Oxidation of methane at about 400°C. Part 1.—Reaction catalyzed by the photodecomposition of acetone

Abstract

The reaction of methane and oxygen at about 400°C catalyzed by the photodecomposition of acetone has been studied using a static system. With a new vessel at 414°C the thermal reaction was negligible and the maximum rate of pressure change Lax) was proportional to and independent of the intensity of the ultra-violet radiation. The products were carbon monoxide, carbon dioxide, water, formaldehyde and methanol, the concentration of the last two rising to a maximum and the alcohol not being formed until after the initial stages of the reaction. After many expsriments the thermal reaction became relatively rapid and the temperature was reduced to 395°C. Both the rate and the products of oxidation now depended on the pretreatment of the surface. After a number of photo-initiated oxidations (" activated " surface) appreciable amounts of hydrogen peroxide and methylhydroperoxide were present in the products and the yield of methanol was considerable. After a number of thermal C h -0 2 reactions (" deactivated " surface) the rate of a subsequent photo-initiated reaction and the alcohol yield was much less and no peroxides were detected in the products. With an " activated " surface the kinetics were different ; in particular, pmax rose to a flat maximum on increasing the methane concentration.The oxidation is almost certainly a chain process with degenerate branching. Possible elementary reactions occurring in the system and the effect of surface are discussed. Most of the C02 probably comes from the reaction of CH3COCH2 radicals and oxygen. It is likely that with a " deactivated "

The gaseous oxidation of acetone - II. The low-temperature reaction

“…formaldehyde, it probably comes directly from the breakdown of the acetonyl peroxy radical Exam ination of a scale molecular model shows th a t this reaction involves rela tively little movement of the oxygen atom, which has to be followed only by the necessary electron redistribution to allow the products to separate. This is a certain am ount of supporting evidence for this reaction which has also been proposed by Hoare & Wellington (i960) and by Fisher & Tipper (1963 a). Thus, Wenger & K utschke (1959) have suggested a formally similar reaction to explain their results on the oxidation of azomethane CH3.N :N .C H 3 + 0 2 = CH3 + N 20 + H C H 0 and Dunn & K utschke (1958) have shown by tracer studies th a t about 90% of the carbon dioxide formed in the photoxidation of acetone a t 120 to 175 °C came from the central carbon atom.…”

“…We may, however, estimate the lifetime of the hydroperoxide a t 284 °C to be interm ediate between th a t of ethyl hydroperoxide (about 15 s) and hydrogen peroxide (5*6 x 106 s). The only other information upon which an estimate may be based are the experiments by Fisher & Tipper (19636) which lead to the half-life of methyl hydroperoxide being ~ 30 s at 395 °C; assuming the activation energy for the homogeneous decomposition is the same as for other organic hydroperoxides, namely 38 kcal, this leads to a life-time of about 104 s a t 284 °C.…”

On the mechanism of thermal oxidation of methane

“…Similar relationships were observed in ref. 20, and it was concluded that the detected organic peroxide is methylhydroperoxide, which is formed according to the following reactions:…”

“…In ref. 20 it was mentioned that when the surface was "nonactive" peroxides were not observed and the rate of the reaction was lower than that found in reactors having an "active" surfacewhich obtains when the reaction is photoinitiated. On "active" surface hydrogen peroxide, methyl hydroperoxide and methanol were formed.…”

“…As a leading active center, the HOz radical has been considered, while for chain-branching, the reaction CHzO + O2 = HCO + H02. Data accumulated recently [15][16][17][18][19][20][21][22][23], however, make it necessary to reevaluate the existing point of view on the mechanism of the reaction, especially with respect to the branching act.…”