Synthesis, characterization, and gas adsorption performance of an efficient hierarchical ZIF-11@ZIF-8 core–shell metal–organic framework (MOF)

Abstract

In this paper, the solvent-assisted linker exchange (SALE) technique was successfully applied for the synthesis of porous ZIF-11@ZIF-8 core–shell composite structure metal–organic framework (MOF). The RHO-type ZIF-11 owing to larger cavities along with higher freedom in linker rotation, acts as the core and contributes in adsorption capacity enhancement. On the other hand, the SOD-type ZIF-8 provides greater molecular sieving features due to its lower degree of linker flexibility and thereby results in an improvement of the adsorption selectivity. The morphology and structure of the fabricated MOFs were characterized by XRD, FTIR, FESEM, EDS, TEM, BET, and TGA analyses and formation of the core–shell structure was confirmed. The BET surface area and micropore volume of the synthesized ZIF-11@ZIF-8 MOF were obtained as high as 1023.4 m²/g and 0.435 cm³ g⁻¹, respectively. Gas adsorption measurements were carried out for CO₂, N₂, CH₄, C₂H₆, and C₂H₄ gases at 298 and 328 K and equilibrium pressures up to 4 bar. The results revealed a remarkable rise (∼100 %) in CO₂ adsorption capacity of ZIF-11@ZIF-8 nanoparticles (8.21 mmol g⁻¹), compared to the pristine ZIF-11 (4.35 mmol g⁻¹) at 298 K. The CO₂/N₂ and CO₂/CH₄ adsorption selectivity values were also augmented by 131 % and 92 %, respectively. In addition, the fabricated core–shell MOF, exhibited greater ethane (C₂H₆) adsorption capacity by ∼ 65 %, along with ethane to ethylene (C₂H₄) adsorption selectivity enhancement by ∼ 50 %. The higher adsorption capacity values without sacrificing adsorption selectivity suggests that the controlled core–shell MOF structures would be promising candidates for effective separation of CO₂ from CH₄ and N₂ as well as C₂H₆ from C₂H₄.