MOF-5 (IRMOF-1): structure, synthesis and properties

MOF-5 is built from two parts. The node is a Zn₄O cluster: one oxygen atom at the centre of a tetrahedron of four zinc atoms, each zinc also bound to three carboxylate oxygens. Six carboxylate groups cap the cluster, so it acts as an octahedral connector, the Zn₄O(CO₂)₆ secondary building unit (SBU).

The linker is terephthalate (BDC²⁻), a benzene ring with a carboxylate at each end. Each linker bridges two clusters, and each cluster takes six linkers, pointing along ±x, ±y and ±z. The result is the simplest three-dimensional net, pcu (primitive cubic): clusters at the corners of cubes 12.9 Å on a side, linkers along the edges.

The flat benzene rings face into some cavities and turn their edges to others, so the cubic cavities are not all equal: they alternate between about 12 Å and 15 Å across (Rowsell et al. 2005). Eight of these small cubes make up the crystallographic unit cell (cubic, Fm-3m, a ≈ 25.8 Å).

1. Solvothermal synthesis. Zinc nitrate tetrahydrate and terephthalic acid are dissolved in N,N-diethylformamide (DEF) and heated in a closed vessel at about 85–105 °C (Eddaoudi et al. 2002). As the acid is slowly deprotonated, Zn₄O clusters form and colourless cubic crystals grow.
2. Washing and solvent exchange. The crystals are washed and the DEF in the pores is exchanged for a more volatile solvent.
3. Activation. The solvent is removed under vacuum, emptying the pores. MOF-5 reacts with water, so this and everything after it is done dry; crystals prepared and handled without air reach a BET surface area of about 3,800 m² g⁻¹ (Kaye et al. 2007).

Before 1999, most open coordination frameworks collapsed when their guest solvent was removed. MOF-5 was reported as a framework that “remains crystalline … and stable when fully desolvated and when heated up to 300 °C” (Li et al. 1999), with a higher apparent surface area and pore volume than most zeolites. It showed that rigid clusters joined by rigid linkers give permanent porosity.

It also started reticular chemistry. Keeping the Zn₄O cluster and the pcu net but swapping BDC for longer or functionalised linkers gave the isoreticular series (IRMOFs), whose pore size and chemistry could be set by design; MOF-5 is IRMOF-1 (Eddaoudi et al. 2002).

• Gas storage. MOF-5 is a reference material for hydrogen and methane storage studies; how it is prepared and handled strongly changes its measured hydrogen uptake (Kaye et al. 2007), and the IRMOF series was designed with methane storage in mind (Eddaoudi et al. 2002).
• Adsorption science. X-ray studies located where argon and nitrogen sit in its pores (Rowsell et al. 2005), making it a model system for gas–framework interactions.
• Teaching and figures. Its simple cubic net makes it the standard example of a MOF in textbooks, reviews and graphical abstracts.
• Limitation. Its sensitivity to moisture rules it out where water is present; water-stable frameworks such as UiO-66 or ZIF-8 are used instead.

For a paper, slide or poster, show the three levels: the Zn₄O(CO₂)₆ cluster (often drawn as a blue tetrahedron or octahedron), one BDC linker, and a 2 × 2 × 2 block of the cubic net with a sphere marking a pore. The 2D figure above has all three and opens as an editable LabCanvas figure; the 3D model opens in the LabCanvas 3D Studio, where you can switch the clusters between atoms and polyhedra, change the linker, add guest molecules and export a high-resolution image.