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.

Mechanochemical dry conversion that only uses zinc oxide and an imidazole ligand proved to be effective and reliable for fabrication of a zeolitic imidazolate framework with a polycrystalline grain boundary and a core-shell structure. The zinc oxide crystals are converted into a zeolitic imidazolate framework to a depth of approx. 10 nm below the surface.

We report here a simple and straightforward method that enables the size-controlled production of zeolitic imidazolate framework-8 in an aqueous system at room temperature. Pure single crystals with the size tuned in the range of ca. 300 nm to 3¯m in mean crystal size can be obtained and exhibit ultrahigh 4 and catalysis. 5 The zirconium(2-methylimidazole) 2 (ZIF-8), with a sodalite (SOD) zeolite-type topology, is a commercially available sample produced by BASF. ZIF-8 possesses large cages of 11.6 ¡ accessible through narrow windows of 3.4 ¡. Most syntheses of ZIFs have been reported so far include using organic solvents such as dimethylformamide (DMF), diethylformamide (DEF), methanol, and DMF/methanol. 6 The use of organic solvents is generally expensive and causes potential environmental pollution and human health problems. Simple techniques to synthesize ZIFs are required for the development of practical applications. Recently, aqueous synthesis of ZIF-8 has been reported. 7 Compared with the synthesis in organic solvents, the aqueous synthesis of ZIFs has particular advantages from economic, operational, and environmental perspectives. However, ZIF-8 prepared in aqueous systems has lower surface area than that prepared in organic systems, suggesting that the product contains dense by-products. 7 In addition, the aqueous synthesis requires excessive imidazole sources to obtain ZIF-8. 7a Further systematic understanding is required to synthesize phase pure ZIF-8 in an aqueous system.In this study, we have developed a simple and sizecontrolled synthesis of ZIF-8 using pure water. We focus on the effects of the imidazole concentration on crystal structure, mean particle size, and particle size distribution.ZIF-8 crystals were prepared from zinc nitrate hexahydrate (Zn(NO 3 ) 2 ¢6H 2 O; Zn) and 2-methylimidazole (mim) in pure water solvent. All chemicals were purchased from SigmaAldrich Chemical Co. and used as received. The molar compositions of the reaction mixtures were in the range of 1 Zn/4100 mim/2228 water. The mim/Zn molar ratio in the precursor solution ranges from 4 to 100. In all runs, to change the mim/Zn molar ratio, only the amount of mim was changed; the amounts of Zn and water were held constant. In a typical preparation, 0.744 g of Zn and 12.3 g of mim were each dissolved in 10 and 90 mL of deionized water, respectively. The former clear solution was poured into the latter clear solution under stirring. All the operations were performed at room temperature. The mixture became cloudy immediately after combining the component solutions. After stirring for 24 h, the milky colloidal dispersion was centrifuged at 6000 rpm for 10 min. The particles obtained were washed with methanol and centrifuged again; the process was repeated five times. The product was dried at 40°C for 48 h under reduced pressure. The products were characterized by powder X-ray diffraction (PXRD), scanning electron microscopy (SEM), dynamic light scattering (DLS), thermogravimetric analysis (TGA), and N 2 sorption measurem...

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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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Pure silica CHA-type zeolite (Si-CHA) membranes were synthesized by a hydrothermal secondary growth method on porous α-alumina supports. Using Si-CHA seed crystals as a crystalline nuclei allowed the formation of a dense Si-CHA layer. The Si-CHA membranes prepared in this study showed excellent gas permeance derived from their large pore volume and effective molecular sieve performance for gas separation. The Si-CHA membranes exhibited excellent H 2 and CO 2 permeance of 1.1 × 10 −6 and 1.7 × 10 −6 mol/m 2 sPa, respectively. The ideal selectivity of H 2 /CH 4 and CO 2 /CH 4 were 34 and 54, respectively. The stability test in the presence of water vapor on the Si-CHA membranes was performed and the Si-CHA membranes demonstrated good water vapor resistance, more so than the topologically analogous aluminosilicate zeolite membrane (SSZ-13). The Si-CHA membranes were unlikely to present pore blockages by water adsorption because of their hydrophobic pore systems.

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