Hydrodehalogenation of haloarenes over Silica supported Pd and Ni: A consideration of catalytic activity/selectivity and haloarene reactivity Original Research Article

The gas phase catalytic hydrodehalogenation of haloarenes (423 K ≤ T ≤ 593 K) is presented as a viable means of treating hazardous concentrated halogenated gas streams for the recovery/reuse of valuable chemical feedstock, i.e. a progressive green processing strategy. The action of a Ni/SiO2 and Pd/SiO2 of comparable metal loading (ca. 5% w/w) was compared where the Pd catalyst delivered specific hydrodehalogenation activities that were up to three orders of magnitude greater. Reduction of Pd/SiO2 was far more facile to generate a supported zero valent phase that was characterized by a narrower distribution of smaller metal particles, delivering a (TEM derived) mean Pd particle diameter = 4.0 nm. Temperature programmed reduction (TPR) analysis of Ni/SiO2 demonstrated higher reduction temperature requirements (723 K as opposed to 523 K for Pd/SiO2) yielding an average Ni diameter = 9.3 nm. Hydrodehalogenation activity/selectivity is strongly dependent on the electron withdrawing/donation properties of the ring substituents where electron donation serves to activate the ring for hydrogen scission of the CX bond. Variations in isomer reactivity can be attributed to steric hindrance limitations. Lumping chloroarene isomers together, the following trend of decreasing hydrodechlorination rates is established: chlorophenol(s) > chlorotoluene(s) > chlorobenzene > dichlorophenol(s), trichlorophenol(s) > dichlorobenzene(s) > trichlorobenzene(s), bromochlorobenzene > pentachlorophenol > hexachlorobenzene. Compensation behavior is established for the dehalogenation of a range of haloarenes over Ni/SiO2, a response that is used to predict a feasible temperature for the treatment of a multi component haloarene feed.