Role of chlorocarbon oligomers in the room-temperature heterogeneous catalytic fluorination of hydrochlorocarbons to hydrochlorofluorocarbons

Abstract

Radiotracer [ 18F] and [ W l ] experiments demonstrate the deposition of oligomeric chlorocarbon species on fluorinated metal oxides and trapping of hydrochlorocarbon monomers, both without loss of surface fluoride lability, thus accounting for the unusual room-temperature heterogeneous catalytic fluorination of CH3CC13 and related species by anhydrous HF.

γ-Alumina-supported boron trifluoride: Catalysis, radiotracer studies and computations

“…Similar behaviour has been observed also on the Brönsted and Lewis acid, g-alumina that had been chlorinated using carbon tetrachloride [19]. On SF 4 -fluorinated g-alumina, these reactions are accompanied by the fluorination of CH 3 CCl 3 to give the HCFC, CH 3 CCl 2 F, an observation that was crucial in the use of the material as a catalyst for room temperature halogen exchange [11,20]. The reactions involved are summarised in Scheme 2.…”

“…The results of sequential BF 2 18 F additions to galumina and to amorphous chromia, prepared by the 'volcano' thermal decomposition of ammonium dichromate, are shown in Table 1. The uptakes of fluorine so determined, after four additions of BF 2 18 F in each case, were 2.05 and 1.45 mg atom F (g solid) À1 , respectively, equivalent to 3.9 and 2.75 wt.% F. The corresponding figure obtained previously for the uptake of fluorine by g-alumina after exposure to SF 3 18 F under comparable conditions is 15 mg atom F (g solid) À1 (28.5 wt.%) [15].…”

“…Scheme 2. Behaviour of CH 3 CCl 3 on SF 4 -fluorinated g-alumina [19,20]. On the basis of our previous model for supported organic layer catalysts [20], it is concluded that both fluorinated precursors have the ability to oligomerise CH 2 CCl 2 (reaction ii in Scheme 2) hence forming an organic layer supported on the partially fluorinated surface that is able to trap CH 3 CCl 3 at the surface.…”

“…Behaviour of CH 3 CCl 3 on SF 4 -fluorinated g-alumina [19,20]. On the basis of our previous model for supported organic layer catalysts [20], it is concluded that both fluorinated precursors have the ability to oligomerise CH 2 CCl 2 (reaction ii in Scheme 2) hence forming an organic layer supported on the partially fluorinated surface that is able to trap CH 3 CCl 3 at the surface. Both fluorinated oxides therefore are capable of behaving as Lewis acids; in the case of the SF 4 -fluorinated oxide the active site has been proposed to involve Al III in a disordered oxide/fluoride environment [14], while the active site in the BF 3 -alumina is likely to be identical or related to one of the species shown in Scheme 1.…”

Investigation of fluorinated surfaces by means of radio-labelled probe molecules

“…Not surprisingly, g-alumina that has been fluorinated by SF 4 is also a suitably acidic surface on which to perform the chemistry. The combination of a pool of labile F, demonstrated by its room temperature [ 18 F] exchange behaviour towards H 18 F, SF 3 18 F and BF 18 F [39], and an oligomeric layer, with which hydrochlorocarbons are miscible, makes the fluorinated alumina-chlorocarbon oligomer composite an active catalyst for room temperature F-for-Cl halogen exchange reactions on C 2 and C 3 hydrochlorocarbons, CH 3 CCl 3 and others, that are susceptible to dehydrochlorination [40].…”