Publication Catalysis 2010
S. Gomez-Quero, E. Diaz, F. Cardenas-Lizana, M. A. Keane, Chem. Eng. Sci. 2010, 65, 3786.
Solvent effects have been studied for the liq. phase (atm. 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 temp. programmed redn., XRD, TEM, BET and pore vol. measurements: the active Pd phase is present as nano-scale particles (<1-6 nm, surface area weighted mean diam.=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 assocd. bond dissocn. energy while the presence of a second halogen substituent has a deactivating effect. Higher initial HDH rates were obsd. with increasing water content in the solvent mixt. and this is principally related to an increase in the dielec. const. of the medium; HDH rate showed a lesser dependence on molar volume.
We link the HDH activity dependency on dielec. const. to the capacity of the solvent to stabilize the arenium intermediate. In contrast, reaction selectivity was unaffected by variations in solvent compn., suggesting that each step in the reaction pathway (stepwise or concerted dehalogenation) is influenced to the same extent. Higher HDH rate with increasing solvent dielec. const. and selectivity invariance with solvent compn. also extended to the HDH of chlorobenzenes.