No abstract

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%.

Methane is of great interest for conversion into high-value hydrocarbons (C2+) and olefins, via oxidative coupling of methane (OCM) using catalysts. In this work, Na2WO4-TiO2/SiO2 catalyst, along with the single catalysts of its components (Na2WO4/SiO2 and TiO2/SiO2), was investigated for OCM reaction to C2+. We found that 5 wt% Na2WO4+ 5 wt% TiO2 on the SiO2 support was a superior catalyst for OCM reaction compared to the single catalysts. The maximum C2+ formation of the Na2WO4-TiO2/SiO2 catalyst was found under test conditions of a N2/(4CH4:1O2) feed gas ratio of 1:1, a reactor temperature of 700 ºC, and gas hourly space velocity of 9,500 h −1 , exhibiting 71.7% C2+ selectivity, 6.8% CH4 conversion, and 4.9% C2+ yield. Moreover, the activity of the catalyst had good stability over 24 h of on-stream testing. The characterizations of the Na2WO4-TiO2/SiO2 catalyst using XRD, FT-IR, XPS, FE-SEM, and TEM revealed that a crystalline structure of α-cristobalite of SiO2 was present along with TiO2 crystals, substantially enhancing the activity of the catalyst for OCM reaction to C2+.