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 synthesized 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 (metal-organic framework) crystallization 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 (HR-TEM), and gas sorption analyses showed that, in the presence of scCO₂, HKUST-1 crystallites with a hierarchical pore structure are generated through a postcrystallization 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 crystallization process for accessing tailored material properties.