MIL-101(Cr): structure, pore size, synthesis and uses

MIL-101 is built in three levels. The node is a chromium trimer: three CrO₆ octahedra share a single central (μ₃) oxygen, and six carboxylate groups bridge pairs of chromium atoms around it. The remaining site on each chromium points outwards and is filled by water or by a fluoride or hydroxide ion, which balances the charge (Cr₃F(H₂O)₂O).

The linker is terephthalate (BDC²⁻), as in MOF-5 and UiO-66. Four trimers and six BDC linkers close up into a small rigid tetrahedron, the supertetrahedron: a trimer at each corner, a linker along each edge.

The supertetrahedra then share corners with one another, just as SiO₄ tetrahedra share corners in a zeolite. They follow the MTN zeolite net, so MIL-101 is called a zeotype. Because each “tetrahedron” is now a whole supertetrahedron, everything is scaled up: the cubic cell (space group Fd-3m, a ≈ 88.9 Å) has a volume of about 702,000 ų, and the net encloses two kinds of mesoporous cage, about 30 and 34 Å across (Férey et al. 2005). In the MTN net the smaller cage has only pentagonal windows and the larger one has pentagonal and hexagonal windows.

Férey’s group found the structure by combining targeted chemistry with computer simulation of possible nets and powder diffraction, the same approach that gave its trimesate cousin MIL-100 a year earlier (Férey et al. 2004).

1. Hydrothermal synthesis. Chromium(III) nitrate nonahydrate, terephthalic acid, hydrofluoric acid and water are sealed in a Teflon-lined autoclave and heated at 220 °C for 8 h (Férey et al. 2005). The product is a green powder of small octahedral crystals.
2. Purification. Unreacted terephthalic acid, which crystallises alongside the MOF, is filtered off, and the solid is washed in hot ethanol to clear the pores.
3. Handling. The synthesis uses HF and a chromium salt, so it needs the matching safety precautions; once made, the framework itself is robust in water and in acid (Ding et al. 2019).

When it appeared in 2005, MIL-101 combined three records in one solid: an exceptionally large cubic cell (about 702,000 ų), cages of 30–34 Å, in the mesoporous range rather than the micropores of most MOFs and zeolites, and a Langmuir surface area of about 5,900 m² g⁻¹ (Férey et al. 2005). Molecules far too big for a zeolite, such as Keggin polyoxometalate anions, fit inside its cages.

It also showed a new way to find frameworks: predicting likely nets by simulation and then matching them to powder diffraction patterns, instead of waiting for single crystals. And because Cr³⁺–carboxylate bonds are strong and inert, MIL-101 is far more resistant to water, acid and base than zinc MOFs such as MOF-5 (Ding et al. 2019), which made it a practical workhorse.

• Host for large guests. Its cages hold Keggin polyanions, and the original paper proposed it as a nanomold for monodisperse nanomaterials (Férey et al. 2005).
• Drug delivery. MIL-100 and MIL-101 were shown to store and release the drug ibuprofen in an early study of MOFs for drug delivery (Horcajada et al. 2006).
• CO₂ capture. Loaded with branched poly(ethylenimine) and 3D-printed into cellulose acetate monoliths, MIL-101(Cr) took up 1.05 mmol g⁻¹ of CO₂ from 400 ppm CO₂ at −20 °C and kept a working capacity of 0.95 mmol g⁻¹ over 14 cycles with regeneration at 60 °C (Wang et al. 2024).
• Adsorption and catalysis. Its large pores and water stability make it a frequent choice for adsorption and catalysis studies.
• Limitation. The classic synthesis uses HF and chromium, and the large cages make it a powder that is usually shaped (pellets, monoliths) before use in beds.

A good MIL-101 figure shows the hierarchy: the Cr₃O trimer (three green CrO₆ octahedra around one oxygen), one supertetrahedron (trimers at the corners, terephthalate along the edges) and a cage of corner-sharing supertetrahedra with a sphere marking the 30–34 Å pore. Draw the cage schematically: the full cell, at about 89 Å on a side, holds far too many atoms to show clearly. The figure above has all three levels plus the synthesis, and opens as an editable LabCanvas figure where you can restyle the polyhedra, change colours and labels, and export a high-resolution image.