Tuning the Pore Environment of MOFs toward Efficient CH₄/N₂ Separation under Humid Conditions

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.

Thermodynamic−Kinetic Synergistic Separation of CH4/N2 on A Robust Aluminum-Based Metal−Organic Framework

“…Obviously, the CH 4 molecules prefers to stay in the pore formed between the two aromatic rings. A similar phenomenon has also been reported in the literature, 55 which has proved that CH 4 molecules and the aromatic rings of framework form C-H•••π interactions. For N 2 , the adsorption sites also stay at the middle of the pore, but the density was much lower than that of CH 4, as confirmed by the probability distributions of the center of mass.…”

Machine Learning-Enabled Framework for High-Throughput Screening of MOFs: Application in Radon/Indoor Air Separation

“…The second commonality of the ligands is the presence of aromatic ring structures in all the selected MOFs. It is elucidated that the electron delocalization characteristic of benzene rings may contribute to Rn adsorption because it enhances van der Waals forces , between Rn and MOFs via the increment of induced dipoles. The DFT calculation for Xe and Kr gases conducted by Zhang et al , determined that electrostatic potential at the location of aromatic ring sites is relatively high, making it easy for noble gases to generate dipole–dipole interactions, as shown in Figure .…”

Pore Environment Optimization of Microporous Metal–Organic Frameworks with Huddled Pyrazine Pillars for C2H2/CO2 Separation

“…Metal–organic frameworks (MOFs) have emerged as an attractive class of functional porous materials in the past 2 decades. − Due to their structural diversity and easy functionalization, plenty of MOFs with novel architectures and intriguing pore chemistry have been constructed and extensively explored as physisorbents for many important separation applications, , ranging from mature ones such as CO 2 /CH 4 (N 2 ) to more challenging C 2 H 6 /C 2 H 4 , C 2 H 2 /CO 2 , and C 3 H 8 /C 3 H 6 separations. − …”