Synthesis of metal-organic framework HKUST-1 via tunable continuous flow supercritical carbon dioxide reactor

Abstract

Scaling-up synthesis of metal–organic framework (MOF) materials is critical for their practical applications. Due to its unique properties, supercritical carbon dioxide (scCO2) is an attractive media single-step synthesis of MOFs. This paper, for the first time, presents a rapid synthesis of the high-quality copper-based MOF HKUST-1 in a continuous flow scCO2 flow reactor, evaluating the effects of pressure, scCO2 injection temperature, scCO2 mass fraction, and the reactor residence time on HKUST-1 properties over 23 experiments. The morphology, crystallinity, and porosity of MOF are characterized by scanning electron microscopy, X-ray diffraction, and physisorption analysis. Four sets of experiments showed excellent repeatability in particle size and specific surface area yielding high-quality HKUST-1 MOFs. The particle sizes ranged from 55 to 300 nm with an average of 135 nm, and the BET surface -- from 1,374 to 2,389 m2·g−1 with an average of 1,712 m2·g−1. Longer residence time increased particle size and led to MOF shape change from spherical to octahedral; however, these did not significantly correlate with MOF surface area. Pressure variation in the 8–10 MPa range did not impact HKUST-1 properties. Reactant temperature variations between 80 and 120 °C showed a weak correlation with particle size. Increasing reactor section temperature independently of the reactant temperature had a statistically significant negative correlation to particle size, r-value = -0.86, and p-value = 0.04. The scCO2-assisted synthesis yielded high-quality MOFs under all studied conditions with residence time in the range of 70–170 s, demonstrating the robustness and high throughput of the method.