No abstract

The aqueous Kolbe−Schmitt synthesis using resorcinol to yield 2,4-dihydroxy benzoic acid was performed in a microreactor rig. This small-scale plant was equipped initially with one capillary reactor and one microstructured cooler only. Later, two upgraded versions were constructed, having in addition a microstructured cooler and a microstructured mixer, respectively. The chemical protocol was significantly varied as compared to standard laboratory operation as described in the literature. Higher temperatures (up to 220 °C) and pressures (up to 74 bar) were employed in a facile manner, termed high-p,T processing. In this way, the reaction time could be shortened by orders of magnitude, from about 2 hours to less than one minute, in some cases to some seconds. This resulted in a remarkable increase of the space-time yield by a factor of 440 at best. Productivity was in the L/h range and yielded at best 111 g/h product, corresponding to 4 t/a. Scale-out solutions are indicated. Drawbacks of the microreactor operation were also identified such as high sensitivity to fouling and delicate regulation of the system pressure, leading to partly unstable plant operation. Possibly even a considerable part of the product was rearranged to 2,6-dihydroxybenzoic acid and then thermally decomposed under the harsh reaction conditions. Solutions to overcome or at least diminish these restrictions are envisaged, and in the hope that this may be achieved, a process innovation and business perspective for the high-p,T microreactor processing is depicted.

Ionic liquids (IL) are the focus of growing interest over the last few years due to their low vapour pressure being beneficial for replacing common organic solvents with high vapour pressure. IL synthesised via alkylation are produced in batch or semi-batch stirred tank reactors. The reaction is highly exothermic and the kinetics was shown to be fast. The heat management during the reactor operation is a crucial point leading to high quality IL product and avoiding thermal runaway. This study reports the use of a microstructured reactor (MSR) system for the production of ethylmethylimidazole ethylsulfate by a solvent-free alkylation reaction. A combination of MSR and two tubular capillary reactors operating at two different cooling temperatures has been proposed. The save and stable operation of this reactor system is proven experimentally rendering the IL of high quality. The specific reactor performance was about 4 kg m −3 s −1 being ca. 3 orders of magnitude higher as compared to more traditional reactors.