Deactivation of Fe-Mo oxide catalyst in industrial plant and simulation tests on laboratory scale

Abstract

The general principles regulating the deactivation of oxidation catalysts based on metal oxides are firstly outlined, showing that the main causes of deactivation for such catalysts are: thermal sintering, solid state reactions and loss of active components. The three corresponding deactivation mechanisms are briefly described. For each of them the recommended type of life test and the accelerating factors are indicated. Then the deactivation mechanism of Fe-Mo oxide catalyst, industrially used for the oxidation of methanol to formaldehyde, is discussed on the basis of extensive characterization of aged catalyst samples discharged from industrial plants. It is shown that two different causes of deactivation are present: accumulation of ∝-Fe 2O 3 in the outer layer of catalyst pellets, with consequent decrease of activity, and MoO 3 volatilization and subsequent recondensation in the colder regions of the catalytic bed, with consequent increase of pressure drop. Formation of ∝-F 2O 3 occurs through reduction of F 2(MoO 4) 3 to FeMoO 4 and subsequent reoxidation. Two different accelerated life tests are proposed. The first consists in the determination by TPO of the oxidation rate of the reduced catalyst, the second in the measurement of weight loss of the catalyst after a drastic treatment. Suitable experimental conditions for such tests are suggested. It is stressed that these life tests have only a relative, not absolute, value and that confirmation of their data on industrial scale is still to be obtained.