Kinetic and Theoretical Study of the Hydrodechlorination of CH4−xClx (x = 1−4) Compounds on Palladium

Abstract

The reaction kinetics of hydrodechlorination (HDCl) for a series of CH(4−x)Cl(x) (x = 1−4) compounds were measured on a Pd/carbon catalyst. The rate of HDCl correlated with the C−Cl bond energy, suggesting scission of this bond in the molecularly adsorbed molecule is rate-determining. The measured reaction kinetics of the CH(4−x)Cl(x) compounds support a previously proposed Langmuir−Hinshelwood type reaction mechanism. Kinetic and isotope exchange experiments demonstrated the following: gas phase H2 and HCl are in equilibrium with surface H and Cl; adsorbed Cl is the most abundant surface intermediate; and irreversible scission of the first C−Cl bond is rate-determining. The overall hydrodechlorination reaction rate can be written as kK(R−Cl)[R−Cl]/(1 + K(HCl)[HCl]/K(H2)(1/2)[H2](1/2)). The activation energy of the rate-determining step was related linearly to the dissociation energy of the first C−Cl bond broken in a Brönsted−Evans−Polanyi relationship. This behavior is in agreement with a previous study of CF(3)CF(3−x)Cl(x) compounds. During the reaction of CH3Cl, CH2Cl2, and CHCl3 with deuterium, H−D exchange occurred in only 2%, 6%, and 9% of products, respectively. The increasing H−D exchange with Cl content suggests the steps which determine selectivity in these multipath, parallel reactions. The density functional theory (DFT)-calculated activation energies for the dissociation of the first C−Cl bond in the family of chlorinated methane compounds are in good agreement with the values extracted from kinetic modeling, suggesting that parameters estimated from DFT calculations may be used to estimate the reactivity of a particular chlorinated compound within a family of chlorocarbons.