New metal-free MOFs show early promise for nuclear chemistry

Metal-free organic frameworks, featuring inexpensive and abundant non-metallic elements, have been designed and then made with the help of computational crystal-structure prediction software.

It is hoped the new materials will have a range of applications such as catalysis, water capture and hydrogen storage and offer an alternative to their metal-based equivalents.

The researchers, who are based at the Universities of Liverpool, Southampton and Nottingham, used computational design methods in combination with synthetic know-how to develop non-metal organic porous framework (N-MOF) materials using non-metallic ions such as chloride.

‘We guided the discovery of these materials using a computational method called crystal structure prediction,’ said Graeme Day, an expert in chemical modelling at the University of Southampton. ‘This allows us to predict which non-metal salts will form stable porous frameworks, which salts will not and to anticipate the precise crystal structure in advance of experimental work. We don’t have to assume a specific geometry for the joints in the framework, which is a fundamental principle in MOF chemistry.’

The researchers said the new frameworks could be thought of as ‘inverted’ MOFs, in which the halide anions are analogous to the metal cations and that, like MOFs, they could be structurally diversified by using other counter-ions such as nitrates, sulfates and phosphates.

The synthesis of these materials is also scalable, as it involves simple acid–base neutralisation, the researchers pointed out. They added that the first examples of these materials already showed ‘practical promise’, outperforming most MOFs for iodine capture, which is important for the nuclear industry.

Other applications could take advantage of the highly charged pore channels in the frameworks for proton conduction, catalysis, water capture or hydrogen storage.