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%.

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The oxidative coupling of methane (OCM) converts CH4 to value-added chemicals (C2+), such as olefins and paraffin. For a series of MnTiO3-Na2WO4 (MnTiO3-NW) and MnOx-TiO2-Na2WO4 (Mn-Ti-NW), the effect of loading of MnTiO3 or MnOx-TiO2, respectively, on two different supports (sol–gel SiO2 (SG) and commercial fumed SiO2 (CS)) was examined. The catalyst with the highest C2+ yield (21.6% with 60.8% C2+ selectivity and 35.6% CH4 conversion) was 10 wt% MnTiO3-NW/SG with an olefins/paraffin ratio of 2.2. The catalyst surfaces with low oxygen-binding energies were associated with high CH4 conversion. Stability tests conducted for over 24 h revealed that SG-supported catalysts were more durable than those on CS because the active phase (especially Na2WO4) was more stable in SG than in CS. With the use of SG, the activity of MnTiO3-NW was not substantially different from that of Mn-Ti-NW, especially at high metal loading.

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