Metal fluorides made to measure
Since the discovery that fluorine incorporation into organic molecules has a number of beneficial effects, the synthesis and application of organofluorine compounds has undergone massive development. Initially, the major driving force of such developments was the pharmaceutical industry. However, organofluorine compounds are nowadays widely used in many other applications including plastics, refrigeration and energy conversion. Especially for energy production and catalytic applications, fluorination techniques have gained considerable interest in improving the performance of such compounds. Alongside that notion, the EU-funded project ‘Functionalised metal fluorides’ (Funfluos) concentrated on developing fluorinated materials with tailored characteristics for specific applications. The objective was to discover the upper limits of surface area, porosity, acidity and thermal stability achievable for these materials. To achieve that, project partners developed highly innovative synthetic approaches to obtain solid fluorides with surface areas 10 times higher than presently known and with extremely high Lewis acidity, paying particular attention to aluminium-containing materials. The first synthesis strategy (non-aqueous sol-gel synthesis) followed by the project involved the reaction of the metal in an organic solvent like alcohol with hydrogen fluoride. An alternative route towards highly distorted metal fluorides was investigated, which proceeded via oxidative decomposition and direct preparation of metal fluoride aerogels. These new routes carefully controlled the chemical composition, the specific electronic and ultraviolet (UV) absorption properties and the crystalline network of the fluorinated metals. In terms of activity, the high surface metal fluorides exhibited interesting catalytic activities due to their improved properties. Incorporation of catalytically interesting metals into these high surface metal fluorides gave very promising catalysts for the oxidative dehydrogenation of propane as well as for the conversion of methanol into formaldehyde. Furthermore, a particularly interesting application of metal fluorides involved their use as UV absorbers for solar protection since the presence of fluorine allowed the tuning of parameters such as the optical band gap and limiting the refractive index in order to get higher transparency in visible range. The Funfluos project succeeded in developing and optimising methods for synthesising high surface metal fluorides for a variety of applications. Being able to control and modify the target characteristics of such compounds is a desirable property that could be exploited commercially and have important economic and environmental impacts.
