ZIF-8: structure, synthesis and properties

ZIF-8 copies a zeolite with metal and organic parts. In an aluminosilicate zeolite, tetrahedral silicon (or aluminium) atoms are bridged by oxygen. In a zeolitic imidazolate framework (ZIF) the tetrahedral atom is a metal ion and the bridge is an imidazolate (Park et al. 2006). In ZIF-8 every Zn²⁺ is bound to four nitrogen atoms of four 2-methylimidazolate linkers, and each linker joins two zinc ions.

The zinc tetrahedra link up in the sodalite (sod) net, the net of the zeolite sodalite. Its building block is the sodalite (truncated-octahedral) cage, which has square four-ring and hexagonal six-ring faces. In ZIF-8 the cages are 11.6 Å across and are reached through six-ring windows only 3.4 Å wide (Park et al. 2006).

The framework is not rigid. The imidazolate rings can rotate, which widens the windows: diffusion measurements put the effective aperture at 4.0–4.2 Å, so molecules larger than the crystallographic 3.4 Å window still enter (Zhang et al. 2012), and under pressure the substituted rings swing open and the pore volume grows (Hobday et al. 2018). The crystallographic cell is cubic (I-43m, a ≈ 17.0 Å) and holds 12 zinc atoms.

1. Solvothermal synthesis. In the first report, crystals of ZIF-8 were grown from a zinc salt and 2-methylimidazole heated in N,N-dimethylformamide (DMF) (Park et al. 2006).
2. Room temperature in methanol. ZIF-8 also forms quickly when methanol solutions of zinc nitrate and 2-methylimidazole are simply combined at room temperature, giving nanocrystals (Cravillon et al. 2009). Water can replace methanol, with nanocrystals forming at room temperature in minutes (Pan et al. 2011).
3. Little or no solvent. ZIF-8 can be made by ball milling (Beldon et al. 2010) and by continuous extrusion (Crawford et al. 2015), and grown as thin films from the vapour phase (Stassen et al. 2016).
4. Activation. The product is separated, washed, and the solvent in its pores is exchanged and removed by heating under vacuum to empty the cages.

ZIF-8 showed that a metal–organic framework could combine permanent porosity with the robustness of a zeolite. In the 2006 study that introduced it, ZIF-8 was stable up to 550 °C, and it kept its full crystallinity after 7 days in boiling benzene, methanol and water and after 24 h in 0.1 M and 8 M sodium hydroxide at 100 °C (Park et al. 2006). Most frameworks known at the time would not survive such treatment.

It also made the zeolite analogy concrete. Because the zinc–imidazolate–zinc unit mimics the Si–O–Si unit of a zeolite, ZIFs can be built on the nets of known zeolites; the 2006 paper reported twelve ZIFs on the nets of seven aluminosilicate zeolites (Park et al. 2006). ZIF-8, the sodalite member, became the most studied of them.

• Membranes and molecular sieving. Its narrow, flexible windows separate molecules by size: thin ZIF-8 membranes separate propylene from propane (Kwon and Jeong 2013), and ZIF-8 grown in polymer hollow fibres has been used the same way (Brown et al. 2014).
• Thin films for devices. ZIF-8 films can be deposited from the vapour phase (Stassen et al. 2016), and vapour-deposited ZIF-8 has been tested as an ultra-low-k dielectric that fills narrow trenches (Krishtab et al. 2019).
• Encapsulation. ZIF-8 grows around nanoparticles and other guests, enclosing them in a porous shell (Lu et al. 2012).
• Porous carbons. Carbonising ZIF-8 directly gives nanoporous carbon for supercapacitor electrodes (Chaikittisilp et al. 2012).
• Model of framework flexibility. The swing of its imidazolate linkers is a standard case for studying how a framework opens to guests (Fairen-Jimenez et al. 2011; Hobday et al. 2018).

For a paper, slide or poster, show three levels: one Zn²⁺ with four 2-methylimidazolate linkers (a blue ZnN₄ tetrahedron), a single sodalite cage with its six-ring windows, and the cubic packing of cages. The synthesis figure above opens as an editable LabCanvas figure. The crystal structure SVG is drawn from the ZIF-8 crystal file by the LabCanvas figure editor, where you can switch between atoms and polyhedra and change the number of cells; the 3D model on this page opens in the LabCanvas 3D Studio, where you can turn it and export a high-resolution image.