Oxidative coupling of methane over sodium promoted praseodymium oxide

Unpromoted and alkali-promoted lanthanide oxides were evaluated in the oxidative coupling of methane to higher hydrocarbons. Methane conversion was carried out catalytically and in a redox mode by cycling methane and air independently over the lanthanide oxides. The sodium-promoted nonstoichiometric oxide, 4% Na on Pr6On, was most active and selective, giving in the redox mode 21% methane conversion and 76% C+2 selectivity at 800 °C and 1.4 WHSV (weight hourly space velocity, g CH4g cat. hr). At comparable conversion catalytic methane conversion had a C+2 selectivity of 64%. This selectivity deficit with respect to redox is attributed to an additional destructive route of the methyl radical, namely the reaction with molecular oxygen to yield a methylperoxy intermediate. Process variable studies support a mechanism whereby methane is activated at the metal oxide surface to form a methyl radical and in the gas phase C+2 hydrocarbon building occurs.