Aqueous-phase hydrodechlorination of 2,4-dichlorophenol over Pd/Al ₂O₃: Reaction under controlled pH

The aqueous-phase hydrodechlorination (HDC) of 2,4-dichlorophenol (2,4-DCP) over a Pd/Al₂O₃ catalyst has been studied with adjustment of the bulk solution pH either by base (NaOH and NH₄OH) addition prereaction or with a continual supply of base during HDC. With a starting pH in the range 7-11.6, the solution pH can be controlled to within ±0.1 with an external supply of NaOH during HDC. A preliminary kinetic analysis suggests that 2,4-DCP HDC is predominately stepwise, yielding 2-chlorophenol (2-CP) as the partially dechlorinated product, which is further converted to phenol and ultimately to cyclohexanone. Pd/Al₂O₃, operating under controlled pH, delivered maximum HDC activity in the pH range 7-8, where the selectivity with respect to 2-CP was lower than that associated with pH = 11.6. In the absence of mass transport constraints, initial HDC rate as a function of 2,4-DCP concentration exhibits Langmuir-Hinshelwood mechanistic behavior involving competitive adsorption of H2 and 2,4-DCP with the surface addition of two H atoms as rate limiting. The pH effect on initial HDC rate can be represented by a generic correlation that demonstrates a 4-fold increase in 2,4-DCP adsorption, where solution pH was lowered from 13 to 7 but there was a pH insensitivity with respect to the rate constant for surface reaction. © 2007 American Chemical Society.