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(PO3)(2), beta-VO(PO3)(2) and amorphous as well as partly crystalline VO(PO3)(2 )) and NH4VP2O7 were synthesized, characterized by chemical and therma l analysis, X-ray diffraction and FTIR spectroscopy and used as cataly sts in the oxidation of n-butane to maleic anhydride (MA) as well as i n the ammoxidation of toluene to benzonitrile. The results are compare d with the catalytic properties of V(PO3)(3) (P/V=3), (NH4)(2)(VO)(3)( P2O7)(2) (P/V=4/3) and (VO)(2)P2O7 (P/V=1). The MA selectivities of th e amorphous VO(PO3)(2) and of the crystalline alpha- and beta-VO(PO3)( 2) are comparable to one another, whereas the specific rate per area o f MA formation of the amorphous as well as partly crystalline VO(PO3)( 2) strongly differs from the rates of the crystalline solids. The amor phous catalyst reveals a rate similar to that of the (VO)(2)P2O7 catal yst, but a lower MA selectivity. Contrary to other studies, only trace s of furan were found and the total oxidation products CO and CO2 were detected on all catalysts at very low conversion. Surprisingly, V(PO3 )(3) exhibits remarkable activity and MA selectivity. The crystalline polyphosphates show a lower activity in the ammoxidation of toluene co mpared with the amorphous VO(PO3)(2) as well as NH4VP2O7 and the benzo nitrile selectivity reaches a value of approximate to 85%. Noticeable benzaldehyde amounts could be detected, especially at low conversion r ates, proving its role as a reaction intermediate. (C) 1998 Elsevier S cience B.V.