Preparation Of Defect-rich Doped And Heterojunction Composite Nb₂O₅-based Catalysts And Their Photocatalytic Oxidation Of Cyclohexane

Saturated alkanes are very important components of fossil energy,and their activation and functionalization are of great significance in industry and research.The inactivity of C-H bond in the selective oxidation of saturated alkanes has become one of the most challenging research topics in catalytic chemistry.Among them,the selective oxidation of cyclohexane(CHA)to cyclohexanol(CHA-ol)and cyclohexanone(CHA-one)(KA oil)is one of the most important and representative reactions.KA oil is an important intermediate for synthetic fibers and fine chemicals,especially as an important raw material for the synthesis of nylon 6 and nylon 66 polymers.Its demand is growing steadily,and supply and demand are becoming increasingly unbalanced.Photocatalytic selective oxidation of cyclohexane is an important green and highly selective way to obtain KA oil,which is widely concerned by researchers at home and abroad.Nb2O5 is a typical n-type semiconductor oxide with a forbidden bandwidth of 3.0～3.4 eV,and its bandgap characteristics enable oxidation reactions to be carried out.Nb2O5 has great potential in photocatalytic oxidation reactions,and has attracted the attention of many researchers.However,Nb2O5 has a wide band gap,can only respond to ultraviolet light,light absorption capacity is insufficient,and the higher photocarrier recombination rate leads to a relatively low quantum efficiency,reducing the photocatalytic reaction efficiency.Heteroatom doping and complex modification are effective ways to improve the photocatalytic performance of Nb2O5.Based on this,this paper mainly carried out the following research:(1)Combining hydrogen,vacuum and air modules with ammonia modules,and considering the calcination sequence and temperature,a preparation strategy of module calcination catalyst was developed,and nitrogen doped Nb2O5 catalyst rich in defects was prepared and screened.In these modified Nb2O5 samples,HN-Nb2O5 calcined in hydrogen and nitrogen in two steps has abundant oxygen vacancy and Nb4+ chemical defects,and has obtained the best photocatalytic performance.The yield of HN-Nb2O5 to CHA-one is 130.35 μmol,which is 2.2 times higher than that of pure Nb2O5 and 26 times higher than that of commercial Nb2O5,and the selectivity for CHA-one is 96.68%.The excellent photocatalytic performance of HN-Nb2O5 can be attributed to the presence of rich chemical defects and NOx species,enhanced light absorption and efficient electron-hole separation.The DFT calculation explains the catalytic effect of chemical defects and N doping on the photocatalytic activity of Nb2O5 at the band electron and band gap levels.The results of EPR analysis and catcher tests found that e-was the main active species leading to photocatalytic activity,while O2·-was the main active species leading to the production of CHA-one during the photooxidation of cyclohexane.This work has developed an effective strategy to construct N-doped metal oxides rich in defects,which has important theoretical reference value for improving the photocatalytic oxidation of saturated alkanes.(2)Due to the layered structure of bismuth oxychloride(BiOCl),which is conducive to photocarrier separation,the composite p-n heterojunction photocatalyst was prepared by loading Nb2O5 on BiOCl.Solvent-free photooxidation of cyclohexane was performed at room temperature by screening different loading ratios.In different x%Nb2O5/BiOCl catalysts,40%Nb2O5/BiOCl obtained the best photocatalytic performance in the photooxidation process of cyclohexane without solvent,the yield of CHA-ol was 39.7 μmol and that of CHA-one was 140.6 μmol,respectively,which were about 4.5 and 3.1 times of that of pure Nb2O5 and pure BiOCl,and the selectivity of KA oil was up to 99.3%.The excellent catalytic performance of 40%Nb2O5/BiOCl in the photooxidation process of cyclohexane can be attributed to enhanced light absorption and efficient electron and pore separation,abundant oxygen vacancies and BiOCl’s highly exposed(001)crystal plane.EPR analysis and trapping agent tests showed that e-,h+ and O2·-were the main active species in the oxidation reaction.The p-n heterojunction x%Nb2O5/BiOCl catalyst developed in this chapter has great potential for practical application in high efficiency photocatalytic oxidation of saturated alkanes.