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., Sr₃Al ₄O₉·2H₂O, double perovskite, SrAl ₄O₇, 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 (Al^IV/Al^VI) 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 Al^IV/Al^VI 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%Na₂WO ₄/SiO₂ under the conditions used (810 °C, CH ₄:O₂ = 108:22, GHSV ∼ 26,000 L/kg_cat h).