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

Vanadium polyphosphates with a molar ratio of P/V=2 (alpha-VO(PO(3))(2), beta-VO(PO(3))(2) and amorphous as well as partly crystalline VO(PO(3))(2)) and NH(4)VP(2)O(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(2)O(7))(2) (P/V=4/3) and (VO)(2)P(2)O(7) (P/V=1). The MA selectivities of the amorphous VO(PO(3))(2) and of the crystalline alpha- and beta-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)(2)P(2)O(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(4)VP(2)O(7) and the benzonitrile selectivity reaches a value of approximate to 85%. Noticeable benzaldehyde amounts could be detected, especially at low conversion rates, proving its role as a reaction intermediate. (C) 1998 Elsevier Science B.V.