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 O2 production and natural gas purification. Here, we present a substituent-induced electron-transfer strategy for improving N2 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 N2 adsorption ability of MIL-101(Cr)-X was improved significantly. For MIL-101(Cr)–NO2, the adsorption enthalpy of N2 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 N2. The adsorption isotherms indicated that MIL-101(Cr)–NO2 had an excellent N2/O2 (79/21, v/v) selectivity of up to 10.8 and a good N2/CH4 separation performance (SN2/CH4 = 2.8, 298 K, 1 bar). Breakthrough curves showed that MIL-101(Cr)–NO2 had great potential for the efficient separation of N2/O2 and N2/CH4.