Exploring cation distribution in ion-exchanged Al,Ga-containing metal-organic frameworks using ¹⁷O NMR spectroscopy

A mixed-metal metal-organic framework, (Al,Ga)-MIL-53, synthesised by post-synthetic ion exchange has been investigated using solid-state nuclear magnetic resonance (NMR) spectroscopy, microscopy and energy dispersive X-ray (EDX) spectroscopy. ¹⁷O enrichment during the ion-exchange process enables site specific information on the metal distribution to be obtained. Within this work two ion-exchange processes have been explored. In the first approach (exchange of metals in the framework with dissolved metal salts), ¹⁷O NMR spectroscopy reveals the formation of crystallites with a core-shell structure, where the cation exchange takes place on the surface of these materials forming a shell with a roughly equal ratio of Al³⁺ and Ga³⁺. For the second approach (exchange of metals between two frameworks), no core-shell structure is observed, and instead crystallites containing a majority of Al³⁺ are obtained with lower levels of Ga³⁺. Noticeably, these particles show little variation in the metal cation distribution between crystallites, a result not previously observed for bulk (Al,Ga)-MIL-53 materials. In all cases where ion exchange has taken place NMR spectroscopy reveals a slight preference for clustering of like cations.