Site Requirements for the Oxidative Coupling of Methane on SiO₂-Supported Mn Catalysts

Abstract

The oxidative coupling of methane (OCM) to C 2 hydrocarbons (C 2 H 4 and C 2 H 6 ) was examined on Mn/SiO 2 and sodium salt-modified Mn/SiO 2 catalysts containing different oxo anions, i.e., WO 4 2-, MoO 4 2-, SO 4 2-, PO 4 3-, P 2 O 7 4-, CO 3 2-, and SiO 3 2-. The catalysts were characterized using X-ray diffraction, X-ray photoelectron spectroscopy, laser Raman spectroscopy, and temperature-programmed reduction with H 2 . The structural and catalytic properties of the catalysts largely depend on the presence of the Na + ions and the identity of the oxo anions of the salts. Mn/SiO 2 consisted of Mn 3 O 4 and amorphous SiO 2 phases. The addition of the sodium salts to Mn/SiO 2 led to the transformation of amorphous SiO 2 exclusively to R-cristobalite, and the concurrent oxidation of Mn 3 O 4 to different Mn species. Mn 2 O 3 was the predominant species as the salts contained oxo anions of WO 4 2-, MoO 4 2-, SO 4 2-, PO 4 3-, and P 2 O 7 4-, whereas the basic sodium salts of CO 3 2and SiO 3 2led to the preferential formation of Mn 4+ species. These effects on the formation of the Mn species were demonstrated indeed to require the coexistence of Na + and the oxo anions. Compared to Mn/SiO 2 , the sodium salt-modified samples with the formation of Mn 2 O 3 showed much higher reducibility, activities, and selectivities to C 2 products. However, the samples with the formation of Mn 4+ species exhibited very low OCM activities, as a result of the strong basicity of Na 2 CO 3 and Na 2 SiO 3 inhibiting the partial reduction of the Mn 4+ species. The observed effects on the structures and catalytic performances suggest that Mn 2 O 3 species act as the active sites responsible for the methane activation, which may provide the rationale for the design of new efficient catalysts for the OCM reaction.

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

“…However, it is also possible that MnO 2 does not have the appropriate geometry for critical interactions with tungsten. The fact that Mn 2+ has been identified after exposure to OCM reaction condition [12,34,35], supports the redox mechanism involving an Mn 3+ /Mn 2+ redox pair. A reaction mechanism involving electron transfer between a W 5+ and nearby Mn 3+ is further supported by results from electrical conductivity measurements.…”

“…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. In contrast, sodium M a n u s c r i p t salts of CO 3 2-and SiO 3 2-anions yielded poorly active Mn 4+ species and catalyst with low reducibility [34].…”

“…In the Mn-Na 2 WO 4 /n-SiO 2 catalyst the conductivity is significantly higher than in any other catalyst tested. To further support a redox mechanism, the addition of Na salts have been shown to increase both reducibility and activity [34]. Therefore, data in the literature appears to support the proposal that the actives site is an oxygen of a distorted tetrahedral Na 2 WO 4 , which is attached to the SiO 2 and has a neighboring octahedral Mn (six-coordinated Mn with O neighbors) [23].…”

“…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.…”

“…Furthermore, sodium salts also stabilize Mn 2 O 3 on the surface, and increases both basicity [9,28] and reducibility of the catalysts [34].…”