Kinetic and thermodynamic analyses of catalytic hydrodechlorinations in supercritical carbon dioxide (SC-CO2) were performed using 5% Pd supported on γ-Al2O3. The selected standard compounds used for the study represented chlorinated wood resins commonly found in pitch deposits; 1-chlorooctadecane (C18-Cl), 9,10-dichlorostearic acid (Stearic-Cl2), and 12,14-dichlorodehydroabietic acid (DHA-Cl2). 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-CO2 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 C18-Cl, Stearic-Cl2, DHA-Cl2 were 43 ± 5, 40 ± 7, and 135 ± 7 kJ mol-1, respectively, in SC-CO2. The relatively high activation energy for DHA-Cl2 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 ± 3 kJ mol-1, leading to the conclusion that the rate-determining 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.

Additional Metadata
Keywords Carbon dioxide, Catalytic hydrodechlorination, Pitch deposits, Supercritical fluids
Persistent URL dx.doi.org/10.1016/S0926-3373(03)00002-X
Journal Applied Catalysis B: Environmental
Citation
Aikawa, B., Burk, R, & Sitholé, B.B. (2003). Catalytic hydrodechlorination of 1-chlorooctadecane, 9,10-dichlorostearic acid, and 12,14-dichlorodehydroabietic acid in supercritical carbon dioxide. Applied Catalysis B: Environmental, 43(4), 371–387. doi:10.1016/S0926-3373(03)00002-X