CORDIS - EU research results

Functionalised metal fluorides

Final Report Summary - FUNFLUOS (Functionalised metal fluorides)

The scientific activities of the FUNFLUOS project were focussed on the development of fluorinated materials with tailored characteristics for specific applications. Hence, the major activities were attributed to the following fields:
- development of new, highly innovative routes for the synthesis of inorganic fluorinated materials with outstanding and controllable surface characteristics;
- key physico-chemical and structural characterisation of new samples synthesised with the aim to understand the underlying processes that are responsible for the enhancement of their characteristics;
- screening and testing of selected functionalised fluorinated materials in processes of application relevance that open new development and innovation perspectives.

New synthesis routes to binary as well as complex metal fluorides and oxide fluorides with interesting, potentially very useful surface and / or optical properties were in the main focus of the preparative work. Three different synthesis strategies towards nanoscopic metal fluorides have been followed in detail which are:
i) non-aqueous sol-gel synthesis;
ii) oxidative decomposition of suitable precursors; and
iii) functionalisation of fluorinated materials with well defined chemical compositions and crystalline networks.

The non-aqueous sol-gel synthesis route comprises basically the reaction of a metal alkoxide or another suitable metal organic compound in an organic solvent like alcohol with an ethereal or alcoholic solution of hydrogen fluoride. This reaction can also be applied to mixtures of different metal compounds. Another route, which is not general applicable, is the direct dissolution of the respective metal in an alcoholic hydrogen fluoride solution. This route is preferentially applicable for aluminium and magnesium. Both routes result in the formation of a non-aqueous metal fluoride sol or gel, which upon drying yields an X-ray amorphous metal fluoride precursor which - depending on the respective metal - may still contain some remaining organics.

An alternative route towards highly distorted metal fluorides was investigated, which proceeds via oxidative decomposition and direct preparation of metal fluoride aerogels. In the first case, activities were focused on the preparation of aluminium and chromium fluorides. Some work was done also on iron and gallium fluorides. The general path consists of the reaction between hydrazinium fluorometalates and fluorine. Reaction between the fluoroaluminates and fluorine is strongly exothermic and difficult to control, especially under gas-solid (heterogeneous) conditions. By performing the reaction in liquid aHF medium a much better control over the decomposition was achieved.

Functionalisation of fluorinated materials with well defined chemical compositions and crystalline networks in connection with their chemical properties such as reactivity, acidic, hydrophobic character, and optical absorption properties has been performed in order to find and develop new applications. Four key-points were identified:
- the development of new routes to prepare diversive inorganic fluorinated materials exhibiting specific or outstanding chemical and electronic properties;
- the correct control of the chemical composition using an adapted synthesis route, for obtaining fluorinated compounds with defined F/OH atomic ratios;
- the accurate determination of the type of crystalline network, local environments and surface features which can be related to the chemical and physical properties of defined compounds;
- the evaluation and analysis of the chemical properties, surface reactivity (nature and strength of acidic sites) and Ultraviolet (UV)-visible absorption properties, as well as the reduction of the refractive index as a function of the chemical composition and structural features.

Because of the very high Lewis acidity of HS-AlF3, which proved to be the second strongest solid Lewis acid of all, topped only by Aluminium chlorofluoride (ACF), all physical and chemical characterisation methods available to the FUNFLUOS consortium have been applied.

To improve the industrial applicability of HS-AlF3, it was supported on Al2O3. The catalytic activity of the supported HS-AlF3 was found to be effective for CHClF2 dismutation as well as for CBrF2CBrFCF3 isomerisation, whereby HS-AlF3/-Al2O3 exhibited superior activity, which was about 10 - 30 times higher than that of HS-AlF3/a-Al2O3, in terms of isomerisation activity.

Binary and guest / host metal fluorides have been tested for several catalytic reactions and showed in several cases very promising activities. Among these reactions, just one will be highlighted here: the hydrodefluorination of CF3CHCF3 since this involves an activation of a CF3-group, a very complicated process but which is possible in case of HS-AlF3 because of its very high Lewis acidity.

Modifying the sol-gel method, highly dispersed noble metals or other catalytically interesting metal species have been successfully incorporated as guests into the lattices of different high surface metal fluorides, and the developed new synthetic approach was used for the preparations of heterogeneous VOx-, Pd- and Pt- catalysts. With catalyst systems consisting of VOx-species supported on HS-AlF3 very promising results have been obtained for the oxidative dehydrogenation of propane as well as for the conversion of methanol into formaldehyde.

Additionally, the team developed and investigated catalyst systems for some important industrial processes using the noble metal catalysts (Pd, Pt) supported on HS-(AlF3, MgF2, ZrF4 or K3AlF6). Further experiments were successfully conducted for the dehydrohalogenation of fluoroalkanes to the respective fluoroethylens as well as for the Suzuki coupling reaction.

As far as optical absorption properties are concerned, the evolution of optical band gap as well as refractive indices are of crucial importance for applications in UV-visual range. UV absorbers for solar protection represent the main field of applications for these inorganic compositions containing Ti4+ cations for instance. Finally, the presence of fluorine in these materials allows to tune parameters such as the optical band gap and to limit the refractive index in order to get higher transparency in visible range. This drastic decrease of refractive index in these fluorinated series compared to TiO2 with rutile or anatase networks leads us to consider these materials to improve the transparency of such UV absorbers in the visible range. Moreover, considering the diffuse reflectance spectra, the band gaps appear around 3.2 eV with a promising UV shielding.