Kinetics and reaction mechanism of Pd-Catalyzed chlorobenzene hydrogenolysis

Chlorinated organic compounds are ubiquitous chemical intermediates, final products, and waste streams found in multiple industries. These compounds must often be dechlorinated prior to disposal to avoid negative consequences for environmental and human health. In this work, we employ kinetic experiments and density functional theory (DFT) to examine Pd-catalyzed chlorobenzene hydrogenolysis, which is highly selective for the formation of benzene (>95 %). Kinetic data (353 K, 10-101 kPa H-2, 0.2-1.1 kPa C6H2Cl, 0.5-1 kPa HCl) on 5 wt% Pd/C catalysts revealed rates that are first-order, half-order, and inverse first-order with respect to PhCl, H-2, and HCl, respectively. These kinetic trends are consistent with PhCl dechlorination as the kinetically relevant step occurring at a single site on surfaces saturated by Cl adatoms (Cl*) in equilibrium with H-2 and HCl, consistent with previous reports in the literature. However, the nature of this C-Cl bond activation has not been well-described in the prior literature, because forming adsorbed Ph* and Cl* typically requires two sites, yet the kinetic trends suggest that only a single site is needed. Here, we resolve this inconsistency using theory, which shows that Cl* saturates Pd surfaces at a Cl*:Pd-surf ratio of ca. 0.3-0.4. This low saturation coverage enables Ph* (and other species) to bind interstitially among the Cl* adatoms without requiring Cl* desorption. Theory furthermore confirms that the reaction occurs through a rate-determining PhCl dechlorination, without any C-H activation of the phenyl ring. This non-competitive binding of Ph* among Cl* adatoms can be described as a two-site mechanism that agrees with the kinetic, isotopic, and theoretical data from this and prior work.