Nanoscopic fluorides

Nanoscopic fluorides (e.g., partly hydroxylated or oxide fluorides) seem to possess appropriate catalytic properties for many industrial important fine chemicals as vitamin E ((all-rac)-a-tocopherol) [78-80], compounds from the vitamin K class (vitamins K K,-chromanol,... 

The presented catalytic results support their unique behavior in a broad area of organic reactions as, for example, biochemical synthesis from renewable raw materials or fine chemical synthesis. Irrespective of the synthesis approach, the use of nanoscopic fluorides as catalysts or catalytically functional support offers not only improved efficiency but also additional green elements in complete agreement with the current trends of the twenty-first-century chemistry. 

Inorganic-polymer nanocomposites characterized by exceptional dielectric constant are often called artificial dielectrics . Artificial dielectrics are created when isolated particles become polarized due to the presence of an applied electric field. These novel nanocomposite artificial dielectrics have the potential to posses high dielectric constants (>100) at high frequencies and the low processing temperature associated with polymers. Such a combination of properties is not found in other capacitor materials [180]. Polymer matrices like PMMA, poly(vinylidene fluoride) (PVDF), PS, and polyurethane (PU) have been used. Owing to their physicochemical properties, they represent suitable polymer components for embedding nanoscopic functional inorganic fillers (Table 2). 

Thus, instead of condensation in case of metal oxide formation, the strong bridging tendency of fluoride ions causes the formation of three-dimensional nanoscopic particles under these conditions. 

This new fluorolytic route was first time published for the synthesis of nanoscopic high-surface-area aluminum fluoride (HS-AIF3) in 2003 [9], which has been proven in the meantime to be a general synthesis approach for nanoscopic metal fluorides [10-25]. For deeper insights into the mechanism of the fluorolytic sol-gel approach see Ref. [26]. 

Figure 6.4 Schematic Representation of the Formation of Highly Distorted Nanoscopic Aluminum Fluoride by Reacting Aluminum Isopropoxide with Anhydrous Hydrogen Fluoride in Isopropanol as Solvent. Reproduced from Ref. [26] with permission of Royal Society of Chemistry.

In this context, nanoscopic, partly hydroxylated fluorides have been found to be able to fulfill all the features needed to develop a more effective and practical catalytic system for an industrial process [78]. Noteworthy results in the presence of nanoscopic, partly hydroxylated fluorides are shown in Table 6.3. 

As expected, MDL can be easily acylated with acetic anhydride to MDD (conversions of 84—89% and selectivities to MDD of >99.0%) in the presence of nanoscopic, partly hydrox-ylated fluorides (Table 6.4) [82]. 

Obviously, if the acidity of the nanoscopic acidic hydroxylated metal fluorides was beneficial for the synthesis of vitamins E, Kj, and Ki-chromanol, the one-pot acetylation of menadione... 

Table 6.4 Catalytic performances of the nanoscopic, partly hydroxylated fluorides in the acetylation of menadiol to MDD.

Nanoscopic, partly hydroxylated aluminum fluoride and noble metal-doped hydroxylated aluminum fluorides are active (conversions of 82.0-91.0%) and selective in the citronellal isomerization to isopulegols (selectivities of 91.0-97.0%) (Table 6.6). Unfortunately, the stereoselectivity to the desired product, ( )-isopulegol, was around 75% starting from racemic citronellal. 

As we have shown, fluorolitic sol-gel-prepared nanoscopic, hydroxylated fluorides exhibit medium to strong Lewis/Brpnsted acidity due to undercoordinated M"+ sites and -OH groups on the surface [80]. Nevertheless, for cellulose hydrolysis such sites can be too weak. On the other hand, strong Lewis acids (e.g., pure HS-AIF3) are instable in water making them not suitable for this reaction [78]. To overcome these problems, novel doped hydroxylated... 

Nanoscopic, partly hydroxylated magnesium fluorides (Mgp2 ,(0H), with x < 0.1) seem to possess appropriate characteristics for this synthesis [147]. Moreover, to further optimize the reaction conditions nonconventional methods to activate reactant molecules such as micro-waves or ultrasound irradiation were used. 

The nanoscopic, partly hydroxylated fluoride screening in the esterification of glycerol with acetic acid using a conventional thermal energy source is shown in Figure 6.32. 

As it has been exemplarily shown in this chapter, the new pure nanoscopic metal fluoride-based materials, the metal hydroxide fluorides, MF (OH) , or their combination with the second metal (e.g., doped fluorides) and metal oxide fluorides, represent potential... 

Consequently, several approaches for the preparation of nanoscopic metal fluorides and metal fluoride thin layers have been developed and proposed. Besides physical methods such as milling, laser dispersion or molecular-beam epitaxy, different chemical methods exist. Basically, three approaches can be distinguished ... 

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. 

In recent work, much of which is reviewed elsewhere in this volume, surface areas of metal fluorides, for example as measured by the BET method, can be increased markedly if the synthetic method leads to aggregates of very small (sometimes called nanoscopic) particles. Such solids will exhibit properties that are very different from the conventionally prepared analogues, for example a molecular dynamics simulation of cubic nanoparticles of a-AIp3 indicated the presence of structural motifs that are found also in the metastable... 

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