Hydrodehalogenation of halogenated hydrocarbons in water with Pd catalysts: Reaction rates and surface competition

Hydrodehalogenation using Pd catalysts has proved to be an efficient reaction for water detoxification. In the present study, reaction rates for a variety of halogenated hydrocarbons with hydrogen as reductant in aqueous catalyst suspensions have been measured. Structure–reactivity considerations allow the prediction of reactivities of various halogenated organic compounds in water clean-up processes. The correlation of CCl bond strength with the hydrodechlorination rate earlier reported was reassessed. In contrast to straightforward structure–reactivity correlations found for chloroalkanes, hydrodehalogenation rates of chloroethenes and halobenzenes do not follow the order of the CX bond strength. Here, the addition of an activated hydrogen species from the catalyst to the unsaturated hydrocarbon seems to be the rate-determining step.For pollutant mixtures, hydrodehalogenation rates were influenced by competition for Pd surface sites. Competitors diminish reaction rates depending on their sorption strength towards the Pd surface. Substances subject to extremely slow hydrodehalogenation (such as methylene chloride and fluorobenzene) showed no marked influence on other competitors. The heavier halogens, bromine and iodine (whether as R–X or as X−), dominate all competing effects. However, no self-poisoning of the catalyst for hydrodehalogenation of iodo- and bromohydrocarbons occurred in the water phase. This makes it possible to reduce these halogenated compounds at reasonable rates over Pd catalysts.Under optimal conditions, the activity of Pd catalysts is so high that external and intraparticle mass transfer limitations cannot be ruled out for hydrodehalogenation of many substrates in aqueous catalyst suspensions.