Obtaining both high CO₂ adsorption capacity and CO₂/N₂ selectivity via blocking N₂ adsorption sites by grafting bulky diamine on metal–organic framework

Functionalizing metal–organic frameworks (MOFs) with alkyl amines has been employed in various studies for CO₂ adsorption because of their high efficiency in CO₂ capture. However, a trade-off between the high CO₂ adsorption and high CO₂/N₂ selectivity has been usually observed. To achieve both, we focused on reducing the amount of N₂ adsorption while maintaining the CO₂ adsorption capacity of alkyl amines. In this study, a bulky diamine was grafted onto coordinatively unsaturated sites of Cr-based MOFs (MIL-101) instead of a non-bulky (normal or linear) diamine of the same length (from amine to amine). The bulky diamine blocked the N₂ adsorption sites (not useful sites for CO₂ adsorption), reducing N₂ adsorption more than the non-bulky diamine. On the other hand, the CO₂ adsorption amount remained unchanged, especially under low pressure (< 20 kPa). This could be explained by the preserved active sites for CO₂ due to the similar quantity of the loaded alkyl amines. Consequently, MIL-101 functionalized with a bulky diamine showed approximately 3 times the CO₂/N₂ selectivity (based on ideal adsorbed solution theory, at 100 kPa) compared to a non-bulky diamine. Moreover, compared with previously reported MOFs (modified with alkyl-amines), MIL-101 functionalized with a bulky diamine (m-xylylenediamine) demonstrated high competitiveness, achieving both high CO₂ adsorption capacity (> 1 mmol·g⁻¹) and large CO₂/N₂ selectivity (> 100) at 100 kPa and 298 K.