Oxidation of Higher Alkanes

Oxidation of Higher Alkanes. New, selective and active catalyst systems were also developed for the metal-catalyzed oxidation of higher alkanes. A carboxylate- 

Efficient oxidation of alkanes with molecular oxygen can be attained using N-hydroxyphthalimide combined with Co(acac) (n = 2,3).1121 According to a new concept, photocatalyzed selective oxidation of small hydrocarbons at room temperature is carried out over alkali or alkaline-earth ion-exchanged zeolites.1122 

Information of the Gif system has been summarized,1055 1123 and new results, including new oxidants such as bis(trimethylsilyl) peroxide,1124 the synergistic oxidation of saturated hydrocarbons and H2S,1125 studies with the Fe3+-picolinate complex encapsulated within zeolites,1126 and the use of Udenfriend s system under Gif conditions1127 were disclosed. Gif-type oxidations were found to be moderately stereoselective.1128 Iron/zinc-containing species involved in Gif-type chemistry were synthesized, and their reactivity and catalytic behavior were studied.1129 

The radical or nonradical chemistry of the Fenton-like systems, including the Fenton reaction and the Fe(III)—ROOH (or H2O2) system, is still hotly debated.1123,1130-1134 New information supporting a free-radical mechanism with carbon- and oxygen-centered radicals1135-1138 and new evidence for a nonradical process1139,1140 have been published. 

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Data on alkyl radical oxidation between 300° and 800°K. have been studied to establish which of the many elementary reactions proposed for systems containing alkyl radicals and oxygen remain valid when considered in a broad framework, and the rate constants of the most likely major reactions have been estimated. It now seems that olefin formation in autocatalytic oxidations at about 600°K. occurs largely by decomposition of peroxy radicals rather than by direct abstraction of H from an alkyl radical by oxygen. This unimolecular decomposition apparently competes with H abstraction by peroxy radicals and mutual reaction of peroxy radicals. The position regarding other peroxy radical isomerization and decomposition reactions remains obscured by the uncertain effects of reaction vessel surface in oxidations of higher alkanes at 500°-600°K. 

Aldehydes, if formed, are obtained as minor products. The oxidation of higher alkanes is accompanied by C —C bond cleavage (oxidations of iodo compounds at the iodine atom are described in Section 3.4.3). 

By contrast to the oxidation of higher alkanes, there is no qualitative way of distinguishing evidence for a diperoxy radical mechanism during the low-temperature oxidation of propane. It is possible that the considerably lower reactivity of propane than that of higher alkanes, as demonstrated by phenomenological aspects (Section 6.4.4), may be explained by the absence of these types of processes. The matter may not be resolved until more insight is obtained from numerical modelling and simulation of experimental results. 

Catalytic oxidation reactions on noble metal surfaces are sufficiently fast and exothermic that they can be operated at contact times on the order of one millisecond with nearly adiabatic temperatures of 1000°C. At short contact times and high temperatures complete reaction of the limiting feed is observed, and highly nonequilibrium products are obtained. We summarize experiments where these processes are used to produce syngas by partial oxidation of methane, olefins by partial oxidation of higher alkanes, and combustion products by total oxidation of alkanes. The former are used to produce chemicals, while the latter is used for high temperature catalytic incineration of volatile organic compounds. 

The oxidation of higher alkanes leads to a large variety of oxidation products, including organic acids, aldehydes, ketones, and alcohols. In most cases the advantage of the cheap alkane feed is eliminated by the large effort required for the tedious separation and purification of the alkane oxidation product mix. This fact has restricted alkane oxidation processes in the chemical industry to few cases, such as, for example, butane oxidation to maleic acid anhydride. 

Table II. Initial Oxidation Products of Higher Alkanes...

In the higher-acidity region, the intensity-potential curve shows two peaks (at 0.9 V and 1.7 V, respectively, versus the Ag/Ag+ system). The first peak corresponds to the oxidation of the protonated alkane and the second the oxidation of the alkane itself. 

Catalytic cracking The decomposition of higher alkanes into alkenes and alkanes of lower relative molecular mass. The process involves passing the larger alkane molecules over a catalyst of aluminium and chromium oxides, heated to 500°C. 

As far as we know, aliphatic methyl ketones as such are unknown as biolipid constituents. It may therefore be assumed that the aliphatic methyl ketones found in the Aleksinac shale bitumen are a product of microbiological oxidation of higher fatty acids or alkanes. This assumption is supported by the fact that methyl ketones have been found in soil and peat as well as in tobacco leaves dried in air and sun for two years. The bitumen of Boux-willer shale was found to contain higher members (C-y-C-,) t 1 s... 

To promote both the conversion of reactants and the selectivity to partial oxidation products, many kinds of metal compounds are used to create catalytically active sites in different oxidation reaction processes [4]. The most well-known oxidation of lower alkanes is the selective oxidation of n-butane to maleic anhydride, which has been successfully demonstrated using crystalline V-P-O complex oxide catalysts [5] and the process has been commercialized. The selective conversions of methane to methanol, formaldehyde, and higher hydrocarbons (by oxidative coupling of methane [OCM]) are also widely investigated [6-8]. The oxidative dehydrogenation of ethane has also received attention [9,10],... 

Another very important feature of such models is their openness they can be built on and on to any direction from any given level. On the one hand, this in principle allows a very detailed elaboration of a given reaction. On the other hand, such openness is very helpful if more and more complicated systems must be analyzed. For instance, any action of additives (promoters, inhibitors, etc.) on hydrocarbon oxidation can be analyzed. On the other hand, a kinetic scheme describing the reaction of a certain compound can be spread on similar reactions of related compounds with gradually increasing complexity (e.g., from methane oxidation to analogous reactions of higher alkanes). 

In oxidative processing of higher alkanes, reactions under discussion also to a considerable degree determine the product distribution. Let us consider reactions of ethyl species as a representative example. First of all, as all reactions of C-centered radicals with oxygen, reaction between ethyl and 02 can proceed as a reversible formation of ethylperoxy radical... 

Effects of Halogens- A study of the partial catalytic oxidation of CH4 over palladium sponge catalysts by Cullis et al. yielded some information quite relevant to its use for complete oxidation in air pollution control. They found that (1) the presence of higher alkanes or of partial oxidation products of methane, HCHO and CH3OH, retard the overall oxidation of the CH4 (2) the wide variety of halomethanes studied retarded the oxidation of CH4 to different extents and (3) some chloromethanes increased the production of HCHO with high selectivity. They attributed the latter to the modification of the elecuonic properties of the catalyst surface by the chloromethane to inhibit the further oxidation of the HCHO. 

These are of two kinds (i) those of moderate character, experienced by very small metal particles on ceramic (i.e. irreducible) oxides having various acid/base characters, and (ii) strong interactions (SMSI) where the support is partially reduced, or a reducible component has been added, and the metal is decorated by species of indeterminate type, stemming from the support. These effects have been outlined in Section 2.6 their consequences for the reactions of higher alkanes will now be considered. The last shall be first. 

The activation of higher alkanes is also being intensively investigated. An example is the oxidative dehydrogenation of ethane, propane, and isobutane to the corre-... 

The oxidation of the alkane (M2BH) by H" gives the carbocation only at pH values below 5.7. In the stronger acids it is the protonated alkane which is oxidized. At pH values higher than 5.7, oxidation of isopentane gives the alkane radical which dimerizes or oxidized in a pH-independent process (Scheme 1). 

Reactivity. From experimental studies, it has been suggested that the activation of methane over oxide catalysts to form methyl radicals for methane to methanol conversion, or the generation of higher alkanes, involves surface oxide species in low oxidation states such as 0 or O2 . Early simulations (421) of this process relied on comparisons with gas-phase MO dimers, in which it is known that a similar partially reduced oxygen species is present. By studying the simple gas-phase reaction... 

Williams, KA, Schmidt, LD. Catalytic autoignition of higher alkane partial oxidation on rhodium coated foams. Appl. Catal. A Gen. 2006399 30-45. 

The nitration of higher alkanes is of limited technical importance. It typically proceeds as a liquid-phase reaction at around 200 °C and produces the desired nitro-alkanes together with significant amounts of oxidation and cracking product. 

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