Protection of palladium catalysts for hydrodechlorination of chlorinated organic compounds in wastewaters

Palladium is an efficient catalyst for hydrodechlorination (HDC) of aliphatic and aromatic chlorinated organic compounds in water. However, its applicability in water treatment processes is limited because of its sensitivity to catalyst poisons. In the present study, Pd/Al2O3 was modified by various silicone-based fillings and coatings. The modified catalysts proved to be active in the HDC of chlorobenzene with APd = 35–65 L gPd−1 min−1 vs. 570 L gPd−1 min−1 for the unprotected catalyst. It was insusceptible against sulphite as a model catalyst-deactivating ionic water constituent. However, under HDC conditions the silicone appeared to be degraded, becoming gradually permeable for sulphite. Hydrolysis of the silicone polymers, initiated and promoted by the HDC product HCl, is assumed to be responsible for the polymer ageing. Release of chloride from the coated catalyst into the bulk water phase has been revealed as a crucial step in this concept of hydrophobic catalyst protection. It could be shown that with Pd/Al2O3, HDC also proceeds in dry organic solvents such as n-hexane at significant reaction rates (APd = 18–28 L gPd−1 min−1).

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► Protection for Pd is effective against ionic catalyst poisons and macromolecules.
► PDMS coats allow several catalytic cycles but the silicone slowly ages.
► Fast and complete release of chloride from coated catalyst is a crucial step.
► Hydrodechlorination proceeds in dry organic solvents such as n-hexane.
► The protection strategy is a big step forwards for the utilization of Pd catalysts.