Silicon distribution in SAPO materials: A computational study of STA-7 Combined to 29 Si MAS NMR spectroscopy I. Deroche a , G. Maurin a, * , P.L. Llewellyn b , M. Castro c , P.A. Wright c a Institut Charles Gerhardt Montpellier, UMR 5253, CNRS-UM2-ENSCM-UM1, Universite ´ Montpellier II, Place E. Bataillon, 34095 Montpellier Cedex 05, France b Laboratoire MADIREL, UMR CNRS 6121, Universite ´ de Provence, Centre St Je ´ro ˆme, Av. Escadrille Normandie Niemen, 13397 Marseille Cedex 20, France c School of Chemistry, University of St. Andrews, The Purdie Building, North Haugh, St Andrews, Fife KY16 9ST, UK Received 4 January 2007; received in revised form 8 March 2007; accepted 9 March 2007 Available online 28 March 2007 Abstract The distribution of silicon atoms in the STA-7 framework has been investigated by combining 29 Si MAS NMR spectroscopy and computational tools based on energy minimization techniques. It was clearly established that only a SM2 silicon substitution mechanism occurs for an atomic silicon content of 0.05 and 0.10, generating preferential sites for silicon atoms in the large channels associated with protons pointing systematically towards the centre of the cavities interacting with a 6-membered ring. The modelling techniques are used to propose the most stable configurations for the STA-7 frameworks of varying silicon content. This procedure allows us to provide a realistic microscopic description of the STA-7 material, a key step before further investigation of the properties of this system. Ó 2007 Elsevier Inc. All rights reserved. Keywords: SAPO materials; Chemical disorder; Silicon distribution; 29 Si NMR; Computational chemistry 1. Introduction In the 1980s, the class of microporous ‘‘zeolite-like’’ materials was enlarged to include organised aluminophos- phates or AlPO’s [1] possessing structures built up of strictly alternated tetrahedral aluminum and phosphorus atoms bridged by oxygen atoms. Unlike aluminosilicate zeolites, the AlPO frameworks are neutrally charged and consequently they do not exhibit intrinsic ion-exchange or catalytic properties due to the absence of compensating extra-framework ions. The introduction of silicon atoms in the structure [2] generates Brønsted acidity, resulting in the silicoaluminophosphate materials so-called SAPO’s. These structures, in general less acidic than their aluminosilicate analogues [3–5] have found practical applications as cata- lysts in industrial processes such as the conversion of meth- anol to light olefins (MTO process) [6–8], representing a possible route for upgrading the natural gas or coal to higher value products and for the selective isomerisation of hydrocarbon chains [9,10]. Although their crystalline structures are well defined, the SAPO and aluminosilicate zeolite materials are generally characterised by a specific chemical disorder created by a number of co-existing alu- minium or silicon distributions for given global Si/P or Si/Al ratios, respectively. This feature of the frameworks may affect some of the properties of the materials such as the acidity [4] and thus also to their reactivity. The problem which consists of characterizing this chemical disorder is not resolved when one considers only experimental data: the X-ray diffraction analysis is very useful to investigate the distribution of the atoms but since the scattering factors of the considered T (Si, P, Al) atoms are too close, it does not lead to a direct distinction between them. In addition, 29 Si NMR spectroscopy can only reach an average 1387-1811/$ - see front matter Ó 2007 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2007.03.021 * Corresponding author. Tel.: +33 467 14 3307; fax: +33 467 14 4290. E-mail address: gmaurin@lpmc.univ-montp2.fr (G. Maurin). www.elsevier.com/locate/micromeso Available online at www.sciencedirect.com Microporous and Mesoporous Materials 107 (2008) 268–275