Synthesis of ZIF-8 in a deep eutectic solvent using cooling-induced crystallisation
Graphical abstract
Introduction
Zeolitic imidazolate frameworks (ZIFs) have neutral zeolite-type framework structures and are constructed from Zn or Co atoms coordinated with imidazole or imidazole derivatives. These materials have become a specialised and rapidly developing subclass of MOF materials, due to their regular porous channels, permanent porosity and large pore diameters [1], [2]. Among ZIFs, ZIF-8 with a SOD topology framework contains zinc linked with 2-methylimidazole (2-MeIm), leading to the formula Zn(MeIm)2; this prototypical and popular ZIF material has potential applications in catalysis, gas separation and storage [3], [4], [5], [6], [7]. Initially, ZIF-8 was synthesised using the liquid-phase diffusion method reported by Huang et al. and a solvothermal method disclosed by Park et al. [3], [4]. Afterward, the synthesis of ZIF-8 has been extensively explored. Synthesising ZIF-8 in a methanolic system at room temperature, as reported by Huber and co-workers, is a popular method for studying the synthesis of ZIF-8 [8], [9]. Undoubtedly, several other prominent methods have also been developed, including the room temperature mechanochemical synthesis adopted by Friscic and co-workers [10], the steam-assisted conversion synthesis reported by Dong and co-workers [11], the sonochemical route applied by Coronas and co-workers [12] and Ahn and co-workers [13] and the aqueous room temperature synthesis carried out by Lai and co-workers and Gross et al. [14], [15]. In addition, the size and shape of the ZIF-8 prepared in the methanolic system could be tuned by adding modulating agents or stabilisers, such as bridging bidentate ligands, simple monodentate ligands or surfactants (e.g., 1-methylimidazole, n-butylamine or poly(diallyldimethylammonium chloride)), etc. [9], [16]. Moreover, the crystallisation mechanism for ZIF-8 nanocrystals synthesised in methanol has also been investigated using various characterisation techniques [17], [18], [19]. Carreon and co-workers [17] utilised PXRD, TEM and SAED to observe the phase transformation of ZIF-8. The crystallisation of ZIF-8 occurs through a solution- and solid-mediated transformation mechanism. Wiebcke and co-workers [18] monitored the crystallisation process for ZIF-8 nanocrystals using in-situ SAXS/WAXS, demonstrating that the crystal growth process involves cluster formation, nucleation, growth and nanocrystal formation. Attfield and co-workers [19] used in-situ AFM to track the surface growth of {1 0 0} facet of ZIF-8, revealing that the crystal growth of ZIF-8 follows “birth and spread” and spiral growth mechanisms on different regions of the face simultaneously. These studies show that the crystal growth mechanism of ZIF-8 in methanolic systems is similar to the crystallisation process for zeolites.
Deep eutectic solvents (DES), a type of ionic solvent, are liquid at near-ambient temperatures below 150 °C [20]. In general, these substances are obtained by mixing a quaternary ammonium salt (e.g., choline chloride) with a hydrogen-bond donor (e.g., amine, citric acids) [21], [22]. The components of a DES can associate with each other through hydrogen bond interactions; these interactions can substantially depress the melting point of the DES. The melting point of a DES is much lower than that of its individual components [20], [21]. DES also exhibit excellent solubility, low volatility, low toxicity, good economy, convenient preparation and other advantages [20], [21], [22]. Currently, DES has become an attractive medium for MOF syntheses. Bu and co-workers synthesised [Nd(bdc)2(choline)](m-urea), [YbIII(bdc)2(urea)] filled with cationic choline guests, a porous anionic C3N4-type ([In3(btc)4]n3n− (btc = 1,3,5-benzenetricarboxylate) framework and a zinc(II)–boron(III)–imidazole framework (Zn2(im)Cl2[B(im)4]) with unusual pentagonal channels and other MOF materials in urea-derived DES solvents [23], [24], [25]. Morris and co-workers synthesised lanthanide/organic-derived MOFs, such as Ln(TMA)(DMU) (Ln: La-Nd-Eu; TMA: C9O6H3; DMU: (CH3NH)2CO)), in a ChCl/dimethylurea DES [26]. During synthetic processes, DES can act as a structure directing agent and a ligand. Furthermore, DES can also stabilise water-sensitive metal-halogen bonds because the reactivity of water is significantly decreased due to the strong binding interactions between the water molecule and the DES through hydrogen bond networks. It offers an approach toward MOFs that cannot be obtained using common methods [20], [23], [24], [25], [26].
Zhu and co-workers [27] have reported the synthesis of ZIF-8 nanocrystals in a urea–choline chloride eutectic mixture. In the synthesis process, as the water and ethanol were added into the clear solution, ZIF-8 product formed from DES immediately. However, in the present work, we synthesised ZIF-8 in DES using cooling-induced crystallisation. When the clear solution was subjected to either rapid or programmed cooling, ZIF-8 crystals could precipitate from DES. In addition, we studied the effect of the synthetic conditions, including the cooling rate of synthesis system and the reaction temperature, on the particle size and morphology of the ZIF-8 sample. We also discussed the precipitation process for ZIF-8 in DES and investigated the performances of the ZIF-8 samples.
Section snippets
Synthesis of ZIF-8
A 50 ml beaker was charged with 6 g of urea and 7 g of choline chloride, in a molar ratio of 2:1 [21]. After the choline chloride–urea mixture melted at 80 °C, 1.23 g of 2-methylimidazole (2-MeIm) and 0.51 g of zinc nitrate hexahydrate (or 0.38 g zinc acetate dihydrate) were added to the hot DES with the following composition: Zn2+:2-MeIm:DES = 1:8:88. After several minutes of electromagnetic stirring, the solution was transferred into a 30 ml Teflon-lined stainless autoclave and heated to the reaction
Synthesis of ZIF-8 in DES
Zinc nitrate hexahydrate and 2-methylimidazole were added to hot DES. After heating at the appropriate temperature, the synthesis solution became transparent; no crystallites were observed. After the autoclave was cooled to room temperature using either rapid or programmed cooling, the clear solution turned cloudy. Subsequently, the solid product could be collected by filtration (Table 1). The PXRD patterns indicate that the product is single-phase ZIF-8, as shown in Fig. 1. The X-ray
Conclusion
In conclusion, we synthesised ZIF-8 in DES and studied its fast cooling crystallisation process. We determined that the particle size and shape of ZIF-8 could be manipulated through controlling the cooling rate and the reaction temperature without adding any extra additives. The present work may provide a simple and fast route toward the preparation of ZIFs. Similar to those synthesised using an alternative method, our ZIF-8 sample also exhibits exceptional thermal stability, as well as
Acknowledgement
This work was surpported by the National Nature Science Foundation of China (Grant Nos. 21001102 and 21373214).
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