A MOF you can scoff

Chemists have accidentally discovered a new type of metal organic framework, or MOF, which is made from edible components. The materials needed to create the new structures are cheap, renewable and widely available and the conditions needed to create the frameworks are benign. That they can be made from molecules that are safe to eat could give the new compounds a role in the development of new food and pharmaceutical products.

MOFs consist of a network of metal-based nodes connected by organic struts. Their huge porosity and the ability to include chemical functionality within the pores have propelled MOFs to the forefront of a number of technologies including gas storage and capture, catalysis and drug delivery.

Fraser Stoddart’s team at Northwestern University in Illinois, US, was attempting to make new interlocked molecular architectures based on the eight-membered sugar ring gamma-cyclodextrin, a low-cost derivative of starch. Dissolving potassium hydroxide with the cyclodextrin in water, then diffusing methanol through the system produced well-defined colourless crystals. ’When we obtained the x-ray structure of the crystals it was clear we had produced something surprising,’ says team member Ron Smaldone.

Working with colleagues from the University of St Andrews in the UK and the University of California Los Angeles, the researchers showed that six cyclodextrin rings had formed the faces of a cubic structure, held together by co-ordinating potassium ions, and these small cubes in turn fitted together in a large three-dimensional cubic framework. Each individual cube has a central pore of around 1.7nm across, with windows in each face of 0.8nm in diameter, which run through the structure as channels connecting the pores. 

’We showed that the structure is stable in the absence of the solvent and that it was capable of absorbing nitrogen on a par with other MOFs,’ says researcher Ross Forgan. 

Most MOFs to date have been constructed from organics derived from petrochemical feedstocks. If MOFs become mass produced, structures derived from a renewable, carbon-neutral source could be attractive, Smaldone says. There is also scope to experiment with other natural macrocycles, using different metal centres and functionalising the internal space of the cubic units. The fact that the new MOF is made from molecules that have been approved for human consumption could make the structures useful for the food and pharmaceutical industries. 

Lee Cronin, an expert on MOFs at the University of Glasgow in the UK, says, ’I think it is a remarkable discovery - something that is so simple but with such good physical properties in terms of nitrogen absorption. The fact that we can make high value materials from sugar is mind-blowing. The organic chemistry to manipulate these molecules is well known, so I think we could now have a whole new sub-field of chemistry in this area.’ 

Simon Hadlington