Selectivity for dehydrogenation

Factors That Determine Selectivity for Dehydrogenation in Oxidation of Light... 

For the 8.2 V/nm sample, the products observed for the pulse reaction at 400°C consisted of only dehydrogenation products (butenes and butadiene) and carbon oxides. No oxygenates were observed, and the carbon balance for each pulse was satisfied within experimental error. The selectivity for dehydrogenation is shown in Fig. 3a as a function of 0. It shows that the selectivity was very low when the catalyst was in a nearly fully oxidized state, but increased rapidly when the catalyst was reduced beyond 0 = 0.15. It should be noted that the dependence of selectivity for dehydrogenation on 0 shown in the figure was not... 

Fig. 3b shows that the selectivity for dehydrogenation (based on detected products) was very low at low values of 0, but increased rapidly as the catalyst was reduced. On this catalyst, small amounts of crotonaldehyde and maleic anhydride were also detected. These amounts decreased slowly with increasing 0. 

The oxidation of butane on these orthovanadates were tested at 500°C in a flow reactor using a butane oxygen helium ratio of 4 8 88. The observed products were isomers of butene, butadiene, CO, and CO2. The carbon balance in these experiments were within experimental errors, thus the amount of any undetected product if present should be small. The selectivity for dehydrogenation (butenes and butadiene) was found to depend on the butane conversion and be quite different for different orthovanadates. Fig. 4 shows the selectivity for dehydrogenation at 12.5% conversion of butane [15,18,19]. Its value ranged from a high of over 60% for Mg3(V04)2 to a low of less than 5% for... 

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... 

This paper summarized our current understanding of the factors that determine selectivity for dehydrogenation versus formation of oxygen-containing products in the oxidation of light alkanes. From the patterns of product distribution in the oxidation of C2 to C6 alkanes obtained with supported vanadium oxide, orthovanadates of cations of different reduction potentials, and vanadates of different bonding units of VO in the active sites, it was shown that the selectivities can be explained by the probability of the surface alkyl species (or the... 

It is interesting to note that on Mg3(V04)2, the selectivity for dehydrogenation was maintained in subsequent pulses up to the end of the experiment when an equivalent of 50% of a monolayer of lattice oxygen was consumed. The activity was decreased by only about 20%. The behavior of Ni orthovanadate with pulse number was similar. For Zn orthovanadate, the decrease in activity was more pronounced. 

The pulse experiments using orthovanadates and V/y-AFCb catalysts showed that high selectivity for dehydrogenation of butane could be obtained by reaction of butane with lattice oxygen. This has also been demonstrated with other oxides, including Mg-Mo oxide (43, 44). 

If the surface reactions of an alkane depicted in Scheme I is common to all catalysts, then the selectivity for dehydrogenation versus formation of oxygen-containing product would be strongly affected by the ability of the catalyst to form a C—O bond with the surface hydrocarbon, that is, the reactivity of the oxygen species, and the number of reactive oxygen available for reaction at the active site. 

The difference between propane and 2-methy]propane on Mg2V207 cannot be explained by the size of the C3 chain. It is then proposed that although with difficulties, a C3 species still has a finite probability to interact with both vanadium ions in a V207 unit in the catalyst to lead to the formation of combustion products. This probability is twice as large for 2-methylpropene (or 2-methylpropyl) than propene (or propyl), which may account for the lower selectivity for dehydrogenation for 2-methylpropane on this catalyst. 

The additional requirement of the size of molecule with respect to the V — V distance in the active site is perhaps the reason behind the fact that propane and butane show not only different selectivity behavior, but also different dependence of the selectivity on the reducibility of the catalyst the selectivity for dehydrogenation in butane oxidation decreases rapidly with increasing reducibility of the catalyst (Figs. 6 and 7), but the selectivity in propane oxidation is much less dependent on it (31). 

The dependence of selectivity for dehydrogenation on the conversion of alkane shows that for the more selective catalysts known, the reaction proceeds with a sequential mechanism. The first step of the reaction is the breaking of a C—H bond of the alkane molecule, which is also the rate-limiting step. For these more selective catalysts, alkene is the primary product. Therefore, high selectivities can be obtained at low conversions. However, as the conversion increases, the selectivity decreases because of the secondary reaction of the alkene. The rate constant for the reaction of the alkene on the most selective catalyst is still about the same in magnitude as the rate constant for the activation of alkane. It is larger for the less selective catalysts. Thus the maximum yield of alkene among the catalysts known to date is still less than about 35%. To improve this yield, catalysts that react with alkene less rapidly than with alkane need to be found. 

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