Formation of defect site on ZIF-7 and its effect on the methoxycarbonylation of aniline with dimethyl carbonate

Article abstract of DOI:10.1016/j.jcat.2019.09.039

The Zeolitic Imidazole Framework ZIF-7, (Zn(benzimidazole)₂), is known to exhibit a unique gate-opening property depending on the temperature and pressure in the presence of guest molecules, making it useful for H₂, CO₂, and paraffine separation technology. Besides this distinctive gas adsorption property, ZIF-7 can be used as a catalyst because it contains a Lewis acid site on Zn, and a Lewis basic site on the benzimidazole. In this study, for the first time, we demonstrate that ZIF-7 is a very promising material as a catalyst for the methoxycarbonylation of aniline with dimethyl carbonate (DMC) to produce methyl phenyl carbamate (MPC), an isocyanate precursor. Fresh ZIF-7 showed a high aniline conversion of over 95% at 190 °C for 2 h, and the yield MPC was 60–70% due to the formation of methylated side products, such as N-methyl aniline and N,N′-dimethylaniline. However, interestingly, when the ZIF-7 was reused, the yield of MPC gradually increased and reached over 94.7% by the catalyst's 6th run. SEM and TEM images revealed the crystalline structure of ZIF-7 collapsed during the reaction due to the leaching of benzimidazole from the ZIF-7, which created defect sites on the Lewis acidic zinc center. Based on ¹H NMR and XPS studies, it can be assumed that the DMC binds to the Zn defect sites on the ZIF-7, activating the carbonyl groups on DMC, resulting in the increased selectivity to methoxycarbonylation compared to methylation. Pretreating ZIF-7 with DMC at 190 °C for 6 h could directly activated the ZIF-7 for this methoxycarbonylation reaction. The activated ZIF-7 showed 91.0% MPC yield and the catalyst was reusable.

Full text of DOI:10.1016/j.jcat.2019.09.039

Journal of Catalysis xxx (xxxx) xxx
Contents lists available at ScienceDirect
Journal of Catalysis
journal homepage: www.elsevier.com/locate/jcat
Formation of defect site on ZIF-7 and its effect on the
methoxycarbonylation of aniline with dimethyl carbonate
Deliana Dahnum a,b, Bora Seo a, Seok-Hyeon Cheong a,b, Ung Lee a,b, Jeong-Myeong Ha a,b, Hyunjoo Lee a,b,
⇑
a Clean Energy Research Center, Korea Institute of Science and Technology, 5, Hwarang-ro 14-gil, Seongbuk-gu, Seoul 02792, Republic of Korea
b Division of Energy & Environment Technology, KIST School, Korea University of Science and Technology, Seoul 02792, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history:
The Zeolitic Imidazole Framework ZIF-7, (Zn(benzimidazole)2), is known to exhibit a unique gate-opening
property depending on the temperature and pressure in the presence of guest molecules, making it useful
for H2, CO2, and paraffine separation technology. Besides this distinctive gas adsorption property, ZIF-7
can be used as a catalyst because it contains a Lewis acid site on Zn, and a Lewis basic site on the ben-
zimidazole. In this study, for the first time, we demonstrate that ZIF-7 is a very promising material as
a catalyst for the methoxycarbonylation of aniline with dimethyl carbonate (DMC) to produce methyl
phenyl carbamate (MPC), an isocyanate precursor. Fresh ZIF-7 showed a high aniline conversion of over
95% at 190 °C for 2 h, and the yield MPC was 60–70% due to the formation of methylated side products,
such as N-methyl aniline and N,N0-dimethylaniline. However, interestingly, when the ZIF-7 was reused,
the yield of MPC gradually increased and reached over 94.7% by the catalyst’s 6th run. SEM and TEM
images revealed the crystalline structure of ZIF-7 collapsed during the reaction due to the leaching of
benzimidazole from the ZIF-7, which created defect sites on the Lewis acidic zinc center. Based on 1H
NMR and XPS studies, it can be assumed that the DMC binds to the Zn defect sites on the ZIF-7, activating
the carbonyl groups on DMC, resulting in the increased selectivity to methoxycarbonylation compared to
methylation. Pretreating ZIF-7 with DMC at 190 °C for 6 h could directly activated the ZIF-7 for this
methoxycarbonylation reaction. The activated ZIF-7 showed 91.0% MPC yield and the catalyst was
reusable.
Received 20 August 2019
Revised 24 September 2019
Accepted 25 September 2019
Available online xxxx
Keywords:
ZIF-7
Defect site
Methoxycarbonylation
Aniline
DMC
Aromatic carbamate
Ó 2019 Elsevier Inc. All rights reserved.
1. Introduction
is known to be the best catalyst, however, during the reaction, zinc
acetate turns into ZnO and loses its activity [6].
Carbamate is one of the attractive green chemicals that can
avoid toxic phosgene during the production of isocyanate. One of
the issues in the development of catalysts for the methoxycarbony-
lation of aromatic amine with DMC is related to their selectivity to
the product. As can be seen in Scheme 1, DMC can act as a methy-
lating agent as well as methoxycarbonylation agent [1,2]. One way
of increasing the selectivity to the methoxycarbonylation reaction
is to use a Lewis acid as a catalyst, which activates the carbonyl
groups on the DMC, thereby facilitating the nucleophilic attack of
amine. Accordingly, many Lewis acid catalytic systems have been
reported, including metal salts based on zinc [3,4], lead [5,6] and
ytterbium [7]. Among these catalysts, homogeneous zinc acetate
To overcome the limitations of a homogeneous catalyst system,
which include difficulties with catalyst separation from the pro-
duct mixture and recycling after the reaction, immobilizing of Zn
(OAc)2 on a solid support has been investigated. Zhao et al. showed
that Zn(OAc)2 supported on activated carbon produced MPC with a
yield of 78% and a selectivity of 98%; however, serious leaching of
the active component was observed [8]. Li et al. also anchored Zn
(OAc)2 onto the surface of SiO2 and found that a high MPC yield
of 93.8% could be achieved, but the catalytic activity decreased
during reuse due to the formation of zinc oxide [9]. Deactivation
of the catalyst due to decreased support area also occurred when
ZnO-TiO2 and ZrO2-SiO2 were used in the reaction [10,11]. The
mesoporous catalyst AlSBA-15 showed recyclability after calcina-
tion at 550 °C in air and yielded 70.3% of MPC [12].
On the other hand, Guo et al. immobilized zinc ions on SBA-15
which contain carboxylate functional groups [13]. Using the syn-
thesized heterogeneous catalyst, the methoxycarbonylation of
4,40-methylenedianiline (MDA) with dimethyl carbonate was
⇑
Corresponding author at: Clean Energy Research Center, Korea Institute of
Science and Technology, 39-1 Hawolgok-dong, Seongbuk-gu, Seoul 02792, Republic
of Korea.
E-mail address: hjlee@kist.re.kr (H. Lee).
https://doi.org/10.1016/j.jcat.2019.09.039
0021-9517/Ó 2019 Elsevier Inc. All rights reserved.
Please cite this article as: D. Dahnum, B. Seo, S. H. Cheong et al., Formation of defect site on ZIF-7 and its effect on the methoxycarbonylation of aniline with
dimethyl carbonate, Journal of Catalysis, https://doi.org/10.1016/j.jcat.2019.09.039

2
D. Dahnum et al. / Journal of Catalysis xxx (xxxx) xxx
Scheme 1. Reaction of aniline with dimethyl carbonate (DMC). MPC: methyl phenyl carbamate, MMPC: methyl N-methyl-N-phenyl carbamate, MA: N-methylaniline, DMA:
N,N-dimethylaniline.
effectively achieved, although catalyst deactivation was observed
during reuse. Similarly, Wang et al. also synthesized zinc alkyl car-
boxylate which was chemically bonded onto a silica surface [14]. It
was found to possess a cyclic single catalytic site which was highly
active and recyclable. Although both catalyst systems had catalytic
activity comparable to homogeneous Zn(OAc)2, their complicated
synthesis processes could be an obstacle to the practical applica-
tion of the catalysts.
Zeolitic Imidazole Frameworks (ZIFs), ZIFs, a subclass of metal-
organic frameworks (MOFs), are attracting great attention due to
their large surface area, facile synthesis as well as high chemical
and thermal stabilities [15,16]. Accordingly, this material has great
applications as sensors for chemical compounds, membrane sepa-
ration, and adsorbents for hydrogen, methane, ethane, and CO2 gas
[17–24]. Furthermore, besides acting as a catalyst support for
metal nanoparticles or transition metal complexes, ZIF itself has
shown good catalytic activities for the Knoevenagel reaction [25],
the coupling reaction of epoxide and CO2 [26,27], olefin epoxida-
tion [28], esterification [29], transesterification [30], and etc.
change from narrow pore phase to large pore phase can occur
depending on the temperature and pressure in the presence of
guest molecules like CH4, CO2 and light olefins [32]. Many struc-
tural investigations have been conducted to understand this gate
opening phenomenon, and it has been employed for the separation
of paraffin/olefin, water/ethanol, CH4 and CO2 from N2.
Like other ZIF materials, ZIF-7 has a Lewis acidic site and basic
site on the zinc metal and benzimidazole, respectively. This means
it should be able to act as a catalyst for both Lewis acid or Lewis
base-catalyzed reactions. However, to the best of our knowledge,
the catalytic activity of ZIF-7 has not been studied in any catalytic
reactions.
In this study, we used ZIF-7 as a catalyst for the methoxycar-
bonylation of aniline and found that defects were formed on the
ZIF-7 during the reaction. The ZIF-7 with defect sites showed
enhanced catalytic performance for the methoxycarbonylation
reaction compared to the fresh ZIF-7.
Frequently, the defect sites on MOFs or ZIFs have played a crit-
ical role in some catalytic reactions [33]. Vermoortele et al.
reported the catalytic activity of zirconium terephthalate UiO-66
(Zr) could be increased by treating the catalyst with trifluoroacetic
acid which affects the formation of the open metal site [34]. Chiza-
llet et al. supposed that the high activity of ZIF-8 for transesterifi-
cation may come from the strong Lewis acid sites of Zn(II), and
ZIF-7 composed of Zn metal connected with
a
1H-
benzimidazole linker is a typical ZIF with sodalite (SOD) frame-
work topology (Scheme 2). Interestingly, it has a very distinctive
property, the gate opening effect [31]. It was reported that, due
to the flexible benzimidazole linker, a non-destructive structural
Scheme 2. Structure of ZIF-7. Zn, yellow; N, blue; C, gray; H, white.
Please cite this article as: D. Dahnum, B. Seo, S. H. Cheong et al., Formation of defect site on ZIF-7 and its effect on the methoxycarbonylation of aniline with
dimethyl carbonate, Journal of Catalysis, https://doi.org/10.1016/j.jcat.2019.09.039

D. Dahnum et al. / Journal of Catalysis xxx (xxxx) xxx
3
Yield of ureas ð%Þ ¼ Conv: of aniline ð%Þ
ꢁ Yield of products ðMPC þ MMPC þ MA þ DMAÞð%Þ
Bronsted acid sites (NH group) [30]. Meanwhile, Tian et al. and Lee
et al. showed the surface of ZIF-8 was covered with various groups
such as carbonate, water/hydroxide, and secondary amines in the
surface of the material, which affected the separation ability of
ZIF-8 [35,36].
For the reuse of the catalyst, after the reaction, the catalyst was
separated from the reaction mixture by centrifugation. A spent cat-
alyst was washed with DMC and dried at 60 °C for 3 h under
vacuum.
We analyzed the used catalyst by SEM, 1H NMR, XPS, and
XANES and proved that benzimidazole was leached from the Zn
center during the reaction, forming a catalytic active site on the
surface of the ZIF-7. Based on this observation, we proposed a cat-
alytic mechanism for this reaction.
2.4. Characterization of catalyst
The XRD pattern of ZIF-7 before and after the reaction was
obtained by Shimazu X-ray diffractometer (XRD-6000, Japan) using
2. Experimental section
nickel-filtered CuKa radiation with the 2h angle between 5 and
80°. GC–MS (HP- 6890 GC with a 5973 Mass spectrometer) was
used to confirm the products. Zinc concentration was measured
by ICP-AES (Thermo scientific iCAP 7000). Proton NMR was carried
out on a Bruker Avance 400 MHz (Bruker Corp.: Billerica, MS, USA).
For the preparation 1H NMR samples, 0.05 g of ZIF-7 was diluted in
0.5 g D2SO4 and 0.5 g H2SO4 including 10 wt% of methanesulfonic
acid as an internal standard. Scanning Electron Microscopy (SEM)
of samples was accomplished using a field-emission scanning elec-
tron microscope (FEI Inc., NovaNano 200, Hillsboro and Teneo VS).
CHN analysis was conducted using Thermo scientific Flash 2000.
Transmission electron microscopy (TEM) images were taken by
FEI Tecnai F20 G2 at an acceleration voltage of 200 kV. XAS was
performed at the beamline 8C of the Pohang Accelerator Labora-
tory (PAL). The incident X-ray had the electron beam energy and
current of 3.0 GeV and 300 mA, respectively. A Si (1 1 1) double
crystal monochromator was used to filter the incident photon
energy, which was detuned by 30% to remove high-order
harmonics.
2.1. General
All chemicals used in this study were of analytical grade, com-
mercially available, and were used without further purification.
Zinc acetate dihydrate and methyl phenyl carbamate were pur-
chased from Kanto and Tokyo Chemical Industries (TCI) Co. Ltd,
respectively. The other chemicals were purchased from Sigma-
Aldrich.
2.2. Catalyst preparation
A series of ZIF-7 was synthesized according to the reported
method [37]. Zinc precursor, including Zn(OAc)2, ZnCl2, and ZnBr2,
benzimidazole, and diethylamine in DMF were heated in the bomb
reactor system with stirring at 130 °C for 48 h, while Zn(NO3)2 was
reacted at RT. After the reaction, the formed solid was filtered and
washed with DMF three times. The isolated solid was dried at 160
°C for 3 h under vacuum to remove DMF. The synthesized material
was stored in the glove box before use. Detailed syntheses methods
and XRD analyses results of produced ZIF-7 according to the used
zinc precursor were described in Supporting Information and
Fig. S1.
3. Results and discussion
3.1. Methoxycarbonylation using ZIF-7
2.3. Methoxycarbonylation
Four kinds of zeolitic imidazole framework-7 (ZIF-7) complexes
were synthesized by the reaction of zinc compounds of Zn(OAc)2,
ZnCl2, ZnBr2, and Zn(NO3)2 with benzimidazole, using DMF as the
solvent as described in the experimental part. The resulting mate-
rials were designated ZIF-7-OAc, ZIF-7-Cl, ZIF-7-Br, and ZIF-7-NO3
according to the zinc precursors used. After synthesis, all products
were activated at 160 °C for 3 h under vacuum to remove the
solvent.
CHNZn analyses results shown in the Supporting Information
reveal the empirical formula of ZIF-7 from ZnCl2, ZnBr2, and Zn
(NO3)2 were very close to Zn(BeIm)2, however, the shapes and sur-
face areas were different according to the zinc precursor used [37].
Catalytic reaction was conducted in a high-pressure Parr reactor
(100 mL) equipped with glass liner and
a magnetic-driven
mechanical stirrer. 12.5 mmol (1.16 g) of aniline and 150 mmol
(13.5 g) of DMC were added in glass liner together with 0.116 g
(10 wt%) of catalyst and 0.5 g of toluene as an internal standard.
After purging the reactor with N2 (>99.9%) twice, the temperature
was increased to 190 °C with a 4 °C/min. After 2 h, the reactor was
cooled to room temperature. Quantitative analysis of product in
solution was performed by using Gas Chromatography (Aligent
7890A) with an HP-5 capillary column (30 m ꢀ 0.32 mm ꢀ 0.25
lm) and a FID detector. Typical GC-chromatogram was shown in
Fig.
1 and Table S1 show that ZIF-7-OAc has a rhombic-
dodecahedral shape with a BET surface area of 75 m2 gꢁ1, while
ZIF-7-Cl and ZIF-7-Br had a rod morphology with BET areas of 12
and 19 m2 gꢁ1, respectively. ZIF-7-NO3 showed the highest surface
area of 110 m2 gꢁ1 which originated from its small spherical parti-
cles and amorphous nature, as shown in Fig. 1. We expected that
those different morphologies and surface areas could result in dif-
ferent catalytic activities during the methoxycarbonylation reac-
tion, but they showed very similar results.
Fig. S2 in Supporting Information.
The conversion of aniline, yields of products including MPC,
MMPC, MA, and DMA were calculated as following equations.
Three kinds of ureas, diphenyl urea, N-methyl-diphenyl urea, and
N,N0-dimethyl-diphenyl urea, were produced together with a
minor concentration. The total yield of ureas was measured by
subtracting the yields of MPC, MMPC, MA, and DMA from the con-
version of aniline.
Fig. 2 revealed when the synthesized ZIF-7s were used as a cat-
alyst for the methoxycarbonylation of aniline with dimethyl car-
bonate (DMC) at 190 °C for 2 h, the aniline was converted almost
quantitatively in all the reactions. They produced methyl phenyl
carbamate (MPC) with similar yields between 61.9 and 70.7%. Con-
trary to our expectation, ZIF-7-NO3 which had the highest surface
area exhibited the lowest MPC yield of 61.9%. As described in the
Conv: of anilineð%Þ
Initial aniline ðmmolÞ ꢁ Unreacted aniline ðmmolÞ
¼
ꢀ 100%
Initial aniline ðmmolÞ
Product ðmmolÞ
Yield of productð%Þ ¼
ꢀ 100%
Initial aniline ðmmolÞ
Please cite this article as: D. Dahnum, B. Seo, S. H. Cheong et al., Formation of defect site on ZIF-7 and its effect on the methoxycarbonylation of aniline with
dimethyl carbonate, Journal of Catalysis, https://doi.org/10.1016/j.jcat.2019.09.039

4
D. Dahnum et al. / Journal of Catalysis xxx (xxxx) xxx
Fig. 1. SEM images of various ZIF-7 synthesized from (a) Zn(OAc)2, (b) ZnCl2, (c) ZnBr2, (d) Zn(NO3)2.
showed the highest selectivity at 30 min reaction, and decreased
after that, while, DMA selectivity increased constantly with the
reaction time. In the catalyst-free reaction, although the conver-
sion of aniline and selectivity to MPC were much lower than in
the ZIF-7-catalyzed reaction, similar selectivity changes beween
MPC-MMPC and MA-MMA were observed according to reaction
time (Fig. S3(b)).
Over all, these results indicate ZIF-7 promoted methoxycar-
bonylation rather than methylation, but its yield and selectivity
to MPC were poorer compared to the homogeneous catalyst sys-
tem, which reduces the advantages of the heterogeneous catalyst.
However, interestingly, when the catalyst was reused, the yield
and selectivity to MPC increased significantly.
Fig. 2. Methoxycarbonylation of aniline with DMC using various ZIF-7s. Reaction
condition: aniline 1.16 g, DMC 13.5 g, catalyst 0.116 g, 190 °C, 2 h.
introduction section, DMC also acted as a methylating agent to pro-
duce methylated MPC (MMPC), N-methylaniline (MA) and N,N-
dimethylaniline (DMA). They were formed with yields between
4.5 and 12.4%. Diphenyl urea and its methylated derivatives made
from MPC and aniline were also detected. In the absence of a cat-
alyst, aniline conversion was 55% and MPC yield was 22.9%. The
formation of methylated side products, MA and DMA, were slightly
higher than those of the ZIF-7-catalyzed reaction. They were 13.0%
and 14.6% yields for MA and DMA, respectively.
On the other hand, as S. Grego et al proposed, the formation of
MMPC and DMA seems to be formed via MPC and MA, respectively,
as shown in Scheme 1 [38]. The effect of reaction time on the selec-
tivities of MPC, MMPC, MA, and DMA support this assumption
(Fig. S3). For the ZIF-7-catalyzed reaction shown in Fig. S3(a),
MPC selectivity reached its maximum after 2 h reaction, and
decreased at 4 h with an increase in MMPC. In the case of MA, it
Fig. 3. Reuse of ZIF-7 for the methoxycarbonylation of aniline with DMC. Reaction
condition: aniline 1.16 g, DMC 13.5 g, catalyst 0.116 g, 190 °C, 2 h.
Please cite this article as: D. Dahnum, B. Seo, S. H. Cheong et al., Formation of defect site on ZIF-7 and its effect on the methoxycarbonylation of aniline with
dimethyl carbonate, Journal of Catalysis, https://doi.org/10.1016/j.jcat.2019.09.039

D. Dahnum et al. / Journal of Catalysis xxx (xxxx) xxx
5
3.2. Catalyst reuse
run, respectively. The same phenomena were observed with the
other ZIF-7s prepared from ZnCl2, ZnBr2, and Zn(NO3)2 (Fig. S4).
For the catalyst reuse experiment, ZIF-7 synthesized from Zn
(OAc)2 was used. After the 1st reaction, the catalyst was filtered
out, washed with DMC, and dried at RT under vacuum for 3 h
and then used again. Fig. 3 show that with increasing catalyst
reuse, the yields of MPC increased. When the ZIF-7 isolated from
the 1st reaction was used again, the yield of MPC increased to
77.6% from 70.7%. Furthermore, the MPC yield increased further
and reached 94.7% by the 6th run. This MPC yield is similar to those
from homogeneous Zn, Pb, and Yb-based methoxycarbonylation
reaction and belongs to one of the highest MPC yield reported at
the heterogeneous catalyst reaction (Table 1). Whereas, the yields
of methylated products decreased by a substantial degree with the
catalyst reuse. The formation of MA and DMA, which were 5.9 and
12.4% in the first reaction, were reduced to 0.7 and 0.6% in the 6th
3.3. Characterization of the used ZIF-7
The increased selectivity to MPC with catalyst reuse indicates
that some properties of the ZIF-7 changed during the reaction. To
elucidate the reason for the increased catalytic activity, the mor-
phologies of the catalyst before and after the reaction were
observed using SEM and TEM. Fig. 4 and Fig. S5 show, fresh ZIF-7
made from Zn(OAc)2 had a very regular rhombic-dodecahedral
structure. However, after the reaction, the crystal structure of
ZIF-7 was destroyed. The structures were worn and broken, and
the surface of the ZIF-7 was rough and had cracks. In addition,
some particles appeared to be agglomerated. As the number of uses
increased, the degree of structural changes increased.
Table 1
Comparison of catalytic activities of various catalysts at the methoxycarbonylation of aniline or its derivatives with DMC
Entry.
Catalyst
Reaction Condition
Carbamate Yield (%)
Remarks
Ref.
(Temp., reaction time, catalyst amount)
Homogeneous
1
2
3
4
5
Zn(OAc)2ꢂ2H2O
120 °C, 190 min, 4 mol%
180 °C, 2 h, 0.25 mol%
160 °C, 1 h, 12 mol%
170 °C, 4 h, 2 mol%,
80 °C, 8 h, 5 mol%
85
96
96.7
97.7
96
Deactivation to ZnO
–
–
Deactivation to Pb3(CO3)2(OH)2
Extraction of catalyst using CH2Cl2
3
4
5
6
7
Zn4O(O2CCH3)6
PbO
Pb(OAc)2
Yb(OTf)3
Heterogeneous
6
7
8
9
10
11
12
13
ZIF-7 (DMC-treated)
Zn(OAc)2/AC
Zn(OAc)2/SiO2
ZnO-TiO2
190 °C, 2 h, 10 wt%
150 °C, 4 h
91.0
78.0
93.8
66.7
79.8
71.0
86.5
91.6
Reusable
This work
8
9
10
11
12
13
14
Leaching of Zn(OAc)2
Deactivation to ZnO
Deactivation
170 °C, 7 h, 30 wt%
170 °C, 7 h, 25 wt%
170 °C, 7 h, 25 wt%
100 °C, 3 h, 53 wt%
170 °C, 4 h, 20 wt%
170 °C, 2 h, 0.5 mol% Zn
ZrO2/SiO2
Al/SBA-15
Deactivation
Reusable after calcination
Slight deactivation with reuse
Reusable
Zinc carboxylate/SBA-15c
Zinc alkyl carboxylate on silica
Fig. 4. SEM images of ZIF-7 made from Zn(OAc)2 (a) fresh, (b) after 2nd run, (c) after 4th run, and (d) after 6th run.
Please cite this article as: D. Dahnum, B. Seo, S. H. Cheong et al., Formation of defect site on ZIF-7 and its effect on the methoxycarbonylation of aniline with
dimethyl carbonate, Journal of Catalysis, https://doi.org/10.1016/j.jcat.2019.09.039

6
D. Dahnum et al. / Journal of Catalysis xxx (xxxx) xxx
XRD spectra, which are shown in Fig. 5, also gave similar results.
coordinated with oxygen atoms like Zn acetate, and peak II is sig-
nificant in the Zn imidazole complex, where Zn is coordinated to
four N atoms [43,44]. After the 4th run, peak I increases and peak
II decreases, suggesting increased Zn–O coordination and
decreased Zn–N coordination, respectively.
The amount of leached benzimidazole from the catalyst and the
types of attached chemical species on the vacant zinc sites could be
estimated using 1H NMR and ICP analyses. When ZIF-7 is dissolved
in D2SO4, it is destructed to zinc ion and benzimidazole, which
enable the quantification of the zinc and benzimidazole in the
ZIF-7. As Fig. 7 and Table S2 show, in 1H NMR the fresh ZIF-7 only
had benzimidazole, and the amount of benzimidazole with respect
to Zn was 2.06, which was close to the empirical formula of ZIF-7,
Zn(BeIm)2. However, after the 2nd run, the molar ratio of benzim-
idazole to Zn became 1.86 and it decreased further to 1.44 after the
6th run.
The intensities of the characteristic peaks of ZIF-7 at 13.3°, 15.4°,
16.2°, and 19.8° which correspond to the (0 3 0), (2 2 0), ( ꢁ1 3
2), and (3 1 2) planes [15], decreased with increasing catalyst
reuse, confirming the destruction of the crystalline phase in ZIF-7
after the reaction. However, interestingly, the intensity of the peak
at 7.7°, which is the (1 1 0) plane, increased with reuse.
Previously, ZIFs like ZIF-7 and ZIF-8 have been known to have a
reversible structural change called ‘gate opening’ or ‘breathing’
under CO2 pressure, C2-C3 alkane adsorption, or solvent substitu-
tion [23,24,39,40]. During the structural changes, the pores in
ZIF-7 transform from a narrow pore phase (np-phase) to a large
pore phase (lp-phase), which can be distinguished in the XRD
peaks, at 8.6° and 9.5° for the np-phase and 7.7° for the lp-phase.
In our study, the np-phase was also detected at 8.6° and 9.5° in
the fresh ZIF-7, but they shifted slightly to 8.3° and 9.2° after 2nd
run, indicating the pores had become larger during the reaction.
After the 4th run, these np peaks disappeared and the lp-phase
became stronger.
Interestingly, new peaks with apparent intensities were
observed in the reused catalysts, whose positions overlapped the
peaks of MPC and methanol. MPC and methanol species are reac-
tion products of this methoxycarbonylation reaction. Therefore,
The collapse of the crystal structure of the ZIF-7 during the reac-
tion can be attributed to a change in the chemical composition of
ZIF-7 by the leaching of the ligand, benzimidazole, and/or zinc
component. This creates a defect site on the catalyst, which can
act as an active catalytic site for the methoxycarbonylation
reaction.
Zhang et al. computationally examined the thermodynamic sta-
bility and kinetic accessibility of various point defects in ZIF-8, Zn
(2-methylimidazolate)2, and found that several of the defect struc-
tures were lower in energy relative to the defect-free parent crystal
[41]. They suggested three kinds of point defects; linker vacancy,
zinc vacancy, and dangling linker. In this study, we deduced the
formation of a linker vacancy in the ZIF-7 during the reaction, by
the leaching of benzimidazole, which resulted in the exposure of
a Lewis acidic zinc site in the reaction media.
The ligand change in ZIF-7 after the reaction was also observed
in X-ray absorption near-edge structure (XANES) spectra. Fig. 6
shows that the edge lines of the spectra for the catalyst before
and after the reactions are overlapped, indicating that the oxida-
tion state of Zn was almost retained [42]. There was a negligible
shift in the binding energies of zinc before and after the reactions.
The white line peaks that appeared around 9665 eV and 9672 eV
(denoted I and II, respectively) are related to the 1 s ? 4 s transi-
tion, and their intensities are affected by the coordination environ-
Fig. 6. X-ray Absorption Near Edge Spectra (XANES) of ZIF-7.
ment [42]. More specifically, peak
I dominates when Zn is
Fig. 7. 1H NMR spectra of (a) fresh ZIF-7, (b) ZIF-7 after 2nd run, (c) ZIF-7 after 4th
run, (d) ZIF-7 after 6th run, (e) MeOH, (f) MPC. All NMR spectra were measured
a
in D2SO4.
Detailed explanation about the 1H NMR of MPC was described
Fig. 5. XRD patterns (a) fresh ZIF-7 and used ZIFs (b) after 2nd run, (c) after 4th run,
in Supporting information.
and (d) after 6th run.
Please cite this article as: D. Dahnum, B. Seo, S. H. Cheong et al., Formation of defect site on ZIF-7 and its effect on the methoxycarbonylation of aniline with
dimethyl carbonate, Journal of Catalysis, https://doi.org/10.1016/j.jcat.2019.09.039

D. Dahnum et al. / Journal of Catalysis xxx (xxxx) xxx
7
they could occupy the vacant sites in Zn formed by the leaching of
benzimidazole.
ppm, a methanol peak, while no peak was detected for the MeOH-
treated catalyst (Fig. S7).
These results indicate MPC and/or methanol might be the spe-
cies that facilitate the leaching of benzimidazole, thereby enhanc-
ing the yield of MPC on ZIF-7. However, a pretreatment study
revealed the critical species in the activation of ZIF-7 was DMC,
instead of MPC and methanol.
Therefore, it is reasonable to assume that the detected metha-
nol species in the 1H NMR spectrum of the used catalyst originate
from the DMC, which occupies a position of the leached benzimi-
dazole. The reaction between DMC and ZIF-7 seems to be caused
by the small amount of water contained in the DMC, as described
in the first reaction in Scheme 3. Formed Zn-OH can react with
DMC to produce Zn-OCO2CH3 with the liberation of methanol. In
the presence of D2SO4 for 1H NMR measurement, it could be
destructed to methanol and CO2.
3.4. Pretreatment of ZIF-7 with DMC
When ZIF-7 was pretreated with DMC, aniline, MPC, and metha-
nol at 190 °C for 6 h, respectively, and used as a catalyst, only the
catalyst treated with DMC showed an increased MPC yield of
91.0% with decreased methylated product formation comparable
to the value of ZIF-7 at the 4th run (Fig. 8). Meanwhile, the aniline,
MPC, and MeOH didn’t show any positive effect on the MPC yields
and methylation products. This result indicates the DMC plays a
critical role in the activation of ZIF-7 for this reaction. Furthermore,
1H NMR analysis of the DMC-treated ZIF-7 showed a peak at 4.19
The XPS analyses shown in Fig. 9 also supports the presence of
carbonyl groups on the ZIF-7 after the DMC treatment. After the
4th run, a new peak at the 289.0 eV was detected in the C1s XPS
spectrum, which correspond to OA(C@O)AO. Similarly, when ZIF-
7 was treated with DMC for 6 h, the same carbonate peak was
observed, suggesting the carbonate group was attached to the
ZIF-7 by the treatment with DMC.
Although we couldn’t observe any deactivation tendency with
catalyst reuse, as shown in Fig. 3, the continuous leaching of ben-
zimidazole by DMC from the ZIF-7 could result in the complete
deconstruction of the catalyst. However, interestingly, when fresh
ZIF-7 was treated with excessive DMC at 190 °C for 48 h, the molar
ratio of benzimidazole to zinc was 1.48, which was a value similar
to that of ZIF-7 after the 6th run.
On the other hand, the formation of methylated side product
seems due to the dissociated benzimidazole in the solution. When
the reaction was proceeded by adding benzimidazole, methylated
products were major reaction products (Fig. S8). Therefore, it can
be assumed that, with repeated catalyst use, the benzimidazole
concentration in the solution decreased and the reactions hap-
pened on the Lewis acidic Zn catalytic centers. As a result,
methoxycarbonylation became dominant over the methylation
reaction.
3.5. Optimization of reaction condition
Based on the results mentioned earlier, when ZIF-7 is pretreated
with DMC before the reaction, a higher MPC yield can be obtained
from the first reaction. To determine the best pretreatment condi-
tions, fresh ZIF-7 was pretreated with DMC at various tempera-
Fig. 8. Effect of treatment reagent on the methoxycarbonylation reaction. Pretreat-
ment condition: 190 °C, 6 h. Reaction condition: aniline 1.16 g, DMC 13.5 g, catalyst
0.116 g, 190 °C, 2 h.
Scheme 3. A plausible mechanism of methoxycarbonylation reaction on ZIF-7. Zn, yellow; N, blue; C, gray; H, white; O, red.
Please cite this article as: D. Dahnum, B. Seo, S. H. Cheong et al., Formation of defect site on ZIF-7 and its effect on the methoxycarbonylation of aniline with
dimethyl carbonate, Journal of Catalysis, https://doi.org/10.1016/j.jcat.2019.09.039

8
D. Dahnum et al. / Journal of Catalysis xxx (xxxx) xxx
Fig. 9. XPS analyses ZIF-7 (a) fresh, (b) after 4th run, and (c) after pretreatment with DMC for 6 h at 190 °C.
tures and times before the methoxycarbonylation reaction. Fig. 10
shows that the optimum pretreatment temperature and time were
190 °C and 6 h; MPC yield was 91.0% which is comparable to the
best heterogeneous catalyst reported (Table 4). However, when
the ZIF-7 was pretreated at 150 °C for 6 h, a substantial degree of
methylated side products were formed; selectivity to MPC was
72.1%. Interestingly, the MPC yield of the pretreated catalyst at
210 °C was 91.9%, which is similar to the value at 190 °C. This
result suggests the DMC-treated ZIF-7 is stable at this temperature.
Meanwhile, increasing in the pretreatment time to longer than 6 h
didn’t affect the yield of MPC.
The catalytic activities of the ZIF-7 pretreated with DMC at 190
°C for 6 h were investigated under the various reaction conditions.
Fig. 11(a) shows that the MPC yield is highly dependent on the
reaction temperature. At 150 and 170 °C, the conversion of aniline
was 24.4 and 66.6%, respectively. Meanwhile, at the higher reac-
tion temperature of 210 °C, the MPC yield was only 47.3% due to
the formation of methylated side products. With a shorter reaction
time, less than 2 h, insufficient aniline conversion was observed,
less than 85%. The longer reaction time of 4 h gave increased the
formation of methylated byproducts and ureas, resulting in a
decreased MPC yield of 69.8% (Fig. 11(b)). The effect of the catalyst
amount shown in Fig. 11(c) revealed that 10 wt% of ZIF-7 to aniline
was the best ratio. At the catalyst amounts of 5 wt% and 20 wt%,
the formation of side products were increased. The effect of ani-
line/DMC molar ratio was found to greatly affect to the reaction.
As shown in Fig. S9, when the DMC/aniline molar ratio was 12
and 10, the amount of MPC and methylated side products formed
were almost similar. However, at the DMC/aniline molar ratios of
8 and 5, the yield of methylated side products increased remark-
ably. Furthermore, increasing the aniline concentration also
resulted in an increase in the urea component. Therefore, to obtain
a high MPC yield, the molar ratio of DMC/aniline apparently needs
to be equal to or higher than 10.
The DMC-pretreated ZIF-7 catalyst was found to be very reusa-
ble. Fig. 11(d) shows a slight increase in MPC yield to 94.7% was
obtained at the 4th catalyst reuse.
3.6. Reaction mechanism
Fig. 10. Effects of (a) pretreatment temperature and (b) time using DMC on the ZIF-
7-catalyzed methoxycarbonylation reaction. aPretreatment time was 6 h. bPretreat-
ment temp. was 190 °C. Reaction condition: aniline 1.16 g, DMC 13.5 g, catalyst
0.116 g, 190 °C, 2 h.
As Chizallet et al. mentioned, the first step in the dissociation of
the ligand from ZIF-7 seems to start with the reaction between
Please cite this article as: D. Dahnum, B. Seo, S. H. Cheong et al., Formation of defect site on ZIF-7 and its effect on the methoxycarbonylation of aniline with
dimethyl carbonate, Journal of Catalysis, https://doi.org/10.1016/j.jcat.2019.09.039

D. Dahnum et al. / Journal of Catalysis xxx (xxxx) xxx
9
Fig. 11. Effect of (a) reaction temperature, (b) reaction time, (c) catalyst loading, and (d) catalyst reuse on the methoxycarbonylation of aniline with DMC reaction of ZIF-7-
OAc treated DMC 6 h 190 °C. aCatalyst/aniline. Reaction condition: aniline 1.16 g, DMC 13.5 g, catalyst 0.116 g, 190 °C, 2 h.
ZIF-7 and water in the DMC, forming the species (I) depicted in
Scheme 3 [30]. Then, the OH groups are turned into methoxycar-
bonyloxy groups by the reaction with DMC, species (II). At this
step, according to the 1H NMR analysis in Table 2, it seems some
OH groups remain. The benzimidazole groups attached to the near
Zn site could be replaced by DMC, species (III), which induces car-
bonyl group activation in DMC. The nucleophilic attack of aniline
on the carbonyl group in DMC produces MPC, species (IV) and
the produced MPC can be replaced by the abundant DMC for fur-
ther methoxycarbonylation reactions.
of ZIF-7 with DMC activated the catalyst directly and the best pre-
treatment condition was 190 °C for 6 h. The activated catalyst
showed 91.0% MPC yield and the yield increased to 94.7% at the
4th reuse.
Acknowledgements
This work was supported by C1 Gas Refinery Program through
the National Research Foundation of Korea (NRF) (NRF-
2015M3D3A1A01065435) and KIST internal program (2E29500).
Appendix A. Supplementary material
4. Conclusion
Supplementary data to this article can be found online at
https://doi.org/10.1016/j.jcat.2019.09.039.
Various versions of ZIF-7, Zn(BeIm)2 were synthesized using the
zinc precursors Zn(OAc)2, ZnCl2, ZnBr2, Zn(NO3)2 and benzimida-
zole. The synthesized ZIF-7s showed similar catalytic activity for
the methoxycarbonylation reaction of aniline, and dimethyl car-
bonate, irrespective of their surface area and morphologies. The
yields of methyl phenyl carbamate (MPC) were 60–70% with
>95% aniline conversion at 190 °C for 2 h reaction. The reusability
study revealed that MPC yield increased with increasing catalyst
reuse, and reached 94.7% after the 6th run, with suppressed
methylation side products. The increase in catalytic activity of
the ZIF-7 with increasing catalyst reuse could be ascribed to the
formation of an active site from the defect sites or vacant sites
made by leaching of the benzimidazole ligand from the zinc center.
The DMC seemed to be activated by this catalyst site. Pretreatment
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Please cite this article as: D. Dahnum, B. Seo, S. H. Cheong et al., Formation of defect site on ZIF-7 and its effect on the methoxycarbonylation of aniline with
dimethyl carbonate, Journal of Catalysis, https://doi.org/10.1016/j.jcat.2019.09.039