Pressure conditions, isobutane oxidation

Isobutane shows the usual NTC and cool flame phenomena (78,154,157,158). As the pressure is iacreased, the expected iacrease ia oxygenated products retaining the parent carbon skeleton is observed (96). Under similar conditions, isobutane oxidizes more slowly than / -butane (159). There are stUl important unresolved questions concerning isobutane VPO (160). 

Figure 7. Effects of 02 partial pressure on isobutane oxidation over reduced H3PMol2O40(Py) at 300°C. Symbols and reaction conditions except oxygen partial pressure are the same as those in Figure 6.

The catalytic performance depends a great deal on the reaction conditions, and specifically on the isobutane-to-oxygen ratio in the feed. Usually isobutane-rich conditions are claimed to be more selective, and the reason for this is that under these conditions the operative POM is a partially reduced one, and a more reduced POM is intrinsically more selective than a fully oxidized one. High isobutane partial pressures help to improve the selectivity, avoiding further oxidation of methacrylic acid. 

Oxidation of n-butane. In the presence of oxygen, Co(l 1) is converted into Co(lll), the actual catalyst for oxidation of alkanes by oxygen thus oxidation of n-butane by Co(lll) ion at 100° at a pressure of 17-24 atm. gives acetic acid (83.5% yield) together with traces of n-butyric acid, propionic acid, and methyl ethyl ketone. Oxidation of n-pentane under similar conditions gives acetic acid (48% yield) and propionic acid (27% yield). Isobutane is relatively inactive. The reaction involves electron transfer in which cobalt ions function as chain carriers. 

There is no direct experimental evidence for this complex decomposition and it may well occur by several steps [107]. However, substantial yields of unsaturated carbonyl compounds are formed particularly at high pressures [78] under initial reaction conditions where cool flames propagate. For example, the cool-flame oxidation of 2-methylpentane at 525 °C and 19.7 atm in a rapid compression machine [78] yields no less than 14 unsaturated carbonyl compounds viz acrolein, methacrolein, but-l-en-3-one, pent-2-enal, pent-l-en-3-one, pent-l-en-4-one, trans-pent-2-en-4r one, 2-methylbut-l-en-3-one, 2-methylpent-l-en-3-one, 4-methylpent-l-en-3-one, 2-methylpent-l-en-4-one, 2-methylpent-2-en-4-one, 2-methyl-pent-2-enal and 4-methylpent-2-enal. Spectroscopic studies of the preflame reactions [78] have shown that the unsaturated ketones account for ca. 90 % of the absorption which, occurs at 2600 A. At lower initial temperatures and pressures acrolein and crotonaldehyde are formed from n-pentane [69, 70] and n-heptane [82], and acrolein is also formed from isobutane [68]. 

In the work of Baumgartner, isobutane is oxidized at conditions significantly higher than the and Pc of isobutane and also above the critical pressure of the reaction mixture. The reactor operating variables must be carefully optimized and controlled to attain enhanced TBHP selectivities. Also, as with the case of -butane oxidation, enhanced TBHP formation is observed when the reaction is run homogeneously in the dense phase, compared with the corresponding formation obtained when the reaction is run in the liquid phase (Baumgartner, 1983). 

Chromium oxide catalysts on support polymerize isoprene-like butadiene to solid polymers. Here too, however, during the polymerization process, polymer particles cover the catalyst completely within a few hours from the start of the reaction and retard or stop further polymer formation. The polymerization conditions are the same as those used for butadiene. The reactions can be carried out over fixed bed catalysts containing 3% chromium oxide on Si02-Al203. Conditions are 88°C and 42 kg/cm pressure with the charge containing 20% of isoprene and 80% isobutane [122]. The mixed molybdenum-alumina catalyst with calcium hydride also yields polyisoprene. 

The slurry reactor was developed by Hochst to make polyethylene using Ziegler catalysts. The reaction medium, called a diluent , is a hydrocarbon that is a solvent for the monomer but not for the polymer. The product is thus formed as a suspended powder. Bimodal products, i.e., products that are, in effect, blends of two polymers having distinctly different molecular weight distributions, can be made using a cascade of two reactors in which the reaction conditions are substantially different [116]. Phillips Petroleum later developed a pipe-loop slurry reactor for use with its chromium oxide catalyst, which required moderately high temperatures and pressures to accommodate the isobutane diluent used. 

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