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≤T≤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