Methane oxidative transformations

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 Oxidative Transformation of Methane over the Nickel-based Catalysts Modified by Alkali Metal Oxide and Rare Earth Metal Oxide... 

The large amounts of natural gas (mainly methane) found worldwide have led to extentive research programs in the area of the direct conversion of methane [1-3]. Ihe oxidative transformation of methane (OTM) is an important route for the effective utilization of the abundant natural gas resources. How to increase catalyst activity is a common problem on the activation of methane. The oxidation of methane over transition m al oxides is always high active, but its main product is CO2, namely the product of deep oxidation. It is because transition metal oxides have high oxidative activity. So, they were usually used as the main corrqtonent of catalysts for the conqilete oxidation of alkane[4]. The strong oxidative activity of CH4 over tran on metal oxides such as NiO indicates that the activation of C-H bond over transition metal oxides is much easier than that over alkaline earth metal oxides and rare earth metal oxides. Furthermore, the activation of C-H bond is the key step of OTM reaction. It is the reason that we use transition metal oxides as the mam conq>onent of the OTM catalysts. However, we have to reahze that the selectivity of OTM over transition metal oxides is poor. 

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. 

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... 

Phenylthio-l-trimethylsilylalkanes are easily prepared by the alkylation of (phenylthioXtrimethylsilyl)mcthane as shown in Scheme 10 [40], The treatment of (phenylthio)(trimethylsilyl)methane with butyllithium/tetramethylethylene-diamine (TMEDA) in hexane followed by the addition of alkyl halides or epoxides produces alkylation products which can be oxidized electrochemically to yield the acetals. Since acetals are readily hydrolyzed to aldehydes, (phenylthioXtrimethylsilyl)methane provides a synthon of the formyl anion. This is an alternative to the oxidative transformation of a-thiosilanes to aldehydes via Sila-Pummerer rearrangement under application of MCPBA as oxidant [40, 41]. 

According to Lunsford, most of the observations on methane oxidation over oxide catalysts may be interpreted in terms of methyl radical chemistry.41 Most experimental data support the role of surface O- ions in the formation of methyl radicals. The latter are transformed by reductive addition to methoxide ions, which decompose to formaldehyde or react with water to form methanol. Methyl radicals may desorb to the gas phase and participate in free-radical reactions to yield non-selective oxidation products. 

For nonheme enzymes that fiuther activate dioxygen, it is apparent that the diferrous forms also bind O2 to eventually generate the active species responsible for the oxidative transformations. In the case of MMO, the first intermediate has been labeled compound P (Scheme 2), which subsequently converts to compound Q both are kinetically competent to hydroxylate methane. In the case of RNR, compound X (Scheme 1) is responsible for the one-electron oxidation of a tyrosine residue to generate a tyrosyl radical. Based on chemical considerations and its Mossbauer properties, it has been proposed that compound P is a diferric peroxide species. To date, three model complexes of compound P, with comparable spectroscopic properties, have been structurally characterized (Figure g). In two of these models O2 is bound in a cis... 

Because of its abundance in anoxic aquatic environments and its importance as a greenhouse gas, methane transformation by anaerobic oxidation has been the subject of numerous studies. The rates of anaerobic methane oxidation and the environments where it has been found were reviewed by Spormann and Widdel (2000). In marine systems, sulfate reduction has been shown to be an important part of the methane oxidation process. Landhlls, however, not hydrocarbon contaminations per se, are the main source of anthropogenic methane emissions in the US and, therefore, methane degradation processes are not discussed further in this chapter (see Chapter 9.16 for a discussion of methane generation from landfills). 

In this work the oxidative transformations of methane were studied with a catalyst system that combines an oxide and a metal component. The presence of both components gave rise to complex oscillation phenomena. The influence of pretreatment and reaction conditions over a wide range of parameters (temperature, total pressure, and oxygen concentration) on the oscillatory process was studied. The possible role of mass transfer and the balance of heat in the reactor were analyzed, and a model for the role of the components in the binary catalyst system is suggested. 

The cooperative effects observed during methane oxidation over a binary oxide-metal system are due to the formation of active intermediates (free methyl radicals) over the oxide component, their escape from the grains of oxide, and transformation into the final products (including CO and H2) over the metal component, which proceeds in a non-steady-state oscillatory regime. 

In Section V.A we will provide arguments for the joint kinetic description of oxidative transformations of methane and C2 hydrocarbons. Regarding molecules containing more than two carbon atoms, their influence on the overall kinetics and on the formation of many important products is below the anticipated accuracy of simulations (Arutyunov et al., 2005). This is why their formation and transformations can be not accounted for in methane and ethane oxidation models for many applications. At least it would not compensate the excessive complication of the model accounting for reactions of C3+ species. 

The sequential oxidative transformation of symmetrical and unsymmetrical tetraarylethylenes with 1 and 2 equiv. of DDQ in CH2CI2 in the presence of methane-sulfonic acid produced substituted 9,10-diarylphenanthrenes and dibenzochrysenes, respectively, in excellent yields. The formation of 9,10-diarylphenanthrenes, without... 

The scope of the present contribution is to describe and analyze the developments made over the last 10 years in oxidative transformations of methane into value-added products over heterogeneous catalysts. Particularly, we focus on (i) reaction engineering concepts for the OCM reaction (ii) mechanistic aspects of direct methane oxidation to methanol, its derivatives, and acetic acid and (iii) novel approaches for designing OCM catalysts. 

The gas-phase selective oxidative transformation of light alkanes is an important challenge as it could reduce the number of process steps, decreasing both the energy required and CO2 emissions, and improve the atom economy. In this chapter a summary of both the oxidative dehydrogenation and the O-insertion for C2-C4 alkanes is presented. In addition, an alternative method for abetter selective oxidative transformation of methane and a description of the best catalytic systems are discussed. 

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