Amine and fluorine co-functionalized MIL-101(Cr) synthesized via a mixed-ligand strategy for CO2 capture under humid conditions

Abstract

The selective capture of CO2 in humid conditions on metal–organic frameworks is challenging due to competitive adsorption of H2O and CO2 on the active sites. Herein, amine and fluorine co-functionalized MIL-101(Cr) was synthesized and evaluated for use as CO2 adsorbent to overcome this issue. MIL-101(Cr)–NH2-F0.5 (CrA-F0.5) was prepared using a mixed-ligand strategy, and contains 2.9% nitrogen and 0.5% fluorine while maintaining the MIL-101(Cr) structure. Numerically, one 4F ligand per 20 amino ligands was included in the CrA-F0.5 structure. The calculated selectivity for CO2/N2 of CrA-F0.5 is 108 (CO2/N2 = 15/85) at 20 ℃ and 1 bar, and this value was 3.7 and 2.1 times higher than that of MIL-101(Cr) and MIL-101(Cr)–NH2, respectively. The results of CrA-F0.5 adsorption of CO2 indicated that amine and fluorine groups served as adsorptive sites for CO2 in this structure. The moderate isosteric heat of CO2 adsorption value (45 kJ mol−1) of CrA-F0.5 merits in desorption of CO2. The result of a hydrophobicity index test, indicated that CrA-F0.5 was 2.5 times hydrophobic than MIL-101(Cr)–NH2. In a CO2 and N2 breakthrough test, compared to the dry condition, the CO2 breakthrough time at 30 ℃ and 1 bar was reduced by 40% for MIL-101(Cr)–NH2 and by 10% for CrA-F0.5 in 60% relative humidity. The CO2 and N2 breakthrough results under humid conditions were consistent with the hydrophobicity index value, and the excellent water repellency of CrA-F0.5 was confirmed. CrA-F0.5 is considered a candidate suitable for CO2 adsorbent in an actual post-combustion CO2 adsorption process.