Superacid scale

Measuring super acids requires something beyond the normal pH scale from 0 - 14. The superacid scale, or Hammett acidity function, is based on experimental evidence. For example, in this equation... 

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. 

Out of the metal oxides, sulfated titania and tin oxide performed slightly better than the sulfated zirconia (SZ) catalyst and niobic acid (Nb205). However, SZ is cheaper and readily available on an industrial scale. Moreover, it is already applied in several industrial processes (7,8). Zirconia can be modified with sulfate ions to form a superacidic catalyst, depending on the treatment conditions (11-16). In our experiments, SZ showed high activity and selectivity for the esterification of fatty acids with a variety of alcohols, from 2-ethylhexanol to methanol. Increasing... 

Zeolites are the main catalyst in the petrochemical industry. The importance of these aluminosilicates is due to their capacity to promote many important reactions. By analogy with superacid media (1), carbocations are believed to be key intermediates in these reactions. However, simple carbocationic species are seldom observed on the zeolite surface as persistent intermediates within the time-scale of spectroscopic techniques. Indeed, only some conjugated cyclic carbocations were observed as long living species, but covalent intermediates, namely alkyl-aluminumsilyl oxonium ions (2) (scheme 1), where the organic moiety is bonded to the zeolite structure, are usually thermodynamically more stable than the free carbocations (3,4). 

A quantitative determination of the strength of Lewis acids to establish similar scales (Ho) as discussed in the case of protic (Br0nsted-type) superacids would be most useful. However, to establish such a scale is extremely difficult. Whereas the Brpnsted acid-base interaction invariably involves a proton transfer reaction that allows meaningful comparison, in the Lewis acid-base interaction, involving for example Lewis acids with widely different electronic and steric donating substituents, there is no such common denominator.25,26 Hence despite various attempts, the term strength of Lewis acid has no well-defined meaning. 

Up to a // value of — 10, all indicators are primary amines and are therefore suitable for the measurement of the Hammett H() function. For stronger acids, new indicators such as nitro compounds have to be used. Although the acidity function scale based upon nitro compounds as indicators may not be a satisfactory extension of the aniline indicator scale, Gillespie and Peel18 have shown that the most basic nitro compound indicator, para-nitrotoluene overlaps in a satisfactory manner with the weakest indicator in the aniline series, 2,4,6-trinitroaniline. Thus, the acidity measurements using the nitro compounds may be considered to give the best semiquantitative picture of the acidity of the various superacid systems. 

In principle, the A 0(H) function is of limited interest for kinetic applications because the indicators are chemically very different from the organic substrates generally used. On the other hand, as the measurements are based on pH determination, the length of the acidity scale is limited by the pA" value of the solvents. However, very interesting electrochemical acidity studies have been performed in HF by Tremillon and co-workers, such as the acidity measurement in anhydrous HF solvent and the determination of the relative strength of various Lewis acids in the same solvent. By studying the variation of the potential of alkane redox couples as a function of acidity, the authors provide a rational explanation of hydrocarbon behavior in the superacid media.48... 

Table V summarizes several reactions that have been demonstrated on a laboratory scale 1 know of no industrialized chemical process using Nafion as a superacid catalyst. Although many of the reactions were carried out with stirring a mixture of reactants and Nafion-H, several alkylation, disproportionation, rearrangement, and esterification reactions were performed by means of the flow-reaction method in the liquid or gas phase. For instance, in the esterification of carboxylic acids with alcohols, when a mixture of the acid and alcohol was allowed to flow over a Nafion-H catalyst at 95-125°C with a contact time 5 s, high yields, usually S90%, of the corresponding ester were obtained (82). It had been found that no reactivation of the catalyst was needed because the catalytic activity of the Nafion remained unchanged for prolonged periods of operation.

The new indicators have been used to obtain a value of H0 for 100% H2S04 which is -11-93 (Gillespie et al., 1971). Although the acidity function scale based on the use of the nitrocompounds may not form a completely satisfactory extension of the scale based upon the anilines, the use of these indicators does give at least a semi-quantitative picture of the acidity of various superacid systems. Moreover the H0 values for... 

In summary, the observation of carbocations in superacids has provided a wealth of previously inaccessible information on stmcture and reactivity. The infinite lifetime of the cations precludes, of course, any stereochemical studies. On a scale of activation energies, the rearrangements studied in superacids overlap in part with those observed in more nucleophilic solvents and extend toward higher energy barriers. 

Different solid acid catalysts like zeolite Y [2-6], beta [7-9], MCM-22 [10], solid superacids [11-13], sulphonic acid resins [14], etc. have been proposed as potential alkylation catalysts and some of them are being tested at a pilot plant scale. Zeolites and solid superacids of sulfated zirconia type were found to be the most active but they suffer rapid deactivation after an initial period. Among different zeolites studied large-pore zeolites are prefered over medium-pore type because the former favors the formation and diffusion of bulkier tri-methylpentane isomers. Beside pore size and zeolite structure, the fiamework composition (Si/Al ratio) and acid strength distribution also play an important role on the activity, selectivity and deactivation of the catalysts. It is known that the adsorption behavior of the zeolite and the extent of hydrogen transfer capacity (a crucial factor of alkylation activity) both depend on the aluminium concentration in the framework [15-16]. 

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