Propane, oxidative behavior

Figure 1 is the catalytic behavior of VSU545 in propane oxidative dehydrogenation to propylene. Selectivities to propylene in the range of60-80% are obtained up to propane conversions of about 20-25% and reaction temperatures up to around 450- 500 C. For higher reaction temperatures and conversions the selectivity decreases due both to the formation of carbon oxides and of aromatics. As compared to pure silicalite, a significant increase in both the selectivity to propylene and the activity in propane conversion is observed. 

Figure 2. Comparison of the catalytic behavior of VSil samples in propane oxidative dehydrogenation to propylene. Conversion of propane and selectivity to propylene at 470 C. Exp. conditions as in Fig. 1.

Figure 4. Comparison of the behavior of VSil545 in propane oxidative dehydrogenation using N2O or O2 as oxidizing agents. Exp. conditions as in Fig. 1. The dotted lines represent the propane conversion and propylene selectivity observed in the absence of the catalyst (homogeneous gas phase). The activity of the catalyst in the absence of O2 or N2O is similar to that observed in the homogeneous gas phase, but the selectivity to propylene (around 50-60%) is lower.

The good catalytic behavior of V-containing silicalite may be associated with the presence of the tetrahedral V species stabilized by the interaction with the zeolite framework as regards both redox and coordination changes. In fact, ESR and TPR data indicate the lower rate of reduction of this species as compared to that of supported vanadium-oxide, and V-NMR data indicate the stability against changes in the coordination environment. Catalytic data (Fig.s 2 and 3) indicate the better catalytic performances of this species in propane oxidative dehydrogenation as compared to supported polynuclear vanadium-oxide which can be removed by treatment with an ammonium acetate solution. 

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). 

Nevertheless, there is a possibility to solve restricted tasks using approaches described above. In particular, the utility principle (see Section II.C) can be fruitfully utilized. For instance, such an approach was utilized by Vedeneev et al. (1997a, b) to describe the NTC behavior in propane oxidation. Generally speaking, if we are interested in the optimization of some particular product yield, we can select a block of reactions in which it is formed and further transformed and analyze it taking into account qualitative or even semi-quantitative notions about the reaction environment. 

An interesting observation of their studies on propane oxidation relates to the observed behavior at higher loads as the cell approaches a steady state. The group suggests that the formation of oxide films on the anode electrode structure affects the voltage response and steady state behavior of the cell above a critical current density, being similar to the type of oxide films observed as early as the 1920s with the electrolysis of formic acid, methanol, and formaldehyde. 

Ml phase " represents the clearest example of a multifunctional catalyst in which each element, in close geometrical and electronic synergy with the surrounding elements, plays a specific role in turn, as an isolated active site, in every reaction step for the alkane transformation into the partial oxidation product desired. The flexibility of the structure allows modification of the catalyst composition and hence its catalytic behavior. Moreover, this type of mixed-metal oxide catalyst has the ability to catalyze other different oxidation reactions starting from alkanes, such as propane oxidation to acrylic acid, " oxidative dehydrogenation of ethane to ethylene, and n-butane selective oxidation. ... 

Ivars, F., Solsona, B., Rodriguez-CasteUon, E., etal. (2009). Selective Propane Oxidation over MoVSbO Catalysts. On the Preparation, Characterization and Catalytic Behavior of Ml Phase, J. Catal, 262, pp. 35-43. 

It has been shown that the presence of particular structure, redox properties, and the levels and type of acidity determine the catalytic behavior for each of the three types of catalysts. One or another reaction pathway becomes more favorable depending on these properties [106-108]. Scheme 13.2 schematizes possible reaction pathways for propane oxidation ... 

V-Sb-oxide based catalysts show interesting catal)dic properties in the direct synthesis of acrylonitrile from propane [1,2], a new alternative option to the commercial process starting from propylene. However, further improvement of the selectivity to acrylonitrile would strengthen interest in the process. Optimization of the behavior of Sb-V-oxide catalysts requires a thorough analysis of the relationship between structural/surface characteristics and catalytic properties. Various studies have been reported on the analysis of this relationship [3-8] and on the reaction kinetics [9,10], but little attention has been given to the study of the surface reactivity of V-Sb-oxide in the transformation of possible intermediates and on the identification of the sxirface mechanism of reaction. 

In the direct ammoxidation of propane over Fe-zeolite catalysts the product mixture consisted of propene, acrylonitrile (AN), acetonitrile (AcN), and carbon oxides. Traces of methane, ethane, ethene and HCN were also detected with selectivity not exceeding 3%. The catalytic performances of the investigated catalysts are summarized in the Table 1. It must be noted that catalytic activity of MTW and silicalite matrix without iron (Fe concentration is lower than 50 ppm) was negligible. The propane conversion was below 1.5 % and no nitriles were detected. It is clearly seen from the Table 1 that the activity and selectivity of catalysts are influenced not only by the content of iron, but also by the zeolite framework structure. Typically, the Fe-MTW zeolites exhibit higher selectivity to propene (even at higher propane conversion than in the case of Fe-silicalite) and substantially lower selectivity to nitriles (both acrylonitrile and acetonitrile). The Fe-silicalite catalyst exhibits acrylonitrile selectivity 31.5 %, whereas the Fe-MTW catalysts with Fe concentration 1400 and 18900 ppm exhibit, at similar propane conversion, the AN selectivity 19.2 and 15.2 %, respectively. On the other hand, Fe-MTW zeolites exhibit higher AN/AcN ratio in comparison with Fe-silicalite catalyst (see Table 1). Fe-MTW-11500 catalyst reveals rather rare behavior. The concentration of Fe ions in the sample is comparable to Fe-sil-12900 catalyst, as well as... 

Nature of Vanadium Species in Vanadium-Containing Silicalite and Their Behavior in Oxidative Dehydrogenation of Propane... 

V-containing silicalite, for example, has been shown to have different catalytic properties than vanadium supported on silica in the conversion of methanol to hydrocarbons, NOx reduction with ammonia and ammoxidation of substituted aromatics, butadiene oxidation to furan and propane ammoxidation to acrylonitrile (7 and references therein). However, limited information is available about the characteristics of vanadium species in V-containing silicalite samples and especially regarding correlations with the catalytic behavior (7- 6). 

The soot concentration, hydrocarbon species and soot temperature in the immediate vicinity of the droplet displayed closely coupled behavior. Hydrocarbon species, listed in approximate order of concentration were C2H2, CH4, C2H4, C2H6 and C3fs. Resolution of the propane and propylene peaks was not possible under current GC procedure, and these concentrations are reported merely as C3 compounds. The hydrocarbons were found to decay in the same time frame as the growth of the soot concentration. The zone of chemical activity, defined as where the vaporized hydrocarbon products react to form soot, is approximately 2 cm, which corresponds to 13 msec, after which the soot concentration decays due apparently to oxidation. 

The average oxidation state of a metal in a catalyst during reaction was found to be related to the presence of carbonaceous deposits on the surface. As the feed for propane ODH was depleted in O2, the catalyst was readily reduced (Mul et al., 2003) and amorphous carbon-containing deposits formed. This behavior was corroborated by UV-vis DRS (Mul et al., 2003 Puurunen and Weckhuysen, 2002) and by combination of UV-vis DRS and Raman spectroscopy (Kuba and Knozinger, 2002 Nijhuis et al., 2003). 

Over the years the range of uses of the K-L model has been extended to chemical processes that can not be described by first order kinetics. For these problems no anal3dical solution can be obtained so the resulting set of DAE equations are solved numerically. Gascon et al [48], for example, investigated the behavior of a two zone fluidized bed reactor for the propane dehydrogenation and n-butane partial oxidation processes emplo3ung the K-L model framework. 

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