Cyclohexane Oxidation Over Supported Nano Gold Catalysts

The commercial process of cyclohexane oxidation to cyclohexanone and cyclohexanol shows low conversion, poor selectivity, high-energy consumption and environmental pollution. It is of great academic and practical importance to study and develop highly efficient and environmentally friendly catalytic cyclohexane oxidation processes. In this dissertation, metal oxide supported nano gold catalysts were applied to catalyze selective oxidation of cyclohexane to cyclohexanone and cyclohexanol using oxygen. A series of supported gold catalysts were prepared by using common alumina and silica gel as the support materials in combination with surface modification method. These catalysts were characterized by ICP-AES, TEM, XRD and BET methods. The effects of catalyst preparation conditions and oxidation reaction parameters on their catalytic performances were investigated in cyclohexane oxidation to cyclohexanone and cyclohexanol using oxygen as the oxidant. Some major results are listed as follows.In preparing gold catalysts supported on alumina or ceria and zirconia modified alumina by the direct anion exchange (DAE) method, it shows considerable simplicity, flexibility and reproducibility. Tuning the gold loadings and washing procedures could control the sizes of gold nanoparticles below 6 nm.The Au/Al₂O₃ catalyst could ignite the oxidation reaction at 130℃, demonstrating good low-temperature activity and significant nano-sized effect. Gold catalysts with gold particles of 2-6 nm gave high activity. Under the reaction conditions of 150℃, 1.5 MPa and 3 h, a cyclohexane conversion of 12.6% was obtained over the Au/Al₂O₃ catalyst with a gold loading of 0.19%, affording a selectivity of 84.7% for cyclohexanone and cyclohexanol, and the TOF value of the gold catalyst was up to 16136 h^-1. The interaction between gold and the support was relatively strong. Therefore, the Au/Al₂O₃ catalysts show high stability.A novel surface sol-gel post-modification method was put forward, which facilitated fine-tuning of the surface composition and acidity of the Au/SiO₂-Al₂O₃ catalyst, and thus its stability as well as catalytic performance could be appreciably enhanced. The introduction of zirconia modified the catalyst structure and showed a promotion effect for the Au/Al₂O₃ catalyst. The zirconia and ceria modification showed a synergetically catalytic effect for Au/Al₂O₃ catalysts. The catalytic performances of Au/CeO₂-ZrO₂/Al₂O₃ catalysts vary appreciably with different molar ratio of ceria to zirconia. Under the reaction conditions of 150℃, 1.5MPa and 3 h, a cyclohexane conversion of 14.7% was accomplished over the Au/CeO₂-ZrO₂/Al₂O₃ catalyst with a molar ratio of 1:1 for ceria and zirconia, affording the selectivity up to 90% for cyclohexanone and cyclohexanol.TiO₂-modified silica gel supported Au/TiO₂/SiO₂ catalysts show high catalytic activity. Under the reaction conditions of 150℃, 1.5 MPa and 3 h, a cyclohexane conversion of 8.4% was gained over the Au/TiO₂/SiO₂ catalyst with Au content of 0.05% and titania content of 0.26%, affording a selectivity of cyclohexanone and cyclohexanol up to 91.7% and the TOF value as high as 40133 h^-1.The role of gold is to activate molecular oxygen and promote the decomposition of cyclohexyl hydroperoxide to target products, accounting for the high activity and selectivity of gold catalysts in cyclohexane oxidation.