Calix[4]arene@MIL-101 as host@MOF for cage-in-cage pore space partitioning for enhanced CO2 separation and catalysis†

Highly stable para-sulfonated calix[4]arene (SCA), a bowl-shaped macrocycle possessing intrinsic porosity, was incorporated into the spherical voids of the micro–mesoporous MIL-101(Cr) metal–organic framework by adsorptive loading from a solution. The pore-space partitioning in the MOF by polar functionalized macrocyclic molecules, which can also act as hosts, led to the host@MOF composite SCA@MIL-101 which demonstrated a high affinity to CO₂ without the involvement of alkaline amino functionalities. The SCA@MIL-101-w materials with w = 5, 10 and 30 wt% SCA showed high stability (including in aqueous medium, at least under non-basic conditions), and slow leaching kinetics due to the near match of the SCA size and the pore entrances of the MOF. Despite the lower surface area and pore volume for w = 30 wt% SCA in MIL-101 (S_BET = 1073 m² g⁻¹ and V_pore = 0.52 cm³ g⁻¹) vs. MIL-101 (2660 m² g⁻¹ and 1.0 cm³ g⁻¹), the pore-space partitioning approach allows the improvement of the CO₂ uptake capacity to 103 cm³ g⁻¹ for SCA@MIL-101-30 over MIL-101 with 66 cm³ g⁻¹ (273 K and 1 bar). This also increases the CO₂/N₂ selectivity, such that SCA@MIL-101-30 has a selectivity of 11 vs. 4 for MIL-101 for a 15 : 85 v : v CO₂/N₂ mixture at 293 K and 1 bar. Additionally, the SCA@MIL-101-30 composite showed good catalytic activity in the esterification of carboxylic acids, giving quantitative conversion on par with H₂SO₄ under the chosen conditions.