Synthesis of hierarchical metal-organic frameworks in DMSO/CO2-gas expanded solvent systems

In this study, a porous material HKUST-1 was synthesised using supercritical CO2 (scCO2) with reduced amounts of conventional solvents. Our results show that with the presence of scCO2, HKUST-1 crystallites with hierarchical pore structure are formed. This material, with a mixture of large and small pore sizes, has potential applications in energy storage and catalysis. Scanning electron microscopy (SEM) and high resolution transmission electron microscopy (HRTEM) showed that scCO2-expanded synthesis had an effect on the porosity of the structure. Macroporous faceted crystallites are seen in the sample synthesised under scCO2 in 24 hours, compared to the perfectly smooth faces of the conventionally-synthesised HKUST-1. A higher proportion of macropores with some mesopores can be seen after 48-hour synthesis in scCO2. A finely divided microstructure with networks of macro- and mesopores is seen in the material after 72 hours. These findings suggest promising control over MOF formation via CO2 pressure for a variety of applications with reduced environmental impact.