Conversion of chlorophenols into cyclohexane by a recyclable Pd-Rh catalyst

Mono and polychlormated phenols that undergo only partial detoxification by conventional hydrogenation methods undergo exhaustive hydrogenation in the presence of a combined silica sol-gel entrapped catalyst composed of metallic palladium and chloro(1,5-cyclooctadiene)rhodium dimer. While the mono-, di-, tri- and tetrachlorophenols are converted into cyclohexane in a variety of solvents at 120 degrees C under 27.6 bar H-2, the pentachlorophenol requires the presence of toluene. Experiments conducted under milder conditions revealed that the hydrodechlorination of the chlorophenols proceeds via gradual evolvement of the chlorine atoms forming HCl and phenol. The C=C bonds of the latter compound are saturated stepwise. The 1-cyclohexenol accumulates as cyclohexanone and is further hydrogenated to cyclohexanol. HCI-catalyzed dehydration yields cyclohexene which is readily transformed to cyclohexane. The initial hydrodechlorination of 2-chlorophenol follows a first order rate law with rate constant k = 5.0 x 10(-4) s(-1) at 100 degrees C. The ceramic combined catalyst is leach-proof and recyclable and can be used in at lest six runs without loss in catalyst activity. (c) 2005 Elsevier B.V. All rights reserved.