Catalytic hyrodechlorination of chlorophenols in aqueous solution under mild conditions Original Research Article

Hydrogen-saturated water or sodium borohydride were inefficient sources of active hydrogen species for the palladium-alumina accelerated hydrodechlorination (HDC) and subsequent hydrogenolysis of 4-chlorophenol (4-CP) or 2,6-dichlorophenol (2,6-DCP) in water at ambient temperature and pressure. By contrast, hydrogen gas in the presence of Pd0/Al2O3 accelerator mediated both hydrodechlorination and partial hydrogenolysis to cyclohexanone. 4-CP or pentachlorophenol (PCP) was converted quantitatively to cyclohexanone in 1 h and subsequently reduced further to cyclohexanol (∼25 mol%) after 3 h. With ammonium formate, HDC of 4-CP was quantitative but hydrogenolysis to cyclohexanol was sluggish (∼14 mol% after 1 h). Whereas zero-valent iron or magnesium generated only traces (∼3 mol%) of phenol, nickel metal powder (Ni0) and Raney-type nickel (Ni–Al) but not NiCl2 mediated HDC/hydrogenolysis in the presence of NaBH4 but not H2. In these cases, hydrogenolysis, at somewhat elevated temperatures, generated cyclohexanol (14–35 mol%) and no cyclohexanone was observed. At elevated H2 pressure (414 kPa) and 333 K over Pd0/γ-Al2O3, hydrogenolysis of PCP to cyclohexanone was complete after 30 min. Further increase in pressure (to 4137 kPa) with N2 seemed to accelerate the reaction rate for shorter reaction times (10 min) but if CO2 served to increase P, the rates but not the course of reaction were retarded appreciably consistent with competitive inhibition of catalytic sites by CO2.