Synthesis Of Pd-based Nanocatalyst And Their Catalytic Performance For 4-nitrophenol Hydrogenation

4-Aminophenol is an important class of fine organic chemical intermediates,which is widely used not only in the fields of synthetic dyes,pharmaceuticals,pesticides,but also in the synthesis of photographic developers,polymer material stabilizers,antioxidants,and petroleum additives.In industry,the reduction of 4-nitrophenol is often used to prepare aniline compounds.Although 4-nitrophenol plays an important role in many fields,it is highly toxic and difficult to degrade as an industrial pollutant.The commonly used methods for the preparation of 4-aminophenol include iron powder reduction,sulfide alkali reduction,electrochemical reduction,transfer hydrogenation,and catalytic hydrogenation.Taking the iron powder reduction method as an example,iron powder is used to hydrogenate 4-nitrophenol,but this method requires a large amount of iron powder and acid,which is costly,and the content of iron in the product is too high to be separated,resulting in significant environmental pollution.Therefore,it is particularly critical to develop a low cost,easy to recover,and environmentally friendly catalyst with high catalytic performance for the catalytic hydrogenation of 4-nitrophenol to 4-aminophenol.In this paper,supported Pd based catalysts were prepared using different catalyst carriers and doped with different non noble metals.The catalytic performance of the catalysts for 4-nitrophenol was investigated,the reasons for the differences in catalytic performance were studied,and the effects of non noble metal doping on the catalysts were studied.The specific research content mainly includes the following three aspects:1.A Schiff base based COFs material（TAPB-PDA COF）was synthesized.Using TAPB-PDA COF as a carrier,Pd nanoparticles with a narrow particle size distribution（5.09±1.30nm）were successfully loaded onto COFs through a sodium borohydride reduction method with the assistance of functional groups distributed in an ordered skeleton structure,and Pd/COFs catalysts with different loading amounts were prepared.Under relatively mild reaction conditions（reaction temperature 40℃,reaction pressure 3.0 MPa H₂）,2.11 wt%Pd/COFs catalysts exhibit excellent catalytic performance for the hydrogenation of 4-nitrophenol(conversion 99.3%,selectivity>99.0%,conversion frequency（TOF）16.5 min^-1).The catalytic performance of 2.11 wt%Pd/COFs catalysts is superior to 0.67 wt%Pd/COFs catalysts(TOF11.6 min^-1).In addition,the repeatability test showed that the Pd/COFs catalyst had high stability.Characterization results such as X-ray diffraction,X-ray photoelectron spectroscopy,scanning electron microscopy,transmission electron microscopy,high-resolution transmission electron microscopy,and Brunauer-Emmett-Teller showed that Pd nanoparticles in 2.11 wt%Pd/COFs catalysts exhibited a higher degree of dispersion on COFs carriers,and highly dispersed Pd nanoparticles could further improve the adsorption capacity for H₂,which was beneficial for improving catalytic activity.2.Bimetal catalysts（Pd Co/COFs and Pd Ni/COFs）with different loading amounts and atomic ratios were prepared by sodium borohydride reduction method,and their catalytic performance for the hydrogenation of 4-nitrophenol was tested.Pd Co/COFs catalysts were prepared by fixing Pd Co alloy particles in the cavity of COFs,with a narrow particle size distribution（3.79±1.11 nm）.The Pd Co/COFS-1 catalyst（with a Pd loading of 1.72 wt%）exhibited excellent catalytic performance for the hydrogenation of 4-nitrophenol for 10 min under mild reaction conditions at a reaction temperature of 40℃and a reaction pressure of 3.0MPa H₂(conversion 90.5%,selectivity>99.0%,TOF 29.9 min^-1).Comparing the catalytic performance of Pd Co/COFs bimetallic catalysts with that of Pd/COFs and Co/COFs monometallic catalysts under the same reaction conditions,it was found that the catalytic performance of Pd Co/COFs catalysts was much better than that of monometallic catalysts,indicating that the formation of Pd Co alloys was beneficial to the activation of 4-nitrophenol.The activity of the catalyst increases with the increase of the Pd/Co ratio（with a constant total load）.Comparing the catalytic hydrogenation performance of Pd Co/COFs-1 and Pd Ni/COFs-1,it was found that Co was superior to Ni in improving the catalytic performance.The catalytic performance of Pd Co/COFs-1 catalyst was superior to that of Pd Ni/COFs-1 catalyst(conversion 76.5%,selectivity>99.0%,TOF=26.7 min^-1)when reacting at 40℃and reaction pressure 3.0 MPa H₂ for 10 minutes.The corresponding catalyst was prepared with a theoretical loading amount of Pd:Co ratio of 1:1.It was found that the catalytic performance of Pd Co/COFS-1 was the best.The characterization results were analyzed by X-ray diffraction,X-ray photoelectron spectroscopy,Inductively coupled plasma-optical emission spectrometer,scanning electron microscopy,transmission electron microscopy,high-resolution transmission electron microscopy,Brunauer-Emmett-Teller,and Aberration-corrected scanning transmission electron microscopy,the Pd Co alloy nanoparticles in the catalyst are highly dispersed on the carrier,enhancing their adsorption and activation abilities for H₂.The stability test of the catalyst showed that there was no significant change in conversion and selectivity,indicating that the catalyst had high stability.3.Pd/g-C₃N₄ catalysts with different loading amounts（1.55 wt%,0.14 wt%,0.04 wt%）were prepared by sodium borohydride reduction method.Among them,1.55 wt%Pd/g-C₃N₄catalyst has more excellent catalytic activity for the catalytic hydrogenation of 4-nitrophenol.Under relatively mild reaction conditions of reaction temperature 40℃and reaction pressure2.0 MPa H₂,the reaction lasted for 5 minutes,and 4-nitrophenol was completely converted.When the reaction time was 1 min,the conversion rate was 40.7%,The selectivity is>99.0%,and the TOF is 149.2 min^-1,which is superior to other Pd/g-C₃N₄ catalysts with a loading amount.Comparing the catalytic activity of Pd/g-C₃N₄ catalyst and support g-C₃N₄ for the hydrogenation of 4-nitrophenol,it was found that g-C₃N₄ only participated in the adsorption of reactants but not in the activation of hydrogen.By analyzing the characterization results of X-ray diffraction,X-ray photoelectron spectroscopy,scanning electron microscopy,and transmission electron microscopy,it was found that Pd nanoparticles were highly dispersed and loaded on the carrier g-C₃N₄.The interfacial synergistic effect between Pd nanoparticles and g-C₃N₄ enabled the effective activation of H₂ on Pd nanoparticles,promoting the catalytic hydrogenation of 4-nitrophenol,and improving their catalytic activity.The stability test of the catalyst shows that the catalyst has excellent stability and can be reused.