Potassium/calcium/nickel oxide catalysts for oxidative coupling of methane

A series of potassium/calcium/nickel oxides were tested for the oxidative coupling of methane (OCM) at 843-943 K and water addition to the feed at O-65 mol-%. The K/Ni ratios varied from 0.0-0.6 and Ca/Ni from 0.0-11; catalysts with no nickel were also tested. At least 10% water in the feed and temperatures lower than ca. 940 K were necessary to keep nickel-containing catalysts from converting to purely steam reforming/combustion behavior. Catalysta of low (less than 4) Ca/Ni were the most active, with a maximum Cz selectivity, which increased with temperature, of 37% at 11% C2 yield (28% water in feed, 933 K). Catalysts of high (4-11) Ca/Ni ratio were less active, with a maximum C, selectivity of 32% at 7.1% Ca yield (43% water in feed, 923 K). Previous claims of higher sel&ivities for similar catalysta could not be reproduced except at short times on stream, in the absence of substantial carbonate buildup. These transient selectivities were not associated with the nickel-containing phase. At steady state, both C, yields and selectivities increased with respect to the water content of the feed, while decreasing with C/O, ratio. The latter behavior is unusual and may be related to the reduction of higher valent potassium/calcium/nickel oxide. The existence of such material as an active phase for OCM was suggested by the resulta of X-ray diffraction, differentional scanning calorimetry, temperature-programmed reduction, and laser Raman spectroscopic charactarixations of fresh and used catalysta.