Enhancing catalytic performance via structure core-shell metal-organic frameworks

Article abstract of DOI:10.1016/j.jcat.2019.06.031

A core-shell structure metal-organic framework based on the Zr clusters bridging with BDC linkers (UiO-66) as a core-structure and BPYDC linkers (UiO-67-BPY) as a shell-structure was developed (UiO-67-BPY@UiO-66). The combination of several techniques suc

Full text of DOI:10.1016/j.jcat.2019.06.031

Journal of Catalysis 375 (2019) 371–379
Contents lists available at ScienceDirect
Journal of Catalysis
journal homepage: www.elsevier.com/locate/jcat
Enhancing catalytic performance via structure core-shell metal-organic
frameworks
Yanyan Gong a,b, Ye Yuan , Cheng Chen , Pan Zhang a,b, Jichao Wang a,b, Anish Khan , Serge Zhuiykov ,
Somboon Chaemchuen
a
a
a
e
d
a,c,
⇑
a,b,c,d,
⇑
, Francis Verpoort
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
School of Materials Science and Engineering, Wuhan University of Technology, Wuhan 430070, PR China
National Research Tomsk Polytechnic University, Lenin Avenue 30, 634050 Tomsk, Russian Federation
Ghent University, Global Campus Songdo, 119 Songdomunhwa-Ro, Yeonsu-Gu, Incheon, Republic of Korea
b
c
d
e
Center of Excellence for Advanced Materials Research, King Abdulaziz University, P.O. Box 80203, Jeddah 21589, Saudi Arabia
a r t i c l e i n f o
a b s t r a c t
Article history:
A core-shell structure metal-organic framework based on the Zr clusters bridging with BDC linkers
Received 23 March 2019
Revised 5 June 2019
Accepted 15 June 2019
(
(
UiO-66) as a core-structure and BPYDC linkers (UiO-67-BPY) as a shell-structure was developed
UiO-67-BPY@UiO-66). The combination of several techniques such as XRD, FTIR, SEM, TEM, and surface
area analysis etc. were applied for the characterization and confirmed a core-shell structure of
UiO-67-BPY@UiO-66. Taking advantage of the high porous stability of the core-structure (UiO-66) and
the presence of active Lewis basic sites from the bipyridinic linker in the shell layer (UiO-67-BPY) could
be advantageous for basic-catalyzed reactions. The synthesized core-shell material was applied as a
heterogeneous catalyst for the Knoevenagel condensation as a model reaction. An excellent catalytic
performance was obtained by the core-shell material over traditional MOFs and other previous reports
based on MOFs. The excellent dispersion of the active sites (Lewis basic) in the outer layer of the designed
core-shell structure was a breakthrough to prevent mass diffusion limitation during catalysis.
Additionally, the catalyst can be recycled and maintained its high catalytic performance at least for four
cycles.
Keywords:
MOF@MOF
Core-shell structure
Heterogeneous catalysis
Knoevenagel condensation
MOFs
Ó 2019 Elsevier Inc. All rights reserved.
1
. Introduction
In the class of porous crystalline materials, Metal-organic
modification of either ligands or metal clusters to meet some par-
ticular requirements. However, still many difficulties arise using
the traditional MOFs methodology. Thereby, explorations to
increase the structural and functional complexity of MOFs to form
multifunctional, purpose-designed MOFs is still developing at a
high pace [10,11]. One kind of these explorations is the implemen-
tation of the core-shell strategy where MOF is encapsulated within
another MOF generating a multifunctional hybrid MOF material,
denoted as MOF@MOF [12,13]. The hybrid MOF@MOF materials
may combine individual functions of core- and/or shell-MOF to
exhibit novel synergetic chemical and physical properties suitable
for selected applications [14–16]. For example, Lee et al. reported a
core-shell nanocomposite ZIF-L@ZIF-8 through heterogeneous sur-
Frameworks (MOFs), constructed from metal ions or clusters and
organic ligands possessing an infinite network, are currently
receiving considerable attention. These MOFs exist as porous
framework materials and possess outstanding properties for exam-
ple high surface area, inherent porosity, multiple coordination
sites, ease of functionalization, well-defined structures, and tun-
able pores size and volume etc. These materials are applied in mul-
tiple fields such as gas storage [1], separation [2], chemical sensing
[
3], catalysis [4–7], and drug delivery [8,9]. The post-synthetic
modification is a strategy to enhance or access diversities of
applications of MOF materials. Other strategies typically involve
face growth applied for CO
dynamic and equilibrium CO
found on the core-shell material and presented a hybridized CO
2
2
adsorption [17]. The realization of
2
adsorption characteristics were
⇑
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@
ghent.ac.kr (F. Verpoort).
uptake and diffusivity of parent crystals. Ren et al. fabricated
core-shell nanocrystals MIL-101@UiO-66 which exhibited a high
degree of moisture tolerance and hydrogen storage capacity 26%
https://doi.org/10.1016/j.jcat.2019.06.031
0
021-9517/Ó 2019 Elsevier Inc. All rights reserved.

3
72
Y. Gong et al. / Journal of Catalysis 375 (2019) 371–379
and 60% higher than that of the individual MIL-101 and UiO-66,
respectively [18]. Hirai et al. demonstrated the design of core-
shell Zn (bdc) -(dabco) @Zn (adc) (dabco) which could extract
2 2 n 2 2 n
2
ing 1,4-benzene dicarboxylic acid (H BDC, 0.12 g, 0.73 mmol).
Thereafter, glacial acetic acid (1.25 mL) as a modulator was added
to the solution with further sonication for 20 min. The obtained
homogeneous solution was transferred into a 40 mL Teflon-lined
stainless steel autoclave and heated to 120 °C for 24 h under auto-
genic pressure. After cooling to room temperature, the resultant
white product was isolated by centrifugation and washed with
DMF (3 ꢀ 10 mL), followed by immersion in methanol (MeOH)
for 3 days and exchange with fresh MeOH every day. After immer-
sion, the solids were collected and dried at 60 °C under vacuum for
24 h (activation) before further used.
cetane from its branched isomer even at a low concentration of
cetane (<1%). This separation could not be demonstrated indepen-
dently by either traditional core- or shell-MOFs [19].
The Knoevenagel condensation between activated methylene
compounds and aldehydes is a significant classic reaction. This
reaction provides a CAC coupling enabling the preparation of sub-
stituted alkenes, coumarin derivatives, applied in cosmetics, per-
fumes, polymers, and pharmaceuticals [20–24]. This reaction is
traditionally catalyzed by alkali metal hydroxides or bases like pri-
mary, secondary, tertiary amines [25–28] and ammonium salts
2.1.2. Synthesis of UiO-67-BPY
[
29] under homogeneous conditions. Unfortunately, these catalysts
UiO-67-BPY was synthesized according to the formerly reported
are difficult to separate and to recover and consequently generate
large amounts of waste. Hence, there is an existing need for the
development of heterogeneous catalysts with evident advantages
like suppressed side reactions, simple separation process, less cor-
rosiveness and reusability with high turn-over number resulting in
better selectivity and yield. Metal-organic frameworks (MOFs) as
novel crystalline materials with tunable structure and properties
have been identified as promising heterogeneous catalysts for the
Knoevenagel condensation [30–34]. Previous reports by Seo et al.
procedure [39]. Typically, ZrCl
4
(0.233 g, 1 mmol) was dissolved in
0
60 mL DMF under sonication for 15 min before 2,2 -bipyridine-5,
0
5 -dicarboxylic acid (H
2
BPYDC, 0.244 g, 1 mmol) and 1.91 mL of
glacial acetic acid were added. The mixture was sonicated further
for 20 min and transferred into a 100 mL Teflon-lined stainless
steel autoclave before heating to 120 °C for 24 h. After cooling to
room temperature, the precipitate was isolated and washed simi-
larly as the described procedure for UiO-66.
[
35], Park et al. [36], and Hasegawa et al. [37] have demonstrated
2.1.3. Synthesis of UiO-67-BPY@UiO-66
that the implementation of a pyridyl group and amide groups in
the organic ligand could generate MOFs acting as basic catalysts.
Keeping in mind all those considerations, herein, we synthe-
sized a core-shell MOF@MOF using UiO-66 as a core-structure with
UiO-67-BPY as the shell-structure. Incorporating dipyridyl groups
The powder of synthesized UiO-66 (80 mg), used as core-
structure, was added to a mixture prepared from ZrCl
4
(46.6 mg,
0.2 mmol), H BPYDC (48.8 mg, 0.2 mmol), and glacial acetic acid
2
(0.4 mL) in 25 mL DMF. The obtained suspension was transferred
into a 40 mL Teflon-lined stainless steel autoclave and sonicated
for 40 min before heating at 120 °C for 24 h. After cooling to room
temperature, the product was collected by centrifugation and
washed similarly as the described procedure for UiO-66.
(
BPYDC), well known as basic sites, in the shell-structure which
could be a promising approach for base-catalyzed reactions. The
Knoevenagel condensation reaction of aldehydes with activated
methylene compounds (malononitrile) was selected as a model
reaction to evaluate the synthesized material as a heterogeneous
catalyst. So far, the literature of MOF@MOF as a heterogeneous cat-
alyst for this reaction is scarce. The synthesis route of the core-
shell structured UiO-67-BPY@UiO-66 is demonstrated in Scheme 1.
2.2. Catalytic reaction
A 15 mL glass reactor was charged with aldehyde (1 mmol),
malononitrile (1.2 mmol), DMSO-d (2 mL) and activated catalyst
(18 mg) under ambient atmosphere. The mixture was stirred at
room temperature (RT) for 1 h. After completion of the reaction,
the suspension was centrifuged and the liquid phase was used to
examine the conversion while the solid catalyst was recovered
for further recycling tests.
Firstly, UiO-66 (1,4-dicarboxybenzene (H
core is synthesized and then UiO-67-BPY (2,2 -bipyridine-5,5 -dic
arboxylic acid (H bpydc) is used as a ligand to grow the shell struc-
ture around the UiO-66 core.
2
bdc) as a ligand) as the
0
0
2
2
. Materials and methods
2.3. Recycling procedure
2
2
.1. Materials preparation
After completion of the reaction, the catalyst was isolated by
centrifugation and adequately washed with methanol
.1.1. Synthesis of UiO-66
All reagents were of analytical grade obtained from commercial
(
5 ꢀ 15 mL) before drying under vacuum at 80 °C for 24 h. There-
after, the recovered catalyst was added to the reaction mixture
for the next cycle applying reaction conditions described for the
catalytic reaction.
suppliers (Sigma-Aldrich, Aladdin, and TCI) and used without fur-
ther purification. The solvothermal synthesis of UiO-66 was con-
ducted with a small modification of the reported procedure [38].
4
Briefly, the ZrCl (0.17 g, 0.73 mmol) was dissolved in 20 mL of N,
N-dimethylformamide (DMF) and sonicated for 15 min before add-
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-K radiation and a scanning rate
a
ꢁ1
of 0.1° s . The size and morphology of the materials were
investigated by using a field-emission scanning electron micro-
scopy (FE-SEM, Zeiss Ultra Plus) with an X-Max 50 energy disper-
sive spectrometer (EDS) and a transmission electron microscopy
(
2
TEM, JEM-2100F). The surface area, porosity, and N adsorption-
desorption isotherms (77 K) were measurement on micrometrics
instrument (ASAP 2020). Fourier transformed infrared spectra
(
FT-IR) were recorded on a Nicolet 6700 FT-IR spectrometer with
ꢁ1
Scheme 1. Schematic diagram of the synthesis of core-shell UiO-67-BPY@UiO-66.
KBr pellets in the range from 4000 to 400 cm . The metal contents

Y. Gong et al. / Journal of Catalysis 375 (2019) 371–379
373
were obtained by inductively coupled plasma optical emission
spectrometry (ICP-OES, Prodigy 7). The CHN elements were ana-
lyzed by Element analysis (Vario EL cube). The reaction products
were analyzed using a nuclear magmatic resonance (Bruker NMR
the simulated crystal patterns. The observed PXRD pattern of syn-
thesized UiO-66 and UiO-67-BPY (traditional structure core- and
shell-MOFs) matched perfectly with their simulated patterns
(Figs. S1 and S2). While the obtained crystal pattern of core-shell
UiO-67-BPY@UiO-66 is in good agreement with the combination
of the simulation of both structures UiO-66 and UiO-67-BPY. More-
over, not only a good agreement of the peak position but also of the
intensity ratios were observed indicating a core-shell structure of
the synthesized material (Fig. 2).
5
00 MHz). The acidic and basic properties of MOFs were examined
via temperature programmed desorption (TPD) (AutoChem II
920) using NH and CO as probe gases. Before TPD analysis, the
sample was first placed inside the instrument and pretreated at
50 °C for 3 h under a He flow. The desorption process was temper-
2
3
2
1
ature programmed (10 °C/min) and the signal was detected via a
thermal conductivity detector (TCD).
3.1.3. Functional group and porosity analysis
The existence of a core-shell structure was further examined by
Fourier transform infrared spectroscopy (FTIR) as in Fig. S3. The
3
. Results and discussion
ꢁ
1
bands located at 2500–3000 cm represent characteristic bands
of OH— stretching vibration from organic ligands. These bands dis-
appeared in the MOF structures due to the deprotonation of the
organic linker followed by coordination with metal clusters during
the framework construction [40,41]. Furthermore, a significant
3
3
.1. Characterization core-shell UiO-67-BPY@UiO-66
.1.1. Morphology analysis
The UiO-67-BPY@UiO-66 MOFs was constructed to obtain the
ꢁ1
shift is observed for the C@O stretch vibration (1680 cm ) in the
core-shell structure. Firstly, the Zr clusters bridging with 1,4-
benzene dicarboxylic acid (H BDC) ligands (UiO-66) were synthe-
sized as a core-structure followed by the addition of the second lin-
synthesized material compared to the free original ligand. The
2
0
0
ker 2,2 -bipyridine-5,5 -dicarboxylic acid (BPYDC) with Zr clusters
assembly to construct (UiO-67-BPY) the shell-structure. The mor-
phology and crystal size of synthesized materials were determined
via field emission scanning electron microscope (FE-SEM) and field
emission transmission electron microscope (FE-TEM) analysis. The
FE-SEM images demonstrated an octahedral shape morphology of
UiO-66 with particle size ꢂ150 nm (Fig. 1a), while the core-shell
UiO-67-BPY@UiO-66 observed a spherical-like shape with particle
size ꢂ200 nm (Fig. 1d). The TEM images confirmed the achieve-
ment of a core-shell structure UiO-67-BPY@UiO-66 as additional
layer coating (shell-structure of UiO-67-BPY) on UiO-66 crystal
(Fig. 1e and f). Furthermore, a larger particle size of UiO-67-
BPY@UiO-66 compared with the UiO-66 crystal size evidences
the core-shell structure and further notifies about the thickness
of the shell-layer UiO-67-BPY (ꢂ50 nm).
3.1.2. Crystallinity analysis
Powder X-ray diffraction (PXRD) was performed to evaluate the
crystal structure of the synthesized materials and compared with
Fig. 2. The XRD pattern of as-synthesized UiO-67-BPY@UiO-66 compared with the
simulated structure of UiO-66 and UiO-67-BPY.
Fig. 1. The crystal morphology images of UiO-66 (a–c) and UiO-67-BPY@UiO-66 (d–f) via FE-SEM (a and d) and TEM (b, c, e and f).

3
74
Y. Gong et al. / Journal of Catalysis 375 (2019) 371–379
ꢁ
1
ꢁ1
characteristic bands centered at 1590 cm and 1417 cm were
found in all three synthesized MOFs and are attributed to the car-
boxylic ligand in the framework of the MOFs [42–46,32]. Though
amount desorbed during temperature raising and the temperature
range of desorption correspond to the concentration and the distri-
bution of the strength of basic or acid sites, respectively. From the
ꢁ
1
the peaks observed at 1590 and 1417 cm
cannot be reliably
CO
observed and correspond to a different strength of the basic sites.
molecules
2 2
-TPD profile (Fig. 4a), two distinct CO desorption peaks are
assigned to the C@N stretching vibration of the dipyridyl groups
of UiO-67-BPY@UiO-66, an increase in the intensity of these peaks
in contrast to that of UiO-66 and UiO-67-BPY is observed and may
be attributed to the C@N stretching mode of the dipyridyl from the
shell layer. The nitrogen adsorption measurements at 77 K were
performed on UiO-66, UiO-67-BPY, and UiO-67-BPY@UiO-66 and
revealed an isotherm of type I for all samples. Moreover, a larger
BET surface area and pore volume of UiO-67-BPY@UiO-66
The peak centered at ꢂ300 °C was assigned to CO
2
adsorption on basic sites with a moderate strength, whereas the
peak positioned at ꢂ710 °C was attributed to stronger interactions
2
with CO molecules and represent the strong basic sites [49–52].
The total amount of basic sites was integrated from the area under
the curve for UiO-67-BPY@UiO-66 and UiO-67-BPY and revealed
an amount of 634 mmol/g and 385 mmol/g, respectively. The
amount of basic sites is of significance for the catalytic perfor-
mance in the Knoevenagel condensation as will be discussed in
2
ꢁ1
ꢁ1
3
ꢁ1
2
ꢁ1
(
0
1470 m ꢃg
and 0.56 cm ꢃg ) than UiO-66 (853 m ꢃg
and
3
.33 cm ꢃg ) was observed (Fig. 3). The higher surface area of
the core-shell UiO-67-BPY@UiO-66 could be attributed to the addi-
3
the catalytic performance section. The NH -TPD patterns of the
2
ꢁ1
tional larger porosity of the UiO-67-BPY shell (1700 m ꢃg ) [39].
Additionally, the BET surface area of the core-shell material was
located between those of UiO-66 and UiO-67-BPY as reported in
previous studies [15,47,48].
synthesized materials are displayed in Fig. 4b. Similar desorption
profiles were found on both synthesized materials, though, the
total amount of acid sites in UiO-67-BPY was larger than in UiO-
67-BPY@UiO-66 (984 mmol/g and 556 mmol/g, respectively). Com-
bining both TPD results, the UiO-67-BPY@UiO-66 MOF revealed a
better acid-base balance, and consequently, a concerted and syner-
gic action might take place during the reaction which is beneficial
to increase the catalytic performance [53]. It could be inferred that
there exist two kinds of acid sites with different acidic strengths.
3
.1.4. Temperature-programmed desorption (TPD) and element
analysis
The temperature-programmed desorption (TPD) using CO
NH as probe gases was applied to investigate the amount of basic
and acid sites in the synthesized materials (Fig. 4). The CO or NH
2
and
3
The peaks located at ꢂ200 °C indicated the NH
adsorbed at weak acid sites, while the other peak centered
desorption from moderate acid sites
54–58]. Table 1 gives the results obtained from element analysis
C, H, N) and clearly indicates that the N content of the core-shell
3
molecules
2
3
ꢂ380 °C derives from NH
3
[
(
UiO-67-BPY@UiO-66 (3.43%) is lower than UiO-67-BPY (5.84%).
Although the content of N is lower in the core-shell material, the
amount of basic sites in the core-shell UiO-67-BPY@UiO-66 is
higher than in UiO-67-BPY and could indicate that ‘‘more accessi-
ble” basic sites are present in the core-shell material. It has to be
2
mentioned that the pyridyl groups of H BPYDC mainly generate
the basic sites present in UiO-67-BPY and core-shell UiO-67-
BPY@UiO-66. For this reason, an acid/base analysis of only those
two materials was performed since UiO-66 lacks the basic sites.
Table 1
Elemental analysis (C H N) results for different samples.
Sample
N %
C %
H %
Fresh UiO-67-BPY@UiO-66
UiO-67-BPY
Recovered UiO-67-BPY@UiO-66
3.43
5.84
3.41
31.65
31.57
32.93
3.13
3.05
2.84
2
Fig. 3. The N isotherms of UiO-66, UiO-67-BPY and UiO-67-BPY@UiO-66.
2 3
Fig. 4. The CO -TPD (a) and NH -TPD (b) profiles of UiO-67-BPY@UiO-66 and UiO-67-BPY.

Y. Gong et al. / Journal of Catalysis 375 (2019) 371–379
375
3
.2. Catalytic performance
als, small sized UiO-67-BPY particles (denoted as UiO-67-BPY-S,
ꢂ200 nm based on the FESEM images, Fig. S8) were synthesized
via tuning of the synthesis conditions. The UiO-67-BPY-S has a sim-
ilar particle size as compared with core-shell UiO-67-BPY@UiO-66
and was further used to investigate the influence of particle size on
the catalytic performance. The results revealed a conversion of 61%
(30 min) and 80% (60 min) (Fig. S8), which is still lower than the
conversion obtained using core-shell UiO-67-BPY@UiO-66 catalyst
under similar conditions.
In addition, as displayed in Table 3 (Entry 7–8), 4-tert-
butylbenzaldehyde with large molecular size was used as the sub-
strate to evaluate the effect of core-shell structure on catalytic
activity. Yields of 75% and 55% were achieved for UiO-67-
BPY@UiO-66 and UiO-67-BPY, respectively, confirming the excel-
lent improved effect of core-shell structure on the catalytic activity.
The catalytic performance of core-shell UiO-67-BPY@UiO-66 for
the Knoevenagel condensation of benzaldehyde and malononitrile
was compared with previous reports based on MOF catalysts
(Table 2). For more than half of the reported MOF catalysts harsh
reaction conditions were needed which was indispensable for an
appropriate conversion. Here in, the core-shell UiO-67-BPY@UiO-
66 demonstrated an excellent conversion of benzaldehyde under
milder reaction conditions (RT) and short time (1 h). The results
fully reflect the excellent catalytic performance of the core-shell
MOF compared to other MOF catalysts.
To demonstrate the capability the core-shell UiO-67-PBY@UiO-
66 as a catalyst for the Knoevenagel condensation reaction the sub-
strate scope using different aldehydes applying was investigated.
Therefore, the catalytic performance using aromatic aldehydes with
different electronic and steric substituents was explored (Table 3).
For the benzaldehyde, up to 98% conversion was achieved after 1 h
(Entry 1). Faster conversion rates were obtained for aldehydes with
electron-withdrawing groups, which could reach completion
within only 30 min (Entry 2–5). However, benzaldehyde with
electron-donating groups (Entry 6–7) retarded the reaction rate
and hence, a lower conversion was obtained after 1 h. Those results
indicated that electron-withdrawing groups promote the reaction,
as expected in this reaction based on the nucleophilic attack of
the basic sites of the core-shell MOF at the carbonyl group of the
aldehyde. The NMR spectra of the products obtained from the dif-
ferent aromatic aldehydes are depicted in Fig. S6.
The robust core-shell structure (MOFs@MOFs) is designed to
eliminate the diffusion limitation that generally occurs in porous
heterogeneous catalysts and thus also in MOFs. Definitively, well-
dispersed active sites near the MOF surface could enhance the sub-
strates to react with the active sites as well as provide a faster dif-
fusion out of the pores of the product. The core-shell UiO-67-
PBY@UiO-66 was applied as a heterogeneous catalyst for the Kno-
evenagel condensation of aldehydes with compounds containing
active methylene groups. The conversion of benzaldehyde with
malononitrile was used as a model reaction to evaluate the cat-
alytic performance of the synthesized materials: UiO-66, UiO-67-
PBY@UiO-66, and UiO-67-PBY. The discrepancy in catalytic perfor-
mance of UiO-67-PBY@UiO-66 depends on the catalyst amount
and reaction time as shown in Fig. 5 and Fig. S4. The conversion
increased along with the catalyst loading as well as reaction time
and conversion of up to 98% is obtained using 18 mg UiO-67-
PBY@UiO-66 after 1 h reaction time. The core-shell MOFs also
exhibited an excellent reactivity for the Knoevenagel condensation
of benzaldehyde and malononitrile even in a short period of 20 min
(
83% conversion) or 40 min (95%).
The effect of the core-shell structure on the catalytic perfor-
mance was investigated by comparing with the individual core-
and shell-MOF under identical reaction conditions. The core-shell
UiO-67-BPY@UiO-66 revealed the highest catalytic activity (98%
conversion) followed by the shell UiO-67-BPY (87%) and the core
UiO-66 with 25% conversion after 1 h. Furthermore, without a cat-
alyst, this model reaction achieved a very low conversion (29%).
Basically, the conversions obtained by UiO-66 and without catalyst
are similar indicating that active sites (acid sites) in UiO-66 hardly
promoted this reaction. The higher total amount of basic sites in
2
the core-shell UiO-67-BPY@UiO-66 (see CO -TPD analysis) com-
pared with UiO-67-BPY, seems to be the main cause for the higher
catalytic performance. Interestingly, the core-shell UiO-67-
BPY@UiO-66 catalyst contains a lower N content (3.43%) compared
to UiO-67-BPY (5.84%) still, a higher reactivity was obtained. This
clearly points out that the basic sites (N-atoms) in the core-shell
structure are easily accessible and that diffusion is not limited. In
addition, the FESEM images of UiO-67-BPY demonstrated a particle
size of 1 mm (Fig. S5) similar as previous reported [39], which is
considerably bigger than the core-shell MOF particles (200 nm).
These findings clearly demonstrate that the synergy between the
core and shell has a crucial effect on the catalytic activity.
3.3. Catalytic mechanism
In order to better understand the influence of core-shell struc-
ture on the catalytic performance of UiO-67-PBY@UiO-66 materi-
A plausible catalytic mechanism only involving basic sites of the
core-shell UiO-67-PBY@UiO-66 catalyst is given in Fig. S7 (left). In
Fig. 5. The conversions of benzaldehyde after 1 h using different amounts of UiO-67-BPY@UiO-66 (0, 5, 10, 18 mg) (a) and the conversions of benzaldehyde using different
MOFs after 20, 40, 60 min with 18 mg catalyst (b).

3
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Y. Gong et al. / Journal of Catalysis 375 (2019) 371–379
Table 2
Reported MOFs for the Knoevenagel condensation of benzaldehyde and malononitrile (model reaction).
Entry
Catalyst
T/°C
t/min
Yield
Ref.
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
UiO-66
UiO-66-NH
MIL-100 (Al)
Cu-BTC
Zn@ZIF-67
100
100
100
100
RT
40
80
80
RT
40
RT
130
130
RT
40
80
RT
RT
270
240
270
270
90
5%
[59]
[59]
[59]
[59]
[34]
[60]
[61]
[61]
[32]
[62]
[63]
[64]
[64]
[65]
[66]
[67]
[68]
This work
2
13%
30%
23%
50%
98%
99%
87%
97%
94%
100%
100%
100%
97%
99%
61%
99%
98%
UiO-66-NH
NH (50%)-MIL-53
2
40
2
300
300
120
60
180
60
180
60
120
30
MIL-53
UiO-66-NH-RNH
CAU-1-NH
ZIF-8
CuBTC
FeBTC
DETA-MIL-101
IRMOF-3
Al-MIL-101-NH
ZIF-9
2
0
1
2
3
4
5
6
7
8
2
2
240
60
UiO-67-BPY@UiO-66
As we demonstrated that only the basic sites play a role in the
catalysis, a catalytic mechanism using the basic sites of the core-
shell UiO-67-BPY@UiO-66 is proposed. Herein, the N present in
Table 3
The catalytic activity of UiO-67-BPY@UiO-66 using different aromatic aldehydes.a
2
the bipyridine linker (H BPYDC) offers the basic sites for the cat-
alytic system. In principle, it is believed that a higher N loading
could provide a higher catalytic activity for base-catalyzed reac-
tion. However, the opposite phenomenon was observed in this
work, a lower N content of the core-shell UiO-67-BPY@UiO-66 (N
content 3.43%wt, ICP) exhibited a higher reactivity than UiO-67-
BPY (N content 5.84%wt, ICP). Additionally, the molecular dimen-
sion of benzaldehyde (6.0 ꢀ 4.3 Å), malononitrile (2.8 ꢀ 4.3 Å)
and the product benzylidenemalononitrile (3.7 ꢀ 7.7 Å) are smaller
than the pore sizes of UiO-67-BPY@UiO-66 (13.01 Å) and UiO-67-
BPY (13.07 Å), so these species can enter inside the pores of both
catalysts. However, the higher reactivity of the core-shell UiO-
Entry
1
Substrate
T (°C)
t (min)
Conversionb
98%
RT
60
2
3
4
RT
RT
RT
30
30
30
100%
100%
100%
6
7-BPY@UiO-66 can be explained by the higher total amount of
5
6
7
RT
RT
RT
30
92%
98%
basic sites in the core-shell MOFs as mentioned in TPD analysis
in addition to a synergistic effect of the core-shell structure for
the diffusion rate of substrate and/or product molecules through
the pores of the shell of the core-shell catalyst. Compared with
UiO-67-BPY@UiO-66, due to the large particle size of UiO-67-BPY
(1 mm), the substrate and product molecules might experience a
difficult diffusion through the pores. Finally, due to a larger particle
size of UiO-67-BPY, a smaller surface interaction between the sub-
strate and the active sites will occur. Those reasons might imply
that the accessibility of the active sites inside of UiO-67-BPY is lim-
ited by diffusion difficulties of the substrate and/or product mole-
cules. Whereas, the diffusion issues in the shell layer UiO-67-BPY
of the core-shell UiO-67-BPY@UiO-66 structure having a smaller
thickness (50 nm) were prevented. Since the diffusion was avoided
in the core-shell structure a higher catalytic performance could be
observed for the Knoevenagel condensation of benzaldehyde and
malononitrile even at short reaction time.
6
0
60
88%
96%
1
20
60
75%
8
RT
60
55%
(UiO-67-BPY as catalyst)
a
Reaction conditions: aldehyde (1 mmol), malononitrile (1.2 mmol), catalyst
(
18 mg), DMSO-d (2 mL), room temperature (RT).
b
The conversions of aldehydes were determined by 1H NMR analysis.
the reaction mechanism, an H-atom of the methylene group in
malononitrile is abstracted by the basic sites, producing a carban-
ion. Then a nucleophilic attack of the carbanion on the carbon atom
of the carbonyl group from the aldehyde gives the intermediate.
Finally, after an appropriate rearrangement together with a dehy-
dration process, the condensation product is generated [44–
3.4. Reusability UiO-67-PBY@UiO-66
4
6,59,69,70]. The catalytic mechanism is related only to the basic
sites of the core-shell UiO-67-BPY@UiO-66. This could be stated
from the fact that low conversions were obtained for the reaction
with UiO-66 (contains only acid sites) and the reaction without a
catalyst. These results indicate that the acid sites in UiO-66 had
no effect on the reactivity [32]. Furthermore, when using dipyridyl
An important property of a heterogeneous catalyst is the cata-
lyst recyclability and stability. The obtained results of reusability
study of the core-shell UiO-67-PBY@UiO-66 heterogeneous cata-
lyst is given in Fig. 6. Therefore, after the completion of the reac-
tion, the catalyst UiO-67-PBY@UiO-66 was isolated from the
reaction mixture by centrifugation, washed with plenty of metha-
nol, and finally dried at 80 °C for 24 h under vacuum. The catalyst
was recovered and reused in successive runs for the Knoevenagel
condensation with a small loss in catalytic activity after four recy-
cles. The spent catalyst was characterized on its crystal morphol-
(
only containing basic sites) as a homogeneous catalyst for this
Knoevenagel condensation reaction of benzaldehyde and malonon-
itrile, the product was generated as demonstrated in Fig. S7 (right).
These two pieces of evidence proved that the mechanism during
the reaction might only involve the basic sites of the catalyst.

Y. Gong et al. / Journal of Catalysis 375 (2019) 371–379
377
Fig. 6. The recycling performance (left) and 1H NMR spectra (right) of the reaction mixture for the Knoevenagel condensation of benzaldehyde and malononitrile using UiO-
7-BPY@UiO-66 (at room temperature; reaction time: 1 h).
6
Fig. 7. The FESEM images (a) and PXRD patterns of core-shell catalyst after reaction (b).
ogy, crystallinity and porosity via TEM, SEM, PXRD, and N
2
adsorp-
4. Conclusions
tion at 77 K, respectively (The new results are applied in the
revised manuscript). The FESEM image (Fig. 7) shows that the size
and morphology of the catalyst were almost completely preserved
A core-shell UiO-67-BPY@UiO-66 was successfully synthesized
and exhibited excellent catalytic performance for the Knoevenagel
condensation of aldehydes with malononitrile. Interestingly, the
core-shell structure exhibited a higher catalytic performance over
traditional core- and shell-MOFs (UiO-66 and UiO-67-BPY, respec-
tively). The basic sites in the shell are key for the catalytic perfor-
mance. The shell structure is of key importance to avoid diffusion
issues. Moreover, the catalyst could be reused for at least four
cycles with a small loss in catalytic performance. The synergistic
effect between the core-shell structures was suggested to enhance
their properties. The present work potentially promotes the devel-
opment of novel high-performance heterogeneous catalytic sys-
tems by forming a core-shell structure. Further work is in
progress to broaden the extent of application of this heterogeneous
catalyst for other organic transformations.
after the reaction. The N
2
isotherms revealed a slightly decreased
adsorption which represents a small reduction of the surface area
2
ꢁ1
2
ꢁ1
from 1470 m ꢃg of the fresh catalyst to 1244 m ꢃg of the spent
catalyst (Fig. S9). The PXRD pattern (Fig. 7) of spent UiO-67-
BPY@UiO-66 definitely exhibits peaks of UiO-67-BPY (2h = 5.8,
6
.7), however, the intensity of those peaks reduces relatively com-
pared to the peaks of UiO-66 (2h = 7.4, 8.5). Additionally, the TEM
images (Fig. S9) of the spent catalyst display a similar size and
morphology compared with the fresh catalyst except for the irreg-
ular edges of the crystal particles, which could indicate a small
degree of degradation of the catalyst after the reaction. Conse-
quently, these results could represent a small loss in shell-
structure (UiO-67-BPY) after the catalytic reaction. Nevertheless,
the shell structure was generally preserved on the UiO-67-
BPY@UiO-66 catalyst as confirmed by the similar particle size
obtained from SEM and TEM images (Fig. 7 and Fig. S9). Addition-
ally, the elemental analysis of N-species revealed a similar N con-
tent on fresh and spent catalyst and the absence of leached Zr in
the solution. In parallel, the recovered core-shell catalyst was
examined by element analysis (C, H, N) and the N-content revealed
a neglectable change (3.41% for recovered, 3.43% for fresh), which
proved the relatively stable structure of UiO-67-PBY@UiO-66.
Author contributions
Y.Y.G., S.C., and F.V. conceived the original idea and designed the
experimental program. Y.Y.G. synthesized all the materials, carried
out all the reactions, and some characterizations. P.Z. and J.C.W.
performed part of BET and XRD analyses. C.C. and Y.Y. character-
ized reaction mixtures via NMR. S.C. and F.V. supervised the pro-

3
78
Y. Gong et al. / Journal of Catalysis 375 (2019) 371–379
ject. Y.Y.G. wrote the initial draft of the manuscript. S.C. and F.V.
finalized the draft version. All authors approved the final version
of the manuscript.
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