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

The hydrogenolysis of CCl3F over 1 wt.% palladium, platinum, rhodium, ruthe nium, 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, prop ane, methyl chloride, and CH2ClF were also observed. The behaviour of the i nvestigated 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: ov er 90% has been measured in large-range conditions. Rhodium catalyst exhibi ts an intermediate behaviour with high selectivities for methane. The adsor ption of chlorine in the surface plays an important role in the product dis tribution: strong chlorine adsorption leads to an increase in the selectivi ty for CHCl2F. No important deactivation was found in the period studied (8 0 h), except in the case of iridium catalyst;platinum and ruthenium even in crease their activity with time-on-stream. The effect of the temperature and H-2/CFC ratio were also studied. The main conclusion is that the Cl coverage decreases with the concentration of H-2 and, associated herewith, the selectivity for Cl-containing products, in p articular CHCl2F decreases; selectivity for methane and alkanes is not very much affected by the conditions studied. The results show that the dehalog enation reactions do not occur according to serial kinetics and that the al kane formation takes place according to a parallel pathway. Apparently, the same reaction network applied in the hydrogenolysis of dichlorodifluoromet hane [E.J.A.X. Van de Sandt, A. Wiersma, M. Makkee, H. Van Bekkum, J.A. Mou lijn, Red. Trav. Chim. Pays-Bas 115 (1996) 505] can explain the product spe ctra of the hydrogenolysis of CCl3F, considering that the key carbene inter mediate (CClF2 .) is not so stable as difluorocarbene, key intermediate in CCl2F2 hydrogenolysis. (C) 2001 Elsevier Science B.V. All rights reserved.