Catalytic hydrodechlorination of chlorophenols

Feasibility studies have shown that complete destruction of chlorophenols by hydrodechlorination over palladium/carbon is possible under mild reaction conditions. When hydrogen was used as the reducing agent, all chlorophenols investigated could be cleanly hydrodechlorinated to phenol at 35°C and 35 psi (1 PSI=6.895 kPa). In general, increasing the degree of chlorine substitution on the phenol ring decreased the rate of dechlorination. Base was required for the reaction to occur, and ammonium hydroxide was found to be superior to sodium hydroxide, sodium acetate, and triethylamine. Aging studies using 4-chlorophenol as substrate showed dramatic improvement in catalyst lifetime when a 50/50 mixture of ethanol/water was used as the reaction solvent instead of ethanol. In the case of 4-chlorophenol which has significant water solubility, hydrodechlorination also worked well when water was the solvent. When hydrazine (sulfate or hydrate) was used as the reducing agent, all chlorophenols investigated could be cleanly dechlorinated to phenol without the need for a pressure apparatus. Except for pentachlorophenol which required a higher reaction temperature (50°C), dechlorinations were accomplished at room temperature. Using 4-chlorophenol as substrate, both water and 50/50 ethanol/water were found to be superior reaction solvents to ethanol.