Towards carbon dioxide utilisation: dense phase carbon dioxide and its mixtures with ionic liquids

Abstract

Compressed carbon dioxide, either by itself or in combination with an ionic liquid, can serve as a promising reaction/separation medium for many potential applications. This work addresses the subject of solubility of carbon dioxide in relatively new, or less explored, families of ionic liquids, and its influence on particular properties, i.e. volume expansion or melting point depression. For the purpose of measuring carbon dioxide solubility at high pressures, a new, especially designed apparatus, was custom-built and its detailed description is provided here. The application of carbon dioxide as a solvent for a process and subsequent separation of product is discussed, giving a perfect example of the currently developing field of carbon dioxide utilisation. Ionic liquids based on tris(pentafluoroethyl)trifluorophosphate and tetracyanoborate anions coupled to 1-alkyl-3-methylimidazoilum cations presented one of the greatest capacities for carbon dioxide capture by physisorption mechanism among ionic liquids investigated in the literature. Exceptional volume expansions of the liquid phase upon carbon dioxide dissolution, reaching values of approximately 55 and 60 % for ionic liquids bearing fluoroalkylphosphate or tetracyanoborate anion, respectively, have been found. Results seem to confirm that the high carbon dioxide solubility is a consequence of the weak coordination between anion and cation, which allows for easy creation of cavities and more CO2 molecule accommodation. Although still much smaller than commonly observed for mixtures of carbon dioxide and organic solvents, this notably high volume expansion of IL-CO2 systems validates the free volume effect and the interspace filling mechanism of dissolution. The solubility of carbon dioxide in ammonium-based ionic liquids, which display abnormally high carbon dioxide-induced melting point depressions was carried out. These solubilities do not show any positive correlation with previously measured melting temperature depressions. Finally, a successful example of an application of carbon dioxide as both reaction and extraction medium in the process of C-H insertion of diazoacetamides is given. The process produces the expected lactam in yields over 97 %. Recycling experiments proves that carbon dioxide is capable of selectively extracting the product, leaving the catalyst behind for its reutilisation.