Aluminium-based Fluoride Materials

In this section, a description of the use of the microwave-assisted synthesis for the preparation of nanosized and crystalline aluminium-based fluorides is proposed. A detailed characterization of the solid obtained is also presented. Using this method, three different crystalline forms can be obtained (i) an aluminium (hydroxyl)-fluoride adopting the HTB type structure and labelled as /S-AlFs, (ii) a hydroxyfluoride adopting 

Structural type Chemical composition HF/AI molar ratio aP+ precursor Solvents Synthesis duration/ temp. 

Pyrochlore AIF i,7(OH)i, 3 2 Isopropoxyde Water isopropanol ether 1 h160°C 

KeUs Alo.82Uo.l8F 2.46lH2U)o,54 3.6 Chloride Water isopropanol 1 h140°C 

Some synthesis parameters have been proved to be decisive for achieving both the chosen final form and nanosized crystallites. The following part will show the impact of the various synthesis parameters on the phase purity, crystal structure and surface areas of the obtained materials. 

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The first part of this chapter undertakes an introduction to microwave synthesis. After presenting the origin of this technique, some basic concepts of microwave heating are described and the advantages offered by this route are briefly discussed. Finally, a typical microwave oven dedicated to the synthesis is shown. Some examples of nanosized metal fluorides prepared by microwave irradiation are presented in the subsequent section. Particular attention is given to the preparation of nanosized aluminium-based fluoride materials. The impact of some synthesis parameters is discussed and a detailed characterization of the Al-based compounds is proposed to highlight the potential of this preparation route. 

The above mentioned studies on aluminium-based fluorides compounds prepared by microwave-assisted synthesis have enabled the improvement of the knowledge on these materials as well as the development of different compounds having unusual properties in... 

Another way to increase resolution while avoiding an excessively long experimental time is to constmct a two-dimensional one pulse (TOP) spectrum [21-23] from a conventional ID MAS spectrum. The TOP spectmm evidences the well-known narrowing of the inner satellite transition line width when the components are not well resolved on the N = 0 cross-section, the projection over the satelhte transitions (N 0) shows a much better resolution as shown in HS pyrochlore aluminium hydroxyfluoride [20] (Figure 5.6), nanostructured Al-based fluoride-oxide materials [24] and aluminium fluoride hydrate with cationic vacancies [25]. 

There are, however, certain acid-based materials which can primarily be construed as cleaners. One such type of material is used in the cleaning of aluminium cans prior to treating and lacquering. Such cleaners are normally based on sulphuric or phosphoric acid, with, generally, additions of hydrofluoric acid and surfactants. These materials are sprayed on to pre-formed cans to remove the lubricant used during the can-forming operation. The fluoride is present to enhance the removal of fines of metal swarf in the cans as well as to remove the oxide film. 

The method based on the use of DAM has been applied for determining titanium in biological materials [11], silicate rocks [32,99,100], cast iron and steel 33], molybdenum and tungsten [6,18], vanadium [18], zirconium, hafnium, and niobium [101], lithium fluoride [102], nickel, aluminium, and molybdenum alloys [11], and ferrotitanium [103]. Titanium was determined in aluminium alloys with the use of DAM in the presence of SnCh [35,104]. 

Aluminium cations are well known to form very strong fluoride complexes [251], and, therefore, fluoride melts interact to an appreciable degree with the Al203-containing constructional materials. This leads to dissolution of alumina in molten mixtures containing F, followed by the formation of fluoro-complexes. Addition of the Lux bases favours the interaction s running in the reverse direction, i.e. it results in precipitation of A1203 from its solutions in the fluoride melt. These ideas allow us to assume that the... 

Fireclay bricks (alumina silica bricks, alumina calcium oxide silica bricks, and other silicate bricks) are not optimal barrier materials for A1 reductirMi cells. As we have mentioned, cryolite-based electrolyte for A1 reduction is a substance that dissolves alumina better than anything else. Certainly, it will dissolve all alumina-based refractory compositions and almost all other oxides similar in chemical structure to alumina. From a chemical point of view, the effective refractory barriers against the penetration of cryolite might be tin oxide, nickel oxide, compounds of nickel oxide, iron oxide, or zinc oxide (such as spinel Fe NiOs). These oxides almost do not react with NaF and aluminium fluoride [175]. Yet the cost of these materials, which is 50-100 times higher than that of firebrick, provides the impetus to find less costly variants of alumina silica materials. 

Recent progress in the development of synthetic routes to metal fluorides, particularly those of aluminium and magnesium, which have high surface areas, has been described in earlier chapters of this volume. The significant catalytic properties of these high surface area (HS) materials have also been emphasized. Here we examine the surface acidities, both Lewis and Brpnsted, of newly synthesized fluorides based on Al and Mg centres and pose the question What is meant by a strong acid in an heterogeneous context ... 

In the second part (Section 5.3), the strong potential of these techniques for the study of functionalized Al-based inorganic fluorides, synthesized for their possible application as catalysts (fluorinated alumina and zeolites, high surface area aluminium trifluorides and nanostructured aluminium fluoride hydrates and hydroxyfluorides), is shown. The compounds used as model for the assignment of the V and Al NMR lines in these materials are also presented. 

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