Oxidative coupling of methane on fluorite-structured samarium-yttrium-bismuth oxide

The catalytic properties of dense fluorite-structured Bi_1.5Y_0.3Sm_0.2O_3-δ (BYS) pellets for oxidative coupling of methane (OCM) to ethane and ethylene (C₂ products) were studied in a packed-bed reactor in co-feed mode. The effects of temperature, feed flow rate, CH₄/O₂ and He/(CH₄ + O₂) ratios on the OCM performance over BYS were systematically investigated. The appropriate operating conditions were found to be: temperature, 900-1000°C; total flow rate, 90-150 ml (STP)/g min; CH₄/O₂ ratio, 2-3.5; He/(CH₄ + O₂) ratio, 0.75-1.75. Under these conditions, C₂ yields of 20-27%, C₂ selectivities of 50-62% and C₂ space-time yield of up to 6.9 μmol/g s were achieved. The good OCM catalytic properties of BYS are believed to be related to its fluorite-type phase structure and large oxygen ion mobility at high temperatures. OCM kinetics of BYS oxide was studied with the assumption that no C₂ products were converted to CO_x under low to medium methane conversion range. Lumped rate equations for C₂ and CO_x formation were obtained as functions of temperature, O₂ and CH₄ partial pressures. The solid phase oxygen ions are responsible for the C₂ formation, while the gas phase oxygen molecules enhance the CO_x formation.