(478c) Mixed-Linker Zeolitic Imidazolate Frameworks: Tuning Molecular Diffusion and Adsorption Properties

Kiwon Eum, Fereshteh Rashidi, Christopher W. Jones, and Sankar Nair

School of Chemical & Biomolecular Engineering, Georgia Institute of Technology, Atlanta, GA

Metal-organic frameworks (MOFs) are a relatively novel and promising class of nanoporous materials consisting of organic linkers coordinated to metal ions in crystalline structures. They are attractive for use in separations or catalysis, among many other applications.^1,2 A subclass of MOFs, known as zeolitic imidazolate frameworks (ZIFs), form structural topologies equivalent to zeolites.³ ZIFs have many advantageous properties including good thermal and chemical stability, high microporosity, and high surface area.¹ It is possible to tune the properties of MOFs for specific applications via chemical or structural modifications.⁴ A post-synthetic method is one approach for introducing functional groups into MOFs.⁵ Structural modification can be also achieved by mixed-linker synthesis, using more than one type of linker. In previous work, the synthesis of ZIF 8-90 mixed-linker frameworks with a combination of ZIF-8 and ZIF-90 linkers was demonstrated.⁶

The present work discusses the synthesis of large (over 50 μm size) ZIF 8-90 mixed-linker crystals varying the ratio of 2-methylimidazole/carboxaldehyde-2-imidazole linkers, and demonstrates that the ZIF-8-90 mixed-linker materials allow tuning of adsorption and diffusion properties of certain hydrocarbons. Volumetric uptake methods are used to study intracrystalline adsorption and diffusion phenomena of different gases in mixed-linker ZIF materials, facilitated by the ability to control the crystal size of the ZIFs. Particle size distributions are measured using dynamic light scattering or optical microscopy, and crystal composition analysis was performed using ¹H-NMR spectroscopy and micro-Raman spectroscopy. Specifically, it is shown that: (1) the hydrocarbon adsorption properties of ZIF 8-90 mixed-linker ZIFs change significantly with composition, and (2) the diffusion properties and selectivities show very large changes with mixed-linker composition. This first demonstration clearly shows the potential of this approach for use in a variety of molecular separations.

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