OXIDATIVE COUPLING OF METHANE OVER LITHIUM-PROMOTED MAGNESIUM-OXIDE CATALYSTS IN FIXED-BED AND FLUIDIZED-BED REACTORS

The oxidative coupling of methane to higher hydrocarbons over lithium-promoted magnesium oxide (Li/MgO) catalysts has been conducted in fixed- and fluidized-bed reactors. The fixed-bed reactor gave encouraging results, but methane conversion was limited to 15% due to the need to avoid excessive temperature gradients being generated over the catalyst bed. By contrast, the fluidized-bed reactor operated essentially isothermally at methane conversions in excess of 40% using methane/oxygen mixtures (without diluents) well inside the explosive limits. However, agglomeration problems occurred unless the lithium content of the catalyst was <0.4% w/w. Comparison of the results from the two reaction systems showed that some loss in hydrocarbon selectivity occurred in translating from fixed-to fluidized-bed reactors, probably due to their different gas/solid contacting modes. The performance of the fluidized-bed reactor with 5-10% v/v ethane in the feed was significantly improved with respect to ethylene production compared with the case where the feed was methane and oxygen alone. The presence of CO2 in the feed gas was shown to reduce the activity of Li/MgO catalysts, indicating that basic sites are involved in methane coupling. Increasing pressure in a fixed-bed reactor resulted in an increase in methane conversion rate but a reduction in hydrocarbon selectivity. © 1990, American Chemical Society. All rights reserved.