Monitoring an ionic liquid synthesis with in-situ IR-spectroscopy – The intricacy of solvent effects

Abstract

This work focuses on the ionic liquid (IL) synthesis starting from dimethyl carbonate and 1-ethylimidazole in methanol. In-situ IR-spectroscopy is used to gain time-dependent concentrations for kinetic analysis. The reaction is found to be self-accelerating, since the solvent polarity increases with the formation of the ionic product. This corresponds to a salting-out effect. Since the reaction rate is increasing in the beginning, pre-dosing of the product is proposed to enhance the overall productivity. Therefore, the space-time-yield is calculated as the arithmetic mean of the reaction rate with a varying starting point. It turns out that pre-dosing does not enhance the productivity here, since the space-time-yield of the complete reaction progress is below the initial reaction rate. Beyond that, two sharp bends can be observed in the progress of the reaction rate coefficient that point towards sudden changes in the reaction environment. The first bend can be assigned to a phase separation of the ionic liquid on a microscopical scale. The second bend takes place when the IL concentration surpasses roughly one sixth of the methanol concentration. Above that threshold, the whole amount of methanol is captured in the solvation shell of the anion monomethyl carbonate, as quantum chemical calculations suggest. Finally, the reaction rate coefficients were calculated using quantum chemical methods with respect to the solvent environment. The theoretical results show a good accordance with experimental values for the starting compositions. Thus, theoretical calculations help to estimate productive reaction mixtures.