Abstract
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 2,5-diformylfuran (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.
Highlights
The depletion of fossil sources urges a switch to renewable feed stocks
In the absence of acetaldehyde at 90 ◦C, no hydroxymethyl furfural (HMF) conversion and product formation were observed in the microreactor for residence times up to 5 min, showing the slow reaction kinetics under such con ditions
The homogeneously catalyzed aerobic oxidation of HMF to DFF, formylfurancarboxylic acid (FFCA) and furandicarboxylic acid (FDCA) was performed for the first time in microreactors
Summary
The depletion of fossil sources urges a switch to renewable feed stocks. Biomass is an abundantly available renewable source of carbon that is essential for the production of fuels and chemicals [1]. The reaction was performed in PTFE capillary microreactors operated under gas–liquid slug flow, with Co/Mn/Br as the homogeneous catalyst in the acetic acid solvent and air or pure O2 as the oxidant Both atmospheric pressure conditions (at 90 ◦C) in the presence of acetaldehyde as a cooxidant and at a slightly elevated temperature and pressure (5 bar and up to 165 ◦C; i.e., above the vapor pressure of acetic acid being 3.5 bar) without acetaldehyde were investigated. Mass transfer limitations could be diminished by wetted slug flow processing in small inner diameter microreactors and oxygen depletion was prevented by increasing the partial oxygen pressure (e.g., using pure O2 and/or increasing the pressure to 5 bar) This allowed to investigate the effect of temperature, partial oxygen pressure and initial HMF concentration under the kinetic regime. For the efficient synthesis of FDCA in microreactors, its precipitation at high yields remains a challenge to tackle
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