Vanadium phosphorus oxides with P/V=2 used as oxidation and ammoxidation catalysts

Abstract

Vanadium polyphosphates with a molar ratio of P/V=2 (α-VO(PO 3) 2, β-VO(PO 3) 2 and amorphous as well as partly crystalline VO(PO 3) 2) and NH 4VP 2O 7 were synthesized, characterized by chemical and thermal analysis, X-ray diffraction and FTIR spectroscopy and used as catalysts in the oxidation of n-butane to maleic anhydride (MA) as well as in the ammoxidation of toluene to benzonitrile. The results are compared with the catalytic properties of V(PO 3) 3 (P/V=3), (NH 4) 2(VO) 3(P 2O 7) 2 (P/V=4/3) and (VO) 2P 2O 7 (P/V=1). The MA selectivities of the amorphous VO(PO 3) 2 and of the crystalline α- and β-VO(PO 3) 2 are comparable to one another, whereas the specific rate per area of MA formation of the amorphous as well as partly crystalline VO(PO 3) 2 strongly differs from the rates of the crystalline solids. The amorphous catalyst reveals a rate similar to that of the (VO) 2P 2O 7 catalyst, but a lower MA selectivity. Contrary to other studies, only traces of furan were found and the total oxidation products CO and CO 2 were detected on all catalysts at very low conversion. Surprisingly, V(PO 3) 3 exhibits remarkable activity and MA selectivity. The crystalline polyphosphates show a lower activity in the ammoxidation of toluene compared with the amorphous VO(PO 3) 2 as well as NH 4VP 2O 7 and the benzonitrile selectivity reaches a value of ≈85%. Noticeable benzaldehyde amounts could be detected, especially at low conversion rates, proving its role as a reaction intermediate.