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

摘要

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 0–66 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. Catalysts of low (less than 4) Ca/Ni were the most active, with a maximum C2 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 C2 selectivity of 32% at 7.1% C2 yield (43% water in feed, 923 K). Previous claims of higher selectivities for similar catalysts 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 C2 yields and selectivities increased with respect to the water content of the feed, while decreasing with C1/O2 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 results of X-ray diffraction, differentional scanning calorimetry, temperature-programmed reduction, and laser Raman spectroscopic characterizations of fresh and used catalysts.