THE OXIDATIVE COUPLING OF METHANE AND THE OXIDATIVE DEHYDROGENATION OF ETHANE OVER A NIOBIUM PROMOTED LITHIUM DOPED MAGNESIUM-OXIDE CATALYST

The promoting effect of niobium in a Li/MgO catalyst for the oxidative coupling of methane (OCM) and for the oxidative dehydrogenation of et hane (ODHE) has been studied in some detail. It has been found that a Li/Nb/MgO catalyst with 16 wt % niobium showed the highest activity fo r the C2 production in the OCM reaction; the activity at 600-degrees-C was ten times that of the Li/MgO catalyst at the same temperature. Th e Li/Nb/MgO catalyst was also slightly more active for the ODHE reacti on than was the Li/MgO catalyst. However, the Li/Nb/MgO catalyst produ ced considerably more carbon dioxide in the both reactions. Structural investigation of the catalyst showed that the addition of niobium to the Li/MgO catalyst increased the surface area and gave an increase in the lithium content of the calcined catalysts. Two niobium phases, Li NbO3 and Li3NbO4, were formed; it is shown that the first of these pro bably causes the increased activity. Ageing experiments showed that th e activity of the catalyst was lost if the catalyst was used above 720 -degrees-C, the melting point of the lithium carbonate phase. The cata lyst showed a decrease of surface area after ageing and a sharp decrea se of the amount of the two niobium phases. The addition of carbon dio xide to the feed could not prevent the deactivation of the Li/Nb/MgO c atalyst.