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 AlF 3 . In CF 2 Cl 2 hydrogenation, CH 2 F 2 and CH 4 represented more than 95% of the products. The catalytic properties of Pd/AlF 3 samples are unchanged with time, but Pd/graphite. Pd/Al 2 O 3 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 Al 2 O 3 to AlF 3 . At the steady state, the kinetics of CF 2 Cl 2 hydrogenation can be described either by a halogenation/dehalogenation of the Pd surface by CF 2 Cl 2 and H 2 , respectively, or by a classical Langmuir-Hinshelwood mechanism. It was concluded that at 453 K the interaction between the Pd surface and CF 2 Cl 2 or H 2 is of the same order of magnitude. The CH 2 F 2 /CH 4 selectivity ratio was the lowest on Pd/graphite and the highest on Pd/AlF 3 . It is proposed that adsorbed, or absorbed, halide species are responsible for the loss of CH 2 F 2 selectivity. The high selectivity ratio on Pd/AlF 3 is ascribed to a cooperative effect between Pd and AlF 3 .Copyright 1993, 1999 Academic Press