Morphological changes of vanadyl pyrophosphate due to thermal excursions

Abstract

Vanadyl pyrophosphate (VPP) is the active catalytic phase that converts n-butane to maleic anhydride in fixed beds, fluidized beds, and DuPont’s circulating fluidized bed (CFB). The original CFB concept was based on the Mars van-Krevelen reaction mechanism in which the solid lattice contributes all of the oxygen to partially oxidize the n-butane. However, to maintain high activity and selectivity requires that the surface lattice is oxidized; thus, DuPont co-fed pure oxygen through 926 nozzles at three levels in the CFB. Since pure oxygen was fed independently of the butane stream, the gas composition at the nozzle crosses the explosion envelope and the local temperature could rise several 100 ∘C. Furthermore, the gas composition in the exhaust pipe downstream of the cyclone was within the explosion envelope. The temperature in these two regions periodically exceeded 700∘C, which was sufficiently high to deactivate the catalyst and turn it black. Here, we reproduced the high temperature conditions to examine the changes in catalyst morphology. A TA-Q500 heated the fresh catalyst to 800 ∘C under nitrogen and air flow. The VPP catalyst turned black above 700 ∘C in air but not in nitrogen. The catalytic surface area and pore volume of the fresh calcined catalyst decreased from 23 m2g−1 and 0.14 cm3g−1 to 12 m2g−1, and 0.09 cm3g−1 in the equilibrated catalyst. New bonds form and the active VPP reacts with silica to form VO(P2SiO8).