Oxidative coupling of methane over sodium promoted praseodymium oxide

Unpromoted and alakli promoted lanthanide oxides were evaluated in the oxidative coupling of methan to higher hydrocarbons. Methane conversion was carried out catalytically and in redox mode by cycling methane and air independently over the lanthanide oxides. The sodium promoted non-stoichiometric oxide, 4% Na on Pr₆O₁₁, was most active and selective towards C₂⁺ products. A two minute cumulative redox run at 800°C and 1.4 WHSV gave 21% methane conversion and 76% C₂⁺ selectivity. In the catalytic reaction, activity comparable to that of redox was obtained at a run temperature 100°C higher. For example at 900°C and 1.4 WHSV and a co cofeed of 50% air in methane, 22% methane conversion and 64% C₂⁺ selectivity were realized. At comparable conversion the catalytic results had a 12% C₂⁺ selectivity deficit with respect to redox. This is attributed to an additional destructive route of the methyl radical, namely the reaction with molecular oxygen to yield a methylperoxy intermediate. Compared to other catalytic data obtained from the partial oxidation of methane over lanthanide oxides, as reported in the literature, the catalyst of 4% Na or Pr₆O₁₁ is at least twice as productive at similar run temperatures and has C₂⁺ selectivity better than or competitive with that of the literature.