Ionic Liquids in-vacuo; marrying Surface Science with Solution Chemistry.

点击翻译按钮获取中文摘要

The Scientific Problem: Scientific, environmental and commercial pressures are all forcing chemists to invent ever better catalytic reactions that generate the desired products with minimal waste. Biological systems achieve this goal very efficiently using chemically complex catalysts, so-called enzymes. Synthetic chemists are becoming increasingly able to design and prepare complex molecular catalysts; however, they are extremely costly in both monetary worth and environmental burden. Thus it is essential that the catalyst is conserved and not lost in the product mixture. There are several strategies for immobilising catalysts to prevent their loss and one of the more exciting new approaches is the application of ionic liquids as a selective solvent facilitating the entrapment of the catalytic species. The catalyst solution does not dissolve in the reaction mixture; hence it can be easily decanted away from the products at the end of the reaction and then reused. Although research groups throughout the world are studying ionic liquids, there is a lack of understanding about how catalysts function in ionic liquid solutions and indeed about the fundamental nature of ionic liquids as solvents. Such information is essential if catalysis is to be refined and reaction selectivity optimised. This project will address this imbalance and generate information vital to the design of better catalysts. Originality of Approach: Ionic liquids are, in general, organic based salts with melting points below room temperature. Because they are composed entirely of ions, they have an almost zero vapour pressure; they do not evaporate even under vacuum. This means that ionic liquids are difficult to ignite, unlike most of the solvents that are conventionally used for chemical synthesis. We have recognised that this lack of volatility allows ionic liquids to be used in a whole range of analytical instruments that require high vacuum for their operation. Thus ionic liquids could enable solutions and liquids to be used in high vacuum instruments for the first time, thereby allowing us to apply such techniques to probe the behaviour of catalysts in ionic liquid solutions. In addition, we can obtain an understanding of the fundamental physical chemistry of the ionic liquids themselves. Our preliminary experiments have demonstrated that this idea works in practice; we have obtained high quality X-ray photoelectron spectroscopy (XPS) data of the pure liquids as well as the liquids doped with simple metal salts. XPS provides information about the electronic structure of different elements within a compound and has sufficient sensitivity to distinguish between atoms of the same element situated in chemically distinct environments. We will extend the application of UHV spectroscopies to allow the in-situ simultaneous characterisation of REDOX active substrates by both XPS and electrochemical methods including cyclic voltammetry, providing a direct link between the electronic structure of the metal to REDOX behaviour.Application of Research: During our preliminary investigations, we noted that the conductivity of Ionic liquids changes quite dramatically as the substrate is frozen to form a solid. We observed that when the cooled surface was irradiated with a focused beam of ions (FIB) an image was written to the substrate. The image, composed of localised areas of +ve charge was found to be remarkably stable, furthermore it was noted that the image could be easily erased by one of two simple methods, i) by warming the surface to increase mobilisation that facilitated dissipation of the charge throughout the bulk, or ii) by simply exposing the frozen surface to a supply of low energy electrons from an electron flood gun. The process is rewritable and there is no detrimental effect on the substrate surface.