Doped samarium oxide xerogels for oxidative coupling of methane - Effects of high-valence dopants at very low concentrations

The effects of high-valance dopants on the catalytic properties of samarium oxide xerogels were investigated at concentrations of 0.1 and 1.0 % (by mol) in the oxidative coupling of methane (OCM). Gd, Y, Zr, and V dopants were selected to examine the influence of oxidation states between +3 and +5 on the OCM performance. Even at these low loadings, the high-valance dopants were observed to have a significant impact on the OCM reaction. At the lowest loading, 0.1 mol %, and below 700 °C, all dopants improved the activity over that of the pure Sm2O3 xerogel, and most also improved the selectivity. In particular, the ethylene yield was significantly improved over these catalysts between 500 and 700 °C. However, the stability of the doped catalysts compared to the undoped Sm2O3 xerogel were inferior above 700 °C, and the higher concentration (1.0 mol %) resulted in catalysts with a lower stability. Time-on-stream experiments revealed that the 0.1 mol % high valence dopants improved the stability of the Sm2O3 xerogel at 700 °C. As a result of the higher stability, the doped catalysts retain more of the original specific surface area and appear to stabilize the more active cubic Sm2O3 phase compared with the undoped catalyst. Therefore, the doped catalysts have a higher number of available basic sites during reaction. This study reveals that high-valence dopants have potential to improve the low temperature (500–700 °C) activity of OCM catalysts. However, the concentrations must be kept very low, as dopants that increase the activity and selectivity at concentrations of 0.1 mol % can result in inferior catalysts at 1.0 mol %, and temperatures above 700 °C must be avoided or rapid deactivation can occur.