Research On The Preparation Of Porous Material Supported Nano-palladium Catalyst By Ultrasonic-assisted Reduction Method

The main factors that influence the catalytic performance of supported palladium catalyst include the structure and morphology of palladium nano-particles?PdNPs?,distribution of PdNPs on the support and interaction between PdNPs and the support.Synthesis method and process control are the key to researches of supported palladium catalyst.In this paper,using Na₂PdCl₄ as the palladium source,applying activated carbon,mesoporous molecular sieve SBA-15,microporous molecular sieve SAPO-11and SAPO-34 as four different supports,supported palladium catalysts were prepared by ultrasound assisted reduction approach.The characterization for structure and morphology of as-prepared catalysts and the analysis of the preparation were monitored by UV-Vis,N₂ physical absorption,FTIR,TG,XRD,TEM approaches.The catalytic activity was tested using Suzuki coupling reaction as probe reaction.Supported Pd/C catalyst was synthesized by ultrasonic reduction with activated carbon?AC?as the support,ethylene glycol as the reducing agent and Na₂PdCl₄ as the source of palladium.The chosen preparation condition was under the ultrasound power of 300 W with the theoretical loading amount of 3%in Pd/C.The as-prepared Pd/C catalyst contained PdNPs of face-centered structure uniformly dispersing on AC.The size distribution of PdNPs ranged from 3.29 nm to 10.81 nm with the average size of6.16 nm.The yield of Suzuki coupling reaction between 4-bromotoluene and phenylboronic acid catalyzed by 0.1%of Pd/C was 98.22%.The catalytic activity maintained high with the yield of 80.75%after repeating five cycles.The hot filtration experiment result with 10.05%of the yield proved few Pd shed from the support.Supported Pd/SBA-15 catalyst was synthesized by ultrasonic reduction with mesoporous silica SBA-15 as the support,ethylene glycol as the reducing agent and Na₂PdCl₄ as the source of palladium.The chosen preparation condition was under the ultrasound power of 600 W with the theoretical loading amount of 1%in Pd/SBA-15.PdNPs in the prepared Pd/SBA-15 distributed orderly in the narrow channels of SBA-15 with a size distribution from 1.71 nm to 4.17 nm and a mean size at 2.98 nm.The yield of Suzuki coupling reaction between 4-bromotoluene and phenylboronic acid catalyzed by 0.1%of Pd/SBA-15 was 98.01%.The yield remained 80.77%after the catalyst was reused for five times.The hot filtration experiment result with only 1.71%of the yield also proved the stability of Pd/SBA-15 catalyst.Pd supported by zeolite molecular sieve SAPO series were synthesized by ultrasonic reduction with ethylene glycol as the reducing agent and Na₂PdCl₄ as the source of palladium.The chosen preparation condition was under the ultrasound power of 600 W with the theoretical loading amount of 1.0%in Pd/SAPO-11 and Pd/SAPO-34.In Pd/SAPO-11,PdNPs were uniformly ranged in the support SAPO-11 with a size distribution from 1.26 nm to 6.01 nm and 3.31 nm in average.The yield of Suzuki coupling reaction between 4-bromotoluene and phenylboronic acid catalyzed by 0.1%of Pd/SAPO-11 was 93.49%.The yield dropped to 80.32%after five repeated cycles.The yield of hot filtration experiment result was 7.74%.PdNPs with an average particle size of 4.02 nm were uniformly distributed in the inner pores of SAPO-34.The size distribution of PdNPs was from 2.43 nm to 7.42 nm.The yield of Suzuki coupling reaction between 4-bromotoluene and phenylboronic acid catalyzed by 0.1%of Pd/SAPO-34 was 86.20%.The yield dropped to 79.67%after five repeated cycles.The yield of hot filtration experiment result was 1.05%.The results showed that in the supported Pd/C catalyst,due to the destruction of ultrasound in the structure of AC and the small size of microporesin AC,part of PdNPs were adsorbed on the surface of AC,leading to the larger size and easier leaking of PdNPs in the structure.In the prepared Pd/SBA-15,PdNPs could easily reach inside the channels of SBA-15 by ultrasonic diffusion,meanwhile the interaction between EG and SBA-15 helped stable PdNPs in the structure of SBA-15.Therefore,Pd/SBA-15exhibited well stability and the size of PdNPs was restricted by pore diameter of SBA-15.Pd/SAPO-11 and Pd/SAPO-34 could be acquired by ultrasound assisted reduction.The two catalysts displayed PdNPs of small particle size uniformly distributed in both surface mesopores and inner micropores of the supports.Especially in Pd/SAPO-34,PdNPs could grow inside the micropores of three-dimensional structured SAPO-34under ultrasonic diffusion,leading to the well stability of Pd/SAPO-34 catalyst.