Tubular electrochemical reactors for the biphasic oxidation of HMF to FDCA

Tubular reactors offer increased surface-to-volume ratios compared to conventional planar reactors. Yet, they are seldom utilized in electrochemical applications. This is primarily due to the challenges associated with membrane placement and the lack of concepts for cell-stacking and integrating mixer elements in such designs. This study introduces two innovative tubular reactor designs which address these limitations, while the biphasic electrooxidation of hydroxymethylfurfural (HMF) to 2,5-furandicarboxylic acid (FDCA) serves as case study: the Mixer Electrode Reactor (MER) and the Swiss-roll Reactor (SRR). The MER leverages a tubular 3D-printed stainless steel or nickel foam electrode to enhance mass transfer and active electrode area within the flow cell. The SRR, similar to spiral-wound membrane modules, employs a rolled-up assembly of nickel foam electrodes separated by polymeric spacers. In testing, the MER exhibited low FDCA yields ($<$60%) due to an inhomogeneous electric field caused by non-uniform electrode spacing. In contrast, the SRR maintained a uniform electrode distance, resulting in a homogeneous electric field significantly improving performance. The SRR achieved higher FDCA yields (up to 73% at 15 mA cm⁻²) and significantly increased space–time yields (437 mol_FDCA m⁻³ h⁻¹), surpassing both the MER and conventional planar reactor designs. This work highlights the potential of the SRR as an efficient and scalable tubular reactor, particularly for the integrated biphasic oxidation of HMF to FDCA.