Fluorinated alumina

Mote stable catalysts ate obtained by using fluorinated graphite or fluorinated alumina as backbones, and Lewis acid halides, such as SbF, TaF, and NbF, which have a relatively low vapor pressure. These Lewis acids ate attached to the fluorinated soHd supports through fluorine bridging. They show high reactivity in Friedel-Crafts type reactions including the isomerization of straight-chain alkanes such as / -hexane. 

The fluorination of chromium oxide caused an increase of surface site Lewis acidity. Kemnitz and al.[12] as well as Peri [13], showed that on fluorinated alumina the progressive substitution of F for O and OH led, thanks to inductive attracting effect of fluorine, to an increase of the Lewis acidity of a sites. Hence the dehydrofluorination reaction was ftivoured on strong acide sites. 

A similar type of catalyst including a supported noble metal for regeneration was described extensively in a series of patents assigned to UOP (209-214). The catalysts were prepared by the sublimation of metal halides, especially aluminum chloride and boron trifluoride, onto an alumina carrier modified with alkali or rare earth-alkali metal ions. The noble metal was preferably deposited in an eggshell concentration profile. An earlier patent assigned to Texaco (215) describes the use of chlorinated alumina in the isobutane alkylation with higher alkenes, especially hexenes. TMPs were supposed to form via self-alkylation. Fluorinated alumina and silica samples were also tested in isobutane alkylation,... 

A. Mantilla-Ramirez, G. Ferrat-Torres, J.M. Dominguez, C. Aldana-Rivero, and M. Bernal. Influence of reaction parameters and comparison of fluorinated alumina and silica supports in the heterogeneous alkylation of isobutane with olefins. Appl. Cat. A, 143 203-214, 1996. 

Trichloroethene (0.69mol h ). 2-chloro-l,l.l-trifluoroelhene (0.17 mol h ) and IIF (5.14mol li" ) were led to a 300 C reactor packed with a catalyst prepared from fluorinated alumina which had been treated with CrCI,. After a contact time of 5 s, trichloroethene was converted with no sign of catalyst degradation after 70h. 2-Chloro-l.l,l-trifluoroethene was formed yield 94%. 

Since isobutylene is a very reactive olefin, its oligomerization can be promoted by almost any electrophilic catalyst. More recently fluorinated alumina,6 cation-exchange resins,7 benzylsulfonic acid siloxane,8 pentasil zeolites,9 and perfluori-nated resinosulfonic acids10 were studied. Some of these catalysts may bring about improved oligomerization. 

The first generation of LSR isomerization catalysts consisted of Pt supported on chlorinated or fluorinated alumina. These catalysts are used at temperatures below 200°C. However, they suffer from three major drawbacks ... 

This mechanism was confirmed by various experiments carried out over a series of Pt/fluorinated alumina catalysts (24). In particular, the comparison of the reactivities of ethylbenzene and of ethylcyclohexane, the values of the kinetic orders with respect to ethylbenzene and to hydrogen are in favour of ethyl and dimethylcyclohexene intermediates rather than ethyl and dimethylcyclohexane which are, however, formed in much more significant amounts. 

HFC-134a has been found to be especially useful as a substitute for CFC-12 in automobile air conditioners. A fairly direct preparation method is shown in eqs (7) and (8) TCE = trichloroethylene. The reactions shown are typically done separately in several reaction zones. HCFC-133a can be prepared using catalysts such as Cr/Mg [34], Cr203/A1F3 [35], AlCUF [36], Zn/fluorinated alumina [37], and Zn/Cr [38, 39]. 

Even if this conditions represent an important improvement, this technology is not applicable on an industrial scale as the catalyst deactivates irreversibly by fluorination (ref. 12). Moreover, if the use of fluorinated aluminas leads to the improvement of initial selectivity, it can not avoid the catalyst deactivation which is transformed during the course of the reaction to aluminium fluoride, a non selective catalyst (ref. 12). 

The Fries rearrangement of phenyl acetate (PA) over solid-acid catalysts was first studied in a fixed bed reactor at 400 °C by Pouilloux et al. [9,10]. o- and p-Hydro-xyacetophenone (o- and p-HAP), p-acetoxyacetophenone (p-AXAP), and phenol (P) were the main reaction products. Fluorinated alumina and H-FAU zeolites afforded approximately the same product distribution, o-HAP being highly favored over the para isomer. The reaction scheme proposed was that PA dissociates into phenol (P) and ketene and that o-HAP results partly from an intramolecular rearrangement of PA and partly from transacylation (Eq. 2) whereas p-HAP results from the latter reaction only [10]. 

Finally, NMR has been used to characterize the structural features of surface-fluorinated aluminas and amorphous aluminosilicates. Both the F NMR chemical shifts and spin-spin relation times differentiate sensitively between Si-F and Al-F bonds. Lineshape and spin echo decay data suggest that the homonuclear F- F dipolar interactions are much weaker than in AlFj-hydrate, indicating that in samples containing up to 5 wt% fluorine, most of the Al-F species are isolated from each other [78]. 

From the above it could be concluded that acid catalysts with strong acidity will be preferred in order to carry out the process at the lowest possible temperature. Pt on chlorinated or fluorinated alumina are used as catalysts at temperatures below 20(FC. However this type of catalysts have three major draw backs ... 

Zeolites, by being in a half way between amorphous silica-alumina and fluorinated alumina, were early recognized as potential components of bifunctional isomerization catalysts. The earliest report of zeolites being used in this application deal on Pt containing X and Y structures (3). They found that the activity increased from the Na to the Ca to the decationized forms. The residual sodium content, and therefore the final acidity of the Pt or Pd HY zeolite catalysts was critical for these type of catalysts (2,4-6). However, to make a successful commercial catalyst the following characteristic have to be accomplished by the zeolitic component ... 

Flourine-complexed acids such as SbFs-fluorinated graphite and SbFj-fluorinated alumina have been used for hydrocarbon isomerizations. 

Fluorinated silica and alumina have also been used for isobutane alkylation with olefins in a batch reactor at 0°C. The fluorinated alumina was active only when mixed with BF3 H2O, but the fluorinated silica was active by itself The selectivity to trimethylpentanes obtained with these catalysts is much lower than that obtained with H2SO4 as a catalyst (48). At 80°C, however, F/AI2O3 catalysts are active for isobutane/2-butene alkylation, though relatively low butene conversions (27% for the most active catalyst containing 1.3% F) are obtained (49). As expected, octenes are the predominant hydrocarbons in the Cg fraction at such a low conversions. The F/AI2O3 catalysts contained both Bronsted and Lewis acid sites in a proportion that depends on the F loading. A good correlation between... 

Complexes with Cu(CFjCOCHCOCFj)2. [73Evrl, 73Evr2,75Evrl] Complexes with alumina, fluorinated alumina, and alumino silicates, silica, zeolites. [73Evr2,76Evel, 77Rigl] ... 

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

Although mechanistic studies of catalytic reactions are hampered by the extreme corrosive nature of the surface, a substantial body of data relating to C2 CFCs indicates that two different reaction pathways can operate depending on the substrate molecule involved and the nature of the surface pretreatment [24]. The two possibilities are sketched in Figure 7.3. Direct replacement of bonded C-Cl by F is not unexpected however, the reverse process, replacement of C-F by surface Cl, deposited from an earlier F-for-Cl substitution, is surprising. Similar events do not occur at fluorinated alumina or at an Al -F-based catalyst, consistent with the thermodynamic rationale developed in Ref. [14]. 

M. Moreno, A. Rosas, J. Alcaraz, M. Hernandez, S. Toppi, P. Da Costa, Identification of the active acid sites of fluorinated alumina catalysts dedicated to n-butene/isobutane alkylation, App. Catalysis A General, 251, 369 (2003). 

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