Dehydrogenation of isopentane

Examples of reverse spillover (or backspillover) are the dehydrogenation of isopentane and cyclohexane on active carbon. Deposition of a transition metal on the active carbon accelerates the recombination of H to H2 due to a reverse spillover or backspillover effect.72... 

It seems that very few investigations concern the oxidation or oxidative dehydrogenation of C5 alkanes. Oxidative dehydrogenation of isopentane to isoprene has been mentioned. Two articles deal with Mn02, CoO/CaOs, Na0H/Al203, but in the presence of HI [51,52] this obviously suggests the intervention of gas-phase reactions. The yields (Y) in isobutene were relatively hi (e.g., Y = 50-60% with a selectivity of 65 to 95%). Pentane can also produce maleic a y de and phthalic anhydride [53-57]. 

Isoprene can also be obtained from the C5 fraction of petroleum by dehydrogenation of isopentane. 

The discussion to this point has emphasized kinetics of catalytic reactions on a uniform surface where only one type of active site participates in the reaction. Bifunctional catalysts operate by utilizing two different types of catalytic sites on the same solid. For example, hydrocarbon reforming reactions that are used to upgrade motor fuels are catalyzed by platinum particles supported on acidified alumina. Extensive research revealed that the metallic function of Pt/Al203 catalyzes hydrogenation/dehydrogenation of hydrocarbons, whereas the acidic function of the support facilitates skeletal isomerization of alkenes. The isomerization of n-pentane (N) to isopentane (I) is used to illustrate the kinetic sequence associated with a bifunctional Pt/Al203 catalyst ... 

Isoprene (2-methylbuta-1,3-diene) is produced mainly by the dehydrogenation of isoamylenes (2-methylbutenes) or isopentane (2-methylbutane), in processes similar to those used in the U.S.A. for butadiene manufacture. 

Brenner and Emmett examined the cracking of isopentane over silica-tdumina catalysts and found the main initial product to be pentenes. This indicates that the first step is dehydrogenation of the alkane, the breaking of carbon — carbon bond being a... 

Some isopentane is dehydrogenated to isoamylene and converted, by processes analogous to those which produce methyl /-butyl ether [1634-04-4] (MTBE) to /-amyl methyl ether [994-05-8] (TAME), which is used as a fuel octane enhancer like MTBE. The amount of TAME which the market can absorb depends mostly on its price relative to MTBE, ethyl /-butyl ether [637-92-3] (ETBE), and ethanol, the other important oxygenated fuel additives. 

With propene, n-butene, and n-pentene, the alkanes formed are propane, n-butane, and n-pentane (plus isopentane), respectively. The production of considerable amounts of light -alkanes is a disadvantage of this reaction route. Furthermore, the yield of the desired alkylate is reduced relative to isobutane and alkene consumption (8). For example, propene alkylation with HF can give more than 15 vol% yield of propane (21). Aluminum chloride-ether complexes also catalyze self-alkylation. However, when acidity is moderated with metal chlorides, the self-alkylation activity is drastically reduced. Intuitively, the formation of isobutylene via proton transfer from an isobutyl cation should be more pronounced at a weaker acidity, but the opposite has been found (92). Other properties besides acidity may contribute to the self-alkylation activity. Earlier publications concerned with zeolites claimed this mechanism to be a source of hydrogen for saturating cracking products or dimerization products (69,93). However, as shown in reaction (10), only the feed alkene will be saturated, and dehydrogenation does not take place. 

Similar to the processes used in the manufacture of 1,3-butadiene, isoprene can be prepared from isopentane, isoamylenes, or a mixed isoC5 feed.172 176 177 The Shell process177 dehydrogenates isoamylenes to isoprene in the presence of steam with 85% selectivity at 35% conversion, over a Fe203—K2CO3—Cr2Oj catalyst at 600°C. 

Corma and co-workers152 have performed a detailed theoretical study (B3PW91/6-31G level) of the mechanism of the reactions between carbenium ions and alkanes (ethyl cation with ethane and propane and isopropyl cation with ethane, propane, and isopentane) including complete geometry optimization and characterization of the reactants, products, reaction intermediates, and transition states involved. Reaction enthalpies and activation energies for the various elemental steps and the equilibrium constants and reaction rate constants were also calculated. It was concluded that the interaction of a carbenium ion and an alkane always results in the formation of a carbonium cation, which is the intermediate not only in alkylation but also in other hydrocarbon transformations (hydride transfer, disproportionation, dehydrogenation). 

Among commercial methods of isoprene production, one of the main methods is the process of two-stage isopentane dehydrogenation. Isopentane and isopentene dehydrogenation is carried out at elevated temperatures followed by the formation of a number of by-products among which a great part is piperylene (pentadiene-1,3) [9, 10]. Piperylene yield is 10-15% of the sum of the diene monomers during isoprene production. 

Piperylene occurs in the Cs cut of a cracked naphtha fraction alongside, but in smaller amounts than isoprene. At least one isopentane dehydrogenation process used for making isoprene also yields some piperylene. 

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