Methane controlled oxidation

Carbon monoxide may be prepared by several methods. Large scale production is carried out by controlled oxidation of natural gas or by the catalytic steam reforming of methane or light petroleum fractions. The products obtained are mixtures of CO, H2, and CO2. It also is made by gasification of coal and coke with oxygen at about 1,500°C. 

In this paper, we summarize results from a small scale methane direct oxidation reactor for residence times between lO and lO seconds. For this work, methane oxidation (using air or oxygen) was studied over Pt-10% Rh gauze catalysts and Pt- and Rh-coated foam and extruded monoliths at atmospheric pressure, and the reactor was operated autothermally rather than at thermostatically controlled catalyst temperatures. By comparing the steady-state performance of these different catalysts at such short contact times, tiie direct oxidation of methane to synthesis gas can be examined independent of the slower reforming reactions. 

Methanation of C02 over Pd on zirconia and Ni on zirconia catalysts derived form amorphous Pd-Zr-, Ni-Zr-, and Ni-containing multicomponent alloys prepared by controlled oxidation-reduction treatment or generated under reaction conditions have been studied in detail. 

Methane also is used in the production of olefins on a large scale. In a controlled-oxidation process, methane is used as a raw material in the production of acetylene. 

Chan SI, Yu SSF. Controlled oxidation of hydrocarbons by the membrane-bound methane monooxygenase the case for a tricopper cluster. Acc Chem Res. 2008 41 969-79. 

In industry many selective oxidations are carried out in a homogeneously catalyzed process. Heterogeneous catalysts are also applied in a number of processes, e.g. total combustion for emission control, oxidative coupling of methane, the synthesis of maleic acid from butanes, the epoxidation of ethylene. Here we focus upon heterogeneous catalysis and of the many examples we have selected one. We will illustrate the characteristics of catalytic oxidation on the basis of the epoxidation of ethylene. It has been chosen because it illustrates well the underlying chemistry in many selective oxidation processes. 

The chemical mechanisms involved in the controlled oxidation of hydrocarbons are extremely complex. A simplified interpretation can nevertheless provided by considering methane. The following transfonnations are considered in this case ... 

Steam reforming is based essentially on the controlled oxidation, by water, of methane or, more generally, hydrocarbons. The main reactions are as foUows ... 

Methane is oxidized under aerobic conditions by a group of bacteria called methanotrophs. These widespread bacteria play an important role in the global cycling of methane. Two types of methane oxidation systems are known, a ubiquitous particulate methane monooxygenase (pMMO) and a cytoplasmic soluble methane monooxygenase (sMMO) found in only a few strains. These enzymes have different catalytic characteristics, and so it is important to know the conditions under which each is expressed. In those strains containing both sMMO and pMMO, the available copper concentration controls which enzyme is expressed. However, the activity of the pMMO is also affected by copper. Data on methane oxidation in natural samples suggest that methanotrophs are not copper-limited in nature and express the pMMO predominantly. 

Fig. 6.18 (a) Variation of atmospheric CH4 concentration with input flux (from methanogenesis) for varying 02 levels (controlling oxidative destruction), (b) Influence of atmospheric methane on mean surface temperature (assuming constant 02 and C02 levels, but different solar luminosity relative to present (S/S0)) during the Palaeoproterozoic (2300Ma) and Neoproterozoic (700Ma). (After Pavlov et al. 2003.)... 

Methanol (CH3OH) can be made by the controlled oxidation of methane ... 

The anodic oxidation of alkanes in anhydrous hydrogen fluoride has been studied at various acidity levels from basic medium (KF) to acidic medium (SbFs) to establish optimum conditions for the formation of carbenium ions . The oxidation potential depends on the structure of the hydrocarbon methane is oxidized at 2.0 V, isopentane at 1.25 V vs Ag/Ag. Three cases of oxidation can be distinguished. In basic medium, direct oxidation of the alkane to its radical cation occurs. In a slightly acidic medium, the first-formed radical cation disproportionates to cation, proton and alkane. The oxidation is, however, complicated by simultaneous isomerization and condensation reactions of the alkane. In strongly acidic medium, protonation of the alkane and its dissociation into a carbenium ion and molecular hydrogen occurs. In acidic medium n-pentane behaves like a tertiary alkane, which is attributed to its isomerization to isopentane. The controlled potential electrolysis in basic medium yields polymeric species. 

Although this process offers the advantage of driving a low temperature, carefully controlled oxidation of methane, thereby increasing the yield of methanol, it also utilizes sulfuric acid to produce the intermediate methyl bisulfate. The need for acid resistant containers to perform these reactions may raise costs of the process. And although the sulfuric acid is recovered and recycled into the process, the environmental benefits of this methane conversion are somewhat offset by the need to ship and store hazardous sulfuric acid. The trade-off between safer methane transport versus increased sulfuric acid transport and storage needs to be considered from the perspective of accidental releases. 

Akin, F. and Lin, Y. (2002). Controlled Oxidative Coupling of Methane by Ionic Conducting Ceramic Membrane, Catal Lett., 78, pp. 239-242. 

Gesser HD, Hunter NR, Prakash CB. The direct conversion of methane to methanol by controlled oxidation. Chem Rev 1985 85 235-44. 

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