Synthesis, characterization, and selective gas adsorption performance of hybrid NH2-MIL-101(Fe)/ZIF-8 metal organic framework (MOF)

In this paper, an efficient NH2-MIL-101(Fe)/ZIF-8 metal organic framework (MOF) with tunable pore architecture was synthesized by an internal extended growth approach, using polyvinylpyrrolidone (PVP) as a regulator. In the resultant structure, the larger-pore MOF (NH2-MIL-101(Fe)) provides the facilitated transport pathways and the smaller-pore MOF (ZIF-8) acts as the shell, enhancing the molecular sieving properties. The synthesized MOFs were characterized by FESEM, EDS, HRTEM, FTIR, XRD, TGA, and BET analyses, and the hybrid structure formation was accordingly approved. Furthermore, the pure gas adsorption measurements were conducted for CO2, N2, CH4, C2H6, and C2H4 gases at 298 and 328 K and equilibrium pressures up to 4 bar. The CO2 adsorption capacity of 3.982 mmol g−1 at 298 K for NH2-MIL-101(Fe)/ZIF-8 was obtained, which is ∼43% and ∼127% higher than the values for the individual MOFs, respectively. This increase is mainly attributed to the favorable CO2 interactions with the unpaired electrons of nitrogen (amine functional groups and imidazole nitrogen) as well as oxygen atoms along with unsaturated metal sites present in the both MOFs. In addition, the beneficial size exclusion effect of the shell MOF, resulted in improved gas adsorption selectivity values. The synthesized hybrid structure displayed considerably higher CO2/N2 and CO2/CH4 adsorption selectivities, which were ∼3.8 and 1.7 times greater than those of the pristine NH2-MIL-101(Fe) MOF, respectively. Furthermore, the C2H6/C2H4 selectivity of NH2-MIL-101(Fe)/ZIF-8 was raised by ∼48%, compared to the NH2-MIL-101(Fe) MOF, owning to the molecular sieving effect of ZIF-8 for C2H6 with larger kinetic diameter compared to C2H4.