Size-controlled Synthesis of Zeolitic Imidazolate Framework-8 (ZIF-8) Crystals in an Aqueous System at Room Temperature

Abstract

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...

Size Control and Stability Study of Zeolitic Imidazolate Framework-8 to Prepare Mixed Matrix Membrane

“…With higher Hmim concentration (molar ratio = 1:70), the deprotonation rate of imidazole was anticipated to increase. 12 Henceforth, more nuclei were formed at the beginning of the reaction (≤ 2 min) at higher Hmim concentration. In this case, the overall ZIF-8 particle sizes were found smaller prior to the extensive growth of the particles.…”

“…13 Hmim deprotonates to provide imidazolate nitrogen atom, which is then coordinated with the Zn 2+ ion to construct the neutral framework structure of ZIF-8, as displayed in Figure 2. 12 At the early stage of the reaction, the deprotonated imidazole was consumed for the nuclei formation rather than the particle growth. With higher Hmim concentration (molar ratio = 1:70), the deprotonation rate of imidazole was anticipated to increase.…”

“…13 In the work carried out by Tanaka et al, the ZIF-8 size in the range of 0.32-3.4 µm was achieved by adjusting the imidazole/Zn molar ratio from 40 to 100 with the reaction occurring in an aqueous system. 12 In their study, higher monodispersity was acquired at higher imidazole/Zn molar ratio. 12 Furthermore, Venna et al demonstrated a simple way of ZIF-8 size control by manipulating the reaction time.…”

“…12 In their study, higher monodispersity was acquired at higher imidazole/Zn molar ratio. 12 Furthermore, Venna et al demonstrated a simple way of ZIF-8 size control by manipulating the reaction time. 13 With the reaction time of 10 min to 24 h, ZIF-8 in the size of 50-500 nm were obtained.…”

“…10 Hence, the size control of ZIF-8 is necessary for its effective application in gas separation. The particle size of ZIF-8 can be regulated by different strategies including incorporation of additives, 10,11 alteration of reactant ratio 12 and adjustment of reaction time. 13 In the work carried out by Tanaka et al, the ZIF-8 size in the range of 0.32-3.4 µm was achieved by adjusting the imidazole/Zn molar ratio from 40 to 100 with the reaction occurring in an aqueous system.…”