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Metal-organic frameworks as catalysts

Inorganic and organic luminescent materials have been exhaustively explored for their applications in lighting, display and sensing devices. Recently, EU-funded researchers introduced new functionalities with the use of polyoxometalates.
Metal-organic frameworks as catalysts
The multi-faceted nature of metal-organic frameworks' (MOFs), which includes both an organic ligand and a metal ion, enables phenomena found in few other materials. In particular, porous MOFs offer a unique platform for the development of solid-state luminescent materials.

Although they have been compared to zeolites, MOFs lack ion-exchange properties. Researchers working on the EU-funded LAMP (Luminescence amplification by metal organic framework-polyoxometalate composites) project have demonstrated this key feature. They boosted the versatility of zeolites with guest molecules occluded in the pores of MOFs.

Specifically, MOFs' functionalisation was achieved with polyoxometalates. These polyatomic ions were used to occlude the pores in the charge-neutral Hong Kong University of Science and Technology-1 (HKUST-1) framework. As a result, their negative charges were compensated by the exchangeable extra-framework copper cations in the MOF.

Keggin-type polyoxometalates induce hydrolysis and dimerisation of copper ions at room temperature, which in turn trigger the spontaneous nucleation and growth of the HKUST-1 framework. Under the same conditions, but in the absence of these polyatomic ions, HKUST-1 is not formed.

On the other hand, stoichiometric amounts of polyoxometalates result in highly crystalline materials with their occupation of large pores. What was named COK-16, which stands for cation-exchanging MOF hybrid, is one of the very few examples of MOFs with documented cation-exchange behaviour.

Several thousand organic-inorganic hybrid materials have been synthesised to date, with many of them applied in industry. In the search for more polyoxometalate-MOF composites with potential industrial applications, the researchers also found niobium-based polyoxometalates.

Details of the synthesis of polyoxometalate-MOF composites are included in a paper published in ChemPlusChem while a second one is under preparation.

Related information


Metal-organic framework, luminescent materials, polyoxometalates, ion-exchange, LAMP
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