Methane oxidative coupling

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). 

A few examples of chemoautolithotrophic processes have been mentioned in this chapter, namely anaerobic methane oxidation coupled to sulfate reduction and the ones listed in Table 12.2 involving manganese, iron, and nitrogen. Another example are the microbial metabolisms that rely on sulfide oxidation. Since sulfide oxidation is a source of electrons, it is a likely source of energy that could be driving denitrification, and manganese and iron reduction where organic matter is scarce. 

M. (1992) Methane production and its fate in paddy fields. II. Oxidation of methane and its coupled ferric oxide reduction in subsoil. Soil Sci. Plant Nutr. 38 673-679... 

Enhanced Q yields from methane oxidative coupling by means of a separative chemical reactor (with A.L. Tonkovich and R.W. Carr). Science 262,221-223 (1993). 

Beside methanol and formaldehyde, the oxidation of methane may be directed to another route, leading to the formation of its condensation products, for example, ethane, ethylene and benzene. This route may provide an alternative way for the chemical use of natural sources of methane. Here, various catalysts were also tested using both 02 and N20 as the oxidants [22], The general picture observed by most authors was similar to that with methane oxidation to oxygenates. The conversion of methane was always higher with 02 than with N20. However, the selectivity to the coupling products showed an opposite trend. 

Selective oxidations, e.g., propane to acroleine, butane to maleic anhydride, ethylene to ethylene oxide Oxidative dehydrogenations of hydrocarbons Oxidative coupling of methane Methane oxidation to syngas... 

Devol, A.H., Anderson, J.J., Kuivila, K., and Murray, J.W. (1984) A model for coupled sulfate reduction and methane oxidation in the sediments of Saanich Inlet. Geochim. Cosmochim. Acta 48, 993-1004. 

A search for organisms with novel metabolic and bioenergetic pathways, particularly pathways involved in carbon dioxide and carbon monoxide reduction and methane oxidation coupled with electron acceptors other than oxygen ... 

The methane oxidation to methanal is thus realized in the catalytic cycle in which atmospheric 02 is the oxidant and the OH radicals are the catalyst, and which is coupled to photoassisted dissociation of nitrogen dioxide (Figure 9.7). The latter process yields two ozone molecules per photocatalytic cycle. 

Industry experts today suggest conversions of 40-50 percent and selectivities above 80 percent based on methane and oxygen as the minimum needed for commercial consideration after fixed and variable costs are added. Nonetheless, methane oxidative coupling holds the most promising combination of process simplicity, product slate versatility and low cost, and worldwide raw material availability not offered now by practiced fuel and chemical feedstock technologies. 

While Eqn. (1) predicts OH levels in the remote troposphere in reasonably good agreement with the predictions of more elaborate photochemical models which properly treat the HO2/OH coupling, for conditions appropriate for less remote regions where enhanced NOx levels are commonly encountered Eqn. (1) does not accurately calculate the OH concentrations. This is because as NOx levels increase, a greater fraction of the HO2 radicals produced from the methane oxidation reaction sequence react with NO via (R8) to regenerate OH. Thus as illustrated in Figure 4, the levels of OH calculated in a complete photochemical model increase substantially as NOx levels increase from the pptv level (typical of remote marine conditions) to the more polluted ppbv level. For NOx levels in... 

On the other hand, Ito et al. (99) found that the oxidative dimerization of methane to yield ethylene and ethane can be achieved with a high yield and good selectivity on Li-doped MgO catalysts. Since this pioneering work, many oxidic systems have been studied. Anpo et al. (100) found that surface sites of low coordination produced by the incorporation of Li into MgO play a vital role in the methane oxidative coupling reaction. Thus, although it was known that MgO acts as an acid-base catalyst, both the catalytic and photocatalytic activities of the MgO catalysts seem to be associated with the existence of surface ions in low coordination located on MgO microcrystals. 

In the methane oxidative coupling reaction, a small amount of Li doping of the MgO catalyst leads to an enhancement of the yield of C2 compounds (mainly C2H4 and C2He) as well as O2 and CH4 conversions (99, 100). The... 

Figure 58 shows the photoluminescence spectrum of the undoped MgO degassed at the same temperature as the methane oxidative coupling reaction together with the photoluminescence speclrmn of the 3 mol% Li-doped MgO (Fig. 58, 2) and its deconvoluted curves (Fig. 58, 2-a and 2-b). In addition to a characteristic photolumincscence spectrum at around 370 nm, attributed to the surface sites in low coordination on MgO, the Li-doped MgO exhibits a new photoluminescence band at about 350-550 nm with a at about 450 nm (Fig. 58, 2-b). The intensity of this new emission depends on the amount of Li doped. The excitation spectr um corresponding to this new emission is evident at about 260-290 nm 100, 240), which suggests that surface sites with a coordination number of four may be associated with this new photoluminescence. 

These findings show that the newly formed surface sites in low coordination on the Li-doped MgO are more active than those on the undoped catalyst, which explains the high activity of the Li-doped MgO catalyst for methane oxidative coupling. Ilo et al (99) detected the (Li-O ) or O sites on the Li-doped MgO catalyst by EPR and suggested that these sites play a significant role in the formation of CH3 radicals from CH4. It is unclear whether the newly produced surface sites in low coordination are directly associated with the existence of such active sites. 

Three primary mechanisms have been offered to explain anaerobic CH4 oxidation in marine sediments. The process may be carried out by a single organism that couples methanotrophy to S04 reduction. Anaerobic methane oxidation coupled to sulfate reduction is thermodynamically... 

Coronas J., Menendez M. and Santamaria J., Methane oxidative coupling using porous ceramic membrane reactors. Part II. Reaction studies, Chem. Engng. Sci. 49 2015 (1994). Coronas J., Menendez M. and Santamaria J., Development of ceramic membrane reactors with non-uniform permeation pattern. Application to methane oxidative coupling, Chem. Eng. Sci. 49 4749 (1994). 

Lafaiga D., Santamaria J. and Menendez M., Methane oxidative coupling using porous ceramic membrane reactors. Part I. Reactor development, Chem. Eng. ScL 49 2005 (1994). Sloot H.J., Smolders C.A., van Swaaij W.P.M. and Versteeg G.F., High-temperature membrane reactor for catalytic gas-solid reactions, AIChE J. J5 887 (1992). 

Indirect evidence of the mechanism of methane formation was reported in the early part of the twentieth century (Sohngen, 1910), and in the 1930s (Stephenson and Strickland, 1931, 1933 Fischer, Lieske, and Winzer, 1931, 1932). Sohngen found that enrichment cultures can couple the oxidation of hydrogen with the reduction of carbon dioxide according to ... 

This section will address the following problems loss of matter in the methane dimeriza-ticm catalysts, the role of sintering, the loss of surface area, and finally, structure sensitivity of some of the methane-oxidative coupling catalysts. 

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