Sol-gel entrapped Au0- and Ag0-nanoparticles catalyze reductive de-halogenation of halo-organic compounds by BH4−

Abstract

This study investigated the reductive de-halogenations of toxic Br3CCO2−, Br2CHCO2−, BrCH2CO2−, CH3CHBrCO2−, CH2BrCH2CO2−, CH2BrCHBrCO2−, Cl3CCO2−, Cl2CHCO2- and ClCH2CO2− by sodium borohydride catalyzed by sol-gel silica entrapped Au0 and Ag0 nanoparticles. The results indicate that the mechanism of reduction of Br3CCO2− differs from that of Cl3CCO2−. Calculated by DFT, the source of this difference lies in the larger bond strength of CCl compared to that of CBr and the weaker M0C bond strength in Au0-CBr2CO2− compared to those of Au0-CCl2CO2− and Au0-CH2CO2−. Furthermore, the de-halogenation mechanisms catalyzed by Ag0-NPs differ from those catalyzed by Au0-NPs. The latter observation is attributed to the different AgC and AuC bond strengths and to the different over-potentials for H2 release of these M0-NPs. In addition, product composition depends on the rate of BH4− addition. Proton labeling experiments prove that nearly all the hydrogen atoms in the products originated from the water solvent and not from the BH4−. The detailed mechanistic conclusions that can be drawn from these results differ considerably from those commonly accepted for de-halogenation processes.