Preparation And Catalytic Performances Of Highly Dispersed Palladium Catalysts

Due to their excellent catalytic properties,Pd supported catalysts have been widely used in many fields such as the selective hydrogenation,oxidation,hydrogen evolution,and Suzuki coupling reactions.Many studies have found that the size and dispersion of Pd particles on the surface of the supported catalyst were mainly dependent on the preparation method,and would have great impact on the catalytic performances.In recent years,the green and easily controlled polyol method has aroused great interest.The typical procedures include preparing a Pd nanoparticles?NPs?supported catalyst in the pure polyol?e.g.ethylene glycol,EG?at relatively high temperature?near the boiling point of the polyol?,and using additional surfactants as capping agents to avoid agglomeration of Pd NPs.However,in the polyol process,the recycling rate of polyol is extremely low,and the reaction temperature near the boiling point easily causes rapid thermal decomposition of polyol.Those surfactants also need to be removed before performing catalytic measurements for excellent catalytic activity.In response to the above problems,the following works has been carried out:1.A novel method for the synthesis of highly dispersed Pd supported catalysts?CAR method?was developped,namely,Pd/Al₂O₃-CAR_Mn,Pd/Al₂O₃-CAR_Zn and Pd/Al₂O₃-CAR_Cu could be prepared at 50?in the EG-water solution through the addition of MSO₄?M=Cu,Zn and Mn?,and the residual filtrate liquid separated from the catalyst could be recycled to prepare other catalysts.The reduction mechanism of the CAR method was also discussed by the characterization of UV-vis,XPS,TEM and FT-IR.It was found that the sizes of Pd NPs on Pd/Al₂O₃-CAR_Cu,Pd/Al₂O₃-CAR_Zn and Pd/Al₂O₃-CAR_Mn were 27.4,5.5 and 3.6 nm in turn.When the filtrate from the separation of Pd/Al₂O₃-CAR_Mn was reused,the average sizes of Pd NPs on Pd/Al₂O₃-CAR_Mn-1,Pd/Al₂O₃-CAR_Mn-2 and Pd/Al₂O₃-CAR_Mn-3 were 3.6,3.2 and3.9 nm respectively,and Pd NPs were well dispersed on the surface of Al₂O₃.The cation can react with HOCH₂CH₂OH and SO₄^2-to form a ternary complex HOCH₂CH₂OH···[M?H₂O?_?n-1?]²⁺···SO₄^2-,and promotes the continuous dehydrogenation of EG to aldehyde,which can reduce Pd²⁺to the metal Pd.The prepared catalysts were applied to the solvent-free oxidation of benzyl alcohol.The results showed that Pd/Al₂O₃-CAR_Mn exhibited the highest activity among Pd/Al₂O₃-CAR_Mn,Pd/Al₂O₃-CAR_Zn and Pd/Al₂O₃-CAR_Cu.When the amount of Pd/Al₂O₃-CAR_Mn?based on benzyl alcohol?was 0.19 wt%,the conversion of benzyl alcohol and the selectivity of benzaldehyde reached 27.0%and 92.1%within 1 h,respectively,and the turnover frequency?TOF?was 21309 h^-1.When the catalyst dosage was reduced to 0.095 wt%,the TOF value increased to 29810 h^-1.And the catalyst prepared by filtrate circulation showed the same activity as Pd/Al₂O₃-CAR_Mn.2.Rod-shaped Al₂O₃ carriers with different alkali center numbers were prepared by hydrothermal method with different molar ratios of Al³⁺/urea?1:6,1:7,1:8 and 1:9?,and used to fabricate Pd/1:6-Al₂O₃-CAR_Mn,Pd/1:7-Al₂O₃-CAR_Mn,Pd/1:8-Al₂O₃-CAR_Mn and Pd/1:9-Al₂O₃-CAR_Mn through the CAR method with Mn SO₄ as the cationic source.The catalysts were characterized by TEM,XPS,XRD,BET,CO₂-TPD and H₂-O₂ titration.It was found that the structures of Al₂O₃ carriers were ascribed to the?phase with low crystallinity.The specific surface area of the catalysts and the base number on the surface of the carriers decreased with the reduction of the molar ratio of Al³⁺/urea,on the contrary,the average size and dispersion of Pd NPs on their surface increased.And the more the number of base centers on the surface of the carrier,the smaller the size of Pd NPs and the higher the dispersion.Four catalysts had high activity in the solvent-free oxidation of benzyl alcohol to benzaldehyde.And the smaller the particle size of Pd NPs,the higher the degree of dispersion,the higher the conversion of benzyl alcohol and the higher the selectivity of benzaldehyde.When Pd/1:6-Al₂O₃-CAR_Mn was used as the catalyst under the conditions of reaction temperature of 140?,stirring speed of 1260 rpm,catalyst amount of 0.19 wt%and reaction time of 6 h,the conversion of benzyl alcohol and the selectivity of benzaldehyde were 69.8%and 96.0%,respectively.3.Using commercial?-Al₂O₃ as support,Pd/?-Al₂O₃-CAR,Pd/?-Al₂O₃-H₂ and Pd/?-Al₂O₃-EG were prepared by the CAR,H₂ thermal reduction and traditional polyol method,respectively.The results of TEM,XPS,XRD,NH₃-TPD and H₂-O₂ titration showed that the average sizes of Pd NPs on the surface of Pd/?-Al₂O₃-CAR,Pd/?-Al₂O₃-H₂and Pd/?-Al₂O₃-EG were 4.2,2.6 and 2.9 nm,and the contents of Pd⁰ on their surface were85.2%,68.1%and 79.5%,respectively.Since the carboxylic acid from the oxidation of EG was adsorbed on the surface of the catalysts,the number of acid centers on the surface displayed the following order:Pd/?-Al₂O₃-H₂<Pd/?-Al₂O₃-CAR<Pd/?-Al₂O₃-EG.In the selective hydrogenation of benzonitrile,the conversion of benzonitrile increased with the increase of the content of Pd⁰ on the surface.The selectivity of benzylamine decreased with the increase of the acid centers on the surface.The benzylamine yield on the Pd/?-Al₂O₃-CAR was the highest,reaching 79.2%.