Fluorinated chromia catalytic fluorination

Direct evidence for a combination of catalytic fluorination and chlorination [4] was obtained from radiotracer studies in which fluorinated chromia catalysts were labelled with the short-lived (t /2 = 110 min) / + emitting isotope fluorine-18 [11]. Using this isotope it was possible to probe the interactions between HF and various fluorinated chromia catalysts more directly than had been possible hitherto. Three types of surface F-containing species were differentiated, weakly adsorbed HF which was easily removed by an inert gas flow, non-labile F, believed to be bound directly to surface Crin, and catalytically active F which could be incorporated into the organic products [12]. The controversy between dismutation (concerted F-for-Cl and Cl-for-F transfers) and non-concerted halogen exchange processes has been resolved more recently and the evidence is described later in the chapter. What is clear from this early work however, is the importance of aluminium and chromium(III) oxides as catalyst precursors. Fluorination of the surfaces of these oxides is slow (cf [12]) and although there are many references to alu-... 

On the basis of temperature programmed reduction and oxidation (TPR-TPO) measurements, it was proposed that the calcination of chromia used as a catalyst for the selective fluorination of CF3CH2C1 into CF3CH2F depends strongly on the gas used to calcine the precursor [50], Furthermore, the fluorination of CF3CH2C1 depends directly on the number of reversibly oxidizible chromium atoms in the catalysts. The oxidation/reduction properties are related closely to the atmosphere of pre-treatment and a linear relationship between the catalytic activity and the hydrogen uptake during the second reduction step has been found [51],... 

The derivation of a mechanism for a chemical reaction is by its very nature an uncertain process, being dependent critically on the nature and extent of the experimental evidence. Mechanisms that have at their heart a surface process or processes are even more uncertain and when the constraints imposed by the manipulation of HF are also taken into account, it is not surprising that there have been relatively few mechanistic studies made of heterogeneous catalytic fluorination. However a catalytic process cannot be said to be understood fully without a mechanism based on the experimental evidence available and such studies are helpful in the design of the next generation of catalysts. In most cases the work described below involves chromia or y-alumina based catalysts that have been pretreated according to the methods described above. Studies involving C2 and Q compounds are described in turn. 

Resume Criteria for an Active Fluorinated Chromia Catalyst 203 Catalytic Fluorination Leading to Hydrofluoroolefins 204 Production of Chloromethane A Green Perspective 205... 

Figure 7.3 Two Possible Pathways for Catalytic Reactions at a Fluorinated Chromia Surface Reversible Halogen Exchange, that is, F-for-CI and CI-for-F Oehydrochlorination Followed by...

Treatment of chromia under flow conditions and at moderate temperatures with a variety of C, CFCs or hydrochlorofluorocarbons (HCFCs) or Cj HFCs is an alternative way of producing a fluorinated chromia surface that is catalytically active [24]. It has the advantage that corrosive reagents such as anhydrous HE are avoided. XPS and X-ray excited Auger electron spectroscopy (XAES) demonstrate clearly the points in the processes where activation occurs formation of amorphous highly fluorinated regions at the surface is indicated. Description of local structure is less easily made, although a... 

It will be apparent from the topics described in the sections above that fluorinated chromia has subtleties that are not completely understood and in some catalytic systems have so far been described incompletely. Some factors are clear however. The surface properties depend crucially on the extent of surface prefluorination. Although many laboratory studies have employed CFC or better HFC reagents, such surfaces are normally lightly fluorinated only, and in the case of CFCs will inevitably contain surface chloride species that complicate interpretation of the catalysis. For large-scale applications, anhydrous HF is the prefluorination (and fluorination) reagent of choice. The process is slow and, even at the surface level, is probably never complete. This is beneficial, since complete conversion to a chromium(in) fluoride layer, it is generally agreed, would result in a catalyst whose activity was very low or which was inactive. 

Many different reaction schemes, as described in Figure 7.5, are considered and both homogeneous and heterogeneous catalysts are featured. Fluorinated chromia appears as the catalyst for the catalytic fluorination step that leads to the intermediate, CF3C1C=CH2, HFO-1233xf it is apparent that catalyst deactivation is a problem and for this reason addition of a basic molecule such as di-isopropylamine to the vapor feed is recommended. 

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