Studies in hydrotreatment as a unit process to destroy 4-chlorophenol in aqueous stream over Ru-Pd/TiO2 catalyst

Catalytic hydrodechlorination of 4-chlorophenol in an aqueous phase over a 4% Ru-1% Pd/TiO2 catalyst was studied in a three-phase slurry reactor. The intermediate phenol formed during reaction was further hydrogenated to cyclohexanol, which is less toxic than 4-chlorophenol and phenol, thereby suggesting the effectiveness of this catalyst in destruction of chlorophenol contaminated waste streams, by hydrotreatment. The effect of sodium chloride on the course of hydrodechlorination was studied. The destruction of 4-chlorophenol in presence of this salt was rapid. This effect was enhanced further in an alkaline medium. It was observed that the disappearance of toxic phenol was the rate-determining step in the hydrotreatment of 4-chlorophenol. The kinetics of phenol hydrogenation over this heterogeneous catalyst in presence of sodium chloride was therefore studied in a strongly alkaline medium in the range of temperatures 373-423 K and hydrogen partial pressures 0.69-2.76 MPa. The kinetic data were modelled using the power law rate expression. The reaction orders with respect to phenol and hydrogen increased with temperature from 0.22 to 0.40 and from 0.81 to 0.95, respectively. A Langmuir-Hinshelwood (L-H) type mechanistic model suggesting non-competitive adsorption of phenol and hydrogen could satisfactorily represent the experimental rates. The catalyst activity for hydrodechlorination remained constant even after three recycles. (C) 2004 Elsevier B.V. All rights reserved.