CD-MOFs can be prepared entirely from edible ingredients: combining food-grade γ-CD with salt substitute (KCl) or potassium benzoate (food additive E212) in bottled water and Everclear grain spirit (EtOH) yields porous frameworks which constitute edible MOFs.

—Smaldone et al.

Fraser Stoddart at Northwestern and his colleagues set themselves a challenge to synthesize MOFs from natural products.

The problem is that natural building blocks are generally not symmetrical. This lack of symmetry seems to prevent them from crystallizing as highly ordered, porous frameworks.

—Fraser Stoddart

γ-Cyclodextrin provided the solution to this problem: it comprises eight asymmetrical glucose residues arranged in ring, which is itself symmetrical. In many countries (for example the USA and Japan), cyclodextrins are approved for use as food additives.

The second ingredient in the frameworks is an alkali metal salt. Suitable candidates include ordinary table salt (sodium chloride), the common salt substitute potassium chloride, or potassium benzoate, an approved preservative. These ingredients are dissolved in water and then crystallized by vapor diffusion with an alcohol. It is even possible to use commercially available sources such as grain alcohol. These ingredients are all substances that can be obtained cheaply, in high quality, and of food-grade purity, says Stoddart.

The resulting crystals consist of cubes made from six γ-cyclodextrin molecules that are linked in three dimensions by potassium ions. These cubes are perfectly arranged to form a porous framework with easily accessible pores. This arrangement is a previously unknown one, according to Stoddart. The resultant pore volume encompasses 54% of the solid body.

Particularly atypical of porous materials is the fact that when dissolved in water, the framework simply dissociates back to its components, which can then be crystallized again with alcohol. In this way a degraded framework can easily be recycled or regenerated, Stoddart says.

We believe that CD-MOFs constitute the forerunners of a large class of porous crystals which can be synthesized under benign conditions using building units derived from renewable natural products.

—Smaldone et al.

Resources

• Ronald A. Smaldone, Ross S. Forgan, Hiroyasu Furukawa, Jeremiah J. Gassensmith, Alexandra M. Z. Slawin, Omar M. Yaghi, and J. Fraser Stoddart (2010) Metal-Organic Frameworks from Edible Natural Products. Angewandte Chemie International Edition, doi: 10.1002/anie.201002343