Conversion under hydrogen of dichlorodifluoromethane and chlorodifluoromethane over nickel catalysts

We have studied the conversion of CFC-12 and HCFC-22 with hydrogen between 523 and 563 K at atmospheric pressure, over Ni, graphite-supported Ni (NiGr ), activated-carbon-supported Ni (NiAC), Ni-potassium oxide (NiKAC), Ni-cop per (NiCuAC) and Ni-aluminium oxide (NiAlAC) catalysts reduced at 623 K. Bu lk nickel, NiAC, and NiAlAC catalysts suffered an increase in activity duri ng the first 15 h on stream, while NiGr, NiCuAC and NiKAC suffered a decrea se in activity during this first period for the hydrodehalogenation reactio n of HCFC-22. The bulk nickel catalyst showed the highest activity, measure d as TOF, for the conversion of CFC-12 and HCFC-22 with hydrogen. This coul d be due to the different morphology of the nickel particles for this catal yst. During this first 15 h on stream, metal nickel phase is totally transf ormed into nickel carbide only for the NiAlAC catalyst. The new Ni3C phase seems to be more active than the nickel phase for the hydroconversion of CF C-12 and HCFC-22. This Ni3C phase has not been detected by XRD for the othe r catalysts. However, their formation at the surface level is not excluded. The best hydrodehalogenation reactions are those which first allow the rem oval of one chlorine atom during one sojourn on the surface of the catalyst s, and then allow the removal of two new halogen atoms. More dimerization c ompounds are also obtained as products, mainly for the hydrodechlorination of CFC-12. These dimerization reactions strongly compete with the hydrodeha logenation reaction with nickel catalysts. We propose a consecutive mechani sm for the hydrogenation of CFC-12 and HCFC-22 using nickel catalysts. (C) 1999 Elsevier Science B.V. All rights reserved.