Catalysis of Organic..

Pohlmann et al 479Experimental SectionAll catalysts were prepared using activated powder carbons using slurry methods.After precipitation of the metal salt, some of the catalysts were reduced. Somecatalysts were dried after preparation and subsequently stored in a dry environment.Liquid phase hydrogenation of cinnamic acid or nitrobenzene at low pressures wasperformed to investigate the hydrogenation activity of each catalyst. The leachingresistance of the catalysts was investigated by stirring the catalysts in a solution ofammonium chloride. The resulting mixtures were filtered and the mother liquor wasanalyzed for Pd by inductively coupled plasma (ICP). Precious metal dispersionsexperiments by CO chemisorption at 25 °C on the catalysts and temperatureprogrammed reduction (TPR) experiments were carried out using a MicromeriticsAutochem 2910 unit by heating the sample from –35 °C to 700 °C at a rate of30 °C/min in a 5% H 2 /95% Ar mixture. The effluent gases were analyzed using aTCD to monitor the changes in the composition of the reducing gas as a function ofthe temperature.ConclusionsSeveral carbon supported palladium catalysts were tested for hydrogenation activity,metal dispersion and metal leaching. These tests were repeated over a period of eightweeks. While the amount of metal leached reduces over time, activity and metaldispersion of the catalysts remains relatively constant. This trend was observed foreggshell and uniform type catalysts. Reduced catalysts showed a higher amount ofpalladium leached. The amount of palladium leaching was reduced over the period ofthe investigation. This reduced leaching effect was stronger for the catalysts thatwere stored in a wet form. TPR experiments of the reduced catalysts showed that thewet catalysts were slowly oxidized during the storage time, while the degree ofoxidation of the dry catalysts was minimal. This oxidation effect is thus suspected tobe the cause for the lower amount of palladium leaching of reduced catalysts after astorage time of several weeks. In a second part of this investigation, it was shownthat palladium on activated carbon catalysts show a stronger resistance against metalleaching when heat-treated at temperatures higher than 200 °C compared to the nonheat-treated analogues. Since the hydrogenation activity at low pressures and themetal dispersion was also reduced for the heat treated samples, it is probable that thiseffect is caused by sintering of the palladium crystallites of the catalyst.References1. The previous study was presented as a poster at the 19 th North AmericanCatalysis Society Meeting in 2005 (P-182)2. A. J Bird, D. T. Thompson, Catalysis in Organic Syntheses, 91, 61-106 (1980).3. T. J. McCarthy, B. Chen, M. L. Ernstberger, F. P. Daly, Chemical Industries(Dekker), 82, (Catalysis of Organic Reactions), 63-74 (2001).