Synthesis of Value-Added Chemicals via Oxidative Coupling of Methanes over Na2WO4–TiO2–MnOx/SiO2 Catalysts with Alkali or Alkali Earth Oxide Additives

摘要

Na_(2)WO_(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_(2)O_(3), α-cristobalite, Na_(2)WO_(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%.