| Phenol is an important organic intermediate,mainly used in the production of bisphenol A,phenolic resin and other chemical products.With the development of social economy,the market demand for phenol has increased steadily and the development prospect of phenol is immense.At present,the cumene method,which is mainly used in the industrial production of phenol,has many disadvantages,such as complicated process,high energy consumption,low one-way yield of phenol and price restriction of co-product acetone.From the perspective of green chemistry,the route of directly preparing phenol from benzene with H2O2 as oxidant skips the step of synthesizing the intermediate product cumene,which improves the atomic utilization rate.The reaction conditions are mild and the only by-product water is environmentally friendly,so it is worth to in-depth research and development.In addition,graphite-like carbon nitride(g-C3N4)is a new type of carbon-based polymer material,which has a unique 2D planar stacking structure,excellent stability and rich active sites.It is widely used in the fields of CO2 reduction,water decomposition,organic synthesis,etc.In particular,g-C3N4 also has the ability to adsorb and activate benzene and other aromatic molecules.Therefore,g-C3N4 material was applied to the reaction of selective oxidation of benzene and modified g-C3N4 catalysts were designed and prepared to achieve efficient and highly selective conversion of benzene to phenol.Based on the comparison of different synthesis methods,g-C3N4 material was firstly prepared by direct calcining of melamine by thermal polymerization method,which has the advantages of low cost and simple operation.Then,the metal Cr was loaded on g-C3N4 substrate by impregnation method and a series of Cr/g-C3N4 catalysts were prepared by changing the calcination temperature.Afterwards,Cr/g-C3N4 catalysts were characterized by a variety of methods,including XRD,FT-IR,SEM,TEM,XPS,N2 adsorption and desorption,etc.,to determine the structure composition and morphology characteristics of each catalyst.Then,with selective oxidation of benzene to phenol as the probe reaction and H2O2 as the oxidant,the single factor experiment was carried out by changing reaction temperature,reaction time,the amount of catalyst and the amount of oxidant to obtain the appropriate reaction conditions.Under the optimized conditions,the amorphous Cr/g-C3N4-300 catalyst calcined at 300? has the highest activity,which can achieve up to 31.1%benzene conversion and 99.5%phenol selectivity.The main active valence states of the active component Cr were determined by analyzing the structure-activity relationship of the catalyst.Subsequently,the stability of the catalyst was tested under the optimal reaction conditions.The results showed that with the increase of cycle times,although the benzene conversion gradually decreased,the selectivity to phenol remained over 99%.Finally,based on the characterization of the catalyst and the results of the activity evaluation,the reaction mechanism of benzene hydroxylation with chromium peroxy radical as the intermediate was proposed.In addition,on the basis of single-metal Cr/g-C3N4 catalyst,six metals of V,Fe,Mn,Co,Cu,and Zn were selected to prepare different bimetallic/g-C3N4 catalysts according to the similar impregnation method.The catalysts were characterized by means of XRD,FT-IR,XPS and used for the selective oxidation of benzene to phenol.The results showed that the activity of catalysts was basically unchanged after doping Fe,Mn,Co,Cu and Zn.However,when doped with V,the activity of CrV/g-C3N4 catalyst was greatly improved by the interaction between the metals,achieving benzene conversion of 36.9%and phenol selectivity of 99.6%. |
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