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

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 Microwave assisted Aqueous Kolbe-Schmitt 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 Conventionally Heated (oil bath) aqueous Kolbe-Schmitt synthesis (CHKS) without use of microwave irradiation. The highest yield (before isolation and purification) obtained by MAKS was 52% (at 161) degrees C, 8 bar, 1 L/h, 90 s), exceeding the yield of CHKS by 5% (at 140 degrees 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 CO2 donating agents a diluted (1.57 mol/L) and concentrated one (3.2 mol/L) with ethyl based methyl imidi oalium hydrogen carbonate (EMIM C) and a concentrated one (2.7 mol/L) with butyl based methyl imidi oalium hydrogen carbonate (BMIM Q. The yield obtained for the ethyl based ionic liquid (e-C IL S) operation was 44% (at 180 degrees C, 35 bar, 0.17 L/h 130 s) at a productivity of 5.9 g/h. The yield of b-C IL S was 59% under the same conditions yet with a higher productivity of 14.2 g/h. For the Microwave assisted butyl Ionic Liquid olbe-Schmitt synthesis (e-MILKS) a yield of 58% at a productivity of 25.3 g/h was derived.