OXIDATIVE COUPLING OF METHANE OVER SOL-GEL MAGNESIUM-OXIDE CATALYSTS - EFFECT ON SELECTIVITY TO OLEFIN FORMATION

A comparative study on the partial oxidation of methane was made in Li-doped magnesium oxide catalysts prepared by the sol-gel method, (i.e. gelation of magnesium ethoxide) or by wet impregnation of commercial MgO support. Activities were comparable in both preparations. However, selectivities to ethylene are up to five times higher in sol-gel than in impregnated catalysts. Theoretical calculations on selected model clusters were made through semi-empirical and ab-initio methods. Semi-empirical results show a trend towards the formation of [Li+ O-] centers when the Li concentration increases. HF-SCF calculations using large gaussian basis sets on linear clusters indicate a loss of more than 1 electron on oxygen when a magnesium atom is replaced by lithium.