Kinetics of the Hydrodechlorination Reaction of Chlorinated Compounds on Palladium Catalysts (2003)

"\nHydrodechlorination is the reaction of a chlorinated organic compound (R-Cl) with hydrogen to form a carbon-hydrogen bond and HCl: R-Cl + H2 = R-H + HCl. This reaction is used in refrigerant manufacturing, industrial by-product reclamation and waste management. These practical applications require in-depth understanding of hydrodechlorination reaction. In this research work, we studied four families of chlorinated compounds; CF3CF3-xClx(x=1-3), CH4-xClx (x=1-4), CF4-xClx (x=1-4) and dichloropropanes (1,1-, 1,2-, 1,3-, 2,2-), on supported palladium catalysts to create a theory capable of predicting the hydrodechlorination rate on chlorinated compounds and to explore the reaction mechanism.\nA possible set of elementary reaction steps of hydrodechlorination reaction was proposed from our kinetics study of all these compounds. In this set of reaction steps, the irreversible scission of the first C-Cl bond in a chlorinated compound was proposed to be the rate-determining step; gas phase H2 and HCl were suggested to be in equilibrium with surface H and Cl species; adsorbed Cl was assumed to be the most abundant surface intermediate. The overall rate of hydrodechlorination reaction could be derived from these reaction steps as r=k'[R-Cl]/(1+K'[HCl]/[H2]0.5). In this rate equation, k'is the product of the adsorption equilibrium constant of the chlorinated compound on catalyst surface times the rate constant for the scission of the first C-Cl bond scission step, and K'is the square root of the equilibrium constant for the equilibrium between H2, HCl and their corresponding surface species: 2HCl + 2* = H2 + 2Cl*.\nThe hydrodechlorination reaction of CF3CFCl2 was performed in the presence of H37Cl to study the reversibility of C-Cl bond scission, and the removal of the first Cl atom from CF3CFCl2 was found to be an irreversible step. Hydrodechlorination experiments of CF3CFCl2 with D2 and HCl mixture revealed that D2 and HCl were in equilibrium with surface adsorbed hydrogen and chlorine during reaction. The forward rate and reverse rate of this equilibrium were at least 400 times higher than the overall hydrodechlorination rate. This result supported the assumption of equilibrium for 2HCl + 2* = H2 + 2Cl*. Additionally, the activation energy for the rate determining step was extracted from hydrodechlorination reaction kinetics results of CH4-xClx (x=1-4), CF4-xClx (x=1-4) and dichloropropanes (1,1-, 1,2-, 1,3-, 2,2-) compounds. It was found that for each of the series compounds, a linear relationship existed between C-Cl bond scission activation energy and gas phase C-Cl bond strength. This observation corroborates our assumption that the removal of the first Cl atom from a chlorinated compound is the rate-determining step in the hydrodechlorination reaction. Thus, all kinetic and isotope experimental results obtained from this study are consistent with the proposed reaction steps for the chlorinated compounds tested. This set of reaction steps can also be used to predict the hydrodechlorination reaction rate of a chlorinated compound, once its gas phase C-Cl bond energy is calculated and the turnover rate of a reference chlorinated compound with similar structure is known.\nSome work has been done to study hydrodechlorination reaction steps and reaction intermediates beyond the rate-limiting step. Isotope tracing experiments with D2 indicated that CH3-, CH2- groups adjacent to a C-Cl bond could undergo deuterium exchange. The study of reactions steps using ab initio methods, including calculation of rate constants, is also under way. Calculations for the CH4-xClx (x=1-4) family showed that the heat of adsorption and C-Cl bond dissociation energy on a Pd surface were linearly related to their gas phase C-Cl bond strength."