Cucurbit[6]uril@MIL-101-Cl: loading polar porous cages in mesoporous stable host for enhanced SO ₂ adsorption at low pressures

The robust cucurbituril-MOF composite CB6@MIL-101-Cl was synthesized by a wet impregnation method and a concomitant OH-to-Cl ligand exchange {CB6 = cucurbit[6]uril, 31 wt% content in the composite, MIL-101-Cl = [Cr₃(O)Cl(H₂O)₂(BDC)₃], BDC = benzene-1,4-dicarboxylate}. MIL-101-Cl was formed postsynthetically from standard fluorine-free MIL-101 where Cr-OH ligands were substituted by Cl during treatment with HCl. CB6@MIL-101-Cl combines the strong SO₂ affinity of the rigid CB6 macrocycles and the high SO₂ uptake capacity of MIL-101, and shows a high SO₂ uptake of 438 cm³ g^-1 (19.5 mmol g^-1) at 1 bar and 293 K (380 cm³ g^-1, 17.0 mmol g^-1 at 1 bar and 298 K). The captured SO₂ amount is 2.2 mmol g^-1 for CB6@MIL-101-Cl at 0.01 bar and 293 K (2.0 mmol g^-1 at 298 K), which is three times higher than that of the parent MIL-101 (0.7 mmol g^-1) under the same conditions. The near zero-coverage SO₂ adsorption enthalpies of MIL-101 and CB6@MIL-101-Cl are -35 kJ mol^-1 and -50 kJ mol^-1, respectively, reflecting the impact of the incorporated CB6 macrocycles, having higher affinity towards SO₂. FT-IR spectroscopy confirms the interactions of the SO₂ with the cucurbit[6]uril moieties of the CB6@MIL-101-Cl composite and SO₂ retention for a few minutes under ambient air. Comparative experiments demonstrated loss of crystallinity and porosity after dry SO₂ adsorption for MIL-101, while CB6@MIL-101-Cl exhibits nearly complete retention of crystallinity and porosity under the exposure to both dry and wet SO₂. Thus, CB6@MIL-101-Cl can be an attractive adsorbent for SO₂ capture because of its excellent recycling stability, high capacity and strong affinity toward SO₂ at low pressure.