Reductive dechlorination of carbon tetrachloride in water catalyzed by mineral-supported biomimetic cobalt macrocycles

Article abstract of DOI:10.1021/es00002a021

Reductive dehalogenation, mediated by nonspecific biomimetic Co macrocycles, was studied in aqueous systems using carbon tetrachloride as a model compound. Two water-soluble macrocycles, cobalt tetrakis(N-methyl-4- pyridiniumyl)porphyrin (CoTMPyP) cation and cobalt tetrasulfophthalocyanine (CoPcTs) anion, were used as homogeneous and mineral-supported catalysts. The supported catalysts were prepared by exchanging CoTMPyP on the hectorite, fluorohectorite, and amorphous silica surface and by exchanging CoPcTs on the layered double hydroxide surface. Supported macrocycles were catalytically active in the dechlorination of CCl₄ and the initial reaction rates followed Michaelis-Menten kinetics. The value of V(max) was correlated to the previously reported orientation of macrocycles in the interlayers and to the accessibility and electronic state of the metal center, following the order: CoTMPyP-silica > CoPcTs-layered double hydroxide > CoT-MPyP-fluorohectorite > CoTMPyP-hectorite. In both heterogeneous and homogeneous systems, volatile reaction products accounted for less than 30% of CCl₄ degraded. In short- term experiments (2 h), homogeneous CoTMPyP was more active than heterogeneous catalysts, while homogeneous CoPcTs was deactivated due to aggregation, and degraded less CCl₄ than supported CoPcTs. In long-term experiments (3 days), where large CCl₄/macrocycle ratios were used, silica- supported CoTMPyP was more active than homogeneous CoTMPyP, suggesting that adsorption stabilized the catalyst.