Superacid-Intercalated Graphites

The isomerization of a series of cyclic and bicyclic saturated hydrocarbons over SbF,-intercalated graphite was achieved at or below room temperature without the ring opening and cracking reactions, and the thermodynamic equilibrium was reached for the isomers in all cases (39). Interconversion between cyclohexane and methylcyclopentane also yielded the thermodynamic equilibrium mixture. 

It was shown that the SbF5-intercalated graphite efficiently promotes disproportionation of various alkylbenzenes by simple mixing at room temperature (41). The isomerization of methylpentanes was carried out over the catalyst at room temperature, -30, and - I7°C in a continuous flow system a careful study of the kinetically controlled product distribution was performed to obtain information for the reaction path (42, 43). The skeletal rearrangements of l3C- abeled 2-methyl, 3-methylpentane and 

3-dimethyIbutane were studied using the same catalyst under similar conditions 44). The isomerization process, which involves only intramolecular rearrangements of the hexyl cations, was fully described by considering 1,2-alkyl shifts of methyl and ethyl groups and rearrangements via (protonated) cyclopropane rings. 

Lalancette et al. studied the catalytic activity of AlCl3-intercalated graphite for the alkylation of aromatics with ethyl bromide, ethylene, propylene, and isobutylene and compared it with pure A1C13 the intercalate was a milder catalyst than AICI3 and gave less polysubstituted products 

The major drawback in the extended use of this catalyst system is its relatively short lifetime and ease of deactivation. An example for such a deactivation is shown in Table III, where reactions were carried out in a flow system, in the gas phase, and in the temperature range of 160-180°C at atmospheric pressure (49). Although the initial conversions were high, the catalyst was totally deactivated after a period of 6-8 h. A possible reason is leaching of the metal halide from the graphite layers by the feed. Three possible reasons for such a deactivation have been given by Heinermann and Gaaf in the case of the SbF, catalyst (40)  

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The difficulties encountered in handling liquid superacids and the need for product separation from catalyst in batch processes have stimulated research in the isomerization of alkanes over solid superacids. The isomerization of 2-methyl- and 3-methylpentane and 2,3-dimethylbutane, using SbF5-intercalated graphite as a catalyst, has been studied in a continuous flow system. ... 

Solid Superacids. Most large-scale petrochemical and chemical industrial processes ate preferably done, whenever possible, over soHd catalysts. SoHd acid systems have been developed with considerably higher acidity than those of acidic oxides. Graphite-intercalated AlCl is an effective sohd Friedel-Crafts catalyst but loses catalytic activity because of partial hydrolysis and leaching of the Lewis acid halide from the graphite. Aluminum chloride can also be complexed to sulfonate polystyrene resins but again the stabiUty of the catalyst is limited. 

The HF-SbFs system is known to be a superacid H34). The possible relevance of this to the intercalation process was pointed out by Vogel V12), who first reported on the extremely high electrical conductivity of graphite-SbFj measured normal to the crystallographic c-axis. The measured conductivity was approximately 40 times that of pristine graphite, and 50% greater than that of pure copper. Other workers... 

Whereas superacid (HF-BF3, HF-SbFs, HF-TaFs, HSOiF-Sbf s, etc.)-cata-lyzed hydrocarbon transformations were first explored in the liquid phase, subsequently solid acid catalyst systems, such as those based on Nafion-H, longer-chain perfluorinated alkanesulfonic acids, fluorinated graphite intercalates, and so on, were also developed and utilized for heterogeneous reactions. The strong acidic nature of zeolites was also explored. 

Ways have been found to immobilize and/or to bind su-peracidic catalysts to an otherwise inert solid support. These include graphite intercalated superacids. Graphite possessing a layered structure can form intercalation compounds with Lewis acids such as AsFs and SbFs. These... 

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