COMPARISON OF THE PERFORMANCE OF ACTIVATED CARBON-SUPPORTED NOBLE-METAL CATALYSTS IN THE HYDROGENOLYSIS OF CCL2F2

The hydrogenolysis of CCl2F2 over 1 wt% palladium, platinum, rhodium, ruthenium, iridium, and rhenium on activated carbon has been studied i n a micro-flow reactor, in a temperature range of 450-540 K, H-2/CCl2F 2 feed ratios between 1.5 and 6, a pressure of 0.4 MPa, and a WHSV of 1 g/(g.h). The main products of the reaction for all investigated cata lysts were CHClF2, CH2F2, and methane. According to their performance, the catalysts could be divided into four groups: rhenium showing no c onversion of CCl2F2, palladium with a high selectivity for CH2F2, irid ium and ruthenium with a high selectivity for CHClF2 and platinum and rhodium with moderate selectivity for CHClF2 and CH2F2. The adsorption of chlorine on the metal surface plays an important role in the selec tivity. Strong chlorine adsorption leads to a higher selectivity for C HClF2. These results are consistent with a reaction mechanism in which difluorocarbene is the key intermediate. Apparently, the same kinetic network applies to all metals studied. The performance of the catalys ts changed as a function of time on stream. Palladium, rhodium, and es pecially ruthenium deactivated during reaction, whereas the activity o f iridium and platinum increased. This can be explained by two opposit e effects. On the one hand, the dispersion of all catalysts increased during reaction, which can explain an increase in activity as a functi on of time on stream. Apparently, CCl2F2 hydrogenolysis conditions are suitable for dispersing noble metal catalysts on activated carbon. On the other hand, deactivation takes place by the adsorption of chlorin e and deposits of heavy halogenated products. (C) 1998 Academic Press.