Tuning the Pore Environment of MOFs toward Efficient CH4/N-2 Separation under Humid Conditions

Adsorption separation technology using adsorbents is promising as an alternative to the energy-demanding cryogenic distillation of natural gas (CH4/N-2) separation. Although a few adsorbents, such as metal-organic frameworks (MOFs), with high performance for CH4/N-2 separation, have been reported, it is still challenging to target the desired adsorbents for the actual CH4/N-2 separation under humid conditions because the adsorption capacity and selectivity of the adsorbents might be mainly dampened by water vapor. Except for the high CH4 uptake and CH4/N-2 selectivity, the adsorption material should simultaneously have excellent stability against moisture and relatively low-water absorption affinity. Here, we tuned the ligands and metal sites of reticular MOFs, Zn-benzene-1,4-dicarboxylic acid-1,4-diazabicyclo[2.2.2]octane (Zn-BDC-DABCO) (DMOF), affording a series of isostructural MOFs (DMOF-N, DMOF-A(1), DMOF-A(2), and DMOF-A(3)). Because of the finely engineered pore size and introduced aromatic rings in the functional DMOF, gas sorption results reveal that the materials show improved performance with a benchmark CH4 uptake of 37 cm(3)/g and a high CH4/N-2 adsorption selectivity of 7.2 for DMOF-A(2) at 298 K and 1.0 bar. Moisture stability experiments show that DMOF-A(2) is a robust MOF with low water vapor capacity even at similar to 40% relative humidity (RH) because of the presence of more hydrophobic aromatic rings. Breakthrough experiments verify the excellent CH4/N-2 separation performances of DMOF-A(2) under high humidity.