Ultrasound-assisted Preparation Of Supported Palladium Nano-catalysts And Their Catalytic Performance

The supported palladium nanocatalyst has excellent photoelectric and mechanical properties,and it is often used in the fields of organic synthesis,petrochemical industry,and degradation of pollutants.The controllabe design of the morphology,structure,particle size for the nanocatalysts and their catalytic performance can be achieved by choosing the appropriate supports,reducing agents and preparation methods.In this paper,a series of supported palladium-based nanocatalysts are synthesized by ultrasound-assisted reduction with the coconut shell activated carbon and magnesium-aluminum hydrotalcite as the supports,and the ethylene glycol,ethanol and hydroxyl groups on the surface of hydrotalcite as the reducing agents.Their catalytic properties for the Suzuki coupling reaction,nitrobenzene hydrogenation reaction and p-nitrophenol reduction reaction are studied.The Pd/C-EG catalysts are synthesized by the ultrasonic-assisted controlled reduction with the coconut shell activated carbon as the support,the ethylene glycol?EG?as the reducing agent and the PdCl₂ as the source of palladium.The optimal preparation conditions are the ultrasound power of 300 W,the reaction temperature of25?and the reduction time of 25 min.The as-prepared Pd/C-EG catalysts contains PdNPs of face-centered structure,mainly composed of?111?crystal plane,uniformly dispersing on the support.The size distribution of PdNPs ranges from 2.62 nm to 13.89nm with an average size of 7.00 nm.The yield and the TOF value of Suzuki coupling reaction between 4-bromonitrobenzene and phenylboronic acid catalyzed by the Pd/C-EG are 98.27%and 1965.4 h^-1,respectively.The TOF value of the hydrogenation of nitrobenzene over Pd/C-EG is 331.6 h^-1.The Pd/C-EtOH catalysts are synthesized by the ultrasonic-assisted controlled reduction with the coconut shell activated carbon as the support,the absolute ethanol?EtOH?as the reducing agent and the PdCl₂ as the source of palladium.The optimal preparation conditions are the ultrasonic power of 500 W,the reaction temperature of60?,the reduction time of 90 min and the absolute ethanol dosage of 5 mL.The as-prepared Pd/C-EtOH catalysts contains PdNPs uniformly dispersing on the support.The size distribution of PdNPs ranges from 1.46 nm to 12.68 nm with an average size of5.71 nm.The yield of Suzuki coupling reaction between 4-bromonitrobenzene and phenylboronic acid over the Pd/C-EtOH is 97.83%,which is significantly higher than that over the commercial Pd/C-C catalyst.The Pd/C-EtOH catalyst prepared by ultrasound-assisted method shows a good catalytic performance for p-nitrobenzene hydrogenation in a wide temperature range.The apparent activation energy and preexponential factor of the Pd/C-EtOH on the hydrogenation of nitrobenzene are 21.20kJ�mol^-11 and 372.41 h^-1,respectively.The Pd/LDH-OH catalysts are synthesized by the ultrasonic-assisted controlled reduction with the magnesium-aluminum hydrotalcite?LDH?as the support,the hydroxyl group on the surface of the hydrotalcite?LDH-OH?as the reducing agent and the PdCl₂ as the source of palladium.The optimal preparation conditions are the ultrasonic power of 300 W,the reaction temperature of 25?and the reduction time of20 min.The as-prepared Pd/LDH-OH catalysts contains PdNPs uniformly dispersing on the surface and the mesoporous channels of LDH.The size distribution of PdNPs ranges from 1.15 nm to 2.81 nm with an average size of 1.49 nm.The yield of Suzuki coupling reaction between 4-bromonitrobenzene and phenylboronic acid over the Pd/LDH-OH is96.58%.The reaction rate of 4-NP reduction over the Pd/LDH-OH is 0.5 min^-1.From the above experimental results,it can be known that the particle size of PdNPs on the Pd/C-EG with AC as the support and EG as the reducing agent is largest,and there are problems such as large amount of reducing agent,etc.When a small amount of EtOH is used as the reducing agent,the particle size of PdNPs on the Pd/C-EtOH catalyst reduces,but there are problems such as high reaction temperature,etc.The small-sized PdNPs are prepared with the LDH as the support and its surface hydroxyl group as the reducing agent under mild reaction conditions,and it shows a good catalytic performance for the Suzuki coupling reaction and 4-NP reduction.