| As an important chemical production process,the selective hydrogenation reaction has been widely studied.One-step selective hydrogenation to produce the target product is favored by researchers because of its high efficiency,but it is often difficult to achieve high conversion and selectivity at the same time,so the design of the catalyst is particularly important.There are many factors that affect the catalytic activity of metal catalysts,such as specific surface area,acid-base property,groups on the surface of the support and the nature of the metal,etc.Among them,the size and distribution of metal particles and the interaction between the metal and the support are particularly important.The most commonly used metal catalysts are(1):traditional catalysts(metal/support).The activity of this catalyst is not high.It usually uses modifiers or other metals to change the properties of the catalyst,or changes the morphology and pore structure of the catalyst through special preparation methods to improve the performance of the catalyst;(2):bimetallic(multi)metal catalysts(Metal-metal/support).This catalyst(including doping,alloy and core-shell structure catalysts)has higher activity than traditional catalysts,mainly using metal-to-metal electron transfer and interaction to improve the catalytic activity of the catalyst;(3):single-atom catalyst(metal1/support).The maximum efficiency of this catalyst utilizes metal atoms and shows high catalytic activity in the selective hydrogenation reaction,but the stability and loading of the catalyst are challenges for single-atom catalysts.Therefore,the precise design of catalysts with high activity,selectivity and stability has always been a core issue in the development of metal catalysts.This study proposes a new method for preparing a novel structured precious metal catalyst with high-efficiency catalytic selective hydrogenation and conducts research on the application of the catalyst.The detail contents include the following:(1)Developed a hybrid nano-structured Pd/@-MeOx/AC(Me:Ti,Ce,La,Zr)catalyst and studied the catalytic performance in the selective hydrogenation of phenol to cyclohexanone.The phenol conversion of the Pd/@-Ti O2/AC,Pd/@-Ce O2/AC and Pd/@-La2O3/AC catalysts was 100%,97.2%and 100%,respectively.However,Pd/AC and Pd/MeOxcatalysts only achieved 76.6%and<10%conversion,respectively.The catalytic activity of hybrid nanostructured Pd catalyst in methylene chloride was higher than that in water.After prolonging the reaction time,Pd/@-La2O3/AC reached 100%phenol conversion.It was found that phenol was mainly adsorbed on the Pd catalyst in a“non-coplanar”manner,which was beneficial to the production of cyclohexanone,and a small amount was adsorbed in a“coplanar”manner,cyclohexanone was further hydrogenated to cyclohexanol.The reason of catalyst deactivation was the aggregation of Pd nanoparticles,and the clogging of reactants and products in the catalyst.Among the Pd/@-Zr O2/AC catalyst,the different calcination temperatures of Zr O2/AC resulted in different activities of Pd/@-Zr O2/AC(450-550)catalysts,the order of activity was:Pd/@-Zr O2/AC(500)>Pd/@-Zr O2/AC>Pd/@-Zr O2/AC.It is found that the reason for the difference in activity was the basicity of the catalyst.The stronger the basicity,the higher the activity.(2)The catalytic performance of the Pd/@-MeOx/AC catalyst in the selective hydrogenation of benzaldehyde to benzyl alcohol was studied.It was found that these hybrid nano-structured Pd/@-Ti O2/AC,Pd/@-Ce O2/AC,Pd/@-La2O3/AC and Pd/@-Zr O2/AC catalysts exhibited synergistic catalysis,and the activity was much higher than that of the traditional catalysts(Pd/AC,Pd/Ti O2,Pd/Ce O2,Pd/La2O3and Pd/Zr O2).Different oxides in the hybrid nano-structured Pd/@-MeOx/AC catalyst showed different catalytic activities.The influencing factors were the particle size and dispersibility of Pd nanoparticles.The smaller the particle size,the better the dispersibility and the higher the activity.According to the preparation method of Pd catalyst,in order to improve the stability of the hybrid nano-structured catalyst,the Pd/@-Zr O2/CNTs catalyst was prepared.Because the CNTs has a regular pore structure,it effectively avoids the clogging of organic matter in the catalyst pores and improves the reusability of the catalyst.(3)The catalytic performance of Pt/@-MeOx/SBA-15(Me:Ti,Ce,La,Zr)catalyst in the benzaldehyde hydrogenation was studied.It was found that this Pt catalyst also exhibits excellent catalytic activity and synergistic catalysis,and also has high stability,whether the Pt/@-MeOx/SBA-15 with high-load Pt(0.85%)or low-load Pt(0.16%),it still maintain a high catalytic activity after several cycles of use.This catalyst demonstrated the structural advantages,the SBA-15 with regularly ordered mesoporous structure stabilized the Pt nanoparticles,avoided the agglomeration and clogging of organic after repeated use.It was found that the content of metal oxides would affect the activity of the Pt/@-MeOx/SBA-15 catalyst.XRD and XPS characterization confirmed that the proper amount of Zr O2was monolayer dispersed in the channel of SBA-15.The loading of Zr O2in the range of 0-5wt.%was like a modifier,the low loading of Zr O2was not enough to cover the surface of the SBA-15,the Pt nanoparticles were major anchored on the SBA-15 directly.Which means that the Pt/@-x Zr O2/SBA-15(x:0-5wt.%)consisted of major Pt/SBA-15 and slight Pt/@-Zr O2/SBA-15.Therefore,the Pt/@-x Zr O2/SBA-15(x:0-5wt.%)showed higher activity than that of Pt/SBA-15(58.7%),but the conversion(78.8%)was not satisfactory.When the loading of Zr O2in the range of 5-15wt.%,the Zr O2can spontaneous mono-layer distribution on the support to form a perfect hybrid nano-structure,then utilize the synergistic effect and the unique properties and obtained satisfying catalytic activity.When the loading of Zr O2was higher than 15wt.%,the Zr O2might block the channel of SBA-15 and affect the selectivity.(4)The catalytic performance of Pt/@-MeOx/SBA-15 catalyst in the hydrogenation of nitrobenzene and chloronitrobenzene was studied.It was found that the Pt/@-Zr O2/SBA-15 catalyst showed super high catalytic activity and stability.Due to the steric hindrance of the substituents,the-Cl at the o-position of NO2group with a comparatively larger size would bring stronger steric repulsion,so that the attack of hydrogen to NO2group became more difficult under this circumstance.Therefore,the m-CNB and p-CNB were more active than o-CNB with this Pt/@-Zr O2/SBA-15catalyst.Different noble metals(Pt,Pd,Au,and Ag)showed different catalytic performances,the order was:Pt/@-Zr O2/SBA-15>Pd/@-Zr O2/SBA-15>Au/@-Zr O2/SBA-15?Ag/@-Zr O2/SBA-15.The interaction between Pt and Zr O2/SBA-15 is the strongest,so the Pt/@-Zr O2/SBA-15 catalyst has the best activity.It was found that when using Pt/@-Zr O2/SBA-15 catalyst,m-chloronitrobenzene and p-chloronitrobenzene are easier to be hydrogenated than o-chloronitrobenzene due to steric hindrance of substituents.When used microporous ZSM-5 and MCM-22,the Pt/@-Zr O2/ZSM-5 and Pt/@-Zr O2/MCM-22 catalysts also showed synergistic catalysis,but their activities were slightly lower than Pt/@-Zr O2/SBA-15,the main reasons may be the specific surface area and micropores.Found that Pt/@-Zr O2/SBA-15 can be widely used in the selectivity of methyl-nitrobenzene,bromo-nitrobenzene,fluoro-nitrobenzene,nitrophenol,styrene,methyl-benzaldehyde and acetophenone hydrogenation reaction.(5)Exploring the product distribution of Pt/@-Zr O2/SBA-15 in the hydrogenation of 3-nitrostyrene,it was found that this catalyst can completely convert3-nitrostyrene,and-C=C was preferred hydrogenation,the product distribution was mainly ethylaniline and ethylnitrobenzene,and a relatively small amount of vinylaniline.After added the alkali,the product distribution changed,the selectivity of vinylaniline increased,and the selectivity of ethylaniline and ethylnitrobenzene decreased,it indicated that alkaline conditions were beneficial for the hydrogenation of-NO2.(6)It was found that the reason of the excellent performance of Pt/@-Zr O2/SBA-15 catalyst can be mainly attributed to the following aspects:(a)the synergistic effect of the catalyst.Compared with traditional Pd or Pt catalysts(Pd/support,Pt/support),hybrid nano-structured Pd or Pt catalysts showed super high catalytic activity;(b)special structure of the catalysts.The loading of metal oxides in hybrid nano-structured catalysts was very important.At low loading,metal oxides likes auxiliaries,which improved the activity of the catalyst.At high loading,metal oxides were dispersed in multiple layers on the support,blocked the catalyst pores,affected the adsorption of reactants and desorption of products,resulted in a decrease in catalyst activity and selectivity.Only within a certain range,the oxides spontaneous monolayer dispersion on the support,then the catalyst exhibited the best catalytic performance.It is found that the reason of the high selectivity of the catalyst was attributed to the electronic effect and interface effect.(7)Opened up a new path for the design of high activity,high selectivity and high stability catalysts.Using a support with an ordered regular channel structure and large specific surface area,the semiconductor metal oxide molecular layer was dispersed on it.The assembled hybrid nanostructured noble metal catalyst greatly improves the utilization rate of the noble metal,achieves extremely high catalytic activity under the low noble metal loading,and achieves high stability due to the particularity of its structure. |
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