A Kinetic and Mechanistic Study of the Cl/F Exchange Reaction of CCl₃F, CCl₂F₂, and CClF₃ with Prefluorided Chromia

Abstract

Chromium oxide has been fluorided to several monolayers depth by interaction with HF at 625 K. The interaction of CCl 3 F and CCl 2 F 2 with these surface chromium fluoride layers has been studied by temperatureprogrammed reaction. In the case of the reaction of CCl 3 F, mono-and bi-exchange occurs with the surface monolayer fluoride, forming CCl 2 F 2 and CClF 3 simultaneously at a peak maximum temperature of 520 K. Exchange with the subsurface fluoride layers occurs at a peak maximum temperature of 680 K, as the fluoride ions from the bulk migrate to the surface and chloride ions on the surface migrate inward. In both temperature regions, mono-exchange predominates by a factor of about 10 2 . Line-shape analysis of the temperature dependence of the rates of production of CCl 2 F 2 and CClF 3 gave activation energies of 53 and 54 kJ mol -1 for exchange with the surface fluoride, so the difference in rates must be the need for special dimer-type surface sites for the bi-exchange. The activation energy for exchange of F ions evolving from the bulk calculated by line-shape analysis is 83 kJ mol -1 for mono-exchange and 100 kJ mol -1 for bi-exchange. These higher values than those obtained by reaction with the surface fluoride ion derive from the added activation energy for migration of the fluoride ion from the bulk to the surface.

Zinc promoted alumina catalysts for the fluorination of chlorofluorocarbons

“…Steady state catalyst activity data clearly demonstrate that there is an optimum loading of zinc for the fluorination of CF2ClCFCl2 (Figure 3 [22]. This suggests that, on the chromia catalysts, mono and di-fluorinated products are simultaneously produced with CClF3 being created at a single site where two F atoms can be simultaneously delivered from the surface.…”

A review of CFC and halon treatment technologies – The nature and role of catalysts

“…widely studied, because it is the most widely used CFC and is estimated to account for 45% of all CFCs [111]. The other most commonly used CFC is CCl 3 F (CFC-11) which can be converted selectively to CCl 2 F 2 via Cl/F exchange reactions [112,113]. As shown in table 2, mainly Group VIII metals have been used for these reactions and Pd, in particular, is considered as the catalyst of choice due to its properties of selectively replacing chlorine with hydrogen and its resistance to hydrogen halides produced during reaction.…”