We report here the synthesis of a zeolitic imidazolate framework-8 (ZIF-8) in an aqueous solution. ZIF-8 crystals were prepared by mixing 2-methylimidazole (Hmim) with Zn nitrate hexahydrate (Zn) in deionized water. The products prepared at high Hmim/Zn molar ratios were assigned to a sodalite (SOD)-type structure whose morphology consisted of a rhombic dodecahedron with truncated corners. The crystals possessed ultrahigh surface areas and micropore volumes. At low Hmim/Zn molar ratio, some zinc hydroxide and basic zinc nitrate were observed in the products. We also focused on the formation process of ZIF-8 crystals in an aqueous system by observing the change in the pH value as a function of synthesis time. We tried to calculate the stability constant of ZIF-8 by fitting the calculated pH values to the measured pH values. When the molar fractions of the zinc compounds in the equilibrium state were calculated, high fractions of the Zn(mim) 2 complex were observed at high concentrations of Hmim. On the other hand, some zinc cations were present at low concentrations of Hmim. This finding would support a causal relationship between these zinc cations and the formation of some by-products.

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.

The synthesis of zeolitic imidazolate framework-8 (ZIF-8) films in an aqueous system was achieved. ZIF-8 films with controllable thickness were successfully grown on a modified substrate at room temperature. The 3-(2-imidazolin-1-yl)propyltriethoxysilane (IPTES) was used to first form a pseudo-surface of ZIF-8 on a glass substrate, followed by layer-by-layer growth. The film thickness of ZIF-8 was controlled within the range from 220 to 640 nm per growth cycle by changing the reactivity of the zinc source. Notably, the use of a preorganized zinc source led to drastic changes in the formation rate of ZIF-8. The use of a low-reactivity growth solution containing zinc acetate thus allowed the preparation of dense ZIF-8 films.

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