In isomerization of butanes

The isomerization of butane to iso butane in superacids is illustrative of a protolytic isomerization, where no intermediate olefins are present in equilibrium with carbocations. 

Isomerization. Stmctural isomerization of / -butane to isobutane is commercially useful when additional isobutane feedstock is needed for alkylation (qv). The catalysts permit low reaction temperatures which favor high proportions of isobutane in the product. The Butamer process also is well known for isomerization of / -butane. 

Before discussing the kinetics of reactions in biphasic systems, the basics of kinetics in homogeneous reactions will be briefly revised. In all systems, the rate of a reaction corresponds to the amount of reactant that will be converted to product over a given time. The rate usually refers to the overall or net rate of the reaction, which is a result of the contributions of the forward and reverse reaction considered together. For example, consider the isomerization of -butane to Ao-butane shown in Scheme 2.1. 

Tn commercial petroleum refining, isomerization of light paraffins has been A applied for many years. Until recently the scope of the process was limited, however, and its main application was isomerization of butane as feed preparation for alkylation processes. Generally, except for a few specific cases, no commercial justification could be found for isomerization of pentane and hexane fractions since in most cases the quality requirements for motor gasoline could be met by alternative processing routes and by addition of various additives, such as lead tetraalkyls, to improve fuel burning characteristics. 

Although isomerization of butane requires a bimolecular mechanism, the mechanism proposed in Ref. 9 for hexane does not seem to give a more straightforward explanation of the phenomena than the classical dual function mechanism. 

In certain reactions, such as the isomerization of butane and the alkylation of isoparaffins, problems of handling hydrogen chloride and acidic sludge are encountered. The corrosive action of the aluminum chloride-hydrocarbon complex, particularly at 70 to 100°C, has long been recognized and various reactor liners have been found satisfactory. 

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... 

The work of Petrov (288) and of Moldavskil (236) on the isomerization of some paraffins in the presence of aluminum chloride points to the similarity in temperature required for cracking and for isomerization of these paraffins in the presence of the catalyst used. Moldavskil used aluminum chloride-hydrogen chloride for isomerization of butanes and pentanes and observed a redistribution of methyl groups (241). 

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... 

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. 

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. 

Catalysts active in the isomerization of n-butane have been synthesized by depositing sulfate ions on well-crystallized defective cubic structures based on ZrOz. This technique for introduction of sulfates does not result in any significant changes in the bulk properties of zirconium dioxide matrix. Active sulfated catalysts were prepared on the basis of cubic solid solutions of ZrOz with calcium oxide and on the basis of cubic anion-doped ZrOz. The dependence of the catalytic activity on the amount of calcium appeared to have a maximum corresponding to 10 mol.% Ca. Radical cations formed after adsorption of chlorobenzene on activated catalysts have been used as spin probes for detection of strong acceptor sites on the surface of the catalysts and estimation of their concentration. A good correlation has been observed between the presence of such sites on a catalyst surface and its activity in isomerization of n-butane. 

Wang et al. [5] reported the dehydrogenation and isomerization of -butane on Cr-supported W03-Zr02 and, in his case, the catalytic activity decreased with reaction time by deposition of carbon. In our study, the catalyst performance for -butane conversion and the C4 products selectivity over Cr/H-SSZ-35 catalyst (Si/Al2=500) remained unchanged at 500°C for 6 h. From these findings on the catalytic activity and lifetime, H-SSZ-35 was expected to be one of the promising supports for the production of isobutene by hydrogenation and isomerization of n-butane. 

The most important catalysts found for this reaction are aluminum chloride and aluminum bromide. The early investigators reported that these aluminum halides are catalysts for the isomerization of n-butane to isobutane later investigators found, however, that the reaction proceeds only when traces of water or hydrogen halide are used in conjunction therewith. More recent work indicated that under certain controlled conditions, even aluminum halides-hydrogen halides do not catalyze the isomerization of butanes unless traces of olefins or their equivalent are present. 

The isomerization of n-butane to isobutane and the reverse isomerization proceed in the presence of catalysts containing either aluminum chloride or aluminum bromide the latter, by virtue of its high solubility in hydrocarbons (Heldman and Thurmond, 1 Boedeker and Oblad, 2) and its higher activity, causes the isomerization of butanes to proceed at lower temperatures. 

The alkyl halide produced may act then as a chain starter for the isomerization of butane as given in eq. (11) and (12). 

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