Solvent effects in the catalytic hydrotreament of haloaromatics over Pd/Al₂O₃ in water+organic mixtures

Solvent effects have been studied for the liquid phase (atmospheric pressure, T=303 K) hydrodehalogenation (HDH) of a range of haloarenes in methanol, THF, water+methanol and water+THF promoted by Pd/Al₂O₃. The catalyst has been characterized in terms of temperature programmed reduction, XRD, TEM, BET and pore volume measurements: the active Pd phase is present as nano-scale particles (<1-6 nm, surface area weighted mean diameter=2.4 nm). In the absence of transport limitations, catalyst deactivation and secondary reactions, the following sequence of increasing dehalogenation rate has been established: dichlorophenol(s)<monochlorophenol(s)<dibromophenol<monobromophenol(s). This is consistent with an electrophilic mechanism involving an arenium intermediate. The hydrogenolytic cleavage of C-Br bond(s) is promoted to a greater extent (relative to C-Cl) due to the lower associated bond dissociation energy while the presence of a second halogen substituent has a deactivating effect. Higher initial HDH rates were observed with increasing water content in the solvent mixture and this is principally related to an increase in the dielectric constant of the medium; HDH rate showed a lesser dependence on molar volume. We link the HDH activity dependency on dielectric constant to the capacity of the solvent to stabilize the arenium intermediate. In contrast, reaction selectivity was unaffected by variations in solvent composition, suggesting that each step in the reaction pathway (stepwise or concerted dehalogenation) is influenced to the same extent. Higher HDH rate with increasing solvent dielectric constant and selectivity invariance with solvent composition also extended to the HDH of chlorobenzenes. © 2010 Elsevier Ltd. All rights reserved.