Reaction isobutane alkylation

The principal use of the alkylation process is the production of high octane aviation and motor gasoline blending stocks by the chemical addition of C2, C3, C4, or C5 olefins or mixtures of these olefins to an iso-paraffin, usually isobutane. Alkylation of benzene with olefins to produce styrene, cumene, and detergent alkylate are petrochemical processes. The alkylation reaction can be promoted by concentrated sulfuric acid, hydrofluoric acid, aluminum chloride, or boron fluoride at low temperatures. Thermal alkylation is possible at high temperatures and very high pressures. 

Alkylation in the petroleum industry, a process by which an olefin (e.g., ethylene) is combined with a branched-chain hydrocarbon (e.g., isobutane) alkylation may be accomplished as a thermal or a catalytic reaction. 

TABLE 13.2 Isobutane Alkylation Catalyzed by Beta Zeolite and Sulfated Zirconia at Different Reaction Temperatures... 

A clear example of the possible use of acid and/or superacid solids as catalysts is the alkylation of isobutane with butenes. Isobutane alkylation with low-molecular-weight olefins is one of the most important refining process for the production of high-octane number (RON and MON), low red vapor pressure (RVP) gasoline. Currently, the reaction is carried out using H2SO4 or HF (Table 13.1), although several catalytic systems have been studied in the last few years. 

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. 

Carlier fundamental studies of autoxidations of hydrocarbons have concentrated on liquid-phase oxidations below 100 °C., gas-phase oxidations above 200°C., and reactions of alkyl radicals with oxygen in the gas phase at 25°C. To investigate the transitions between these three regions, we have studied the oxidation of isobutane (2-methylpropane) between 50° and 155°C., emphasizing the kinetics and products. Isobutane was chosen because its oxidation has been studied in both the gas and liquid phases (9, 34, 36), and both the products and intermediate radicals are simple and known. Its physical properties make both gas- and liquid -phase studies feasible at 100°C. where primary oxidation products are stable and initiation and oxidation rates are convenient. 

Such reactions can take place predominantly in either the continuous or disperse phase or in both phases or mainly at the interface. Mutual solubilities, distribution coefficients, and the amount of interfadal surface are factors that determine the overall rate of conversion. Stirred tanks with power inputs of 5-10 HP/1000 gal or extraction-type equipment of various kinds are used to enhance mass transfer. Horizontal TFRs usually are impractical unless sufficiently stable emulsions can be formed, but mixing baffles at intervals are helpful if there are strong reasons for using such equipment. Multistage stirred chambers in a single shell are used for example in butene-isobutane alkylation with sulfuric acid catalyst. Other liquid-liquid processes listed in Table 17.1 are numbers 8, 27, 45, 78, and 90. 

PFAS were obtained with 2 moles of water, for each mole of acid and they could not be dehydrated with physical methods. Hydrated acids, both as such and supported on silica using water as solvent, were not active in isobutane alkylation. Therefore the effect of different dehydrating solvent was studied, in order to remove residual water. The catalysts obtained by supporting perfluoroethanedisulphonic acid on Si02 (PFES-Si02) after dissolution in various dehydrating solvents were tested in the reaction and resulted active with high butene conversion (Table 1). 

All four butene isomers ore believed to be capable of undergoing isomerization and polymerization under alkylation conditions. These ionic reactions are extremely raoid and precede or accompany isobutane alkylation. 

Chain Initiotion. The theory pxKtulated by a number of investigators (Cupit etal., 1961, Schmerling, 1955) is that carbonium ions are generated by addition of a proton (H+) to an olefin molecule in the presence of HF. Albright and Li, 1970, and Hofmann and Schriesheim, 1962, indicate that initiation steps with H2SO4 catalyst may involve red oil hydrocarbons. However, only the tertiary butyl carbonium ion performs the chain carrying function in isobutane alkylation. Reactions follow ... 

Thus, the most direct route to chain-carrying, tertiary butyl carbonium Ions is offered in isobutene-isobutane alkylation (Equation I). When initiating with either a linear butene or propylene, a second step is necessary to form the tertiary butyl carbonium ion, i.e., abstraction of a hydride Ion from an isobutane molecule while forming a molecule of normal alkane. (Equation 2, 2-A, 3, 3-A). Reaction sequences in these equations are often referred to as hydrogen- or hydride transfer reactions and will be discussed subsequently. 

Chain Termination. Chain termination in isobutane alkylation Is any reaction sequence which results in the elimination of a tertiary butyl carbonium ion. Specifically, two tertiary butyl carbonium ions are consumed... 

Olefin Isomerization. Olefin isomerization plays an important role in butene-isobutane alkylation reaction mechanisms. Normal butenes are largely isomerized to isobutene before alkylation. This is believed to take place in ionic form, i.e., immediately following olefin protonation, since a number of olefins have been found to odd HF across their double bonds quite readily at room temperature (Grosse and Linn, 1938). Thus, the likelihood of olefin molecules being present for very long under alkylation conditions is not great. 

Excess Polymerization. A small amount of high-boiling heavy "tail" or residue Is formed in Isobutane alkylation, even urxJer the most favorable reaction conditions. The polymer molecule is in reality on isoparaffin formed from two or more molecules of olefin plus one molecule of Isobutane. Polymer is formed because of the inherent tendency of larger carbonium ions, e.g., Cj or C0 ions, to complete with tertiary butyl carbonium ions for addition of olefin molecules before abstracting hydride ions and becoming isoparaffin molecules. Reactions follow ... 

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 ... 

Normal Butene Reactions. Under alkylation conditions, all four butene isomers are believed to undergo isomerization, dimerization, and co-dimerization when first coming in contact with HF catalyst, i.e., immediately following protonation. These are very rapid, Ionic reactions and take place competitively along with isobutane alkylation. Alkylate compositions from the four butenes are basically similar (see Table VII). However, l-butene produces a C3 fraction containing nearly two times... 

Propylene Reactions. The following reaction mechanisms are generally 7ecognTzeTM tlTeprinci pa I ones occurring in propylene-isobutane alkylation with hydrofluoric acid catalyst (Ciapetta, 1945). In pnirenthe-ses are shown amounts of oroducts from each mechanism these are from Table VII for propylene ... 

Effects of Water in HF Catalyst. A number of investigators have pointed out that water has an important role in alkylation catalysts. Schmer-ling (1955) stated that the use of HF catalyst with one percent water produced a favorable result In propylene-isobutane alkylation, whereas, with a catalyst containing ten percent water, isopropyl fluoride was the principal product and no alkylate was formed. (Both reactions were at 25C.) Albright et al. (1972) found the water content of sulfuric acid to be "highly important" In affecting the quality and yield of butene-isobutane alkylate. They postulated that the water content of sulfuric acid controlled the level of ionization and hydride transfer rate In the catalyst phase. It appears that dissolved water affects HF alkylation catalyst similarly and also exerts further physical influence on the catalyst phase such as reducing viscosity. Interfacial tension, and isobutane solubility. 

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... 

The second group of papers (chapters 13-20) discusses the more practical aspects of isobutane alkylation including mixing, reaction variables, computer modeling, recovery of catalyst, and an alternate fuel to alkylate. 

The regeneration of Y-zeolite catalysts used in isobutane alkylation with C4 olefins was studied. The coke formed on these catalysts during this reaction needs temperatures higher than 500°C to be burnt off with air. Ozone was used in this study to eliminate most of the coke at a much lower temperature. After a treatment at 125 C with ozone, the small amount of coke remaining on the catalyst can be removed with air at 250°C. The ozone not only eliminates coke from the catalyst, but also modifies its burning characteristics as measured by Temperature Programmed Oxidation, shifting the peak to lower temperatures. This allows a combined treatment with ozone at 125°C followed by air at 250°C to restore the activity and stability of Y-zeolite catalysts for isobutane alkylation. 

In this work, the regeneration with ozone of Y-zeolite catalysts, exchanged with lanthanum, is studied. The objective is to find a low temperature regeneration procedure for the solid acid catalysts used in the isobutane alkylation reaction. 

The activity of Nafion composites of greater surface area was investigated in different organic reactions, e. g. Friedel-Crafts alkylation and acylation, the Fries rearrangement, the dimerization of a-methylstyrene, esterification reactions, and isobutane alkylation. 

Isobutane Alkylation. The deactivation of solid acid catalysts due to coke deposition is the cause of not having as yet, a commercially available process for isobutane alkylation with C4 olefins, using solid acid catalysts. The coke on these catalysts have been characterized with TPO analyses . The TPO profiles on zeolites used in this reaction, displayed two well defined burning zones. One peak below 300°C, and the other at high temperatures. The relative size of these peaks depends on the zeolite and the reaction temperature. In the case of the mordenite, the first peak was the most important, and in the case of the Y-zeolite, at 50°C or... 

The BET surface area was determined for both fresh and spent catalysts, during the isobutane alkylation with 1-butene . LaY and Lap zeolites displayed a decrease in the BET area of 45 % approximately due to the coking, while the amorphous silica alumina a decrease of 33 %. This is the case where the pretreatment of the coked catalysts before the BET determination will eliminate some of the carbonaceous deposits, since the reaction temperature is typically below 100°C, and the pretreatment for BET determination with zeolite catalysts, is usually around 250°C. TPO studies clearly demonstrated that this treatment under vacuum eliminates a fraction of the coke, and therefore the real decrease in surface area due to coke deposition is larger than that measured by BET. 

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