Supercritical CO2 Assisted Aerosol Synthesis of HKUST-1 Nanoparticles in a Continuous Flow Reactor

Metal-organic Frameworks (MOFs) are very promising materials for gas sensing, catalysis, energy storage, water purification and drug delivery due to their high porosity, tunable pore sizes, adequate chemical and thermal stability, and various structures and compositions. Downsizing MOFs to the nanoscale brings superior properties over their bulk analogs, such as high surface-to-volume ratio, rich exposed metals and ligands, short diffusion path for reactants, which all contribute to improved performances. While great efforts have been made to reduce the particle sizes by controlling the reaction kinetics or terminating the particle growth with additives, large-scale synthesis of MOF nanoparticles with simple methods remains a challenge. Here, we report supercritical CO2 assisted synthesis of HKUST-1 nanoparticles in a continuous flow reactor. This method yields pure and thermal stable HKUST-1 nanoparticles with median sizes of 98-212 nm and BET surface area of 1613-1887 m²/g in the residence time of just 3 seconds without any additives. Supercritical CO2 and ethanol with a mole ratio of 9:1 are used as co-solvents for the fast precipitation of HKUST-1 nucleus and crystal growth. A typical dry yield of 53.7 wt% is achieved with 0.1 M copper precursor at 75 °C and 13 MPa. Size analysis of the products obtained at different copper concentrations shows the influence of supersaturation and fluid phase behavior to the nanoparticle formation. Fractal dimension analysis indicates that the growth is caused by aggregation of primary nanocrystals, indicating a non-classical crystal growth mechanism. The use of supercritical CO2 saves the use of organic solvents. In addition, super critical CO2 has physical properties such as natural abundance, non-flammability, and low toxicity, making this synthesis a green and sustainable method for the scalable production of MOF nanoparticles.