Alkylation of isopentane

Theoretically, even the direct alkylation of carbenium ions with isobutane is feasible. The reaction of isobutane with a r-butyl cation would lead to 2,2,3,3-tetramethylbutane as the primary product. With liquid superacids under controlled conditions, this has been observed (52), but under typical alkylation conditions 2,2,3,3-TMB is not produced. Kazansky et al. (26,27) proposed the direct alkylation of isopentane with propene in a two-step alkylation process. In this process, the alkene first forms the ester, which in the second step reacts with the isoalkane. Isopentane was found to add directly to the isopropyl ester via intermediate formation of (non-classical) carbonium ions. In this way, the carbenium ions are freed as the corresponding alkanes without hydride transfer (see Section II.D). This conclusion was inferred from the virtual absence of propane in the product mixture. Whether this reaction path is of significance in conventional alkylation processes is unclear at present. HF produces substantial amounts of propane in isobutane/propene alkylation. The lack of 2,2,4-TMP in the product, which is formed in almost all alkylates regardless of the feed (55), implies that the mechanism in the two-step alkylation process is different from that of conventional alkylation. 

Although the preceding discussion of the sulfuric and hydrofluoric acid processes has been confined to butene alkylation, isobutane has also been alkylated commercially with other olefins. Ethylene, propylene, pentenes, and dimers of butenes have been used for this purpose. It is also possible to use these olefins for the alkylation of isopentane. Such an operation, however, has not achieved commercial acceptance because it produces an inferior alkylate with a high catalyst consumption, and because isopentane is a satisfactory aviation gasoline component in its own right. 

Alkylation of isopentane, on the other hand, has been chiefly of academic interest since this isoalkane is already a liquid with a relatively high octane number. 

It is apparent that the ratio of reacting isoparaffin to olefin may thus be greater than unity and the yield of alkylate based on the olefin may be higher than theoretical. Such high yields have been obtained in the alkylation of isopentane in the presence of sulfuric acid or hydrogen fluoride (see below). 

Alkylation of isopentane with 2-butene at 10° in the presence of 100% sulfuric acid resulted in a 264% yield of alkylate based on the olefin (McAllister et al., 12). The theoretical yield for the simple alkylation reaction is 228%. Isobutane, hexanes (chiefly 2-methylpentane), nonanes, and decanes were formed. Destructive alkylation and hydrogen transfer occurred (Section III). 

Similarly, alkylation of isopentane with i-butyl alcohol under the same conditions resulted in the formation of alkylate (boiling above pentane) to the extent of 305% by weight of the available isobutylene. The product contained about 21% by weight hexanes, 6% heptanes, 9% octanes, 27% nonanes, and 25% decanes. Nonanes were thus formed in 36% of the theoretical yield. Decanes and isobutane, respectively, were isolated in 30% and 111% of the yields obtainable if all of the <-butyl alcohol had undergone hydrogen transfer and none simple alkylation to form nonanes. Extensive destructive alkylation of the decanes apparently occurred. 

Alkylation of isopentane with s-butyl alcohol at 24° resulted in 44% of the theoretical yield of nonanes and 18 and 72% yield decanes and isobutane, respectively, as well as substantial amounts of hexanes and octanes. 

Alkylation of isopentane with propene at 10° resulted in a 272% yield of liquid alkylate by weight of the olefin 19% of isobutane was also formed (Linn and Grosse, 19). The yield of alkylate (the theoretical is 272%) and... 

Table I gives the compositions of alkylates produced with various acidic catalysts. The product distribution is similar for a variety of acidic catalysts, both solid and liquid, and over a wide range of process conditions. Typically, alkylate is a mixture of methyl-branched alkanes with a high content of isooctanes. Almost all the compounds have tertiary carbon atoms only very few have quaternary carbon atoms or are non-branched. Alkylate contains not only the primary products, trimethylpentanes, but also dimethylhexanes, sometimes methylheptanes, and a considerable amount of isopentane, isohexanes, isoheptanes and hydrocarbons with nine or more carbon atoms. The complexity of the product illustrates that no simple and straightforward single-step mechanism is operative rather, the reaction involves a set of parallel and consecutive reaction steps, with the importance of the individual steps differing markedly from one catalyst to another. To arrive at this complex product distribution from two simple molecules such as isobutane and butene, reaction steps such as isomerization, oligomerization, (3-scission, and hydride transfer have to be involved.

In 1989, the NDF Company opened a facility in Georgetown, South Carolina to produce low density polyethylene. Manufacturing of the polyethylene is done in two 50-ton reactors that are encased individually within their own 8-story-high process unit. The main raw materials for the manufacturing operations include ethylene, hexane, and hutene. The polymerization is completed in the presence of a catalyst. The hase chemicals for the catalyst are aluminum alkyl and isopentane. The reactor and catalyst preparation areas are on a distributed control system (DCS). A simplihed process flow diagram is attached. 

In the catalyst preparation area where the fire occurred, aluminum alkyl and isopentane are mixed in a batch blending operation in three 8000-gallon kettles. The flow rates of components are regulated by an operator at the control room. Temperature, pressure, and liquid level within the kettles are monitored by the control room operator. The formulated catalyst is stored in four 12,000-gallon vertical storage tanks within this process unit. Aluminum alkyl is a pyrophoric material and isopentane is extremely flammable. Each vessel was insulated and equipped with a relief valve sized for external fire. 

In fact, the C-H bond activation by the zirconium or tantalum hydride on 2,2-dimethylbutane can occur in three different positions (Scheme 3.5) from which only isobutane and isopentane can be obtained via a P-alkyl transfer process the formation of neopentane from these various metal-alkyl structures necessarily requires a one-carbon-atom transfer process like an a-alkyl transfer or carbene deinsertion. This one-carbon-atom process does not preclude the formation of isopentane but neopentane is largely preferred in the case of tantalum hydride. 

Isopentane and trimethylpentanes (C8 alkanes) are inevitably formed whenever isobutane is alkylated with any alkene. They are often called abnormal products. Formation of isopentane involves the reaction of a primary alkylation product (or its carbocation precursor, e.g., 3) with isobutane. The cation thus formed undergoes alkyl and methyl migration and, eventually, P scission ... 

Both Cp Re(CO)3 and Cp Re(Bpin)2(CO)2 catalyze the photochemical borylation of alkanes and alkyl ethers (Table 1). The yield of the photochemical borylation of alkanes depends on the steric hindrance around the methyl groups. For example, the borylation of pentane proceeds with higher yields than the borylation of methylcyclohexane (Table 1, entries 1 and 2), and the functionalization of the less hindered 4-position of isopentane occurs more than three times faster than the borylation of the more hindered 1-position (Table 1, entry 3). The greater yield for borylation of n-butyl ether compared with tert-butyl ethyl ether (Table 1, entries 4 and 5) also indicates that the efficiency of borylation depends on the steric accessibility of the methyl groups. The borylation of alkanes with H Bpin as reagent, which occurs under thermal conditions, was not observed under photochemical conditions. 

Disproportionation of primary Cy alkylate (from propylene) with isobutane under alkylation conditions probably accounts for the near-equal amounts of isopentane and isohexanes found in propylene alkylate. 

Disproportionation. Disproportionation is believed to ploy only a minor role in the formation of alkylate components. What does occur is probably via a carbonium ion mechanism, i.e., when the precursor is in ionic form. Disproportionation reactions could account for the formation of the small concentrations of isopentane, isohexanes, and isoheptanes which are usually found in butene-isobutane alkylates. An example follows ... 

The compositions of the P P s and LE s as determined for this Investigation showed Important differences as compared to the compositions calculated for alkylates produced In conventional processes ooerated at 10 to 15 C. The alkylates of this Investigation produced from sec-butyl sulfates contained larger relative amounts of 2,3,4- and 2,3,3-TMP s but lesser amounts of 2,2,4-TMP. The ratios of 2,3-dlmethylbutane to Isopentane were much higher In the alkylates of the present Investigation. 

Figure 23 shows one arrangement of the fractionation system for an Isomate unit. The washed stripper bottoms pass to a depentanizer, where normal pentane and lighter hydrocarbons are separated from hexanes and heavier hydrocarbons. The overhead passes to a debutanizer, where butanes are taken off and sent to alkylation. The bottoms mixture of isopentane and normal pentane is split to produce isopentane for use in... 

Several points need to be emphasized. Eirst, the /-CgHis (frequently indicated in the literature as TMP) is really a mixture of C5-C16 isoparaffins, often with RON values in the 93-94 range. This mixture (or alkylate) is formed basically by mechanism 2 reactions. Second, when n-olefins (propylene, n-butenes, and n-pentenes) are used, the light n-paraffins formed are propane, n-butane, or n-pentane, respectively none are suitable in the gasoline pool. Third, isobutane consumption per production of a given quantity of alkylate is often increased by 6-10% when HE is employed because of the importance of mechanism 4, as compared to little or most likely no importance for alkylations with sulfuric acid. Eourth, C5 olefins are not usually used in the feedstocks when HE is the catalyst because of the large amounts of isopentane and n-pentane produced further, isobutane consumption increases. 

Since tertiary alkyl radicals are formed more readily than primary, it is to be expected that more isopentane than n-pentane will be obtained by the alkylation of propane, and more neohexane than isohexane will be produced from isobutane. The experimental results show this to be the case. 

For example, the reaction of propane with ethylene at 510° and 4500 p.B.i. pressure resulted in a liquid product containing 55.5% by weight isopentane and 16.4% ii-pentane, 7.3% hexanes and 10.1% heptanes 7.4% alkenes were also present. The formation of alkenes and other by-products indicates that thermal cracking occurred. The alkylation of isobutane with ethylene under the same conditions yielded a liquid product containing 44.3% by weight neohexane, 11.6% isohexane, 1.1% n-hexane, 4.5% heptanes, 9.6% octanes as well as minor amounts of other alkanes and alkenes. This reaction has served as a means for producing neohexane commercially. 

In virtually all cases, the alkylation of an isoparaffin with an olefin yields not only the products to be expected from the condensation of one molecule of the isoparaffin with one or more molecules of olefin but also paraffins of intermediate molecular weight. Thus, for example, pentanes, heptanes, and other alkanes containing an odd number of carbon atoms are obtained as by-products of the alkylation of isobutane with ethylene in the presence of aluminum chloride. Indeed, isopentane is usually formed in alkylation reactions involving isobutane regardless of the olefin or catalyst employed. 

When isobutane was alkylated with 1- or 2-butene in the presence of aluminum chloride monomethanolate, very little or no n-butane was formed despite the fact that appreciable amounts of 2,2,4-trimethylpentane were produced (Schmerling, 14d). Similarly, no n-butane or n-pentane, respectively, was obtained by the alkylation of isobutane with 2-butenc and with 2-pentene in the presence of sulfuric acid although trimethylpentanes were formed in both cases (McAllister et al., 12 cf. Marschner and Carmody, 24). This apparent discrepancy in the alkylation mechanism may be explained readily. The n-alkylcne is converted not into n-alkane, but into isoalkane. The resulting isobutane cannot, of course, be differentiated from that charged the resulting isopentane, on the other hand, can be and was actually found in substantial yield. In other words, the proton transfer reaction is accompanied by rearrangement of the carbon skeleton of the carbonium ion. 

Alkylation of n-butane and isobutanc with methyl or ethyl bromide in the presence of aluminum bromide has been claimed (Heldman, 36). It was found, for example, that the action of aluminum bromide (0.00158 mole as Al2Br ) on a solution of 0.0392 mole methyl bromide in 0.0750 mole n-butane at 25° for 120 hours yielded 0.0052 mole isopentane (13% of the theoretical based on the methyl bromide) isobutane was also formed. In a similar experiment carried out at 78° for 65 hours, the yield of isopentane was 33%. The reaction of isobutane (0.0520 mole) with methyl bromide (0.0392 mole) in the presence of aluminum bromide (0.00142 mole AUBrj) at 25° for 283 hours yielded low boiling material (0.0009 mole), n-butane (0.0062 mole), unreacted isobutane (0.0410, 79% of that charged), isopentane (0.0034 mole, 9% of the theory) and higher boiling material (0.0024 mole). 

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