Isomerization of n-butane to isobutane

These reactions can also be performed over a strong acid catalyst at reaction temperatures that are lower than over zeolites. Because of this, isomerization of M-butane over Zr02-supported sulfate catalysts was initially proposed by Hino and Arata. They proposed these catalysts as being effective in butane isomerization at room temperature, a reaction that does not take place, even in 100% sulphuric acid. For this reason, these catalysts were considered as solid superacids, since they are active and selective in the isomerization of n-butane to isobutane at... 

Figure 13.4 Isomerization of n-butane to isobutane on beta zeolite and sulfated zirconia catalysts at different reaction temperatures.

Kumar, N., Villegas, J.I., Salmi, T., Murzin, D.Y., and Heikkila, T. (2005) Isomerization of n-butane to isobutane over Pt-SAPO-5, SAPO-5, Pt-H-morden-ite and H-mordenite catalysts. Catal. Today, 100, 355-361. 

D.Y. (2006) Isomerization of n-butane to isobutane over Pt-modified beta and ZSM-5 zeolite catalyst catalyst deactivation and regeneration. Chem. Eng. ]., 120, 83-89. 

AH these processes faced competition from the on-demand production of isobutene through a combined process of isomerization of n-butane to isobutane... 

According to an early report, sulfated zirconia promoted with 1.5% Fe and 0.5% Mn increased the rate of isomerization of n-butane to isobutane by several orders of magnitude at modest temperature (28°C).299 This reactivity is surprising, since the isomerization of n-butane in strong liquid acids takes place at a rate much lower than that of higher alkanes, which is due to the involvement of the primary carbocationic intermediate. In addition, other solid acids, such as zeolites, did not show activity under such mild conditions. Evidence by isotope labeling studies with double-labeled n-butane unequivocally shows, however, that the isomerization of... 

The combination of Pt or Pd with CS25H0.5PW12O40 (Cs2.5) is also very effective for the isomerization of n-butane to isobutane (381). The reaction rate and selectivity for conversion to isobutane are summarized in Table XXXIV (381, 382). The activity in the presence of H2 changed little with time. Pt- and Pd-Cs2.5 show very high selectivities (94-96%) relative to those of Pt/SO -/ Zr02 (47%) and Pt/HZSM-5 (34%), whereas the activities of Pt- and Pd-Cs2.5 for the formation of isobutane are comparable to those of Pt/HZSM-5 and Pt/S04 /Zr02. Pt-Cs2.5 catalyzes the reaction even at 473 K and 0.05 atm of H2. 

Clear evidence for a C-C protonated C4H1C ion (55) (which would resemble 52) has been obtained by Siskin/ while studying the I II TaFs catalyzed ethylation of excess ethane with ethylene in a flow system [Eq. (5.12)]. n-Butane was obtained as the only product no isobutane was detected. This remarkable result can be explained by C-H bond ethylation of ethane by the ethyl cation, thus producing the hypercoordinate 55 carbocation intermediate, which, subsequently, by proton elimination, yields n-butane (56). The use of a flow system that limits the contact of the product n-butane (56) with the acid catalyst is essential. Prolonged contact causes isomerization of n-butane to isobutane to occur (see Chapter 6). 

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 early investigators of the isomerization of n-butane to isobutane did not take special precautions to purify the reagents used or to carry out the reaction under conditions which would exclude the contact of air, moisture, or other impurities. For this reason the results obtained are not always reproducible. 

It was found (Pines and Wackher, 7) that aluminum chloride-hydrogen chloride did not cause the isomerization of n-butane to isobutane at 100°. The presence however of 1 part butenes per 10,000 parts n-butane was sufficient to cause the isomerization. When the concentration of butenes was increased to 2.5% side reactions occurred, as evidenced by the formation of higher hydrocarbons and a small amount of a viscous dark layer the latter was produced by a complex formation between the catalyst and unsaturated hydrocarbons. 

The following steps were proposed for the isomerization of n-butane to isobutane 1. The butane contacts the catalyst mass in such a manner that the hydrogen of the hydrogen chloride in combination with aluminum chloride (designated as Ho in the diagram) enters the bonding sphere of the carbon atoms (designated as Ci) of the butane. 

Recent experiments (Beeck el al., 25a) on the isomerization of propane-1-C to propane-2-C further demonstrate that the isomerization proceeds through an intramolecular rearrangement. No propane containing more than one C carbon atom per molecule was foimd. These experiments were made by contacting vapors of propane at approximately 25 and 450 mm. pressure with a catalyst prepared by the addition of 0.023 g. water to 0.40 g. anhydrous aluminum bromide for periods of time varying from 0 to 1074 hours. It was found that the rate of isomerization of propane-1-C to propane-2-C is comparable to the rate of isomerization of n-butane to isobutane under similar conditions. The equilibrium distribution of the propanes-l-C and -2-C was found to be statistical, that is, propane-l-OVpropane-2-C = 2. 

During World War II, the great demand for aircraft fuel necessitated the production of large quantities of isobutane, a basic raw material in the production of high octane aviation gasoline. (See chapter on Alkylation of Alkanes. ) Various processes have been developed for the isomerization of n-butane to isobutane all of them employed aluminum chloride-hydrogen chloride as catalyst. The difference between the various processes consisted either in the method of introduction of aluminum chloride to the reaction zone, the catalyst support, or the state of the catalyst. The following summary describes some of the main features of the various processes which were developed ... 

Before the war, using one of the earliest samples of tritium from the Radiation Laboratory c.t Berkeley, he found the rate of isomerization of n-butane to isobutane, over aluminum chloride promoted with water, was proportional to the rate of exchange of hydrogens between the hydrocarbon and a catalyst promoted with tritiated water. This observation may have been part of the stimulus for the more detailed studies with isotopic tracers of acid catalysis after the war. 

Consider the reaction and distillation operations for the isomerization of n-butane to isobutane according to the reaction... 

The isomerization of n-butane to isobutane is of substantial importance because isobutane reacts under mild acidic conditions with olefins to give highly branched hydrocarbons in the gasoline range. A variety of useful products can be obtained from isobutane isobutylene, t-butyl alcohol, methyl t-butyl ether and t-butyl hydroperoxide. A number of methods involving solution as well as solid acid catalysts have been developed to achieve isomerization of n-butane as well as other linear higher alkanes to branched isomers. 

It is important to point out that thermodynamic equilibria of hydrocarbons and those of derived carbocations are substantially different. Under appropriate conditions (traditional acid catalysts, longer contact time), the thermodynamic equilibrium mixture of hydrocarbons can be reached. In contrast, when a reaction mixture in contact with excess of strong (super) acid is quenched, a product distribution approaching the thermodynamic equilibrium of the corresponding carbocations may be obtained. The two equilibria can be very different. Since a large energy difference in the stability of primary < secondary < tertiary carbocations exists, in excess of superacid solution, generally the most stable tertiary cations predominate. This allows, for example, isomerization of n-butane to isobutane to proceed past the equilibrium concentrations of the neutral hydrocarbons, as the er -butyl cation is by far the most stable butyl cation. 

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. 

MTBE is commercially produced by the reaction of isobutylene with methanol in the presence of an acidic ion-exchange resin as catalyst, usually in the liquid phase and at temperatures below 100°C. A typical catalyst is sulfonated styrene/divinylbenzene resin catalyst. Other solid acid catalysts such as bentonites are also effective and other novel catalysts have recently been discovered. Isobutylene is obtained from field butane by initial isomerization of n-butane to isobutane, followed by dehydrogenation to isobutylene. Commercial preparations of MTBE are 95.03 to 98.93% pure. Impurities are methanol (<0.43%), t-butyl alcohol (<0.80%), and diisobutylene (<0.25%). 

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