Effect of promoter in the oxidative coupling of methane over synthesized Mn/SiO2 nanocatalysts via incipient wetness impregnation

Characterization of Mn–Na2WO4/SiO2 and Mn–Na2WO4/MgO catalysts for the oxidative coupling of methane

“…Replacing the WO 4 2-anion with MoO 4 2-and SO 4 2-, PO 4 3-, or P 2 O 7 4-also yield active OCM catalysts [32,34]. Anions of stronger acids appear to be more effective in stabilizing Mn 2 O 3 on the catalyst, and resulted in catalysts with increased reducibility.…”

“…Also the Mn 2 O 3 can be substituted for other transition metal oxides, such as those of Fe, Co, Nb, and Zn [22,32], or even rare earth oxides, such as CeO 2 [10], but Mn 2 O 3 is the most effective of the metal oxides tested in the OCM reaction.…”

“…However, it appears that the Mn 2 O 3 -Na 2 WO 4 /α-cristobalite SiO 2 is a very effective combination due to strong interactions between each component, which serve to stabilize the active phases and thus also the Na-O-W [5,7,13,14,28], this is a very complex system, and all components; Mn, Na, W, and Si, are important and appear to interact strongly, via oxygen bridges. For example, Na not only induces crystallization of α-cristobalite SiO 2 , it also interacts closely with SiO 2 as is evidenced in the XPS binding energy shifts discussed above, and also in the IR bands of α-cristobalite SiO 2 [32]. In addition, sodium interacts with and stabilizes both the tetrahedral WO 4 2-group and Mn 2 O 3 on the surface [9,28].…”

“…It has been suggested that the W 6+ is regenerated by the transfer of an electron from W 5+ to Mn 3+ . This forms a Mn 2+ species, which is later reoxidized by gas phase oxygen [32]. The reason Mn 2 O 3 is so effective may be due to its ability to accept an electron from W 5+ and reoxidize the tungsten to W 6+ .…”

“…Consistent with this, the Na 2 WO 4 /n-SiO 2 reference catalyst also has a high Na surface concentration. Furthermore, data from the literature indicates that Na can interact strongly with SiO 2 [32], and the presence of strong sodium-silica interactions M a n u s c r i p t can explain the slight shifts in the O 1s and Si 2p peaks obtained from the Na-containing SiO 2 -supported catalysts compared with the MnO x /n-SiO 2 catalyst with no added Na. The Si 2p peaks of SiO 2 are typically reported around 103.5 eV, and the O 1s peak of SiO 2 at 533 eV [30,31].…”