Stability of Iron-Molybdate Catalysts for Selective Oxidation of Methanol to Formaldehyde: Influence of Preparation Method

Iron molybdate/molybdenum oxide catalysts with varying content of Mo (Mo/Fe = 1.6 and 2.0) were synthesized by a mild hydrothermal method and structurally characterized by XRD, XPS, Raman spectroscopy, SEM–EDX, BET and ICP-OES. The stability of the prepared catalysts in selective oxidation of methanol to formaldehyde was investigated by catalytic activity measurements for up to 100 h on stream in a laboratory fixed-bed reactor (5% MeOH, 10% O₂ in N₂, temp. = 380–407 °C). Excess MoO₃ present in the catalyst volatilized under reaction conditions, which lead to an initial loss of activity. Interestingly, the structure of the excess MoO₃ significantly affected the stability of the catalyst. By using low temperature hydrothermal synthesis, catalysts with the thermodynamically metastable hexagonal h-MoO₃ phase was synthesized, which yielded relatively large crystals (2–10 µm), with correspondingly low surface area to volume ratio. The rate of volatilization of MoO₃ from these crystals was comparatively low, which stabilized the catalysts. It was furthermore shown that heat-treatment of a spent catalyst, subject to significant depletion of MoO₃, reactivated the catalyst, likely due to migration of Mo from the bulk of the iron molybdate crystals to the surface region.