| The common name of phenol is carbolic acid,also known as hydroxybenzene,which is an important homolog of benzene.Phenol is an important organic chemical raw material,mainly used in the production of phenolic resins,caprolactam,etc.,and can also be used as solvents,reagents,etc.,and it has a wide range of applications in synthetic fibers,plastics,medicine and so on.Traditional industrial methods for preparing phenol are mainly cumene three-step process,but there are problems such as harsh reaction conditions,difficulty in product separation and purification,and noncompliance with green chemistry.In heterogeneous system,direct oxidation of benzene as a raw material to produce phenol is not only harmless by-products,but also has fewer reaction steps,high atom utilization,and low process cost.Oxygen is the preferred oxidant for hydroxylation of benzene because of its low price,wide sources,and environmentally friendly.However,it is difficult to break the O-O of molecular oxygen and often requires higher temperatures and energy.The benzene ring is a large π bond system.Due to the conjugation of the large π bond,the C-H bond has a high energy and is difficult to activate.Therefore,the direct oxidation of benzene to phenol is a subject with both industrial value and academic research.In response to the above problems,this article focuses on the preparation of supported rhodium-based catalysts.Using zeolite H-ZSM-5 as the carrier,the Rh atom is anchored in the pores of the zeolite by ion exchange to achieve a high degree of metal dispersion,and selective catalytic oxidation of benzene is the target reaction.The effects of catalyst support,metal loading,calcination temperature and atmosphere on the catalytic performance of supported Rh-based catalysts have been comprehensively studied.And the reaction conditions have been optimized.It was finally determined that the better the catalytic performance of the zeolite with the lower the amount of substance of SiO2/Al2O3,the best zeolite selected in this paper has a SiO2/Al2O3 of 24.And the catalytic performance of the catalyst has a volcanic relationship with the metal loading,in which the highest phenol production is the catalyst with 1.0 wt%metal loading.The best calcination atmosphere for the catalyst is hydrogen,and the temperature is 550℃.At a low temperature of 90℃,the catalyst still has good activity.With the increase of temperature,the output of phenol also increases.The target reaction basically reaches equilibrium at 3 h.The liquid phase product is only phenol.The structure and surface species of the catalyst were analyzed by XRD,TEM,SEM,XPS,FT-IR,CO-TPD,NH3-TPD,TG and other characterization methods.The metal of the catalyst prepared by the ion exchange method is highly dispersed on the carrier,and the framework crystal form of the carrier is not changed.In order to study the catalytic activity center,compared with the catalyst prepared by hydrothermal synthesis method,it is found that the catalytic reaction center is the Rh+species that replaces the Br?nsted acid site of the zeolite ZSM-5.H2O and CO or O2 can be coadsorbed on Rh+,and CO and O2 form a competitive adsorption.Because of the strong coordination ability of CO,the most stable adsorption species is Rh(CO)2(H2O).Although CO hinders the adsorption of O2,the carbonyl group coordinated with Rh can effectively help the activation and cleavage of O-O in O2 and the"sacrifice" is oxidized to CO2 and leaves the system.The remaining O atom in O2 coordinates with Rh to form Rh(O)(CO)(H2O)surface active species.Rh-O,a special oxygen species with oxidizing properties,can be directly inserted into the C-H bond of the benzene ring to form a phenol release system.When the system lacks CO,O2 and H2O coordinate with Rh+ to form Rh(H2O)2(O2)surface active species.Due to the lack of CO promoter,it is difficult for Rh to activate the O-O bond in O2 and form superoxygen species.The benzene ring can hardly be activated by Rh-O-O under mild conditions,and the reaction can not proceed. |
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