Methane, oxidative behavior

GP 8] [R 7] Dilution with the inert gas argon served to simulate the oxidation behavior when using air. Methane conversion and H2 and CO selectivity remain constant for a long range of dilution until they finally drop at inert gas contents above 50% [CH4/O2 2.0 10 - 57 vol.-% Ar 0.15 MPa 7.8 10 h (STP) 105 W] [3]. Oxygen conversion is near-complete for all experiments. The micro channels outlet temperatures drops on increasing the amount of inert gas. 

The methyl radical formed in these reactions is the key species in methane oxidation and it is responsible for the unusual oxidation behavior of this fuel. Unlike other hydrocarbon radicals, the CH3 radical is comparatively unreactive and may build up in fairly high concentrations. Several consumption reactions compete for CH3, and the overall oxidation rate for methane is determined by this competition. Rapid oxidation of CH3 to CH2O requires high temperatures and a sufficient concentration of O2. Ignition is very sensitive to the reaction between CH3 and O2, in particular, the product channel leading to methoxy (R39). This step is part of the chain-branching sequence... 

Doping Tb into CeC>2 can move the oxygen releasing peaks into lower temperature (see Section 5.1) and that may promote the oxidation methane. Figure 44 demonstrates the catalytic result of the methane oxidation by Ceo.sTbo.202-5 oxide. The Ceo.8Tbo.202-5 oxide has better catalytic behavior than pure CeC>2. 

Recently we observed the effect which supports the conclusion about the substantial role of the radical reaction outside of the catalyst grains. When a very efficient OCM oxide catalyst (10% Nd/MgO) was placed into the reactor together with an inactive metal filament (Ni-based alloy) the sharp increase of conversion accompanied by the selectivity shift from oxidative coupling to the formation of CO and H2 was observed [19]. Since the metal component has a low activity also with respect to ethane oxidation, this behavior is not due to successive oxidation or decomposition of C2 hydrocarbons on the metal surface, but should be attributed to the reactions of methane oxidation intermediates. Almost total disappearance of ethane (which is a product of CH3 radicals recombination) and acceleration of the apparent reaction rate by the addition of an "inert material indicate that the efficiency of methane oxidative transformations can be substantially increased if the radicals have a chance to react outside the zone where they formed and the role of reaction (-1) decreases. Although the second (metal) surface is not active enough to conduct the reaction of saturated hydrocarbon molecules (methane and ethane), the radicals generated by the oxide can react further on the metal surface. As a result, the fraction of the products formed from methane activated in the reaction (1) increases, and the formation of the final reaction mixture of different composition takes place. 

The analysis of critical phenomena, such as hysteresis and self-oscillations, gives valuable information about the intrinsic mechanism of catalytic reactions [1,2], Recently we have observed a synergistic behavior and kinetic oscillations during methane oxidation in a binary catalytic bed containing oxide and metal components [3]. Whereas the oxide component (10% Nd/MgO) itself is very efficient as a catalyst for oxidative coupling of methane (OCM) to higher hydrocarbons, in the presence of an inactive low-surface area metal filament (Ni-based alloy) a sharp increase in the rate of reaction accompanied by a selectivity shift towards CO and H2 takes place and the oscillatory behavior arises. In the present work the following aspects of these phenomena have been studied ... 

The methane oxidation cycle is depicted in Fig. 1. A fundamental feature of the behavior of the methane oxidation chain is the competition... 

Although alkanes are not strongly adsorbed over noble metals, it seems that ethane and propane may shift light-off temperatures for CH4 oxidation to higher values. This behavior was clearly observed over Pd catalysts and ascribed to a change of the reduction state of Pd in the presence of ethane and propane. Inhibition of methane oxidation by C2-C3 alkanes is virtually not observed over Pt, whereas strongly adsorbed hydrocarbons such as alkenes and acetylene may strongly affect alkane oxidation. ... 

Many authors have shown that the support could play a role, not only in changing particle size but also in modifying adsorption properties of the metals. Ceria could stabilize ionic species of platinum leading to a strong metal-support interaction. Bera et al. have compared the behavior of Pt/Ce02 and Pt/Al203 in TW catalysis." The enhanced activity observed in several reachons (CO-I-O2, CO - - NO and HC -f O2, Table 1.10) has been attributed to the formation of new sites (-0 Ce" +-0 Pt"+-0 with = 2 or 4). Ceria-supported catalysts are more active than alumina ones for all the reactions. NO as an oxidant is more sensitive in nature to support than O2. Moreover, ceria is a better promoter for oxidation of CO and propane than that of methane. Whatever the oxidant (NO or O2), methane oxidation remains difficult with a modest promotion by ceria. 

It is presumed that the global-quenching criteria of premixed flames can be characterized by turbulent shaining (effect of Ka), equivalence ratio (effect of 4>), and heat-loss effects. Based on these aforemenhoned data, it is obvious that the lean methane flames (Le < 1) are much more difficult to be quenched globally by turbulence than the rich methane flames (Le > 1). This may be explained by the premixed flame shucture proposed by Peters [13], for which the premixed flame consisted of a chemically inert preheat zone, a chemically reacting inner layer, and an oxidation layer. Rich methane flames have only the inert preheat layer and the inner layer without the oxidation layers, while the lean methane flames have all the three layers. Since the behavior of the inner layer is responsible for the fuel consumption that... 

We expected to control the direction of OTM reaction over NiO by sur ce modification, namely making use of the interaction between NiO and other conq>onents to beget a synergistic effect. In this paper, two completely different behaviors of the oxidative transformation of methane were performed over the nickel-based catalysts because of the different modifications by alkali metal oxide and rare earth metal oxide and the different interactions between nickel and supports. Furthermore, the two completely different reactions were related with the acid-base properties of catalysts and the states of nickel present. 

Effects of Li content on the catalytic behaviors and structures of LiNiLaOx catalysts The dpendence of performance of LiNiLaOx catalysts on Li content at 1073K was shown in Fig.l. When D/Ni mole ratio was 0, the relatively acidic LaNiOx had the highest CH4 conversion(92.0%), but no C2 yielded. The products were CO, CO2 and H2, and CO selectivity was 98.3%. It is not an OCM catalyst but a good catalyst for partial oxidation of methane(POM). With Li content and the baric property of LiNiLaOx catalysts increasing, CH4 conversion and CO selectivity decreased, but there was still no C2 formed imtil Li/Ni mole ratio was 0.4. There was a tumpoint of catalytic behavior between 0.2 and 0.4 (Li/Ni mole... 

Two conqiletely different behaviors of oxidative transformation of methane, namely the Oxidative Coupling of Methane to C2 Hydrocarbons(OCM) and the Partial Oxidation of Methane to Syngas(POM), were performed and related over the nickel-based catalysts due to different modification and different supports. It is concluded that the acidic property favors keeping the reduced nickel and the reduced nickel is necessary for POM reaction, and the bade property frvors keeping the oxidized nickel and the oxidized mckel is necessary for OCM reaction. POM and OCM reactions proceed at different active sites caused by different... 

FIGURE 6.24 Redox behavior of the methano-dimer of a-tocopherol (bis(5-tocopheryl) methane, 28) temperature dependence of the oxidation with bromine. 

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... 

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