Ionic liquids modified cobalt/ZSM-5 as a highly efficient catalyst for enhancing the selectivity towards KA oil in the aerobic oxidation of cyclohexane
AbstractThe industrial oxidation of cyclohexane is currently performed with very low conversion level, i.e. 4-6% conversion and poor selectivity for cyclohexanone and cyclohexanol (K-A oil), i.e.70-85%, at above 150oC reaction temperature and above 10atm reaction pressure using molecular oxygen oxidant and homogeneous catalyst. Several disadvantages are, however, associated with the process, such as, complex catalyst-product separation, high power input, and low safe operation. Therefore, the oxidation of cyclohexane using heterogeneous catalyst oxygen oxidant from air at mild conditions has received particular attention. Aerobic oxidation of cyclohexane over ionic liquids modified cobalt/ZSM-5 (IL-Co/ZSM-5) in absence of solvents was developed in this article. The prepared catalysts were characterized by XRD, FT-IR, N2 adsorption-desorption, SEM, TEM and XPS analyses. The influence of reaction parameters on the oxidation of cyclohexane was researched, such as the various catalysts, reaction temperature, reaction time, and the reaction pressure, on the process. Highly selective synthesis of KA oil was performed by aerobic oxidation of cyclohexane using ionic liquids modified cobalt/ZSM-5 (IL-Co/ZSM-5) as the catalyst in absence of solvents for the first time. A selectivity of up to 93.6% of KA oil with 9.2% conversion of cyclohexane was produced at 150℃ and 1.5 MPa after 3 h, with about 0.1 mol cyclohexane, C7mimHSO4-Co/ZSM-5 catalyst equal to 6.0 wt%, respectively. The induction period of oxidation was greatly shortened when the ionic liquid was supported on ZSM-5. The catalyst was easy to centrifuge and was reused after five cycles. It was found that both the characterization and performance of the catalysts revealed that both the presence of oxygen vacancies with incorporation of Co ions into the framework of ZSM-5 and the introduction of C7mimHSO4 into the ZSM-5 leads to the both satisfactory selectivity and robust stability of the C7mimHSO4-Co/ZSM-5 heterogeneous catalyst.
