Methyl radicals, oxidation

Some control over the spHt between methyl radical oxidation (to HCHO) and dimerization in heterogeneous oxidation can be achieved by varying conditions (116). For homogeneous oxidation, an efficiency of 70—80% to methanol has been claimed at 8—10% conversions (110). This is the high end of the reported range and is controversial. Even so, such technology appears unlikely to be competitive for regular commercial use until further advances are made (117). The critical need is to protect the products from further oxidation (118). 

Hoare and Wellington (22) produced CH3O radicals from the photochemical (50° and 100°C.) and thermal (135°C.) decompositions of di-terf-butyl peroxide in the presence of 02. The initially formed tert-butoxy radicals decomposed to acetone plus methyl radicals, and the methyl radicals oxidized to methoxy radicals. Formaldehyde and CH3OH were products of the reaction the formation of the former was inhibited, and the latter was enhanced as the reaction proceeded. If the sole fate of CH3O were either... 

The work on acetone photooxidation shows that acetyl radicals may yield acetic acid or carbon dioxide, carbon monoxide, and products of methyl radical oxidation. The carbon dioxide came mostly from the carbonyl group but the carbon monoxide was not so specific in origin. 

Three experiments performed using oxygen containing about 12% Qi6Qig indicated that the carbon dioxide was formed predominantly from carbonyl group oxidation, while the formaldehyde and methanol were formed by methyl radical oxidation and the acetic acid was formed by oxidation of an acetyl radical. The results for carbon monoxide were unreliable. 

The rate coefficient, ko x > for the methyl radical oxidation is given by fe32[M]fe33[X]... 

Hoare and Pearson presented both sides of a debate as to certain mechanistic features of methyl radical oxidation. Different workers have... 

In addition to the above pathways, which proceed through methyl radicals, oxidative dehydrogenation of ethane in the feedstream, as represented by Eq. (7.32) to (7.34), can also occur. 

Since the detailed mechanism of methyl radical oxidation is unknown, an undehned stoichiometric coefficient, x is introduced here. The further oxidation of ethane follows the same mechanism as that for methane oxidation as ... 

Lanthanum oxide has often been used as a major constituent of patented OCM formulations. Lacombe et al. [94] proposed that on La203 catalysts CO2 is formed on its surface, involving a slow step of methyl radical oxidation, while CO issues from C2-1- hydrocarbons. Two types of active sites have been identified on theLa203 surface. Low coordination metal sites localized on step edges are related to the total oxidation pathway, while basic sites associated with oxygen vacancies enable the oxygen activation that leads to the initial methane activation. Proper modification of the lanthana surface by specific additives inhibits CO2 formation. Thus, Sarkany et al. [95] reported the lowest temperature for methane coupling, which in turn... 

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). 

A typical oxidation is conducted at 700°C (113). Methyl radicals generated on the surface are effectively injected into the vapor space before further reaction occurs (114). Under these conditions, methyl radicals are not very reactive with oxygen and tend to dimerize. Ethane and its oxidation product ethylene can be produced in good efficiencies but maximum yield is limited to ca 20%. This limitation is imposed by the susceptibiUty of the intermediates to further oxidation (see Figs. 2 and 3). A conservative estimate of the lower limit of the oxidation rate constant ratio for ethane and ethylene with respect to methane is one, and the ratio for methanol may be at least 20 (115). 

This ladical-geneiating reaction has been used in synthetic apphcations, eg, aioyloxylation of olefins and aromatics, oxidation of alcohols to aldehydes, etc (52,187). Only alkyl radicals, R-, are produced from aliphatic diacyl peroxides, since decarboxylation occurs during or very shortiy after oxygen—oxygen bond scission in the transition state (187,188,199). For example, diacetyl peroxide is well known as a source of methyl radicals (206). 

The oxidation of hydrocarbons involves the sequential formation of a number of similar reactions in which various intermediate radicals which are combinations of carbon, hydrogen and oxygen are formed. In the simplest case, the oxidation of medrane, the methyl radical CH3 plays an important part both in direct oxidation to CO(g) and in indirect oxidation duough the formation of higher hydrocarbons such as CaHe before CO is formed. The chain reactions include... 

The lithium oxide-promoted barium oxide also functions as a catalyst for the methane coupling reaction, but the mechanism is not clearly understood at the present time. The only comment that might be offered here is that the presence of ions on the surface of this material might etdrance the formation of methyl radicals drrough the formation of hydroxyl groups thus... 

The reaction of OH radicals with dimethyl sulfoxide in aqueous solution was studied already in 1964 by Norman and coworkers37 38. They used the system T1m-H202 to produce OH radicals and using ESR/rapid mixing techniques they were able to demonstrate elimination of a methyl radical during the OH induced oxidation. Further studies showed the formation of sulfmic radicals in this reaction either directly or by spin trapping experiments39-44. 

The photolysis of dimethyl sulphoxide (at 253.7 nm) in a wide range of solvents has been studied in detail176. Three primary reactions occur, namely (i) fragmentation into methyl radicals and methanesulphinyl radicals, equation (60), (ii) disproportionation into dimethyl sulphone and dimethyl sulphide, equation (61) and (iii) deactivation of the excited state to ground state dimethyl sulphoxide. All chemical processes occur through the singlet state. Further chemical reactions of the initial photochemical products produce species that have been oxidized relative to dimethyl sulphoxide. 

The methyl radical reacts further to form either formaldehyde CH2O or methyl hydroperoxide CH3OOH, depending upon the nitric oxide NO concentration ... 

Direct conversion of methane to ethane and ethylene (C2 hydrocarbons) has a large implication towards the utilization of natural gas in the gas-based petrochemical and liquid fuels industries [ 1 ]. CO2 OCM process provides an alternative route to produce useful chemicals and materials where the process utilizes CO2 as the feedstock in an environmentally-benefiting chemical process. Carbon dioxide rather than oxygen seems to be an alternative oxidant as methyl radicals are induced in the presence of oxygen. Basicity, reducibility, and ability of catalyst to form oxygen vacancies are some of the physico-chemical criteria that are essential in designing a suitable catalyst for the CO2 OCM process [2]. The synergism between catalyst reducibility and basicity was reported to play an important role in the activation of the carbon dioxide and methane reaction [2]. 

Recently, Stair and coworkers [10, 11] developed a method to produce gas-phase methyl radicals, and used this to study reactions of methyl groups on Pt surfaces [12] and on molybdenum oxide thin films [13]. In this approach, methyl radicals are produced by pyrolysis of azomethane in a tubular reactor locat inside an ulttahigh vacuum chamber. This method avoids the complications of co-adsorbcd halide atoms, it allows higher covraages to be reached, and it allows tiie study of reactions on oxide and other surfaces that do not dissociate methyl halides effectively. 

Investigation of direct conversion of methane to transportation fiiels has been an ongoing effort at PETC for over 10 years. One of our current areas of research is the conversion of methane to methanol, under mild conditions, using li t, water, and a semiconductor photocatalyst. Research in our laboratory is directed toward ad ting the chemistry developed for photolysis of water to that of methane conversion. The reaction sequence of interest uses visible light, a doped tungsten oxide photocatalyst and an electron transfer molecule to produce a hydroxyl i cal. Hydroxyl t cal can then react with a methane molecule to produce a methyl radical. In the preferred reaction pathway, the methyl radical then reacts with an additional wata- molecule to produce methanol and hydrogen. 

The EPR intensity of the ethyl radicals is irreversibly attenuated above 50 K and falls below the detection limit above 80 K. This can be explained by assuming the ethyl radicals to diffuse and recombine at these temperatures, as has been observed for methyl radicals above 45 K [ 124] and for NO2 radicals on an oxide surface above 75 K [125]. 

There are examples in which base radicals undergo reaction with adjacent base residues. The 5-(2 -deoxyuridinyl)methyl radical (63, Scheme 8.30) can forge an intrastrand cross-link with adjacent purine residues. Cross-link formation is favored with a guanine residue on the 5 -side of the pyrimidine radical and occurs under low-oxygen conditions. A mechanism was not proposed for this process, but presumably the reaction involves addition of the nucleobase alkyl radical to the C8-position of the adjacent purine residue. Molecular oxygen likely inhibits crosslink formation by trapping the radical 63, as shown in Scheme 8.24. The radical intermediate 89 must undergo oxidation to yield the final cross-linked product 90,... 

Intriguing results were obtained for photolysis of chemisorbed layers of methyl radicals on oxide films [18]. Experiments confirmed the hypothesis [19] concerning the natime of surface compounds produced in chemisorption of elementary radicals on metal oxides. 

---