ZIF-8
is a flexible zeolitic imidazole-based metal–organic framework
whose narrow pore apertures swing open by reorientation of imidazolate
linkers and expand when probed with guest molecules. This work reports
on the crystal size dependency of both structural transitions induced
by N2 and Ar adsorption and dynamic adsorption behavior
of n-butanol using well-engineered ZIF-8 crystals
with identical surface area and micropore volume. It is found that
the crystal downsizing of ZIF-8 regulates the structural flexibility
in equilibrium adsorption and desorption of N2 and Ar.
Adsorption kinetics of n-butanol in ZIF-8 are strongly
affected by the crystal size, however, not according to a classical
intracrystalline diffusion mechanism. Our results suggest that structural
transitions and transport properties are dominated by crystal surface
effects. Crystal downsizing increases the importance of such surface
barriers.
An efficient and simple mechanosynthesis was proposed to synthesize zeolitic imidazolate-based metal organic framework ZIF-8 with hierarchical superstructure. The synthesis strategy is based on a concept that only uses precursor powders for ease of handling in the solid−solid mechanochemical reaction. The solvent-free conversion of ZnO to ZIF-8 could be accelerated by an addition of zinc acetate dihydrate as both a Zn source and a catalyst. The acid dissolution of ZnO and complex forming reaction between ionized Zn and 2-methylimidazole were facilitated by the hydrated water and acetic acid generated sequentially from zinc acetate dihydrate during the reaction process. The mechanochemically synthesized ZIF-8 with trimodal hierarchical porous structure exhibited excellent adsorption rate capabilities. The large-scale rapid production and enhanced adsorption performances make this salt-assisted mechanosynthesis a very promising candidate for the industrial application and shaping of metal organic frameworks.
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