Oxygen reaction with isobutane

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. 

Inserting oxygen into the C-H bond of an alkane initially leads to hydroperoxides. When this reaction is performed with atmospheric oxygen it is also called autooxidation. It usually leads to a multitude of products, because of further spontaneous reactions, so this reaction is of limited synthetic use. An exception is oxidation of isobutane with oxygen, which leads to 70 % yield of tert-butyl hydroperoxide at a conversion of 80% (Table 1, entry 7). Hydrogen bromide is used, among other compounds, as an initiator [15]. tert-Butyl hydroperoxide is used as an oxidant in propylene oxide production by the Halcon process. In the formation of phenol by the cumene process cumene is oxidized into the corresponding hydroperoxide in a similar way. 

More recent studies by Irvine and Knox [50] on the competitive oxidation of isobutane with ethane and propane at 300 °C have also led them to conclude that at low rates of reaction of isobutane a heterogeneous component leading to isobutene does indeed occur in parallel, but independently of the homogeneous reaction under most experimental conditions used in slow oxidation studies. They have suggested, however, in agreement with Semenov, that the reaction responsible probably involves the direct reaction of oxygen with isobutane adsorbed on the surface of the reactor (see p. 263), viz. 

A similar increase in reactivities in the methyl-methylene-methine series is found in the free-radical oxidations of lower alkanes with oxygen in the presence of hydrogen bromide as an initiator of the reaction. Ethane gives a 64% yield of acetic acid at 220 °C, propane gives a 72% yield of acetone at 189 °C, and isobutane gives a 69.5% yield of terf-butyl hydroperoxide, a 10% yield of fm-butyl alcohol, and a 6% yield of di-rm-butyl peroxide at 163 °C [54],... 

On the other hand, the reaction of ethane, propane, isobutane, and pentanes with oxygen described until now are poorly selective at high, and even at moderate conversions. One cannot... 

Isobutane reacts with high purity oxygen noncatalytically at about 300°F and 500 psig in accordance with Eq. (13) to form tertiary butyl hydroperoxide (TBHP). A small amount of tertiary butyl alcohol is also formed by the reaction of isobutane with oxygen, as shown in Eq. (14). The TBHP then reacts with propylene in the presence of a molybdenum catalyst, as shown in Eq. (15), at about 250°F and 600 psig to yield propylene oxide and tertiary butyl alcohol as a by-product. 

At last, in the cases of isobutane, n-butane and isopentane, the presence of oxygen increases the relative weight of the deme-thanation" reaction (with regard to the other decomposition reactions of the alkane). This indicates that the free-radical becomes less easily oxidized than other chain carriers of the alkane decomposition. 

Results obtained in an investigation of the pyrolyses of four alkanes (ethane, n-butane, isobutane and isopentane) in the presence of trace amounts of oxygen, at low extent of reaction and around 500 C, are reported. The organic products of the primary oxidation are mainly olefins. According to experimental conditions (particularly to wall conditions of the reaction vessel), oxygen accelerates or inhibits the alkane decomposition. Walls inhibit the oxygen consumption. These experimental facts are interpreted and compared with recent results in literature. 

Because the protonation of ozone removes its dipolar nature, the electrophilic chemistry of HOs, a very efficient oxygenating electrophile, has no relevance to conventional ozone chemistry. The superacid-catalyzed reaction of isobutane with ozone giving acetone and methyl alcohol, the aliphatic equivalent of the industrially significant Hock-reaction of cumene, is illustrative. 

Schubert, C.C., Pease, R.N. (1956) The oxidation of lower paraffin hydrocarbons. I. Room temperature reaction of methane, propane, n-butane and isobutane with ozonized oxygen. J. Am. Chem. Soc. 78, 2044—2048. 

POM composed of (NH4)3PMoi204o data were collected at a reaction temperature of 380°C, with an isobutane-rich feedstock (26 mol % isobutane, 13% oxygen, 12% steam, remainder helium), and a residence time of 3.6 s. At the very beginning of its lifetime, the fresh POM was completely unselective and inactive. After approximately 100 hours reaction time, it was 6.5% converted, with a selectivity to methacrylic acid of 42% and to methacrolein of 13%. The main by-product was carbon dioxide. Therefore, the equilibration time was necessary for the generation of the active and selective sites. 

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