Aluminium triplets in dealuminated zeolites detected by 27 Al NMR correlation spectroscopy Nicolas Malicki a , Gregor Mali b , Anne-Agathe Quoineaud c , Patrick Bourges c , Laurent J. Simon c , Frédéric Thibault-Starzyk a , Christian Fernandez a, * a Laboratoire Catalyse et Spectrochimie, ENSICAEN, Université de Caen-Basse Normandie, CNRS, 6 Bd Maréchal Juin, 10450 Caen, France b National Institute of Chemistry, Hajdrihova 19, SI-1001 Ljubljana, Slovenia c IFP-Lyon, CEDI René Navarre, BP No. 3, 69390 Vernaison, France article info Article history: Received 16 September 2008 Received in revised form 4 September 2009 Accepted 4 September 2009 Available online 10 September 2009 Keywords: USY zeolite Extra-framework aluminium 27 Al solid-state NMR Enhanced acidity abstract Arrangements associating three aluminium atoms in dealuminated Y zeolites are for the first time detected using a combination of multiple-quantum MAS (MQMAS) and 2D double-quantum homonu- clear NMR correlation spectroscopy. From these results, we propose a model describing the enhanced Brønsted acid sites in dealuminated Y zeolite, consisting in a configuration where framework aluminium pairs are interacting with cationic extra-framework aluminium atoms. Ó 2009 Elsevier Inc. All rights reserved. 1. Introduction Ultra-stabilised Y zeolite (USY) prepared from high temperature hydrothermal steaming (around 1000 K) exhibit acidity and stabil- ity that make this aluminosilicate material of great importance in petro-chemistry, e.g., for oil and gas refinement. During such a steaming treatment, significant dealumination of the framework occurs, leading to the generation of some extra-framework alumin- ium (EFAL) species [1]. This dealumination strongly modifies the properties of the zeolite. Especially, the decrease of the aluminium density in the zeolite framework upon dealumination is, in particu- lar conditions, accompanied by a subsequent increase of the acidity. Some studies of such dealuminated zeolites, using FTIR spectros- copy techniques, showed the presence of Brønsted acid sites with enhanced strength, characterized by a OH vibration band at 3600 cm À1 [2–7]. When dealuminated acidic zeolites are partially exchanged with sodium cations, the number and strength of the remaining Brønsted acid sites are significantly modified, and in par- ticular, the site (at 3600 cm À1 ) with enhanced acidity disappears. In some models reported in literature, these acid sites, which only appear in dealuminated zeolite, are explained by the synergistic interactions between framework hydroxyl groups and an extra- framework aluminium atom, whose nature and localisation remain, however, very much a controversial issue [2–10]. It has been very recently proposed that the enhanced acid strength observed for bridging OH groups in zeolites Al,Na–X and Al,Na–Y in comparison with zeolites H,Na–X and H,Na–Y, may be due to a polarising effect of cationic EFAL species in the vicinity of Brønsted acid sites [8]. Moreover, it was also suggested that the creation of aluminium pairs in the framework upon dealumination could explain the in- creased acidity of the attached protons [10]. In this paper, we shall present experimental results obtained by NMR spectroscopy, which clearly show that arrangements of three aluminium atoms can be found in HUSY zeolite. From these observations, a new model explaining the enhanced acidity in dealuminated Y zeolite is proposed. 2. Experimental details 2.1. Samples The zeolite studied in this work is an ultra-stable Y zeolite (H- form, denoted as HUSY in the following), with a framework Si/Al ratio of 21 as measured by 29 Si MAS NMR and a global Si/Al ratio of 15.9 as measured by X-ray fluorescence spectroscopy. A sample (denoted HNaUSY) with 61% sodium exchange was also prepared from the parent HUSY zeolite using an aqueous solution of NaNO 3 . 29 Si NMR measurements show that the exchange process does not significantly modify the framework aluminium ratio. However, X- ray fluorescence spectroscopy shows that the global Si/Al ratio is 1387-1811/$ - see front matter Ó 2009 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2009.09.003 * Corresponding author. E-mail address: christian.fernandez@ensicaen.fr (C. Fernandez). Microporous and Mesoporous Materials 129 (2010) 100–105 Contents lists available at ScienceDirect Microporous and Mesoporous Materials journal homepage: www.elsevier.com/locate/micromeso