OXIDATIVE COUPLING OF METHANE OVER TETRAGONAL BI2O3-LN(2)O(3) PHASES

Solid solutions Ln(2)O(3)-Bi2O3 (Ln=Er, Sm, Lu) have been prepared by the ceramic method. In all cases, a tetragonal phase of the beta-Bi2O3 type has been characterized by X-ray diffraction and Raman spectrosco py. The sequence of basicity of the catalysts has been derived from th e energy of the 0 2p-->Bi 6d transition and Bi-O distance. The activit y and selectivity towards C-2 hydrocarbons in the oxidative coupling o f methane have been measured in a fixed-bed flow reactor. From the cor relation of the catalytic tests and the characterization results it ha s been concluded that the activity to C-2 increases with catalyst basi city, 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 increa se in the Lewis acidity of the bismuth cation led to an enhancement of the ethene production. All these results imply that optimization of t he activity and selectivity towards C-2 can be attained by fine tuning of the Lewis acid-base properties of the surface pairs responsible fo r the activation of C-H bonds.