OXIDATIVE COUPLING OF METHANE OVER ALKALI-METAL-COMPOUND-PROMOTED ZIRCONIA CATALYSTS

Upon promoting ZrO2 with alkali metal compounds (chlorides, nitrates, carbonates, or acetates), effective catalytic systems are obtained which are more active, selective, and stable with time-onstream than unpromoted ZrO2 or unsupported promoter compounds at 750°C, PCH = 243.2 Torr, CH4/O2 = 4, and a space velocity of 7500 cm3 g-1 h-1. The performance of the catalysts is found to strongly depend on the nature of the alkali salt, the promoter content, the method of catalyst preparation, and the reaction conditions (temperature, partial pressures. CH4/O2 ratio, and contact time). The most effective catalytic system is obtained with Na+-ZrO2-Cl- prepared via the sol-gel method, which leads to a methane conversion of 13.8 mol%, total C2 selectivity of 77 mol% (C2 yield 10.6%), and an ethylene-to-ethane ratio of 3.3 for at least 30 h under the aforementioned conditions and atmospheric pressure. Addition of carbon tetrachloride as a source of chlorine species to the feedstream improves the performance of the catalyst and its stability with time-on-stream. In the latter case, a CH4 conversion of 22.0% and a C2 yield of 17.5% with a C2H4/C2H6 molar ratio of 2.7 are obtained at a space velocity of 3750 cm3 g-1 h-1. In contrast, Na+-ZrO2-Cl- prepared via impregnation, gel-precipitation, physical mixing, or ion-exchange, and the Li-, K-, Rb-, or CsCl-promoted ZrO2 catalysts prepared by any of the above methods, were less effective, even upon the addition of carbon tetrachloride to the feedstream. The relatively high performance of Na+-ZrO2-Cl- (sol-gel) is attributed to the incorporation of Na+Cl- into the ZrO2 matrix, the exposure of an appreciable amount of Na+ and Cl- on the surface, and the increase in basicity and base strength. The relationship between the catalytic performance and the physico-chemical characteristics of the catalysts revealed by XRD, XPS, and basicity (stepwise thermal desorption of CO2) measurements is explored. © 1993 Academic Press, Inc.