SrO·Al2O3 mixed oxides: A promising class of catalysts for oxidative coupling of methane

Varying the composition of Sr–Al oxides, mixed as well as single phase compounds were prepared and characterized extensively by XRD, NMR, TEM, EPR, XPS, and TPD. Depending on synthesis method and precursors, Sr/Al-based materials with different crystalline phases (e.g., Sr3Al4O9·2H2O, double perovskite, SrAl4O7, SrAl2O4, and SrCO3) were obtained. Apart from these crystalline phases, several other species were present, as identified by solid state NMR and TG–MS. The ratio of tetrahedrally to octahedrally coordinated Al (AlIV/AlVI) in the Sr/Al mixed oxides increased from 0.5 (pure alumina) to 5.2 in Sr/Al = 1.25, as determined by solid state NMR. The performance of these catalysts in the oxidative coupling of methane depended on the AlIV/AlVI ratio, both activity as well as C2+ selectivity increased with increasing Sr-content. The catalyst with a Sr/Al ratio of 1.25 showed a performance comparable to the well-known catalyst 1.9%Mn–4%Na2WO4/SiO2 under the conditions used (810 °C, CH4:O2 = 108:22, GHSV ∼ 26,000 L/kgcat h).

Methane conversion rate increases exponentially with increasing ratio of tetrahedral AlIV to octahedral AlVI sites while surface area decreases, indicating higher population density or higher activity of sites at higher AlIV/AlVI.Figure optionsDownload high-quality image (109 K)Download as PowerPoint slideHighlights
► Sr–Al mixed oxides are efficient catalysts for the oxidative coupling of methane.
► They consist of crystalline Sr3Al2O6 and a non-crystalline Sr–Al containing phase.
► Catalytic activity per unit surface increases strongly with loading of Sr.
► Activity correlates with ratio of tetrahedrally to octahedrally coordinated Al (AlIV/AlVI).
► Oxide ion radical formation is suggested to be linked with AlIV environment.