Performance of activated carbon-supported noble metal catalysts in the hydrogenolysis of CCl3F

The hydrogenolysis of CCl3F over 1 wt.% palladium, platinum, rhodium, ruthenium, and iridium on activated carbon has been studied over a wide range of conditions. The main products of the reaction were methyl fluoride (CH3F), methane, and dichlorofluoromethane (CHCl2F); small amounts of ethane, propane, methyl chloride, and CH2ClF were also observed. The behaviour of the investigated catalysts is quite different. Platinum and palladium exhibit a high selectivity for CH3F, whereas ruthenium and iridium are more selective for CHCl2F. The latter exhibits a remarkably high selectivity to CHCl2F: over 90% has been measured in large-range conditions. Rhodium catalyst exhibits an intermediate behaviour with high selectivities for methane. The adsorption of chlorine in the surface plays an important role in the product distribution; strong chlorine adsorption leads to an increase in the selectivity for CHCl2F. No important deactivation was found in the period studied (80 h), except in the case of iridium catalyst; platinum and ruthenium even increase their activity with time-on-stream. fect of the temperature and H2/CFC ratio were also studied. The main conclusion is that the Cl coverage decreases with the concentration of H2 and, associated herewith, the selectivity for Cl-containing products, in particular CHCl2F, decreases; selectivity for methane and alkanes is not very much affected by the conditions studied. The results show that the dehalogenation reactions do not occur according to serial kinetics and that the alkane formation takes place according to a parallel pathway. Apparently, the same reaction network applied in the hydrogenolysis of dichlorodifluoromethane [E.J.A.X. Van de Sandt, A. Wiersma, M. Makkee, H. Van Bekkum, J.A. Moulijn, Recl. Trav. Chim. Pays-Bas 115 (1996) 505] can explain the product spectra of the hydrogenolysis of CCl3F, considering that the key carbene intermediate (CClF2) is not so stable as difluorocarbene, key intermediate in CCl2F2 hydrogenolysis.