Direct Evidence of CO2 Capture under Low Partial Pressure on a Pillared Metal-Organic Framework with Improved Stabilization through Intramolecular Hydrogen Bonding

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/N-2 (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/N-2 (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) FJU40-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 H2O 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.