Structure of the Mn-Na2WO4/SiO2 catalyst for Oxidative Coupling of Methane (OCM) at real reaction conditions

Abstract

The OCM performance of the Mn-Na2WO4/SiO2 catalyst has been ascribed to crystalline phases that are not present at real reaction conditions. This work studies the W and Mn sites structure on the 2wt.% Mn-5wt.% Na2WO4/SiO2, 5wt.% Na2WO4/SiO2, and 3.1wt. % WO3/SiO2 catalysts for OCM at ambient, in situ, and operando conditions via XRD, Raman, and XANES spectroscopies. The crystalline phases identified at room temperature transform at heating under oxidizing conditions: γ→β→α-WO3, α→βcristobalite, and cubic→orthorhombic→molten-Na2WO4. The bond order of the W sites with octahedral (Oh) and tetrahedral (Td) symmetry changes during the γ→β→α-WO3 and cubic→orthorhombic→molten-Na2WO4 transitions, respectively. Such bond order changes are due to distortion degree variations because all samples preserve essentially W6+ valence at heating. Oh-W6+ sites are inactive and Td-W6+ sites are less distorted and more active towards methane activation in the presence of Oh-Mn3+ sites. The Td-W6+ and Oh-Mn3+ sites are present even at heating under an OCM gas mixture and perform OCM reactions at steady-state. Finally, the Mn-Na2WO4/SiO2 catalyst performs OCM reactions in the absence of gas-phase oxygen due to the Mn reducibility.