Tuning the Pore Environment of MOFs toward Efficient CH<sub>4</sub>/N<sub>2</sub> Separation under Humid Conditions
Tong Li, Xiaoxia Jia, Hui Chen, Zeyu Chang, Libo Li, Yong Wang, Jinping Li
Abstract
Adsorption separation technology using adsorbents is promising as an alternative to the energy-demanding cryogenic distillation of natural gas (CH4/N2) separation. Although a few adsorbents, such as metal–organic frameworks (MOFs), with high performance for CH4/N2 separation, have been reported, it is still challenging to target the desired adsorbents for the actual CH4/N2 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/N2 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-A1, DMOF-A2, and DMOF-A3). 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 cm3/g and a high CH4/N2 adsorption selectivity of 7.2 for DMOF-A2 at 298 K and 1.0 bar. Moisture stability experiments show that DMOF-A2 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 CH4/N2 separation performances of DMOF-A2 under high humidity.