Electrochemical vanillin reduction in a plane parallel flow reactor: Characterization, modeling and process improvement

The application of electrochemical flow reactors for electrosynthetic processes on a laboratory or pilot scale has recently gained increased attention as they provide reliable and improved performance and facilitate a further scale-up. We investigated in this study the electrochemical reduction of vanillin (4-hydroxy-3-methoxybenzaldehyde) to its pinacolization product hydrovanilloin, a building block in the synthesis of biobased polymers, in a divided plane parallel flow reactor in batch recycle operation mode. First, the mass transport of the flow reactor was characterized with dimensionless groups using the ferricyanide ion / ferrocyanide ion system as model reaction. The mass transport was further improved by 3D-printed turbulence promoters. We then studied the electrochemical vanillin reduction monitored by an online UV-cell setup at various electrolyte flow rates up to 20 cm s−1 and current densities up to 50 mA cm−2 with total conversions of 95%. Key figures of merit such as Faradaic efficiencies, space-time-yields and specific energy consumptions were calculated and a simple model was used to describe vanillin concentration decays and courses of these key figures of merit. The model distinguishes between a kinetic and mass transport-controlled zone, which transition point was predicted based on the mass transport characterization results. Lastly, we developed based on the model an operational process improvement strategy for fast and energy efficient production of hydrovanilloin and validated it experimentally. This study reports a first engineering approach for the production of vanillin-based polymer building blocks in an electrochemical flow reactor highlighting the ability of simple prediction models.