Methane oxidation high temperature

Reactions of methane and ethane with HF recyclable metal fluorides to give fluorocarbons have been reported briefly in the patent and journal literature [16-18], Reaction of methane with hydrogen fluoride in the presence of oxygen and the salt or oxide of a variable valency metal as catalyst yielded small amounts of fluoromethane and difluoromethane at temperatures above 500°C. Olsen et al. [17] reacted copper(ll) fluoride with methane at high temperatures (>600°C) and found products that always included copper metal, hydrogen fluoride, fluoromethane and carbon. Although activity was first detected around... 

Chouddhury and co-worker[6] oxidized methane at high temperatures ranging from 300-900°C over Ni/CaO. High methane conversion (90%) and high synthesis gas selectivity (92%) were found when the reaction took place over reduced Ni catalyst [6], Schmidt et al. [7], studied the catalytic partial oxidation of CH in air and pure O2 at atmospheric pressure over Pt and Rh coated monoliths. High selectivity for H2 and CO (90 s%) were achieved at 950°C over Rh catalyst when pure O2 was used with air, the selectivity s were 70% and 40% over Rh and Pt, respectively. 

Such a mechanism is postulated to operate in the activation of methane at high temperatures in the process of the oxidative coupling (65). Catalysts which are both active and selective for the oxidative coupling of methane may be classified as strongly basic metal oxides. Substitution of lower-valent cations in their lattice generates oxygen vacancies, which constitute electron acceptor levels and are responsible for the appearance of electron holes in the valence band. These holes diffuse to the surface, because the lone pair orbitals of surface oxide ions are the HOMOs of the oxide and their energy levels form the top of the valence band. Localization of a hole on such lone pair orbital is equivalent to the formation of a surface O- species. 

An Overview. During the past decade, much work has been reported ii.volving attempts to activate methane and convert it to ethylene and other higher hydrocarbons. Two main routes have been proposed. The first involves passing methane at high temperatures (800 c+) over transition metal oxides at variable oxidation states. The oxides are reduced to lower valence oxides, and the methane is oxidatively dehydrogenated and coupled to form ethane and ethylene as well as water and carbon dioxide. 

Langguth J, Dittmeyer R, Hofmann H and Tomandl G (1997), Studies on oxidative coupling of methane using high-temperature proton-conducting membranes ,... 

Methane oxidations occur only by intermediate and high temperature mechanisms and have been reported not to support cool flames (104,105). However, others have reported that cool flames do occur in methane oxidation, even at temperatures >400 ° C (93,94,106,107). Since methyl radicals caimot participate in reactions 23 or 24, some other mechanism must be operative to achieve the quenching observed in methane cool flames. It has been proposed that the interaction of formaldehyde and its products with radicals decreases their concentrations and inhibits the whole oxidation process (93). 

The reported characteristics of methane oxidation at high pressures are interesting. As expected,the reaction can be conducted at lower temperatures eg, 262°C at 334 MPa (3300 atm) (100). However, the cool flame phenomenon is observed even under these conditions. At high pressures. 

As a chemical compound, methane is not very reactive. It does not react with acids or bases under normal conditions. It reacts, however, with a limited number of reagents such as oxygen and chlorine under specific conditions. For example, it is partially oxidized with a limited amount of oxygen to a carbon monoxide-hydrogen mixture at high temperatures in presence of a catalyst. The mixture (synthesis gas) is an important building block for many chemicals. (Chapter 5). 

It is usual to protect carbon from oxidation at high temperature by the use of alternative gas atmospheres — these are generally hydrogen, nitrogen, argon or helium. The first two will react at temperatures above 1700°C to form methane and cyanogen, respectively. 

However, many reactions of commercial interest have chemistry, mechanical, or system requirements that preclude the use of cross-flow reactors. Processes cannot use a cross-flow orientation primarily because of high temperatures and the need to internally recuperate heat such as steam methane reforming (SMR) [12, 13] and oxidation reactions [14]. Counter- and coflow devices require a micromanifold to dehver sufficiently uniform flow to each of the many parallel channels. 

The application of ly transition metal carbides as effective substitutes for the more expensive noble metals in a variety of reactions has hem demonstrated in several studies [ 1 -2]. Conventional pr aration route via high temperature (>1200K) oxide carburization using methane is, however, poorly understood. This study deals with the synthesis of supported tungsten carbide nanoparticles via the relatively low-tempoatine propane carburization of the precursor metal sulphide, hi order to optimize the carbide catalyst propertira at the molecular level, we have undertaken a detailed examination of hotii solid-state carburization conditions and gas phase kinetics so as to understand the connectivity between plmse kinetic parametera and catalytically-important intrinsic attributes of the nanoparticle catalyst system. 

Interaction of chlorine with methane is explosive at ambient temperature over yellow mercury oxide [1], and mixtures containing above 20 vol% of chlorine are explosive [2], Mixtures of acetylene and chlorine may explode on initiation by sunlight, other UV source, or high temperatures, sometimes very violently [3], Mixtures with ethylene explode on initiation by sunlight, etc., or over mercury, mercury oxide or silver oxide at ambient temperature, or over lead oxide at 100°C [1,4], Interaction with ethane over activated carbon at 350°C has caused explosions, but added carbon dioxide reduces the risk [5], Accidental introduction of gasoline into a cylinder of liquid chlorine caused a slow exothermic reaction which accelerated to detonation. This effect was verified [6], Injection of liquid chlorine into a naphtha-sodium hydroxide mixture (to generate hypochlorite in situ) caused a violent explosion. Several other incidents involving violent reactions of saturated hydrocarbons with chlorine were noted [7],... 

These results confirm that cobalt oxides particles (C03O4 and CoOx) have a very important role in the direct oxidation of methane to C02. On the other hand, at high temperature, cationic cobalt (Co2+) appears to be able to reduce NO to N2, even in the presence of an excess of oxygen. 

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