2.2.4- Trimethylpentane, from alkylation

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). 

The following is an example of an alkylation reaction that is important in the production of isooctane (2,2,4-trimethylpentane) from two components of crude oil isobutane and isobutene. Isooctane is an antiknock additive for gasoline. 

Pecsar and Martin have measured the activity coefficients of homologous series of alkanes, chloromethanes, alkyl formates, aldehydes, amines, and alcohols in the solvents H water, 2-pentanone, and 2,3,4-trimethylpentane from 293 to 313 K. The solvents used were rather volatile and pre-saturators were essential. The work is not of high accuracy the quoted average error was 5 per cent but it is probably higher. The range of systems measured, however, makes the work interesting. The results were adequately correlated by relationships proposed by Pierotti et who have measured activity coefficients of... 

The effect of butene isomer distribution on alkylate composition produced with HF catalyst (21) is shown in Table 1. The alkylate product octane is highest for 2-butene feedstock and lowest for 1-butene isobutylene is intermediate. The fact that the major product from 1-butene is trimethylpentane and not the expected primary product dimethylhexane indicates that significant isomerization of 1-butene has occurred before alkylation. 

The alkylate contains a mixture of isoparaffins, ranging from pentanes to decanes and higher, regardless of the olefins used. The dominant paraffin in the product is 2,2,4-trimethylpentane, also called isooctane. The reaction involves methide-ion transfer and carbenium-ion chain reaction, which is cataly2ed by strong acid. 

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.

Only large-pore zeolites exhibit sufficient activity and selectivity for the alkylation reaction. Chu and Chester (119) found ZSM-5, a typical medium-pore zeolite, to be inactive under typical alkylation conditions. This observation was explained by diffusion limitations in the pores. Corma et al. (126) tested HZSM-5 and HMCM-22 samples at 323 K, finding that the ZSM-5 exhibited a very low activity with a rapid and complete deactivation and produced mainly dimethyl-hexanes and dimethylhexenes. The authors claimed that alkylation takes place mainly at the external surface of the zeolite, whereas dimerization, which is less sterically demanding, proceeds within the pore system. Weitkamp and Jacobs (170) found ZSM-5 and ZSM-11 to be active at temperatures above 423 K. The product distribution was very different from that of a typical alkylate it contained much more cracked products trimethylpentanes were absent and considerable amounts of monomethyl isomers, n-alkanes, and cyclic hydrocarbons were present. This behavior was explained by steric restrictions that prevented the formation of highly branched carbenium ions. Reactions with the less branched or non-branched carbenium ions require higher activation energies, so that higher temperatures are necessary. 

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). 

In the production of aviation and motor alkylates, both propylene and amylenes are inferior feed stocks when compared with the butylenes. These feeds produce lighter and heavier alkylates, respectively, than the butylenes, both alkylates having a lower octane than the trimethylpentane produced from butylenes. Also, acid consumption when sulfuric acid catalyst is used is two or more times as great with propylene or amylenes than with butylenes. Hydrofluoric acid catalyst, on the other hand, is not consumed at a higher rate on propylene and amylene feeds but does make a higher percentage of tar. 

Low temperatures in the alkylation reaction zone favor the isomerization of butylene-1 to butylene-2 during the alkylation reaction. Where low temperatures are not used for economic reasons, however, as is the case with many HF alkylation units, this isomerization is sometimes carried out in a separate unit and the isomerized product then charged to the alkylation unit. Catalysts for this isomerization reaction are phosphoric acid and silica-alumina. Some polymer is usually made when isomerizing with these catalysts, but when the polymer production is limited to diisobutylene, it will be alkylated to produce the trimethylpentanes. Any polymer made from normal butylenes, however, is an inferior feed for alkylation units. When this type of polymer is made, it is usually removed by fractionation before charging the isomerized butylene feed to the alkylation unit. 

The enhanced diffusivity of polynuclear compounds in sc C02 has been utilized to enhance catalyst lifetimes in both 1-butene/isoparaffin alkylations (Clark and Subramaniam, 1998 Gao et al., 1996). The former may be catalyzed using a number of solid acid catalysts (zeolites, sulfated zeolites, etc.), and the use of sc C02 as a solvent/diluent permits the alkylations to be carried out at relatively mild temperatures, leading to the increased production of valuable trimethylpentanes (which are used as high-octane gasoline blending components). The enhancement of product selectivity in the latter process is believed to result from rapid diffusion of ethylbenzene product away from the Y-type zeolite catalysts, thus preventing product isomerization to xylenes. 

Alkylene A process for making alkylate (see previous two entries) by reacting olefins with isobutene, using a proprietary solid acid catalyst called HAL-100. The major constituents of this alkylate are branched trimethylpentanes. Developed by UOP from 1995. 

The "homogeneous" alkylate is distinctly different from typical alkylate made in a well stirred pilot unit or commercial reactor. In particular it exhibits a very high ratio of dimethyl-hexanes to trimethylpentanes, and this ratio is higher in weaker acid than in stronger. Table HI ... 

In addition to saving acid the additive appeared to improve the octane number by more than 0.1 MDN as was Indicated by a few spot checks of alkylate during the run. The improvements generally arose from a slight Increase In the Cg fraction, a rise In the trimethylpentane concentration and changes of the trl-methylpentane distribution. The octane analyses are not nearly as extensive as the tltratable acidity determinations and the Improvements are noted as being consistent with what would be estimated from plant correlations and the observed reduction In acid composition. 

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. 

Trimethylpentane and its ionic form are not primary reaction products or intermediates of isabutane and 2-butene alkylotion. Found in alkylation product in very small concentrations, 2,2,3-trimethylpentane probably results mainly from isomerization of 2,2,4-trimethylpentyl carbonium ions. 

The following facts are the basis for butene isomerization (I) There is a basic similarity in the composition of alkylates produced from all four butene isomers. (2) Alkylate molecules, once formed, are relatively stable under alkylation conditions and do not isomerize to any appreciable extent alkylate fractions having the same carbon number ore not equilibrated (see Table I). (3) Thermodynamic equilibrium between the butene olefins highly favors isobutene formation at alkylation temperatures. (4) Normal butenes p>roduce only small and variable amounts of normal butane, thus indicating only a small and variable amount of chain initiation from normal butenes. Yet the alkylate composition shows a high concentration of trimethylpentanes and a low concentration of dimethylhexanes. (5) A few of the octane isomers can be explai.ned only by isomerization of the eight-carbon skeletal structure this isomerization occurs while isobutene dimer is in ionic form. For example, 2,3,3- and 2,3,4-trimethylpentanes... 

Reaction sequence I, 4, and 4-A, and sequence 6 and 6-A shown above are believed to represent the predominant ones involved in isobutane-butene alkylation, since the greater portion (80 to 90 percent) of Cs fractions from all four butenes is made up of trimethylpientanes which are predominantly 2,2,4-trimethylpentane. Hofmann and Schriesheim... 

The experimental studies reported here in Tables VII-XI indicate that os the propane-to-propylene ratio in alkylation feed was increased from 0 to 3.6/1, alkylate yield and isobutane consumption decreased significantly. The main effect on alkylate composition was an increase in the isoheptanes at the expense of the trimethylpentanes. Since, with propylene feed, trimethylpentane formation is a result mainly of hydrogen transfer reactions, the synthetic propane would be expected to decrease. This decrease was observed. A similar result occurred when normal butane was used as a diluent in alkylation feed. 

Thus, good dispersion or mass transfer favors olefin isomerization (to isobutene), isobutene dimerization, and maximizes hydrogen transfer and primary alkylation reactions, i.e., yielding the greatest amount of high-octane-number trimethylpentanes, and minimizing low-octane-number byproducts from secondary reactions such os excess polymerization. 

Tetramethylpiperidin-l-oxyl (8) has also been used as a very fast trap for cyclopropyl-alkyl radicals, and rearranged species generated from peroxides.Peroxide decomposition is induced by 8 which then combines with the transient radicals. This work included a study of the decomposition of bis[(bicyclo[2.1.0]pentan-2-yl)carbonyl]peroxide (7) in chlorobenzene and 2,2,4-trimethylpentane good yields of trialkylhydroxylamines 9 and 10 were reported. The rate of the trapping reaction was solvent dependent and the rate constant of the extremely fast clock rearrangement of the bieyclo[2.1.0]pentan-2-yl radical was determined from the yields of the two products. 

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