Oxidation Of Cyclohexane Catalyzed By Molecular Sieve In Ionic Liquid

Oxidation of cyclohexane is significant and economical to the chemical industry. In the present industrial oxidation process, cyclohexane conversion is about 3.5～4.2% and the selectivity to cyclohexanol and cyclohexanone is 76～81%. The obvious drawback of this process is the high energetic demand and the large volume of effluents. Therefore, it is very important to develop an effective and environmentally benign catalytic system. Recently, molecular sieves such as ZSM-5, titanium silicalite 1 (TS-1) has attracted great interest in the selective oxidation of cyclohexane due to their redox ability, shape-selectivity and recyclable properties. However, most of these reported procedures used volatile organic compounds such as acetonitrile, acetone and acetic acid as molecular solvents. The development of clean catalytic method to performed cyclohexane oxidation selectively has received increasing. Ionic liquids are new generation solvents, which have received great attention as promising substitutes for traditional organic solvents. An effectively and environmentally friendly oxidative conversion of cyclohexane might be realized in ionic liquids compared with in molecular solvents.In this paper, the most widely used alkylimdazolium-based ionic liquids were synthesized. On the basis of study on their physiochemical properties, ionic liquids were firstly used as solvents in the catalytic oxidation system of "molecular sieves- tert-butyl-hydroperoxide". The new combinations of "molecular sieves-ionic liquid-TBHP" system were firstly applied to the cyclohexane oxidation reaction and the highly effective, environment-friendly oxidation reaction of cyclohexane, which has a high yield and selectivity, was obtained.A series of room temperature ionic liquids based on 1-alkyl-3-methylimidazolium tetrafluoroborate ([C_2～7mim]BF₄) and 1-alkyl-3-methylimidazolium hexafluorophosphate ([C_4～7mim]PF₆) were synthesized, and the structures were characterized and confirmed by ¹H NMR, IR and MS. Water adsorption properties and surface tension of the series of ionic liquids were measured. The results show that the amount of saturated water adsorbed in series [C_2～7mim]BF₄ ionic liquids drops with increasing alkyl chain length n. Among them, the [C₂mim]BF₄ possessed the highest water adsorption properties and the highest amount of adsorptive water was 8.114% at room temperature. While there is a little amount of saturated water in the series of [C(4～7)mim]PF₆ ionic liquids and they are all lower than 0.9%. There is a linear decrease of surface tension with increasing temperature. The values of surface tension span an unusually wide range for compounds of similar structure, from 50.4 mJ/m² ([C₂mim]BF₄) to 36.1 mJ/m² ([C₇mim]BF₄) at 298 K. In the sametemperature, the surface tension decreases as the alkyl chain in the 1-position of the cation is lengthened for the two series of ionic liquids.ZSM-5 molecular sieves with different Si/Al ratio were first applied to catalyze the oxidation of cyclohexane with tert-butyl-hydroperoxide in ionic liquids and acetone. Higher oxidation activities were obtained in ionic liquids than in molecular solvent. In the same ionic solvent, the activity of cyclohexane oxidation decreases with the increase of Si/Al ratio of ZSM-5. the molecular sieves with a Si/Al ratio of 25 shows the highest catalytic activity in [C₂mim]BF₄ and a conversion of 15.8% and a selectivity of 97.0 % of desired products were obtained. While in acetone, the selectivity was sharply decreased and only in the range of 70.2～74.7%. In contrast to the reaction in water-immicible ionic liquids, the cyclohexane oxidation in water-miscible ionic liquids shows higher activity. In the case of tetrafluoroborate ionic liquids, a reduction of activity occurs with increasing cation size for a specific molecular sieve. The order of activity is as follows: [C₂mim]⁺ > [C₃mim]⁺ > [C₄mim]⁺ > [C₅mim]⁺ > [C₆mim]⁺ > [C₇mim]⁺. The conversion of cyclohexane decreases from 15.8% to 3.10%.Two series of modified ZSM-5 with different metals were prepared by using ion-exchange and incipient wetness impregnation method. The synthesized metal-containing molecular sieves were characterized by XRD, EDX and N₂ adsorption. In ionic liquid [C₂mim]BF₄ media, cyclohexane oxidation reactions were studied using the obtained two series metal-containing ZSM-5 as catalysts. Results indicate that the activities span a relatively wide range for the catalysts prepared by using incipient wetness impregnation method and the total yield was in the range of 9.5～20.5%. While for the catalysts synthesized by ion-exchange, a different results were obtained, and the total yields were in a narrow range of 15.0～16.5%. For the specific metal with the same amount of loading, different preparation method have different influence on their catalytic activity. The activities of ZSM-5 loading Ni,Cu and Co prepared by incipient wetness impregnation method were lower than those of ZSM-5 loading Ni,Cu and Co prepared by ion-exchange method.The oxidations of cyclohexane with tert-butyl-hydroperoxide catalyzed by TS-1 in series of ionic liquids were performed. A higher cyclohexane conversion and higher selectivity of products were found in the ionic liquid compared with in acetone. The catalytic activities of TS-1 in tetrafluoroborate ionic liquids are higher than those in either molecular solvent acetone or no solvent. In the case of [C₄mim]PF₆, the catalytic activity of TS-1 is higher than that no solvent and lower than that in acetone. There has the highest catalytic activity of TS-1 in ionic liquid [C₂mim]BF₄ and the total yield of oxidation products reaches 14.9%. Cyclohexane oxidation reaction with ZSM-5 and TS-1 molecular sieves in ionic liquids proceeds by free-radical mechanism. For TS-1 catalyst, the higher activity of the ionic liquid compared with the organic solvent may suggest that it can activate the active site on the catalyst and the activation can be understood from consideration of the acidity of 2-H of the imidazolium cation. This interaction induces a polarization of the Ti-0 bond, increasing the overall Lewis acidity of the catalytic site, and, as a consequence, the activity of the peroxo species toward the cyclohexane increases.The process of cyclohexane oxidation catalyzed by ZSM-5, metal-containing ZSM-5 and TS-1 in ionic liquids has a series of advantages. The mild reaction conditions and the simple product isolation combined with increased reaction rates, ease of recovery and reuse of ZSM-5/ionic liquid system make this procedure useful and attractive for cyclohexane oxidation.