Efficient C_2H_2/CO_2 Separation in Ultramicroporous Metal-Organic Frameworks with Record C_2H_2 Storage Density

摘要

Physical separation of C_2H_2 from CO_2 on metal-organic frameworks (MOFs) has received substantial research interest due to the advantages of simplicity, security, and energy efficiency. However, that C_2H_2 and CO_2 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 C_2H_2 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- Ni_2(L-asp)_2(bpy) (MOF-NH_2) and Ni_2(L-mal)_2(bpy) (MOF-OH)-we targeted efficient C_2H_2 uptake and C_2H_2/CO_2 separation, which outperforms most benchmark materials. Explicitly, MOF-OH adsorbs substantial amount of C_2H_2 with record storage density of 0.81 g mL~(-1) at ambient conditions, which even exceeds the solid density of C_2H_2 at 189 K. In addition, MOF-OH gave IAST selectivity of 25 toward equimolar mixture of C_2H_2/CO_2, which is nearly twice higher than that of MOF-NH_2. Notably, the adsorption enthalpies for C_2H_2 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-NH_2), which to our knowledge are the lowest among efficient rigid C_2H_2 sorbents. The efficiencies of both MOFs for the separation of C_2H_2/CO_2 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.