Selective Oxidation Of Methylbenzenes

Aromatic aldehydes are important intermediates in the production of pharmaceuticals, agricultural chemicals,and dyes,traditionally manufactured mainly by the chlorination of methylbenzenes to form benzylchlorides which are then subjected to acid hydrolysis, leading to serious pollution.Selective oxidation of methylbenzenes with oxygen as a cheap and clean oxidant still remains a chemical challenge and is a strategic subject,which,in recent years,attracts much attention.Using the strategy of liquid-phase oxidation,the following aspects have been explored in the present work.(1)Selective oxidation of p-chlorotoluene catalyzed by Co(OAc)₂/Mn(OAc)₂/KBr in acetic acid-water mixed solvent was studied.Acetic acid-water was used as the reaction medium in place of the commonly used pure acetic acid,which avoids the nuisance of the acetic acid separation and is beneficial to the recover and reuse of the solvent.In acetic acid-water medium,the occurrence of precipitate leads to the great reduction of the catalytic activity.The precipitate has been analyzed by XRD,FT-IR and AAS,the results showing that the precipitate is manganese dioxide(MnO₂).The mechanism of the precipitate formation has been supplemented and consummated.The fact that direct oxidation of Mn(â…¡)and Mn(â…¢)by oxygen barely contributes to the formation of the precipate has been verified by a series of experiments.The factors which influence the precipitate formation have been investigated.The reaction conditions have been optimized.The optimum reaction conditions were as follows: initial water concentration of the solvent 10 wt.%,Co/(Co+Mn)mole ratio 0.4, Br/(Co+Mn)mole ratio 0.4,amount of the catalyst accounting for 4 wt.%of the substrate,p-chlorotoluene/solvent volume ratio 1.5,reaction temperature 106℃, reaction time 10h,and the oxygen flow rate 10 mL/min.Under these conditions, 14.3%yield of p-chlorobenzaldehyde was obtained at 19.7%conversion of p-chlorotoluene with 72.4%selectivity.(2)To solve the problem of precipitation in the Mn(OAc)₂/Co(OAc)₂/KBr catalytic system, the three-component MC catalytic system has been systematically studied using the selective oxidation of p-chlorotoluene as a probe.By the selection of the second metal component,different metal salts and complexes,different solvents,and different initiators,a superior three-component MC catalytic system has been defined.It was found that when MnSO₄ is used in place of Mn(OAc)₂,precipitate barely forms,and the catalytic activity greatly enhances.Moreover,at similar conversions,there is no significant difference in selectivity between the Co(OAc)₂/MnSO₄/KBr catalytic system and the Co(OAc)₂/Mn(OAc)₂/KBr catalytic system.The reaction conditions of the Co(OAc)₂/MnSO₄/KBr catalytic system have been optimized,and the optimum Co/(Co+Mn)mole ratio,Co/(Co+Mn)mole ratio and reaction temperatures are 0.2,0.3, and 106℃respectively.The initial water concentration of the mixed solvent is an extremely important factor.Due to the complex double effects of water on the reaction, with the change of the initial water concentration,the catalytic activity shows complex changing trend.During the reaction,the bromide component has been converted to catalytically inactive benzylic bromide.In pure acetic acid medium,the "catalyst failure" occurs owing to the formation of benzylic bromide while in acetic acid-water medium,the solvolysis of benzylic bromide by water maintains the catalytic activity to some extent.And in certain range,the increase of the initial water concentration enhances the selectivity.It was demonstrated that it is beneficial to control the initial water concentration in the range of 5-10wt.%.Under the optimum reaction conditions, after 10h of reaction,22.4%yield of p-chlorobenzaldehyde was obtained at 33.7% conversion of p-chlorotoluene with 66.6%selectivity and the results are reproducible. The recycle experiments show that the recovered catalyst could be reused for many times provided the total bromide component and 5%metal component(Co and Mn) were replenished.This catalytic system is applicable in a wide range of substrates, showing good catalytic properties in the oxidation of toluene,o-chlorotoluene,and 2, 4-dichlorotoluene.(3)A biomimetic catalytic system M(acac)_n/CH₃CHO has been used in the selective oxidation of p-chlorotoluene.The effects of the center metal ions,the amount of catalyst and ethanal,the oxygen pressure,the reaction time,and the reaction solvents on the selective oxidation were investigated.The results show that among the acetoacetonate complexes,Co(acac)₃ has the best catalytic activity.The optimum reaction conditions were as follows:0.764 mmol of Co(acac)₃,mole fraction of ethanal 10%based on the substrate,room temperature,oxygen pressure 2 MPa,reaction time 11h and solvent acetonitrile.Under these conditions,13.3%p-chlorobenzaldehyde was obtained with 15.3%conversion and 87.2%selectivity.The oxidation can also be carried out without solvent with 8.1%conversion,89.1%selectivity and 7.2%yield respectively.