Methane reaction intermediates

Concerning the reaction pathway, two routes have been proposed the sequence of total oxidation of methane, followed by reforming of the unconverted methane with CO2 and H2O (designated as indirect scheme), and the direct partial oxidation of methane to synthesis gas without the experience of CO2 and H2O as reaction intermediates. The results obtained by Schmidt and his co-workers [4, 5] indicate that the direct reaction scheme may be followed in a monolith reactor when an extremely short contact time is employed at temperatures in the neighborhood of 1000°C. However, the majority of previous studies over numerous types of catalysts show that the partial oxidation of methane follows the indirect reaction scheme, which is supported by the observation that a sharp temperature spike occurs near the entrance of the catalyst bed, and that essentially zero CO and H2 selectivity is obtained at low methane conversions (<25%) where oxygen is not fully consumed [2, 3]. A major problem encountered... 

Chen et al. [70] suggested that temperature gradients may have been responsible for the more than 90 % selectivity of the formation of acetylene from methane in a microwave heated activated carbon bed. The authors believed that the highly nonisothermal nature of the packed bed might allow reaction intermediates formed on the surface to desorb into a relatively cool gas stream where they are transformed via a different reaction pathway than in a conventional isothermal reactor. The results indicated that temperature gradients were approximately 20 K. The nonisothermal nature of this packed bed resulted in an apparent rate enhancement and altered the activation energy and pre-exponential factor [94]. Formation of hot spots was modeled by calculation and, in the case of solid materials, studied by several authors [105-108],... 

As-described compounds have also been proposed to be formed as intermediates in the gas phase in the traditional two-component MOCVD process (pre-reactions). For instance, the deposition of AlN from AlMe3 and NH3 [11] most likely proceeds through a multistep-reaction mechanism including both the adduct Me3Al-NH3 and the heterocycle [Me2AlNH2]3, that is formed after elimination of one equivalent of methane, as more or less stable reaction intermediates. This is supported by the fact that both compounds have been successfully used for the deposition of AIN in the absence of any additional NH3 [12]. The same was found for the deposition of InP from InMe3 and PH3 [13]. 

Figure 14.3 shows results from flow reactor experiments on NO sensitized oxidation of methane in the 800 to 1200 K temperature range. In the absence of NO, temperatures of about 1100 K are required to initiate rapid oxidation of CH4 [31], but in the presence of NO, reaction occurs at temperatures as low as 850 K. The results indicate three different temperature regimes a low-temperature region (900-1000 K) with partial oxidation of methane, an intermediate-temperature regime with little reaction (1000-1150 K), and a high-temperature regime (>1150 K) with complete oxidation. 

Scheme 5 illustrates the photochemical rearrangement of 4//-1,2-diazepine 23 to 6//-1,4-diazepine 25, which occurs via an aza-di-7i-methane reaction with l,2-diazabicyclo[3.2.0]hepta-2,6-diene 24, as the intermediate <1996CRV3065>. 

In closed tube experiments of ethane oxidation by CuO, methane was a reaction intermediate (Table III). 

Sugars (fructose) Reaction Intermediate 4-Hydroxy-5-Methyl -3-(2H)-Furanone+ Methanal... 

Within the area of SET-promoted di-7t-methane reactions, recent studies have shown that irradiation of 1-aza-1,4-dienes, such as 32, and the 1,4-diene 34, using AW-dimethylaniline (DMA) as electron-donor sensitizer, leads to production of the corresponding cyclopropane derivatives 33 and 35 resulting from 1-ADPM and DPM rearrangements, respectively, in reactions that take place via radical-anion intermediates (Sch. 11) [25]. 

A detailed study of the oxidation of alkenes by O on MgO at 300 K indicated a stoichiometry of one alkene reacted for each O ion (114). With all three alkenes, the initial reaction appears to be the abstraction of a hydrogen atom by the O ion in line with the gas-phase data (100). The reaction of ethylene and propylene with O" gave no gaseous products at 25°C, but heating the sample above 450°C gave mainly methane. Reaction of 1-butene with O gives butadiene as the main product on thermal desorption, and the formation of alkoxide ions was proposed as the intermediate step. The reaction of ethylene is assumed to go through the intermediate H2C=C HO which reacts further with surface oxide ions to form carboxylate ions in Eq. (23),... 

Rostrup-Nielsen and Pedersen (209) recently studied sulfur poisoning of supported nickel catalysts in both methanation and Boudouard reactions by means of gravimetric and differential packed-bed reactor experiments. In their gravimetric experiments a synthesis mixture (H2/CO/He = 5/7/3) containing 1-2 ppm H2S was passed over a catalyst pellet of 13% Ni/Al203-MgO at 673 K and 1 atm. The rates of Boudouard and methanation reactions were determined from weight increases and exit methane concentrations respectively. In the presence of 2 ppm H2S a factor of 20 decrease was observed in both methanation and Boudouard rates over a period of 30-60 min. However, the selectivity or ratio of the rates for Boudouard and methanation reactions was constant with time. From these results the authors concluded that the methanation and Boudouard reactions involve the same intermediate, carbon, and that sulfur blocks the sites for the formation of this intermediate. 

An alternative approach to partial oxidation of methane is oxyhydrohalogenation— usually oxyhydrochlorination.31 In this alternative chemistry modification, methane is converted first to methyl chloride in a reactor fed a mixture of HC1, steam, and methane. The intermediately formed methyl chloride then is converted in a second catalytic (zeolite) reactor to low-to-medium range hydrocarbons. The net reaction is ... 

Older IR-imwstigalions by Kolbel et at. led to the assumption of surface formyl or hydroxycarbene groups [113]. More recent measurements on supported ruthenium under synthesis conditions showed absorptions for mr ecularly adsorbed CO and for formate and CH species. As a result of deuieration experiments, however, the latter were concluded not to be reaction intermediates (113. 106], It was also shown that the production of methane and ethane continued for a significant period after CO had been removed from tlic reaction mixture and after the disappearance of all IR-observable COads species [106J. It was concluded that product formation occurs via carbidic intermediates. /n-situ IR studies at higher pressures (3 bar) revealed formation of CHjf species with absorptions in the 3000 cm region (114). 

The use of mass spectrometers suggests itself as a convenient method for the investigation of unstable reaction intermediates. The mass spectrometer can not only measure the concentration of an intermediate but also establish its chemical identity. Eltenton first applied the technique to the study of free radicals, and from the earliest experiments useful results have been obtained. Thus Eltenton in his first paper showed that the pyrolysis of methane on a carbon filament leads to the formation of methyl radicals, and not methylene 2is believed formerly, and he... 

We are particularly concerned about assumption (3), as surface spectroscopy has recently shown that in addition to growing chains a substantial amount of carbidic carbon develops on the catalyst surface (35-37). While only part of it might be reaction intermediate under steady state conditions, it would be converted almost completely to methane and small amounts of higher hydrocarbons when the surface is exposed to hydrogen in the absence of CO (37). 

The limited yield of biphenyl (<5%), the trace amounts of trapped phenyl radical, and the absence of oxygenated phenyl products are consistent with the reaction sequence of Eqs. (7-17) and (7-18), and with the reaction intermediate of Eq. (7-18). The reaction of O2 - with methylhydrazine is analogous to that for phenylhydrazine with methane the major product. The slower rate is consistent with the enhanced stabilization of radical intermediates by aromatic substituents. 

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 photochemical production of vinylcyclopropane derivatives from compounds having two 7t-moieties bonded to an sp3-hybridized carbon648 is termed the din-methane rearrangement, also known as the Zimmerman reaction.649 A very broad spectrum of di-7t-systems can lead to photoproducts that are usually not obtainable by alternative routes.632,633 The reaction may be classified formally as a [l,2]-shift but, according to the proposed stepwise biradical mechanism,650 651 1,3- and 1,4-biradical (BR) intermediates and also the second 7t-bond may be involved652 (Scheme 6.29). A concerted (pericyclic) pathway for the di-7t-methane reaction from the excited singlet state is, however, not excluded. Typically, the singlet state reaction occurs upon direct... 

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