Factors influencing activity/selectivity of La-promoted MgO catalyst prepared from La- and Mg- acetates for oxidative coupling of methane

Influence of La/Mg ratio (0.0–1.0), catalyst calcination temperature (600–1200°C), gas atmosphere used in the catalyst calcination (viz. static air, flowing air, flowing moist air, flowing N2 or flowing CO2) on the activity and selectivity of La-promoted MgO, prepared from mixed La- and Mg- acetates by calcination in the oxidative coupling of methane both at lower (600–700°C) and higher (750–850°C) temperatures and low contact time (space velocity: 51,360 cm3 g−1 h−1) has been investigated. Influence of these parameters on the strong basicity (measured in terms of CO2 chemisorbed at 500°C) and surface area of the catalyst has also been studied. The parameters have a strong influence on both the basicity and surface area; the basicity is passed through a maximum and the surface area is decreased continuously with increasing La/Mg ratio or catalyst calcination temperature. However, the parameters have strong influence on the activity and selectivity of the catalyst only at the lower reaction temperatures (≤700°C); their influence is decreased gradually with increasing temperature, indicating a change in the controlling mechanism from heterogeneous to homogeneous reaction. The catalysts with La/Mg ratio of 0.005–0.25 show a good activity/selectivity in the OCM process at low temperatures (600–700°C).