Conversion under hydrogen of dichlorodifluoromethane and chlorodifluoromethane over nickel catalysts

摘要

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-copper (NiCuAC) and Ni-aluminium oxide (NiAlAC) catalysts reduced at 623 K. Bulk nickel, NiAC, and NiAlAC catalysts suffered an increase in activity during the first 15 h on stream, while NiGr, NiCuAC and NiKAC suffered a decrease in activity during this first period for the hydrodehalogenation reaction of HCFC-22. The bulk nickel catalyst showed the highest activity, measured as TOF, for the conversion of CFC-12 and HCFC-22 with hydrogen. This could be due to the different morphology of the nickel particles for this catalyst. During this first 15 h on stream, metal nickel phase is totally transformed 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 CFC-12 and HCFC-22. This Ni_3C phase has not been detected by XRD for the other catalysts. However, their formation at the surface level is not excluded. The best hydrodehalogenation reactions are those which first allow the removal of one chlorine atom during one sojourn on the surface of the catalysts, and then allow the removal of two new halogen atoms. More dimerization compounds are also obtained as products, mainly for the hydrodechlorination of CFC-12. These dimerization reactions strongly compete with the hydrodehalogenation reaction with nickel catalysts. We propose a consecutive mechanism for the hydrogenation of CFC-12 and HCFC-22 using nickel catalysts.