Inducing hierarchical pores in nano-MOFs for efficient gas separation

The synthesis of metal-organic frameworks (MOFs) and their processing into structures with tailored hierarchical porosity is essential for using MOFs in the adsorption-driven gas separation process. We report the synthesis of modified Cu-MOF nanocrystals for CO₂ separation from CH₄ and N₂, prepared from DABCO (1,4-diazabicyclo[2.2.2] octane) and 9,10 anthracene dicarboxylic acid linkers with copper metal salt. The synthesis parameters were optimized to introduce mesoporosity in the microporous Cu-MOF crystals. The volumetric CO₂ adsorption capacity of the new hierarchical Cu-MOF was 2.58 mmol g⁻¹ at 293 K and 100 kPa with a low isosteric heat of adsorption of 28 kJ mol⁻¹. The hierarchical Cu-MOF nanocrystals were structured into mechanically stable pellets with a diametral compression strength exceeding 1.2 MPa using polyvinyl alcohol (PVA) as a binder. The CO₂ breakthrough curves were measured from a binary CO₂-CH₄ (45/55 vol%) gas mixture at 293 K and 400 kPa pressure on Cu-MOF pellets to demonstrate the role of hierarchical porosity in mass transfer kinetics during adsorption. The structured hierarchical Cu-MOF pellets showed stable cyclic CO₂ adsorption capacity during 5 adsorption-desorption cycles with a CO₂ uptake capacity of 3.1 mmol g⁻¹ at 400 kPa and showed a high mass transfer coefficient of 1.8 m s⁻¹ as compared to the benchmark zeolite NaX commercialized binderless granules, suggesting that the introduction of hierarchical porosity in Cu-MOF pellets can effectively reduce the time for CO₂ separation cycles.