Study On Preparation Of Nano Gold Catalysts And Application In Cyclohexane Oxidation

The selective oxidation of cyclohexane is one of the most attractive subjects in the field of catalysis chemistry. The system of cyclohexane oxidation using molecular oxygen as oxidant is also a research hotspot because of its importance in both academy and economy. The peocess of cyclohexane selective oxidation to cyclohexanone and cyclohexanol is of importance in chemical industry, as these compounds are utilized in the manufacture of nylon-6 and nylon-66. Meanwhile, cyclohexanone and cyclohexanol are also important industrial chemicals, solvents and important intermediates for fine chemicalindusty, such as the synthetic additives of food, medicine, dye stuff, coating materials, and so on.In the present work, a series of gold catalysts supported on alumina oxide, silica and cobaltosic oxide were prepared and characterized by ICP-AES, XRD and TEM methods. These catalysts were applied to catalyze selective oxidation of cyclohexane to cyclohexanone and cyclohexanol using oxygen as oxidant. The influnences of gold loadings, the catalyst composition and preparation methods on the catalytic performance were investigated. Meanwhile, the factors influencing the reaction like the temperature, the pressure and the time were studied carefully. The major results are listed as follows.Au/Al2O3 and Au/MxOy/Al2O3 (M=Co, Zr, Ce) catalysts were prepared by an impregnation-ammonia washing method. Under the reaction conditions of 150℃,1.5 MPa and 3 h,8.96% conversion of cyclohexane and 93.52% selectivity to cyclohexanone, cyclohexanol and CHHP were obtained over 1.0%Au/Al2O3 catalyst. The selectivity to CHHP was 11.16% and the ratio of cyclohexanone to cyclohexanol was 0.69. This suggested that Au/Al2O3 catalysts obtained by impregnation-ammonia washing method exhibited good catalytic performance in cyclohexane oxidation.In Au/MxOy/Al2O3 (M=Co, Zr, Ce) catalysts, the addition of Co3O4 facilitated the enhancement of cyclohexane conversion and selectivity to cyclohexanone and cyclohexanol; the introduction of ZrO2 or CeO2 improved the ratio of cyclohexanone to cyclohexanol. In the presence of both Co3O4and MxOy(M=Zr, Ce), good catalytic activity, good selectivity and high ratio of cyclohexanone to cyclohexanol could be obtained over Au/Co3O4-MxOy/Al2O3 catalysts. At 150℃,1.5 MPa and 3 h, using Au/Co3O4-ZrO2/Al2O3 catalyst with 1.0% gold loading and w(Co3O4):w(ZrO2)=2:1, the cyclohexane conversion could reach 9.69%, the selectivity to cyclohexanone and cyclohexanol was 86.13%, the CHHP selectivity was 7.18% and the ratio of cyclohexanone to cyclohexanol was 1.02.The introduction of TiO2 on SiO2 can enhance the isoelectric point so as to facilitate the deposition of gold. The Au/Co3O4-TiO2/SiO2 catalysts obtained by the deposition of Co3O4 and Au on TiO2/SiO2 exhibited good catalytic activity and selectivity. At 150℃,1.5 MPa and 3 h, using 1.0%Au/Co3O4-TiO2/SiO2 catalyst, the cyclohexane conversion could reach 9.55%, the selectivity to cyclohexanone and cyclohexanol was 91.64% and the CHHP selectivity was only 1.52%. The ratio of cyclohexanone to cyclohexanol was 0.78.Higher cyclohexane conversion and cyclohexanone and cyclohexanol selectivity were obtained over Au/Co3O4 catalysts prepared by the coprecipitaion method than that over Co3O4. Under the reaction conditions of 150℃,1.5 MPa and 3 h,9.07% conversion of cyclohexane,91.90% selectivity to cyclohexanone and cyclohexanol 1.24% selectivity to CHHP and 0.70 ratio of cyclohexanone to cyclohexaneol using 3.0% Au/Co3O4 catalyst.The results above provide a good basis for the deep research of cyclohexane oxidation and its industrial application and also provide good materials for exploring the catalytic mechanism of gold nanoparticles.