Paired Electrosynthesis 2,5-Furandicarboxylic Acid (FDCA) and 2-Butanone in a Flow-Cell

Abstract

Biobased platform chemicals and energy carriers can replace conventional, fossil-based chemicals and fuels to decrease the footprint of the chemical industry in the future. Promising chemicals for this purpose are 2,5-furandicarboxylic acid (FDCA), a monomer for biobased polymers, and 2-butanone, a solvent and energy carrier. Both molecules can be synthesized in electrochemical processes: FDCA is a product of the oxidation of Hydroxymethylfurfural (HMF) and 2-butanone is a product of the reduction of acetoin. In an electrochemical reactor, the oxidation and the reduction reaction can be coupled, so that the anode and the cathode yield a value-added product. The increased efficiency is accompanied by an increased complexity. A crucial aspect is the migration of ions between anolyte and catholyte, which impacts the pH in both electrolyte compartments thereby influences the reactions. The direction of this migration is determined by the type of ion exchange membrane, the effect of which is much less pronounced when analyzing a single reaction.This work develops a coupled process for these reactions. The individual reactions have been demonstrated prior [1,2]. The oxidation takes place on an Ni-foam electrode with a Ni(OH)2/NiOOH catalyst in 0.1 mol/L KOH in a zero-gap assembly. The reduction is carried out on a lead plate electrode in a potassium phosphate buffer at pH 9 with an electrolyte gap of 3 mm. We analyze the impact of the three different ion exchange membranes on the migration of ions and the pH in the electrolyte compartments, using anion exchange membranes (AEM), cation exchange membranes (CEM), and bipolar membranes (BPM) in a flow-cell reactor (flex-E-cell). The electrode area was 25cm². No stable operation was possible with a CEM due to the acidic pH at the interface of the CEM and the anode, which deteriorated the nickel-based catalyst. With an AEM, a stable process was made possible through pH control in the electrolytes to counteract ion migration across the membrane. With a BPM, we showed the most stable process with little to no ion permeation. However, the cell-voltage was highest for the BPM.At an increased current density to 120 mA/cm2 and a reactant concentration to 0.45 mol/L of HMF and 0.9 mol/L acetoin, respectively, we were able to maintain a high yield, for FDCA on the anode side above 90 % and for 2-butanone above 60 % on the cathode side with a BPM. The combined faraday efficiency (FE) was above 150 %. The increased product concentration of FDCA required the stabilization of the alkaline pH through the redosing of base. With an AEM however, no stable process was possible at the elevated current densities due to the extensive crossover of anions, which resulted in drastically changing pH and conductivity. To increase the economic prospect of the process, we employed a background electrolyte on the cathode side (0.5 M Na2SO4) to decrease the cell voltage by 15 % and employed NaOH instead of KOH on the anode side without significantly affecting the reaction metrices. The elevated current densities in combination with the economically interesting electrolyte system moves the process towards economic competitiveness, which we will assess in the future. With this work, we hope to establish a paired electrolysis process for the efficient and economic electrochemical valorization of biomass.