Elucidating the structure of the W and Mn sites on the Mn-Na2WO4/SiO2 catalyst for the oxidative coupling of methane (OCM) at real reaction temperatures

Abstract

[EN] The performance of the Mn-NaWO/SiO catalyst for oxidative coupling of methane (OCM) has been ascribed to crystalline phases that are not present at reaction temperatures (>700 °C). The evolution of W and Mn sites structure was monitored via in situ TPO-XRD, -Raman, and -XANES spectroscopies. TPO-XRD shows that the crystalline phases identified on the Mn-NaWO/SiO, NaWO/SiO, and WO/SiO catalysts at room temperature do not exist at relevant OCM temperatures. The γ → β → α-WO, α → β-cristobalite, and cubic → orthorhombic → molten-NaWO phase transitions occur upon heating. TPO-Raman spectra show that the bond order of W sites with octahedral (O) and tetrahedral (T) symmetry changes during the δ → γ → β → α-WO and cubic → orthorhombic → molten-NaWO transitions, respectively. TPO-XANES spectra show that bond order changes are due to distortion degree variations because all samples preserve essentially W valence and O-Mn sites are always present on Mn-NaWO/SiO catalyst. Steady-state OCM tests show that O-W sites are inactive and T-W sites are less distorted and more active towards methane activation in the presence of O-Mn sites.