Direct Evidence of CO₂ Capture under Low Partial Pressure on a Pillared Metal-Organic Framework with Improved...

Direct structural observation of CO₂ -loaded MOFs is helpful for revealing the specific binding interactions to allow the design of better CO₂ sorbents, but such direct structural evidence is almost always observed for pure-component CO₂ under a pressure of 1 atm or more, which does not really represent practical CO₂ capture and separation under low partial pressure (≤1 atm) in the presence of other gases. Herein, a series of isoreticular MOFs [Zn(Trz)(R-BDC)_1/2 ] (FJU-40-R, R=H, NH₂ , Br, or OH) are synthesized. Among them, FJU-40-NH₂ exhibits the highest robustness, and good heat and water resistance, attributed to its intramolecular hydrogen-bonding interactions. A CO₂ /N₂ (15:85, v/v) mixture can be separated efficiently through a column packed bed of FJU-40-NH₂ solid. The structures of CO₂ -loaded FJU-40-NH₂ at 1 atm under various atmosphere conditions, including pure CO₂ , CO₂ /N₂ (15:85, v/v), and air, are observed, and it is found that: 1) the mechanism for CO₂ loading into the cages depends on the CO₂ partial pressure; 2) FJU-40-NH₂ can capture CO₂ directly from air, and CO₂ will have priority to occupy hydrophobic cage-I, whereas hydrophilic cage-II containing the amino group is occupied by H₂ O molecules; 3) the triazolate C-H groups, rather than the amino groups in past observations in dry ice, act as predominant functional sites here under low CO₂ partial pressure.