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Polyfluorinated arenes are increasingly used in industry and can be considered emerging contaminants. Environmentally applicable degradation methods leading to full defluorination are not reported in the literature. In this study, it is demonstrated that the heterogeneous catalyst Rh/Al2O3 is capable of fully defluorinating and hydrogenating polyfluorinated benzenes in water under mild conditions (1 atm H2, ambient temperature) with degradation half-lives between 11 and 42 min. Analysis of the degradation rates of the 12 fluorobenzene congeners showed two trends: slower degradation with increasing number of fluorine substituents and increasing degradation rates with increasing number of adjacent fluorine substituents. The observed fluorinated intermediates indicated that adjacent fluorine substituents are preferably removed. Besides defluorination and hydrogenation, the scope of the catalyst includes dehalogenation of polychlorinated benzenes, bromobenzene, iodobenzene, and selected mixed dihalobenzenes. Polychlorobenzene degradation rates, like their fluorinated counterparts, decreased with increasing halogen substitution. In contrast to the polyfluorobenzenes though, removal of chlorine substituents was sterically driven. All monohalobenzenes were degraded at similar rates; however, when two carbon-halogen bonds were in direct intramolecular competition, the weaker bond was broken first. Differences in sorption affinities of the substrates are suggested to play a major role in determining the relative rates of transformation of halobenzenes by Rh/Al2O3 and H2.

Fluorinated organic compounds are increasingly used in many applications, and their release to the environment is expected. It is therefore important to find suitable methods for degradation of fluorinated compounds under environmentally relevant conditions. In this study, a simple heterogeneous rhodium-based catalytic system (Rh/Al(2)O(3) and H(2)) for hydrodefluorination and hydrogenation of fluorobenzene under mild aqueous conditions (1 atm of H(2), ambient temperature) was developed and the underlying reaction mechanism was investigated. Fluorobenzene degraded rapidly (t(1/2) ≈ 0.2 h) to form cyclohexane and fluoride (F(-)) as the stable end products, with benzene and cyclohexene observed as intermediates. Cyclohexadiene intermediates were not observed but were expected to form during the hydrogenation of benzene. Three postulated but unobserved fluorinated intermediates were subjected to the catalytic reaction conditions, and it was concluded that they most likely do not form during the fluorobenzene degradation reaction. Isotope labeling experiments showed that the unsaturated intermediates undergo rapid and reversible hydrogenation/dehydrogenation under the reaction conditions and also that fully saturated compounds are unreactive in the catalytic system. Both molecular hydrogen and water were sources of hydrogen in the final cyclohexane product. Kinetic fitting indicated that sorption/desorption of fluorobenzene onto the catalyst surface plays an important role in the mechanism.