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

Solvent effects in the liquid phase (0.1 MPa; 303 K) hydrodechlorination (HDC) of 2,4‐dichlorophenol have been established over Pd/Al₂O₃. 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 (ε) and a decrease in the molar volume ( $\overline {\nu}$ ) of the solvent, where the major (ca. 80%) contribution is due to ε. We attribute this response to the increased solvent capacity to stabilize the arenium intermediate at higher/lower ε/$\overline {\nu}$ , an effect that extends to reaction in water + organic combinations. We provide, for the first time, a reliable quantification of solvent effects that can be potentially applied to other catalytic hydrogenolysis systems. © 2009 American Institute of Chemical Engineers AIChE J, 2010