Core-shell metal-organic frameworks and metal functionalization to access highest efficiency in catalytic carboxylation

Article abstract of DOI:10.1016/j.jcat.2019.01.036

A core-shell metal-organic frameworks (MOF@MOF) based on the Zr-MOFs assembly from core-structure UiO-66 combined with shell-structure UiO-67-BPY were explored. The synthesized materials were characterized via XRD, FTIR, SEM, TEM, and surface area analysis, etc. indicating the presence of a core-shell structure of UiO-66@UiO-67-BPY. Furthermore, incorporation of the bipyridinic (BPY) group in the linker used to construct the shell layer (UiO-67-BPY) could coordinate with active metal species and thus create an advantage for site-selective metal incorporation in the core-shell structure. Silver (Ag) was selected for the selective metal incorporation and an excellent Ag-dispersion via coordination with the bipyridinic groups in the UiO-67-BPY layer of the core-shell material was obtained. The synthesized material (UiO-66@UiO-67-BPY-Ag) was successfully applied as a heterogeneous catalyst for the CO₂ fixation via carboxylation of terminal alkynes. The catalytic material showed excellent yields using at a low Ag-loading under mild reaction condition (50 °C, 1 bar). Moreover, the catalyst can be recycled for at least 5 times maintaining a stable catalytic performance. Interestingly, the high catalytic activity of the synthesized material demonstrated clearly the beneficial advantage of the metalated core-shell structure over the reported routes to synthesize silver catalysts such as encapsulated Ag nanoparticles (AgNP@MOF) or Ag-bidentately coordinated on traditional MOFs applying the same reaction model.

Full text of DOI:10.1016/j.jcat.2019.01.036

Journal of Catalysis 371 (2019) 106–115
Contents lists available at ScienceDirect
Journal of Catalysis
journal homepage: www.elsevier.com/locate/jcat
Core-shell metal-organic frameworks and metal functionalization to
access highest efficiency in catalytic carboxylation
Yanyan Gong a,b, Ye Yuan a, Cheng Chen a, Pan Zhang a,b, Jichao Wang a,b, Serge Zhuiykov e,
Somboon Chaemchuen a,c, , Francis Verpoort a,c,d,e,
⇑
⇑
a Laboratory of Organometallics, Catalysis and Ordered Materials, State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University
of Technology, Wuhan 430070, PR China
b School of Materials Science and Engineering, Wuhan University of Technology, Wuhan 430070, PR China
c National Research Tomsk Polytechnic University, Lenin Avenue 30, 634050 Tomsk, Russian Federation
d College of Arts and Sciences, Khalifa University of Science and Technology, PO Box 127788, Abu Dhabi, United Arab Emirates
e Ghent University, Global Campus Songdo, 119 Songdomunhwa-Ro, Yeonsu-Gu, Incheon 406-840, South Korea
a r t i c l e i n f o
a b s t r a c t
Article history:
A core-shell metal-organic frameworks (MOF@MOF) based on the Zr-MOFs assembly from core-structure
UiO-66 combined with shell-structure UiO-67-BPY were explored. The synthesized materials were char-
acterized via XRD, FTIR, SEM, TEM, and surface area analysis, etc. indicating the presence of a core-shell
structure of UiO-66@UiO-67-BPY. Furthermore, incorporation of the bipyridinic (BPY) group in the linker
used to construct the shell layer (UiO-67-BPY) could coordinate with active metal species and thus create
an advantage for site-selective metal incorporation in the core-shell structure. Silver (Ag) was selected for
the selective metal incorporation and an excellent Ag-dispersion via coordination with the bipyridinic
groups in the UiO-67-BPY layer of the core-shell material was obtained. The synthesized material
(UiO-66@UiO-67-BPY-Ag) was successfully applied as a heterogeneous catalyst for the CO2 fixation via
carboxylation of terminal alkynes. The catalytic material showed excellent yields using at a low Ag-
loading under mild reaction condition (50 °C, 1 bar). Moreover, the catalyst can be recycled for at least
5 times maintaining a stable catalytic performance. Interestingly, the high catalytic activity of the synthe-
sized material demonstrated clearly the beneficial advantage of the metalated core-shell structure over
the reported routes to synthesize silver catalysts such as encapsulated Ag nanoparticles (AgNP@MOF)
or Ag-bidentately coordinated on traditional MOFs applying the same reaction model.
Received 16 November 2018
Revised 22 January 2019
Accepted 25 January 2019
Keywords:
MOF@MOF
Core-shell structure
Heterogeneous catalysis
Carboxylation
CO2 fixation
Propiolic acids
Ó 2019 Published by Elsevier Inc.
1. Introduction
drug delivery [10], and sensors [11,12] etc. Furthermore, the
post-synthetic modification (PSM) via metal encapsulation or func-
For over a decade, porous coordination polymer (PCP) materials
also well-known as metal-organic frameworks (MOFs), have been
synthesized and the development of new structures is still ongo-
ing. Their structures are of interest due to the diversity of metal
species/clusters that can be combined with an enormous variety
of organic linkers to construct a 2D or 3D framework. Their high
crystallinity and tunability of porosity by applying different
organic ligands have broadened their scope of potential application
such as catalysis [1–6], separation [7,8], gas storage/capture [9],
tionalization changes their properties which could further enhance
MOF applications. For instance, Zhou et al. constructed a MOF-253-
Pt material through immobilizing a platinum complex in MOF-253
by post-synthetic modification [13]. The obtained MOF-253-Pt
exhibited an excellent reactivity approximately five times higher
than that of the corresponding complex for the hydrogen evolution
under visible-light irradiation. Crake et al. prepared MOF/TiO2
bifunctional materials via an in-situ growth method, which
allowed to form a heterojunction between TiO2 and NH2-UiO-66
through a tight interaction [14]. The nanocomposites were proven
durable and significantly more effective in reducing CO2 to CO than
their single components. Ezugwu et al. synthesized Pd@MOF
(Pd(II)-NHC@MOF) by post-synthetic modification of an azolium-
containing MOF framework with palladium acetate [15]. Their
⇑
Corresponding authors at: Laboratory of Organometallics, Catalysis and Ordered
Materials, State Key Laboratory of Advanced Technology for Materials Synthesis and
Processing, Wuhan University of Technology, Wuhan 430070, PR China.
E-mail addresses: sama_che@hotmail.com (S. Chaemchuen), francis.verpoort@
ku.ac.ae (F. Verpoort).
https://doi.org/10.1016/j.jcat.2019.01.036
0021-9517/Ó 2019 Published by Elsevier Inc.

Y. Gong et al. / Journal of Catalysis 371 (2019) 106–115
107
synthesized material showed an excellent catalytic performance
towards the Sonogashira cross-coupling and Knoevenagel conden-
sation reaction. From time to time, it is hard to meet some special
requirements for conventional MOFs so that incorporation of some
other functional groups or components to generate a hybrid and
multifunctional MOF material is required [16–18]. In this respect,
the core-shell heterostructures were developed by enveloping a
core MOF with another shell MOF, the so-called MOF@MOF. The
hybridization of different materials to some degree often tends to
cause some changes in their chemical/physical properties over that
of a single type of MOF materials. Additionally, sometimes there
may be some intriguingly unexpected synergies or other effects
[19]. Zhuang et al. successfully prepared UiO-66@ZIF-8 core-shell
composites in the presence of CTAB [20]. ZIF-8 revealed an out-
standing molecular sieving behavior so that the core-shell MOF@-
MOF realized an excellent catalytic application in size-selective
catalysis of ethylene/cyclohexene hydrogenation. Li et al. designed
a core-shell bio-MOF-11/14@bio-MOF-14 which exhibited a 30%
enhancement of CO2 adsorption over ordinary bio-MOF-14 [21].
Moreover, an improvement in water-stability was also obtained
by synthesizing a core-shell structure (bio-MOF-11/14@bio-MOF-
14) since the shell bio-MOF-14 serves to prevent the degradation.
Obviously, relieving and comprising the negative impact of
excessive carbon dioxide (CO2) emission on the global environ-
ment are in high demand. CO2 as a nontoxic, renewable, and
low-cost C1 building block source to synthesize high-value chem-
icals and fuels has gained a huge momentum and also complies
with the principles of ‘‘Green Chemistry” [22–28]. One of the most
valuable strategies is the CO2 conversion to propiolic acids which
are then applied further in the field of pharmaceutics. Propiolic
acids are synthesized via the insertion of CO2 into the carbon-
metal bond of the acetylide intermediate [29–33]. Currently,
among the reported applied metals for the activation of alkynes,
silver (Ag) is preferred. Ag serves as one of the most powerful acti-
vators owing to the unique d10 electron arrangement making it
selected as a model reaction to evaluate the synthesized material
as a heterogeneous catalyst. The synthesis route of the core-shell
structured UiO-66@UiO-67-BPY-Ag is demonstrated in Scheme 1.
Firstly, UiO-66 (1,4-dicarboxybenzene (H2bdc) as a ligand) as the
core was synthesized and then UiO-67-BPY (2,20-bipyridine-5,50-
dicarboxylic acid (H2bpydc) as a ligand, Fig. 1) was grown as the
shell structure before the Ag was loaded via metallation method.
The metalated core-shell catalyst exhibited superior catalytic per-
formance compared with any report studying the carboxylation of
terminal alkynes with CO2 based on the Ag@MOF. Finally, this
core-shell silver-based catalyst is reusable for at least 5 cycles
without degradation of its structure.
2. Materials and methods
2.1. Materials preparation
2.1.1. Synthesis of UiO-66
All reagents were used in analytical grade obtaining from com-
mercial suppliers (Sigma-Aldrich, Aladdin, TCI) without further
purification. The synthesis of UiO-66 through solvothermal route
was conducted with a small modification based on reported [39].
Briefly, the ZrCl4 (0.17 g, 0.73 mmol) was dissolved in 20 mL N,N-
dimethylformamide (DMF) under sonication for 15 min before
adding 1,4-benzene dicarboxylic acid (H2bdc, 0.12 g, 0.73 mmol).
The glacial acetic acid (1.25 mL), acting as a modulator, was added
in the mixture with further sonication for 20 min. A homogeneous
solution was transferred into a 40 mL Teflon-lined stainless steel
autoclave and heated to 120 °C for 24 h. After cooling to room tem-
perature, the resultant white product was isolated by centrifuga-
tion and washed with DMF (3 ꢁ 10 mL), followed by soaking in
methanol (MeOH) for 3 days and exchanged with fresh MeOH
every day. After soaking, the solids were collected and dried under
vacuum at 60 °C for 24 h before further used.
easier to interact with the CꢀC bound to form an Ag-
p complex
2.1.2. Synthesis of UiO-67-BPY
[19,34,35]. Nevertheless, this reaction is mostly reported in a
homogeneous phase and recovering of the catalyst is practically
impossible. Moreover, the reaction mixture makes it even more
complicated to separate the homogeneous catalyst and requires
extra purification processes. Hence, the development of an efficient
heterogeneous catalyst for the incorporation of CO2 into terminal
alkynes is imminent and challenging. Catalysis by MOFs is one of
the most widely studied MOF application, including the CO2 con-
version with terminal alkynes [36,37]. Additionally, it is possible
to synthesize MOF structures with nearly any desired metal, large
pore size, and surface area, and the easy design and synthesis of
MOFs give them an extra advantage compared with other materi-
als [38].
UiO-67-BPY was synthesized according to the formerly reported
[40]. Typically, ZrCl4 (0.233 g, 1 mmol) was dissolved in 60 mL
DMF under sonication for 15 min before 2,2-bipyridine-5,5-
dicarboxylic acid (H2bpydc, 0.244 g, 1 mmol) and 1.91 mL of glacial
acetic acid were added. The mixture solution was sonicated further
for 20 min, and transferred into a 100-mL Teflon-lined stainless
steel autoclave and heated to 120 °C for 24 h. After cooling to room
temperature, the precipitate was isolated and washed which sim-
ilar procedure as synthesized UiO-66.
2.1.3. Synthesis of UiO-66@UiO-67-BPY
The powder of synthesized UiO-66 (80 mg) as core-structure
was added in mixture solution of ZrCl4 (46.6 mg, 0.2 mmol), H2-
bpydc (48.8 mg, 0.2 mmol) and glacial acetic acid (0.4 mL) in
25 mL DMF. The suspension solutions were transferred into
Inspired by the aforementioned, herein, we successfully pre-
pared a core-shell MOF with metal functionalization in the shell
structure. The carboxylation of terminal alkynes with CO2 was
Scheme 1. Schematic diagram of the synthesis of core-shell UiO-66@UiO-67-BPY-Ag.

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Y. Gong et al. / Journal of Catalysis 371 (2019) 106–115
2.4. Characterizations
The crystallinity of the samples was analyzed by powder XRD
using a Bruker D8 advance diffractometer (Bragg-Brentano geome-
try) at 40 kV and 45 Ma with Cu-Ka radiation and a scanning rate
of 0.1°sꢂ1. The size and morphology of the materials were investi-
gated by using field-emission scanning electron microscopy (FE-
SEM, Zeiss Ultra Plus) with an X-Max 50 energy dispersive spec-
trometer (EDS) and a transmission electron microscopy (TEM,
JEM-2100F). The surface area and porosity obtained from N2
adsorption-desorption isotherms (77 K) were measured on
a
Micrometrics instrument (ASAP 2020). Fourier transformed infra-
red spectra (FT-IR) were recorded on a Nicolet 6700 FT-IR spec-
trometer with KBr pellets in the wave range of 4000–400 cmꢂ1
.
The metal loading were obtained by Inductive coupled plasma
optical emission spectrometry (ICP-OES, Prodigy 7). The reaction
products were analyzed on a nuclear magmatic resonance (Bruker
NMR 500 MHz: 1H and 13C).
3. Results and discussion
Fig. 1. The crystal structure of UiO-67-BPY MOFs with uncoordinated bpy-moieties
in the channels.
3.1. Characterization of UiO-66@UiO-67-BPY-Ag
40 mL Teflon-lined stainless steel autoclave and sonicated for
40 min before heated to 120 °C for 24 h. After cooling to room tem-
perature, the product was collected by centrifugation and washed
which similar procedure as synthesized UiO-66.
3.1.1. XRD, TEM and FESEM analysis
The assembly of MOF@MOF structure using Zr as metal with
two organic linkers (1,4-benzene dicarboxylic acid (BDC) and 2,2-
bipyridine dicarboxylic acid (BPYDC)) was explored for the first
time. To obtain the core-shell UiO-66@UiO-67-BPY structure,
firstly Zr clusters bridging with BDC ligand (UiO-66) were synthe-
sized as a core-structure followed by the second linker (BPYDC)
assemblage to construct (UiO-67-BPY) as a shell-structure. Powder
X-ray diffraction (XRD) was performed to evaluate the crystal
structure and compared to the simulated structures. Using the sin-
gle linker, Zr-MOFs (UiO-66 and UiO-67-BPY) were synthesized
and XRD analyses indicated that their crystal structure matches
excellently with their simulation (Figs. S1 and S2). The XRD analy-
sis of the crystal structure of the synthesized core-shell UiO-
66@UiO-67-BPY confirmed that the XRD patterns of both crystal
structures, UiO-66 and UiO-67-BPY, are present as shown in
Fig. 2. Fourier transform infrared spectroscopy (FTIR) analysis con-
firmed the presence of metal-linker coordination in the MOFs
structure. The disappearance of the OHꢂ stretching vibration from
the organic ligands at 2500–3000 cmꢂ1 indicated the occurrence of
the linker deprotonation and bridge formation with metal clusters
(Fig. S3). Furthermore, a significant shift of the C@O stretch vibra-
2.1.4. Synthesis of UiO-66@UiO-67-BPY-Ag
The route to synthesize UiO-66@UiO-67-BPY-Ag was adapted
from the reported procedure [41]. Silver nitrate (520 mg,
3.06 mmol) was dissolved with 80 mL MeOH in 100 mL round flask
under and sonicated for 10 min. A metallation followed by the add-
ing core-shell UiO-66@UiO-67-BPY (130 mg) to the silver solution.
The suspension solutions were stirred at a constant temperature of
40 °C for 48 h. The resulting brownish product was collected by
centrifugation and then immersed in MeOH for 3 days by exchang-
ing with fresh MeOH (10 mL) every day.
2.2. Catalytic reaction
A 35 mL reactor was charged with alkynes (1 mmol), Cs2CO3
(1.5 mmol), DMF (2 mL) and activated catalyst (0.73 mol% Ag)
under an inert atmosphere. The system was fast purged 5 times
with CO2 and then stirred at 50 °C for 24 h under CO2 (1 atm)
wrapped in aluminum foil to create darkness. After completion,
the suspension solutions were isolated by centrifugation and the
liquid phase was transferred to a separation funnel while the solid
catalyst was recovered for further recycling tests. A 2 mL water
was added in the separation funnel before liquid phase extraction
with dichloromethane (3 ꢁ 15 mL). The aqueous layer was acidi-
fied with concentrated HCl (pH = 1) and then extracted with
diethyl ether (3 ꢁ 20 mL). The combined organic layers were
washed with saturated brine, dried over anhydrous Na2SO4 and fil-
tered. The solvent was removed under vacuum to calculate the
yield of pure product.
2.3. Recycling procedure
After completion of the reaction, the catalyst was isolated by
centrifugation and adequately washed with methanol
(3 ꢁ 15 mL) before drying under vacuum at 50 °C for 24 h. Where
after, the recovered catalyst was used for the following reaction
cycle as described for the first catalytic reaction.
Fig. 2. The XRD pattern of as-synthesized UiO-66@UiO-67-BPY compared with
simulated structure of UiO-66 and UiO-67-BPY.

Y. Gong et al. / Journal of Catalysis 371 (2019) 106–115
109
tion (1680 cmꢂ1) in the synthesized MOFs compared to the vibra-
tion of the original ligand was observed. Moreover, the intense
peak centered at 1590 cmꢂ1 became acuter in UiO-66@UiO-67-
BPY with respect to UiO-66 owing to the overlapping with the
C@N stretching vibration present in the BPYDC ligand [41].
The core-shell morphology of the synthesized material was con-
firmed via scanning electron microscopy (FE-SEM) and transmis-
sion electron microscope (TEM) analysis. The first step, the
synthesis of UiO-66, resulted in cubic shape like particles having
a particle size ꢃ150 nm (Fig. 3a and c). The second step, the syn-
thesis of the shell-structure (UiO-67-BPY) to construct a core-
shell UiO-66@UiO-67-BPY, resulted in a more spherical particle
shape (Fig. 3b and d). Interestingly, the UiO-67-BPY was success-
fully grown as a shell (thickness, 50 nm) on a core/seed-structure
(UiO-66) to create a core-shell UiO-66@UiO-67-BPY material with
particle size ꢃ200 nm, as demonstrated by TEM analysis.
distributed evenly throughout the core-shell material (Fig. 4d
and e). A silver loading of 4.38 wt% was determined through induc-
tively coupled plasma-optical emission spectroscopy (ICP-OES) of
the synthesized material (UiO-66@UiO-67-BPY-Ag). It is worth
noting that aggregation of silver atoms might occur as silver
nano-clusters are observed (Figs. S6 and S9). The presence of silver
nano-clusters was confirmed by XPS, H2-TPD, and TEM images. The
Ag aggregation in the shell material might result in a reduced crys-
tallinity as observed from the lower intensity in the XRD pattern
after Ag metallation. Powder-XRD analysis indicated some changes
of the crystal pattern (Fig. S4). However, the XRD pattern of UiO-
66@UiO-67-BPY-Ag (2h = 5.8, 6.6 and 7.4, 8.7) exhibits a similar
pattern as before Ag-metallation (UiO-66@UiO-67-BPY). Neverthe-
less, the metallation process, as lower intensities were observed,
probably causes the loss in crystallinity. The partial degradation
of the crystal structure might occur during two steps; firstly during
the post-synthetic modification (Ag-coordination) and secondly,
during the catalytic reaction. However, the change in crystal struc-
ture did not influence the catalytic performance, as the activity
remains stable during the recycle tests Additionally, the spherical
shape-like morphologies are maintained as well as the particle size
compared to the pre-metallization stage size and morphologies. In
a previous study of an Ag-Co-MOF by Molla et al. it was reported
that the overall reflection intensities of the Ag@Co-MOF decreased
compared to the original structure (Co-MOF) [37]. The Ag-
immobilization on the surface of Co-MOF could contribute to the
reduction of crystallinity of the host support (Co-MOF). The Ag-
incorporation in the core-shell materials was also confirmed via
the loss of porosity and surface area of PSM materials. The nitrogen
isotherms of the different materials were determined (Fig. 5). The
surface area (BET) and pore volume of UiO-66, UiO-66@UiO-67-
Likewise, it has been widely reported that the bipyridine group
can act as a bidentate chelator with metals via a metallization pro-
cess [42]. This opens the opportunity to afford catalytic active
single-sites on the synthesized material. The metal-incorporation
on bipyridine linkers in MOFs has been explored and a more supe-
rior performance over the original MOFs was reported [43–45]. Sil-
ver as a catalyst has been reported to be an active catalyst for
several chemical reactions as well as the Ag metallization of the
bipyridine linker in MOFs [46]. In this study, Ag-coordination in
the shell of synthesized core-shell MOFs (UiO-66@UiO-67-BPY-
Ag) was explored. The presence of the pyridyl groups in the BPYDC
linker, which are free coordinating sites in the MOF structure (UiO-
67-BPY), opens the possibility for metal-coordination via post-
synthetic modification. The silver-metallization via PSM on these
free pyridyl coordination sites would provide UiO-66@UiO-67-
BPY-Ag in which the Ag-ions could be applied as catalytic sites.
BPY, and UiO-66@UiO-67-Ag were 853 m2ꢄgꢂ1 and 0.33 cm3ꢄgꢂ1
,
,
1470 m2ꢄgꢂ1 and 0.56 cm3ꢄgꢂ1, and 491 m2ꢄgꢂ1 and 0.19 cm3ꢄgꢂ1
respectively. The higher surface area of the core-shell MOF (UiO-
66@UiO-67-BPY) over UiO-66 is contributed due to the additional
large porosity of the UiO-67-BPY shell. However, the surface area
and porosity decreased severely after Ag-metallation in the core-
shell MOFs. The Ag-coordination in the shell-linker could occupy
the pores of the material resulting in a loss of the pore volume. Fur-
thermore, the partial degradation results in a reduced crystallinity,
as confirmed by XRD, can lead to a lower surface area of UiO-
66@UiO-67-BPY-Ag. The CO2 sorption capability was investigated
also and an excellent CO2 sorption of 61 cm3ꢄgꢂ1 at 273 K and
1 atm, was obtained on core-shell UiO-66@UiO-67-BPY-Ag
(Fig. S5). The ability of CO2 adsorption could enhance the concen-
tration of CO2 around Ag active sites and this can contribute to
its improved reactivity in the subsequent carboxylation of terminal
alkynes with CO2.
3.1.2. EDS and BET analysis
The Ag-metallization of UiO-66@UiO-67-BPY could be observed
easily by a color change from white to brown for the final product
(UiO-66@UiO-67-BPY-Ag) (Fig. 4g). The FE-SEM and TEM images
reveal that the spherical-like morphologies are preserved after
Ag-metallization (Fig. 4a–c). The Ag is well dispersed as high elec-
tron density dark spots all over the core-shell UiO-66@UiO-67-BPY
sample in the EDS image (Fig. 4f). The Zr and O elements also are
3.1.3. XPS analysis
The chemical state of the metals and elements present in UiO-
66@UiO-67-BPY-Ag was investigated using X-ray photoelectron
spectroscopy (XPS). According to the XPS survey spectrum of
UiO-66@UiO-67-BPY-Ag, Ag, O, C, Zr, and N were all detected
(Fig. 6a). The spectral fitting of Ag 3d species are displayed in
Fig. 6b revealing the presence of two doublets assigned to Ag(I)
(3d5/2-3d3/2) and Ag0 (3d5/2-3d3/2) having binding energies of
367.5–373.5 and 368.3–374.3 eV respectively (Fig. 6b) [47,48]. In
addition, the higher ratio of Ag(I) over Ag0 confirmed that mainly
silver is anchored to the pyridine of the BPY ligand. Moreover, this
correlates perfectly with the EDS analysis (Fig. 4) where well-
dispersed Ag was observed. However, using Ag in excess compared
with the available coordination sites delivered from the BPY linkers
combined with suitable reduction condition (methanol solvent), Ag
nanoparticle could be generated as revealed in Fig. S6 where black
dots of metallic silver or Ag0 nanoparticle are present in a tiny
Fig. 3. The observed morphologies of UiO-66 (a, c) and UiO-66@UiO-67-BPY (b, d)
via FE-SEM image (a, b) and TEM image (c, d).

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Y. Gong et al. / Journal of Catalysis 371 (2019) 106–115
Fig. 4. The crystal morphologies of UiO-66@UiO-67-BPY-Ag using FESEM (a) and TEM (b, c). The elemental distribution of Zr (d), O (e), Ag (f), and element plot (g). The insert
pictures are optical images of UiO-66@UiO-67-BPY (left) and UiO-66@UiO-67-BPY-Ag (right).
shell structure (Fig. 6d) [49]. Fig. 6c exhibits the C1s XPS spectrum
which contains four main peaks. The peak at 284.8 eV was stan-
dardized as the reference for the other specimen. The peak related
to the carboxylate carbon was found at 288.3 eV, the peaks at
284.1 eV and 285.7 eV were attributed to the carbon of the ben-
zene ring [50,51]. Furthermore, the XPS analysis of O1s (Fig. 6e)
revealed that the envelope band could be deconvoluted into three
peaks centered at 529.9 eV, 531.3 eV, and 532.5 eV. The binding
energy at 529.9 eV is indicative for the metal-oxygen (ZrAO)
bonds, while the peak centered at 531.1 eV is assigned to the
O@CAO bonds of the organic linker and the peak of 532.5 eV cor-
responds to the contributions from the hydroxyl groups of surface
adsorbed H2O [50,52]. Fig. 6f presents the binding energy spectrum
of Zr in UiO-66@UiO-67-BPY-Ag The XPS spectrum of Zr contains
one doublet with the 3d5/2 and 3d3/2 peaks appearing at 182.3 eV
and 184.7 eV, respectively [53].
3.1.4. H2-TPR and UV–Vis
The metal stability (Ag) in the MOF framework was examined
via temperature program reduction using hydrogen gas (H2-TPR
analysis), which is performed usually to estimate the redox proper-
ties of materials. The corresponding H2-TPR profiles of UiO-
66@UiO-67-BPY-Ag and UiO-67-BPY-Ag are presented in Fig. 7.
Three main peaks of H2 consumption, at approximately 80 °C,
255 °C, and 550 °C, are observed on both samples indicating that
three different types of silver species are participating during the
reduction process. Furthermore, from the obtained H2-TPR results
Fig. 5. The N2 isotherms of UiO-66, UiO-66@UiO-67-BPY, and UiO-66@UiO-67-BPY-
Ag.
amount in the sample. Moreover, the shift in the binding energy of
the N1s peak to 398.9 eV in UiO-66@UiO-67-BPY-Ag from 395.5 eV
for the uncoordinated bipyridyl nitrogen atoms, reveals the trans-
formation of the chemical environment and can imply that the
Ag(I) ions are coordinating to bipyridyl moieties present in the

Y. Gong et al. / Journal of Catalysis 371 (2019) 106–115
111
Fig. 6. XPS spectra of UiO-66@UiO-67-BPY-Ag sample: (a) survey scan, (b) Ag 3d, (c) C1s, (d) N 1s, (e) O1s and (f) Zr 3d region.
ious interactions of Ag+ ions/Ag2O with the support (framework
structure) or/and fixed to different lattice sites which exhibit a dif-
ferent reducibility [58].
UV–Vis diffuse reflectance spectroscopy was performed to dis-
tinguish the Ag species in the sample. The UV–Vis profile of UiO-
66@UiO-67-BPY-Ag (Fig. S7) showed an absorption peak at
213 nm which represent the 4d10 to 4d95s1 electronic transition
of Ag+. While the absorbance in the region between 240 and
350 nm, having a relatively high intensity, corresponds to small
charged Ag+n clusters. Finally, the absorbance above 350 nm is
assigned to metallic silver clusters in the sample [54,56,59–61].
3.2. Catalytic activity
Core-shell UiO-66@UiO-67-BPY-Ag was applied as a heteroge-
neous catalyst for the CO2 carboxylation of terminal alkynes. The
yield of 3-phenyl propiolic acid (desired product) from the reaction
of phenylacetylene and CO2 was used as a model reaction to eval-
uate the catalytic performance of the synthesized materials. As
shown in Table 1, the yield discrepancy depends on the catalyst
loading. The yield of the product increased along with the catalyst
loading and a yield of up to 96% could be obtained using 0.73 mol%
Ag (Entry 1–5). The influence of the shell, bulk and the presence of
silver regarding the catalytic activity were investigated under sim-
ilar reaction conditions (Entry 5–8). The result reveals that the
core-shell UiO-66@UiO-67-BPY-Ag demonstrated a higher activity
(96% yield) compared with the bulk UiO-67-BPY-Ag reaching only
75% yield. Interestingly, a higher reactivity was revealed using a
lower Ag content in the core-shell UiO-66@UiO-67-BPY-Ag
(0.73 mol% of Ag) compared to the bulk UiO-67-BPY-Ag (1.72 mol
% of Ag). In addition, the SEM images and elemental mapping of
UiO-67-BPY-Ag demonstrated a particle size of 1 mm and a uniform
distribution of Ag, Zr, and O (Fig. S8). Finally, applying core-shell
UiO-66@UiO-67-BPY (in absence of Ag) and no catalyst only very
Fig. 7. H2-TPR profiles of UiO-66@UiO-67-BPY-Ag (black) and UiO-67-BPY-Ag (red),
using a TCD detector, the samples were heated at a rate of 10 °C/min up to 900 °C.
combined with literature data, the low-temperature peak observed
in the H2-TPR profile at nearly 80 °C can be assigned to the reduc-
tion of Ag2O to form metallic Ag0, the peak at 255 °C is due to the
+
reduction of Ag(I)-ions to generate Agn clusters. While the peak
about 550 °C is attributed to the reduction of silver clusters species
producing Ag0 [54–57]. The area-% obtained after integration of the
peaks revealed that for UiO-66@UiO-67-BPY-Ag, containing two
weak-reduction peaks at 80 °C and 255 °C, the values amount to
11% and 21%, respectively. While for UiO-67-BPY-Ag 9% and 11%
is reached, respectively. These results indicate that UiO-66@UiO-
67-BPY-Ag contains more Ag(I)-ions but less metallic silver could
lead to a better dispersion and might foster a higher catalytic per-
formance compared with UiO-67-BPY-Ag. It has to be noted that
the peaks contained several shoulders due to the presence of var-

112
Y. Gong et al. / Journal of Catalysis 371 (2019) 106–115
Table 1
%-Yield for the carboxylation of phenylacetylene using different catalyst loadings.a
Entry
1
Catalyst
%-Yieldb
50
UiO-66@UiO-67-BPY-Ag
(4 mg, 0.16 mol% Ag)
UiO-66@UiO-67-BPY-Ag
(8 mg, 0.32 mol% Ag)
UiO-66@UiO-67-BPY-Ag
(10 mg, 0.41 mol% Ag)
UiO-66@UiO-67-BPY-Ag
(30 mg, 1.22 mol% Ag)
UiO-66@UiO-67-BPY-Ag
(18 mg, 0.73 mol% Ag)
UiO-67-BPY-Ag
2
3
4
5
6
67
75
97
96
75
(18 mg, 1.72 mol% Ag)
UiO-66@UiO-67-BPY (18 mg)
No catalyst
7
8
7
9
a
Reaction conditions: phenylacetylene (1 mmol), CO2 (0.1 MPa), Cs2CO3 (1.5 mmol), DMF (2 ml), catalyst, 50 °C, 24 h.
The yields were determined by NMR with 1,3,5-trimethoxybenzene as the internal standard.
b
Table 2
Reported catalysts and their comparison of catalytic performance.
Entry
1
Catalyst
Reaction conditions
Ag loading
2.7 mol%
Yield
TOF (hꢂ1
2.38
)
Ref.
Ag@MIL-101
1-ethynylbenzene (1.0 mmol), Cs2CO3 (1.5 mmol),
CO2 (1.0 atm), 50 °C, DMF (5 mL), 15 h
96.5%
[36]
2
3
Ag@Co-MOF
1-ethynylbenzene (1.0 mmol), Cs2CO3 (1.5 mmol),
CO2 (1.0 atm), 80 °C, DMF (5 mL), 14 h
1-ethynylbenzene (1.0 mmol), Cs2CO3 (1.5 mmol),
CO2 (1.0 atm), 50 °C, DMF (2 mL), 24 h
2.04 mol%
0.73 mol%
98%
96%
3.43
5.48
[37]
Core-shell UiO-66 @UiO-67-BPY-Ag
This work
low product yields were achieved (7% and 9% of the desired pro-
duct, respectively). These findings clearly demonstrate that the cat-
alytic activity essentially occurs in the presence of Ag sites.
It stands to reason that a higher silver loading could provide a
higher reactivity/yield. However, the opposite outcome surfaced
in this work, UiO-66@UiO-67-BPY-Ag exhibited a higher reactivity
than UiO-67-BPY-Ag with an Ag loading of 0.73 mol% and 1.72 mol
%, respectively. The enhanced catalytic performance using a low
Ag-loading in core-shell MOFs is attributed to a synergistic effect
obtained by the core-shell strategy. The active sites (Ag) are
located in the shell structure (selected coordination sites with lin-
ker) of which the later promotes the diffusion rate of substrate and
product molecules through only pores of the shell layer of the cat-
alyst. With a smaller thickness e.g. the shell layer UiO-67-BPY
(50 nm) of the core-shell UiO-66@UiO-67-BPY structure, diffusion
issues are avoided and hence a higher catalytic performance can
be obtained. In the case of UiO-67-BPY-Ag, the substrate and pro-
duct molecules can hardly diffuse through the pores implying that
the main part of the Ag inside UiO-67-BPY-Ag (1 mm) might not
participate as active sites during the reaction. Furthermore, the
H2-TPR profile indicates that more Ag+ is present over Ag0 in
UiO-66@UiO-67-BPY-Ag demonstrating a good dispersion of Ag.
Hence, the core@shell strategy could lead to better and effective
contact between active catalyst and substrates, and consequently,
demonstrate improved catalytic performance over UiO-67-BPY-Ag
and of any previously reported systems such as Ag@MIL-101 [31]
and Ag@Co-MOF [32].
by a simple liquid impregnation method [31]. Later, the Ag@Co-
MOF showed an excellent catalytic efficiency and the catalyst
could be recycled conveniently without appreciable loss of cat-
alytic activity [32]. However, those reported catalysts applied a
high Ag loading which was indispensable for a good yield as dis-
played in Table 2. In this work, Ag-coordination on organic linkers
of MOF is reported demonstrating a TOF value higher than the
encapsulated core-shell MOF (Ag@MIL-101 and Ag@Co-MOF).
Moreover, the lower Ag loading applied to achieve a similar cat-
alytic effect could possibly be due to an enhanced effective contact
area of the Ag ions than the silver nanoparticles.
It was reported that Ag coordinated (Ag+) with an organic ligand
such as N-heterocyclic carbene (NHC) etc., demonstrated an excel-
lent catalytic activity for the carboxylation reaction [62–66].
Therefore, the compound [Ag+(2,20-bipyridine)]NO3 was synthe-
sized according to previous reports [67–69]. The structure was
confirmed by HR-MS and 1H NMR (Fig. S9) The compound was
used as a homogeneous catalyst under the same reaction condi-
tions and Ag-loading as applied for UiO-66@UiO-67-BPY-Ag for
the carboxylation and exhibited 97% in yield. Besides, the Ag
nanoparticles (Ag0) encapsulated in MOFs or supported on a poly-
mer also demonstrated good activities for this reaction [37,70,71].
Therefore, the active sites for the reaction could be both of Ag+ and
Ag0. However, Ag+ coordinated with the 2,20-bipyridine ligand can
provide an excellent dispersion of silver and demonstrate strong
interactions which reduce the leaching problem compared to Ag0.
The aim of this work was to selectively coordinate Ag with H2-
bpydc and use it as a shell layer with a low Ag-loading and high
stability for recycling to promote the carboxylation of terminal
alkynes.
Comparable catalysts reported in the literature for the CO2
carboxylation of terminal alkynes based on MOF materials are
given in Table 2. Liu and co-worker reported the Ag@MIL-101(Cr)

Y. Gong et al. / Journal of Catalysis 371 (2019) 106–115
113
3.3. Suitability and mechanism of UiO-66@UiO-67-BPY-Ag
Further, the synthesized catalyst UiO-66@UiO-67-BPY-Ag was
applied in a range of terminal alkynes with different electronic
and steric substituent groups as summarized in Table 3. The
electron-withdrawing and electron-donating substituent groups
on terminal alkynes had a distinct effect on the yield of the desired
substance, but all substrates ensured in good to remarkable yields.
In general, the reaction mechanism is completed in three steps, (I)
the deprotonation of alkynes occurs with the aid of Cs2CO3 fol-
lowed by the formation of the key intermediate, silver acetylide,
by coordination between terminal alkynes and Ag active sites. (II)
CO2 is inserted into the C-Ag bond of silver acetylide leading to
the generation of silver propiolate. (III) silver propiolate reacts with
alkyne and Cs2CO3 producing cesium propiolate which could be
further converted into the final product by simple acidification
meanwhile another silver acetylide intermediate to proceed the
cycle (Fig. 8) [72–74]. The first step was more prone to substances
with strong electron-withdrawing groups, while the third part was
more facile to perform for terminal alkynes with strong electron-
donating groups. Therefore, moderated electron donating or with-
drawing groups on the terminal alkynes afforded a higher yield
(Entry 1–5) compared to alkynes bearing strong withdrawing
group (ANO2) (Entry 6).
Fig. 8. Mechanism of CO2 carboxylation of alkyne using UiO-66@UiO-67-BPY-Ag.
after five recycles (Fig. 9). Hence, the catalyst was isolated from
the reaction mixture by centrifugation then washed with water
and methanol to remove Cs2CO3 before drying the catalyst at
60 °C under vacuum for 24 h. It should be mentioned that a small
weight loss of the spent catalyst occurred which might be due to
partial degradation or catalyst lose. The spent catalyst (after 5 recy-
cles) was characterized via SEM, TEM, and XRD analysis. The crys-
tal morphologies of the spent catalyst UiO-66@UiO-67-BPY-Ag was
preserved (Fig. S10), although a certain degree of loss in crys-
tallinity could be observed from the XRD pattern (Fig. S11). Inter-
estingly, the partial degradation of the crystal structure might
occur during two steps; firstly during the post-synthetic modifica-
tion (Ag-coordination) and secondly, during the catalytic reaction.
3.4. Reusability of UiO-66@UiO-67-BPY-Ag
A significant feature of a heterogeneous catalyst that should be
considered is the reusability of UiO-66@UiO-67-BPY-Ag. The cata-
lyst was recovered and reused in successive reactions of CO2 car-
boxylation without a perceptible loss in catalytic activity even
Table 3
The carboxylation activity of UiO-66@UiO-67-BPY-Ag using different terminal alkynes.a
Entry
1
Substrate
Yieldb
96%
2
97%
3
4
98%
98%
5
96%
6
7
84%
97%
a
Reaction conditions: phenylacetylene (1 mmol), CO2 (0.1 MPa), Cs2CO3 (1.5 mmol), DMF (2 ml), catalyst (18 mg, 0.73 mol%Ag), 50 °C, 24 h.
b
The yields were determined by 1H NMR with 1,3,5-trimethoxybenzene as the internal standard.

114
Y. Gong et al. / Journal of Catalysis 371 (2019) 106–115
Fig. 9. The recycling performance (left) and 1H NMR spectroscopy (right) of catalyst UiO-66@UiO-67-BPY-Ag for CO2 carboxylation of terminal alkynes.
However, the change in crystal structure did not influence the cat-
alytic performance, as the activity remains stable during the recy-
cle tests (Fig. 9). Afterward, the filtrate solution of the reaction
mixture was analyzed by ICP-AES, and just less than 0.013% Ag
and 0.002% Zr were found in the solution, which proved the rela-
tively stable structure of UiO-66@UiO-67-BPY-Ag.
and procedure for the catalytic reactions of terminal alkynes, some
supplementary characterization analysis, characterization of
desired products (1H, 13C NMR and HRMS) and references. Supple-
mentary data to this article can be found online at https://doi.org/
10.1016/j.jcat.2019.01.036.
References
4. Conclusions
[1] H.G.T. Nguyen, N.M. Schweitzer, C.Y. Chang, T.L. Drake, M.C. So, P.C. Stair, O.K.
Farha, J.T. Hupp, S.B.T. Nguyen, ACS Catal. 4 (2014) 2496–2500.
[2] T. Araya, M. Jia, J. Yang, P. Zhao, K. Cai, W. Ma, Y. Huang, Appl. Catal. B-Environ.
203 (2017) 768–777.
[3] S.W. Li, Z. Yang, R.M. Gao, G. Zhang, J.S. Zhao, Appl. Catal. B-Environ. (2018)
221.
[4] N. Wei, Y. Zhang, L. Liu, Z.B. Han, D.Q. Yuan, Appl. Catal. B-Environ. (2017) 219.
[5] J. Qiu, X. Zhang, Y. Feng, X. Zhang, H. Wang, J. Yao, Appl. Catal. B-Environ.
(2018) 231.
[6] J.C. Cardoso, S. Stulp, J.F.D. Brito, J.B.S. Flor, R.C.G. Frem, M.V.B. Zanoni, Appl.
Catal. B-Environ. (2018) 225.
[7] B. Hu, L. Huang, M. He, B. Chen, J. Mater. Chem. A 3 (2015) 11587–11595.
[8] S. Chaemchuen, N.A. Kabir, K. Zhou, F. Verpoort, Chem. Soc. Rev. 42 (2013)
9304–9332.
[9] Y. Zhao, M. Seredych, Q. Zhong, T.J. Bandosz, ACS Appl. Mater. Inter. 5 (2013)
4951–4959.
[10] P. Horcajada, C. Serre, G. Maurin, N.A. Ramsahye, F. Balas, M. Vallet-Regí, M.
Sebban, F. Taulelle, G. Férey, J. Am. Chem. Soc. 130 (2008) 6774–6780.
[11] S.K. Bhardwaj, N. Bhardwaj, G. Mohanta, P. Kumar, A. Sharma, K.H. Kim, A.
Deep, ACS Appl. Mater. Inter. 7 (2015) 26124–26130.
[12] Y. Zhang, X. Bo, A. Nsabimana, C. Han, M. Li, L. Guo, J. Mater. Chem. A 3 (2014)
732–738.
A novel composite material was fabricated via PSM of an active
metal-coordinated in the shell-structure of the core-shell MOF@-
MOF (UiO-66@UiO-67-BPY-Ag). The synthesized materials were
applied as heterogeneous catalysts in the model reaction of CO2
carboxylation reaction of terminal alkynes. A high-performance
carboxylation reaction with different electronic and steric sub-
stituent groups on the terminal alkynes catalyzed by the synthe-
sized materials under mild conditions (50 °C and 1 atm of CO2)
was observed. Moreover, the UiO-66@UiO-67-BPY-Ag could be
recycled and reused at least 5 times without losing in catalytic
activity. The synergetic design of active metal bonded within the
core-shell structure proved that an enhancement of the catalytic
activity of the heterogeneous catalyst could be achieved even with
a lower active metal loading. The present work potentially pro-
motes the development of novel high-performance heterogeneous
catalytic systems by forming core-shell structure MOFs and broad-
ens the extent of other applications.
[13] T. Zhou, Y. Du, A. Borgna, J. Hong, Y. Wang, J. Han, W. Zhang, R. Xu, Energy
Environ. Sci. 6 (2013) 3229–3234.
[14] A. Crake, K.C. Christoforidis, A. Kafizas, S. Zafeiratos, C. Petit, Appl. Catal. B-
Environ. 210 (2017) 131–140.
[15] C.I. Ezugwu, B. Mousavi, M.A. Asraf, Z. Luo, F. Verpoort, J. Catal. 344 (2016)
445–454.
Conflict of interest
[16] M.L. Foo, R. Matsuda, S. Kitagawa, Chem. Mater. 26 (2014) 310–322.
[17] J. Liu, L. Chen, H. Cui, J. Zhang, L. Zhang, C.Y. Su, Chem. Soc. Rev. 45 (2015)
6011–6061.
The authors declare no competing financial interest.
[18] Q.L. Zhu, Q. Xu, Chem. Soc. Rev. 43 (2014) 5468–5512.
[19] K. Zhou, B. Mousavi, Z. Luo, S. Phatanasri, S. Chaemchuen, F. Verpoort, J. Mater.
Chem. A 5 (2017) 952–957.
Acknowledgement
[20] J. Zhuang, L.Y. Chou, B.T. Sneed, Y. Cao, P. Hu, L. Feng, C.K. Tsung, Small 11
(2015) 5551–5555.
The authors are grateful to the State Key Lab of Advanced Tech-
nology for Materials Synthesis and Processing for financial support
(Wuhan University of technology). S.C. acknowledges the support
of the National Natural Science Foundation of China (No.
21502146). F.V. appreciates the financial support from Tomsk
Polytechnic University Competitiveness Enhancement Program
grant (VIU-195/2018).
[21] T. Li, J.E. Sullivan, N.L. Rosi, J. Am. Chem. Soc. 135 (2013) 9984–9987.
[22] K. Huang, C.L. Sun, Z.J. Shi, Chem. Soc. Rev. 40 (2011) 2435–2452.
[23] C. Maeda, Y. Miyazaki, T. Ema, Chem. Inform. 45 (2014) 1482–1497.
[24] M. Aresta, Salud Pã°blica De MãÓxico 56 (2010) 417.
[25] S.N. Riduan, Y. Zhang, Dalton Trans. 39 (2010) 3347–3357.
[26] Y. Tsuji, T. Fujihara, Chem. Commun. 48 (2012) 9956–9964.
[27] L. Zhang, Z. Hou, Chem. Inform. 84 (2012) 1705–1712.
[28] L. Zhang, Z. Hou, Chem. Sci. 4 (2013) 3395–3403.
[29] M. Arndt, E. Risto, T. Krause, L.J. Gooßen, Chem. Inform. 4 (2012) 484–487.
[30] M. Bararjanian, S. Balalaie, F. Rominger, B. Movassagh, H.R. Bijanzadeh, J. Org.
Chem. 75 (2010) 2806–2812.
Appendix A. Supplementary material
[31] A.V. Dubrovskiy, R.C. Larock, Org. Lett. 12 (2010) 3117–3119.
[32] K. Inamoto, N. Asano, K. Kobayashi, M. Yonemoto, Y. Kondo, Org. Biomol.
Chem. 43 (2012) 1514–1516.
General information, the preparative procedure for UiO-66,
UiO-67-BPY, UiO-66@UiO-67-BPY, UiO-66@UiO-67-BPY-Ag MOFs

Y. Gong et al. / Journal of Catalysis 371 (2019) 106–115
115
[33] F. Manjolinho, M. Arndt, K. Goossen, L.J. Goossen, Chem. Inform. 43 (2012)
2014–2021.
[53] P. Ji, X. Feng, S.S. Veroneau, Y. Song, W. Lin, J. Am. Chem. Soc. 139 (2017)
15600–15603.
[34] G. Fang, X. Bi, Chem. Soc. Rev. 44 (2015) 8124–8173.
[35] K. Sekine, T. Yamada, Chem. Soc. Rev. 45 (2016) 4524–4532.
[36] X.H. Liu, J.G. Ma, Z. Niu, G.M. Yang, P. Cheng, Angew. Chem. 127 (2015) 1002–
1005.
[54] K.A. Bethke, H.H. Kung, J. Catal. 172 (1997) 93–102.
[55] A. Ausavasukhi, S. Suwannaran, J. Limtrakul, T. Sooknoi, Appl. Catal. A-Gen.
345 (2008) 89–96.
[56] J. Shibata, K. Shimizu, Y. Takada, A. Shichi, H. Yoshida, S. Satokawa, J. Catal. 227
(2004) 367–374.
[37] R.A. Molla, K. Ghosh, B. Banerjee, M.A. Iqubal, S.K. Kundu, S.M. Islam, A.
Bhaumik, J. Colloid Interface Sci. 477 (2016) 220–229.
[57] H.K. Beyer, P.A. Jacobs, Stud. Surf. Sci. Catal. 12 (1982) 95–102.
[58] H. Berndt, M. Richter, T. Gerlach, J. Chem. Soc. Faraday Trans. 94 (1998) 2043–
2046.
ˇ
[38] A. Dhakshinamoorthy, M. Opanasenko, J. Cejka, H. Garcia, Catal. Sci. Technol. 3
(2013) 2509–2540.
ˇ
[39] J.J. Zhou, R. Wang, X.L. Liu, F.M. Peng, C.H. Li, F. Teng, Y.P. Yuan, Appl. Surf. Sci.
346 (2015) 278–283.
[40] H. Fei, S.M. Cohen, Chem. Commun (Camb) 50 (2014) 4810–4812.
[41] B. Mortada, T.A. Matar, A. Sakaya, H. Atallah, A.Z. Kara, P. Karam, M. Hmadeh,
Inorg. Chem. 56 (2017) 4739–4744.
[42] Y. Kikkawa, E. Koyama, M. Aoyagi, N. Schneider, M. Takahashi, K. Fujiwara, M.
Kanesato, Supramol. Chem. 23 (2011) 9–12.
[43] E.D. Bloch, D. Britt, C. Lee, C.J. Doonan, F.J. Uriberomo, H. Furukawa, J.R. Long, O.
M. Yaghi, J. Am. Chem. Soc. 132 (2010) 14382–14384.
[44] C. Wang, Z. Xie, K.E. Dekrafft, W. Lin, J. Am. Chem. Soc. 133 (2011) 13445–
13454.
[59] K. Arve, L. Capek, F. Klingstedt, Top. Catal. 30–31 (2004) 91–95.
[60] L. Chen, W. Zhao, Y. Jiao, X. He, J. Wang, Y. Zhang, Spectrochim. Acta A 68
(2007) 484–490.
[61] M. Richter, A. Abramova, U. Bentrup, J. Appl. Spectrosc. 71 (2004) 400–403.
[62] Z. Zhou, C. He, L. Yang, Y. Wang, T. Liu, C. Duan, ACS Catal. 7 (2017) 2248–2256.
[63] J. Jover, F. Maseras, J. Org. Chem. 79 (2014) (1987) 11981–11991.
[64] S. Li, J. Sun, Z. Zhang, R. Xie, X. Fang, M. Zhou, Dalton Trans. 45 (2016) 10577–
10584.
[65] W. Wang, G. Zhang, R. Lang, C. Xia, F. Li, Green. Chem. 15 (2013) 635.
[66] Y. Yuan, C. Chen, C. Zeng, B. Mousavi, S. Chaemchuen, F. Verpoort, Chem. Cat.
Chem. 9 (2017) 882–887.
[45] T. Jacobs, R. Clowes, A.I. Cooper, M.J. Hardie, Angew. Chem. Int. Ed. Engl. 51
(2012) 5192–5195.
[67] M.G. Cowan, W.M. McDanel, H.H. Funke, Y. Kohno, D.L. Gin, R.D. Noble, Angew.
Chem. Int. Ed. 54 (2015) 5740–5743.
[46] X. Lu, J. Ye, D. Zhang, R. Xie, R.F. Bogale, Y. Sun, L. Zhao, Q. Zhao, G. Ning, J.
Inorg. Biochem. 138 (2014) 114–121.
[68] G.A. Bowmaker, Effendy, S. Marfuah, B.W. Skelton, A.H. White, Inorg. Chim.
Acta 358 (2005) 4371–4388.
[47] M.P. Suh, H.R. Moon, E.Y. Lee, J. Am. Chem. Soc. 128 (2006) 4710–4718.
[48] H.R. Moon, J.H. Kim, M.P. Suh, Angew. Chem. 117 (2005) 1287–1291.
[49] Y. Zhou, B. Yan, Chem. Commun. (Camb) 52 (2016) 2265–2268.
[50] H. Guo, Y. Zhang, Z. Zheng, H. Lin, Y. Zhang, Talanta 170 (2017) 146–151.
[51] M.D. Firouzjaei, A.A. Shamsabadi, G.M. Sharifian, A. Rahimpour, M. Soroush,
Adv. Mater. Interfaces 5 (2018) 1701365.
[69] F. Hung-Low, A. Renz, K.K. Klausmeyer, J. Chem. Crystallorgr. 41 (2011) 1174–
1179.
[70] X.H. Liu, J.G. Ma, Z. Niu, G.M. Yang, P. Cheng, Angew. Chem. Int. Ed. 54 (2015)
988–991.
[71] D. Yu, M.X. Tan, Y. Zhang, Adv. Synth. Catal. 354 (2012) 969–974.
[72] Y. Dingyi, Z. Yugen, Green Chem. 13 (2011) 1275–1279.
[73] J.D. Mccowan, Trans. Faraday Soc. 59 (1963) 1860–1864.
[74] R. Vestin, E. Ralf, E. Saluste, Acta Chem. Scand. 3 (1949) 101–124.
[52] A. Wang, Y. Zhou, Z. Wang, M. Chen, L. Sun, X. Liu, RSC Adv. 6 (2016) 3671–
3679.