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

Abstract

Physical\nseparation of C₂H₂ from CO₂ on metal–organic\nframeworks (MOFs) has received substantial\nresearch interest due to the advantages of simplicity, security, and\nenergy efficiency. However, that C₂H₂ and CO₂ exhibit very close physical properties makes their separation\nexceptionally challenging. Previous work appeared to mostly focused\non introducing open metal sites that aims to enhance the C₂H₂ affinity at desired sites, whereas the reticular manipulation\nof organic components has rarely been investigated. In this work,\nby reticulating preselected amino and hydroxy functionalities into\nisostructural 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,\nwhich outperforms most benchmark materials. Explicitly, <b>MOF-OH</b> adsorbs substantial amount of C₂H₂ with record\nstorage density of 0.81 g mL^–1 at ambient conditions,\nwhich even exceeds the solid density of C₂H₂ at 189 K. In addition, <b>MOF-OH</b> gave IAST selectivity\nof 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\nC₂H₂ at zero converge in both MOFs are remarkably\nlow (17.5 kJ mol^–1 for <b>MOF-OH</b> and 16.7\nkJ mol^–1 for <b>MOF-NH</b>_<b>2</b>), which to our knowledge are the lowest among efficient rigid\nC₂H₂ sorbents. The efficiencies of both MOFs\nfor the separation of C₂H₂/CO₂ are\nvalidated by multicycle breakthrough experiments. DFT calculations\nprovide molecular-level insight over the adsorption/separation mechanism.\nMoreover, <b>MOF-OH</b> can survive in boiling water for at\nleast 1 week and can be easily scaled up to kilograms eco-friendly\nand economically, which is very crucial for potential industrial implementation.