OXIDATIVE COUPLING OF METHANE OVER LA2O3-CAO CATALYSTS - EFFECT OF BULK AND SURFACE-PROPERTIES ON CATALYTIC PERFORMANCE

The oxidative coupling reaction of methane to higher hydrocarbons over lanthanum-calcium-oxide catalysts was studied in a microcatalytic fixed-bed reactor in the temperature range from 973 to 1013 K applying inlet partial pressures of methane and oxygen of 93 and 7 kPa, respectively. Lanthanum to calcium atom ratios of the catalyst were varied as follows: 0/100, 1/99, 5/95, 10/90, 14/86, 20/80, 27/73, 30/70, 50/50, 70/30, and 100/0. Maximum C2+ selectivities of 76% and C2+ yields of 9% were achieved (1013 K). The bulk-phase and surface compositions of the catalysts were determined by XRD and XPS, respectively. A relative measure of surface basicity and acidity was derived from the product distribution of the isopropyl alcohol conversion used as a probe reaction. The catalytic performance of the differently composed materials is affected by their surface composition and not by their bulk composition. Maximum selectivity is achieved with a surface content of lanthanum between 60 and 90%. It is tentatively suggested that a catalyst which is primarily basic in nature requires acidic sites also for achieving high C2+ selectivity. © 1991.