Surface alteration of (VO)(2)P2O7 by alpha-Sb2O4 as a route to control then-butane selective oxidation

The catalytic properties of (VO)(2)P2O7/alpha -Sb2O4 mixed oxides system fo r n-butane mild oxidation have been investigated on two mechanical mixtures (M1 and M2) of the same well crystallized (VO)(2)P2O7 (reference vanadyl p yrophosphate) with two different morphologies of alpha -Sb2O4. The M1 mixture of (VO)(2)P2O7 with alpha -Sb2O4 (1), prepared by oxidation of Sb2O3, leads to the oxidative dehydrogenation (ODH) of n-butane, whereas the M2 mixture of (VO)(2)P2O7 With a commercial alpha -Sb2O4 (2) (Aldrich) with a different morphology improves the maleic anhydride selectivity as c ompared to the reference (VO)zP207 catalyst (synergetic effect). After reac tion, no ternary VPSbO phase is detected by XRD and DTA and it was controll ed that the two alpha -Sb2O4 oxides are catalytically inactive. The (VO)(2)P2O7 reference catalyst which produced only maleic anhydride as mild oxidation product shows by XPS a slightly oxidized surface (14% V5+-86 % V4+). Contamination of the (VO)(2)P2O7 phase by migration of Sb species occurs af ter catalytic reaction in the case of the M1 mixture as shown by XPS, LEIS and TEM-EDX analysis. XPS showed that (VO)(2)P2O7 is partially superficiall y reduced (86% V4+-14% V3+). This feature is consistent with the decrease o f acidity as observed by pyridine adsorption-desorption. In opposition with the M1 mixture, no contamination of the (VO)(2)P2O7 phas e is observed after catalytic reaction in the case of the M2 mixture. The X PS study shows, in this case, that (VO)(2)P2O7 is partially oxidized (30% V 5+-70% V4+) at a higher level than for the reference (VO)(2)P2O7 catalyst. This situation is associated with the increase of selectivity observed for maleic anhydride (synergetic effect). The difference in the catalytic results for the two M1 and M2 mixtures, as compared to the (VO)(2)P2O7 reference catalyst, can be explained by the alt eration of the surface composition of (VO)(2)P2O7 and the distribution of v anadium oxidation state due to different interaction between Sb2O4 and (VO) (2)P2O7, depending on the orientation of the alpha -Sb2O4, crystals. (C) 20 01 Elsevier Science B.V, All rights reserved.