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