Oxidative coupling of methane over a Sr-promoted La₂O₃ catalyst supported on a low surface area porous catalyst carrier

Abstract

Oxidative coupling of methane (OCM) to higher hydrocarbons over Sr-promoted La₂O₃ supported on commercial low surface area porous catalyst carriers (containing mainly alumina and silica) at 800 and 850°C and a space velocity of 102 000 cm³.g^-1.h^-1 has been thoroughly investigated. Effects of support, catalyst particle size, linear gas velocity (at the same space velocity), Sr/La ratio, CH₄/O₂ ratio in the feed, and catalyst dilution by inert solid particles on the conversion, yield, or selectivity and product ratios (C₂H₄/C₂H₆ and CO/CO₂) in the OCM process have been studied. The catalysts have been characterized for their basicity, acidity, and oxygen chemisorption by the TPD of CO₂, ammonia, and oxygen, respectively, from 50 to 950°C and also characterized for their surface area. The supported catalysts showed better performance than the unsupported one. The best OCM results (obtained over Sr-La₂O₃/SA-5205 with a Sr/La ratio of 0.3 at a space velocity of 102 000 cm³.g^-1.h^-1) are 30.1% CH₄ conversion with 65.6% selectivity for C₂₊ (or 19.7% C₂₊-yield) at 800°C (CH₄/O₂ = 4.0) and 12.8% CH₄ conversion with 85.1% selectivity for C₂₊ (or 10.9% C₂₊-yield) at 850°C (CH₄/O₂ = 16.0). The basicity is strongly influenced by the Sr/La ratio; the supported catalysts showed the best performance for their Sr/La ratio of about 0.3. The methane/O₂ ratio also showed a strong influence on the OCM process. However, the influence of linear gas velocity and particle size is found to be small; it results mainly from the temperature gradient in the catalyst. The catalyst dilution has little or no effect on the conversion and selectivity. 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.