The catalytic hydrodechlorination of mono-, di- and trichlorobenzenes over supported nickel

The gas phase hydrodechlorination (HDC) of chlorobenzene (CB), chlorotoluene(s) (CT), 3-chlorophenol (3-CP), dichlorobenzene(s) (DCB) and trichlorobenzene(s) (TCB) over the temperature range 473 K≤T≤573 K has been studied using 1.5% and 6.1% (w/w) Ni/SiO2 catalysts; the catalytic data have been obtained in the absence of any appreciable short-term deactivation. HDC of DCB and TCB generated the partially or the fully dechlorinated aromatic product + HCl and there was no significant cyclohexene or cyclohexane in the effluent stream. The conversion of mono-chloroarenes yielded the following reactivity sequence CB < 2-CT < 3-CT < 4-CT < 3-CP, i.e. the presence of an electron donating ring substituent enhances HDC and steric hindrance lowers reactivity. HDC kinetics have been adequately represented by a pseudo-first order approximation. Chlorine removal from DCB and TCB isomers proceeded through sequential and concerted routes, the relative importance of each dependent on the nature of the isomer and reaction conditions; apparent HDC activation energy increases in the order CB < DCB < TCB. The relationship between dechlorination selectivity and residence time/fractional conversion is addressed. The higher Ni loaded catalyst delivered consistently higher (specific) dechlorination rates and higher benzene yields from a polychlorinated feedstock. Catalytic HDC over Ni/SiO2 is presented as a viable means of treating/detoxifying concentrated chlorinated gas streams and the best strategy for generating the parent benzene or a target partially dechlorinated product is discussed.