Synthesis And Performance Of Novel Nano-Gold Catalysts For Cyclohexane Oxidation

Selective oxidation of cyclohexane to cyclohexanone and cyclohexanol (KA oil) is one of the most challenging research topics, with huge industrial application value. Nano-gold catalysts have high catalytic activity in wide reactions at low temperature. Two new synthesis routes to nano-gold catalysts supported on mesoporous silica were put forward according to relevant literatures, namely nano-gold catalysts embedded in amorphous silica without template in the synthesis process (Au/M-SiO2), and nano-gold catalysts supported on mesoporous silica spheres (Au/MMSS). The catalysts were characterized by BET, UV-vis, TEM and XRD, and their catalytic activities were investigated for selective oxidation of cyclohexane.The characterization results displayed that the catalysts synthesized by two methods were both mesoporous materials, with uniform gold nanoparticles. Moreover, these catalysts exhibited high catalytic activity for selective cyclohexane oxidation. For Au/M-SiO2catalysts, the ever-reported highest TOF value of21097h-1was obtained over0.2%Au/M-SiO2catalyst, and1.5%Au/M-SiO2catalyst exhibited22.66%cyclohexane conversion and83.10%selectivity to cyclohexanone and cyclohexanol at423K,1.5MPa O2for3h using acetone as a solvent. As for Au/MMSS catalysts,0.2%Au/MMSS-0.49catalyst displayed19.88%cyclohexane conversion and76.76%selectivity to cyclohexanone and cyclohexanol at423K,1.5MPa O2for5h using acetone as a solvent. In addition, results also revealed that solvents played very important roles on cyclohexane oxidation, which was that dipolar aprotic solvents were more favorable to the reaction than that protic solvents are. Effects of Au loading on the reaction showed that catalyst with the most active sites exhibited the best performances. The reaction process of cyclohexane oxidation could be conjectured through the relationship of selectivities of three products versus cyclohexane conversion. Cyclohexane was firstly oxygenated to cyclohexyl hydroperoxide, and then cyclohexyl hydroperoxide converted to cyclohexanone and cyclohexanol. Cyclohexanol was consecutively oxygenated to cyclohexanone, while cyclohexanone and cyclohexanol were both oxidized to depth oxides at high conversion value of cyclohexane.