Oxidative coupling of methane over tetragonal Bi2O3-Ln2O3 phases

Abstract

Solid solutions Ln₂O₃-Bi₂O₃(Ln = Er, Sm, Lu) have been prepared by the ceramic method. In all cases, a tetragonal phase of the β-Bi₂O₃ type has been characterized by X-ray diffraction and Raman spectroscopy. The sequence of basicity of the catalysts has been derived from the energy of the O 2p→Bi 6d transition and Bi–O distance. The activity and selectivity towards C₂ hydrocarbons in the oxidative coupling of methane have been measured in a fixedbed flow reactor. From the correlation of the catalytic tests and the characterization results it has been concluded that the activity to C₂ increases with catalyst basicity, ethene being a secondary product of the reaction. At the lowest temperature tried (988 K) an alternative reaction route, which results in the formation of ethene as primary product, is proposed. An increase in the Lewis acidity of the bismuth cation led to an enhancement of the ethene production. All these results imply that optimization of the activity and selectivity towards C₂ can be attained by fine tuning of the Lewis acid-base properties of the surface pairs responsible for the activation of C–H bonds.