Insights into the Ultra-High Volumetric Capacity in a Robust Metal–Organic Framework for Efficient C₂H₂/CO₂ Separation

The efficient separation of C2H2/CO2 is a challenging problem due to their similar physical properties. This is in addition to the risk of explosion when pressuring C2H2 over 2.07 bar. Besides, understanding the framework–guest and guest–guest interactions is also critical to achieving an optimal separation. Herein, we report the development of a robust and low-cost adsorbent, MIL-160(Al), with an extremely high acetylene volumetric capacity of 227 cm3(STP)/cm3 and high C2H2/CO2 selectivity of 7.1 under ambient conditions. We combine in situ solid-state nuclear magnetic resonance spectroscopy and in-depth theoretical calculations to unravel the synergistic interactions, driven by the hydrogen bonding of C2H2-MOF, and the electrostatic interactions between C2H2 molecules in the confined ångström-scale pores. This suitable pore confinement is also reflected in the rotation of the MOF ligands and the resultant evolution in the interacting distances between the metal–organic framework (MOF) backbone and C2H2 molecules. Acetylene is adsorbed as a unique dimer in MIL-160(Al), leading to an extremely efficient packing inside the pores. Breakthrough experiments confirm the efficient performance of MIL-160(Al) in real C2H2/CO2 mixtures, showing a C2H2-volumetric breakthrough performance superior to any previous MOF.