Propene oxidation mechanism

In the following scheme, an oxidation pathway for propane and propene is proposed. This mechanism, that could be generalized to different hansition metal oxide catalysts, implies that propene oxidation can follow the allylic oxidation way, or alternatively, the oxidation way at C2, through acetone. The latter easily gives rise to combustion, because it can give rise to enolization and C-C bond oxidative breaking. This is believed to be the main combustion way for propene over some catalysts, while for other catalysts acrolein overoxidation could... 

Catalysis is a special type of closed-sequence reaction mechanism (Chapter 7). In this sense, a catalyst is a species which is involved in steps in the reaction mechanism, but which is regenerated after product formation to participate in another catalytic cycle. The nature of the catalytic cycle is illustrated in Figure 8.1 for the catalytic reaction used commercially to make propene oxide (with Mo as the catalyst), cited above. 

Mechanical mixtures of Bi2Mo30i2 and solid solution FexCoi.xMo04 phases of variable relative composition and variable x value have been studied for propene oxidation to acrolein. A huge synergy effect was observed when Bi, Co and Fe were present and a maximum in... 

The incremental reactivity of a VOC is the product of two fundamental factors, its kinetic reactivity and its mechanistic reactivity. The former reflects its rate of reaction, particularly with the OH radical, which, as we have seen, with some important exceptions (ozonolysis and photolysis of certain VOCs) initiates most atmospheric oxidations. Table 16.8, for example, also shows the rate constants for reaction of CO and the individual VOC with OH at 298 K. For many compounds, e.g., propene vs ethane, the faster the initial attack of OH on the VOC, the greater the IR. However, the second factor, reflecting the oxidation mechanism, can be determining in some cases as, for example, discussed earlier for benzaldehyde. For a detailed discussion of the factors affecting kinetic and mechanistic reactivities, based on environmental chamber measurements combined with modeling, see Carter et al. (1995) and Carter (1995). 

Mechanism. The mechanism outlined for the propene oxidation over metal oxides is, in general, fully applicable to bismuth molybdate. The occurrence of a symmetrical allyl intermediate and the participation of lattice oxygen is well established (Hucknall [160], Voge and Adams [343]). 

A more complicated reaction scheme is proposed by the authors to include the formation of the by-products acetonitrile, acetaldehyde and ethylene. However, appropriate rate coefficients cannot be given as the reactions appear to be partially homogeneous gas phase reactions, implying that factors like the reactor geometry are also involved. Regarding the oxidation mechanism, the authors assume that two hydrogen atoms are first abstracted from propene, followed by reaction with surface oxygen or NH species. 

With respect to the mechanism, the adsorption measurements of Matsuura and Schuit [207—209] are of interest. The assumption of A-and B-sites is reported in some detail for the propene oxidation in Sect. 2.2.2(d)(i). As for the oxidation of propene, the abstraction of (both) H-atoms is assumed to occur on the molybdenum layers by initially forming HOb groups, followed by H-transfer from Ob to Oa and desorption of water (H2Oa). 

For specific cases such as olefin oxidation over Bi-Mo oxide combinations some information concerning the oxidation mechanism is available. The work of Adams and Jennings (2), of Sachtler (16), and of Adams (1) has led to the general acceptance of an allylic intermediate. The discoverers of the Bi-Mo catalyst system (21) showed that propene is converted to acrolein, while Hearne and Furman (9) proved that butene forms butadiene. The allylic intermediate therefore can in principle react in two different ways (1) formation of a conjugated diene... 

Measurements of these relatively minor species will not only complete the budget of NO, but will also indicate if our understanding of the hydrocarbon oxidation schemes in the atmosphere is complete. The organic nitrates that completed the NO, budget in the example in Figure 9 arose primarily from the oxidation of the naturally emitted hydrocarbon, isoprene (2-methylbutadiene). To demonstrate the oxidation mechanisms believed to be involved in the production of multifunctional organic nitrates, a partial OH oxidation sequence for isoprene is discussed. The reaction pathways described are modeled closely to those described in reference 52 for propene. The first step in this oxidation is addition of the hydroxyl radical across a double bond. Subsequent addition of 02 results in the formation of a peroxy radical. With the two double bonds present in isoprene, there are four possible isomers, as shown in reactions 2-5 ... 

The molecular mechanism of the selective oxidation pathway is believed to be the one shown in Scheme 2 (Section I). Adsorbed butene forms adsorbed 7r-allyl by H abstraction in much the same way as xc-allyl is formed from propene in propene oxidation (28-31). A second H abstraction results in adsorbed butadiene. Indeed, IR spectroscopy has identified adsorbed 71-complexes of butene and 7t-allyl on MgFe204 (32,33). On heating, the 7r-complex band at 1505 cm 1 disappears between 100-200°C, and the 7t-allyl band at 1480 cm-1 disappears between 200-300°C. The formation of butadiene shows a deuterium isotope effect. The ratio of the rate constants for normal and deuterated butenes, kH/kD, is 3.9 at 300°C and 2.6 at 400°C for MgFe204 (75), 2.4 at 435°C for CoFe204, and 1.8 at 435°C for CuFe204 (25). The large isotope effects indicate that the breaking of C—H (C—D) bonds is involved in the slow reaction step. 

Ressler et al. (2002) XAS Mo02 Oxidation mechanism comparison to Mo03 + + + Propene oxidation... 

Following the discussion from the preceding section, consideration will be given to the oxidation of ethene and propene (when a radical pool already exists) and, since acetylene is a product of this oxidation process, to acetylene as well. These small olefins and acetylene form in the oxidation of a paraffin or any large olefin. Thus, the detailed oxidation mechanisms for ethane, propane, and other paraffins necessarily include the oxidation steps for the olefins [28]. 

The basic oxidation mechanism for propene and isobutene appears almost unique and apart from OH and HO2 addition involves the formation, through abstraction, of highly inert radicals which are almost completely consumed through radical-radical processes. The tremendous acceleration (over a factor of 100) shown between the initial and maximum rates for both alkenes (Eig. 1.17) can be explained through secondary initiation involving three reactions for each alkene. 

There is now a considerable body of evidence to indicate that propene oxidation to acrolein occurs via a Mars and van Krevelen [123] mechanism whereby the reacting hydrocarbon, or a species derived from it, extracts lattice oxygen from the encapsulating surface layer of bismuth molybdate. In a separate step this lattice oxygen is replenished at least in part by lattice oxygen transfer from the encapsulated phases. These phases in turn are reoxidized by gas phase oxygen. This process is shown in Scheme 5.2. 

In summary, the available experimental evidence suggests that an adsorbed form of molecular oxygen is involved in partial oxidation while lattice oxygen is required for carbon dioxide production. This proposed mechanism is directly opposed to that generally accepted for propene oxidation over mixed oxide catalysts such as bismuth molybdate. In this case, lattice oxygen is responsible for acrolein formation while adsorbed oxygen results in complete combustion. This means that the fully oxidized phase is the selective catalyst while the reaction is first order with respect to alkene. 

The best yield reported in the literature is 13.3%, with selectivity of about 60% obtained with silica-supported K-promoted iron oxide catalysts modified by amines [43c]. The same catalyst is inactive in propene oxidation with air. However, the use of ammonia/air mixtures leads to a considerably enhanced conversion with respect to air only, with 60% selectivity for the epoxide. This observation suggests a mechanism whereby ammonia is first oxidized to nitrous oxide, which subsequently produces the active oxygen species for epoxidation. 

Let us consider what are the intermediates in the formation of carbon dioxide. Isaev, Margolis, and Sazonova (116) attempted to elucidate the mechanism of propene oxidation to acrolein using the kinetic tracer method. 

Variations in the selectivity of propene oxidation as a function of the catalyst composition are shown in Fig. 19a and b. If the suggested electronic mechanism of the action of mixed catalysts is true, the electron work function () of mixtures should be higher than that of pure molybdenum and bismuth oxides. The dependence of A on the composition of a molybdenum-bismuth catalyst is shown in Fig. 19b. The maximum change in the electron work function corresponds to highest selectivity. Such a proportional change in catalytic and electronic properties seems to provide evidence for the electronic mechanism of the effect of these mixed catalysts. 

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