Alkanes oxidative pathway

Respiratory, or oxidative, metaboHsm produces more energy than fermentation. Complete oxidation of one mol of glucose to carbon dioxide and water may produce up to 36 mol ATP in the tricarboxyHc acid (TCA) cycle or related oxidative pathways. More substrates can be respired than fermented, including pentoses (eg, by Candida species), ethanol (eg, by Saccharomjces), methanol (eg, by Hansenu/a species), and alkanes (eg, by Saccharomjces lipoljticd). 

Synthesis of PHAMCL from fatty acids such as octanoic acid or from the corresponding alkanes such as octane was first detected in P. oleovorans [119]. The alkanes are oxidized to the fatty acids the latter are activated by thiokinases and then degraded via the fatty acid /1-oxidation pathway. Obviously intermediates of this pathway accumulate under conditions favorable for the synthesis of PHA and are subsequently converted into substrates for the PHA synthase. Many reactions for the conversion of an intermediate of the -oxidation cycle into R-(-)-3-hydroxyacyl-CoA were considered. These were ... 

Lageveen et al. [41] showed that the monomer composition of aliphatic saturated poly(3HAMCL) produced by P. oleovorans is depended on the type of n-alkane used. It appeared that the n-alkanes were degraded by the subsequent removal of C2-units and it was therefore proposed that the /1-oxidation pathway was involved in poly(3HAMCL) biosynthesis. Preusting et al. [42] confirmed these results but also showed that with hexane as substrate some 3-hydroxyoctanoate and 3-hydroxydecanoate were produced, indicating that additional pathways were involved in poly(3HAMCL) biosynthesis (Table 1). 

Many Pseudomonas strains accumulate MCL-PHAs from alkane, alkene, al-kanoate, alkenoate, or alkanol [5,6,14,96]. The composition of the PHAs formed by the pseudomonads of the rRNA homology group I is directly related to the structure of the carbon substrate used [6]. These results suggested that MCL-PHAs are synthesized from the intermediates of the fatty acid oxidation pathway. In almost all pseudomonads belonging to the rRNA homology group I except Pseudomonas oleovorans, MCL-PHA can also be synthesized from acetyl-CoA through de novo fatty acid synthetic pathway [97]. The -oxidation pathway and de novo fatty acid synthetic pathway function independently in PHA biosynthesis. 

Scheme 9.1 shows a generalized sequence of reactions for the oxidation of an alkane, via alcohol, ketone and carboxylic acid, to the completely oxidized products, water and carbon dioxide. The latter are often referred to as combustion products as they are the same as those formed by burning hydrocarbons. These are not normally desirable chemical products unless it is necessary to destroy a toxic, hazardous or otherwise unwanted waste material. Oxidation itself is not difficult to achieve, and is a highly exothermic or even explosive process. Selective oxidation, however, is a much greater challenge, as it is important to stop the sequence at the desired product without proceeding further down the oxidation pathway. 

Oxidation of unfunctionalized alkanes is notoriously difficult to perform selectively, because breaking of a C-H bond is required. Although oxidation is thermodynamically favourable, there are limited kinetic pathways for reaction to occur. For most alkanes, the hydrogens are not labile, and, as the carbon atom cannot expand its valence electron shell beyond eight electrons, there is no mechanism for electrophilic or nucleophilic substitution short of using extreme (superacid or superbase) conditions. Alkane oxidations are therefore normally radical processes, and thus difficult to control in terms of selectivity. Nonetheless, some oxidations of alkanes have been performed under supercritical conditions, although it is probable that these actually proceed via radical mechanisms. 

Porphyrines and phthallocyanines suffer from oxidative degradation and oxidative dimerization [68]. The improved activity of the zeolitic systems is due to the effective site isolation within the pores, which prevents any bimolecular pathways to catalyst destruction [63]. Therefore, deactivation is more severe for the homogeneous catalysts than for the heterogenized TMPc. FePc itself is a poor catalyst for alkane oxidation with a high initial turn-over, but after less than 45 minutes it becomes completely inactive. On the contrary, FePc encaged in zeolite Y is stable for 24 hours [63]. 

The data presented above showed that the oxidative dehydrogenation reactions of the various alkanes share many common features. Thus it is tempting to discuss selectivity for alkane oxidative dehydrogenation with a common scheme. The reaction scheme for ethane oxidation [Eqs. (5)-(7)] provides a useful basis for such a discussion. It shows that the primary reaction of alkane oxidation can take on three different pathways depending on the reaction temperature (Scheme I). The first step in all three pathways is breaking a C—H bond, which is the rate-limiting step. The three pathways are described below. 

On the other hand, above 20mol% SbF5, a small but increasing amount of unionized SbF5 can be observed, which may rationalize the change in the mechanism of alkane activation from the protolytic to the oxidative pathway, when the concentration of SbF5 increases over 20mol% (see Section 5.1.1). 

The Oxidative Pathway. For a long time, one of the difficulties in understanding the mechanism of the superacid-catalyzed transformations of alkanes was that no... 

An important goal is, therefore, to develop effective methods for catalytic oxidations with dioxygen, under mild conditions in the liquid phase. Two substrates which are often chosen as models for alkane oxidations are cyclohexane and adamantane. Cyclohexane is of immense industrial importance as its oxidation products - cyclohexanone and adipic acid - are the raw materials for the manufacture of nylon-6 and nylon-6,6. Adamantane is an interesting substrate as the ratio of oxidation at the secondary versus the tertiary C-H bonds is used as a measure of radical versus nonradical oxidation pathways. Industrial processes for the oxidation of cyclohexane, to a mixture of cyclohexanol and cyclohexanone, generally involve low conversions (under 10%). Even at such low conversions, selectivities are modest (70-80%) and substantial amounts of overoxidation products, mostly dicarboxylic acids, are formed. 

A photochemically driven reaction that mimics biological photosynthesis, electron-transfer, and hydrocarbon-oxidation reactions is described. The reaction occurs at room temperature and uses 2 as the ultimate oxidant. Most importantly, the reaction can be run for hours without significant degradation. This means that the oxidation of low molecular weight alkanes by O2, which proceeds at a lower rate than for hexane, can be investigated. Further studies are underway to determine the detailed reaction mechanisms involved in the photochemical reaction and the relative contributions of various oxidative pathways. Transient absorption and Raman spectrocopic techniques will also be applied to determine reaction rates. 

The fatty acid oxidation pathway comprises a sequence of steps frequently encountered in biology (1) oxidation of an alkane to produce an alkene (2) hydration of the alkene to form a hydrojcyl group and (3) oxidation of the hydroxyl group to form a kelo group. This three-step sequence is also found in the Krebs cycle and the isoJeucine catabolic pathway. 

Oil-eating bacteria can oxidize long-chain alkanes. In the first step of the pathway, the enzyme monooxygenase catalyzes a reaction that converts the long-chain alkane into a primary alcohol. Data from research studies indicate that three more reactions are required to allow the primary alcohol to enter the p-oxidation pathway. Propose a pathway that would convert the long-chain alcohol into a product that could enter the p-oxidation pathway. 

Olefins are not epoxidized in Gif-systems, allylic oxidation to ketone and alcohol is a major pathway in the case of cylohexene, cyclohexene being oxidized at about the same rate as cyclohexane29. in the case of 1,1-disubstituted olefins the C=C bond is cleaved giving ketone and formaldehyde35. Here it is worth mentioning that alkane oxidation is not suppressed by the presence of other easily oxidizable compounds, such as alcohols, thiols, etc.213637 this peculiarity is discussed below. 

FIGURE 19.2 Verdezyne process for production of bio-adipic acid from sugars, fats, and alkanes with Candia twpicalis via the w-oxidation pathway (Picataggio et al., 1992 Picataggio and Beardslee, 2012). When alkanes or fatty acids are used as feedstock, these typically long-chain snhstrates are depending on the number of carbons, repeatedly shortened via the p-oxidation pathway to obtain hio-adipic acid. 

Pseudomonas oleovorans and most pseudomonads belonging to the rRNA homology group I can accumulate MCL-PHAs using 3-hydroxyacyl-CoA intermediates of P-oxidation pathway when grown on various alkanes, alkanols, or fatty acids. Aeromo-nas sp. utilize P-oxidation pathway to supply PHA precursors, especially 3HB and... 

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