Effect of metal dispersion on the liquid-phase hydrodechlorination of 2,4-dichlorophenol over Pd/Al2O3

The effect of metal particle size on the aqueous-phase (T = 303 K) catalytic hydrodechlorination (HDC) of 2,4-dichlorophenol (2,4-DCP) over Pd/Al2O3 has been investigated. A range of palladium dispersions (8%-67%) was achieved using a H-2 thermal treatment over the temperature range of 423 K :5 T:5 1273 K. The catalyst samples have been characterized in terms of transmission electron microscopy (TEM), X-ray diffraction (XRD), surface area/porosity, H-2 chemisorption, and point-of-zero-charge measurements. The HDC of 2,4-DCP is predominately a stepwise process, yielding 2-chlorophenol (2-CP) as the partially dechlorinated product, which is further converted to phenol and, ultimately, to cyclohexanone. The temporal dependence of product distribution is recorded and correlated to metal particle size/bulk solution pH changes. Structure sensitivity has been established where smaller palladium particles (<= 5 nm) exhibit intrinsically higher specific activities. The HDC of 2,4-DCP has also been investigated under conditions of controlled pH (pH(acid) = 3 and pH(basic) = 13). At pH(acid), a greater palladium dispersion delivers greater 2-CP selectivities, as a result of repulsion between chlorophenolic species in solution and a supported Pd-H+ adduct; HDC selectivity at pH(basic) is insensitive to palladium particle size. Bulk palladium was inactive under the same reaction conditions, necessitating lower C1/Pd ratios, but it promoted the removal of both chlorines to a greater extent than that which was observed for Pd/Al2O3. We express, for the first time, HDC performance in terms of solution ecotoxicity and record an 81% decrease in toxicity for reaction at a higher palladium dispersion and pH(basic) value. The results establish that catalytic HDC is an effective means of detoxifying chlorophenol-contaminated water.