CARBON-DIOXIDE CHEMISTRY DURING OXIDATIVE COUPLING OF METHANE ON A LI/MGO CATALYST

Reactor tests, temperature-programmed reaction/desorption (TPR/D) carried out in a thermogravimetric balance and FT-IR were employed to investigate the course of CO2 formed during oxidative coupling of methane (OCM) on a 7-wt.-% Li/MgO catalyst between 600-800 degrees C. Initially, the carbonate free Li/MgO catalyst showed good OCM activity even at 600 degrees C. However, its activity diminished considerably after about 20 min on stream. This coincided with the appearance of CO2 in the OCM products and the disappearance of ethylene. During OCM, a substantial amount of the available lithium was converted to a stable carbonate which did not decompose easily even at 800 degrees C. FT-IR and TPR revealed that carbonate formation began at 400 degrees C and that the lithium carbonate existed in the form of monodentate (LiOCO2) and perhaps mixed bridged [Mg(Li)O2CO] carbonates. Simultaneous existence of a commonly accepted Li2CO3 cannot be excluded. The role of CO2 produced during OCM in modifying the catalytic performance of Li/MgO is discussed.