INVESTIGATION OF OXIDATIVE COUPLING OF METHANE OVER BISMUTH OXYCHLORIDE, SAMARIUM CHLORIDE, OR MANGANESE CHLORIDE SUPPORTED ON LITHIUM-CARBONATE MAGNESIA SYSTEMS

The magnesia-supported bismuth oxychloride with lithium carbonate present is significantly more effective and stable with time-on-stream than the unsupported or supported systems free of Li2CO3 in the oxidative coupling of methane at 750°C, PCH4 = 20.2 kPa, CH4/O2 = 4, and aspace velocity of 15,000 cm3 g-1 h-1. The most effective catalytic system is obtained when 10 mol% BiOCl is supported on MgO with 10 mol% Li2CO3, which leads to a methane conversion of 18%, C2 selectivity of 83%, and an ethylene-to-ethane molar ratio of 2.9 for at least 12 h under the aforementioned conditions and atmospheric pressure. The unsupported or supported samarium chloride free of Li2,CO3 was less effective under similar conditions. Only when the samarium chloride and lithium carbonate contents on the support were sufficiently increased (28 and 40 mol%, respectively) the system became effective towards high C2 selectivity and stability. In contrast, none of the manganese-chloride-based systems exhibited good C2 selectivity and stability except initially, but they mainly promoted the reaction of methane to carbon oxides. The presence of chlorine, the nature of the cation associated with it and lithium are found to be key factors in determining the performance of the catalysts. The introduction of Li2CO3 is found to retard the loss of surface chlorine and prevent the decrease of surface bismuth and samarium, resulting thereby in more stabilized systems. In addition, the bulk and surface modifications, which are more prominent in the presence of Li2CO3, caused by calcination and reaction conditions are believed to influence the formation of the catalytically active or inactive sites. The performance of the catalysts is related to its surface composition, bulk modification, decomposition, and reducibility of the metal chlorides or oxychloride on the basis of XPS, XRD, SEM, and thermal analysis (TPD and TPR) investigations. © 1992.