Intensification of the Capillary-Based Kolbe−Schmitt Synthesis from Resorcinol by Reactive Ionic Liquids, Microwave Heating, or a Combination Thereof

Abstract

The continuous Kolbe−Schmitt synthesis of 2,4-dihydroxybenzoic acid from resorcinol was carried out using a setup with a capillary reactor of mm-internals and a micro heat exchanger. The process intensification potential of microwave irradiation for heating up the reactant solution and/or of using ionic liquids as carbonating reactants was evaluated. For the M icrowave assisted A queous K olbe− S chmitt synthesis ( MAKS ), power-to-temperature graphs were calibrated with water and real-case (ion-containing) solutions, revealing several features relevant for process control. Due to higher mean reaction temperatures inside the reactor thanks to faster heating, the yields of all MAKS experiments were higher as compared to the C onventionally H eated (oil bath) aqueous K olbe− S chmitt synthesis ( CHKS ) without use of microwave irradiation. The highest yield (before isolation and purification) obtained by MAKS was 52% (at 160 °C, 8 bar, 1 L/h, 90 s), exceeding the yield of CHKS by 5% (at 140 °C, 40 bar, 84 mL/h, 390 s) at a concurrent reduction of reaction time. The MAKS productivity, although lower than possible due to setup limitations (large internal volumes), was up to 67 g/h being in the same range as the CHKS productivity from 25 g/h (39%, 43 s) to 86 g/h (34%, 11 s). Two solutions of ionic liquids were used as CO 2 donating agents, a diluted (1.57 mol/L) and concentrated one (3.2 mol/L) with ethyl based methyl imidizoalium hydrogen carbonate (EMIMHC) and a concentrated one (2.7 mol/L) with butyl based methyl imidizoalium hydrogen carbonate (BMIMHC). The yield obtained for the ethyl based ionic liquid (e- CHILKS ) operation was 44% (at 180 °C, 35 bar, 0.17 L/h, 130 s) at a productivity of 5.9 g/h. The yield of b- CHILKS was 59% under the same conditions, yet with a higher productivity of 14.2 g/h. For the M icrowave assisted butyl I onic L iquid K olbe− S chmitt synthesis (e- MILKS ) a yield of 58% at a productivity of 25.3 g/h was derived.