Other Solid Superacids

In these (and other) solid superacid catalyst systems, bi- or multi-dentate interactions are thns possible, forming highly reactive intermediates. This amounts to the solid-state equivalent of protosolvation resulting in superelectrophilic activation. 

Acylium ions can be formed in superacid solutions from carboxylic acids and acyl halides (8). They are among the best characterized carbenium ions, and single-crystal X-ray structures of a number of them have been determined as BFf, SbFg, or TaClfi salts (135-139). Solid-state NMR characterization of these species on AlBr3 and other solid superacids was described earlier in this review. 

For the activation of methane, SO4 /Zr02 was found to be active for the reaction of methane with ethylene to form C3, t-C4, 1-C5 [64] In Table 5, the product distribution is compared with that of the other solid superacid, TaFs/AEC [65] For the chlorination of methane, the selectivity of S04 /Zr02 for the formation of methyl chloride exceeds 90% with 30% of chlorine conversion at 473 K and is above 80% with 70-90% conversion at 439 K The selectivity is enhanced by adding Pt, while Fe203-Mn02-S04 /Zr02 exhibits excellent sclectivities (99-100%) at lower conversions [66] The electrophilic insertion is said to involve electron-deficient metal-coordinated chlorine at the methane C H bond... 

Hull and Conant in 1927 showed that weak organic bases (ketones and aldehydes) will form salts with perchloric acid in nonaqueous solvents. This results from the ability of perchlonc aad in nonaqueous systems to protonate these weak bases. These early investigators called such a system a superacid. Some authorities believe that any protic acid that is stronger than sulfunc aad (100%) should be typed as a superaad. Based upon this criterion, fluorosulfuric arid and trifluoro-methanesulfonic acid, among others, are so classified. Acidic oxides (silica and silica-aluminai have been used as solid acid catalysts for many years. Within the last few years, solid acid systems of considerably greater strength have been developed and can he classified as solid superacids. 

Considering the impressive amount of literature on sulfated zirconia and solid superacids,125 134-139 it will be difficult to impose a definition a posteriori. On the other hand, due to the large difference in acidity and in structure between various liquid superacids, there is no unique chemistry of hydrocarbons in liquid superacids. For this reason it is not possible to suggest a unequivocal definition of solid superacidity at the present stage. Nevertheless, it seems clear from all the data presently available that at high temperatures the chemical reactivity of the proton bound to the surface shows a close resemblance to the one observed at low temperature in liquid superacidic media as will be seen in Chapter 5. 

Zeolites such as HZSM-5 were considered as superacids on the basis of the initial product distribution in accord with C-H and C-C bond protolysis when isoalkanes were reacted at 500°C (the Haag and Dessau mechanism).135 The reactivity was assigned to superacidic sites in the zeolite framework.136 The superacid character of other solid acids was claimed on the basis of Hammett indicator color change137,138 or on the basis of UV spectrophotometric measurements.139,140 In 2000, a special issue of Microporous and Mesoporous Materials141 was devoted to the superacid-type hydrocarbon chemistry taking place on solid acids as suggested by the late Werner Haag. 

As already pointed out earlier (see Section 1.4.8), a clear definition of solid superacidity is needed. On the other hand, for catalysts to be able to activate alkanes at low temperatures, such as sulfated zirconias and heteropoly acids, the redox properties should not be neglected in the activation step.142,143... 

It should be mentioned that a solventless method of hydrolytic modification of starch has recently been developed. The method employs solid superacids, per-fluorinated resin-sulfonic acids, which successfully catalyze hydrolysis of starch (and other polysaccharides) and offer the possibility of continuous-process applications in plug-flow reactors.34... 

Solid catalysts can be used at elevated temperatures, though their acidities are much weaker than those of liquid ones. From this point of view, solid superacids based on Lewis acids and liquid superacids discussed in Sections II—1V are not sufficiently stable Nafion-H is also unsatisfactory, its maximum operating temperature being below 200°C. A new type of the sulfate-supported metal oxides is more stable because of preparatory heat treatment at high temperatures, but elimination of the sulfate is sometimes observed during reaction, thus it is hoped to synthesize superacids with the system of metal oxides. Another type of superacid, tungsten or molybdenum oxide supported on zirconia, has been prepared by a new preparation method, and its stability is satisfactory so far. It is hoped that the preparation method will be extensively applied to other metal oxides for new solid superacids. 

The solid superacids of the second group which have a possibility of leaching or evaporating of halogen compounds seem to be environmentally undesirable as catalysts. These superacids and Nafion-H have been extensively reviewed [1-5]. H-ZSM-5 and heteropolyacids are discussed in Sections 2.3.3 and 2.1.7 of this handbook, respectively. Therefore, the superacids of the first group for which many papers have been contributed recently and the industrial application of which is promising, are mainly described here and the other superacids are dealt with only briefly. 

H S (adsorption at 673 K) docs not generate super-acidity. It is interesting that SC /FeiC or HiS/FeiOj docs not show any acidity, but exhibits supcracidity when oxidized with O2 at 773 K [14]. On the other hand. SO4 /FC1O3 loses its acidity when reduced with Hi at 773 K. These facts indicate that oxidation and reduction influence the acidity of sulfur-containing superacids. It should be noted that the solid superacids cannot be used in the presence of reducing reagents... 

Another type of solid superacid is based on perfluorinated resin sulfonic acids, such as the acid form of DuPont s Nafion resin, a copolymer of a perfluorinated epoxide and vinylsulfonic acid, or higher perflu-oroalkanesulfonic acids such as perfluorodecanesulfonic acid, CF3(CF2) 03H. Such solid catalysts were found to be very efficient in alkylation of aromatic hydrocarbons and other Friedel-Crafts reactions. A comprehensive review is available on the application of Nafion-H in organic catalysis. ... 

In homogeneous solutions, fluorosulphonic acid combined with antimony pentafluoride is used as a superacid having some useful catalytic properties in organic chemistry. In its protonated form, perfluorosulphonic Nafion powders are used as the catalyst for a variety of organic syntheses. The catalytic power is higher than that of other solid phase superacid catalysts. It enables lower temperatures and pressures to be used and its specificity is higher. Nafion is used as the solid superacid catalyst for the gas phase alkalination of aromatic hydrocarbons and liquid phase esterification or Friedel-Craft reactions ... 

Another area of difficulty is measuring the acid strength of solid superacids. Since solid superacid catalysts are used extensively in the chemical industry, particularly in the petroleum field, a reliable method for measuring the acidity of solids would be extremely useful. The main difficulty to start with is that the activity coefficients for solid species are unknown and thus no thermodynamic acidity function can be properly defined. On the other hand, because the solid by definition is heterogeneous, acidic and basic sites can coexist with variable strength. The surface area available for colorimetric determinations may have acidic properties widely different from those of the bulk material this is especially true for well-structured solids such as zeolites. 

Transition states in proton-catalyzed reactions that occur in zeolites and other solid acids proceed through activated intermediates close to the carbenium or carbonium ions found in superacid solutions. A carbenium ion is a positively charged ion, that can be formed by the protonation of an alkene. The positive charge is localized on an sp -hybridized C atom. A carbonimn ion is a protonated saturated alkane, that forms non-classical valencies such as a protonated a C-C bond or a five-coordinated C atom. In zeolites, the positively charged intermediates are compensated for by the negatively charged zeolite framework... 

The most stable of all alkyl cations is the tert-butyl cation. Even the relatively stable tert-pentyl and fen-hexyl cations fragment at higher temperatures to produce the tert-butyl cation, as do all other alkyl cations with four or more carbons so far studied. Methane,ethane, and propane, treated with superacid, also yield ten-butyl cations as the main product (see 2-17). Even paraffin wax and polyethylene give the ten-butyl cation. Solid salts of frrf-butyl and rerf-pentyl cations (e.g., MeaC" SbFg ) have been prepared from superacid solutions and are stable below -20°C. ... 

The reader is referred the recent book by Bell and Pines [2] for a more complete overview of the various methods and objectives in NMR studies of solid acids and other heterogeneous catalysis. In the present contribution we illustrate the application of H, and MAS NMR to two archetypal solid acids, Brpnsted sites in zeolites and solid metal halides such as aluminum chloride and bromide powders which exhibit "Lewis superacidity". An important characteristic of the more recent work is the integration of quantum chemical calculations into the design and interpretation of the NMR experiments. 

A variety of solid acids besides zeolites have been tested as alkylation catalysts. Sulfated zirconia and related materials have drawn considerable attention because of what was initially thought to be their superacidic nature and their well-demonstrated ability to isomerize short linear alkanes at temperatures below 423 K. Corma et al. (188) compared sulfated zirconia and zeolite BEA at reaction temperatures of 273 and 323 K in isobutane/2-butene alkylation. While BEA catalyzed mainly dimerization at 273 K, the sulfated zirconia exhibited a high selectivity to TMPs. At 323 K, on the other hand, zeolite BEA produced more TMPs than sulfated zirconia, which under these conditions produced mainly cracked products with 65 wt% selectivity. The TMP/DMH ratio was always higher for the sulfated zirconia sample. These distinctive differences in the product distribution were attributed to the much stronger acid sites in sulfated zirconia than in zeolite BEA, but today one would question this suggestion because of evidence that the sulfated zirconia catalyst is not strongly acidic, being active for alkane isomerization because of a combination of acidic character and redox properties that help initiate hydrocarbon conversions (189). The time-on-stream behavior was more favorable for BEA, which deactivated at a lower rate than sulfated zirconia. Whether differences in the adsorption of the feed and product molecules influenced the performance was not discussed. 

Fluorinated polymers, especially polytetrafluoroethylene (PTFE) and copolymers of tetrafluoroethylene (TFE) with hexafluoropropylene (HFP) and perfluorinated alkyl vinyl ethers (PFAVE) as well as other fluorine-containing polymers are well known as materials with unique inertness. However, fluorinated polymers with functional groups are of much more interest because they combine the merits of pefluorinated materials and functional polymers (the terms functional monomer/ polymer will be used in this chapter to mean monomer/polymer containing functional groups, respectively). Such materials can be used, e.g., as ion exchange membranes for chlorine-alkali and fuel cells, gas separation membranes, solid polymeric superacid catalysts and polymeric reagents for various organic reactions, and chemical sensors. Of course, fully fluorinated materials are exceptionally inert, but at the same time are the most complicated to produce. 

It is generally admitted that skeletal transformations of hydrocarbons are catalyzed by protonic sites only. Indeed good correlations were obtained between the concentration of Bronsted acid sites and the rate of various reactions, e g. cumene dealkylation, xylene isomerization, toluene and ethylbenzene disproportionation and n-hexane cracking10 12 On the other hand, it was never demonstrated that isolated Lewis acid sites could be active for these reactions. However, it is well known that Lewis acid sites located in the vicinity of protonic sites can increase the strength (hence the activity) of these latter sites, this effect being comparable to the one observed in the formation of superacid solutions. Protonic sites are also active for non skeletal transformations of hydrocarbons e g. cis trans and double bond shift isomerization of alkenes and for many transformations of functional compounds e.g. rearrangement of functionalized saturated systems, of arenes, electrophilic substitution of arenes and heteroarenes (alkylation, acylation, nitration, etc ), hydration and dehydration etc. However, many of these transformations are more complex with simultaneously reactions on the acid and on the base sites of the solid... 

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