ZIF-8@Zn-MOF-74 core–shell metal–organic framework (MOF) with open metal sites: Synthesis, characterization, and gas adsorption performance

Abstract

In this paper, an efficient hierarchical metal–organic framework (MOF), namely ZIF-8@Zn-MOF-74 with core–shell structure and coordinately open/unsaturated metal sites (OMS) was successfully synthesized. For this purpose, the facile post-synthetic ligand exchange (PSE) method at ambient temperature was applied. In the fabricated core–shell porous structure, the SOD-type ZIF-8 acts as the core, providing molecular sieving features, resulting in greater adsorption selectivity values. On the other hand, Zn-MOF-74 as the shell part, owing to the larger pore size, makes entrance and diffusion of the guest gas molecules more convenient. In addition, due to the presence of OMS in the Zn-MOF-74 structure, a greater adsorption capacity would be achieved. The prepared MOFs were characterized by FESEM, EDS, HRTEM, FTIR, XRD, BET, and TGA analyses, and formation of the core–shell structure was confirmed. The FESEM and HRTEM images demonstrated rather smooth coverage of the core (ZIF-8) with a layer of the shell (Zn-MOF-74). The rather high BET surface area (633.2 m2/g) and total pore volume (0.671 cm3 g−1) of the core–shell structure, make it promising for gas adsorption applications. The gas adsorption measurements were conducted for CO2, N2, H2, O2, CH4, C2H6, C3H8, C2H4, and C3H6 pure gases at 308 K and equilibrium pressures up to 4 bar. It was found that the CO2 adsorption capacity of ZIF-8@Zn-MOF-74 nanoparticles (3.27 mmol g−1) has enhanced ∼63.5 % compared to the pristine ZIF-8 (∼2 mmol/g). The CO2/N2 and CO2/CH4 selectivity values were also increased by 85.4 % and 74.4 %, respectively, that indicates overcoming the trade-off between adsorption capacity and selectivity.