Improved N ₂O Capture Performance of Chromium Terephthalate MIL-101 Via Substituent Engineering

Abstract

The capture of N₂O from CO₂ by an adsorption-based mechanism remains a significant challenge due to their similar molecular sizes and physical properties. Although substituent engineering has been widely employed for the selectivity regulation and improvement of gas mixtures in metal-organic frameworks, its effect on N₂O/CO₂ separation has yet not been explored. Herein, four isoreticular MTN-type metal-organic frameworks (MOFs) with different substituents (-H, -NH₂, -Br or -NO₂) are reported for the capture of N₂O from a N₂O/CO₂ mixture. As revealed by isothermal measurements, thermodynamic studies and ideal adsorbed solution theory computations, the N₂O/CO₂ separation potential can be effectively improved by an electron-donating group (-NH₂), whereas electron-withdrawing groups (-NO₂ or -Br) exhibit the opposite effect. Notably, MIL-101(Cr)-NH₂ exhibits, by far, the highest ideal adsorbed solution theory equilibrium adsorption selectivity of 1.91 for 50/50 N₂O/CO₂. The N₂O/N₂ separation ability of the modified materials is also studied and functionalization with a -Br group yields an improved separation potential compared to the parent MIL-101(Cr) material. The strategy of using substituents to enhance the N₂O/CO₂ separation efficiency of MOFs could also be extended to other porous materials for chemical separation processes.

Keywords: Metal-organic frameworks, Substituents, N2O/CO2 separation, Electron-donating groups