Direct Synthesis Of Hydrogen Peroxide From H₂/O₂ And Integration Thereof Into Oxidative Reactions Over Au/TS-1

Hydrogen peroxide is an important green oxidizing agent.It is used in many industrial areas,such as textile industry,papermaking,chemical industry and environmental protection. The current industrial production of H₂O₂ involves a circular reaction pathway by the sequential hydrogenation and oxidation of alkyl-anthraquinone.However,this process has several obvious drawbacks,including complex operation steps,high cost and safety issue in transportation and storage.Consequently,alternative routes for H₂O₂ synthesis are highly desirable.The direct synthesis of H₂O₂ from O₂ and H₂ over a precious metal catalyst is an attractive alternative,which has been investigated for almost one century.Recently,supported gold nanoparticles proved to be a potential catalyst for direct H₂O₂ formation from H₂ and O₂. Familiar supports for gold catalyst are TiO₂,Al₂O₃,SiO₂,Fe₂O₃,active carbon,et al.Since titanium silicalite TS-1 was successfully prepared in 1983,it has exhibited the unique catalytic performance in many selective oxidation reactions involving H₂O₂ as the oxidant.Several reactions exhibit the potential for an industrial process,including propene epoxidation,ammoxidation of cyclohexanone and hydroxylation of phenol et al.However,the high cost and the problem of transportation and storage of commercial H₂O₂ have limited the industrial application of TS-1.In this paper,a bifunctional catalyst Au/TS-1 is prepared and employed in the direct synthesis of H₂O₂ from O₂ and H₂.The conditions of catalyst preparation and reaction are systematically studied.On the basis of the direct synthesis of H₂O₂ from O₂ and H₂,oxidative desulfurization and 1-butylene epoxidation are attempted over Au/TS-1 in the presence of O₂ and H₂.The following results have been obtained:1.Several supported gold catalysts are prepared by the method of deposition-precipitation with urea(DP urea) and used in the direct syntheis of H₂O₂ using deionized water as reaction medium at room temperature.TiO₂ supported gold catalyst exhibits the highest activity,followed by Au/TS-1,Au/Al₂O₃,Au/Fe₂O₃,and SiO₂. Preparation methods of the Au/TS-1 catalyst influence the catalytic activity in the direct synthesis of hydrogen peroxide.The catalyst,which contains smaller gold particles,.exhibits a higher catalystic activity.2.Effects of the preparation condition of Au/TS-1 catalysts and reaction conditions on the direct synthesis of H₂O₂ are systemically investigated.Results show that Au³⁺ and Au⁺ are mainly gold species on the Au/TS-1 catalysts dried at 40 and 100℃.These samples can not catalytze the formation of hydrogen peroxide.With increasing of calcination temperature from 200～400℃,the Au³⁺ and Au⁺ species are gradually reduced to metallic Au⁰,and the catalytic activity increases.Metallic gold species Au⁰ is proposed as the active species in the direct synthesis of hydrogen peroxide.With the amount of urea increasing during the preparation process of the Au/TS-1 catalyst,the final pH value of solvent increases,and samller gold nanoparticles are formed.The catalytical activity and the stability of the Au/TS-1 catalyst are also improved.The increase of Ti content in Au/TS-1 can result in the increase of catalytical activity.Instead of deionized water as reaction solvent,methanol is a rather suitable medium for H₂O₂ synthesis.However,a higher catalytic activity can be obtained using a water-methanol mixture as the solvent,and the optimal water/methanol volume ratio is 3:2.3.Oxidative desulfurization is investigated over bifunctional Au/TS-1 catalyst in the presence of H₂ and O₂.Results show that methyl sulfide(DMS) can be very seaily oxidized by H₂O₂ generated in situ.For the effect of steric hindrance,the oxidation rate of 2-methyl thiophene is lower than DMS and thiophene.With increasing reaction time,the removal of sulfide can be improved continuously.The temperatrue has a singnificant effect on the removal of thiophene.With increasing reaction temperature,the removal is higher than that at room temperature,and the optimum temperature is 40℃.The mechanism of oxidative desulfurization by in situ generated H₂O₂ over Au/TS-1 is similar with that in the system TS-1/H₂O₂.The sufides are oxidized to sub-sulfone,sulfone,sulfur dioxide and sulfuric acid.4.Using bifunctional Au/TS-1 as catalyst,epoxidation of 1-butylene is investigated in the presence of H₂ and O₂.The result shows that high temparature is disadvantageous to the synthesis of hydrogen peroxide,but favor the 1-butylene epoxidation.A highest formation rate of 1,2-butylene oxide(BO) is obtained at 40℃.With increasing reaction time,the amount of BO increases and the fomation rate keep at about 0.54～0.6 g/(g_Au h).Increasing the Ti content of Au/TS-1 can also improve the formation of BO.The selectivity of BO is 100% when the integration is performed at room temperature.