(36f) Ionic Liquids Like Micro – Fast and Safe Production of a Colourless Imidazolium-Based Ionic Liquid

The manufacturing of ionic liquids usually involve the strongly exothermic alkylation of an amine or a phosphine with haloalkanes. During these reactions disperse systems like liquid/liquid mixtures emerge. In conventional production modes (inefficient batch procedures) often additional solvents are added to handle the strong exothermicity, to reduce viscosity and to avoid multi phase operation. A general problem in ionic liquid synthesis is the generation of coloured impurities (Nockemann et al. 2005, Earle et al. 2007) especially in high temperature synthesis procedures. To obtain a colourless ionic liquid - an additional purification step is required.

Recently is has been shown that in micro reactor systems the solvent-free synthesis of ionic liquids can be considerably intensified with respect to space time yield maintaining high product quality (Waterkamp et al. 2007). Based on further investigations this process had been improved.

Here we present a high temperature method (100 - 150°C) to produce the ionic liquid precursor 1-butyl-3-methylimidazolium bromide (a liquid/liquid phase system) by using micro reaction technology involving micro mixers and capillary reactors. In this system the heat of reaction could be removed efficiently leading to nearly isothermal processing avoiding thermal runaway behaviour. During the synthesis of imidazolium-based ionic liquids without solvents, the diffusion of the reactant 1-methylimidazole from the product phase to the reaction phase limits the reaction velocity. In the liquid/liquid slug flow-regime with intensified mixing this mass transfer limitation can be overwhelmed. Furthermore the main reaction is supported while the side reactions that lead to discoloured impurities are suppressed. Additional to the mixing intensity also the stochiometry of the reactants is decisive for the product quality. With the presented concept the reaction time could be reduced from hours to a minute. As a result a colourless IL-precursor could be produced with a space time yield that is around three orders of magnitude higher compared to the conventional batch process.

Nockemann, P., K. Binnemans, and K. Driesen, Purification of imidazolium ionic liquids for spectroscopic applications. Chemical Physics Letters, 2005. 415(1-3): p. 131-136.

Earle, M.J., et al., Decolorization of ionic liquids for spectroscopy (vol 79, pg 758, 2007). Analytical Chemistry, 2007. 79(11): p. 4247-4247.

Waterkamp, D.A., et al., Synthesis of ionic liquids in micro-reactors - a process intensification study. Green Chemistry, 2007. 9(10): p. 1084-1090.