Microreactor processing for the aqueous Kolbe-Schmitt synthesis of hydroquinone and phloroglucinol

Hydroquinone and phloroglucinol were used as substrates for the aqueous Kolbe-Schmitt synthesis, using a novel processing methodology, termed high-p,T processing, recently demonstrated for the carboxylation of resorcinol. By the high-p,T approach, the temperature limitations of classical batch synthesis, e.g., set by reflux conditions (solvent boiling point), can be overcome by simple technical expenditure, e.g., the use of a few microstructured components, a capillary, and a needle valve at very low internal holdup. In this way, favorable speed-up of chemical reaction is achieved at temperatures normally outside the useful range for organic synthesis. While the hydroquinone synthesis gave only very low yields, phloroglucinol was converted to the corresponding carboxylic acid at a high yield of 50 %, which is about 20 % higher than for a laboratory batch synthesis. Process intensification was achieved by reducing the reaction time from 2 h for the batch synthesis to 50 s for the micro processing. However, the most favorable temperature is limited to ca. 130 °C, since at higher temperatures, decarboxylation of the product back to phloroglucinol outpaces the speed-up of the reaction, while at lower temperatures, the reaction rate and conversion are insufficient