Studies of Deactivation of Methanol to Formaldehyde Selective Oxidation Catalyst

Formaldehyde (CH₂O) may be synthesized industrially by selective oxidation of methanol over an iron-molybdate (Fe-Mo) oxide catalyst according to: CH₃OH + ½O₂ →CH₂O + H₂O. The reaction is normally carried out in a multitubular reactor with excess of air at 250-400 °C (yield = 90-95 %), known as the Formox process [1]. The average lifetime of the industrial catalyst is only 1–2 years depending on the operating conditions. The catalyst consists of a bulk phase of Fe₂(MoO₄)₃ and a surface layer phase of MoO3. The MoO₃ surfaceis selective towards formaldehyde while the iron in the sublayer increases the activity of the catalyst [2]. Pure MoO₃ in itself has low activity. Literature from the last decades agrees that the major reason for the deactivation is loss of molybdenum from the catalyst. Molybdenum forms volatile species with methanol, which can leave behind Mo poor zones. The catalyst is usually prepared with excess MoO₃ (Mo/Fe > 1.5) to counter the loss of Mo. This work focuses on understanding the structural changes occurring in the catalyst and its behavior during deactivation via prolonged activity testing and spectroscopic investigations.