Oxidative coupling of methane using non-stoichiometric lead hydroxyapatite catalyst mixtures

Lead hydroxyapatite (PbxCa10−x(PO4)6(OH)2, 0 < x ⩽ 10) is one of the most active catalysts for oxidative coupling of methane (OCM) to produce ethane and ethylene from natural gas (methane). In this study, we investigated how the OCM activity of a precipitated lead hydroxyapatite is associated with its mixture and non-stoichiometric properties. Lead hydroxyapatite was prepared through aqueous precipitation in the presence of chlorine anions and calcination under helium background at 1073 K, which resulted in a mixture of non-stoichiometric lead hydroxyapatites, each having a different cationic composition (PbxCa10−x). The mixture could be diversified by extracting the chlorines thermally from the chloro-hydroxyapatites of various chlorine contents. The formation of an apatite structure was confirmed through Fourier transform infrared (FT-IR) spectroscopy, X-ray diffraction (XRD), X-ray fluorescence (XRF) and inductively coupled plasma (ICP) analyses. The non-stoichiometric characters of the prepared hydroxyapatites were analyzed through Raman/FT-Raman spectroscopy. The C2 selectivities of the prepared catalysts were evaluated at 1048 K with a fixed CH4 conversion of 35%. Among the tested catalysts, lead hydroxyapatite, which was obtained by removing chlorines from Pb2Ca8(PO4)6(OH)0.5Cl1.5, showed a C2 selectivity of 62%, and achieved a C2 yield of 22% at 1048 K. The OCM activity of the catalyst was mainly associated with its surface basicity, which was investigated using CO2-temperature-programmed desorption (TPD) and X-ray photoelectron spectroscopy (XPS) analyses.