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

• DOI: 10.1016/j.jcat.2021.06.021
• Publication Date: 2021-06-28T05:41:43Z
• Authors: Carlos A. Ortiz-B...; Santiago J.A. Fig...; Raquel Portela; Carlos A. Chagas; Miguel A. Bañares; Fabio Souza Toniolo
• Keywords: Orthorhombic crystal system; Scheelite; XANES

The performance of the Mn-Na2WO4/SiO2 catalyst for oxidative coupling methane (OCM) has been ascribed to crystalline phases that are not present at reaction temperatures (>700 °C). evolution W and Mn sites structure was monitored via in situ TPO-XRD, -Raman, -XANES spectroscopies. TPO-XRD shows identified on Mn-Na2WO4/SiO2, Na2WO4/SiO2, WO3/SiO2 catalysts room temperature do exist relevant OCM temperatures. γ → β α-WO3, α β-cristobalite, cubic orthorhombic molten-Na2WO4 phase transitions occur upon heating. TPO-Raman spectra show bond order with octahedral (Oh) tetrahedral (Td) symmetry changes during δ α-WO3 transitions, respectively. TPO-XANES due distortion degree variations because all samples preserve essentially W6+ valence Oh-Mn3+ always catalyst. Steady-state tests Oh-W6+ inactive Td-W6+ less distorted more active towards activation presence sites.