Robust Bifunctional Core-Shell MOF@POP Catalyst for One-Pot Tandem Reaction

Article abstract of DOI:10.1021/acs.inorgchem.8b02303

A new bifunctional acid-base catalyst, core-shell UiO-66@SNW-1, with robust chemical and thermal stability, recyclability, and durable catalytic activity is synthesized by a convenient, universal strategy. Interestingly, this hybrid material can effectively catalyze deacetalization-Knoevenagel condensation reaction in the presence of excellent compartmentalization to spatially isolate opposing acid-base sites.

Full text of DOI:10.1021/acs.inorgchem.8b02303

Communication
pubs.acs.org/IC
Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
Robust Bifunctional Core−Shell MOF@POP Catalyst for One-Pot
Tandem Reaction
Mei-Hong Qi, Ming-Liang Gao, Lin Liu, and Zheng-Bo Han*
College of Chemistry, Liaoning University, Shenyang 110036, P. R. China
S
* Supporting Information
neous catalysis. Recently, Zhang and co-workers successfully
ABSTRACT: A new bifunctional acid−base catalyst,
core−shell UiO-66@SNW-1, with robust chemical and
thermal stability, recyclability, and durable catalytic
activity is synthesized by a convenient, universal strategy.
Interestingly, this hybrid material can effectively catalyze
deacetalization-Knoevenagel condensation reaction in the
presence of excellent compartmentalization to spatially
isolate opposing acid−base sites.
developed a new type of NH2-MIL-68@TPA-COF core−shell
hybrid material and applied it to degradation of rhodamine
B.12 Xie et al. reported a MOF@POP nanocomposite (UNM)
that showed great potential in biomedical fields and cancer
treatment.13 The Hu group also developed a series of MIL-
101@Pt@FeP-CMP composite materials that possess Lewis
acid sites to activate CO bond.14 To the best of our
knowledge, the performances of these core−shell MOFs@
POPs hybrid materials are superior to the individual
components.
On the basis of the above-mentioned strategies, the use of
MOFs@POPs hybrid materials as a single catalytic system
could combine their respective advantages while effectively
eliminating their weakness. Therefore, the fabrication of
MOFs@POP hybrid materials with multifunctional catalytic
sites would be of great promise for extending the application of
MOFs and POPs. In this work, a new MOF@POP (UiO-66@
SNW-1) was prepared under solvothermal conditions and
presented as an original platform for catalyzing the one-pot
tandem deacetalization-Knoevenagel condensation reaction
(Scheme 1). Delightedly, the core−shell UiO-66@SNW-1,
ne-pot tandem reactions have attracted great attention
Oowing to their natures of simple synthetic procedures
and atom efficiency.1 The multifunctional solid catalysts
possessing different catalytic sites are extremely applicable to
catalyze tandem reactions.2 For example, metal−organic
frameworks (MOFs), montmorillonite, porous organic poly-
mers (POPs), mesoporous silicas, etc., which contain
coexistent acidic and basic sites, have been applied for tandem
reaction.3
MOFs, a promising class of porous crystalline materials,
assemble with metal centers and various organic linkers.4
Because of their large surface areas, tunable pore size,
recoverability, and chemical diversity, MOFs usually show
great potential for versatile applications, such as gas storage
and separation, carbon capture, sensing, drug delivery,
catalysis,5 etc.6 Recently, the catalytic property of MOFs has
attracted a great attention by researchers. Most MOFs are
appropriate for single-site catalysis, thus preparing multifunc-
tional MOFs catalyst has remained challenging.7 The main
limitation in the catalytic field depended on how to introduce
other functional sites and maintaining the natural features.8
MOF-based hybrid materials are an emerging multifunctional
material that combines MOFs with other MOFs, enzymes,
magnetic metal oxides, and polymers.9 These hybrid materials
demonstrate admirable properties compared with single
species in a catalytic field. Porous organic polymers (POPs),
as a new kind of developed porous material, are entirely
constructed from organic building blocks with reticular
chemistry through covalent bond formation.10 POPs possess
high surface areas, variable functional groups, and adjustable
pore sizes that offer unprecedented possibilities for catalysis.11
However, to prepare acid−base bifunctional catalysts of POPs
is not straightforward, especially when Lewis acid and base
sites tend to quench each other. Therefore, to develop new
types of hybrid MOFs@POPs is of great strategic significance
owing to their potentially improved performance over those of
individual components and extensive applications in heteroge-
Scheme 1. Schematic Illustration Showing the Fabrication
of UiO-66@SNW-1
consisting of Lewis acid sites (the Zr clusters in UiO-66) and
Brønsted base sites (aminal groups in SNW-1), can act as a
highly efficient bifunctional catalyst for the one-pot tandem
reaction. Impressively, the bifunctional core−shell UiO-66@
SNW-1 catalyst exhibited more preeminent catalytic perform-
ance than the the parent UiO-66, SNW-1, and UiO-66-NH2.
To investigate the crystallinity and structural stability of
UiO-66@SNW-1 hybrid material, the comprehensive charac-
Received: August 15, 2018
© XXXX American Chemical Society
A
DOI: 10.1021/acs.inorgchem.8b02303
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Communication
terization was performed by powder X-ray diffraction (PXRD).
As shown in Figure S1, the identically strong Bragg diffraction
peaks of UiO-66 and UiO-66@SNW-1 revealed that SNW-1 is
amorphous and the crystalline structure of UiO-66 does not
alter in the process of growth of POPs. The FT-IR spectrum of
UiO-66@SNW-1 matched well with UiO-66 as well as SNW-1
and further indicated that the UiO-66@SNW-1 was success-
fully formed. The presence of the distinct stretching band for
the triazine ring at ≈1550 and 1480 cm−1 indicates the
formation of SNW-1 (Figure S2 and S3). In addition, the
chemical and thermal stability of core−shell UiO-66@SNW-1
hybrid material were investigated by PXRD. When the hybrid
material was soaked in different solvents for 48 h, PXRD
patterns remained unchanged, which indicated no framework
collapse during chemical stability tests (Figure 1 and S4). FT-
Figure 2. SEM images of (a) UiO-66, (b) SNW-1, (c) UiO-66@
SNW-1 hybrid material. (d) TEM image of UiO-66@SNW-1 hybrid
material.
drawn much attention. The UiO-66@SNW-1 hybrid material
contained abundant unsaturated Zr centers, which can be used
as Lewis acid sites for catalyzing deacetalization reaction and
the aminal groups in SNW-1 as a weak Brønsted base sites can
be utilized for catalyzing Knoevenagel reaction. Thus,
deacetalization-Knoevenagel reaction was selected to explore
the catalytic performance of core−shell UiO-66@SNW-1
hybrid material. The catalytic activity of hybrid material was
evaluated in the presence of 50 mg of activated UiO-66@
SNW-1 as a model reaction, wherein benzaldehydedimethyla-
cetal (a) (2.0 mmol) reacts with malononitrile (2.1 mmol) in
DMSO-d6 (2.0 mL) under a N2 atmosphere at 80 °C for 12 h.
The conversion and yield were monitored by 1H NMR
spectroscopy. In this case, excellent yield (99.6%) of the
desired product 2-benzylidenemalononitrile (c) was observed
(Table 1). Compared with other previous catalysts such as Yb-
BCD-NH2, MIL-101(Al)-NH2, and PCN-124 (Table S2),15
UiO-66@SNW-1 hybrid material also exhibited good catalytic
Figure 1. Chemical stability tests for UiO-66@SNW-1 monitored by
PXRD.
IR spectrum further proved that the structure is intact without
damage (Figure S5). The variable-temperature PXRD and
thermogravimetric analysis (TGA) were also investigated, and
the results revealed that UiO-66@SNW-1 hybrid material
could be stable up to 400 °C in air (Figure S6 and S7).
The core−shell UiO-66@SNW-1 hybrid material was
investigated by scanning and transmission electron microscopy
(SEM and TEM, in Figure 2). As shown in Figure 2a−c, SEM
images showed that UiO-66 has an angulated nanocrystal with
smooth surfaces and the surfaces of the pristine UiO-66
become rough after growing SNW-1. The SNW-1 coating in
the TEM images can be distinguished by a lighter contrast than
that of the inner MOF. TEM image analysis of UiO-66@SNW-
1 revealed that the surface of the UiO-66 is coated with SNW-
1 material (∼25 nm thickness) (Figure 2d). The weight ratio
of UiO-66:SNW-1 in UiO-66@SNW-1 is determined to be
≈1:0.56 on the basis of the TGA analyses. The N2 sorption
properties of core−shell UiO-66@SNW-1 material were
analyzed by the Brunauer−Emmett−Teller (BET) method at
77 K. The results of core−shell UiO-66@SNW-1 hybrid
material exhibited type IV isotherms, and the surface area was
measured as 354 m2 g−1. The N2 sorption properties of the
UiO-66@SNW-1 revealed that the microporous characteristics
of the hybrid material with a very high degree of cross-linking
by SNW-1 (Figure S8).
Table 1. One-Pot Tandem Deacetalization-Knoevenagel
a
1
Condensation Reaction and Monitored by H NMR
entry
conv of a (%)
yield of b (%)
yield of c (%)
UiO-66@SNW-1
second cycle
third cycle
fourth cycle
fifth cycle
UiO-66-NH2
UiO-66
SNW-1
99.6
99.5
99.4
99.6
99.5
75.2
99.5
∼0
trace
trace
trace
trace
trace
0.9
83.3
trace
trace
17.5
99.6
99.5
99.4
99.6
99.5
74.3
16.2
trace
trace
81.9
no catalyst
UiO-66+SNW-1
∼0
99.4
a
Reaction conditions: benzaldehyde dimethylacetal (2.0 mmol),
Recently, the development of bifunctional heterogeneous
catalysts for promoting one-pot tandem reactions has currently
malononitrile (2.1 mmol), DMSO-d6 (2 mL), and catalyst (50 mg);
reaction temperature, 80 °C; reaction time, 12 h.
B
DOI: 10.1021/acs.inorgchem.8b02303
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Inorganic Chemistry
Communication
activity for deacetalization-Knoevenagel reaction. In detail, the
kinetic investigation revealed that the reaction completes
within 12 h (Figure S9). On the basis of the control
experiments, the tandem reaction can be considered as two
sequential steps: first, the unsaturated Zr clusters catalyzed
benzaldehyde dimethylacetal (a) to generate benzaldehyde
(b); second, the aminal groups in SNW-1 catalyzed
Knoevenagel condensation reaction to produce 2-benzylide-
nemalononitrile (c). To better comprehend the necessity of
the catalyst for this tandem reaction, the same model reaction
was carried out without catalyst. The reaction without UiO-
66@SNW-1 can scarcely react to product over 12 h. When
UiO-66 used as catalyst to boost this tandem reaction, the first
step deacetalization reaction was efficient but the second step
Knoevenagel condensation reaction did not take place. When
only SNW-1 was used as catalyst, the target product could not
be detected. In addition, as contrasted with UiO-66-NH2, the
catalytic effect of UiO-66@SNW-1 was better on deacetaliza-
tion-Knoevenagel condensation reaction. The activity on the
physical mixture of UiO-66 and SNW-1 (UiO-66: SNW-1 =
1:0.56) was relatively lower than UiO-66@SNW-1 which
confirmed the advantages of the core−shell structural design of
the UiO-66@SNW-1 material. The reason was assigned to the
two catalytic parts being joined together, making the mass
transfer process more efficient.16 A leaching test indicated that
no active species leached into the solution and the
heterogeneity of the catalyst is directly proved (Figures S10).
After tandem reaction, the catalyst could be removed by
centrifuging and reused at least for five cycles without a
noticeable change in its activity (Figures S11). The crystallinity
and structural integrity of core−shell UiO-66@SNW-1 hybrid
material did not change even after five cycles, indicating its
great recyclability and excellent stability for the one-pot
tandem deacetalization-Knoevenagel condensation reaction
(Figures S12, S13, S14).
In conclusion, a bifunctional acid−base catalyst, core−shell
UiO-66@SNW-1 hybrid material, has been successfully
developed to boost the catalytic performance toward one-pot
tandem deacetalization-Knoevenagel condensation reaction.
The core−shell UiO-66@SNW-1 hybrid material contained
both Lewis acid and Brønsted base sites and showed large
surface areas as well as high chemical and thermal stability. In
the meantime, high chemical stability and thermal stability,
microporous characteristics of core−shell UiO-66@SNW-1
hybrid material is markedly beneficial for heterogeneous
catalysis. Finally, this work highlighted the prominent catalytic
performance of core−shell UiO-66@SNW-1 hybrid material
for one-pot tandem deacetalization-Knoevenagel condensation
reaction. This strategy would be extensively employed in the
development of functional MOFs hybrid materials and high
performance heterogeneous catalyst. Our ongoing work will
deal with systematically preparing MOFs@POPs hybrid
materials that can be applied in tandem reaction.
filtration reactions, recycle capacity tests, SEM and TEM
images, and data from condensation reactions (PDF)
AUTHOR INFORMATION
Corresponding Author
*Z.-B. Han. E-mail: ceshzb@lnu.edu.cn.
ORCID
■
Zheng-Bo Han: 0000-0001-8635-9783
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was granted financial support from the National
Natural Science Foundation of China (Grant 21671090 and
21701076).
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ASSOCIATED CONTENT
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S
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.8b02303.
Materials and general methods, PXRD pattern, FT-IR
spectra, TGA curves, adsorption−desorption isotherms,
1H NMR spectra, conversions and yields from hot
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