A comparative study of ultrasound-, microwave-, and microreactor-assisted imidazolium-based ionic liquid synthesis

Abstract

Synthesis of ionic liquid 1-heptyl-2,3-dimethylimidazolium bromide was accomplished with the assistance of ultrasound, microwave irradiation, and a continuously operated microreactor and was compared with a conventional laboratory scale process applying magnetic stirring and water-bath heating. Results were compared with respect to process productivity, energy consumption, and product colourisation as an indicator of its purity. By using nonconventional technologies, volumetric productivity was 10- to 30-fold superior, while energy consumption was reduced by 45%-65%. Among the alternatives tested, ultrasound-assisted synthesis was shown as the most efficient one in terms of volumetric productivity (4.40 moll-1 h-1) and specific power consumption (909.1 W h mol-1), while microwave-assisted process was the least favourable. However, only a microreactor system enabled the synthesis of a noncoloured product resulting from very efficient mixing and temperature control. Due to significant process intensification along with high product quality and superior industrial perspectives, a continuous quaternisation within microchannels could be selected as the most promising green approach among the alternatives tested in this study. Integration of ultrasound and microreactor technology including miniaturised heat exchanger is foreseen for process intensification.