The Preparation Of New Chromium Catalysts And Their Catalytic Performance Of Photocatalytic Cyclohexane Oxidation

The oxidation of cyclohexane to cyclohexanone and cyclohexanol?as KA-oil?is the key step of the commercial production of nylon-6,nylon-66 polymers and other fine chemicals.C urrently,the conventional thermocatalysis of cyclohexane oxidation to KA oil has been relatively mature,but this reaction generally couses high energy consumption,low selectivity and serious environmental pollution problems.Photocatalytic oxidation of cyclohexane is an energy-saving and green environmental route,which attracts many researchers attention.Visible light-induced partial oxidation of cyclohexane has many advantages,such as the high utilization rate of solar energy,green environmental and less by-produce.Therefore,the development of visible-light-driven catalyst is the focus of current research.In this paper,the C-N co-doped chromium oxide?C/N/Cr₂O₃?photocatalysts were synthesized from metal-organic framework MIL-101?Cr?using a two-step method including carbonization and calcination.Besides,the carbon doped chromium oxide?C/Cr₂O₃?photocatalysts were obtained after directly calcining of MIL-101?Cr?and the C/Cr₂O₃ photocatalyst grafted by Fe?III?ions were achieved through an impregnation technique.The structure and performance of C/N/Cr₂O₃ and Fe???/C/Cr₂O₃ catalysts were characterized by XRD,BET,IR,PL,SPV,XPS,UV-vis,TEM and SEM characterization methods.Moreover,the effects of carbonization temperature,reaction time,catalyst dosage and other reaction conditions on the photocatalytic oxidation of cyclohexane under visible light irradiation have been investigated in detail.The results show that the nitrogen doping of C/N/Cr₂O₃ photocatalysts occurs in the carbonization process while the carbon doping happens during the carbonization and calcination steps.The C /N/Cr₂O₃ photocatalysts possess a regular morphology and outstanding catalytic activity for cyclohexane oxidation under visible light.The carbonization temperature exerts an important effect on the doping amount and the C,N total doping amount of catalysts increase with the carbonization temperature.The photocatalysts with the more doping amount possess better phototcatalytic activity due to the more effective separation of surface electronic-hole pair inducing the higher surface photovoltage.The C/N/Cr₂O₃-1000 photocatalyst has the highest catalytic activity,and its yields of cyclohexanol and cyclohexanone are 1.43 ? mol and 3.26 ? mol.When the catalyst concentration is 5 g/L,the C/N/Cr₂O₃ photocatalysts possess highest catalytic activity.Besides,the C/N/Cr₂O₃ catalysts have a low stability due to the light corrosion.The surface of C/Cr₂O₃ are modified with Fe?III?ions in the form of Fe2O3 or FeOOH in order to improve its catalytic activity.The morphology and phase structure of Fe?III?-modified C/Cr₂O₃ catalyst are the same with those of C/Cr₂O₃ catalyst.The Fe?III?/C/Cr₂O₃ catalysts possess higher photocatalytic activity than C/Cr₂O₃ and the catalyst of C/Cr₂O₃ with 0.3wt% Fe?III?shows the highest catalytic activity for cyclohexane oxidation under visible light irradiation.Using dichloromethane as solvent,the conversion rate of the cyclohexane oxidation is improved and 4.3 ? mol cyclohexanone without any cyclohexanol is generated in the reaction system.This is because the Fe?III?species existing on the surface improve the separation of photo-generated electron hole pair,which is assigned to the interfacial charge transfer from the valence band of Cr₂O₃ to the surface Fe?III?species.