Chemical separations and conversions can be accomplished by selective adsorption of an adsorbate onto a given porous adsorbent based on its molecular shape, kinetic diameter, and supramolecular interactions with the adsorbent host. Flexible metal-organic frameworks (MOFs) are microporous crystalline adsorbents constructed by self-assembly of metal nodes and organic linkers that allow for structural mobility in response to uptake or release of adsorbates. Means to easily modify or alter the mechanical properties of MOFs are essential for their successful transition to industry, where high pressures cause framework collapse and loss of crystallinity. Herein, modified nanocrystals of ZIF-8 were generated in triethylamine via an in-situ coordination modulation method. We demonstrate that using triethylamine modulator during synthesis results in a conspicuous decrease in elastic modulus of individual nanocrystals, measured by atomic force microscopy nanoindentation. Furthermore, argon gas sorption analysis showed significant differences in the switchable characteristics after addition of triethylamine compared to unmodified ZIF-8. The more flexible, modified crystals exhibited gradual and cooperative gate-opening phenomena compared to the observed immediate and step-wise characteristics of ZIF-8 without amine modifier. The differences in argon pressure-induced hysteretic deformation correspond to the decrease in elastic modulus of base-modified ZIF-8 nanoparticles. Thus, simple incorporation of a commonly used modulator during synthesis of the metal-organic framework nanocrystals led to increased mechanical flexibility and modified gas sorption properties while retaining the crystal structure, size, and morphology.

Effect of base modulator on switchable adsorption characteristics and mechanical properties of microporous ZIF-8 nanocrystals

ACS Spring 2022

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