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

Abstract

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) (<b>MOF-NH</b>_<b>2</b>) and Ni₂(l-mal)₂(bpy) (<b>MOF-OH</b>)-we targeted efficient C₂H₂ uptake and C₂H₂/CO₂ separation, which outperforms most benchmark materials. Explicitly, <b>MOF-OH</b> 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, <b>MOF-OH</b> gave IAST selectivity of 25 toward equimolar mixture of C₂H₂/CO₂, which is nearly twice higher than that of <b>MOF-NH</b>_<b>2</b>. Notably, the adsorption enthalpies for C₂H₂ at zero converge in both MOFs are remarkably low (17.5 kJ mol^-1 for <b>MOF-OH</b> and 16.7 kJ mol^-1 for <b>MOF-NH</b>_<b>2</b>), 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, <b>MOF-OH</b> 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.