COMMENTS ON THE NATURE OF THE ACTIVE-SITE OF VANADIUM PHOSPHATE CATALYSTS FOR BUTANE OXIDATION

The effect of structure activity relationships for vanadium phosphorus catalysts for the oxidation of butane to maleic anhydride are describ ed and discussed. A range of characterisation techniques including in situ laser Raman spectroscopy and transmission electron microscopy hav e been used on catalysts prepared by using three different methods: (i ) VPA, prepared by using the standard aqueous HCl method followed by a water extraction step; (ii) VPO, prepared by the reaction of V2O5 wit h H3PO4 in isobutanol followed by a water extraction step; and (iii) V PD, prepared by the reaction of VOPO4 . 2H(2)O with isobutanol. The ke y results indicate that: (a) The active site can be formed on a wide r ange of vanadium phosphorus containing phases due to the observation t hat VPA, VPO and VPD catalysts with different compositions give simila r specific activity for maleic anhydride formation. (b) The in situ LR S and TEM studies show that the hemihydrate to final catalyst transfor mation is complex and that after a short exposure to butane/air at tem peratures >370 degrees C the hemihydrate is transformed mainly to a di sordered material. At the onset of maleic anhydride formation, there i s a complex mixture of phases present, including (VO)(2)P2O7 and a var iety of VOPO4 phases. (c) For Co2+ promoted by VPO and VPD catalysts, the activity for maleic anhydride formation is a function of the conce ntration of the V(V) phases present. From these studies, we conclude t hat the active center for butane activation and maleic anhydride forma tion comprise a V4+/V5+ couple that is well dispersed on the surface o f a range of VPO phases, which for well-equilibrated catalysts will be (VO)(2)P2O7. This hypothesis is discussed in relation to other recent data which also indicate the importance of V5+ centers in the selecti ve activation of butane to maleic anhydride. (C) 1998 Elsevier Science B.V.