Methane oxidative coupling reaction with

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. 

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. 

Non-metallic catalysts — MgO, Li/MgO and La203 — known to produce methyl radicals during the methane oxidative coupling reaction have been shown to be active for NO reduction by CH4. Li-promoted MgO in the absence of O2 produces N2 and N2O with a (N2/N2O) selectivity below 2 at low temperature but which increases to 3 or more at higher temperatures. Unpromoted MgO is less active but produces almost 100% N2 at high temperatures. La203 is more active and selective for NO reduction to N2 by CH4 than MgO and Li/MgO catalysts. The activity of La203 continuously increases with temperature, at least up to 973 K, and selectivity for N2 rather than N2O... 

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). 

Much recent research (7-5) has been devoted to converting methane to products that are more easily transported and more valuable. Such more valuable products include higher hydrocarbons and the partial oxidation products of methane which are formed by either direct routes such as oxidative coupling reactions or indirect methods via synthesis gas as an intermediate. The topic of syngas formation by oxidation of CH4 has been considered primarily from an engineering perspective (7-5). Most fundamental studies of the direct oxidation of CH4 have dealt with the CH4 + O2 reaction system in excess O2 and at lower temperatures (6-10). 

As can be seen from the above equation, formation of HCN is in reality a hetero-bimolecular oxidative coupling reaction of methane with ammonia. The ammoxidation reactor construction is a simple fixed-bed multi-tube and the catalyst is usually a platinum or sometimes a Group V or VI metal oxide on a silica or alumina support. The HCN product is recovered by condensation and fractionation. With the reaction simplicity and yield, and widespread availability of starting materials, in-situ HCN generation is an ideal industry solution to HCN supply. (See Chapter 29 for more details.)... 

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). 

In this section, we again select the case of light alkanes functionalization in our attempt to discuss fruitful lines of research. It is well known that gas-phase reactions play an important role in methane oxidative coupling. This is expected, as this occurs at very high temperature. But this is a general phenomenon. Contrary to the case of olefins, homogeneous catalytic oxidations of alkanes with more than 3 carbon atoms proceed at temperatures similar to those of the catalytic reaction and these are relatively low. This probably has... 

The coincidence of maxima in the methane oxidation rate and the sulfate reduction rate in Saanich Inlet strongly suggests that the methane oxidizing agent was sulfate, either via direct reaction, or coupled indirectly through reactions with other substrates (Devol, 1983). A methane-sulfate coupled reaction diffusion model was developed to describe the inverse relationship commonly observed between methane and sulfate concentrations in the pore waters of anoxic marine sediments. When fit to data from Saanich Inlet (B.C., Canada) and Skan Bay (Alaska), the model not only reproduces the observed methane and sulfate pore water concentration profiles but also accurately predicts the methane oxidation and sulfate reduction rates. In Saanich Inlet sediments, from 23 to 40% of the downward sulfate flux is consumed in methane oxidation while in Skan Bay this value is only about 12%. 

Other Selected Studies. In addition to the work by ARCO and UCC, numerous catalysts (mostly metal oxides of various groups, singly or mixed, promoted or unpromoted, with or without supports) have been explored for oxidative coupling reaction of methane by a number of researchers in different countries. To... 

The influence of COj on the oxidative coupling of methane with LijCOj/MgO catalysts was studied by Korf et al. It was observed that at 800 C, the yield of C -hydrocarbons decreased from 13% to 3% after 40 hours of reaction. The introduction of COj after the reaction increased C2 yield by 12%. Thus, the catalyst had been restored to its initial behavior by the introduction of COj, despite the loss of Li. It was concluded that the active sites created on the Li/MgO catalyst as a direct result of the loss of carbonate species were not stable in the atmosphere of the oxidative coupling reaction. The authors were able to achieve a yield of 18% for a period of 13.5 hours by carefully choosing reaction conditions for Li/MgO. 

To test our hypothesis that the generation of methyl radicals can enhance the reaction between CH4 and NOx compounds, we first examined NO reduction by CH4 over MgO and Li/MgO, which is the most characterized catalyst system for methane oxidative coupling. This study showed that Li/MgO catalysts as well as pure MgO are indeed active for NO reduction by CH4 and the activity continuously increased with temperature up to 953K. Both N2 and N2O as nitrogen-containing products were observed, and selectivity to N2 increased with temperature. N2 mass balances showed that litde or no NO2 formation occurred. Control experiments demonstrated that there was no reaction of either NO or CH4 in an empty quartz tube reactor with quartz wool inserted. Further experiments in the absence of CH4 showed that there was no direct NO decomposition over these catalysts, which implies that NO reduction can proceed only in the presence of gaseous CH4. 

Many metal oxides are able to perform the oxidative coupling reaction of methane molecules. Methane can also be directly converted with oxygen via oxidative coupling (OCM) into ethane and ethylene according to the following reaction ... 

---