THE KINETICS AND MATHEMATICAL MODEL OF THE DEACTIVATION OF FERRIC-MOLYBDATE CATALYSTS (REACTOR MODELLING, FORMALDEHYDE SYNTHESIS, METHANOL OXIDATION)

Ferric molybdate catalysts, used in formaldehyde synthesis, deactivate in six to twelve months due to decomposition of ferric molybdate and sintering at reactor hot spots. At temperatures greater than ca. 500(DEGREES)C, ferric molybdate, an active catalytic phase in formaldehyde synthesis from methanol, decomposes to form ferrous molybdate, an inactive phase, and molybdena vapor. To simulate deactivation, catalysts were pretreated at 600 to 700(DEGREES)C for between 1 and 2.5 hours and characterized for catalytic activity in formaldehyde synthesis and for physical and chemical properties (e.g., surface area and composition). For analysis of activity decay, kinetic models for formaldehyde and byproduct carbon monoxide synthesis were developed. The models account for loss of reaction sites due to ferric molybdate decomposition to ferrous molybdate. The number of reaction sites was found to be proportional to the ferric molybdate content of the catalyst. Deactivation kinetics were found to be described by the rate of mass transfer to molybdena from the catalyst surface under pretreatment conditions. Molybdena vapor pressure data were used to describe the driving force for mass transport. To further investigate the deactivation phenomenon in ferric molybdate catalysts, integral reactor experiments were performed under conditions such that hot spots developed and catalysts deactivated at those hot spots. A reactor model was developed for data analysis that combined reaction, deactivation, and sintering kinetics developed in differential reactor studies, with literature transport data, resulting in a model with no adjustable parameters. The maximum reactor temperature increased with increased methanol feed concentration and the hot spot location shifted upstream with increased flow rate. Reactor temperature decreased and formaldehyde selectivity increased with increased time on stream as deactivation progressed. At the hot spot, catalyst surface area decreased, ferric molybdate content decreased, and ferrous molybdate content increased. No changes in selectivities, temperature profiles, or catalyst properties were observed after the hot spot temperature decreased to below ca. 500(DEGREES)C.