Synthesis of value-added hydrocarbons via oxidative coupling of methane over MnTiO3-Na2WO4/SBA-15 catalysts

Abstract

Methane, the main component of natural gas and considered a greenhouse gas, was converted to value-added hydrocarbons (C2+) using synthesized Na2WO4 and MnTiO3 supported on SBA-15 through the oxidative coupling of methane. The MnTiO3 co-impregnated with Na2WO4 on SBA-15 (MnTiO3-NW/SBA-15) was more active than catalysts that were prepared using a simple one-pot impregnation of Mn2+, Ti4+, and Na2WO4 over fumed-SiO2, MCM-41, or SBA-15. Characterizations of the catalysts suggested that the presence of MnTiO3 was more important in the activation of methane relative to Mn2O3. The highest C2+ yield obtained from MnTiO3-NW/SBA-15 was 24.9 % with 63.0 % C2+ selectivity and 39.4 % CH4 conversion at a reaction temperature of 700 °C, a feed ratio of CH4:O2:N2 = 3:1:4, and a space velocity of 18,500 h−1. A time-on-stream test of MnTiO3-NW/SBA-15 over 25 h revealed that the C2+ yield slowly decreased from 24.9%–20.2% at the end of the 25 h run, because of a decrease in the catalyst’s surface area, a loss of active Na2WO4 from the catalyst, and the destruction of α-cristobalite. Experiments using in-situ diffuse reflectance infrared Fourier transform spectroscopy with mass spectrometry, operated at 475 °C, revealed that the MnTiO3-NW/SBA-15 catalyst enhanced the formation of CH3 and H radicals, while no products or intermediates were found when only SBA-15 was tested.