Preparation Of CdS/ZnO Composites And Photocatalytic Hydroxylation Of Benzene To Phenol

Phenol is an important chemical widely applied in the preparation of resins,fibers,nylon,herbicides and drugs,etc.Currently,90%of phenol is primarily produced through the three-step cumene process,which associates with several drawbacks including the low one-pass yield of phenol（around 5%）,harsh reaction conditions and large energy consumption.Nowadays,the visible-light photocatalytic hydroxylation of benzene to phenol using H₂O₂as oxidant has gained tremendous attention owing to its advantages of high atom utilization and environmentally benign process,from which water is generated as the only by-products.It is noteworthy,phenol is prone to be over-oxidized since its reactivity is much higher than that of benzene,which will certainly increase the difficulty in the introduction of-OH group into benzene.Therefore,the design and fabrication of a versatile photocatalyst for the activation of H₂O₂with excellent selectivity is crucial and very challenging for the photocatalytic benzene hydroxylation reaction.Cr-CdS/ZnO and Cr-MgO/Bi₂O₃were employed for catalyzing photocatalytic hydroxylation of benzene to phenol and the effects of constructing semiconductor heterojunctions and transition metal doping on their photocatalytic activity was explored,the main content is as follows:（1）CdS/ZnO heterojunction was prepared by solvothermal method,and then Cr was doped on CdS/ZnO to afford Cr-CdS/ZnO composite.The performance of Cr-CdS/ZnO for photocatalytic hydroxylation of benzene to phenol was studied.A series of methods such as XRD,SEM,HRTEM,UV-Vis DRS,XPS,PL,EIS and EPR were used to characterize the Cr-CdS/ZnO and explore the reaction mechanism.The introduction of Cr favored the generation of a trapping level in the band gap of the Cr-CdS/ZnO composite,which would enhance visible-light absorption capacity.Additionally,the introduced Cr could also act as an electron trapping center,thus minimizing the rapid recombination of the photo-generated charge carriers.Finally,the effects of various reaction conditions including the catalyst dosage,solvents and benzene to H₂O₂volume ratio were systematically explored in this study,the yield and selectivity of phenol were increased to 11.1%and 98%,respectively.（2）MgO/Bi₂O₃composites were prepared by solvothermal method,and the effects of the molar ratio of Bi to Mg and crystallization temperature on the catalyst activity were investigated.Cr-MgO/Bi₂O₃was prepared by doping MgO/Bi₂O₃with Cr,and its catalytic performance for the photocatalytic hydroxylation of benzene to phenol was explored.10%Cr-MgO/Bi₂O₃showed the best photocatalytic activity.A series of characterization methods,such as XRD,SEM,HRTEM,UV-Vis DRS,XPS,PL,EIS and EPR,were used to explore the relationship between the structure and catalytic activity of 10%Cr-MgO/Bi₂O₃materials.The results showed that the unique Lewis alkaline properties of MgO in 10%Cr-MgO/Bi₂O₃composites could enhance the adsorption ability of benzene on the catalyst surface.In addition,Cr deposition on the surface of MgO/Bi₂O₃forms a large area of surface defects,effectively expanding the range of visible light absorption.Cr could also act as an electronic trap on MgO/Bi₂O₃to accelerate the transfer rate of photogenerated electrons to the catalyst surface.Finally,the yield and selectivity of phenol were increased to 13.8%and 98.5%by optimizing the reaction conditions,including the amount of catalyst,solvent and volume ratio of benzene to H₂O₂.In this paper,5%Cr-CdS/ZnO and 10%Cr-MgO/Bi₂O₃semiconductor composites have been prepared successfully.Both of the two kinds of composites exhibit highly enhanced and stable visible-light activity,which opens up a new way for the design of catalysts with tunable phase composition and excellent photocatalytic selectivity.