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

Abstract

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 (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 N₂/O₂ and N₂/CH₄.

Keywords: Lewis acidity of Cr(III) site; N2 removal; adsorption separation; linker substituent; metal−organic framework.