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 (600-1 200 degrees C), gas atmosphere used in the catalyst calcination (viz. stati c air, flowing air, flowing moist air, flowing N-2 or flowing CO2) on the a ctivity 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 degrees C) and higher (750-850 degrees C) temperatures and low co ntact time (space velocity: 51,360 cm(3) g(-1) h(-1)) has been investigated . Influence of these parameters on the strong basicity (measured in terms o f CO2 chemisorbed at 500 degrees C) and surface area of the catalyst has al so been studied. The parameters have a strong influence on both the basicit y and surface area; the basicity is passed through a maximum and the surfac e area is decreased continuously with increasing La/Mg ratio or catalyst ca lcination temperature. However, the parameters have strong influence on the activity and selectivity of the catalyst only at the lower reaction temper atures (less than or equal to 700 degrees C); their influence is decreased gradually with increasing temperature, indicating a change in the controlli ng 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 proc ess at low temperatures (600-700 degrees C). (C) 2000 Elsevier Science Ltd. All rights reserved.