Synthesis of Value-Added Chemicals via Oxidative Coupling of Methanes over Na ₂WO ₄–TiO ₂–MnO _x /SiO ₂ Catalysts with Alkali or Alkali Earth Oxide Additives

Abstract

Na ₂WO ₄–TiO ₂–MnO _x /SiO ₂ (SM) catalysts with alkali (Li, K, Rb, Cs) or alkali earth (Mg, Ca, Sr, Ba) oxide additives, which were prepared using incipient wetness impregnation, were investigated for oxidative coupling of methane (OCM) to value-added hydrocarbons (C ₂₊). A screening test that was performed on the catalysts revealed that SM with Sr (SM–Sr) had the highest yield of C ₂₊. X-ray photoelectron spectroscopy analyses indicated that the catalysts with a relatively low binding energy of W 4f _7/2 facilitated a high CH ₄ conversion. A combination of crystalline MnTiO ₃, Mn ₂O ₃, α-cristobalite, Na ₂WO ₄, and TiO ₂ phases was identified as an essential component for a remarkable improvement in the activity of the catalysts in the OCM reaction. In attempts to optimize the C ₂₊ yield, 0.25 wt % Sr onto SM–Sr achieved the highest C ₂₊ yield at 22.9% with a 62.5% C ₂₊ selectivity and a 36.6% CH ₄ conversion. A stability test of the optimal catalyst showed that after 24 h of testing, its activity decreased by 18.7%.