Nano-sized metal fluorides in catalysis
The unique sol-gel fluorination synthesis of metal fluorides, developed in our Lab, opens a variety of possibilities for the preparation of diverse catalysts.
One consequence of the typically high surface area of the fluorides is that they can exhibit high Lewis acidity, and HS-AlF3 prepared this way is the second strongest Lewis acid there is. With mixed metal fluoride systems, easily prepared via our sol-gel route, the Lewis acidity can be tuned as needed for special reactions. Tested applications are isomerisations, dismutations, dehydrohalogenations, hydrofluorinations and Friedel-Crafts reactions.
By graded hydroxylation in course of the sol-gel synthesis, bifunctional Lewis and Brønsted acidic solids are accessible, whereby the Brønsted acidity can be tuned down to basicity. Tested applications are Michael additions and tocopherol synthesis.
Metal fluorides can be used for supporting/ distributing other catalytically active materials, whereby the performance of the latter can be boosted. Examples of these are VOx oxidation catalysts and noble metals on metal fluoride, tested for, e.g., propane partial oxidation, and Suzuki coupling and hydrodehalogenation, respectively.
All of these catalysts can be supported without loss of activity.
Metal fluorides can combine low index of refraction, the lowest of solids, with high transparency up to the UV range. Some examples are MgF2, AlF3 and Na3AlF6. The highly disordered, often X-ray amorphous nano-sized materials obtained via our sol-gel fluorination route can be easily pressed into dense, optically clear glasses for diverse applications, avoiding problems linked with melting. Another application already realised is a thin layer with an index of refraction lower than that of silica-based glasses used as, for example, an anti-reflection layer. Nano-sized metal fluorides can also be used as additives to polymers to alter the optical properties. To do this, modifying the metal fluoride nano-particles via linkage to selected organic groups, which is possible, might be useful.
The primary product of sol-gel fluorination synthesis of nano-sized metal fluorides is a non-aqueous metal fluoride sol. Such sols are ideally suited for coating all wettable materials employing simple techniques such as, dip-coating, blade-coating or spin-coating for example. Low-temperature drying results in stable, optically clear metal fluoride layers. Potential and already tentatively tested applications of the fluoride layers are in the following fields:
optical layers (see optics); protective coating to protect against mechanical and/or chemical and/or sanitary attack; friction reduction; adsorbent; sintering aid; flux for metal soldering/welding; energy (gas) storage; and opto-electronics (inverse opals).