Tuning the Pore Environment of MOFs toward Efficient CH/N Separation under Humid Conditions.

Adsorption separation technology using adsorbents is promising as an alternative to the energy-demanding cryogenic distillation of natural gas (CH/N) separation. Although a few adsorbents, such as metal-organic frameworks (MOFs), with high performance for CH/N separation, have been reported, it is still challenging to target the desired adsorbents for the actual CH/N separation under humid conditions because the adsorption capacity and selectivity of the adsorbents might be mainly dampened by water vapor. Except for the high CH uptake and CH/N 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, DMOF-A, and DMOF-A). 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 CH uptake of 37 cm/g and a high CH/N adsorption selectivity of 7.2 for DMOF-A at 298 K and 1.0 bar. Moisture stability experiments show that DMOF-A is a robust MOF with low water vapor capacity even at ∼40% relative humidity (RH) because of the presence of more hydrophobic aromatic rings. Breakthrough experiments verify the excellent CH/N separation performances of DMOF-A under high humidity.