Comparison of the surface and catalytic properties of rare earth-promoted CaO catalysts in the oxidative coupling of methane

Abstract

Rare earth (viz. La, Ce, Sm, Nd and Yb) promoted CaO catalysts have been investigated, comparing their surface properties (viz. surface area and basicity/base strength distribution) and catalytic activity/selectivity in the oxidative coupling of methane at different reaction conditions (temperatures, 650-800°C, CH₄/O₂ ratios, 2.0-8.0 and space velocity, 51 360 cm³ g^-1 h^-1). The surface properties and catalytic activity/selectivity are strongly influenced by the rare earth promoter and its concentration. Apart from the Sm-promoted CaO catalyst, both the total and strong basic sites (measured in terms of CO₂ chemisorbed at 50° and 500°C respectively) are decreased due to the promotion of CaO by rare earth metals (viz. La, Ce, Nd and Yb). The catalytic activity/selectivity is strongly influenced by the temperature, particularly below 700°C, whereas at higher temperature no further effect is seen. The La₂O₃CaO, Nd₂O₃CaO and Yb₂O₃CaO catalysts showed high activity and selectivity, and also their results are comparable. Among the catalysts, Nd-promoted CaO (with Nd/Ca = 0.05) showed the best performance (19.5% CH₄ conversion with 70.8% C₂₊ selectivity) in the oxidative coupling of methane. A close relationship between the surface density of total and strong basic sites (measured in terms of CO₂ chemisorbed at 50° and 500 °C respectively) and the C₂₊ selectivity and/or C₂₊ yield has been observed.