ORIGIN OF CARBON OXIDES DURING THE OXIDATIVE COUPLING OF METHANE
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Carbon oxides (CO and CO2), which are generated as side products during the oxidative coupling of methane, can potentially be derived from CH4, C2H4, or C2H6. The source of COx was determined by adding 13C2H6 or 13C2H4 to the reacants and analyzing the isotopic composition of the COx. In order to eliminate the extent of reaction as a factor, a parameter, R has been defined as the ratio of the percentage of 13C in the COx products to that in the feed gas. If the amount of COx derived from C2H6 is small, it can be shown that R is approximately the ratio of the rate constant for COx formation from C2H4 to that from CH4. Values of R have been obtained for several oxide catalysts by introducing measured amounts of 13C2H4 into a CH4/O2 reaction mixture and then determining the percentage of 13C in the COx reaction product. Additional experiments, involving 13C2H6 addition to the reaction mixture, have established that the primary coupling product, C2H6, is selectively converted into C2H4; i.e., virtually no direct conversion of C2H6 into COx occurs during the methane coupling reaction. For selective oxidative coupling catalysts, the experimentally determined values of R at 700C were 2.8 for Sr/La2O3 and Ba/MgO and 4.7 for Li+/MgO. The value of increased with increasing reaction temperature, along with the C2 selectively and yield, indicating that the direct conversion of CH4 into COx is less important at higher temperatures. At 850 C, for example, the value of R for Ba/MgO was 6.2. It has been noted previously that, over a Li+/MgO catalyst, lower reaction favor the formation of COx from CH4. This occurs because of decreased CH4 conversion and, thus, lower partial pressures of C2H4, not because of a particularly small value of R. At 650 C in fact, the value of R for Li+MgO was 304. Over a wide range of operating conditions, the rate constants for COx formation from C2H4 were several times as large as those for COx formation from CH4. This observation is consistent with the moderate C2 yields that have been achieved during the oxidative coupling of CH4, although large values of R may also indicate that CH4 is converted into C2H6 very selectively.
