Isobutane-isobutylene alkylation

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... 

Two reports demonstrate that Bronsted (Olah et al., 1999) or Lewis acids (Oakes et al., 1999) may be more reactive in sc C02 than in organic solvents. The isobutane-isobutylene alkylation (eq. 2.2), which is used commercially to increase the octane numbers in automotive fuels, has been demonstrated to occur in sc C02 with several liquid acid catalysts (Olah et al., 1999). Increased selectivity for Cg-alkylates is observed in sc C02 relative to neat acid media when anhydrous HF, pyridine poly(hydrogen fluoride) (PPHF),... 

Olah GA, Mathew T, Goeppert A et al (2005) Ionic liquid and solid HF equivalent amine-poly(hydrogen fluoride) complexes effecting efficient environmentally friendly isobutane-isobutylene alkylation. J Am Chem Soc 127 5964—5969... 

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). 

The alkylation of alkanes by olefins, from a mechanistic point of view, must be considered as the alkylation by the carbenium ion formed by the protonation of the olefin. The well-known acid-catalyzed isobutane-isobutylene reaction demonstrates the mechanism rather well (Scheme 5.18). 

The cyclic voltammograms at vitreous carbon electrodes for 2-iodooctane, r-butyl bromide and -butyl iodide show two waves [e.g., -1.6 V and -1.8 V (see) for r-butyl bromide] indicating stepwise generation of alkyl radicals and carbanions. The products of large-scale electrolyses of r-butyl bromide (isobutane, isobutylene, 2,2,3,3-tetramethyl-butane) are indicative of the involvement of both radical and carbanion species214. 

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. 

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. 

Isopentane and trimethylpentanes (Cg alkanes, often called abnormal products) are always formed when isobutane is alkylated even when it reacts with propene or pentenes. Formation of isopentane involves the reaction of a primary alkylation product (or its carbocation precursor) with isobutane followed by alkyl and methyl migrations and, eventually, j8-scission. Formation of Cg alkanes is explained by the ready formation of isobutylene either by olefin oligomerization-cleavage reaction or hydrogen transfer converting isobutane to isobutylene. Isobutylene thus formed may participate in alkylation jdelding Cg alkanes. 

Alkylation combines lower-molecular-weight saturated and unsaturated hydrocarbons (alkanes and alkenes) to produce high-octane gasoline and other hydrocarbon products. Conventional paraffin-olefin (alkane-alkene) alkylation is an acid-catalyzed reaction, such as combining isobutylene and isobutane to isooctane. 

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

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 ... 

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. 

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. 

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. 

Besides ethylene and propylene, the steam cracking of naphtha and catalytic cracking in the refinery produce appreciable amounts of C4 compounds. This C4 stream includes butane, isobutane, 1-butene (butylene), cis- and trans-2-hutene, isobutene (isobutylene), and butadiene. The C4 hydrocarbons can be used to alkylate gasoline. Of these, only butadiene and isobutylene appear in the top 50 chemicals as separate pure chemicals. The other C4 hydrocarbons have specific uses but are not as important as butadiene and isobutylene. A typical composition of a C4 stream from steam cracking of naphtha is given in Table 8.3. 

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... 

Acid-catalyzed alkylation and isomerization processes all proceed through carbocations. Typical is the isobutylene-isobutane alkylation giving high-octane isooctane. 

Isomerization of straight-chain to branched alkanes also increases the octane number, as do alkylates produced by alkene-isoalkane alkylation (such as that of isobutane and propylene, isobutylene, etc.). These large-scale processes are by now an integral part of the petroleum industry. Refining and processing of transportation fuels became probably the largest-scale industrial operation. 

C4 Alkenes. Several industrial processes have been developed for olefin production through catalytic dehydrogenation138 166 167 of C4 alkenes. All four butenes are valuable industrial intermediates used mostly for octane enhancement. Isobutylene, the most important isomer, and its dimer are used to alkylate isobutane to produce polymer and alkylate gasoline (see Section 5.5.1). Other important utilizations include oxidation to manufacture maleic anhydride (see Section 9.5.4) and hydroformylation (see Section 7.1.3). 

Even the alkylation of isobutane by the ferf-butyl cation 4 despite the highly unfavorable steric interaction has been demonstrated142 by the formation of small amounts of 2,2,3,3-tetramethylbutane 36. This result also indicates that the related five-coordinate carbocationic transition state (or high-lying intermediate) 35 of the degenerate isobutylene-terf-butyl cation hydride transfer reaction is not entirely linear, despite the highly crowded nature of the system (Scheme 5.21). 

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. 

---