Fluorolytic sol-gel synthesis

Metal alkoxides can be regarded as metal salts of alcohols, where the latter are very weak Brpnstedt acids. Acids that are stronger than the respective alcohol can therefore replace alkoxy groups attached to the metal ion under hberation of the alcohol and formation of the metal fluoride according to Equation (1.4). 

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The Fluorolytic Sol-Gel Synthesis of Metal Fluorides 136 Ternary and quaternary metal fluorides 140... 

Modifications of the Fluorolytic Sol-Gel Synthesis 143 Partly hydroxyiated metal fluorides 143... 

The simplified chemical pathway of tho fluorolytic sol-gel synthesis is given by Equations (6.5) and (6.6) ... 

Figure 6.2 Illustration of the Fluorolytic Sol-Gel Synthesis Starting from a Metal Alkoxide or Carboxylate, Respectively. Reproduced from Ref. [26] with permission of Royal Society of Chemistry.

In recent years, much work has been spent in adapting the fluorolytic sol-gel chemistry also to other metals [28-33]. In an attempt to get deeper insights regarding the mechanism of the fluorolytic sol-gel synthesis most efforts have been made in case of AIF3 and MgFj formation. This has been comprehensively reviewed recently in Ref. [26] hence, for further information we refer to this. 

A closely related but distinct different synthesis approach is the competitive fluorolysis/ hydrolysis reaction. This means that hydrogen fluoride will be used in almost stoichiometric portions as in case of the original fluorolytic sol-gel synthesis (Equation (6.3)) but small amounts of water will be added to the reaction system at the same time, thus acting as competitor for HF as shown in Equation (6.8) ... 

However, these nanoscaled metal fluorides and hydroxide fluorides not only represent new, catalytically active classes of compounds with very high surface areas but are also excellent candidates to be used as supports for active components such as precious (e.g., Au, Pd, Pt) or nonprecious (e.g., Sn, Nb) metal deposition by incipient wetness impregnation or by in situ incorporation during the fluorolytic sol-gel synthesis. 

The fluorolytic sol-gel synthesis of metal fluorides is summarized in Scheme 1.3. 

Scheme 1.3 Fluorolytic sol-gel synthesis of high surface area metal fluorides (for explanations of the numbers 1 to 5 see text)...

The fluorolytic sol-gel synthesis of metal fluorides was originally developed and explored for aluminium fluoride, which was a piece of luck since both the stepwise synthesis and the properties of HS-AIF3 showed the influence of the new synthesis process. Thus, almost immediately after exploration of HS-AIF3 other binary metal fluorides have been similarly prepared and attempted syntheses of more complex systems started. 

In conclusion, the new access towards nanoscopic metal fluorides via this recently developed fluorolytic sol-gel synthesis route opens a wide range of applications for metal fluorides due to the distinctive different properties of these nano materials. 

In conclusion, in several catalysed reactions die fluorolytic sol-gel synthesis-derived nanoscopic metal fluorides have already shown strong potential. Although it is probably the case that, for many catalytic reactions, metal oxides might remain the preferred options for many reasons, there are several very promising fields for which nanoscopic metal fluoride-based catalysts are advantageous. For many halogen exchange reactions, in particular, these new catalysts exhibit unique properties. 

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