Oxygen isotope transfer rates during the oxidative coupling of Methane over a Li/MgO catalyst

The rates of various oxygen isotope transfer processes over Li/MgO catalysts have been investigated in a flow system under conditions relevant to methane coupling. Using ¹⁶O₂/¹⁸O₂ mixtures, mixing to form ¹⁶O₁₈O reached equilibrium at 700 and 750°C when helium was used as the carrier. The extent of equilibration was greatly reduced when the same mixture was used to oxidize methane. The inhibition occurs because mixing requires that the catalytic sites be in oxide form and these sites are converted to carbonate by the carbon dioxide formed from methane concurrently with ethane and ethylene. The same inhibition could be demonstrated directly when using an inert carrier by including C¹⁸O₂ in the feed. When C¹⁸O₂ was added during methane coupling with ¹⁶O₂ as the sole oxidant, there was complete equilibration between added C¹⁸O₂ and C¹⁶O₂ derived from methane. In addition some ¹⁸O was transferred into unreacted ¹⁶O₂. This exchange did not produce ¹⁶O₂, ¹⁶O₁₈O, and ¹⁸O₂ in equilibrium. There was a slight excess of ¹⁸O₂ due to the occurrence of some two-place exchange. Transient experiments in which C¹⁸O₂ alone was carried over Li/MgO showed that only a small fraction of the total oxygen pool of the catalyst participated in exchange. It is probably associated with lithium. When C¹⁸O was included in reacting CH₄/¹⁶O₂ mixtures, approximately one-half of it was oxidized to carbon dioxide. Some exchange of ¹⁸O out of unreacted C¹⁸O took place but the rate of this process was much slower than transfer from carbon dioxide to oxygen. Calculations indicate that the catalyst is able to carry out exchange reactions of oxygen molecules faster than it reacts oxygen with methane. Hence methane coupling is unlikely to be limited by the supply of oxygen to the catalyst surface when the Li/MgO system is used.