Conversion Under Hydrogen of Dichlorodifluoromethane over Supported Palladium Catalysts

The conversion of difluorodichloromethane has been studied in the gas phase between 433 and 523 K at atmospheric pressure aver Pd black and Pd supported on alumina, graphite, or AlF3. In CF2Cl2 hydrogenation, CH2F2 and CH4 represented more than 95% of the products. The catalytic properties of Pd/AlF3 samples are unchanged with time, but Pd/graphite. Pd/Al2O3 and Pd black suffered changes of activity and/or selectivity during the first few hours on stream. This was ascribed to the diffusion of halide species into the bulk of palladium, and transformation of Al2O3 to AlF3. At the steady state, the kinetics of CF2Cl2 hydrogenation can be described either by a halogenation/dehalogenation of the Pd surface by CF2Cl2 and H2, respectively, or by a classical Langmuir-Hinshelwood mechanism. It was concluded that at 453 K the interaction between the Pd surface and CF2Cl2 or H2 is of the same order of magnitude. The CH2F2/CH4 selectivity ratio was the lowest on Pd/graphite and the highest on Pd/AlF3. It is proposed that adsorbed, or absorbed, halide species are responsible for the loss of CH2F2 selectivity. The high selectivity ratio on Pd/AlF3 is ascribed to a cooperative effect between Pd and AlF3.