Short communicationZIF-8: A comparison of synthesis methods
Introduction
Zeolitic imidazolate frameworks (ZIFs) are porous hybrid materials with structures analogous to zeolites that are built upon 4-connected nets of tetrahedral units, wherein metal ions, such as Zn2+ or Co2+, are linked through N atoms in ditopic imidazolate anions [1], [2]. ZIFs have emerged as a potential material for H2 storage [3], CO2 adsorption [4], alkane/alkene separation [5], and heterogeneous catalysis [6] owing to their structural flexibility, which allows rational design of the pore sizes and surface functionality, and their relatively high chemical and thermal stability [7]. Among the ZIFs, ZIF-8 (Zn(mIM)2, mIM = 2-methylimidazolate) exhibits a sod topology comprised of 1.16 nm cages connected through six-membered windows, 0.34 nm in size (Fig. 1), and is currently the most widely investigated ZIF material for a range of applications [8], [9], [10].
ZIF-8 can be prepared in high purity through several different synthesis routes [1], [2], [11], [12], [13], [14], [15], [16]. Synthesis in an environmental-friendly manner under facile conditions is desirable, and particular emphasis has been placed upon size-controlled ZIF-8 synthesis [17], [18], [19] and the easy separation of nano- to submicron-sized ZIF-8 by incorporating a magnetic Fe3O4 guest into the ZIF-8 particle [11], [16], [20].
In the present study, ZIF-8 was synthesized using 7 different synthesis methods (conventional solvothermal in different organic solvents of dimethylformamide (DMF) and methanol (MeOH); microwave-assisted; sonochemical; mechanochemical; dry-gel; microfluidic), and their physicochemical properties were compared with those of commercial ZIF-8 prepared using an electrochemical method. The differences in the physicochemical properties among the ZIF-8 samples were manifested in the probe catalytic reaction of the Knoevenagel condensation of benzaldehyde with malononitrile. The feasibility of incorporating Fe3O4 nanoparticles into ZIF-8 produced via the different synthesis routes was also examined.
Section snippets
Synthesis
The detailed synthesis steps of ZIF-8 via solvothermal (in DMF and MeOH), microwave-assisted, sonochemical, mechanochemical, dry-gel, and microfluidic routes (Fig. S1) are reported in Supporting information.
Characterization
The X-ray diffraction (XRD) patterns of the ZIF-8 samples were obtained on a Rigaku diffractometer using Cu Kα (λ = 1.54 Å) radiation. The N2 adsorption and desorption isotherms were obtained on an ASAP-2020 (Micromeritics, USA) sorptometer at a liquid nitrogen temperature. Prior to the
Various synthesis of ZIF-8
The XRD patterns for all the ZIF-8 samples [(a)–(g)] synthesized in this work and that of commercial ZIF-8(h) (Fig. S2) were all in good agreement with the simulated pattern of the ZIF-8 structure. All the ZIF-8 samples showed type-I N2 adsorption isotherms at 77 K with no hysteresis (Fig. S3). Table 1 lists the summary of synthesis conditions and textural properties (BET surface area, total pore volume, and external surface area) and Table 2 shows the SEM images and particle size of the
Conclusions
ZIF-8 was prepared using several methods: solvothermal (DMF, MeOH), microwave-assisted, sonochemical, mechanochemical, dry-gel, and microfluidic method. Few differences in textural properties were detected, but the dry-gel and sonochemical techniques produced significantly smaller particles, which resulted in faster reaction rates in the Knoevenagel condensation reaction of benzaldehyde with malononitrile. The encapsulation of Fe3O4 nanoparticles within ZIF-8 was feasible using solvothermal as
Acknowledgement
This study was supported by a National Research Foundation of Korea (NRF) grant funded by the Korean Government (MEST) (No. 2013005862).
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