Probing acid–base sites in vanadium redox zeolites by DFT calculation and compared with FTIR results Frederik Tielens a,b, *, Stanislaw Dzwigaj a,b, * a UPMC Univ Paris 06, UMR 7197, Laboratoire de Re ´activite ´ de Surface, Tour 54-55, 2e `me e ´tage - Casier 178, 4, Place Jussieu, F-75005 Paris, France b CNRS, UMR 7197, Laboratoire de Re ´activite ´ de Surface, Tour 54-55, 2e `me e ´tage - Casier 178, 4, Place Jussieu, F-75005 Paris, France 1. Introduction Zeolites doped with transition metals ions are an extremely important class of catalysts with remarkable properties [1–8]. The insertion of transition metal ions into the zeolite framework can lead to modify the reactivity of zeolites, either by introduction of redox sites and modulation of acid–base properties. In particular, introduction of vanadium ions into the zeolite allows to obtain catalysts with efficient activity in many selective oxidative processes [2,3]. The incorporation of vanadium ions into the framework was for a long time a very difficult task. Dzwigaj et al. [9–12] have reported that catalytically active vanadium (V) sites can be incorporated in the BEA framework by a two-step postsynthesis method. This method allows to incorporate V as different kinds of tetrahedral V(V) species evidenced by diffuse reflectance UV–vis, 51 V MAS NMR, XAS and photoluminescence spectoscopies [4,9–13]. The models of tetrahedral V(V) species and their probable location in the structure of BEA zeolite have been proposed. To confirm these models, ab initio periodic DFT calculations have been performed [14,15]. Different vanadium framework site models were proposed after a systematic theoretical study of the substitution of a tetrahedral framework site (T-site) by vanadium atoms. The vanadium framework sites were characterized by their calculated geometrical parameters and vibrational frequencies. The results obtained were found to be fully consistent with experimental data reported earlier by Dzwigaj et al. [13,16] and allow to identify the molecular structure of the vanadium sites in the zeolite framework. Using the acquired knowledge on the molecular structure of V sites in zeolites, their reactivity is investigated and more precisely their acid–base properties, taken into account our earlier works on the acidity and adsorption properties of zeolites [14,15,17–25]. We report in this paper protonation and deprotonation energies of different vanadium sites and a geometry of experimentally observed Lewis and Brønsted acid sites is proposed. A inter- conversion path is proposed between the vanadyl site and the penta coordinated V-OH site. These theoretical results are compared with experimental FTIR investigation of pyridine adsorption. 2. Methodology 2.1. Computational details All calculations are performed using ab initio plane-wave pseudopotential calculations implemented in VASP [26,27]. The Perdew–Burke–Ernzerhof (PBE) functional [28–30] has been chosen to perform the periodic DFT calculations with an accuracy on the overall convergence tested elsewhere [31–34]. The valence electrons are treated explicitly and their interactions with the ionic Catalysis Today 152 (2010) 66–69 ARTICLE INFO Article history: Available online 28 October 2009 Keywords: Zeolites Vanadium Pyridine Acidity Basicity DFT FTIR ABSTRACT Ab initio periodic DFT calculations in combination with experimental FTIR of adsorption pyridine investigations have been used to study the acid–base properties of vanadium doped zeolite materials. It is evidenced that VO-H groups of V(V) and V(IV) framework sites are more acidic than SiO-H groups present in siliceous zeolites. It is proposed for the first time that the protonation of the penta coordinated V site can lead to the formation of a stable vanadyl group containing site through proton exchange, which is expected to be the inter-conversion path between both sites. We report in this paper protonation and deprotonation energies of different vanadium sites, and a geometry of experimentally observed Lewis and Brønsted acid sites is proposed. ß 2009 Elsevier B.V. All rights reserved. * Corresponding authors at: UPMC Univ Paris 06, UMR 7197, Laboratoire de Re ´ activite ´ de Surface, Tour 54-55, 2e ` me e ´ tage - Casier 178, 4, Place Jussieu, F-75005 Paris, France. E-mail addresses: frederik.tielens@upmc.fr (F. Tielens), stanislaw.dzwigaj@upmc.fr (S. Dzwigaj). Contents lists available at ScienceDirect Catalysis Today journal homepage: www.elsevier.com/locate/cattod 0920-5861/$ – see front matter ß 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.cattod.2009.09.006