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Selective, catalytic processes to convert methane into higher hydrocarbons or oxygenates are of great industrial importance because they would enable the conversion of abundant, remote natural gas reserves to useful petrochemicals and fuels. 1–6,7a One of the most studied conversion processes is oxidative coupling, the formation of ethane, ethylene, and higher hydrocarbons (abbreviated as C2+) from the reaction of methane with oxygen. 2–14 Alkali-promoted metal oxides are relatively selective catalysts for this conversion. A wide variety of oxides have been studied, including: basic oxides such as MgO, 7 rare-earth oxides such as Sm2O3, 9,10 and transition metal oxides such as Mn oxides 2,3,11 and NiO. 12–14 With all of these materials, deep oxidation of methane to CO2 and H2O competes with oxidative coupling. The highest yields (methane conversion times selectivity to C2+) achieved to date are below 30%. It is estimated that methane conversions in excess of 60% and C2+ selectivities greater than 80% are necessary for an economic conversion process. 1 One of the challenges in achieving better yields is to design or discover catalytic materials that have a high intrinsic selectivity for oxidative coupling, and a minimum activity for deep oxidation. |
