Molecular isobutane alkylation

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. 

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. 

Component analyses of a typical alkylation product are presented in Table I. The formation of all of these compounds cannot be explained by the simple addition of an olefin molecule to an isobutane molecule. Even the formation of some of the specific isomers having the expected molecular weight cannot be accounted for by simple molecular addition. The mechanism of the reaction, therefore, must be rather complex... 

The proposed mechanisms may also be used to explain the formation of paraffins having both lower and higher molecular weights than would be expected from simple addition of olefin molecules to isobutane molecules. A typical example is the formation of heptanes and nonanes when isobutane is alkylated with butene. The first step consists of... 

Alkylation processes usually combine isobutane with an alkene or with mixed alkene streams (C3-C5 olefins from FCC units). The best octane ratings are attained when isobutane is alkylated with butylenes. Alkylation of higher-molecular-weight hydrocarbons (>C5) is less economic because of increased probability of side reactions. Phillips developed a technology that combines its triolefin process (metathesis of propylene to produce ethylene and 2-butenes) with alkylation since 2-butenes yield better alkylate than propylene.290 Since ethylene cannot be readily used in protic acid-catalyzed alkylations, a process employing AICI3 promoted by water was also developed.291... 

Thermal alkylation was never a totally successful commercial process because of the severe operating conditions required. The reaction was carried out in a heater coil with temperatures of 900°-975°F, pressures in the range of 3000-5000 psig, and contact times of 2-7 seconds (16). Polymerization of the olefins occurred readily under these conditions, and low olefin concentrations had to be used to minimize undesirable side reactions. Ethylene could be alkylated more readily than the higher molecular weight olefins, and either normal butane or isobutane could react with the olefin. In general, the yields and quality of the product were not equal to those obtained with catalytic alkylation. 

Typical heterogeneous Ziegler catalysts operate at temperatures of 70 100°C and pressures of 0.1 2 MPa (15 300 psi). The polymerization reactions are carried out in an inert liquid medium (e.g, hexane, isobutane) or in the gas phase. Molecular weights of LLDPE resins are controlled by using hydrogen as a chain-transfer agent. Reactivities of a-olefins in copolymerization with ethylene depend on two factors the size of the alkyl groups attached to their double bonds and the type of catalyst,... 

A condensed version of the ERDL alkylation pilot plant flow plan is shown in Figure 4. Olefin and isobutane feed streams are separately pumped to the unit from large feed storage vessels with Lapp Pulsafeeder diaphragm pumps and metered with turbine flow meters. The streams are then combined, caustic scrubbed, water washed and dried with molecular sieves before being sent to the reactor. The combined feed stream is then injected into the acid-hydrocarbon emulsion in the stirred reactor vessel. In order to maintain a constant temperature environment both reactor and settler are coolant jacketed. 

Only the first and last are liquids. The stronger acids can catalyze some reactions that the weaker ones cannot (e.g., the alkylation of isobutane by 2-butene). As a rule of thumb, it is probably better to use the weakest acid that will do the job to avoid unwanted side reactions. Many solid acids, such as clays and molecular sieves, are shape- and size-selective, so that this also enters into the decision on... 

Superacid DF-SbFs induces protium-deuterium exchange in isobutane [54b]. Strong acids (BF3, BFj-HjO, HF-BFj, CF3SO3H)catalyze carbonylation of alkanes including methane by carbon monoxide [54c], whereas sulfuric acid can induce carbonylation of iso- and cycloalkanes [54e,d]. In both cases, carboxylic acids are obtained. The elimination of molecular hydrogen from alkyl can occur (see a recent theoretical study of the Hz elimination from CzHs [54f]). 

Two groups of investigators (Caesar and Francis, 11 McAllister et al., 12) have proposed mechanisms which are similar in that each assumes that the isoparaffin undergoes carbon to carbon cleavage to yield two alkyl fragments (isopropyl and methyl, in the case of isobutane) which then add to the olehnic double bond to yield a higher molecular weight paraffin. The mechanisms do account for the structures of the products obtained. However, to explain the formation of 2,3-dimethylbutane by the alkylation of isobutane with ethylene, it is necessary to assume that the ethylene acts as if it were ethylidene. A similar postulation for the alkylation of iso-butane with propene (i.e., isopropylidene) would lead to the erroneous conclusion that 2,2,3-trimethylbutane is formed in substantial amount. 

In virtually all cases, the alkylation of an isoparaffin with an olefin yields not only the products to be expected from the condensation of one molecule of the isoparaffin with one or more molecules of olefin but also paraffins of intermediate molecular weight. Thus, for example, pentanes, heptanes, and other alkanes containing an odd number of carbon atoms are obtained as by-products of the alkylation of isobutane with ethylene in the presence of aluminum chloride. Indeed, isopentane is usually formed in alkylation reactions involving isobutane regardless of the olefin or catalyst employed. 

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