Hydrogen-transfer hydrodechlorination of polychlorinated dibenzo-p-dioxins and dibenzofurans catalyzed by supported palladium catalysts

Catalytic dechlorination of 2,7-dichlorodibenzo-p-dioxin (2,7-DCDD), 1,2,3,4-tetrachlorodibenzo-p-dioxin (1,2,3,4-TCDD), and 2,8-dichlorodibenzofuran (2,8-DCDF) was carried out in a solution of NaOH in 2-propanol in the presence of a supported Pd catalyst. At an initial concentration of 100–200 μg/ml, 2,7-DCDD and 2,8-DCDF were converted to chlorine-free products (dibenzo-p-dioxin (DD) and dibenzofuran (DF), respectively) in high yield (>80%) even at 30 °C. The activity of the supported Pd catalyst depended on the catalyst support, decreasing in the order Pd/C>Pd/Al2O3>Pd/TiO2. Catalytic dechlorination of 1,2,3,4-TCDD proceeded successfully, although a higher temperature (45 °C) was required to complete dechlorination. From analysis of the partially dechlorinated species formed during the reaction (trichlorodibenzo-p-dioxin (TrCDD), dichlorodibenzo-p-dioxin (DCDD), and monochlorodibenzo-p-dioxin (MCDD)), we postulated the main dechlorination pathway to be 1,2,3,4-TCDD→1,2,3-TrCDD→1,2-DCDD→1-MCDD→DD. Using deuterium-labeled 2-propanol as a solvent, we found that the α-hydrogen of 2-propanol preferentially transferred to the organic chlorines. The added NaOH was likely to promote the dechlorination reaction, with the rate of dechlorination increasing with increasing NaOH concentration for NaOH/Clsubstrate ratios greater than unity.