Effect of hydroxyapatite-doping in Na-W-Mn/SiO₂ catalysts on oxidative coupling of methane

Sodium-tungsten-manganese supported on silica (Na-W-Mn/SiO₂) and hydroxyapatite (HAp) are representative catalysts for oxidative coupling of methane (OCM). In this work, the effect of the HAp doping in a Na-W-Mn/SiO₂ catalysts on the OCM performance was studied. To enhance the ethylene selectivity of the Na-W-Mn/SiO₂ catalyst, silica supports were coated with HAp containing hydroxyl and phosphate groups as oxygen species. A series of Na-W-Mn/xHAp_SiO₂ (x=1, 3, 5 and 7) catalysts with the different HAp coating cycles were prepared through the alternative soaking method, and X-ray diffraction (XRD) and scanning electron microscopy (SEM) showed that the amount of HAp doping was dependent on the HAp coating cycles. In addition, the change of oxygen species upon HAp doping was examined with X-ray photoelectron spectroscopy (XPS) and oxygen temperature-programmed desorption (O₂-TPD) techniques. With HAp doping, the increase of oxygen species assigned to metal oxide responsible for selective oxidation of methane to ethylene was observed in O 1s XPS spectra. In addition, weakly bound oxygen species were observed with the introduction of HAp doping in O₂-TPD profiles of prepared catalysts. The influence of these oxygen species on OCM catalytic performance was evaluated at an operating temperature of 775 °C and gas hourly space velocity of 18,000 ml/g_cat·h. The amount of HAp doping provided reactive oxygen species for oxidative dehydrogenation of ethane, which resulted in as much as 120% increase in C₂H₄/C₂H₆ ratio over the Na-W-Mn/3HAp_SiO₂ catalyst compared to the Na-W-Mn/SiO₂ catalyst.