HKUST-1 MOF nanoparticles: a non-classical crystallization route in supercritical CO₂

Reducing MOF particles to the nanoscale size range is beneficial due to their increased surface-to-volume ratio, higher defects exposing metals and ligands, and short diffusion path. While great efforts have been made to reduce the particle sizes by controlling the reaction kinetics or terminating the particle growth, large-scale, rapid synthesis of MOF nanoparticles (NPs) remains a challenge. Here, we report supercritical (sc) CO₂-assisted synthesis of HKUST-1 NPs in a continuous flow reactor, which yielded pure and thermally stable MOFs with median sizes of 110-250 nm and BET surface areas of 1610-1890 m² g^-1 under 10 seconds synthesis time. ScCO₂ and ethanol with a molar ratio of 9 : 1 are used as co-solvents for the fast nucleation of HKUST-1 and crystal formation. A typical dry yield of 53.7 wt% is achieved with 0.1 M Cu precursor under mild conditions at 75 °C and 13 MPa. The space-time yields and surface area production rates are 5668 kg m^-3 d^-1 and 1.0 × 10¹⁰ m² m^-3 d^-1. Particle size and morphology analyses indicate that aggregation of nascent structures occurs in the aerosolized state, leading to a non-classical crystal growth mechanism and enabling multiple pathways for tuning the synthesis process. With the ability to recycle CO₂, solvents, and unreacted precursors, the method can be used for the scalable production of MOFs.