Pengwei Wang,Xin Zhang,Guofeng Zhao,Ye Liu,Yong Lu
Mn 2 O 3 -Na 2 WO 4 /SiO 2 is considered as the most promising catalyst for the oxidative coupling of methane (OCM) process; however, it only has a better catalytic performance over 800 °C. To improve its low-temperature performance, an attempt has been made to modify the Mn 2 O 3 -Na 2 WO 4 /SiO 2 catalyst using TiO 2 , MgO, Ga 2 O 3 , and ZrO 2 . Among the synthesized catalysts, the TiO 2 -modified Mn 2 O 3 -Na 2 WO 4 /SiO 2 catalyst shows markedly improved low-temperature OCM performance, achieving a high CH 4 conversion of ~23% and a good C 2 -C 3 selectivity of ~73% at 700 °C (the catalyst bed temperature), along with promising stability for at least 300 h without signs of deactivation. In comparison with the unmodified Mn 2 O 3 -Na 2 WO 4 /SiO 2 catalyst, the TiO 2 modification results in significant improvement in the low-temperature activity/selectivity, whereas the MgO modification has almost no impact and the Ga 2 O 3 and ZrO 2 modifications have a negative effect. The X-ray diffraction (XRD) and Raman results reveal that the formation of a MnTiO 3 phase and a MnTiO 3 -dominated catalyst surface is crucial for the improvement of the low-temperature activity/selectivity in the OCM process.
Oxidative coupling of methane (OCM) is a promising method for the direct conversion of methane to ethene and ethane (C2 products). Among the catalysts reported previously, Mn2O3-Na2WO4/SiO2 showed the highest conversion and selectivity, but only at 800° to 900°C, which represents a substantial challenge for commercialization. We report a TiO2-doped Mn2O3-Na2WO4/SiO2 catalyst by using Ti-MWW zeolite as TiO2 dopant as well as SiO2 support, enabling OCM with 26% conversion and 76% C2-C3 selectivity at 720°C because of MnTiO3 formation. MnTiO3 triggers the low-temperature Mn2+↔Mn3+ cycle for O2 activation while working synergistically with Na2WO4 to selectively convert methane to C2-C3. We also prepared a practical Mn2O3-TiO2-Na2WO4/SiO2 catalyst in a ball mill. This catalyst can be transformed in situ into MnTiO3-Na2WO4/SiO2, yielding 22% conversion and 62% selectivity at 650°C. Our results will stimulate attempts to understand more fully the chemistry of MnTiO3-governed low-temperature activity, which might lead to commercial exploitation of a low-temperature OCM process.
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