Carbon-chlorine and carbon-bromine bond cleavage in the catalytic hydrodehalogenation of halogenated aromatics

The catalytic hydrodehalogenation of chlorobenzene (CB), bromobenzene (BB), CB/BB mixtures, and the three chlorobromobenzene (CBB) isomers was studied over the temperature range 473 K less than or equal to T less than or equal to 603 K using Ni/SiO2 where the Ni loading was varied from 6.2 to 15.2% wt/wt. Each catalyst was 100% selective in terms of hydrodehalogenation and there was no evidence of any aromatic ring reduction. Steady-state conversion was readily achieved but there was a decided decline in activity with prolonged catalyst use. The catalyst samples were characterized, before and after use, by TEM, CO chemisorption/TPD, TPO, and potentiometric analysis. We recorded the levels of reversibly and irreversibly held Cl and/or Br on the spent catalysts. Appreciable Ni particle growth during catalysis was observed and this is attributed to a halide-induced metal agglomeration, while CO TPD revealed a significant disruption to the Ni particle electronic structure. Long-term deactivation is ascribed to a less effective activation of the haloarene and/or hydrogen reactant(s) allied to a halide-promoted carbonaceous deposition; the surface coke is characterized as predominantly amorphous. Reactivity increased in the order 2-CBB < 3-CBB < 4-CBB < BB < CB, which is taken to be diagnostic of an electrophilic mechanism where the presence of a second electron withdrawing a halogen substituent lowers the overall dehalogenation rate and steric hindrance governs reactivity in the case of the CBB isomers. In the hydrotreatment of CB/BB mixtures and CBB isomers, the fractional debromination increased due to a surface hydrochlorination and Cl exchange with the Br substituent on the aromatic ring. The reaction orders with respect to CB, BB, and H-2 were determined, while the experimentally determined rate/pressure profiles were subjected to standard Langmuir-Hinshelwood kinetic modeling. The best overall (meaningful) fit to the experimental data was achieved with a model based on the associative adsorption of the haloarene and the dissociative adsorption of hydrogen on the same sites with no product inhibition. (C) 2002 Elsevier Science (USA).