Hydrothermal Modification Of Graphene Oxide And Its Application In Pickering Emulsion Catalyzed Hydroxylation Of Benzene To Phenol

As an important chemical raw material,phenol is widely used in the fields of coatings,synthetic fibers,resins and pharmaceuticals.Industrially,about 97%of the world’s phenol is produced by the three-step isopropylbenzene process,but this process is energy-intensive,produces toxic intermediates,and produces an equal amount of acetone as a by-product.The direct conversion of benzene to phenol has received much attention because it is more environmentally friendly than industrial isopropene.Hydrogen peroxide（H₂O₂）is considered an important oxidant and only water is produced as a by-product in the oxidation process.However,because H₂O₂is easy to decompose under harsh reaction conditions,the oxidation process of H₂O₂ is usually limited to relatively mild reaction conditions,that is,lower concentration of H₂O₂ and lower reaction temperature.In addition,in this reaction system,it is difficult to quickly achieve the uniform mixing of H₂O₂ and organic reactants,therefore,the reaction efficiency is usually very low,the reaction products are not easy to separate and phenol selectivity is low.As an effective platform for strengthening chemical processes,"milk drop microreactor"has attracted extensive attention due to its large specific surface area,high mass transfer efficiency and easy separation of products.In this paper,surface modified graphene oxide（GO）based material was prepared by hydrothermal method and used as emulsifier to prepare Pickering emulsion.Phenol was synthesized from benzene and H₂O₂ in Pickering emulsion system.Raman spectroscopy（Raman）,X-ray diffraction（XRD）,infrared spectroscopy（FTIR）,scanning electron microscopy（SEM）and other characterization methods were used to analyze the prepared GO materials,and finally,the catalytic performance of the catalytic hydroxylation of benzene was studied.From the following three aspects of exploration:（1）Pickering emulsions were prepared by using GO as emulsifier,and the formation of emulsions was observed and recorded under different oil-water volume ratios,different GO concentrations,different salt ion strengths and different temperatures.Therefore,under the conditions of oil-water volume ratio of 0.8,GO aqueous solution concentration of 6 mg/mL,salt ion concentration of 10 mM,and temperature below 80℃,the Pickering emulsion formed has strong stability and can be stable for about several days.（2）RGO was prepared by hydrothermal method and hydrazine hydrate reduction method,and the catalytic performance of RGO in the Pickering emulsion system was explored.RGO prepared by hydrothermal method had a higher reduction degree,and had a higher catalytic activity in the direct hydroxylation of benzene to phenol.In addition,N-RGO prepared by hydrothermal synthesis method has different reduction degrees at different hydrothermal temperatures.It can be seen from the characterization results of FTIR,Raman,XRD and contact angle that with the increase of hydrothermal temperature,the reduction degree of N-RGO materials is different,the reduction degree of GO was first enhanced and then weakened,which in turn changed the hydrophilicity and hydrophobicity of GO-based materials.The GO-stabilized Pickering emulsion has a low yield of the product phenol in the catalytic benzene hydroxylation reaction,which may be attributed to the fact that the surface structure of GO does not have sufficient catalytic active sites.The RGO-stabilized Pickering emulsions obtained after GO reduction have higher benzene hydroxylation catalytic performance than the GO-stabilized Pickering emulsions.This shows that GO provides more catalytically active sites for the reaction after reduction,and its surface hydrophilicity and hydrophobicity are changed at the same time.In addition,the N-RGO obtained by the hydrothermal reaction at 80°C can well stabilize the Pickering emulsion.Under this condition,the catalyzed benzene hydroxylation reaction can obtain better reaction results,the phenol yield is 0.23%,and the phenol selectivity is up to92.00%.（3）In order to obtain better catalytic activity of benzene hydroxylation,different metal oxide doped RGO composites were prepared by hydrothermal synthesis method,and the performance of V₃O₇·H₂O/RGO composite was better by comparing their catalytic performance of benzene hydroxylation in Pickering emulsion system.Therefore,the series of XRD,Raman,XPS and other characterization results show that the material is V₃O₇·H₂O/RGO complex.The characterization results of XPS,XRD and Raman show that the composite is V₃O₇·H₂O/RGO composite.Compared with V₂O₅,RGO and V₃O₇·H₂O/RGO stable Pickering emulsions,the V₃O₇·H₂O/RGO emulsion system has the best catalytic activity.At the same time,the effects of different reaction temperature,time and H₂O₂ dosage on V₃O₇·H₂O/RGO catalyzed benzene hydroxylation were investigated.Finally,it was concluded that adding 2 mL H₂O₂（30%）aqueous solution into the reaction solution at 40℃for 7 h was the best reaction condition for the hydroxylation of benzene catalyzed by Pickering emulsion.The yield and selectivity of phenol were 3.00%and 90.00%respectively.