Application of aluminum-supported Pd, Rh, and Rh-Pd nanoparticles in supercritical carbon dioxide system for hydrodebromination of polybrominated diphenyl ethers

• Pd, Rh, and Rh-Pd nanoparticles are deposited on Aluminum powder via spontaneous redox reactions in aqueous solutions at room temperature.
• Aluminum supported metals, an easy preparation catalysts, used for aqueous phase polybrominated diphenyl ethers in sc-CO2.
• The conditions for hydrobromination are green, hydrodebrominating 4- and 4,4′-bromodiphenyl ethers into diphenyl ether in sc-CO2 below 100 °C.
• Hydrogenolysis of diphenyl ether producing cyclohexanol and cyclohexane is the minor reaction route.

Al-powder-supported Pd, Rh, and Rh-Pd catalysts were synthesized through a spontaneous redox reaction in aqueous solutions. These catalysts hydrodebrominated 4- and 4,4′-bromodiphenyl ethers in supercritical carbon dioxide at 200 atm CO2 containing 10 atm H2 and 80 °C in 1 h. Diphenyl ether was the major product of Pd/Al. Rh/Al and Rh-Pd/Al further hydrogenated two benzene rings of diphenyl ether to form dicyclohexyl ether. The hydrogenolysis of CO bonds on diphenyl ether over Rh/Al and Rh-Pd/Al was observed to generate cyclohexanol and cyclohexane (<1%). With respect to hydrodebromination efficiency and catalyst stability, Rh-Pd/Al among three catalysts is suggested to be used for ex situ degradation of polybrominated diphenyl ethers in supercritical carbon dioxide.

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