ORIGIN OF CARBON OXIDES DURING THE OXIDATIVE COUPLING OF METHANE

Carbon oxides (CO and CO2), which are generated as side products durin g the oxidative coupling of methane, can potentially be derived from C H4, C2H4, or C2H6 The source of COx was determined by adding (C2H6)-C- 13 or (C2H4)-C-13 to the reactants and analyzing the isotopic composit ion of the COx. In order to eliminate the extent of reaction as a fact or, a parameter, R, has been defined as the ratio of the percentage of C-13 in the COx products to that in the feed gas. If the amount of CO x 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 fro m CH4. Values of R have been obtained for several oxide catalysts by i ntroducing measured amounts of (C2H4)-C-13 into a CH4/O-2 reaction mix ture and then determining the percentage of C-13 in the COx reaction p roduct. Additional experiments, involving (C2H6)-C-13 addition to the reaction mixture, have established that the primary coupling product, C2H6, is selectively converted into C2H4; i.e., virtually no direct co nversion of C2H6 into COx occurs during the methane coupling reaction. For selective oxidative coupling catalysts, the experimentally determ ined values of R at 700 degrees C were 2.8 for Sr/La2O3 and Ba/MgO and 4.7 for Li+/MgO. The value of R increased with increasing reaction te mperature, along with the C-2 selectivity and yield, indicating that t he direct conversion of CH4 into COx is less important at higher tempe ratures. At 850 degrees 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 temperatures favor the formation of COx from CH4. This occur s because of decreased CH4 conversion and, thus, lower partial pressur es of C2H4, not because of a particularly small value of R. At 650 deg rees C, in fact, the value of R for Li+/MgO was 3.4. Over a wide range of operating conditions, the rate constants for COx formation from C2 H4 were several times as large as those for COx formation from CH4. Th is observation is consistent with the moderate C-2 yields that have be en achieved during the oxidative coupling of CH4, although large value s of R may also indicate that CH4 is converted into C2H6 very selectiv ely.