Hydrodechlorination of CCl2F2 (CFC-12) over γ-alumina supported palladium catalysts

Alumina displays low catalytic activity at the initial stage of the reaction of CCl2F2 with hydrogen, giving mainly halogen exchange products, and this activity quickly decays with time-on-stream. In the case of Pd/Al2O3 catalysts, the contribution of the support is negligible at 180 degrees C. Catalytic activity of Pd/gamma-Al2O3 in CCl2F2 hydrodechlorination strongly depends on metal dispersion: poorly dispersed Pd samples exhibit the highest turnover frequencies. The same samples also show the highest selectivities towards the formation of CH2F2. Time-on-stream behaviour and considerable amounts of carbon found in used catalysts suggest that the catalytic properties of Pd/Al2O3 are regulated by incorporation of carbon into Pd lattice, or the formation of Pd carbide. Poorly dispersed Pd catalysts contain a higher proportion of plane atoms and, therefore, are subjected to a more severe carbiding. Such a transformation generates surfaces which bind freon molecules less strongly, resulting in higher activity and selectivity to partial dehalogenation, i.e formation of CH2F2. High-temperature reduction at 600 degrees C does not much change the overall activity of Pd/Al2O3. However, the selectivity to CH2F2 is somewhat increased. It is believed that the Pd-Al2O3 interface changes upon high-temperature reduction, leading to a Pd-Al compound. At corrosive conditions of hydrodehalogenation of CCl2F2, the Pd-Al would be converted to AlFx species much more easily than the Al2O3 species at the Pd-Al2O3 interface of mildly reduced Pd/Al2O3 catalysts. (C) 1998 Elsevier Science B.V.