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

Abstract

Highly stable para-sulfonated calix[4]arene (SCA), a bowl-shaped macrocycle possessing intrinsic porosity, was incorporated in 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 involvement of alkaline amino functionalities. The SCA@MIL-101-w materials with w = 5, 10 and 30 wt% of 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 (SBET = 1073 m2 g–1, Vpore = 0.52 cm3 g–1) vs MIL-101 (2660 m2 g–1, 1.0 cm3 g–1), the pore-space partitioning approach allows to improve the CO2 uptake capacity to 103 cm3 g–1 for SCA@MIL-101-30 over MIL-101 with 66 cm3 g–1 (293 K, 1 bar). Also, the CO2/N2 selectivity increases at low molar CO2 fraction, which is important for practical carbon-capture applications. For example, for a 25:75 molar CO2/N2 mixture SCA@MIL-101-30 has a selectivity of 23 vs 5.6 for MIL-101 at 273 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.