A catalytic strategy for rapid cleavage of C-Cl bond under mild conditions: Effects of active hydrogen induced by Pd nanoparticles on the complete dechlorination of chlorobenzenes

Abstract

It is still very difficult to achieve rapid cleavage of C-Cl bond of highly-chlorinated benzenes by catalytic reduction process under mild conditions. By using pentachlorobenzene (PentaCB) as a representative of highly-chlorinated benzenes, we proposed a strategy to achieve rapid and complete dechlorination of highly-chlorinated benzenes by exploiting palladium with low content deposited g-C3N4 (Pd/g-C3N4) as a catalyst with NaBH4 as a reductant. Under favorable conditions, this strategy yielded a dechlorination efficiency of 100% for PentaCB (0.4 mmol L−1) in methanol solution at 30 °C within 20 min, and especially the consumption of NaBH4 is as low as 10 mmol L−1 with a molar ratio of Cl atoms to NaBH4 of only 1:5. A hydro-dechlorination pathway was confirmed to dominate the dechlorination of PentaCB, which involves active hydrogen species (H*) as being identified by electron spin resonance (ESR) monitoring and isotope labelling experiments. The dechlorination rate constant value of chlorobenzenes (CBs) was only a little dependent on the number of chlorine atoms and the chlorine atom with smaller steric hindrance was prior to be substituted by H*. A catalytic mechanism that Pd nanoparticles played an important role on both the weakening/breaking of C-Cl bond and the formation of active hydrogen species was further clarified: Pd exerts affinity interaction on chlorine atoms at ortho-position of CBs molecules to form adsorbed state [CBs]ads; H* was formed on the Pd surface due to its strong ability in H* retainment; subsequently the generated H* attacks the C-Cl bond in [CBs]ads to achieve hydrodechlorination. The new method provides a highly efficient method to remove halogenated pollutants.