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/N-2 selectivity has been usually observed. To achieve both, we focused on reducing the amount of N-2 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 N-2 adsorption sites (not useful sites for CO2 adsorption), reducing N-2 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/N-2 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<middle dot>g(-1)) and large CO2/N-2 selectivity (> 100) at 100 kPa and 298 K.