Ultramicroporous Building Units as a Path to Bi‐microporous Metal–Organic Frameworks with High Acetylene Storage and Separation Performance

A strategy called ultramicroporous building unit (UBU) is introduced. It allows the creation of hierarchical bi‐porous features that work in tandem to enhance gas uptake capacity and separation. Smaller pores from UBUs promote selectivity, while larger inter‐UBU packing pores increase uptake capacity. The effectiveness of this UBU strategy is shown with a cobalt MOF (denoted SNNU‐45) in which octahedral cages with 4.5 Å pore size serve as UBUs. The C 2 H 2 uptake capacity at 1 atm reaches 193.0 cm 3 g −1 (8.6 mmol g −1 ) at 273 K and 134.0 cm 3 g −1 (6.0 mmol g −1 ) at 298 K. Such high uptake capacity is accompanied by a high C 2 H 2 /CO 2 selectivity of up to 8.5 at 298 K. Dynamic breakthrough studies at room temperature and 1 atm show a C 2 H 2 /CO 2 breakthrough time up to 79 min g −1 , among top‐performing MOFs. Grand canonical Monte Carlo simulations agree that ultrahigh C 2 H 2 /CO 2 selectivity is mainly from UBU ultramicropores, while packing pores promote C 2 H 2 uptake capacity.