Oxidation of Cyclohexane over Metal Nitrogen Doped Carbons and Zeolite-Supported Metal Phthalocyanines

Oxidation of cyclohexane is of industrial importance for production of cyclohexanol and cyclohexanone, together termed “KA oil”. KA oil is a precursor used for synthesis of Nylon-6 and Nylon-6,6. Nylon has wide applications across multiple industries; thus, KA oil synthesis is critical for modern life. Heterogeneous catalysts can be used for cyclohexane oxidation in the liquid phase at mild conditions with high selectivity to the alcohol and ketone products. Metal phthalocyanine catalysts (MPCs) and metal-nitrogen-doped carbon catalysts (MNCs), have similar molecular structures; most notably isolated single metal atom active sites coordinated to four nitrogen atoms which are themselves within carbon rings. MNCs and MPCs deposited onto and encapsulated within faujasite catalysts, MPC/FAU and MPC@FAU, respectively, were prepared and characterized for use in comparative studies of the reaction rate, apparent reaction orders, and product selectivity with respect to the liquid phase batch oxidation of cyclohexane utilizing tert-butyl hydrogen peroxide (TBHP) as the oxidant. Additionally, experiments were conducted to elucidate the effects of the zeolite support. Reactions were performed in a glass batch reactor sealed with a rubber septum and placed within an oil bath held at 303 K and stirred at 350 RPM. Products were identified using a gas chromatograph (GC) equipped with a mass spectrometer and quantified using a GC equipped with a flame ionization detector, using n-propanol as an external standard. Rates measured over sodium faujasite and nitrogen doped carbon without transition metal active sites were measured and subtracted from the measured rates over metal-containing samples. Comparisons are drawn regarding the relatively reactivity of each material by calculating the per metal atom turnover frequencies (TOFs). Our ongoing work indicates that MNC, MPC/FAU, and MPC@FAU catalysts with Fe active sites outperformed all other tested transition metals within these supports. TOFs for each of the sample types followed the following reactivity trends: Fe > Co > Ni for MNCs, Fe > Mn > Co > Cu > Cr > Ni for MPC/FAU, and Fe > Mn > Cr ≈ Cu for MPC@FAU samples. With these results and further study, we hope to further probe the effects of transition metal identities on catalytic performance in cyclohexane oxidation and to determine the effectiveness of studying well-defined MPC@FAU materials as models for the less tunable, less uniform metal active sites of MNCs.