Stable products from methane

The results of these studies are given in Table I. The percent of all primary products is about equal for positive ions and neutral species (45% and 55%, respectively). Thus, it appears that any mechanism for producing stable products from the radiolysis of methane must include positive ions and neutral species. 

Mercuric chloride in methanol also reacts with compounds 8 (in dichloro-methane), forming colorless mercury complexes, which can in turn be reconverted to the cyanines 8. Such addition compounds are stable only as solids, decomposing rather quickly in solution. Mercuric acetate in methanol reacts rapidly with the formation of elemental mercury, where by the phosphamethin-cyanines are destroyed uniform products from this reaction have not as yet been isolated. 

Synthesis gas production. Alqahtany et al.92 have studied synthesis gas production from methane over an iron/iron oxide electrode-catalyst. Although the study was essentially devoted to fuel cell operation, for purposes of comparison some potentiometric work was performed at 950°C. It was found that under reaction conditions Fe, FeO or Fe304 could be the stable catalyst phase. Hysteresis in the rates of methane conversion were observed with much greater rates over a pre-reduced surface than over a pre-oxidised surface possibly due to the formation of an oxide. 

With the exception of formic acid, the lower fatty acids are quite stable np to relatively high temperatures. Cahours and Berthelot early notedos the thermal stability of these acids, and the latter reported that acetic acid did not decompose until above a dull red heat. More recently Senderens showed that acetic, propionic, /i-butyric, isobutyric, and isovaleric acids were perfectly stable at temperatures as high as 460° C.os At higher temperatures these acids undergo pyrogenic decomposition to yield simple and stable substances. In the case of acetic acid, Nef 01 reported the presence of methane, carbon dioxide, carbon monoxide, ethylene, hydrogen, and acetone in the products from decomposition. 

We designed a novel three-compartment source (wide-range radiolysis source) for our research mass spectrometer, which was first used to study the radiolysis of methane. The present technique, employing flow, low pressure, localized ionization, and electric fields appears to be a straightforward approach to the problem, and we hoped that this technique would resolve some of the above discrepancies. Our objectives were to (a) determine the percent abundance of the various reactive primary species—ionic and neutral (b) ascertain the percent abundance of stable products under conditions that would minimize subsequent reactions of reactive stable products (c) calculate G values for these products (d) measure the relative contribution of ion-molecule reactions to the formation of stable products (e) obtain the threshold energies and yield curves for such products to assign their precursors and (f) postulate, from the above information and pressure studies, a mechanism for the production of the radiolytic products from methane. 

Preliminary experiments, extending the pressure studies to final products, were performed. Methane was radiolyzed in Compartment A by 150 e.v. electrons in a positive field. The sample pressure was varied from (0.7 to 4.1) X 10 torr, and the intensities of the resulting final stable products were monitored as usual. 

Anhydrous, monomeric formaldehyde is not available commercially. The pure, dry gas is relatively stable at 80—100°C but slowly polymerizes at lower temperatures. Traces of polar impurities such as acids, alkahes, and water greatly accelerate the polymerization. When Hquid formaldehyde is warmed to room temperature in a sealed ampul, it polymerizes rapidly with evolution of heat (63 kj /mol or 15.05 kcal/mol). Uncatalyzed decomposition is very slow below 300°C extrapolation of kinetic data (32) to 400°C indicates that the rate of decomposition is ca 0.44%/min at 101 kPa (1 atm). The main products ate CO and H2. Metals such as platinum (33), copper (34), and chromia and alumina (35) also catalyze the formation of methanol, methyl formate, formic acid, carbon dioxide, and methane. Trace levels of formaldehyde found in urban atmospheres are readily photo-oxidized to carbon dioxide the half-life ranges from 35—50 minutes (36). 

The result of the fast reactions in the ion source is the production of two abundant reagent ions (CH5+ and C2H5+) that are stable in the methane plasma (do not react further with neutral methane). These so-called reagent ions are strong Brpnsted acids and will ionize most compounds by transferring a proton (eq. 7). For exothermic reactions, the proton is transferred from the reagent ion to the neutral sample molecule at the diffusion controlled rate (at every collision, or ca. 10 9 s 1). 

The most stable of all alkyl cations is the tert-butyl cation. Even the relatively stable tert-pentyl and fen-hexyl cations fragment at higher temperatures to produce the tert-butyl cation, as do all other alkyl cations with four or more carbons so far studied. Methane,ethane, and propane, treated with superacid, also yield ten-butyl cations as the main product (see 2-17). Even paraffin wax and polyethylene give the ten-butyl cation. Solid salts of frrf-butyl and rerf-pentyl cations (e.g., MeaC" SbFg ) have been prepared from superacid solutions and are stable below -20°C. ... 

Presumably, 9 is actually formed from carbene 8 in the pyrolysis zone by a P/C phenyl shift, but then apparently succumbs to fast transformation into the thermodynamically stable final products. Formation of the methane derivative 13 should be preceded by a 1,2-phenyl shift to give the shortlived 10, the production of fluorene (14) by the occurrence of diphenylcarbene (II), and the formation of benzophenone (15) by isomerization to the angle-strained three-membered heterocycle 12, which is followed by elimination of phenylphospbinidene. No direct evidence is available for the intermediacy of 10-12. 

For illustration, we consider a simplified treatment of methane oxidative coupling in which ethane (desired product) and CO, (undesired) are produced (Mims et al., 1995). This is an example of the effort (so far not commercially feasible) to convert CH, to products for use in chemical syntheses (so-called Q chemistry ). In this illustration, both C Hg and CO, are stable primary products (Section 5.6.2). Both arise from a common intermediate, CH, which is produced from CH4 by reaction with an oxidative agent, MO. Here, MO is treated as another gas-phase molecule, although in practice it is a solid. The reaction may be represented by parallel steps as in Figure 7.1(a), but a mechanism for it is better represented as in Figure 7.1(b). 

---