Hydrocarbons, oxidation Activation, Alkanes

Alkanes and Strong Solid Acids. Since the early reports by Nenizetscu and Dragan67 on alkane isomerization on wet aluminum chloride in 1933, all mechanistic studies have led to a general agreement on the carbenium-ion-type nature of the reaction intermediates involved in acid-catalyzed hydrocarbon conversions. In contrast with this statement, the nature of the initial step is still under discussion and a variety of suggestions can be found in the literature among which direct protolysis of C—H and C—C bonds, protonation of alkenes present as traces, and oxidative activation are the most often quoted.54,55... 

Various alkane oxidations are catalyzed by iron complexes. Such reactions are important in view of the action of non-heme iron enzymes, such as methane monooxygenase, in hydrocarbon oxidations in biological systems. For example, the oxo-bridged complex [Fe2(TPA)2(ju,-0)(ju.-0Ac)]3+ [TPA = tris(2-pyridylmethyl)-amine] catalyzes the oxidation of cyclohexane with Bu OOH. Related complexes with an Fein2(/i-0)(/i-0Ac)2 core oxidize cyclohexane or adamantane to give a mixture of alcohols and ketones.159 Less well defined systems, e.g., FeCl3-6H20/ aldehyde/AcOH/02 are similarly active.160... 

In the electrochemical conversion of hydrocarbons the NEMCA (non-faradaic electrochemical modification of catalytic activity) effect has been reported frequently over metal anodes [13] and rarely over metal oxide anodes [14]. The NEMCA effect is known to promote the rate of oxidation and, to the knowledges of the authors, such enhancement in catalytic activity is generally observed over the metal anodes which have original catalytic activity, e.g. Pt, Pd, Rh and Ag, and is also observed as a non-linear function of the electric current. In the present study, we observed an almost linear increase of activity with increase in the electric current. Lacking a reference electrode, it is beyond the scope of this work to elaborate on the work function of the anode material. However, it is likely that the contribution of the NEMCA effect is neglisible and the electrochemically generated ooxygen species operates in the partial oxidation of alkanes. 

Recent reviews (a) references 2a-d, 12a. (b) Arndtsen BA, Bergman RG,Mobley TA, Peterson TH (1995) Acc Chem Res 28 154. (c) Davies JA, Watson PL, Greenberg A, Liebman JF (eds) (1990) Selective hydrocarbon oxidation and functionalization. VCH, New York, Chaps 1-5. (d) Hill CL (ed) (1989) Activation and functionalization of alkanes. Wiley, New York, (e) Shilov AE, Shul pin GB (1997) Chem Rev 97 2879. (f) Shilov AE (1984) Activation of saturated hydrocarbons by transition metal complexes. D. Reidel, Dordrecht... 

Supported metal oxides are currently being used in a large number of industrial applications. The oxidation of alkanes is a very interesting field, however, only until recently very little attention has been paid to the oxidation of ethane, the second most abundant paraffin (1). The production of ethylene or acetaldehyde from this feed stock is a challenging option. Vanadium oxide is an important element in the formulation of catalysts for selective cataljdic reactions (e. g. oxidation of o-xylene, 1-3, butadiene, methanol, CO, ammoxidation of hydrocarbons, selective catalytic reduction of NO and the partial oxidation of methane) (2-4). Many of the reactions involving vanadium oxide focus on the selective oxidation of hydrocarbons, and some studies have also examined the oxidation of ethane over vanadium oxide based catalysts (5-7) or reviewed the activity of vanadium oxide for the oxidation of lower alkanes (1). Our work focuses on determining the relevance of the specific oxide support and of the surface vanadia coverage on the nature and activity of the supported vanadia species for the oxidation of ethane. 

The selective oxidation of hydrocarbons by dry ozonation is not restricted to reaction at tertiary C atoms the CH2 groups adjacent to a cyclopropane ring are smoothly converted into carbonyl groups. This cyclopropyl activation can be explained by the well-known ability of the cyclopropyl group to effectively stabilize a neighboring R3C+ center. Although in the oxidation of alkanes with ozone the first intermediate, the hydrotrioxide, [11] does not arise from carbenium ions, the effect of polarity upon attack of the electronegative ozone molecule, either as a radical or by 1,3-dipolar... 

Although alkanes are the noble gases of organic chemistry in terms of reactivity, it is well known that transition metal systems oxidatively add alkanes to form stable alkylhydride complexes.2-11,112 114 C-H bond insertion by a coordinatively unsaturated fragment has been examined using fast spectroscopic techniques in solid, liquefied, or supercritical rare gases10,11 117 and hydrocarbon solution.118,119 There is now substantial evidence for the intermediacy of alkane complexes in the activation of alkanes via OA [Eq. (12.18)] exactly as for transient H2 complexes in OAof H2. 

In the absence of hydrocarbon or in the presence of alkanes, CO oxidation activity is high. At low temperatures, CO coverage on metallic sites is probably very high [7], It decreases when temperature increases, which allows the adsorption of oxidizing species and the subsequent CO oxidation. Alkanes are poorly adsorbed on metallic sites. They can not compete with CO and do not modify its oxidation rate. They should adsorb only when CO is eliminated. Even under these conditions they have to compete with oxygen adsorption, in the case of platinum catalysts [15]. Tlieir adsorption is favored by an increase of their chain length. 

Whereas the reactions included in the second group require direct contact between a molecule of the C-H compound and the metal complex (albeit via the ligand). In the processes belonging to the third type, the complex activates initially not the hydrocarbon but the other reactant (e.g H2O2 or O2) The reactive species formed (a carbene or radical, e.g hydroxyl radical) attacks then the hydrocarbon molecule without any participation in the latter process of the metal complex which has generated this species. Oxidation of alkanes by Fenton s reagent [4a-c] is an example of a such type process ... 

The current views about aerobic biological oxidation of alkanes and arenes involving various oxygenases. Its chemical simulations are discussed in this chapter, which gives only a brief survey of the most recent data for biological C-H activation and hydrocarbon oxidation. It is noteworthy that, somewhat unexpectedly, important and profound analogies exist between chemical activation by metal complexes and biological C-H oxidation. It is necessary to note that many books [14] and reviews [15] have been devoted to enzymatic oxidations and processes that more or less closely model these oxidations. 

A third topic, not specific to the use of dioxygen as oxidizing agent, concerns selectivity and, in particular, the closely related aspect of over-oxidation. This problem is most severe in the partial oxidation of saturated hydrocarbons, because activation of alkanes is usually more difficult than that of any of the oxygenated products (e.g., alcohols and aldehydes), which are easily oxidized further. The reason is simply that C-H bonds of functionalized alkanes are generally weaker than those of the parent hydrocarbons. Moreover, as far as metal catalysis is concerned, the polar oxidation products can... 

This structural variation notwithstanding, only a few cationic transition-metal ions react efficiently with molecular oxygen under gas-phase conditions (see below). In contrast, many anionic metal complexes and clusters are readily oxidized by O2 to afford various metal-oxide anions [19]. From a conceptual point of view, however, anionic species appear to be inadequate reagents for the activation of hydrocarbons, because they generally require electrophilic attack. At present, only a few oxidations by transition-metal oxide anions have been reported to occur in the gas phase, and they are mostly limited to relatively polar substrates, such as the CH3OH CH2O conversion [20]. Because of the lower reactivity of hydrocarbons, their C-H bond activation by metal-oxide anions is likely to be limited to radical pathways driven purely thermodynamically, i.e., when Z)(0-H) exceeds Z)(C-H) of the substrate [21]. As radical-type pathways are prone to create selectivity problems, and over-oxidation is particularly difficult to control, the anionic route appears less attractive as far as partial oxidation of alkanes is concerned. 

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