of non-catalytic molecular oxygen oxidation of sulfide to sulfoxide was studied.The effects of solvent type,reaction time,reaction temperature,substrate concentration,free radical trapping agent were investigated in this work.It is found that the peroxide radicals for this reaction are originated from thermochemical activation of the active C(sp³)-H bond for the production of carbon free radicals and further activation of molecular oxygen,peroxide radicals activated by molecular oxygen can attack the sulfur to oxidize to form the corresponding sulfoxide or sulfone on the sulfide,thus achieving the selective oxidation of sulfoxide.The results show that the best oxidation effect of sulfide with THN as solvent,and 96.7%conversion of sulfide and 69%selectivity of sulfoxide in optimal reaction conditions(sulfide,0.08 mmol·m L^-1;THN,10 m L;temperature,110?;time,90 min).Meanwhile,the reaction system also available for other organic sulfides and diesel to obtain almost clear products.A variety of THN radicals production had been stopped by quenching experiments and the signals of THN radicals had been directly detectable by ESR spectroscopy.A possible reaction mechanism of catalyst-free oxidation of organic sulfides with O₂was deduced from the both evidence.The work provided a green synthesis approach of sulfoxide and sulfone compounds,and had broad application prospects.(2)The catalytic oxidation performance of sulfide was studied by using molecular oxygen as oxidant,dimethyl sulfide as substrate,vanadium-substituted Keggin polyoxometalates(TBA-PMo_12-nV_nO₄₀)as catalyst.The effects of number of V-atoms in the Keggin unit,amount of catalyst,solvent type,reaction time and temperature were investigated.It is found that the catalyst has the highest catalytic oxidation performance sulfide to sulfoxide using THN as the solvent.Indeed the conversion of dimethyl sulfide increased significantly while increasing the number of V-atoms from 1 to 6.Under optimal reaction conditions(THN,19 m L;dimethyl sulfide,0.15 mol;catalyst,0.02 mmol;temperature,90?;time,12 h):the conversion of dibenzothiophene could reach 88%.Under the condition of TBA-PMo_12-nV_nO₄₀heteropolyoxometalate as the catalyst for the oxidation of sulfide,the sulfoxide selectivity is increased to 100%,which is better than that of the system of non-catalytic molecular oxygen oxidation of sulfide.(3)The catalytic oxidation performance of sulfide by using molecular oxygen as oxidant,vanadium-substituted Keggin polyoxometalates([CTMA]₅PMo₁₀V₂O₄₀)as catalyst,sulfide as substrate,water as solvent.The effects of solvent type,amount of catalyst,reaction time were investigated.The results show the oxidation of sulfide to sulfoxide catalyzed by[CTMA]₅PMo₁₀V₂O₄₀was achieved by water as solvent,oxygen as oxidant in high sulfide conversion,and mild condition with excellent selectivity(99%)while increasing amount of catalyst or reaction time.