Highly efficient catalytic debromination of tetrabromodiphenyl ether with hydrazine as reducing agent: The role of the interaction between the catalyst and the reducing agent

Abstract

It is still urgently needed to clarify how the reductive dehalogenation is accelerated by the interaction between the catalyst and the reducing agent. In the present work, both Pd and Cu were observed to induce the debromination of 2,2′,4,4′-tetrabromodiphenyl ether (BDE47) with hydrazine hydrate (N2H4·H2O) as reducing agent. In the case of Pd system, all the added BDE47 was debrominated to bromine-free products within 20 min. However, much different debromination behaviors of BDE47 were observed when Cu was used as the catalyst: in the initial stage the debromination efficiency of BDE47 could reach up to 55.3% within 0.5 min; nevertheless the complete debromination could not be achieved by prolonging reaction time. With the aid of DFT calculations, the above-mentioned great differences were attributed to the affinity of Pd to both H and N atoms, and strong Cu–N coordination existed between Cu and the adsorbed hydrazine molecules. The dissociation energy for the cleavage of N–H and N-N bonds of N2H4·H2O adsorbed on Cu was very low, and hence the debromination of BDE47 was ultra-fast due to the spontaneously cleavage of N–H bond but it was inhibited at the later stage by the generated ammonium species (poisoning effect to the Cu catalyst) due to N-N bond cleavage. In contrast, the dissociation energy for the cleavage of N–H bond on the surface of Pd was higher and no ammonium was detected. These guarantee the continuously fast and complete debromination of BDE47 and its intermediates. These new insights to the catalytic mechanism through the interaction between the catalyst and the reducing agent will help to construct an efficient system to remove halogenated organic pollutants.