(198c) Continuous Microreactor Processing, Reactive Ionic Liquids and Microwave Heating – Approaches towards Intensified Kolbe-Schmitt Synthesis

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 [1].

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.

[1]   Hessel, V. et al., Org. Proc. Res & Dev, 2005, 9, pp.479-489