Modification of vanadium phosphorus oxides used for n-butane oxidation to maleic anhydride by interaction with niobium phosphate

Abstract

A new family of vanadium phosphorus oxides (VPO) catalysts has been identified. It consists in a mixture of VPO with niobium phosphate (NbPO). The amorphous NbPO material is introduced during the preparation of the VOHPO 4·0.5H 2O precursor. It is observed that the VPO–NbPO catalyst is more rapidly activated and gives better performances ( n-butane conversion and maleic anhydride selectivity) for mild oxidation of n-butane to maleic anhydride. VPO phases and the NbPO material have been identified in the VPO–NbPO precursor and in the VPO–NbPO catalyst. Nb in VPO crystals and V in NbPO particles have been respectively observed by EDX-STEM. No other VPO crystals than the VOHPO 4, 0.5H 2O precursor, and (VO) 2P 2O 7 for the catalyst, have been identified by XRD and 31P NMR. 31P NMR by spin echo mapping and 31P MAS NMR have confirmed an interaction of the VPO precursor with Nb and of the NbPO amorphous material with V, as evidenced by EDX-STEM. This should be the reason for the observed improvement of the catalytic results by a redox effect of niobium on the VPO catalyst modifying the V 5+/V 4+ balance in a favourable way.