One-pot synthesis of 3D-ZIF-7 supported on 2D-Zn–Benzimidazole–Acetate and its catalytic activity in the methoxycarbonylation of aniline with dimethyl carbonate
Graphical abstract
Introduction
Zeolitic Imidazolate Frameworks (ZIFs) are three-dimensional (3D) framework materials composed of tetrahedral units of metals like Co or Zn with imidazolium ligand linkers (Fig. 1a) [1]. Depending on the kinds of center metal and imidazole ligand, numerous ZIFs have been produced with different morphologies and surface properties [1], [2], [3]. In addition, functionalization and/or mixing imidazole ligands can alter the topology and morphology of ZIF, thereby changing the physical and chemical properties of ZIF. These modulated ZIF structure provides an opportunity to create materials tailored for specific applications [4], [5]. Accordingly, ZIFs has been reported to have numerous applications in various research fields, including sensors, adsorbents, matrix fillers, membranes, drug delivery, and catalysis [6], [7], [8], [9], [10].
Besides the center metal and ligand, various reaction factors were known to affect to the morphology of ZIF. Lee et al. reported, depending on the synthesis methods, for example, solvothermal, microwave-assisted, sonochemical, etc., ZIF-8 (Zn(mIM)2, mIM = 2-methylimidazoleate) possess different surface area and textural properties [11]. Similarly, different kind of metal precursor produced different shape/morphology of ZIF. Cai et al. reported ZIF-7s (Zn(BeIM)2, BeIM = benzimidazole) made from Zn(OAc)2 and ZnCl2 have rhombic-dodecahedral and rod structures, while that from Zn(NO3)2 has spherical structure [12]. Solvent also could control the structure of the final product through altering ligand– ligand interactions [3].
It was reported that the use of amine in the synthesis of ZIFs affects the nucleation and growth rates of ZIFs by controlling the rate of deprotonation of the imidazole ligand, indicating the presence of basic amine facilitate the formation of ZIF. Cravillon et al. reported the use of n-butylamine produced smaller ZIF particles due to the increased nucleation rate [13]. Gross et al. also reported the addition of trimethylamine allowed the formation of crystalline ZIF-8 and ZIF-67 even in the presence of water at a lower ligand/metal ratio [14]. Accordingly, inexpensive alkylamine such as n-butylamine, diethylamine, and triethylamine are frequently added to the synthesis mixture to reduce the amount of imidazole to facilitate the formation of ZIF.
On the other hands, Zn(BeIM)OAc is a 2-dimentional (2D) material, in which each Zn ion is coordinated to two benzimidazole (BeIM) ligands and two acetate (OAc−) ligands (Fig. 1b) [15]. The Zn–BeIM–Zn linkages and OAc–Zn–OAc linkages lie along the b axis and c axis, respectively, forming a layered structure that stacks along the a axis owing to van der Waals forces [16]. The synthesis and structural characterization of bulk structured 2D-Zn(BeIM)OAc was performed first by Li et al. Later, Junggeburth et al. and Xue et al. successfully synthesized ultrathin 2D-Zn(BeIM)OAc nanosheets and suggested its application as a gas separation membranes [16], [17].
Here, for the first time, we report that two different materials, 3D-ZIF-7 and 2D-Zn(BeIM)OAc, can be synthesized together using a one-pot reaction process at a specific precursor ratio of Zn(OAc)2/benzimidazole (1/1.5) by controlling the amount of diethylamine during the reaction. Interestingly, the morphology of synthesized mixture of 3D ZIF-7/2D Zn(BeIM)OAc was similar to that of a metal catalyst coated on a supporting material. That is, small ZIF-7 particles were coated on the surface of bulky 2D Zn(BeIM)OAc. Furthermore, the ZIF-7/Zn(BeIM)OAc showed very active and stable catalytic performance at the methoxycarbonylation of aniline with dimethylcarbonate (DMC).
Methoxycarbonylation of aromatic amine using DMC is one of the green reactions used for the synthesis of isocyanate intermediate without using phosgene [18]. Various homogeneous Lewis acid catalytic systems like zinc [19], [20], [21], lead [22], [23], and ytterbium [24] have been reported. However, many of them were found to be deactivated in the form of oxide or carbonate like ZnO and Pb(CO3)2(OH)2 during the reaction [23]. To overcome the limitations of homogeneous catalyst systems such as deactivation and difficulties of reuse and separation, heterogeneous catalysts such as Zn(OAc)2/AC, Zn(OAc)2/SiO2, ZnO/TiO2, ZrO2/SiO2, Al/SBA-15, Zn/carboxylated silica, Zinc carbonate basic, Zn/Al/Ce, Zr-MOF-808@MCM-41, and ZIF-7 have also been presented (Table S1) [25], [26], [27], [28], [29], [30], [31], [32], [33]. Some of them showed comparable activities to homogeneous catalyst systems and were highly recyclable, however, further improvements are required to address issues related to degradation, leaching of the active component, and side product formation, etc.
In this report, the formation route of 3D-ZIF-7/2D-Zn(BeIM)OAc was clearly elucidated along with the role of DEA in the mixture formation. In the following section, the synergistic catalytic performance of ZIF-7/Zn(BeIM)OAc was shown, which was comparable to that of a homogeneous system. The catalyst system showed significant durability, with no reduction in catalytic activity and no structural collapse, even after more than five uses.
Section snippets
General
All chemicals used in this study were of analytical grade, commercially available, and were used without further purification. Zinc acetate dihydrate was purchased from Tokyo Chemical Industries (TCI) Co. Ltd and other chemicals were purchased from Sigma-Aldrich.
Materials preparation
ZIF-7 supported on Zn(BeIM)OAc was synthesized from Zn(OAc)2·2H2O and benzimidazole with a molar ratio of 1 to 1.5. The detailed procedure was modified from the conventional synthesis method of ZIF-7 [12]. 4.48 g (20 mmol) of Zn(OAc)2·2H2
One-pot synthesis of 3D-ZIF-7 supported on 2D-Zn(BeIM)OAc
The one-pot synthesis of 3D-ZIF-7 (Zn(BeIM)2) supported on 2D-Zn(BeIM)OAc was successfully conducted from Zn(OAc)2 and benzimidazole (1/1.5 molar ratio) in the presence of diethylamine (DEA) and solvent DMF (this product is denoted as ZIF-7(1.5)). The reaction was carried out at 130 °C using a round-bottomed flask equipped with a reflux condenser for 24 h, then, the DEA was distilled off and the reaction was continued for an additional 24 h (Scheme 1). The SEM image of a white precipitate in Fig. 2
Conclusion
In the reaction of Zn(OAc)2 and benzimidazole, by controlling the amount of diethylamine during the reaction, two different zinc imidazolate compounds, ZIF-7 and Zn(BeIM)OAc, could be synthesized together. The synthesized material was in the form that small three-dimensional ZIF-7 particles were coated on the surface of a two-dimensional bulky Zn(BeIM)OAc. It was confirmed that ZIF-7 was first synthesized in the presence of diethylamine, and then some part of ZIF-7 changed to Zn(BeIM)OAc by
Conflicts of interest
There are no conflicts to declare.
Acknowledgments
This work was supported by C1 Gas Refinery Program through the National Research Foundation of Korea (NRF) (NRF-2015M3D3A1A01065435) and KIST internal program (Atmospheric Environment Research Program, Project No. 2E31360).
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