(631f) Optimized Activation of MIL-100Cr for Highly Efficient CO2 Capture and N2O Removal

As fantastic candidates as sorbents for gas-mixture adsorption-separation, the open metal sites of metal organic frameworks (MOFs) have attracted a substantial amount of attention because of their high binding energy of a particular component. Typically, MOFs were activated under gentle conditions of comparatively low temperature and vacuum to sustain the framework structures. In certain cases, for example, MIL-100/MIL-101 series, such gentle activation might be not able to remove all the solvent molecules, particularly those strongly bound terminal solvent molecules with metal sites, so their adsorption performance might still have not been optimized as well. Chang et al. found that MIL-100Cr can be regenerated after being activated under a higher vacuum 1 ´ 10^-8 bar and higher temperature 523 K, exhibiting excellent performance for N₂ capture from CH₄ and O₂, and thus for N₂/CH₄ and N₂/O₂ adsorption separation (Nat. Mater., 2016, 16, 526.). In our work, the open Cr³⁺ sites on MIL-100Cr were exposed at a high temperature (573 K) and high vacuum of 1 ´ 10^-10 bar and studied the N₂O and CO₂ capture properties. Herein, we reported MIL-100Cr with open Cr³⁺ sites for a record N₂O capture capacities of 5.78 mmol/g at 298 K, and a high adsorption capacity of 6.09 mol/g at 298 K for CO_2, 2.5 times higher than that of treated under mild activation conditions (423 K). The DFT calculations showed that the static binding energy of N₂O on the open-Cr site is notably higher than that of N₂, 72.5 kJ/mol vs. 51.6 kJ/mol, which enforces MIL-100Cr to exhibit extremely high N₂O/N₂ gas separation selectivity (IAST) up to 1000. The CO₂-Cr³⁺ binding energy was up 63 kJ/mol, the results of selectivity for CO₂/N₂ showed a 100-fold increase was observed after optimized activation.