Design of an Electrocatalytic Flow Reactor for the Electrosynthetic Aldol Reaction of Acetone

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Organic electrosynthesis is a field within electrochemistry that concerns the synthesis of organic products using the electron as a redox agent instead of chemical reductants or oxidants. It offers several important advantages to conventional synthetic methods, such as less aggressive process conditions (reactions can be carried out at ambient temperature and pressure), higher selectivity (precise control of reaction by control of electrode potential), ability to produce unstable or hazardous reagents in situ and less generation of pollutants and waste streams. It is a versatile and inherently environmental-friendly technique, considered a “clean” and “green” process. Its advantages make it an interesting alternative for industrial applications. However, despite its potential, its implementation is limited. A major drawback is the lack of the necessary combined knowledge on organic synthesis, electrochemistry and engineering aspects. This work aims to bridge the gap between the engineering aspects and the fundamental electrochemical mechanistic insights of a reaction. One reaction that exhibits drawbacks to which electrosynthesis offers an answer, is the aldol condensation: an important reaction for the formation of carbon-carbon bonds. The often several possible reaction pathways between the various reactants and products of the aldol condensation make it difficult to promote the desired reaction. The homogeneous catalysts which are often used in industry require an intensive purification afterwards which often destroys the catalyst. Additionally the purification causes large waste streams. In this work, the potential-controlled electrochemical aldol reaction of acetone to diacetone alcohol is studied in a standard batch electrolysis set-up as well as in an electrochemical continuous flow microfluidic reactor. In the batch set-up the influence of a number of parameters such as electrolyte composition, electrode material and electrolysis potential is described. Starting with pure acetone and a quaternary ammonium salt as support electrolyte, a maximum concentration of 15 m% diacetone alcohol is obtained after 2 h electrolysis in a divided set-up with a platinum electrode at -2.5 V [1, 2]. These results are taken into account in the optimisation towards maximum space-time yield and diacetone alcohol selectivity in the electrochemical micro flow reactor. Employing a flow cell system has important benefits which allows for a more efficient reaction such as a high surface-to-volume ratio of the electrode area to the reaction volume and close proximity of the electrodes. In addition optimization of the flow rate and cell height allows high yields of product in continuous operation. Incorporating a membrane in the reactor prevents cross-over of products which is detrimental for the yield. [1] D. Pauwels, B. Geboes, J. Hereijgers, D. Choukroun, K. De Wael, T. Breugelmans, The application of an electrochemical microflow reactor for the electrosynthetic aldol reaction of acetone to diacetone alcohol, Chemical Engineering Research and Design, 128 (2017) 205-2013. [2] D. Pauwels, J. Hereijgers, K. Verhulst, K. De Wael, T. Breugelmans, Investigation of the electrosynthetic pathway of the aldol condensation of acetone, Chemical Engineering Journal, 289 (2016) 554-561. Figure 1: Exploded view of the electrochemical microreactor. 1: Electrode material 2: PEEK housing with hole for the electrode (2a: WE, 2b: CE), 3: POM spacer with serpentine reaction channel. 4: Aluminium parts with holes for the in- and outlet tubing, hole for the reference electrode (4a only) and holes for the electrode connections. Figure 1