OXIDATIVE COUPLING OF METHANE OVER A SR-PROMOTED LA2O3 CATALYST SUPPORTED ON A LOW SURFACE-AREA POROUS CATALYST CARRIER

Oxidative coupling of methane (OCM) to higher hydrocarbons over Sr-pro moted La2O3 supported on commercial low surface area porous catalyst c arriers (containing mainly alumina and silica) at 800 and 850 degrees C and a space velocity of 102 000 cm(3).g(-1).h(-1) has been thoroughl y investigated. Effects of support, catalyst particle size, linear gas velocity (at the same space velocity), Sr/La ratio, CH4/O-2 ratio in the feed, and catalyst dilution by inert solid particles on the conver sion, yield, or selectivity and product ratios (C2H4/C2H6 and CO/CO2) in the OCM process have been studied. The catalysts have been characte rized for their basicity, acidity, and oxygen chemisorption by the TPD of CO2, ammonia, and oxygen, respectively, from 50 to 950 degrees C a nd also characterized for their surface area. The supported catalysts showed better performance than the unsupported one. The best OCM resul ts (obtained over Sr-La2O3/SA-5205 with a Sr/La ratio of 0.3 at a spac e velocity of 102 000 cm(3).g(-1).h(-1)) are 30.1% CH4 conversion with 65.6% selectivity for C2+ (or 19.7% C2+-yield) at 800 degrees C (CH4/ O-2 = 4.0) and 12.8% CH4 conversion with 85.1% selectivity for C2+ (or 10.9% C2+-yield) at 850 degrees C (CH4/O-2 = 16.0). The basicity is s trongly influenced by the Sr/La ratio; the supported catalysts showed the best performance for their Sr/La ratio of about 0.3. The methane/O -2 ratio also showed a strong influence on the OCM process. However, t he influence of linear gas velocity and particle size is found to be s mall; it results mainly from the temperature gradient in the catalyst. The catalyst dilution has little or no effect on the conversion and s electivity. However, it has beneficial effects for achieving a higher C2H4/C2H6 ratio and also for reducing the hazardous nature of the OCM process because of the coupling of the exothermic oxidative conversion reactions and the endothermic thermal cracking reactions and also due to the increased heat transfer area.