Skeletal isomerization of butane

The product distributions obtained over the catalysts are summarized in Table II for the reaction of butane (29, 31, 32). The activities varied with the kinds of metal oxides that were treated with SbF,. SbF,/ Si02-Ti02 showed the highest activity, and SbF,/Ti02 was highly selective for the skeletal isomerization of butane, the selectivity being 72%. On... 

Both catalysts were active for the skeletal isomerizations of butane and isobutane at room temperature (130). The reaction of butane was carried out under pulse reaction conditions the catalytic activities were shown as a function of calcination temperature of the catalyst (Fig. 4) (129). Butane was converted to isobutane and propane, and the catalytic activity of Ti02-I was higher than that of TiOrlI. The maximum activity was observed with calcination at 525°C. The quantity of S was estimated to be 2.11 and 0.01% for the TiOrI catalyst calcined at 525 and 650°C, respectively (56). The activity enhancement of Ti02 by an addition of ammonium sulfate was also reported by Tanabe et at. (137). 

Catalytic activity can be used to rank solid acidity, and activity for the skeletal isomerization of butane is often used to indicate very strong acidity, in particular superacidic strength [95]. A comparative study using the isomerizations of butane... 

The skeletal isomerization of butane to isobutane is a typical reaction catalyzed by superacidity. Early in the history of this work, S04/Fe203, S04/Ti02, and S04/Zr02, were termed superacids owing to their ability to isomerize butane at room temperature or below [32, 37, 39] The formation of isobutane from butane, however, does not necessarily require superacidic strength. A bimolecular reaction pathway based on the intermediacy of butane is energetically lower than a monomolecular mechanism [129-133]. The monomolecular and bimolecular mechanisms are shown in Schemes 17.1 and 17.2, respectively, using pentane as a model. 

Skeletal isomerization of paraffins such as butane, pentane, etc. is not catalyzed even by 100% H2SO4. It was found, however, that Zr02 S04 , Ti02 — S04 , and Fe203 — S04 " catalyzed the skeletal isomerization of butane at 293 — 323 K, the main products being isobutane. The activity of the solid superacids is lowered as the reaction proceeds probably due to coke formation. To prevent the catalyst from its deactivation, a catalyst on which a small amount of Pt, Ni etc. was added was developed. Over a Pt — Z1O2 — S04 catalyst, no deactivation was observed for more than 100 h for the skeletal isomerization of pentane at 413 K under 20 kg/cm of hydrogen pressure. 

The acid-catalyzed reactions are detailed in Fig. 8.9 for skeletal isomerization of butanes. 

The skeletal isomerization of straight-chain paraffins is important for the enhancement of the octane numbers of light petroleum fractions. The isomerization of H-butane to isobutane has attracted much attention because isobutane is a feedstock for alkylation with olefins and MTBE synthesis. It is widely believed that the low-temperature transformation of n-alkanes can be catalyzed only by superacidic sites, and this reaction has often been used to test for the presence of these sites. 

Activity and Selectivity for Skeletal Isomerization of n-Butane in the Presence of H2 and Metal-Promoted Catalysts at 573 K (381. 382)... 

Pt-promoted Cs2.5 catalyst also is efficient for the skeletal isomerization of n-butane, n-pentane, n-hexane, and n-heptane. Pt-Cs2.5 supported on silica is effective for isomerization of cyclohexane and the hydroisomerization of benzene to methylcyclopentane. ... 

In order to confirm the acidity results measured using the indicators shown in Table 17.1, we have investigated as many acid-catalyzed reactions as possible. The reactions are summarized in Table 17.4 [43, 48, 118, 119]. Among them, the skeletal isomerization of light paraffins, in particular butane and pentane, has been the most widely applied. The isomerization of butane at room temperature was a well known test reaction for superacidity at the beginning of this work [43, 48, 118]. The activity for many of the reactions tested correspond to the acidities as determined by use of the Hammett indicators. 

Sulfated zirconias, nowadays a well established class of acid solids first reported by Holm and Bailey [2], and systematically studied by Arata [3] and Tanabe et aL [4], are considered as potential alternative catalysts for the skeletal isomerization of n-butane. These catalysts have recently found a commercial application (Par-Isom Process of UOP) for the isomerization of light naphtha (Cs-Ce), but since they are less active than Pt-chlorinated aluminas, there is a real interest for improving their catalytic performance [5]. 

It is worth mentioning here that, in contrast to what has been previously fovmd for the skeletal isomerization of n-butane to isobutane, the addition of small amounts of Fe, Mn, and Pt promoters to 804 /Zr02 does not have a marked effect on its alkylation performance (188). From their own results, the authors proposed that the alkylation reaction on this kind of solid acids occurs on very strong (ie, superacidic) sites, probably via -butyl cation formed by H abstraction from isobutane on superacidic Lewds acid sites. 

Besides the above reactions, any kind of acid-catalyzed reactions such as cracking of cumene, alkylation of benzene with propene, hydration of olefins, isomerization of cyclopropane, esterification of acetic acid with ethanol, etc. can be used for the estimation of the acidic property of solid acids. Skeletal isomerization of li-butane to -butane is used to check whether a solid acid has superacidity, since the isomerization is known not to be catalyzed even by 100% sulfuric acid. However, it should be noticed that the differentiation between acid strength and acid amount is not easy from the measurement of catalytic activity for an acid-catalyzed reaction. Characterization of acid catalysts by use of model reactions has been reviewed recently by Guisnet. ... 

Zeolites have also been described as efficient catalysts for acylation,11 for the preparation of acetals,12 and proved to be useful for acetal hydrolysis13 or intramolecular lactonization of hydroxyalkanoic acids,14 to name a few examples of their application. A number of isomerizations and skeletal rearrangements promoted by these porous materials have also been reported. From these, we can underline two important industrial processes such as the isomerization of xylenes,2 and the Beckmann rearrangement of cyclohexanone oxime to e-caprolactam,15 which is an intermediate for polyamide manufacture. Other applications include the conversion of n-butane to isobutane,16 Fries rearrangement of phenyl esters,17 or the rearrangement of epoxides to carbonyl compounds.18... 

Low-molecular-weight hydrocarbons (C4 and C5 alkanes) usually undergo isomerization through a simple bond shift. The transformation of [l-l3C]-butane, for instance, yields isobutane via skeletal isomerization and the isotopomer [2-13C]-butane 155... 

The following facts are the basis for butene isomerization (I) There is a basic similarity in the composition of alkylates produced from all four butene isomers. (2) Alkylate molecules, once formed, are relatively stable under alkylation conditions and do not isomerize to any appreciable extent alkylate fractions having the same carbon number ore not equilibrated (see Table I). (3) Thermodynamic equilibrium between the butene olefins highly favors isobutene formation at alkylation temperatures. (4) Normal butenes p>roduce only small and variable amounts of normal butane, thus indicating only a small and variable amount of chain initiation from normal butenes. Yet the alkylate composition shows a high concentration of trimethylpentanes and a low concentration of dimethylhexanes. (5) A few of the octane isomers can be explai.ned only by isomerization of the eight-carbon skeletal structure this isomerization occurs while isobutene dimer is in ionic form. For example, 2,3,3- and 2,3,4-trimethylpentanes... 

Dehydrogenation over Chromia—Alumina. Chromia—alumina catalyst CR-0205 was studied relative to butane dehydrogenation and gave low conversion to butadiene under conditions of appreciable conversion to butenes. In addition, there was an exceptionally small amount of cracked products and essentially no skeletal isomerization. Accordingly, this well-known catalyst was evaluated for n-dodecane dehydrogenation. Conditions of evaluation were temperature of 440°C, atmospheric pressure, hydrogen diluent with a hydrogen to n-dodecane mole ratio of 8 to 1. The results obtained were as follows ... 

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... 

Tbngsten oxide mounted on ZrOz has acid sites as strong as //o= —14.52. The oxide shows activity for acylation of toluene with benzoic anhydride at 303 K, and butane skeletal isomerization to isobutane at 373 K. The maximum activity is obtained when the oxide is calcined at 1073- 1273... 

Paraffin skeletal isomerization is the second of the reaction-producing high-octane hydrocarbons, though TMPs cannot be obtained by using diverse isomerization catalysts, even the Pt/Cl-Al Oj, has been used on the industrial scale in n-butane isomerization... 

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