Oxidative coupling of methane over Ca- and alkali metal-doped ThO2

Th0.8Ca0.2O2−δ and Na/K dispersed Th0.8Ca0.2O2−δ catalysts, synthesized by the citric acid gel method, have been tested concerning their efficiency in the oxidative coupling of methane. The structural properties of the catalysts were characterized by BET, XRD, Raman, TEM, TPD, and EPR techniques. The presence of oxide ion vacancies due to Ca ion incorporation into ThO2 lattice was evidenced by Raman spectroscopy. EPR revealed the formation of radical oxygen in the Ca-doped ThO2 sample after oxygen treatment at 500 °C. Th0.8Ca0.2O2−δ showed high catalytic activity reaching about 24% methane conversion (rate: 2.3 mol g−1 h−1) at 56% selectivity toward Cn+ products at 600 °C, whose performance is comparable to that of the well known Sr doped La2O3 catalyst. Other reference catalysts like 1.9%Mn–4%Na2WO4/SiO2 and 5%Li/MgO showed considerably lower activity under these conditions. The high catalytic activity and selectivity of the Ca-doped sample is likely due to formation of stable active O˙− species in the oxide ion vacancy site in the lattice.

Graphical abstractThe Ca-doped ThO2 catalyst shows high catalytic activity and selectivity toward C2+ products for oxidative coupling of methane due to favorable formation of oxide ion radical in the defect lattice.Figure optionsDownload full-size imageDownload high-quality image (149 K)Download as PowerPoint slide