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

The performance of the Mn-Na2WO4/SiO2 catalyst for oxidative coupling of methane (OCM) has been ascribed to crystalline phases that are not present at reaction temperatures (> 700 degrees 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-Na2WO4/SiO2 , Na2WO4/SiO2 , and WO3/SiO2 catalysts at room temperature do not exist at relevant OCM temperatures. The gamma -> beta -> alpha- WO3 , alpha -> beta-cristobalite, and cubic -> orthorhombic -> molten-Na2WO4 phase transitions occur upon heating. TPO-Raman spectra show that the bond order of W sites with octahedral (Oh) and tetrahedral (Td) symmetry changes during the delta -> gamma -> beta -> alpha-WO3 and cubic -> orthorhombic -> molten-Na2WO4 transitions, respectively. TPO-XANES spectra show that bond order changes are due to distortion degree variations because all samples preserve essentially W6+ valence and O-h-Mn3+ sites are always present on Mn-Na2WO4/SiO2 catalyst. Steady-state OCM tests show that O-h-W6+ sites are inactive and T-d-W6+ sites are less distorted and more active towards methane activation in the presence of O-h-Mn3+ sites. (C) 2021 The Authors. Published by Elsevier Inc.