Iron Location in Fe-Silicalites by Synchrotron Radiation Single Crystal X-ray Diffraction M. Milanesio, ² C. Lamberti,* ,²,‡ R. Aiello, § F. Testa, § M. Piana, | and D. Viterbo | Dipartimento di Chimica IFM, UniVersita ` di Torino, Via P. Giuria 7, I-10125 Torino, Italy, INFM Sezione di Torino UniVersita ` , Dipartimento di Ingegneria Chimica e dei Materiali, UniVersita ` della Calabria, I-87030 Rende (Cs), Italy, and Dipartimento di Scienze e Tecnologie AVanzate, UniVersita ` del Piemonte Orientale, Corso T. Borsalino 54, I-15100 Alessandria, Italy ReceiVed: June 21, 2000; In Final Form: August 30, 2000 We report the first structural evidence, obtained by synchrotron radiation single-crystal X-ray diffraction experiments, of the presence of preferential substitution sites for iron heteroatoms hosted in the MFI framework of a Na-Fe-silicalite sample. The location of Fe III at sites T(9) and T(10) is inferred on the basis of (i) an increase of the average T-O distance greater than 3σ; (ii) the presence of two peaks (2.28 and 0.67 e/Å 3 ) in the electron density map near T(9) and T(10) in a niche of the sinusoidal channel, interpreted as the corresponding Na + counterion coordinated to framework oxygens; (iii) the higher anisotropy of the thermal displacement parameters observed for T(9) and T(10), which is related to the substitutional disorder of iron insertion. Independent data obtained on single crystals of Fe-silicalite samples synthesized with NH 4 + and K + counterions confirm the results obtained on the sodium derivative. In the past decade MFI-type 1 materials have shown a tremendous impact as new shape-selective industrial catalysts having tunable acidic strength. In fact, the isomorphous substi- tution of Si by other tetrahedrally coordinated elements such as B III , 2 Al III (ZSM-5 zeolite), 3 Ti IV (Ti-silicalite or TS-1), 4 Ga III , 5 and Fe III 6 in small amounts (1-3 wt %), provides new materials showing specific catalytic properties in oxidation and hydroxy- lation reactions related to the coordination state of the hetero- atom. 7 Moreover, when trivalent metals are present in tetrahedral (T) sites, the zeolite framework has a net negative charge which can be balanced by a number of bridged Si(OH)M III protons (M ) B, Al, Fe, Ga), effectively yielding microporous solids with Brønsted acidity. Since the acidic strength of the OH group depends on the nature of the trivalent heteroatom, the choice of M III critically affects this property of the material (Al > Fe ∼ Ga . B). 8 The charge can be balanced by other cations (Na + , K + , Cs + , etc.), instead of protons, yielding microporous solids with Lewis acidity. Fe-silicalite proved to be a remarkable catalyst for several reactions; among them we recall: the oxidation of benzene to phenol, 9 the conversion of ethylbenzene, 10 the m-xylene isomer- ization, 10 the NO x decomposition 11 and the conversion of methanol into light olefins. 12 From a structural point of view, the relevant question still remains open whether there are preferential substitution T sites, among the 12 nonequivalent sites of the orthorhombic MFI cell. Because of its industrial relevance, most of the published data refer to TS-1. Since only very small fractions of heteroatoms can be isomorphously inserted into the framework, the deter- mination of the substitution sites is rather difficult. Moreover metal-substituted silicalites are in general powdered materials and almost all structural studies have been performed by the less accurate powder XRD technique. For these reasons the debate concerning the possible presence of preferential substitu- tion T sites has been mainly limited to the field of computational chemistry. 13-18 Some authors concluded that the energy differ- ences for the insertion of the heteroatom in different T sites is too small and thus substitution should be randomly dispersed on all the sites. 13 Others claimed T8 14,15 or T2 and T12 15 as preferential sites for the Ti insertion. The most recent results obtained by Sauer and co-workers, 18 are of relevant novelty since they highlight how all previous computational results, performed by simulating the zeolite structure in a vacuum, should be considered with care; in fact, hydration must be taken into account when discussing Ti substitution in the framework sites because, in the presence of water, the stability differences among different Ti sites are much larger (up to 40 kJ mol -1 ) than in the dehydrated state and also the order of the stability of the different Ti substitution sites in TS-1 changes with hydration. To the best of our knowledge, the only published experimental attempt to contribute to this debate comes from Lamberti et al., 19 who reported a synchrotron radiation powder XRD study on different Ti-silicalites, but no clear evidence was obtained. In a subsequent work 20 they have repeated the experiment at lower temperature and a weak evidence that titanium has a preferential tendency to occupy sites T10 and T11 and to avoid sites T4 and T12 was found. Coming to indirect evidences, we recall that the calorimetric study of Bolis et al. 21 reported an evolution of the heat of adsorption of ammonia on TS-1 typical of heterogeneous surfaces, suggesting either a random distribu- tion or a portioning of Ti in more than one preferential site. A powder neutron diffraction study of Ti- and Fe-silicalites was presented at the last IUCr meeting 22 and 5 substitution sites were indicated for Ti, while only the T8 site was indicated for Fe. A powder neutron diffraction study of Ti-silicalites from our group will be available soon. 23 We have recently synthesized high quality Fe-silicalite samples incorporating a relatively large fraction of iron (higher than 2 wt %) and exhibiting crystals of comparatively large dimensions (∼15 × 15 × 40 μm 3 ). 24 Such dimensions were sufficient to allow an accurate single crystal study using the * Corresponding author. Tel: +39-011-6707841. Fax: +39-011- 6707855. E-mail: Lamberti@ch.unito.it. ² Universita ` di Torino. ‡ INFM Sezione di Torino Universita `. § Universita ` della Calabria. | Universita ` del piemonte Orientale. 9951 J. Phys. Chem. B 2000, 104, 9951-9953 10.1021/jp002238f CCC: $19.00 © 2000 American Chemical Society Published on Web 10/11/2000