Ultramicroporous Building Units as a Path to Bi-microporous Metal-Organic Frameworks with High Acetylene Storage and...

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 C₂ H₂ uptake capacity at 1 atm reaches 193.0 cm³  g^-1 (8.6 mmol g^-1 ) at 273 K and 134.0 cm³  g^-1 (6.0 mmol g^-1 ) at 298 K. Such high uptake capacity is accompanied by a high C₂ H₂ /CO₂ selectivity of up to 8.5 at 298 K. Dynamic breakthrough studies at room temperature and 1 atm show a C₂ H₂ /CO₂ breakthrough time up to 79 min g^-1 , among top-performing MOFs. Grand canonical Monte Carlo simulations agree that ultrahigh C₂ H₂ /CO₂ selectivity is mainly from UBU ultramicropores, while packing pores promote C₂ H₂ uptake capacity.