Homogeneous Co/Mn/Br catalyzed aerobic oxidation of benzyl alcohol in acetic acid to benzaldehyde was performed in polytetrafluoroethylene microreactors operated under slug flow at temperatures up to 150 C and pressures up to 5 bar. Depending on the bubble velocity and length, a wetted or dewetted slug flow was observed, characterized typically by a complete or partially wetting liquid film around the bubble body. The latter flow suffered from a limited interfacial area for mass transfer. Experiments at temperatures up to ca. 90 C were under kinetic control given no product yield difference under wetted and dewetted slug flows and were used to establish a simplified kinetic expression (first order in benzyl alcohol and zero order in oxygen). This allows to develop a mass transfer model combined with an instantaneous reaction regime that well described the experimental results at higher temperatures where mass transfer was limiting in the dewetted slug flow.

Oxidation of 5-hydroxymethylfurfural (HMF) using air or pure oxygen was performed in polytetrafluoroethylene capillary microreactors under gas-liquid slug flow, with Co/Mn/Br as the homogeneous catalyst in the acetic acid solvent. The temperature was varied from 90 to 165 • C at a pressure of 1 or 5 bar. At atmospheric pressure conditions (and 90 • C), acetaldehyde was further added as a co-oxidant to accelerate the reaction. At 150 • C, 5 bar oxygen and a residence time of 2.73 min, an HMF conversion of 99.2% was obtained, with the yields of 2,5diformylfuran (DFF), 5-formylfurancarboxylic acid (FFCA) and 2,5-furandicarboxylic acid (FDCA) being 22.9%, 46.7%, and 23.8%, respectively. By operation under wetted slug flows and elevated partial oxygen pressures, mass transfer limitations and oxygen depletion in the microreactor could be eliminated. This allowed to run the microreactor under kinetically controlled conditions, where both the HMF consumption and DFF formation were found zero order in partial oxygen pressure and roughly first order in HMF. The total selectivity towards DFF/ FFCA/FDCA was ca. 40% at low partial oxygen pressures due to the dominant occurrence of side reactions. By using pure oxygen at 5 bar the total selectivity was improved to 60-94%. The space time yields of DFF and FFCA in the microreactor exceeded those obtained in conventional (semi-)batch reactors at slightly elevated temperatures and pressures, due to the superior mass transfer and higher initial HMF concentrations in the microreactor. For highly efficient FDCA synthesis, more dedicated microreactor operations are needed to tackle its precipitation.

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