Fluorinated chromia

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-... 

Fluorinated chromia used to catalyze the isomerization reaction of CHF2CHF2 to CF3CH2F, also exhibited evidence for the importance of chromium in higher oxidation states. FTIR spectroscopic measurements of CO adsorption confirmed the occurrence of Cr4+ and Cr5+ on the surface of chromia catalysts before being used [52]. During the activation, Cr4+ and Cr5+ sites were reduced and enhanced activity of the catalyst was observed. The reaction pathway proposed for isomerization involves the formation of hydrogen fluoride due to the degradation reaction of the fluoroalkane. 

XPS and TEM measurements indicate that after fluorination, zinc and nickel, on the surface of these catalysts is present as ZnF2 or NiF2, respectively and these findings support the idea that the Zn(II) and Ni(II) species on doped fluorinated chromias do not become integrated into the surface structure but are, in fact, distinct phases on the surface. 

Fig. 13. Intermediates proposed for mono- and di-exchange of Cl by F in CC14 at a fluorinated chromia surface (reproduced with permission from J. Catal. 174 (1998) 219 [104]).

In neither case was it possible to propose definitive mechanisms due to the complexity of the systems in the 7-alumina study, it is suggested that adsorption-desorption processes are slow relative to rapid dismutation between two adsorbed species [105], while from the chromia study mono-molecular halogen exchange reactions with metal halide surface sites are indicated [38], The latter mechanism is reminiscent of the halogen exchange model proposed [95] for C2 CFCs on fluorinated chromia. 

Heterogeneous Catalysts Used for Large-Scale Syntheses of Selected Chlorohydrocarbons and Fluorohydrocarbons Fluorinated Chromia and eta-Alumina... 

Fluorinated Chromia and the Montreal Protocol and its Successors and the Kyoto Protocol 196... 

Fundamental Properties of Fluorinated Chromia 198 Attempts to Describe Active Sites on Fluorinated Chromia 199 Synthesis of 1,1,1,2-Tetrafluoroethane Over Fluorinated Chromia 200 Oxidation States of Chromium 200 Doped Chromia Catalysts 201... 

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

Figure 7.1 Contrasting Pathways from CCI2FCCIF2 at Fluorinated Alumina and Fluorinated Chromia.

The following discussion examines the role of the two materials under consideration. Section 2 reviews fundamental issues connected with the role of fluorinated chromia as an active catalyst for a variety of reactions Section 3 considers physicochemical attributes of the acid site distribution associated with rj-alumina catalysts as applied to a single reaction, namely the synthesis of methyl chloride via the hydrochlorination of methanol. 

The drop-in replacement selected for the most widely used CFC refrigerant, CCljFj, was 1,1,1,2-tetrafluoroethane, CHjFCFj (Figure 7.2). One of the routes to this compound used the existing technology for the fluorinated chromia-catalyzed route to CCljFCFj followed by a hydrodechlorination step the other used a new zinc-doped chromia catalyst precursor in a two-step process to convert CHQ=CCl2 to CH2FCF3 directly [16]. The doped chromia developed is described later in this chapter. 

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...

Attempts to Describe Active Sites on Fluorinated Chromia... 

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... 

Equations (1 )-(3) represent a synthesis scheme from the patent literature, using various catalysts including fluorinated chromia. Equation (4) is a fluorinated chromia-catalyzed reaction receiving... 

Resume Criteria for an Active Fluorinated Chromia Catalyst... 

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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