Isobutane, alkylation with isobutylene

Alkylation of isoalkanes with alkenes is of particular significance. The industrially used alkylation of isobutane with isobutylene to iso-... 

Alkylation of isobutylene and isobutane in the presence of an acidic catalyst yields isooctane. This reaction proceeds through the same mechanism as dimerization except that during the last step, a proton is transferred from a surrounding alkane instead of one being abstracted by a base. The cation thus formed bonds with the base. Alkylation of aromatics with butylenes is another addition reaction and follows the same general rules with regard to relative rates and product stmcture. Thus 1- and 2-butenes yield j -butyl derivatives and isobutylene yields tert-huty derivatives. 

Isomerization. Isomerization of any of the butylene isomers to increase supply of another isomer is not practiced commercially. However, their isomerization has been studied extensively because formation and isomerization accompany many refinery processes maximization of 2-butene content maximizes octane number when isobutane is alkylated with butene streams using HF as catalyst and isomerization of high concentrations of 1-butene to 2-butene in mixtures with isobutylene could simplify subsequent separations (22). One plant (Phillips) is now being operated for this latter purpose (23,24). The general topic of isomerization has been covered in detail (25—27). Isomer distribution at thermodynamic equiUbrium in the range 300—1000 Kis summarized in Table 4 (25). 

With propene, n-butene, and n-pentene, the alkanes formed are propane, n-butane, and n-pentane (plus isopentane), respectively. The production of considerable amounts of light -alkanes is a disadvantage of this reaction route. Furthermore, the yield of the desired alkylate is reduced relative to isobutane and alkene consumption (8). For example, propene alkylation with HF can give more than 15 vol% yield of propane (21). Aluminum chloride-ether complexes also catalyze self-alkylation. However, when acidity is moderated with metal chlorides, the self-alkylation activity is drastically reduced. Intuitively, the formation of isobutylene via proton transfer from an isobutyl cation should be more pronounced at a weaker acidity, but the opposite has been found (92). Other properties besides acidity may contribute to the self-alkylation activity. Earlier publications concerned with zeolites claimed this mechanism to be a source of hydrogen for saturating cracking products or dimerization products (69,93). However, as shown in reaction (10), only the feed alkene will be saturated, and dehydrogenation does not take place. 

Formation of C8 alkanes in the alkylation of isobutane even when it reacts with propene or pentenes is explained by the ready formation of isobutylene in the systems (by olefin oligomerization-cleavage reaction) (Scheme 5.2). Hydrogen transfer converting an alkane to an alkene is also a side reaction of acid-catalyzed alkylations. Isobutylene thus formed may participate in alkylation Cg alkanes, therefore, are formed via the isobutylene-isobutane alkylation. 

Superacid-catalyzed alkylation of adamantane with lower alkenes (ethene, propene, isomeric butenes) has been investigated by Olah et al.151 in triflic acid and triflic acid-B(0S02CF3)3. Only trace amounts of 1 -ferf-butyladamantane (37) were detected in alkylation with 1- and 2-butenes, whereas isobutylene gave consistently relatively good yield of 37. Since isomerization of isomeric 1-butyladamantane under identical conditions did not give even traces of 37, its formation can be accounted for by (r-alkylation, that is, through the insertion of the ferf-butyl cation into the C—H bond (Scheme 5.22). This reaction is similar to that between ferf-butyl cation and isobutane to form 2,2,3,3-tetramethylbutane discussed above (Scheme 5.21). In either case, the pentacoordinate carbocation intermedate, which may also lead to hydride transfer, does not attain a linear geometry, despite the unfavorable steric interactions. 

The trimethylpentanes are easily produced by alkylating isobutane with isobutylene, but unfortunately, the content of isobutylene produced by catalytic cracking is only about one-third of the total butylenes in the C4 stream, the remaining butylenes being butylene-1 and butylene-2. Although most of the butylene-2 tends to form trimethylpentanes, the butylene-1 must be isomerized to butylene-2, either in the alkylation reaction or in a separate previous reaction, before it will form trimethylpentane. If not isomerized, the butylene-1 when alkylated forms the much lower-octane material, dimethylhexane. 

Alkylation of isobutane with isobutylene or butene with sulfuric acid as a catalyst... 

The surface active cations also improve product quality when alkylating with a typical refinery feed. Table V contains a list of additives studied with a feed containing 94 percent Isobutane and 6 percent butenes. The butenes contained 42 percent isobutylene and an equilibrium mixture of 1- and 2-butene. 

More than 20 runs were made to Investigate the reaction between Isobutane and the products of first-effect reactions with Isobutylene. Alkylate was not produced until excess acid was used and larger amounts of excess acid were needed for runs at -30°C as compared to runs at -10 c (3). An acld-to-olefin ratio of about 1.0 was required at -10 C to obtain TMP s whereas a 7 1 ratio was needed at -30 C. 

The alkylation mechanism for second-step reactions with isobutylene as the olefin have been significantly clarified by two runs, each conducted as follows. The acid and hydrocarbon phases produced by first-step reactions involving Isobutylene and sulfuric acid were separated. The acid phase which contained some acid-soluble hydrocarbons or reaction products such as perhaps t-butyl sulfate was then contacted with fresh Isobutane. This resulting mixture of reactants was designated as A below. The hydrocarbon... 

Second, there Is the Indirect evidence DMH s are often formed by routes In which 1-butene does not exist In significant amounts. One such reaction Is alkylation of Isobutane with Isobutylene more DMH s are produced than In comparable reactions... 

Schmerling (9,10) had originally postulated that a hydride Ion transfer from Isobutane was both the most Important method of hydride transfer and also part of the chain set of reactions (see Reaction C of Table I and Reaction M-2 of Table IV). Other hydride transfer steps that have now been suggested Include transfer with the acid-soluble hydrocarbons (RH), see Reaction M-1 of Table IV (8,11), and with Isobutylene, see Reaction M-3 (8). Reaction M-3 Is however considered to be of minor Importance since only trace amounts of free Isobutylene are likely ever present at the acid-hydrocarbon Interface (the probable location of alkylation reactions) Isobutylene quickly protonates to form t-CaHg . Reaction M-1 Is considered to be more Important than Reaction H-2 especially when sulfuric acid Is used as the catalyst for the reasons listed as follows ... 

One possible advantage of the HF process in propylene/ butylene alkylation is the production of isobutylene from isobutane effected by the hydride-ion transfer to propylene. Isobutylene is the C4 olefinic isomer which produces a significantly higher octane alkylate with HF. This shift, however, converts as much as 22% of the propylene to propane and is a debit. 

Some normal butane is also produced from butylenes but this is estimated at only 4-6%. The higher octane isobutylene alkylate and a claimed yield increase must be contrasted with normal paraffin production from olefins and a higher isobutane requirement. The typical mixed 03 = 704= feed can be made to produce a high octane alkylate with either acid catalyst by the optimization of other variables. The highest alkylate octane numbers reported are produced with sulfuric acid catalyst, alkylating with a typical cat cracker butylene olefin. 

Supplemental processes which can be operated in conjunction with alkylation and/or sulfuric acid production can influence the overall economics. Examples are (1) the integration of normal butane-to-isobutane isomerization with alkylation, utilizing common fractionation equipment and (2), utilizing 65% sulfuric acid extraction of isobutylene or isoamylene from olefins fed to alkylation, justified by monetary return on sale of the high purity iso-olefin as a petrochemical feedstock, which reduces quantity of alkylate produced and reduces isobutane required while producing still higher quality alkylate with sulfuric acid catalyst. 

Several processes are used to upgrade the C4 fraction. The isobutydene contained in the C4 cut is removed by reaction with methanol to produce MTBE. The remaining n-butenes in the C4 cut can be alkylated with isobutane catalyzed by liquid HE or H2SO4 or isomcrized into isobutylene in the presence of acid catalysts. 

Thermal alkylation occurs readily with ethylene, less readily with propene and n-butylenes, and with difficulty with isobutylene. (As wiU be shown, the reverse is true in catalytic alkylation.) The reaction of propane with propene at 505° and 6300 p.s.i. yielded a liquid product containing 18.0% 2,3-dimethylbutane, 17.7% 2-methylpentane, and 5.4% n-hexane. An even higher pressure, 8000 p.s.i. was required for the reaction of isobutylene with isobutane at 486° the liquid product, whose yield was only 10-20% of that obtained when ethylene w as used, contained 34.0% octanes but also 32.7% octenes. 

Alkylation of isobutane with isobutylene at 25° in the presence of boron fluoride promoted by water yielded an alkylate which contained 32% octanes and 15% dodecanes (Ipatieff and Grosse, 37). 

The product obtained by the alkylation of isobutane with 2-butene at 10° in the presence of 100% sulfuric acid contained 34-38% 2,2,4-tri-methylpentane and 51-55% 2,3,4- and 2,3,3-trimethylpentane (McAllister ei al., 12). 7i-Butane was not obtained the significance of this observation was discussed above (p. 42). Alkylation of isobutane with isobutylene at 20° in the presence of 98% acid yielded an alkylate which contained 24-28% by weight of 2,2,4-trimethylpentane and 30-34% of 2,3,4- and 2,3,3-trimethylpentane, as well as 7-9% of isopentane and 8-10% of 2,3-dimethyl-butane formed by destructive alkylation (McAllister et al., 12). 

As has already been mentioned, alkylation in the presence of hydrogen fluoride may be carried out over a wide range of temperature. In the alkylation of isobutane with isobutylene, for example, increasing the reaction temperature from —24° to -f32 had little effect on either the yield (198 and 195%, respectively) or the quality (91.5 and 92.0 A.S.T.M. 

Products do not contain 2,2,3-trimethylbutane or 2,2,3,3-tetramethylbutane, which would be expected as the primary alkylation products of direct alkylation of isobutane with propylene and isobutylene, respectively. In fact, the process iavolves alkylation of the alkenes by the carbocations produced from the isoalkanes via intermolecular hydride abstraction. 

On the other hand, under superacidic conditions, alkanes are readily alkylated via front-side CJ-iasertion by carbocationic alkylating agents. The direct alkylation of the tertiary C—H CJ-bond of isobutylene with isobutane has been demonstrated (71). The stericaHy unfavorable reaction of tert-huty fluoroantimonate with isobutane gave a Cg fraction, 2% of which was 2,2,3,3-tetramethylbutane ... 

The alkylation unit in a petroleum refinery is situated downstream of the fluid catalytic cracking (FCC) units. The C4 cut from the FCC unit contains linear butenes, isobutylene, n-butane, and isobutane. In some refineries, isobutylene is converted with methanol into MTBE. A typical modern refinery flow scheme showing the position of the alkylation together with an acid regeneration unit is displayed in Fig. 1. 

The propylene-butylene fraction constitutes a large part of the useful hydrocarbons produced by synthesis. It differs from similar fractions derived from petroleum refining in its high olefin (over 80%) and low isobutylene content, but this is no handicap in converting it to high octane gasoline by polymerization or by alkylation, if isobutane is available from another source. Polymerization is effected readily over a phosphoric acid on quartz catalyst with high conversion of propylene as well as butylene. The polymer... 

The acidity dependence of the isobutane-isobutylene alkylation was studied using triflic acid modified with trifluoroacetic acid (TFA) and water in the range of acidity... 

A comparative study of nanocomposites (16% Nafion-silica and commercial SAC-13) has been performed by Hoelderich and co-workers169 in the alkylation of isobutane and Raffinate II. Raffinate II, the remaining C4 cut of a stream cracker effluent after removal of dienes, isobutane, propane, and propene, contains butane, isobutylene, and butenes as main components. High conversion with a selectivity of 62% to isooctane was found for Nafion SAC-13 (batch reactor, 80°C). Both the quality of the product and the activity of the catalysts, however, decrease rapidly due to isomerization and oligomerization. Treating under reflux, the deactivated catalysts in acetone followed by a further treatment with aqueous hydrogen peroxide (80°C, 2 h), however, restores the activity. 

The primary reactions in the alkylation of isobutane produce octanes from butylenes, heptanes from propylene, and nonanes from amylenes. Also, when dimer and trimer polymers of isobutylene are used with isobutane, the polymer is broken down during the reaction, and the resulting products are branched chain octanes similar to those produced when isobutylene is charged. Sulfuric acid consumption is somewhat higher for the diisobutylenes, however, and there are more side reactions than for isobutylene. 

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