Alkane oxidative dehydrogenation

Other catalysts for alkane oxidative dehydrogenation have also been reported in the patent literature. For example, it was claimed that a Na and Li phosphomolybdate produced 17% butadiene and 5% butenes at 600°C with a 1 1 mixture of butane and oxygen (13). 

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. 

It seems that very few investigations concern the oxidation or oxidative dehydrogenation of C5 alkanes. Oxidative dehydrogenation of isopentane to isoprene has been mentioned. Two articles deal with Mn02, CoO/CaOs, Na0H/Al203, but in the presence of HI [51,52] this obviously suggests the intervention of gas-phase reactions. The yields (Y) in isobutene were relatively hi (e.g., Y = 50-60% with a selectivity of 65 to 95%). Pentane can also produce maleic a y de and phthalic anhydride [53-57]. 

It follows that one may conclude that basic properties as well as atomic arrangements at molecular level play an important role in alkane oxidative dehydrogenation reactions. Such a conclusion could also be reached from the study of VMgO catalysts [42] for oxidative dehydrogenation of several alkanes as ethane, propane and butane under similar conditions (see e g. fig. 5 in ref 42). 

As with the previously discussed methane coupling chemistry, the promise of OTMs for promoting alkane oxidative dehydrogenation reactions such as ... 

Anaerobic oxidation of butane to butadiene Alkane oxidative dehydrogenation... 

Theoretical studies of catalytic alkane-dehydrogenation reactions by [(PCP )IrH2], PCP rf-C6H3(CH2P112)2-l, 3 and [cpIr(PH3)(H)]+, suggest that they proceed through similar steps in both cases namely (i) alkane oxidation, (ii) dihydride reductive elimination, (iii) /3-II transfer from alkyl ligand to metal, (iv) elimination of olefin.402 The calculated barriers to steps (i), (ii), and (iv) are more balanced for the PCP system than for cp(PH3). 

These results suggested that 0 may be an important intermediate in the oxidative dehydrogenation of alkanes. 

Oxidative Dehydrogenation of Ethane. The dehydrogenation of alkanes also occurs, but in a catalytic manner, over molybdenum supported on silica (22,23). In addition to the stoichiometric reactions, the role of the 0 ion in this catalytic reaction is further suggested by the observation that N2O is an effective oxidant at temperatures as low as 280°C, but no reaction is observed at these temperatures with O2 as the oxidant (22). It should be noted that at moderate temperatues N2O gives rise to 0 , whereas O2 yields O2 over Mo/Si02. Under steady-state conditions the rates of formation of C2Hi were in the ratio of 7 1 at 375°C and 3.7 1 at 450°C when N2O and O2 were used as the oxidants, respectively (23). ... 

Metal-oxygen bond, 27 195, 196 insertion reactions, 28 136-141 strength and selectivity, oxidative dehydrogenation of alkanes, 40 26-28... 

The role of adsorbed oxygen species in the mechanism of alkane transformation, on the contrary, is more questionable. The effect induced by the substitution of O2 with N2O and IR indications are in agreement with this interpretation, but, on the other hand, activated electrophilic oxygen species form on reduced sites, preferably in tetrahedral coordination (79). The partial reduction of tetrahedral V =0 with formation of tetrahedral v after propane oxidative dehydrogenation can be observed using UV-Visible diffuse reflectance, ESR and V-NMR spectroscopies. It is thus not possible to assign unequivocally the active species in propane selective activation to a tetrahedral V =0 species or to or V -0-0 species formed in the... 

The selective oxidation of alkanes is cuiTently one of the most widely studied classes of catalytic reactions. This work mainly concentrates on the oxidative dehydrogenation of methane, with some attention paid to the partial oxidation of the product of this reaction, ethane. As regards the latter reaction, higher yields of pai tial oxidation products (acetaldehyde and ethylene) were achieved when N2O was used instead of O2 (1-6). 

The data in Figs. 3 and 4 show that the ease of removal of a lattice oxygen, which can also be expressed in terms of the reducibility of the neighboring cations, has a strong effect on the selectivity for oxidative dehydrogenation of butane. If this is the only factor that determines selectivity, then a catalyst that is selective for dehydrogenation of butane, such as Mg3(V04)2, will be selective for other alkanes as well. Likewise, any catalyst that contains bonds will not be... 

Oxidative Dehydrogenation. Specific reviews deal with oxidative dehydro-genation of lower alkanes, particularly over vanadium oxide-based cata-... 

In spite of significant fundamental studies and its significant economic potential as an alternate route to alkenes, the oxidative dehydrogenation of alkanes to alkenes is not currently practiced.383 The main reason is that the secondary oxidation of the primary alkene products limits severely alkene yields, which becomes more significant with increasing conversion. This is due mainly to the higher energies of the C—H bonds in the reactant alkanes compared to those of the product alkenes. This leads to the rapid combustion of alkenes, that is, the formation of carbon oxides, at the temperatures required for C—H bond activation in alkanes. 

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