Preparation And Catalytic Performance Of Palladium Cluster@Chitosan Microsphere Catalyst

Palladium catalysis is a research hotspot in the field of chemical and chemical catalysis.Palladium nanoparticles have high activity in the catalytic hydrogenation reduction reaction,and at the same time,they also have a good degradation efficiency in the reduction and degradation of dyes.Pollutants such as aromatic aldehyde compounds,nitro compounds and organic dyes can affect the growth process of animals and plants.The use of metal catalysis to control environmental pollution has strong research significance.However,the palladium catalyst has disadvantages such as difficult recovery,poor cycle life,poor atomic economic benefits,high cost,and easy poisoning,which does not conform to the concept of green chemistry.Therefore,the development of recyclable,highly active,and environmentally friendly heterogeneous palladium catalysts has important research value.Chitosan-based carrier has the advantages of a wide range of sources,non-toxic and harmless,and good biocompatibility.Its structure contains a large number of hydroxyl and amino groups,providing a large number of complex sites for metal palladium,and can effectively stabilize palladium nanoparticles It is beneficial to the preparation of chitosan-supported palladium heterogeneous catalyst.Under this research background,this thesis designed and synthesized NaBH₄-Pd@CS,N₂-Pd@CS and H₂-Pd@CS catalysts,and applied them in the reduction and degradation of dyes,conversion of aromatic nitro compounds into aromatic amines,and aromatics.In the three reactions of aldehyde hydrogenation,the details are as follows:1.Preparation of chitosan microspheres（CS）by sol-gel method;using palladium acetate as the precursor of the palladium species and CS as the precursor of the carrier,the chitosan-supported palladium catalyst precursor(Pd²⁺@CS)was synthesized by the impregnation method.),and treated with sodium borohydride,hydrogen and nitrogen to prepare NaBH₄-Pd@CS,N₂-Pd@CS and H₂-Pd@CS catalysts respectively.The catalysts were characterized by atomic absorption,SEM,TEM,FT-IR,TG,XRD,XPS,and the results showed that the SEM test showed that the prepared NaBH₄-Pd@CS,N₂-Pd@CS and H₂-Pd@CS catalysts had better surface than the CS surface.Smooth,and the spherical morphology of the catalyst remains intact;TG test shows that the thermal stability of the catalyst is lower than that of the carrier.TEM test shows that the average particle size of palladium nanoparticles is about 2nm;through XRD analysis,N₂-Pd@CS and H₂-Pd@CS catalysts have diffraction peaks at 2θof 40.41°,which are similar to the（111）crystals of palladium.Correspondingly,the palladium in the N₂-Pd@CS and H₂-Pd@CS catalysts is reduced to elemental palladium nanoparticles,while the elemental palladium nanoion（111）crystal face in the NaBH₄-Pd@CS catalyst does not show a peak.The reason is It may be that the palladium nanoparticles are relatively finely and uniformly dispersed.According to infrared spectroscopy and XPS tests of nitrogen and oxygen elements,it is known that palladium nanoparticles and nitrogen and oxygen elements in the carrier exhibit a coordination complex effect.The above test results prove that chitosan has successfully loaded elemental palladium nanoparticles.2.Use NaBH₄ as the initiator and NaBH₄-Pd@CS as the catalyst to investigate the degradation effect of the catalyst on a variety of dyes.The amount of catalyst is 0.5mol%,and the degradation times of Phenol red,Rhodamine,Methylene blue,Methyl orange,and Congo red are 8,5,10,3,and 3 minutes,respectively.According to the degradation rate calculated by Lambert Beer’s law,the degradation rates of Phenol red,Rhodamine,Methylene blue,Methyl orange,and Congo red are 98%,96%,98%,89%,and 94%,respectively.Methyl blue can be directly degraded within 10 minutes without adding sodium borohydride as an initiator,and the degradation rate is 99%.Using the Eley-Rideal mechanism and comparing the concentration derivative with time,it conforms to the first-order kinetics.NaBH₄-Pd@CS and NaBH₄,NaBH₄-Pd/C,NaBH₄-n Pd and other catalytic systems were also used to degrade the dye.The results showed that,taking phenol red as an example,no degradation occurred when NaBH₄ was added alone.NaBH₄-Pd/C The degradation rates of NaBH₄-n Pd are 20%and 15%respectively.Compared with NaBH₄-Pd/C and NaBH₄-n Pd,the degradation rates of NaBH₄-Pd@CS are increased by 78%and 83%,respectively.3.The NaBH₄-Pd@CS catalyst was applied to the conversion of aromatic nitro groups to aromatic amines,and the influence of reaction temperature,catalyst dosage and type on the yield was investigated.The best reaction conditions are 0.25mmol of aromatic nitro group,1.3mmol NaBH₄,2m L Me OH and 0.2mol%Pd²⁺@CS（Pd content is 2.8wt%）as the catalyst for reaction at room temperature for 1h,and the yield can reach 99%.The adaptability of the substrate was also investigated,and the results showed that the yield of aromatic nitro groups converted to aromatic amines was 92-99%.Using the ultraviolet absorption curve of o-nitroaniline,the kinetics of the reaction of NaBH₄-Pd@CS with Pd/C,n Pd,Pd（OAc）₂ were investigated,and it was found that the four catalysts all accord with the first-order reaction kinetics,among which NaBH₄-Pd@CS catalyst The rate of catalysis is the fastest.After the NaBH₄-Pd@CS catalyst catalyzed the conversion of nitrobenzene and used it for 5 times,the yield of aniline was 97%.It shows that the catalyst has high catalytic activity,good stability,and can be recycled.4.The N₂-Pd@CS was used to catalyze the hydrogenation of benzaldehyde,and the effect of catalyst type,dosage,and solvent on the yield was investigated.The optimal condition was 0.36mol%N₂-Pd@CS（Pd content is 4.2 wt%）,2ml Me OH,reacted under 25 bar hydrogen pressure for 3h,the yield was 99%.Compared with N₂-Pd@CS,NaBH₄-Pd@CS,and H₂-Pd@CS catalysts,N₂-Pd@CS has the best catalytic effect.The substrate adaptability was investigated,and it was found that the reaction has both selectivity and good substrate adaptability.The system can selectively reduce carbonyl groups in substituted benzaldehyde substrates of halogen,alkyl,hydroxyl,methoxy and nitro groups,with a yield of 90-99%.After 5 cycles of the catalyst,the yield can still reach 97%.