Deactivation of a Pd/AC catalyst in the hydrodechlorination of chlorinated herbicides

This work analyses the stability of a palladium on activated carbon (Pd/AC) catalyst in the ambient-like conditions hydrodechlorination (HDC) of the organochlorinated herbicides 4-chloro-2-methylphenoxyacetic acid (MCPA) and 2,4-dichlorophenoxyacetic acid (2,4-D) as well as 2,4-dichlorophenol (2,4-DCP), a precursor in the synthesis of the second. Continuous long term experiments (100 h time on stream) were performed at mild operating conditions (30 °C, 1 atm). The composition of the reaction effluents was analyzed and their ecotoxicity (Microtox) measured. In all cases, a significant decrease of ecotoxicity was observed due to the high dechlorination achieved. The Pd/AC catalyst maintained a constant activity along the HDC of 2,4-DCP, while it suffered an important deactivation in the HDC of 2,4-D and MCPA. From characterization of the fresh and used catalyst the adsorption/deposition of reaction byproducts on the active sites can be recognized as the main cause of deactivation. The use of activated carbon as support reduces the negative effect that the released chloride ions usually provoke on other HDC catalysts.