Study On The Reaction And Mechanism Of Selective Oxidation Of Toluene Co-catalyzed By Metalloporphyrins/NHPI

The controlled oxidation of toluene to benzaldehyde,benzyl alcohol is an important transformation in chemical industry because benzaldehyde and benzyl alcohol are widely used to synthesize a large variety of diverse chemicals such as pharmaceuticals,foodstuff,dyes,and household chemicals.Currently,benzaldehyde and benzyl alcohol are mainly produced via chlorination of toluene followed by hydrolysis,which generates large quantities of waste water containing toluene and chlorinated-organic compounds,thus causing serious pollution problems.More importantly,the products produced in the above process are prohibited to be used in the pharmaceutical and foodstuff fields because they contain some of the chlorinated impurities.As a result many researchers have been striving to develop new routes for the production of benzaldehyde and benzyl alcohol through the controlled oxidation of toluene using molecular oxygen.However,the existing technologies are still troubled with low conversion and low yield problems.Consequently,the controlled aerobic oxidation of toluene to benzaldehyde and benzyl alcohol in good yield and with excellent selectivity remains as a challenge.Metalloporphyrins,which are widely recognized as environmentally benign catalysts,have been used to catalyze the controlled oxygenation of hydrocarbons with molecular oxygen under mild conditions.Our group has investigated the selective aerobic oxidation of toluene using simple metalloporphyrin as the catalyst and selectivity of benzaldehyde and benzyl alcohol up to 60%with the conversion of toluene up to 8.9%were achieved,though the reaction has to be run at relative high temperature with 0.8MPa of air.N-hydroxyphthalimide(NHPI),a free radical initiator and oxidation catalyst,has been shown to be able to catalyze the oxidation of toluene under relative mild conditions.However,the main product is benzoic acid and the yield of benzaldehyde is only 3%.Inspired by the fact that there are literature reports showing NHPI can be used as an effective cocatalyst for the metalloporphyrin catalyzed epoxidation of alkenes,we wonder whether NHPI and metalloporphyrin can be used together in the aerobic oxidation of toluene.The selective oxidation of toluene with oxygen as the oxidant over metalloporphyrin/NHPI has been studied in acetic acid.It has been discovered that suitable aliphatic alcohols can exert remarkable beneficial effect on the aerobic controlled oxidation of toluene over(T(p-Cl)PP)MnF/NHPI and good conversion and product selectivity toward benzaldehyde and benzyl alcohol were achieved.In addition,it has been also investigated that the metalloporphyrin/NHPI cocatalyzed aerobic oxidation of toluene in solvent free conditions.In order to probe the reactions mechanisms,UV-Vis spectra were used to investigate the possible structure of reaction intermediate in the oxidation mixture.The main accomplishments of this thesis are the following:Firstly,the selective aerobic oxidation of toluene has been studied systematically using metalloporphyrin/NHPI as the catalyst in acetic acid and that NHPI can cocatalyze the toluene oxidation with metalloporphyrins with different center metal ions has been found.Best results were achieved with T(p-Cl)PPCo and T(p-Cl)PPMnCl.Effects of catalyst concentration,reaction temperature,the flow rate of air,substrate concentration and additive on the reaction were also investigated.Secondly,the metalloporphyrin/NHPI catalyzed aerobic toluene oxidation using simple alcohol as the additive has been carefully investigated.The results showed that the catalytic activity of(T(p-Cl)PP)MnF/NHPI in the controlled aerobic oxidation of toluene could be remarkably enhanced by adding some aliphatic alcohol such as methanol,ethanol and 1-propanol into the reaction mixture.Both the toluene conversion and the selectivity of benzaldehyde and benzyl alcohol could be significantly improved.Our study indicated that concentration and structure of alcohols,reaction temperature,the flow rate of air,catalyst concentration and substrate concentration all had a profound effect on the toluene oxidation.The effect of methanol on the oxidation is different from those of formaldehyde and formic acid,which can be formed in oxidation reaction from methanol,has also found.Thirdly,metalloporphyrin/NHPI cocatalyzed aerobic oxidation of toluene in solvent free conditions has been examined.It was discovered that phase transfer catalysts such as CTAB can increase the toluene conversion and product yield significantly.Among the various metalloporphyrins tested,T(p?Cl)PPMnCl,T(p-Cl)PPCo,T(p-Cl)PPFeCl and T(p-Cl)PPCu were found to work well with NHPI and CTAB though the highest toluene conversion was obtained using T(p-Cl)PPMnCl/NHPI/CTAB as the catalyst.The improvement after adding CTAB might be due to the fact CTAB could efficiently decompose benzyl hydroge.n peroxides which are generated from the toluene oxidation.Fourthly,in order to gain some information on the reaction mechanism,UV-Vis spectra were used to investigate the possible structure of reaction intermediate derived from the catalyst(T(p-Cl)PP)MnF in the toluene oxidation systems in the presence and absence of additives such as alcohol and organic acids.The results showed that the added methanol or acetic acid can coordinate with catalyst T(p-Cl)PPMnX(X=F-,Cl-,Br-,I-,SCN-,OAc-)axially.Under the reaction conditions used,the coordinating ability of methanol was found to be stronger than the solvent acetic acid.The catalyst was transformed into a high valent manganese-oxo radical cation intermediate[(T(p-Cl)PP)Mn v(0)(CH3OH)]+' in the oxidation systems has also been found.Based on experimental evidence and analysis,we deduce that high valent manganese-oxo complexes and benzyl peroxide coordinated manganese porphyrin complexes could be present in the reaction mixture in the(T(p?Cl)PP)MnF/NHPI cocatalyzed aerobic oxidation of toluene.