Substituent-Induced Electron-Transfer Strategy for Selective Adsorption of N2 in MIL-101(Cr)-X Metal-Organic Frameworks

N2 removal is of great significance in high-purity O-2 production and natural gas purification. Here, we present a substituent-induced electron-transfer strategy for improving N-2 capture performance by controlling the Lewis acidity of Cr(III) metal unsaturated sites in Cr-based metal-organic frameworks. With the enhancement of the electron-withdrawing ability of the modified group on terephthalic acid (-NO2 > -CH3), the N-2 adsorption ability of MIL-101(Cr)-X was improved significantly. For MIL-101(Cr)-NO2, the adsorption enthalpy of N-2 at zero coverage was 30.01 kJ/mol, which was much larger than that of MIL-101(Cr)-CH3 (14.31 kJ/mol). In situ infrared spectroscopy studies, Bader charges, and density functional theory calculations showed that the presence of -NO2 could enhance the Lewis acidity of Cr(III) metal unsaturated sites, which resulted in a strong interaction affinity for N-2. The adsorption isotherms indicated that MIL-101(Cr)-NO2 had an excellent N-2/O-2 (79/21, v/v) selectivity of up to 10.8 and a good N-2/CH4 separation performance (S-N2/CH4 = 2.8,298 K, 1 bar). Breakthrough curves showed that MIL-101(Cr)-NO2 had great potential for the efficient separation of N-2/O-2 and N-2/CH4.