Pulse microreactor studies on conversion of methane, ethane, and ethylene over rare earth oxides in the absence and presence of free oxygen

Abstract

Rare earth oxides and rare earth promoted alkaline earth oxides show high activity in the oxidative coupling of methane (OCM) to C[sub 2] hydrocarbons. Oxidative coupling of methane has been covered in a number of recent reviews. In the authors' recent studies, rare earth oxides have been compared for their acid/base strength distribution and catalytic activity of the OCM process, and a mechanism for the formation of methyl radicals involving acid-base pairs on the catalyst surface has been suggested. According to this mechanism, the formation of the methyl radical involves the transfer of an electron from CH[sub 3][sup [minus]] to an oxygen molecule. Hence, for the rare earth oxide catalyst to be active in the OCM process, the presence of free oxygen is essential. Earlier, Lin et al. have observed the requirement of free O[sub 2] for the formation of methyl radicals on La[sub 2]O[sub 3] (at 773 K) in substantial concentrations. It is therefore very interesting to compare the catalytic activity/selectivity of the rare earth oxides in the conversion of methane and also of ethane and ethylene (which are the desirable products of the OCM) in the absence and presence of free oxygen for understanding the catalytic process. Thesemore » studies could be carried out very conveniently using a pulse microreactor. The present investigation was undertaken for this purpose. The rare earth metal oxide catalysts studied were La[sub 2]O[sub 3], CeO[sub 2], Sm[sub 2]O[sub 3], Eu[sub 2]O[sub 3], and Yb[sub 2]O[sub 3].« less