Study On The Catalytic Oxidation Of Cyclohexene To Adipic Acid In Ionic Liquid System

Adipic acid is widely used in the fields of polyurethane,nylon,leather,polyester foam synthetic resin and so on.At present,the industrial method for adipic acid production is mainly nitric acid oxidation of cyclohexane.However,N₂O emitted during the production of this process seriously pollutes the environment.In recent years,a green process for the preparation of adipic acid by oxidation of cyclohexene with H₂O₂ using cyclohexene as raw material has been developed,and ionic liquid（IL）has been introduced as a green substitute for organic solvents in the reaction of cyclohexene to adipic acid.In this paper,a series of functionalized ionic liquid systems were used to oxidize cyclohexene to adipic acid under the condition of 30%hydrogen peroxide.The research will provide a theoretical basis for the green production of adipic acid.In this paper,two kinds of imidazole dinuclear functionalized acidic ionic liquids（Im-HSO₄,Im-PTSA）were prepared with N-methyl imidazole.The synthesized functionalized acidic ionic liquids was characterized by FT-IR and ¹H NMR.The Hammett acidity（H₀）of two acidic ionic liquids was also measured,and the effect of anion species on the acidity of the ionic liquids was investigated.Without any organic solvents and harmful phase transfer catalysts,Im-HSO₄/Im-PTSA was used together with tungstic acid and sodium tungstate as a catalytic system in the H₂O₂ oxidation of cyclohexene to adipic acid.The reaction conditions such as the type of ionic liquid anion,reaction time,and amount of catalyst on the oxidation reaction of cyclohexene were discussed.The results show that the Im-HSO₄/tungstic acid catalytic system has the better catalytic activity,because Im-HSO₄ shows stronger acidity than that of Im-PTSA in the reaction system,the stronger acidity is conducive to the formation of active structures in the system.Im-HSO₄ can also act as a phase transfer agent to improve reaction efficiency.Under the optimal conditions（85℃,6 h）,the yield of adipic acid is 66.0%,and the catalytic system still has high activity after repeated use of 4 times.Mechanism analysis shows that the reasonable control of the content of hydrogen peroxide and acidic environment has a great influence on the active structure of tungstic acid catalysts.Low oxygen sources and strong acidic environments lead to the lower activity of catalytic system.In order to improve the activity of the catalyst and solve the problem of difficulty recovery of ionic liquid in the above mentioned Im-HSO₄/tungstic acid,a new type of heteropolyacid ionic liquid was constructed.A bifunctional ionic liquids were synthesized by introducing sulfonic acid group to the cation and using tungstic heteropoly acid as anion.This bifunctional ionic liquid can realize the green process of adipic acid from cyclohexene without solvent.The series of synthesized liquids,such as[DPSIM]_1.5PW₁₂O₄₀,[DPSIM]₂Si W₁₂O₄₀,[DPSIM]_1.5PMo₁₂O₄₀,[DPSIM]WO₄,was characterized by FT-IR and ¹H NMR,and its acidity was determined.The synthetic bifunctional ionic liquid was used in the synthesis of adipic acid from cyclohexene hydroxide,and the effects of the type of ionic liquid catalyst and the reaction conditions on the yield of adipic acid were investigated.It is found that the stronger the acidity of the heteropolyacid ionic liquid,the higher its catalytic activity.The acidity of the ionic liquid is related to the type of its anion.Among them,[DPSIM]_1.5PW₁₂O₄₀ has the strongest acidity and shows high catalytic activity in the reaction.Under the optimal conditions（83℃,13 h）,the yield of adipic acid was 72.0%.The bifunctionalized catalyst was easy to recycle and could be reused without solvent.