| Selective oxidation of cyclohexane is of great value in chemical production, due to the large demand for its derivatives such as cyclohexanol and cyclohexanone, known as KA oil, which are important raw materials in the synthesis of adipic acid and caprolactam. In industry, the reaction was operated at 415～435 K with the pressure of 1～1.5 MPa in the presence of cobalt-based homogeneous catalyst, and the conversion rate was kept at 4% to prevent the formation of excessive amounts of by-products, and only 70～85%selectivity toward KA oil was obtained. Since this process suffered the problems of equipment corrosion, catalyst deactivation, and difficulties in catalyst recycling, a number of efficient catalysts had been extensively developed in order to solve these problems, such as:TS-1, Ti-MCM-41, metal-containing SAPO-11, metalloporphyrins, transition metal complex, and transition metals (Sn, Zr, Cr, Fe, Mn, Co, Au, Ce and Cu) incorporated into zeolites. However, those aforementioned catalyst systems showed relatively low selectivity to KA oil, and the utilized volatile organic solvents such as acetonitrile, acetone, acetic acid and methanol, could lead to the product contamination and serious environmental problems. Therefore, there was an urgent need to develop environmentally friendly and high efficient catalysts for the selective oxidation of cyclohexane.Ionic liquids (ILs), which are organic salts having melting points below 100 ℃, had attracted much attention in the last decade for their intriguing properties. Their high thermal and chemical stability, no measurable vapor pressure, non-flammability, and friction reducibility provided them with significantly advantageous reusability. To solve the aforementioned problems in the oxidation process, first, we load metal on the ZSM-5 support to prepare M/ZSM-5(M:Co, Fe, Ni) catalysts, and apply them to the oxidation of cyclohexane at 150℃ for 3h, then optimize and choose the best catalyst Co/ZSM-5, then load heptyl imidazole IL on the aforementioned Co/ZSM-5 catalyst to prepare a heterogeneous ionic liquid functionalized catalyst system Co/ZSM-5-IL(IL:HepmimHSO4, HepmimH2PO4, HepmimNO3, HepmimTSO), and choose the best catalysts under the same conditions, as a result of this, Co/ZSM-5-HepmimHSO4 showed the highest conversion rate, and Co/ZSM-5-HepmimNO3 showed the highest selectivity in this catalytic oxidation of cyclohexane. This catalytic system without adding other solvents is proved to be a gentle, clean, environmentally friendly and efficient route in selective oxidation of cyclohexane.Combined with the preferred Co3O4/ZSM-5-HepmimHSO4 catalyst, a novel heterogeneous catalyst Co3O4/ZSM-5-IL(x) was prepared with different initial adding amount of HepmimHS04. The results showed that the bifunctional Co304/ZSM-5-IL(x) catalysts possessed higher selectivity than ZSM-5 in the synthesis of KA oil from the oxidation of cyclohexane. The Co3O4/ZSM-5-IL(x) (x=11) showed the highest catalytic activity with the cyclohexane conversion ratio of 7.59%, and the selectivity ratio of KA oil and CHHP reached as high as 94.11%. The introduction of Co3O4 and HepmimHSO4 to the surface of ZSM-5 support was confirmed by all kinds of characterization methods. The possible mechanism for the enhanced catalytic properties of the Co3O4/ZSM-5-IL composites was also discussed. This system proved to be a high activity catalyst system with concurrently increased conversion rate and selectivity, because of the hydrophobic properties of these high-silica zeolite support, their shape selective effects, and the catalytically active site of cobalt, also the activation of H bond by the introduction of ionic liquids. Our finding paves a way to design high activity catalysts with concurrently increased conversion rate and selectivity. |
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