Crystalline aluminium hydroxy fluorides—Suitable reference compounds for 19 F chemical shift trend analysis of related amorphous solids Rene ´ Ko ¨ nig a , Gudrun Scholz a , Rainer Bertram b , Erhard Kemnitz a, * a Institut fu ¨r Chemie; Humboldt Universita ¨t zu Berlin, Brook Taylor-Str. 2, 12489 Berlin, Germany b Institut fu ¨r Kristallzu ¨chtung, Max Born-Str. 2, 12489 Berlin, Germany 1. Introduction Alkoxide precursor based sol–gel techniques are well estab- lished for the preparation of inorganic materials with remarkable properties. Studied areas of interests include mainly oxide or phosphate containing materials and their application for ceramics, optics or catalysis [1–5]. The extension on the preparation of fluoride based materials is a topical field of research and different approaches have been suggested, such as using metal trifluor- oacetates [6–8] or fluorophosphates as fluorine source [9]. In 2003 we succeeded in modifying the sol–gel techniques for the preparation of fluoride materials using non-aqueous HF [10,11]. The method has proven to be useful for ‘‘convenient’’ synthesis of complex aluminium based fluorides (e.g. elpasolites or cryolites) [12], alkaline earth metal fluorides [13,14] or modified ‘‘doped’’ systems. The highly disordered metal fluorides obtained feature special characteristics: small particle or crystallite sizes open the application in optics [15] and ceramic fields and high surface (HS) areas combined with the possibility to influence the surface properties enable the utilisation of these materials for hetero- geneous catalysis [16–18]. High surface-AlF 3 is one of the most important of these metal fluorides [19]. Its synthesis route involves two steps: (i) the sol–gel reaction of an aluminium alkoxide with non-aqueous HF, which leads to a xerogel, described as AlF x (OR) 3x ROH, after drying in vacuum. (ii) A post fluorination process with halofluoroalkanes (e.g. CHClF 2 ) gives HS-AlF 3 , exhibiting an extremely high Lewis acidity, comparable to SbF 5 or the solid Lewis acids ACF or ABF [20]. The final HS-AlF 3 and the intermediate xerogel are both X-ray amorphous and comparative vibrational analysis studies of AlF 3 - phases were made in order to understand structural features of the final HS-AlF 3 [21]. Preliminary investigations were concerned with local structural changes in the course of the sol–gel reaction followed by liquid state NMR [22] and MAS NMR studies on the wet and dry aluminium alkoxide fluoride gels [23]. Nevertheless, due to their amorphous highly disordered character, it is still a challenge to attribute local structures in the xerogels and to give a certain definite assignment of different AlF x (OR) 3x species in these solids. For this reason the aims of the present study are: (i) To prepare well defined crystalline aluminium hydroxy fluorides with varying Al/F ratio through the sol–gel route followed by hydrolysis. (ii) To find correlations between experimental MAS NMR-data of the hydroxy fluorides and their average composition of AlF x O 6x species, and also to develop a 19 F chemical shift Journal of Fluorine Chemistry 129 (2008) 598–606 ARTICLE INFO Article history: Received 29 February 2008 Received in revised form 24 April 2008 Accepted 25 April 2008 Available online 15 May 2008 Keywords: Aluminium hydroxy fluorides Amorphous materials Chemical shift trend analysis Sol–gel process Solid-state NMR ABSTRACT On the basis of MAS NMR-data for crystalline AlF x (OH) 3x H 2 O samples in the pyrochlore structure, 19 F chemical shifts correlate with the average chemical composition of the octahedral environment, given by AlF x O 6x in these compounds. The attribution of local structures in sol–gel derived amorphous AlF x (OX) 3x XOH (X = H, R (alkyl)) compounds is of special interest as these or consecutively prepared solids exhibit remarkable features, for example, a high surface (HS) area accompanied by a high Lewis acidity. By transferring this scale of a 19 F chemical shift trend analysis to such compounds a prediction of the chemical nature of the average Al coordination becomes possible. A new synthetic approach to crystalline aluminium hydroxy fluorides involving a sol gel fluorination as the first reaction step and an aluminium alkoxide as precursor compound is presented. Varying the amount of HF leads to different F–OH-ratios in the AlF x (OH) 3x compounds. ß 2008 Elsevier B.V. All rights reserved. * Corresponding author. Fax: +49 30 2093 7277. E-mail address: erhard.kemnitz@chemie.hu-berlin.de (E. Kemnitz). Contents lists available at ScienceDirect Journal of Fluorine Chemistry journal homepage: www.elsevier.com/locate/fluor 0022-1139/$ – see front matter ß 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jfluchem.2008.04.015