Study On High Efficiency Selective Oxidation Catalysts Of Cyclohexane

The catalytic oxidation of cyclohexane to cyclohexanone and cyclohexanol(KA oil)is an important reaction in the production of nylon.Cyclohexanone and cyclohexanol are synthesized via the phenol hydrogenation,cyclohexane oxidation or cyclohexene hydration and cyclohexane aerobic oxidation is the most effective method.However,low conversion rate and environmental pollution are the main problems for the catalytic oxidation of cyclohexane.The present work is carried out to explore the catalytic oxidation mechanism of cyclohexane and prepare a highly efficient catalyst to improve the yield of the product.Density functional theory(DFT)was used to study on cyclohexane autoxidation and catalytic oxidation to cyclohexyl hydroperoxide intermediate(CyOOH),cyclohexanol and cyclohexanone.The activation of cyclohexane and O2 was the rate determining step in the formation of CyOOH due to the relatively high energy barrier of 41.2 kcal/mol,and the subsequent reaction behavior of CyOOH controled the selectivity to cyclohexanol and cyclohexanone.CH3COOH and(CH3COO)2Co as the catalysts would reduce the energy barrier required for activation of cyclohexane and O2 by 3.8 and 10.6 kcal/mol,respectively.Regarding with the effects on the product distribution,CH3COOH improved the CyOOH intramolecular dehydration process,which favored the formation of cyclohexanone.The energy barrier of the decomposition of CyOOH decreased from 35.5 kcal/mol for autoxidation to 25.9 kcal/mol by(CH3COO)2Co catalysis.The computational results agreed with the experimental observation quite well,revealing the underlying role of CH3COOH and Co2+in cyclohexane oxidation.Using iron nitrate and cobalt nitrate as metal sources,sodium hydroxide and urea as precipitants,the active components were loaded onto ZSM-5 support by hydrothermal crystallization method.XRD,SEM,XPS were used to characterize the structures of the catalyst.The results showed that sodium hydroxide as the precipitant favors the formation of CoFe2O4and destroyed the support structure.When using urea as the precipitant,C03O4 and ?-Fe2O3 are synthesized and the structure of the support is preserved.The results of XPS showed that the ratio of Co2+/(Co2++Co3+)decreased and the content of the lattice oxygen in the catalyst increased due to the presence of ?-Fe2O3.The catalyst was applied to cyclohexane aerobic oxidation with acetic acid as co-catalyst.Bare ZSM-5 support could not further improve the conversion of cyclohexane and the catalytic activity of Co3O4 or ?-Fe2O3 was lower than that of mixed crystal phases(Co3O4 and ?-Fe2O3).The time of hydrothermal crystallization and amount of catalysts had little effects on the conversion of cyclohexane.Loading mixed crystal phases on ZSM-5 support could greatly promote cyclohexane oxidation,and there was an optimum loading amount(60%).As contrast,the same amount of CoFe2O4 was loaded on ZSM-5 support,the conversion decreased to 8.0%.Trimethylchlorosilane was used to modify the optimum loading catalyst to increase hydrophobicity?XRD,BET and FT-IR were used to explore the specific surface area and pore volume of the modified catalysts.XRD characterization results showed that the organic functional groups were grafted on the surface of the catalyst.The BET characterization results showed that the specific surface area and pore volume of the catalysts were reduced after modification.The catalyst was applied to cyclohexane aerobic oxidation with acetic acid as co-catalyst.The experimental results show that the catalytic performance of the catalyst has a linear relationship with the amount of the organosilane.When the mass ratio of organosilane/catalysts were 0.05:1,the conversion of cyclohexane increased to 9.9%.Uniform diameter SiO2 pellets were prepared and coated with different thickness of CeO2 shell.The core-shell SiO2@CeO2 was used as the support to load cobalt-iron oxide.XRD,SEM and TEM were used to characterize the synthesized materials.The supports were applied to cyclohexane aerobic oxidation with acetic acid as co-catalyst,there was an optimum mass ratio between core and shell(1:0.2),the conversion of cyclohexane was 8.3%,which was related to the shell thickness of the core-shell structure.The conversion of cyclohexane could not be improved by loading cobalt-iron oxide onto SiO2@CeO2 using hydrothermal method,the reason cobalt-iron oxides accumulated on the outer surface and couldn't enter its innernal.The mechanism of cyclohexane oxidation,the preparation conditions of catalysts and the modification of support were studied.It was found that Co2+and CH3COO-could reduce some energy barriers in the oxidation of cyclohexane.The catalysts with certain hydrophobicity and large specific surface area can largely catalyze the oxidation of cyclohexane.The transition metal are cheap and easy to obtain,and the preparation process of cobalt-iron composite metal oxide is simple and easy to recover.This is a meaningful exploration for the study of multi-phase catalysts,which provides reliable experience for industrial production and selective catalytic oxidation of cyclohexane.