Dechlorination of chlorobenzene in subcritical water with Fe/ZrO2, Ni/ZrO2 and Cu/ZrO2

Abstract

To obtain information about dechlorination of organochlorine compounds in subcritical water catalyzed by metals assisted with ZrO(2), dechlorination of chlorobenzene has been investigated in the presence of Fe/ZrO(2), Ni/ZrO(2) and Cu/ZrO(2) catalysts. The dechlorination efficiency was increased with increasing residence time, temperature and pressure. The order of effectiveness of the catalysts was Cu/ZrO(2) < Ni/ZrO(2) < Fe/ZrO(2). The dechlorination of chlorobenze obeyed pseudo-first-order kinetics models. The rate constants in subcritical water were much greater than that in ambient-temperature water; the activation energies were obtained. ZrO(2) in the catalyst had the power to absorb chlorobenzene onto the catalyst surface and promoted the dechlorination ability of the metal. The primary mechanism for dechlorinaton of chlorobenzene involved the reduction of chlorobenzene by reaction with nascent hydrogen. The nascent hydrogen reacted with the chlorobenzene, which adsorbed on the catalyst in non-planar and co-planar form, and formed benzene and chloride ions.

Reductive dechlorination of chlorobenzene in supercritical water catalyzed by Fe/ZrO2

“…In supercritical water, the dechlorination efficiency reached 0.39, 0.56, 0.76 and 0.86 even at a residence time of 80 s and the temperatures of 410, 440, 470 and 500°C, respectively. However, the dechlorination efficiency in subcritical water with 50 g Fe/ ZrO 2 only reached 0.84 at a residence time of 360 s and the temperature of 350°C (Wei et al 2009). These results hint that the dechlorination efficiency in supercritical water was much greater than that in subcritical water.…”

“…3. Different from the effect of pressure on dechlorination in subcritical water (Wei et al 2009), it was interesting to note that the variation of reaction pressure in supercritical region had no significant influence on dechlorination. In subcritical water, increasing the operating pressures from 22 to 28 MPa resulted in water density increases that leaded to the increase of ion product (Marshall and Franck 1981).…”

“…The flow-type apparatus used for the reductive dechlorination of chlorobenzene had been described previously (Wei et al 2009). At the beginning of the experiment, 35 g Fe/ZrO 2 prepared by co-precipitation method was loaded into the reactor.…”

“…The dried pallets were calcined at 500°C for 5 h, and then were reduced at 500°C for 5 h in a stream of H 2 . The preparation of Fe/ZrO 2 by mechanical mixing method has been described previously (Wei et al 2009), iron powder ([98.0%, 75 lm) mixed with ZrO 2 powder (AR) together in an automatic mortar for 30 min, and the mixed powders were then pressed into pellets using a pelleting machine followed by drying at 120°C for 6 h. The next steps in calcination and reduction were similar to those in preparation of Fe/ZrO 2 by co-precipitation method.…”

“…Fe/ZrO 2 , Ni/ZrO 2 and Cu/ZrO 2 . The reductive dechlorination order was found in Cu/ZrO 2 \ Ni/ ZrO 2 \ Fe/ZrO 2 , and the combination of ZrO 2 with zero metal was more efficient than zero valent metal alone (Wei et al 2009). Even though catalysts mentioned above, prepared by mechanical mixing method, had an appreciable effect in dechlorination of organochlorine compounds, catalyst presented some physical properties needed to be improved, such as low BET surface area, poor dispersity and uniformity of active composition.…”