Controlled formation of hierarchical metal-organic frameworks using CO2 expanded solvent systems

It is shown that a crystalline metal-organic framework (HKUST-1) can be rapidly synthesised from a DMSO/MeOH solution with greatly reduced amounts of organic solvents using a supercritical CO2 (scCO2) solvent expansion technique. The precursor solution is stable for months under ambient conditions, and CO2-driven MOF crystallisation is achieved under mild conditions (40˚C, 40-100 bar) with excellent reproducibility. As the degree of liquid phase expansion drives MOF nucleation and growth, the crystallite size and overall yield can be tuned by adjusting the CO2 pressure. Furthermore, scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HRTEM) and gas sorption analyses showed that in the presence of scCO2 HKUST-1 crystallites with a hierarchical pore structure are generated through a post-crystallisation etching process. These findings demonstrate that scCO2 is a time and material efficient route to MOF synthesis with a high level of control over the crystallisation process for tailoring properties and performance.