Hydrocarbons, activation superacids

The high acidity of superacids makes them extremely effective pro-tonating agents and catalysts. They also can activate a wide variety of extremely weakly basic compounds (nucleophiles) that previously could not be considered reactive in any practical way. Superacids such as fluoroantimonic or magic acid are capable of protonating not only TT-donor systems (aromatics, olefins, and acetylenes) but also what are called (T-donors, such as saturated hydrocarbons, including methane (CH4), the simplest parent saturated hydrocarbon. 

The finding that highly deactivated aromatics do not react with N02 salts is in accord with the finding that their greatly diminished TT-donor ability no longer snffices to polarize NOi. Similarly, (j-donor hydrocarbons such as methane (CH4) are not able to affect such polarization. Instead, the linear nitronium ion is activated by the superacid. Despite the fact that is a small, triatomic cation, the 11011-... 

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. 

Different catalysts bring about different types of isomerization of hydrocarbons. Acids are the best known and most important catalysts bringing about isomerization through a carbocationic process. Brpnsted and Lewis acids, acidic solids, and superacids are used in different applications. Base-catalyzed isomerizations of hydrocarbons are less frequent, with mainly alkenes undergoing such transformations. Acetylenes and allenes are also interconverted in base-catalyzed reactions. Metals with dehydrogenating-hydrogenating activity usually supported on oxides are also used to bring about isomerizations. Zeolites with shape-selective characteristics... 

Alkane isomerization equilibria are temperature-dependent, with the formation of branched isomers tending to occur at lower temperatures (Table 4.1). The use of superacids exhibiting high activity allows to achieve isomerization at lower temperature (as discussed below). As a result, high branching and consequently higher octane numbers are attained. Also, thermodynamic equilibria of neutral hydrocarbons and those of derived carbocations are substantially different. Under appropriate conditions (usual acid catalysts, longer contact time) the thermodynamic... 

The usual way to achieve heterosubstitution of saturated hydrocarbons is by free-radical reactions. Halogenation, sulfochlorination, and nitration are among the most important transformations. Superacid-catalyzed electrophilic substitutions have also been developed. This clearly indicates that alkanes, once considered to be highly unreactive compounds (paraffins), can be readily functionalized not only in free-radical from but also via electrophilic activation. Electrophilic substitution, in turn, is the major transformation of aromatic hydrocarbons. 

Considering the exceptional activity of liquid superacids and their wide application in hydrocarbon chemistry, it is not surprising that work was also extended to solid superacids. The search for solid superacids has become an active area since the early 1970s, as reflected primarily by the existence of extensive patent literature. 

The preparation of new solid acids, their characterization, mechanistic studies, and theoretical approaches to understand the fundamental aspects of acid-catalyzed hydrocarbon conversion constitute a very large fraction of the topics discussed in the last decade in all journals related to catalysis and physical chemistry. However, in contrast with liquid-acid-catalyzed activation processes, many fundamental questions concerning the initial step, the true nature of the reaction intermediates, and the number of active sites remain open for discussion. For this reason, the results obtained in liquid-superacid-catalyzed chemistry, which can be rationalized by classical reaction mechanisms, supported by the usual analytical tools of organic chemists, represent the fundamental basis to which scientist in the field refer. 

It is impossible not to note the remarkable similarity between Figure 1 and the scheme for the formation of stable carbocations presented in Olah s superacids review28. It is only for the activation of the C—H bond in hydrocarbons that trichlorogermane turns out to be an insufficiently strong acid. 

The Fe203 superacid was found to be quite effective for oxidation of hydrocarbons to CO and C02 when the reaction was performed at temperatures above 100°C. The catalyst gave a 29% conversion for the reaction of butane at 300°C to form CO and C02 in the ratio 4 6 under the conditions in which none of the reactions occurred at 300°C over Fe203, without the sulfate treatment (177). The decrease in oxygen of the catalyst surface was observed together with the complete recovery of activity by supply of 02. The catalyst was entirely poisoned by the addition of pyridine, the oxidation being related to the surface acidity. The activity enhancement of oxidation by the sulfate addition was also observed with the Sn02 superacid (135, 145). Iron and tin oxides are known to be oxidation catalysts thus those superacids would be the oxidation catalysts with superacidity. 

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... 

Novel organic syntheses that are possible in usual acidic media can be accomplished in superacids, including syntheses of economically important hydrocarbons. The remarkable ability of superacids to bring about hydrocarbon transformations can open up new fields in chemistry. In consideration of the exceptionally high activity of Hquid superacids, research was extended to prepare solid superacids. As for chemical appHcations of liquid superacids, efforts were made to attach them to soHd materials, and the results are found in extensive patent literature [10-13]. 

Since the early 1960s, superacids have been known to react with saturated hydrocarbons to yield carbocations, even at low temperature [41]. This discovery initiated extensive studies devoted to electrophilic reactions and conversions of saturated hydrocarbons. Thus, the use of superacidic activation of alkanes to their related carbocations allowed the preparation of alkanecarboxylic acids from alkanes themselves with CO. In this respect, Yoneda et al. have found that alkanes can be directly carboxylated with CO in an HF-SbFs superacid system [42]. Tertiary carbenium ions formed by protolysis of C-H bonds of branched alkanes in HF-SbFs undergo skeletal isomerization and disproportionation prior to reacting with CO in the same acid system to form carboxylic acids after hydrolysis (eq. (9)). 

The Cr203/Zr02 catalysts showed activity in the SCR of NO by a propane-butane mixture, which depended on the means of preparation of the zirconium dioxide. Thus, the conversion of NO to N2 was 13-17% at 350 °C on 5-10 wt.% Cr203/Zr02 catalysts obtained by precipitation, while the conversion of NO to N2 was 54% at 300 °C on catalysts with analogous composition obtained through an alcogel step. This more active sample was also tested in the presence of SO2 (0.02%) in the reaction mixture. The conversion of NO in this case was also enhanced and reached 60% at 300-350 °C. This increase in activity by the action of sulfur dioxide may be attributed to the formation of sulfate since sul ted zirconium dioxide is a solid superacid and catalyzes the SCR of NO by hydrocarbons [11]. 

The isomerization of light paraffin using superacid solid catalysts is a clean way to increase the octane number of hydrocarbons. On this basis, sulfated metal oxides have attracted the attention of many research groups owing to their high activity in acid catalyzed reactions [1]. Sulfated zirconia was found to be a promising catalyst in this field and at the industrial level [2],... 

It is well known that zeolites and solid superacid catalysts suffer rapid deactivation within the first few minutes during alkylation reaction [4-7], Therefore, the product distribution during initial few minutes of reaction was monitored on the dealuminated Y zeolites. During preliminary experiments it was found that appreciable alkylation activity is observed only at temperatures above 50 C. Unlike liquid-phase reactions, where alkylates were observed in the product within 1 min time-on-stream [5,7], in our experiment no Cg hydrocarbons was observed during first 5 minutes and the product mainly consists of only Cs fiactions, i.e., cracked products. After 5 minutes, Cg fiactions started showing up in the product, reached a maximum and then again decreased. This indicates that the alkylate formed initially were... 

Ihe activation of hydrocarbons over zeolites is widely held to result from direct protonation at C-C or at C-H bonds (16) (17) as preposed for reaction in superacid media (18) (19). Present results (14) are exettplified by Fig (11) and Table 2. Frctti the limiting slopes of plots of weight selectivity against conversion (20) the products at zero conversion may be estimated (Scheme 1). 

The high acidity and the extremely low nucleophilicity of the counterions of superacidic systems are especially useful for the preparation of stable, electron-deficient cations, including carbocations. Many of these cations, which were formerly suggested only as fleeting metastable intermediates and were detectable only in the gas phase in mass spectrometric studies, can be conveniently studied in superacid solutions. New chemical transformations and syntheses that are not possible using conventional acids can also be achieved with superacids. These include transformations and syntheses of many industrially important hydrocarbons. The unique ability of superacids to bring about hydrocarbon transformations, even to activate methane (the principal component of natural gas) for electrophilic reactions, has opened up a fascinating new field in chemistry. 

It is demonstrated that sulfated zirconia does not act as a superacid, protorating paraffins, but activates the alkanes and other hydrocarbons via a radical cation type. 

Both titania (anatase more than rutile) and, even more, zirconia (tetragonal more than monoclinic), when sulfated or covered with tungsten oxide become very active for some hydrocarbon conversion reactions such as -butane skeletal isomerization [263]. For this reason, a discussion began on whether these materials have to be considered superacidic. Spectroscopic studies showed that the sulfate ions [264] as well as the tungstate ions [265,266] on ionic oxides in dry conditions, are tetracoordinated with one short S=0 and W=0 bond (mono-oxo structure) as shown in Scheme 9.3(11). Polymeric forms of tungstate species could also be present [267]. However, in the presence of water the situation changes very much. According to the Lewis acidity of wolframyl species, it is believed that it can react with water and be converted in a hydrated form, as shown in Scheme 9.3. Residual... 

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