Droplet-based millifluidic synthesis of a proton-conducting sulfonate metal–organic framework

Metal–organic frameworks (MOFs) have emerged as promising candidate materials for proton exchange membranes (PEMs), due to the control of proton transport enabled by functional groups and the structural order within the MOFs. In this work, we report a millifluidic approach for the synthesis of a MOF incorporating both sulfonate and amine groups, termed Cu-SAT, which exhibits a high proton conductivity. The fouling-free multiphase flow reactor synthesis was operated for more than 5 h with no reduction in yield or change in the particle size distribution, demonstrating a sustained space–time yield up to 131.7 kg m−3 day−1 with consistent particle quality. Reaction yield and particle size were controllably tuned by the adjustment of reaction parameters, such as residence/reaction time, temperature, and reagent concentration. The reaction yields from the flow reactor were 10–20% higher than those of corresponding batch syntheses, indicating improved mass and heat transfer in flow. A systematic exploration of synthetic parameters using a factorial design of experiments approach revealed the key correlations between the process parameters and yields and particle size distributions. The proton conductivity of the synthesized Cu-SAT MOF was evaluated in a mixed matrix membrane model PEM with polyvinylpyrrolidone and polyvinylidene fluoride polymers, exhibiting a promising composite conductivity of 1.34 ± 0.05 mS cm−1 at 353 K and 95% relative humidity (RH).