Solvent effects in the hydrodechlorination of 2,4‐dichlorophenol over Pd/Al₂O₃

Abstract

Solvent effects in the liquid phase (0.1 MPa; 303 K) hydrodechlorination (HDC) of 2,4-dichlorophenol have been established over Pd/Al 2 O 3 . In the absence of secondary reactions, catalyst deactivation, and transport limitations, a stepwise HDC yields 2-chlorophenol and phenol, where product selectivity was insensitive to the nature of the solvent. In contrast, the initial HDC rates exhibited a marked dependence on the reaction medium and increased in the order: benzene \ THF \ n-hexane \ cyclohexane \ alcohols \ water. Higher rates result from the concomitant effect of an increase in the dielectric constant (e) and a decrease in the molar volume (m) of the solvent, where the major (ca. 80%) contribution is due to e. We attribute this response to the increased solvent capacity to stabilize the arenium intermediate at higher/lower e/v, an effect that extends to reaction in water þ organic combinations. We provide, for the first time, a reliable quantification of solvent effects that can be potentially applied to other catalytic hydrogenolysis systems. V

Water-promoted catalytic hydrodechlorination of transformer oil-contained PCBs in liquid system under mild conditions

“…For the HDC of COCs, Pd/C catalyst exhibited higher activity in alcohols than in alkanes, arenes and heterocycles [36]. The phenomenon was in agreement with the results obtained by Gómez-Quero [34,35]. Therefore, alcohols were selected as the solvent in the HDC reaction.…”

“…In our previous work, the Pd/C catalyst exhibited higher activity in alcohols than in alkanes, arenes and heterocycles in the HDC of COCs, and the HDC rate could be improved by adding water to the reaction solvents [36,37]. And some literature reported the similar results [34,35,38]. However, there is no agreement on the effect of added water.…”

“…Reaction media plays an important role on reaction activity of the liquid-phase catalytic HDC [34,35,38]. For the HDC of COCs, Pd/C catalyst exhibited higher activity in alcohols than in alkanes, arenes and heterocycles [36].…”

“…Besides catalysts, solvent systems are considerably important factors to influence the liquid-phase HDC reaction [21,[33][34][35]. However, little attention has been paid to solvent system in liquidphase catalytic HDC.…”

“…However, there is no agreement on the effect of added water. Gómez-Quero et al [34,35] have linked the HDC activity dependency on dielectric constant (ε) to the capacity of the solvent to stabilize the arenium intermediate. Xia et al [36,37] have suggested that higher HDC rate was achieved because of better removal of absorbed reaction products from the catalyst surface by added water.…”