Dependence of n-Butane Activation on Active Site of Vanadium Phosphate Catalysts Y. H. Taufiq-Yap Æ C. K. Goh Æ G. J. Hutchings Æ N. Dummer Æ J. Bartley Received: 6 March 2009 / Accepted: 24 April 2009 / Published online: 27 May 2009 Ó Springer Science+Business Media, LLC 2009 Abstract The nature and the role of oxygen species and vanadium oxidation states on the activation of n-butane for selective oxidation to maleic anhydride were investigated. Bi–Fe doped and undoped vanadium phosphate catalysts were used a model catalyst. XRD revealed that Bi–Fe mixture dopants led to formation of a II -VOPO 4 phase together with (VO) 2 P 2 O 7 as a dominant phase when the materials were heated in n-butane/air to form the final catalysts. TPR analysis showed that the reduction behav- iour of Bi–Fe doped catalysts was dominated by the reduction peak assigned to the reduction of V 5? species as compared to the undoped catalyst, which gave the reduc- tion of V 4? as the major feature. An excess of the oxygen species (O 2- ) associated with V 5? in Bi–Fe doped catalysts improved the maleic anhydride selectivity but significantly lowering the rate of n-butane conversion. The reactive pairing of V 4? -O - was shown to be the centre for n-butane activation. It is proposed that the availability and appear- ance of active oxygen species (O - ) on the surface of vanadium phosphate catalyst is the rate determining step of the overall reaction. Keywords Vanadium phosphate Á n-Butane oxidation Á Vanadium Á Oxygen species Á Maleic anhydride Á Activation 1 Introduction Selective oxidation catalysis involved in many productions of important products in petrochemical industry. Among them is the manufacturing of maleic anhydride (MA) from well-known n-butane oxidation over vanadium phosphate (VPO) catalyst. The active phase claimed for this catalyst is vanadyl pyrophosphate, (VO) 2 P 2 O 7 . Several promoters have been incorporated in this catalyst in order to modify the catalytic performance. This reaction is the most com- plicated in selective oxidation process since it involved the largest amount of electrons (14 electrons). Several steps take place in the overall process where the first step, i.e. the abstraction of hydrogen from saturated hydrocarbon is the most critical. This is due to the highly stable C–H bond of light hydrocarbon. Other steps are oxidative dehydroge- nation of alkene, insertion of oxygen to form oxidized products, activation of molecular oxygen, etc. [1, 2]. The aim of our present work is to identify the essential factor for butane activation which involved the nature and the specific role of different vanadium oxidation states and oxygen species, and the structural relationships with cata- lytic performance in selective oxidation of n-butane to maleic anhydride. Bi–Fe doped and undoped (VO) 2 P 2 O 7 catalysts were used as a model catalysts in this study. Previously, VPO catalysts have been found to be promoted by the addition of Bi [3] and Fe [4]. Y. H. Taufiq-Yap Á C. K. Goh Centre of Excellence for Catalysis Science and Technology, University Putra Malaysia, 43400 Serdang, Selangor, Malaysia Y. H. Taufiq-Yap (&) Á C. K. Goh Department of Chemistry, Faculty of Science, University Putra Malaysia, 43400 Serdang, Selangor, Malaysia e-mail: yap@fsas.upm.edu.my G. J. Hutchings Á N. Dummer Á J. Bartley Cardiff Catalysis Institute, School of Chemistry, Cardiff University, Cardiff CF10 3AT, UK 123 Catal Lett (2009) 130:327–334 DOI 10.1007/s10562-009-0003-2