Optimum Performance of Vanadyl Pyrophosphate Catalysts

A scheme is proposed for the dynamic, catalytically active vanadium-phosphorus-mixed oxide surface of industrially used catalysts for the selective oxidation of n-butane to maleic anhydride. Surface species interconvert as function of operation conditions which leads to dynamic changes of the reactor performance on the time scale of hours to days if not controlled properly. This scheme is used as basis for a two-dimensional, heterogeneous reactor model describing the observed performance changes as function of the underlying phosphorus surface dynamics. The dynamic model comprises two reversible reactions: slow phosphorus adsorption, and water adsorption reaching its equilibrium faster. The formation rate of catalytically active species on the surface of vanadyl pyrophosphate is proportional to the actual number of inactive surface sites and the surface concentration of water being in agreement with literature mechanisms according to which water drives the vanadyl pyrophosphate system into a two-dimensional surface state facilitating the mobility of the three oxygen atoms necessary for the conversion of n-butane to MA. This activation process on the other hand is inhibited by a surplus of surface phosphorus increasingly destroying/blocking the sites. The kinetic model distinguishes explicitly between the intrinsic kinetics and phosphorus/water induced activity dynamics. In the presented study all phosphorus and water related processes appeared to be completely reversible, and the developed reactor model fully describes dynamic performance changes up to 400 h on stream. Irreversible long-term changes of catalyst performance, induced by e.g., bulk diffusion of phosphorus, or crystalline phase transitions, are not included in the model and hence need future investigation.