Trimethylpentanes , isobutane alkylation

The main products formed by the catalytic alkylation of isobutane with ethylene (HC1—AICI3, 25-35°C) are 2,3-dimethylbutane and 2-methylpentane with smaller amounts of ethane and trimethylpentanes.13 Alkylation of isobutane with propylene (HC1—AICI3, — 30°C) yields 2,3- and 2,4-dimethylpentane as the main products and propane and trimethylpentanes as byproducts.14 This is in sharp contrast with product distributions of thermal alkylation that gives mainly 2,2-dimethylbutane (alkylation with ethylene)15 and 2,2-dimethylpentane (alkylation with propylene).16... 

Since alkylate compositions from the four butene isomers are basically similar, the butenes are thought to isomerize considerably, approaching equilibrium composition prior to isobutane alkylation. Such a postulation is at variance v/ith previously published alkylation mechanisms. The Isomerization step yields predominantly isobutene which then polymerizes and forms a 2,2,4-trimethylpentyl carbonium ion, a precursor of 2,2,4-trimethylpentane, the principal end product. The 2,2,4-trimethylpentyl ion is also capable of isomerization to other trimethylpentyl ions and thus yields other trimethylpentanes, principally 2,3,4-trimethyl-pentane and 2, 3, 3-trimethylpentane. 

When alkylating isobutane, chain tennination forms primarily, but not entirely, 2,2,4-trimethylpentane the alkylate from chain termination very closely resembles isobutene alkylate. The similarity of alkylate compositions, particularly their C0 fractions, originating from various olefins and the distance from thermodynamic equilibrium composition indicates that alkylate molecules, once formed, are relatively stable under alkylation conditions and undergo little isomerization. Undesirable side products, e.g., dimethylhexanes and residue, are probably formed by buter e isomerization and polymerization (rather than by isomerization of alkylate or by isomerization of the C3 carbonium Ion which subsequently becomes alkylate). 

Olefin Dimerization. Other investigators (Schaad, 1955, Sparks et al., 1939) have reported that the catalytic polymerization of isobutene produced a liquid polymer consisting mainly of isooctenes. Hydrogenation of these isooctenes gave isooctanes consisting of 70 to 90 percent 2,2,4-trimethylpentone the remainder was reported as mainly 2,3,4-trimethyl-pentane. That earlier work used analytical techniques inferior to those now available. Typically, It Is now found that the C0 fraction of Isobutene-isobutane alkylate catalyzed by HF contains about 62.6 percent 2,2,4-trimethylpentane, 13.4 percent 2,3,4-trimethylpentane,... 

Hydrogen transfer resulting in the conversion of more or less of the olefin to paraffin always accompanies the catalytic alkylation of isoparaffins with olefins. At the same time, some of the isoparaffin is converted to paraffin containing twice the number of carbon atoms thus, for example, trimethylpentanes are formed in all isobutane alkylations regardless of the olefin used. 

An afterglow microwave plasma with stabilized pulse power was applied to the activation of zeolite catalysts for isobutane alkylation with butenes. It was found that the pretreatment of zeolite catalysts in a microwave plasma discharge affected their properties. The catalysts exhibited higher activity, stability in operation, and selectivity (the fraction of trimethylpentanes in the alkylate increased). The properties of catalysts after plasma activation depend on the treatment conditions such as plasma temperature and nonequilibrium character and depend only slightly on the initial activity of catalysts, which is primarily controlled by the catalyst preparation conditions. 

Fluorinated silica and alumina have also been used for isobutane alkylation with olefins in a batch reactor at 0°C. The fluorinated alumina was active only when mixed with BF3 H2O, but the fluorinated silica was active by itself The selectivity to trimethylpentanes obtained with these catalysts is much lower than that obtained with H2SO4 as a catalyst (48). At 80°C, however, F/AI2O3 catalysts are active for isobutane/2-butene alkylation, though relatively low butene conversions (27% for the most active catalyst containing 1.3% F) are obtained (49). As expected, octenes are the predominant hydrocarbons in the Cg fraction at such a low conversions. The F/AI2O3 catalysts contained both Bronsted and Lewis acid sites in a proportion that depends on the F loading. A good correlation between... 

Isobutane alkylation with butenes is the process of producing high-octane products, such as trimethylpentanes (TMP) with 2,2,4-trimethylpentane being most desirable product with its 100% octane number. Up to now homogeneous catalysts were used on the industrial scale for this reaction with concentrated sulfuric acid or HF being the unprecedented catalysts. The disadvantages of these catalytic systems are well known and include, inter alia, corrosion problems, which are of environmental concern because of the necessity of utilization of sulfuric and fluoride wastes. 

Propylene. Propylene alkylation produces a product that is rich in dimethylpentane and has a research octane typically in the range of 89—92. The HF catalyst tends to produce somewhat higher octane than does the H2SO4 catalyst because of the hydrogen-transfer reaction, which consumes additional isobutane and results in the production of trimethylpentane and propane. 

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.

The 1-butene conversion and product distribution obtained at 25°C after 1 h of alkylation reaction of isobutane on JML-I50 and Beta catalysts are summarized in Table 6.1. The conversion (97%) with JML-I50 catalyst is higher than that (86%) with zeolite Beta. The primary products with the above catalysts are Cs compounds (59.9% with JML-I50 and 62% with Beta). The Cg products mainly consist of trimethylpentanes (TMPs), 58.7% for JML-I50 and 73% for zeolite Beta. The TMP/DMH (dimethylhexane) ratios are 13.5 for JLM-I50 and 4.1 for Beta, demonstrating that the selectivity of JML-I50 is higher than that of zeolite Beta. The yields of alkylate are 6.6 mL and 5.2 mL for JML-I50 and Beta zeolite, respectively. The weights of alkylate produced per weight of butene fed to the reactor are 1.13 and 0.95 for JML-I50 and zeolite Beta, respectively. 

Paraffin alkylation involves the add-catalyzed addihon of an olefin to a branched paraffin to give a highly branched, paraffinic product The representahve reaction is that of isobutane with 2-butene to give 2,2,4-trimethylpentane ... 

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 ... 

Ipatieff and coworkers observed first that A1C13 catalyzes the destructive alkylation of aromatics with branched alkanes.179 For example, rm-butylbenzene (35%), p-di-rm-butylbenzene (25%), and considerable isobutane are the main products when benzene is reacted at 20-50°C with 2,2,4-trimethylpentane. Toluene and biphenyl are alkylated at 100°C in a similar way.180 Straight-chain alkanes required more severe reaction conditions. n-Pentane reacted at 175°C to yield 8% propylbenzene, 25% ethylbenzene, and 20% toluene.181 Phosphoric acid afforded similar products at higher temperature (450°C).182 Pentasil zeolites and dealumi-nated pentasils have been found to promote alkylation of benzene with C2—C4 alkanes to form toluene and xylenes.183,184... 

Studies with sulfated zirconia also show similar fast catalyst deactivation in the alkylation of isobutane with butenes. It was found, however, that original activities were easily restored by thermal treatment under air without the loss of selectivity to trimethylpentanes. Promoting metals such as Fe, Mn, and Pt did not have a marked effect on the reaction.362,363 Heteropoly acids supported on various oxides have the same characteristics as sulfated zirconia.364 Wells-Dawson heteropoly acids supported on silica show high selectivity for the formation of trimethylpentanes and can be regenerated with 03 at low temperature (125°C).365... 

More information is available about orientation, when a second alkyl group is introduced into the aromatic ring, and about relative rates. As might be expected, propene reacts more easily than ethylene [342,346] and isobutene more easily than propene [342]. Normal butenes are sometimes isomerised in the process practically the same product composition, consisting mainly of 2,2,4-trimethylpentane, is obtained in the alkylation of isobutane whether the olefin component is isobutene or 2-butene [339]. In the alkylation of aromatic hydrocarbons, this side reaction is negligible. 

Silica-supported triflic acid catalysts were prepared by various methods (treatment of silica with triflic acid at 150°C or adsorption of the acid from solutions in trifluoroacetic acid or Freon-113) and tested in the isobutane-1-butene alkylation.161 All catalysts showed high and stable activity (near-complete conversion at room temperature in a continuous flow reactor at 22 bar) and high selectivity to form saturated C8 isomers (up to 99%) and isomeric trimethylpentanes (up to 86%). Selectivities to saturated C8 isomers, however, decreased considerable with time-on-stream (79% and 80% after 24 h). 

Alkylation catalysts may have the ability of breaking down certain olefins and alkylating isobutane with the resulting olefins. A useful example of this ability is the charging of diisobutylene to an alkylation unit, wherein the diisobutylene is broken down and reacts with two molecules of isobutane to form two molecules of trimethylpentane. 

The highest quality component in the alkylation product when making aviation or motor alkylate is a mixture of trimethylpentanes. Although some of the other components have high octane, most are of inferior octane number. It is therefore desirable to make as much of the trimethylpentanes as possible. On a pure component basis, isobutylene and butylene-2 will alkylate isobutane quite readily to form trimethylpentanes. However, butylene-1 has a tendency to form dimethylhexanes. Most of these dimethyl-hexanes are of lower octane number, and their production is to be avoided... 

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. 

PFAS can be supported on silica using a dehydrating solvent, obtaining an active catalyst. Spectroscopic studies suggest that the interaction between PFAS and the support is covalent. PFAS-Si02 catalyses the alkylation of isobutane with n-butenes to yield a mixture of products, of which saturated octanes are the major fraction. Trimethylpentanes are the main constituent of this fraction. The best results were obtained using thionyl chloride as the dehydrating solvent. 

Zeolite Beta has also been studied for isobutane/butene alkylation (65, 66), but it was less selective to the desired TMP than USY, suggesting some diffusional limitations for these highly branched products at the relatively low reaction temperatures used. In fact, an increase of activity was observed when decreasing the crystal size of the Beta zeolite (66). As for USY zeolites, the activity, selectivity and deactivation rate of Beta zeolite were influenced by the presence of EFAL species (67). Medium pore zeolites, such as ZSM-5 and ZSM-11 were also found active for alkylation, but at temperatures above 100°C (68, 69). Moreover, the product obtained on ZSM-5 and ZSM-11 contained more light compounds (C5-C7), and the Os fraction was almost free of trimethylpentanes, indicating serious pore restrictions for the formation of the desired alkylation products. 

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