Oxidative coupling of methane over La-promoted MgO catalysts:Influence of precursors and method of catalyst preparation

摘要

The influence of precursors for La_2O_3(viz.lanthanum acetate and lanthanum nitrate)and MgO(viz.magnesium acetate,magnesium carbonate and magnesium hydroxide)and catalyst preparation methods(viz.physical mixing of the catalyst precursors and coprecipitation using different precipitating agents such as ammonium hydroxide,ammonium carbonate and sodium carbonate)of La-promoted MgO(La/Mg=0.1)catalyst on its surface area,total and strong basicity(measured in terms of CO,chemisorbed at 50 and 500deg C,respectively)and catalytic performance in oxidative coupling of methane(OCM)to C_2-hydrocarbons at different temperatures(750-850 deg C),CH_4/O_2 ratios(4.0 and 8.0)for a high space velocity(51,360 cm~3.h~(-1))has been investigated.The catalyst prepared using the coprecipitation method have higher surface area and basicity(total and strong basic sites measured in terms of CO_2 chemisorbed at 50 and 500 deg C,respectively)but lower activity/selectivity,whereas,the catalyst prepared using the physical mixing method have lower surface area and basicity(both total and strong basic sites)but show higher activity and selectivity in the OCM process,depending upon the La-and Mg-salts used as the catalyst precursors.The catalyst,prepared by physical mixing using lanthanum nitrate and magnesium carbonate(prepared by precipitation from magnesium nitrate and sodium carbonate)as the catalyst precursors showed best catalytic performance in the OCM at different process conditions.