Surface alteration of (VO)2P2O7 by α-Sb2O4 as a route to control the n-butane selective oxidation

Abstract The catalytic properties of (VO) 2 P 2 O 7 /α-Sb 2 O 4 mixed oxides system for n -butane mild oxidation have been investigated on two mechanical mixtures (M1 and M2) of the same well crystallized (VO) 2 P 2 O 7 (reference vanadyl pyrophosphate) with two different morphologies of α-Sb 2 O 4 . The M1 mixture of (VO) 2 P 2 O 7 with α-Sb 2 O 4 (1), prepared by oxidation of Sb 2 O 3 , leads to the oxidative dehydrogenation (ODH) of n -butane, whereas the M2 mixture of (VO) 2 P 2 O 7 with a commercial α-Sb 2 O 4 (2) (Aldrich) with a different morphology improves the maleic anhydride selectivity as compared to the reference (VO) 2 P 2 O 7 catalyst (synergetic effect). After reaction, no ternary VPSbO phase is detected by XRD and DTA and it was controlled that the two α-Sb 2 O 4 oxides are catalytically inactive. The (VO) 2 P 2 O 7 reference catalyst which produced only maleic anhydride as mild oxidation product shows by XPS a slightly oxidized surface (14% V 5+ –86% V 4+ ). Contamination of the (VO) 2 P 2 O 7 phase by migration of Sb species occurs after catalytic reaction in the case of the M1 mixture as shown by XPS, LEIS and TEM–EDX analysis. XPS showed that (VO) 2 P 2 O 7 is partially superficially reduced (86% V 4+ –14% V 3+ ). This feature is consistent with the decrease of acidity as observed by pyridine adsorption–desorption. In opposition with the M1 mixture, no contamination of the (VO) 2 P 2 O 7 phase is observed after catalytic reaction in the case of the M2 mixture. The XPS study shows, in this case, that (VO) 2 P 2 O 7 is partially oxidized (30% V 5+ –70% V 4+ ) at a higher level than for the reference (VO) 2 P 2 O 7 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 P 2 O 7 reference catalyst, can be explained by the alteration of the surface composition of (VO) 2 P 2 O 7 and the distribution of vanadium oxidation state due to different interaction between Sb 2 O 4 and (VO) 2 P 2 O 7 , depending on the orientation of the α-Sb 2 O 4 crystals.