The Kolbe-Schmitt synthesis is a traditionally used carboxylation reaction for phenolic cores and normally carried out batch wise with long reaction times. This presentation will address new approaches to intensify the synthesis using and combining different methods of process intensification: Continuous processing using microstructured devices, microwave heating and the use of reactive ionic liquids.
First conventional heated aqueous Kolbe-Schmitt synthesis was continuously carried out for the synthesis of 2,4-dihydroxybenzoic acid from resorcinol using a capillary reactor and a cross-flow micro heat exchanger for cooling down reaction mixture at the end of the reactor. The achieved yields are comparable with experimental results using a stirred batch; however the latter needs 120 minutes reaction time which demonstrates the process intensification by means of continuous capillary reactor technology .
Next, a continuous microwave processing was developed, with a capillary reactor of similar dimensions. The microwave assisted aqueous Kolbe-Schmitt synthesis was carried out under a pressure of 8 bar at different temperatures. This resulted in considerable higher yields compared to the conventional heated approach. The best product yield obtained was 52%, which is an improvement of 5% compared with the best value obtained by conventional heating even at decreased reaction time.
Then, ionic liquids were used as CO2 donating agents, diluted (2.91 mol/L) or concentrated (5.81 mol/L), thus being reactive solvents which allow to perform solvent-less processing. In the diluted case 1-Ethyl-3-methylimidazolium hydrogen carbonate was used, while in the concentrated case 1-Butyl-3-methylimidazolium hydrogen carbonate was applied. Both ionic liquids were used for the conventional heated Kolbe-Schmitt synthesis. The yield obtained in the diluted case was 44% at 180°C, 35 bar and at 130 s reaction time. Only a relatively low flow rate could be applied due to the limited available amounts of the ionic liquids. The obtained yield in the concentrated case was 59% at the same conditions, but with considerably higher space-time-yield.
Finally experiments using a combination of microwave assisted Kolbe-Schmitt synthesis and reactive ionic liquid were performed at different temperatures and resulting in a yield of 57 % using a reaction time of 130 s, 10 bar and 140°C. Motivated by the optimistic results, more detailed investigations are planned within the project “New process windows for the process intensification of the Kolbe-Schmitt synthesis” which will be performed in the frame of the German Federal Foundation Environment (DBU) funding cluster “Novel Process Windows”. Latest results will be implemented into the presentation.
 Hessel, V. et al., Org. Proc. Res & Dev, 2005, 9, pp.479-489