Programming MIL-101Cr for selective and enhanced CO2 adsorption at low pressure by postsynthetic amine functionalization

Abstract

MIL-101Cr fully or partially (p) postsynthetically modified with nitro (–NO₂) or amino (–NH₂) groups was shown to be a robust, water stable, selective and enhanced carbon dioxide (CO₂) adsorption material with the amine-functionality. The highly microporous amine-modified frameworks (up to 1.6 cm³ g⁻¹ total pore volume) exhibit excellent thermal stability (>300 °C) with BET surface areas up to 2680 m² g⁻¹. At 1 bar (at 273 K) the gases CO₂, CH₄ and N₂ are adsorbed up to 22.2 wt%, 1.67 wt% and 2.27 wt%, respectively. The two amine-modified MIL-101Cr-NH₂ (4) and MIL-101Cr-pNH₂ (5) showed the highest gas uptake capacities in the series with high ratios for the CO₂ : N₂ and CO₂ : CH₄ selectivities (up to 119 : 1 and 75 : 1, respectively, at 273 K). Comparison with non-modified MIL-101Cr traces the favorable CO₂ adsorption properties of MIL-101Cr-NH₂ (4) and MIL-101Cr-pNH₂ (5) to the presence of the Lewis-basic amine groups. MIL-101Cr-NH₂ (4) has a high isosteric heat of adsorption of 43 kJ mol⁻¹ at zero surface coverage and also >23 kJ mol⁻¹ over the entire adsorption range, which is well above the heat of liquefaction of bulk CO₂. Large CO₂ uptake capacities of amine-functionalized 4 and 5, coupled with high adsorption enthalpy, high selectivities and proven long-term water stability, make them suitable candidates for capturing CO₂ at low pressure from gas mixtures including the use as a CO₂ sorbent from moist air.