Aluminium fluoride catalysts

Figure 3.11 Propane-ODH results with (a) VOx/aluminium oxide (VAIO) and (b) VOJ aluminium fluoride catalysts (VAIF) (from AI HF— 1 3) with different V contents (773 K meat = 100 mg N2 02 CjFls—20 50 30) (X) conversion (Y) yield. (Reprinted with permission from 129] Copyright (2008) FIsevier Ltd.)...

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). 

We now report that the fluorodecarboxylation of arylchloroformates can be achieved in an anhydrous hydrogen fluoride vapor phase reaction using aluminium fluoride as a catalyst. 

Without catalyst, we do not obtain at this temperature any fluorinated derivatives (Table 2, entry 1). Several catalysts (Table 1) are suitable for the fluorodecarboxylation of phenylchloroformate to fluorobenzene such as oxyfluoride (Al, entry 2 Cr, entry 4 Zr, entry 5 Ti, entry 10) or fluorides (Al, entry 3 La, entry 6 Ce entry 7 Mg entry 9). The best activity and selectivity is observed using as catalyst aluminium fluoride or oxyfluoride (entries 2 and 3). 

The fluorodecarboxylation of arylfluoroformates in the vapor phase, under anhydrous hydrogen fluoride has been successfully realised by using catalysts such as aluminium fluoride or aluminium or chromium oxyfluoride. This transformation is quite versatile, the main limitation being the stability of substrates or products under the reaction conditions. The catalyst used, deactivates rapidly by coking but can be reactivated by a simple oxidative treatment. This new access to fluorinated aromatics derivatives appears to be an attractive industrial alternative to the diazotisation of anilines. 

Extensive recycling of organic and HF is required and catalyst lifetime is a problem. However, subsequent work by Dupont [8] over a variety of metals on an aluminium fluoride support gave extended catalyst lifetimes. 

In 1995, BP patented the polyisobutene (PIB) synthesis using isobutene-rich C4 cuts as feedstock [43]. This technology offers many advantages compared to aluminium(III) chloride or boron(III) fluoride catalysts simple product separation due to biphasic reaction mode. 

K. ScheureU and E. Kemnitz, Amorphous aluminium fluoride as new matrix for vanadium containing catalysts, J. Mater. Chem., 15,4845-4853 (2005). 

High Surface Area Aluminium Fluoride as Catalyst... 

Although the yield of propene does not exceed about 12% at best, the performance of the catalysts is remarkable insofar as, for the first time, the yield of propene is higher than that of COx, with the 10% vanadium catalyst. The good performance of the aluminium fluoride-supported VOx catalysts is due to the nature of the support and not a result of the sol-gel preparation route as follows from a direct comparison of VOx/aluminium fluoride ( VAIF ) and VOx/aluminium oxide ( VAIO ) catalysts prepared similarly via the sol-gel route. 

In Figure 3.11 compares the performance of VAIF and VAIO catalysts with varied V content for propane-ODH. From Figure 3.11 (a) follows that with aluminium oxide-based catalysts high conversion rates have been obtained, even with only 10 mol% V content, however, predominantly as undesired total combustion with a low yield of propene. A rather different picture was obtained with the aluminium fluoride-based catalysts (Figure 3.11 (b)). Catalysts with low V content still gave acceptable conversion rates but the conversion resulted predominantly in the aimed propene. Only with higher V content did the conversion rate increase to nearly the same values as in the case of VAIO catalysts, however, the yield... 

K. Scheurell, G. Scholz and E. Kemnitz, Structural study of VOx doped aluminium fluoride and aluminium oxide catalysts, J. Solid State Chem., 180, 749-758 (2007). 

In the second part (Section 5.3), the strong potential of these techniques for the study of functionalized Al-based inorganic fluorides, synthesized for their possible application as catalysts (fluorinated alumina and zeolites, high surface area aluminium trifluorides and nanostructured aluminium fluoride hydrates and hydroxyfluorides), is shown. The compounds used as model for the assignment of the V and Al NMR lines in these materials are also presented. 

Prior to 1965, alkylbenzene production was synthesized from petroleum tetrapropylene reacted with an aluminium chloride or hydrogen fluoride catalyst and benzene. The resultant alkylate was a hard branched-chain compound that was considered slowly biodegradable. A straight-chain alkylate, termed LAB (linear alkylbenzene), has been produced since 1965 in the United States. Extensive research has demonstrated biodegradation effectiveness in sewage treatment plants in excess of 95 percent. ... 

Three basic processes have been practiced for linear alkylbenzene manufacture. The most prevalent route of alkylbenzene manufacture is by partial dehydrogenation of paraffins, followed by alkylation of benzene with a mixed olefin/paraffin feedstock, using liquid hydrogen fluoride catalyst. A second route is via partial chlorination of paraffins, followed by alkylation of the chloroparaffin/paraffin feedstock in the presence of an aluminium chloride catalyst. The third process uses partial chlorination, but includes a dehydrochlorination to olefin step prior to alkylation with aluminium chloride or hydrogen fluoride. 

In addition to cryolite and aluminium fluoride, there is a wide range of inorganic fluorides of commercial importance these include sulfur hexafluoride, used as an electrical insulating gas. Sodium fluoride has been used in the fluoridation of water, although it is being replaced by fluorosilicic acid, and its salts. Tin (IV) fluoride (Snp4) is used in toothpaste to prevent dental decay, and boron trifluoride (BF3) is a widely used catalyst for reactions in the petrochemical industry. 

The role of Lewis acids in the formation of oxazoles from diazocarbonyl compounds and nitriles has primarily been studied independently by two groups. Doyle et al. first reported the use of aluminium(III) chloride as a catalyst for the decomposition of diazoketones.<78TL2247> In a more detailed study, a range of Lewis acids was screened for catalytic activity, using diazoacetophenone la and acetonitrile as the test reaction.<80JOC3657> Of the catalysts employed, boron trifluoride etherate was found to be the catalyst of choice, due to the low yield of the 1-halogenated side-product 17 (X = Cl or F) compared to 2-methyI-5-phenyloxazole 18. Unfortunately, it was found that in the case of boron trifluoride etherate, the nitrile had to be used in a ten-fold excess, however the use of antimony(V) fluoride allowed the use of the nitrile in only a three fold excess (Table 1). 

A ubiquitous co-catalyst is water. This can be effective in extremely small quantities, as was first shown by Evans and Meadows [18] for the polymerisation of isobutene by boron fluoride at low temperatures, although they could give no quantitative estimate of the amount of water required to co-catalyse this reaction. Later [11, 13] it was shown that in methylene dichloride solution at temperatures below about -60° a few micromoles of water are sufficient to polymerise completely some decimoles of isobutene in the presence of millimolar quantities of titanium tetrachloride. With stannic chloride at -78° the maximum reaction rate is obtained with quantities of water equivalent to that of stannic chloride [31]. As far as aluminium chloride is concerned, there is no rigorous proof that it does require a co-catalyst in order to polymerise isobutene. However, the need for a co-catalyst in isomerisations and alkylations catalysed by aluminium bromide (which is more active than the chloride) has been proved [34-37], so that there is little doubt that even the polymerisations carried out by Kennedy and Thomas with aluminium chloride (see Section 5, iii, (a)) under fairly rigorous conditions depended critically on the presence of a co-catalyst - though whether this was water, or hydrogen chloride, or some other substance, cannot be decided at present. 

The only solid acidic catalyst which has given high polymers at an appreciable rate at low temperatures, and which has been studied in some detail, is that described by Wichterle [41, 42]. This was prepared as follows A 10 per cent solution in hexane of aluminium tri-(s- or t-butoxide) was saturated with boron fluoride at room temperature, and excess boron fluoride was removed from the precipitate by pumping off about half the hexane. Two moles of boron fluoride were absorbed per atom of aluminium, and butene oligomers equivalent to two-thirds of the alkoxy groups were found in the solution the resulting solid had hardly any catalytic activity. When titanium tetrachloride was added to the suspension in hexane, an extremely active catalyst was formed but the supernatant liquid phase had no catalytic activity. The DP of the polymers formed by the catalyst prepared from the s-butoxide was much lower than that of polymers formed with a catalyst prepared from the r-butoxidc. 

Cleavage of a carbon-fluorine bond can be induced by reaction with a Lewis acid (see Chapter 4, Section VIC). In Friedel-Crafts alkylations, aUcyl fluorides are more reactive than the chlorides [37] with, for example, aluminium halides or boron halides as catalysts (Figure 5.17). 

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