The nature of the active/selective phase in VPO catalysts and the kinetics of n-butane oxidation

Abstract

In this chapter, new results have been discussed from high-speed transient response experiments and Raman spectroscopic studies that provide insights into one of the key unresolved issues— that is, the nature of the active–selective phase. It is well established that vanadyl pyrophosphate (VO) 2 P 2 O 7 is an essential component of the most selective vanadium phosphorus oxide (VPO) catalysts. The reaction of n-butane with “oxygen-treated” (VO) 2 P 2 O 7 based catalysts has been investigated, using high speed transient response techniques and Raman spectroscopy. Results indicate that two types of active oxygen are present on the VPO surface after oxidation. One type is associated with the production of CO 2 and the other with the production of MA and CO 2 . The amount of oxygen consumed during the CO 2 decrease and the MA increase can be estimated from the n-butane conversion. It is assumed that on average, each converted n-butane molecule consumes 10 oxygen atoms. These results also indicate that significant quantities of oxygen can be stored in the VPO lattice and used in the selective conversion of n-butane. Raman spectroscopic data indicates that stored oxygen does not appear to be associated with a crystalline VOPO 4 phase.