Efficient C2H2/CO2 Separation in Ultramicroporous Metal–Organic Frameworks with Record C2H2 Storage Density

Physical separation of C2H2 from CO2 on metal–organic frameworks (MOFs) has received substantial research interest due to the advantages of simplicity, security, and energy efficiency. However, that C2H2 and CO2 exhibit very close physical properties makes their separation exceptionally challenging. Previous work appeared to mostly focused on introducing open metal sites that aims to enhance the C2H2 affinity at desired sites, whereas the reticular manipulation of organic components has rarely been investigated. In this work, by reticulating preselected amino and hydroxy functionalities into isostructural ultramicroporous chiral MOFs—Ni2(l-asp)2(bpy) (MOF-NH2) and Ni2(l-mal)2(bpy) (MOF-OH)—we targeted efficient C2H2 uptake and C2H2/CO2 separation, which outperforms most benchmark materials. Explicitly, MOF-OH adsorbs substantial amount of C2H2 with record storage density of 0.81 g mL–1 at ambient conditions, which even exceeds the solid density of C2H2 at 189 K. In addition, MOF-OH gave IAST selectivity of 25 toward equimolar mixture of C2H2/CO2, which is nearly twice higher than that of MOF-NH2. Notably, the adsorption enthalpies for C2H2 at zero converge in both MOFs are remarkably low (17.5 kJ mol–1 for MOF-OH and 16.7 kJ mol–1 for MOF-NH2), which to our knowledge are the lowest among efficient rigid C2H2 sorbents. The efficiencies of both MOFs for the separation of C2H2/CO2 are validated by multicycle breakthrough experiments. DFT calculations provide molecular-level insight over the adsorption/separation mechanism. Moreover, MOF-OH can survive in boiling water for at least 1 week and can be easily scaled up to kilograms eco-friendly and economically, which is very crucial for potential industrial implementation.