Catalytic hydrodechlorination of 1-chlorooctadecane, 9,10-dichlorostearic acid, and 12,14-dichlorodehydroabietic acid in supercritical carbon dioxide

Kinetic and thermodynamic analyses of catalytic hydrodechlori nations in supercritical carbon dioxide (SC-M) were performed using 5% Pd supported on gamma-Al(2)O(3). The selected standard compounds used for the study represented chlorinated wood resins commonly found in pitch deposits; 1-chlorooctadecane (C(18)-Cl), 9,10-dichlorostearic acid (Stearic-O(2)), and 12,14-dichlorodehydroabietic acid (DHA-Cl(2)). The reaction utilized isopropanol as a hydrogen donor. Pressure, temperature, and the concentrations of isopropanol and palladium were varied to study the effect of each parameter and to optimize the dechlorination yield. The reaction in SC-CO(2) was compared to the one in liquid solvents at atmospheric pressure. By applying a Langmuir-Hinshelwood kinetic model, the rate-determining step of the reaction was deduced to be adsorption of the chlorinated molecules on the palladium surface. The apparent activation energies of the reactions for C(18)-Cl, Stearic-Cl(2), DHA-Cl(2) were 43 +/- 7 40 +/- 7, and 135 +/- 7 mol(-1), respectively, in SC-CO(2). The relatively high activation energy for DHA-Cl(2) was apparently due to structural differences from the other two compounds. The apparent activation energy of dechlorination of C18-Cl in liquid isopropanol at atmospheric pressure was determined to be 35 +/-L 3 kJ mol(-1), leading to the conclusion that the rate-deter-mining step is the same for this compound in both fluid systems. The enthalpies of desorption of stearic acid and dehydroabietic acid were determined to be 18 +/- 2 and 12 +/- 2 kJ mol(-1), respectively. These values being less than half of the apparent activation energies of dechlorination of their corresponding chlorinated compounds indicates that desorption of the dechlorinated products is not the rate-determining step of the reaction. This was consistent with the conclusion that the rate-determining step is adsorption, on the understanding that the reaction mechanism is same in both fluid systems. (C) 2003 Elsevier Science B.V. All rights reserved.