Isobutane, oxidation mechanism

THE MECHANISM OE SELECTIVE ISOBUTANE OXIDATION CATALYZED BY KEGGIN P/Mo POM,... 

Figure 4. Proposed mechanisms for isobutane oxidation on Mo- and W-containing...

Scheme IILIO. Proposed mechanism for isobutane oxidation on W-containing heteropolycompound.

Schubert and Pease (11) proposed a mechanism for isobutane oxidation in the gaseous state, in which the first stage... 

The most pronunent example is the partial oxidation of sc isobutane (Tc = 134.7" C,pc = 36.3 bar. Pc = 0.225 g/cm ) towards tert-butanol via hydroperoxides. The reaction is autocatalytic and follows a similar mechanism as in cyclohexane oxidation. Isobutane oxidation has gained importance because of the applications of the oxidation products, tert-butyl hydroperoxide and tert-butyl alcohol, in the manufacture of important chenucals like propylene oxide and methyl tert-butyl ether (MTBE). A comparative study between supercritical oxidation and liquid-phase oxidation with air, but without a catalyst, provides thermodynamic and kinetic data on isobutane oxidation. Supercritical conditions provided higher rates and selectivities than in liquid phase, because a liquid phase-like mechanism runs at... 

Butane-Naphtha Catalytic Liquid-Phase Oxidation. Direct Hquid-phase oxidation ofbutane and/or naphtha [8030-30-6] was once the most favored worldwide route to acetic acid because of the low cost of these hydrocarbons. Butane [106-97-8] in the presence of metallic ions, eg, cobalt, chromium, or manganese, undergoes simple air oxidation in acetic acid solvent (48). The peroxidic intermediates are decomposed by high temperature, by mechanical agitation, and by action of the metallic catalysts, to form acetic acid and a comparatively small suite of other compounds (49). Ethyl acetate and butanone are produced, and the process can be altered to provide larger quantities of these valuable materials. Ethanol is thought to be an important intermediate (50) acetone forms through a minor pathway from isobutane present in the hydrocarbon feed. Formic acid, propionic acid, and minor quantities of butyric acid are also formed. 

Figure 14.4 Mechanism of the oxidation of isobutane to methacrylic acid catalyzed by Keggin-type POMs.

Simplest Mechanism. The kinetics and products of the liquid-phase oxidation of neat isobutane (tert-BuH) are largely explained by the following steps. (We have found no significant reactions of primary C—H bonds.) Similar steps also apply to the liquid-phase oxidation of cumene (6,12). 

Osmium-catalysed dihydroxylation has been reviewed with emphasis on the use of new reoxidants and recycling of the catalysts.44 Various aspects of asymmetric dihydroxylation of alkenes by osmium complexes, including the mechanism, acceleration by chiral ligands 45 and development of novel asymmetric dihydroxylation processes,46 has been reviewed. Two reviews on the recent developments in osmium-catalysed asymmetric aminohydroxylation of alkenes have appeared. Factors responsible for chemo-, enantio- and regio-selectivities have been discussed.47,48 Osmium tetraoxide oxidizes unactivated alkanes in aqueous base. Isobutane is oxidized to r-butyl alcohol, cyclohexane to a mixture of adipate and succinate, toluene to benzoate, and both ethane and propane to acetate in low yields. The data are consistent with a concerted 3 + 2 mechanism, analogous to that proposed for alkane oxidation by Ru04, and for alkene oxidations by 0s04.49... 

However, it is difficult to reconcile the observed relative reactivities of hydrocarbons with a mechanism involving electron transfer as the rate-determining process. For example, n-butane is more reactive than isobutane despite its higher ionization potential (see Table VII). Similarly, cyclohexane undergoes facile oxidation by Co(III) acetate under conditions in which benzene, which has a significantly lower ionization potential (Table VII), is completely inert. Perhaps the answer to these apparent anomalies is to be found in the reversibility of the electron transfer step. Thus, k-j may be much larger than k2 for substrates, such as benzene, that cannot form a stable radical by proton loss from the radical cation [Eqs. (224) and (225)]. With alkanes and alkyl-substituted arenes, on the other hand, proton loss in Eq. (225) is expected to be fast. 

The oxidation of a branched C4 alkane, isobutane, was carried out to probe the mechanism of the C-H bond activation of alkanes on the VPO catalysts [103]. Maleic anhydride was among the products of oxidation of this branched alkane. In the case of isobutane 29 (Figure 15), the surface-bound peroxo radical would show discrimination in activating first the weaker tertiary C-H bond. The... 

After activation, the catalyst is intrcxiuced into the polymerization reactor as slurry in a saturated hydrocarbon such as isobutane. The precise mechanism of initiation is not known, but is believed to involve oxidation-reduction reactions between ethylene and chromium, resulting in formation of chromium (II) which is the precursor for the active center. Polymerization is initially slow, possibly because oxidation products coordinate with (and block) active centers. Consequently, standard Phillips catalysts typically exhibit an induction period. The typical kinetic profile for a Phillips catalyst is shown in curve C of Figure 3.1. If the catalyst is pre-reduced by carbon monoxide, the induction period is not observed. Unlike Ziegler-Natta and most single site catalysts, no cocatalyst is required for standard Phillips catalysts. Molecular weight distribution of the polymer is broad because of the variety of active centers. 

Manganese is ineffective as a catalyst under the conditions used in high-con-centration cobalt cases [68]. It does not appear to take part in an electron transfer mechanism such as eq. (25). It functions mainly via a free radical pathway [48, 69]. The high concentration cobalt-ion catalyst system also exhibits pronounced steric influences. Hydrogen atoms which are readily abstractable in conventional oxidations are bypassed in favor of less reactive hydrogenswhich are not sterically hindered. For example, isobutane is less readily attacked than -butane. 

We shall first show that it is still far fiom clear which are the families of catalysts to be used for the various reactions mainly oxidative dehydrogenation or oxidation to oxygen-containing molecules of ethane, propane or isobutane. Much research is still necessary for understanding the mechanisms leading to high selectivity. In this context, we shall suggest that many concepts inherited from the development in selective oxidation and ammoxidation of olefins are probably of little use. 

I.6. Higher Alkonium Ions The acid-induced H-D exchange of isobutane (49) in deuterosulfuric acid was studied by Otvos and coworkers. All nine methyl hydrogens were readily exchanged, but not the methine hydrogen. The mechanism for this observation is considered to involve the initial formation of trivalent zerz-butyl cation (50), probably in an oxidative... 

The two different mechanisms which have been proposed for the oxidation of isobutane over i) Mo-containing Keggin-type compounds and ii) W-containing Keggin-type and Wells-... 

A significant feature of this mechanism is the inclusion of step (29), a non-terminating interaction of f-butyl peroxy radicals to give f-butoxy radicals. Thus the oxidation of isobutane proceeds via two competing chain carriers whose relative concentrations depend on the rate of initiation. 

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