Isobutylene, isomerization

CDTECH n-Butenes Mixed C hydrocarbons Isobutylene isomerization to produce normal butenes NA NA... 

Ethers, such as MTBE and methyl / fZ-amyl ether (TAME) are made by a catalytic process from methanol (qv) and the corresponding isomeric olefin. These ethers have excellent octane values and compete on an economic basis with alkylation for inclusion in gasoline. Another ether, ethyl tert-huty ether (ETBE) is made from ethanol (qv) and isobutylene (see Butylenes). The cost and economic driving forces to use ETBE vs MTBE or TAME ate a function of the raw material costs and any tax incentives that may be provided because of the ethanol that is used to produce it. 

Production of maleic anhydride by oxidation of / -butane represents one of butane s largest markets. Butane and LPG are also used as feedstocks for ethylene production by thermal cracking. A relatively new use for butane of growing importance is isomerization to isobutane, followed by dehydrogenation to isobutylene for use in MTBE synthesis. Smaller chemical uses include production of acetic acid and by-products. Methyl ethyl ketone (MEK) is the principal by-product, though small amounts of formic, propionic, and butyric acid are also produced. / -Butane is also used as a solvent in Hquid—Hquid extraction of heavy oils in a deasphalting process. 

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 H2SO4 catalyst produces a high octane product of similar composition from either 2-butene or 1-butene. This fact suggests that the isomerization of 1-butene to 2-butene is more complete than in the HF system. Isobutylene produces a slightly lower product octane than do the / -butenes. The location of a methyl tert-huty ether [1634-04-4] (MTBE) process upstream of the H2SO4 alkylation unit has a favorable effect on performance because isobutylene is selectively removed from the alkylation feed. 

Another process involves a one-step reaction of isobutylene with formaldehyde and acetone under high temperature and pressure (eq. 2) (20). a-MethyUieptenone (2) (6-methyIhept-6-en-2-one [10408-15-8]) is the product, but it is easily catalyticaHy isomerized to P-methyUieptenone (21,22). Unconverted isobutylene and acetone can be recycled to the process, thus making it commercially viable (23,24). Variations of this process have also been described in the Hterature (25—28). 

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 three isomerizations, ct5-2-butene trans-2-huiene, 1-butene 2-butene, and butenes isobutylene, require increasingly severe reaction conditions. When the position of the double bond is shifted, cis—trans isomerization also occurs, and mixtures of butenes result when the carbon skeleton... 

Kennedy, J. P. and Chou,T. Poly (isobutylene-co-fS-Pinene) A New Sulfur Vulcanizable, Ozone Resistant Elastomer by Cationic Isomerization Copolymerization. Vol. 21, pp. 1—39. 

Studies of hydrosilation with trichlorosilane-d (2f) proved that exchange can also take place between SiD and C—H bonds in olefins during hydrosilation. Isobutylene was chosen as the olefin for this study because both it and isobutyltrichlorosilane have H NMR spectra that are easy to interpret, and because movement of the double bond can give rise to no detectable isomerization. Excess trichlorosilane-d with isobutylene and chloroplatinic acid was sealed into a Pyrex tube and kept near 25°C overnight. Deuterosilation was complete in less than 1 hour. Analysis of the product after about 16 hours indicated reactions that can be summed up as follows ... 

ISOFIN [Iso olefins] A cataylytic process for making iso-olefins from normal olefins by skeletal isomerization. The principle example converts //-butenes to isobutylene, needed as a feedstock for making methyl /-butyl ether. Developed by BP Oil Company, Mobil Corporation, and MW Kellogg from 1992. 

When -butenes are used, the initiation produces a secondary carbenium ion/butoxide. This species may isomerize via a methyl shift (Reaction (2)) or accept a hydride from isobutane to form the tertiary butyl cation (Reaction (3)). Isobutylene forms the tertiary cation directly. 

The isomerization of light olefins is usually carried out to convert -butenes to isobutylene [12] with the most frequently studied zeolite for this operation being PER [30]. Lyondell s IsomPlus process uses a PER catalyst to convert -butenes to isobutylene or n-pentenes to isopentene [31]. Processes such as this were in larger demand to generate isobutene before the phaseout of MTBE as a gasoline additive. Since the phaseout, these processes often perform the reverse reaction to convert isobutene to n-butenes which are then used as a metathesis feed [32]. As doublebond isomerization is much easier than skeletal isomerization, most of the catalysts below are at equilibrium ratios of the n-olefins as the skeletal isomerization begins (Table 12.5). 

Therefore, the base-catalyzed reaction of isobutylene yields the same dimer as the acid-catalyzed reaction although the mechanisms are completely different. Since olefin isomerizations are also catalyzed under these conditions, an equilibrium distribution of products is expected for example, the reaction of isobutylene yields 78% of 2,4,4-trimethyl-l-pentene and 22% of 2.4,4-trimethyl-2-pentene. 

The epoxidation of propylene is discussed in Chapter 10, Section 2. Some isobutane can be made by isomerizing -butane. The isomerization of -butenes to isobutylene is also being commercialized. 

Isomerization polymerizations can be associated with coordination catalyst systems, ionic catalyst systems, and free radical systems. The cationic isomerization polymerization of 4-methyl-1-pentene is of interest because the product can be viewed as an alternating copolymer of ethylene and isobutylene. This structure cannot be obtained by conventional... 

Dimerization of propylene is also used to produce isoprene. Several steps are involved. Initially, dimerization of propylene to 2-methyl-1-pentene occurs. Then isomerization to 2-methyl-2-pentene is effected. Finally, the 2-methyl-2-pentene is pyrolyzed to isoprene and methane. Another multistep synthesis starts with acetylene and acetone. Perhaps the most attractive route involves formaldehyde and isobutylene (equation 17.42). 

It is the opinion of the present authors that isomerization of a tertiary alkyl radical to a primary radical as in the formation of II from I is improbable. The formation of IV is similarly unlikely. The cycliza-tion of V by intramolecular alkylation seems quite plausible however, equation 9 does not explain either the formation of V or its subsequent cyclization. The following mechanism has the advantages that, like the generally accepted free radical-initiated mechanisms, it postulates a chain reaction and that the intramolecular alkylation step is directly analogous to that proposed for thermal alkylation, namely addition of an alkyl radical to the double bond of the alkene (Frey and Hepp, 12). The method of formation of the chain initiator, R —, again is not critical since R —, merely starts the first cycle of the chain reaction it may be formed by decomposition of the isobutylene. 

Some carbonyl compounds are unreactive in the photocycloaddition reaction because their excited states undergo molecular rearrangement. Illustrative of this point is the isomerization process involved upon irradiation of 2-methylbenzophenone. This ketone is known to be stable to irradiation in isopropanol,45 and irradiation in the presence of isobutylene yields a complex mixture which contains little, if any,... 

Butenes or butylenes are hydrocarbon alkenes that exist as four different isomers. Each isomer is a flammable gas at normal room temperature and one atmosphere pressure, but their boiling points indicate that butenes can be condensed at low ambient temperatures and/or increase pressure similar to propane and butane. The 2 designation in the names indicates the position of the double bond. The cis and trans labels indicate geometric isomerism. Geometric isomers are molecules that have similar atoms and bonds but different spatial arrangement of atoms. The structures indicate that three of the butenes are normal butenes, n-butenes, but that methylpropene is branched. Methylpropene is also called isobutene or isobutylene. Isobutenes are more reactive than n-butenes, and reaction mechanisms involving isobutenes differ from those of normal butenes. 

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