Crystal Growth & Design
Article
methylimidazole (3 mmol, 246 mg) were mixed and ground in a
mortar about 5 min under ambient atmosphere. Cobalt(II)
acetylacetonate was replaced by zinc acetylacetonate hydrate (1
mmol, 263 mg) to synthesize ZIF-8. For the synthesis of Zn/Co-ZIF,
both metal precursors (50:50 ratio of cobalt(II) acetylacetonate/zinc
acetylacetonate) were mixed. Then the obtained solid mixture was
placed in an alumina boat (30 × 60 × 15 mm3) before transferring
into the quartz tube (OD: 60 mm, length: 1000 mm) inside the
muffle furnace (TL 1200, Nanjing Bo Yun Tong Instrument
Technology Co. Ltd). An Ar or N2 (100 cm3·min−1) stream was
flown in the system. The temperature program was set in two steps;
from room temperature to 100 °C within 30 min, increased to 200 °C
(5 °C·min−1), and then the temperature was kept constant for 1 h.
Finally, the product was collected after cooling and used in further
applications or characterization. The yield of the products was
measured using eq 1.
ability to dissolve the precursors and support in crystal
formation. Using water as a solvent, a green method was
developed at room temperature, generating nanocrystal
products and a high production yield compared to the
solvothermal synthesis.13−15 However, only a limited number
of precursors (metal and ligand) display solubility in water,
resulting in only a few achievements for the synthesis of
MOFs/ZIFs. Improvements in the green approach were
obtained using advanced synthesis techniques such as
microwave, mechanochemical, spray drying, electrochemistry,
etc.16,17 The microwave-assisted method is developed for fast
crystallization MOFs synthesis. Still, a unique instrument with
a complicated operation is required. Nonconventional
methods such as electrochemical deposition, nucleation agent
seeding, etc., have been developed to synthesize ZIFs. Despite
the currently available methods to synthesize these materials,
there are still several drawbacks, e.g., high energy, utilization of
expensive or harmful precursors and organic solvents,
prolonged reaction time, excess of ligand (ligand/metal > 1),
use of additives (acid/base), or application of a complex
instrument, and so on. During the conventional method, the
synthesis and subsequent post-treatment produce various
wastes resulting in a nonenvironmentally friendly synthesis of
the materials. Additionally, the presence of a solvent, salt
residue, or side-products requires further processing as post-
treatment (purification or activation) before introducing the
MOF material in an application. The activation (post-
treatment) is required to access all material’s functionalities
before using it in an application. All these reasons limit the
synthesis and exploitation of ZIFs in new applications.
Moreover, no practical information for the synthesis on a
large scale is available. The reported different methods to
synthesize the materials resulted in ZIFs with specific
structures and properties. The obtained properties can be
correlated with the desired applications.18−20 Therefore,
exploiting a new synthetic method with comparable facile
and green procedures is of ongoing interest and challenge in
the development of MOFs/ZIFs.
Herein, the development of a new method based on a facile
and straightforward one-pot synthesis route to synthesize ZIFs
(ZIF-8, ZIF-67, and ZnCo-ZIF) through the in situ thermal
treatment (IST) method is reported. High porous crystalline
materials were obtained in a short synthesis time under
solvent- and additive-free conditions in this approach. The as-
synthesized materials were demonstrated to be isostructural
and exhibited identical properties (i.e., porosity, surface area,
and thermal stability) compared with traditional synthesized
ZIFs. Moreover, no post-treatment or activation process of the
synthesized material was required. Finally, the obtained ZIFs
applying the straightforward and effective cost IST method
demonstrated advantages for chemical and physical properties.
These advantages were exemplified by adsorption and catalysis
application. Moreover, the novel IST synthesis method for
ZIFs provides the potential for industrial application. All
arguments mentioned above are advantageous compared with
the traditional or earlier reported synthesis methods.
average weight of product
synthesis yield (%) =
× 100
theoretical product
(1)
2.2. Characterization. The X-ray diffraction (XRD) was
performed using a Bruker D8 Advance diffractometer at 40 kV and
45 Ma with Cu Kα radiation source (λ = 1.54056 Å at 40 kV and 45
Ma) and a scanning rate of 2°/min−1. The field-emission scanning
electron microscopy (FE-SEM, Zeiss Ultra Plus) was applied to
determine the materials’ size and morphology. The gas adsorption−
desorption measurement was carried out with an ASAP 2020 Analyzer
(Micrometrics Instruments) using N2, CO2, and CH4 gases of
99.999% purity. In addition, samples were activated under a dynamic
vacuum at 200 °C for 3 h before the adsorption measurement. The
porosity and surface area were analyzed by the Brunauer−Emmett−
Teller (BET) and Langmuir methods. The linearized BET and
Langmuir equations were fitted in the range of 0.003 < P/P0 < 0.05.
Fourier transform infrared (FT-IR) spectra were recorded on a
Nicolet 6700 FT-IR spectrometer with KBr pellets in the wavelength
range of 4000−400 cm−1. Thermogravimetric analysis was carried out
on a TGA from Netzsch (STA449c/3/G) Instrument with a heating
rate of 10 °C·min−1 under a nitrogen atmosphere. The acidity and
basicity measurements were carried out on the Auto Chem II 2920
instrument equipped with a thermal conductivity detector (TCD)
(Micromeritics). NH3 and CO2 were used probe gas for temperature-
programmed desorption tests for acidity (NH3-TPD) and basicity
(CO2-TPD) analysis, respectively. 1H NMR spectra were recorded on
a Bruker Avance III 500 spectrometer in CDCl3 with 1,3,5,-trioxane as
the internal reference, and 13C NMR spectra were recorded in CDCl3
on a Bruker Avance 500 (126 MHz) spectrometer.
2.3. Catalytic Reaction. The addition of CO2 into epichlorohy-
drin was performed using the synthesized material (ZIF-67_1:3) and
compared with the conventionally synthesized ZIF-67 in methanol at
room temperature (ZIF-67-RT). The synthesized catalysts (50 mg)
and substrates (9.2 mmol) were charged in a high-pressure glass tube
(15 mL) with a magnetic stir bar. The reactor’s atmosphere was
exchanged with CO2 (99.9% purities) before pressurizing to 1.5 bar of
CO2. The reactor was immersed in a preheated oil bath at 90 °C
under stirring conditions (400 rpm) for 24 h. The reactor was cooled
to room temperature, and 1,3,5-trimethoxybenzene as an internal
standard (3 mmol) and CDCl3 (1 mL) were added. The reaction
1
mixture was analyzed via H NMR using CDCl3 as a solvent. In the
recyclability experiments, the catalyst was recovered by centrifugation,
washed three times with methanol, and dried at 120 °C under vacuum
for 12 h for the next reuse.
3. RESULTS AND DISCUSSION
3.1. Synthesis and Characterization. Among zeolitic
imidazole frameworks, ZIFs based on the 2-methylimidazole
linker (ZIF-8, ZIF-67, and bimetallic ZnCo-ZIF) have been
mainly synthesized and applied in a broad field of
applications.7,21−23 In particular, ZIF-67 and its derivatives
show outstanding characteristics and performances in various
2. EXPERIMENTAL SECTION
2.1. Materials and Synthesis. All reagents were used as received
from Aladdin Ltd., including cobalt(II) acetylacetonate (97%), zinc
acetylacetonate hydrate (97%), 2-methylimidazole (98%), and
epichlorohydrin (analytical grade grad). In a typical synthesis of
ZIF-67, the cobalt(II) acetylacetonate (1 mmol, 257 mg) and 2-
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Cryst. Growth Des. 2021, 21, 5349−5359
Wang, Jichao
