Influence of precursors of Li₂O and MgO on surface and catalytic properties of Li-promoted MgO in oxidative coupling of methane

Abstract

The influence of the catalyst precursors (for Li₂O and MgO) used in the preparation of Li-doped MgO (Li/Mg = 0.1) on its surface properties (viz basicity, CO₂ content and surface area) and activity/selectivity in the oxidative coupling of methane (OCM) process at 650-750°C (CH₄/O₂ feed ratio = 3.0-8.0 and space velocity = 5140-20550 cm³ g^-1 h^-1) has been investigated. The surface and catalytic properties are found to be strongly affected by the precursor for Li₂O (viz lithium nitrate, lithium ethanoate and lithium carbonate) and MgO (viz magnesium nitrate, magnesium hydroxide prepared by different methods, magnesium carbonate, magnesium oxide and magnesium ethanoate). Among the Li-MgO (Li/MgO = 0.1) catalysts, the Li-MgO catalyst prepared using lithium carbonate and magnesium hydroxide (prepared by the precipitation from magnesium sulfate by ammonia solution) and lithium ethanoate and magnesium acetate shows high surface area and basicity, respectively. The catalysts prepared using lithium ethanoate and magnesium ethanoate, and lithium nitrate and magnesium nitrate have very high and almost no CO₂ contents, respectively. The catalysts prepared using lithium ethanoate or carbonate as precursor for Li₂O, and magnesium carbonate or ethanoate, as precursor for MgO, showed a good and comparable performance in the OCM process. The performance of the other catalysts was inferior. No direct relationship between the basicity of Li-doped MgO or surface area and its catalytic activity/selectivity in the OCM process was, however, observed.