Yolk-Shell-Structured Covalent Organic Frameworks with Encapsulated Metal-Organic Frameworks for Synergistic Catalysis

Article abstract of DOI:10.1021/acs.chemmater.1c01383

Yolk-shell composites offer a promising platform for integrating cores into hollow shells to create unique structures and properties. However, the concomitant functionality and tunability of yolk-shell nanocomposites is still a great challenge but highly desirable. Herein, we demonstrate a rational design for the fabrication of yolk-shell-structured covalent organic framework (COF)@metal-organic framework (MOF) (YS-COF@MOF) nanocomposites with COF as the external shell and MOF as the inner yolk. Series of the YS-COF@MOF composites with different MOF cores and COF shells were readily synthesized via a template-free solvothermal method. Control experiments showed that the formation of the hollow cavity between the core and the shell originated from the amorphous-to-crystalline transformation and the simultaneous shrinkage of the shell under the pyrrolidine-catalyzed conditions. The resultant YS-COF@MOF merges the inherent structure tunability and functionality of both COFs and MOFs. The functions of YS-COF@MOF can be regulated and optimized by judicious selections of metal ions and organic building blocks. Representative YS-TpPa@UiO-66-(COOH)2 with spatially distributed acidic and basic sites exhibited synergistically enhanced catalytic activity in one-pot deacetalization-Knoevenagel cascade reactions.

Full text of DOI:10.1021/acs.chemmater.1c01383

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Article
Yolk−Shell-Structured Covalent Organic Frameworks with
Encapsulated Metal−Organic Frameworks for Synergistic Catalysis
Qiang Dang, Hanlin Huang, Liuyi Li,* Xiaolin Lyu, Shenghong Zhong, Yan Yu,* and Dongsheng Xu*
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ABSTRACT: Yolk−shell composites offer a promising platform for
integrating cores into hollow shells to create unique structures and
properties. However, the concomitant functionality and tunability of
yolk−shell nanocomposites is still a great challenge but highly desirable.
Herein, we demonstrate a rational design for the fabrication of yolk−
shell-structured covalent organic framework (COF)@metal−organic
framework (MOF) (YS-COF@MOF) nanocomposites with COF as the
external shell and MOF as the inner yolk. Series of the YS-COF@MOF
composites with different MOF cores and COF shells were readily
synthesized via a template-free solvothermal method. Control experi-
ments showed that the formation of the hollow cavity between the core
and the shell originated from the amorphous-to-crystalline trans-
formation and the simultaneous shrinkage of the shell under the
pyrrolidine-catalyzed conditions. The resultant YS-COF@MOF merges
the inherent structure tunability and functionality of both COFs and MOFs. The functions of YS-COF@MOF can be regulated and
optimized by judicious selections of metal ions and organic building blocks. Representative YS-TpPa@UiO-66-(COOH)2 with
spatially distributed acidic and basic sites exhibited synergistically enhanced catalytic activity in one-pot deacetalization−
Knoevenagel cascade reactions.
Metal−organic frameworks (MOFs)22 and covalent organic
INTRODUCTION
■
frameworks (COFs),23 featuring the well-defined,24 custom-
Yolk−shell composites consisting of a core inside a hollow
izable,25 and porous structures,26 have received incredible
cavity encircled by an outer shell have attracted considerable
attention as potential candidates for catalysis,27−32 chemical
interest because of their unique structures1 and versatile
sensing,33 gas separators,34 and many other applications.35,36
potential applications in sensing,2 biomedical imaging,3 drug
The systematic design in structure and functionality at the
delivery,4 energy conversion and storage,4−6 and catalysis.7−11
molecular level37 makes them promising candidates for yolk−
They can integrate multiple functionalities from the respective
shell structure engineering.38−40 Through the rational selection
cores and shells to achieve specific purposes.12 The existence
of metal ions, organic ligands, and organic building blocks,
specific properties in the yolk−shell-structured COF@MOF
of the cavity between the shell and the yolk allows sufficient
exposure of the core while retaining the intrinsic properties of
the core and shell in the maximum degree.13 Moreover, the
(YS-COF@MOF, COF as the shell and MOF as the yolk)
composites can be targeted.41 Besides, the well-defined
cavity can serve as a host space to accommodate guests for
special applications.3,9 Particularly, in catalysis, compared to
composite and structure in YS-COF@MOF could conse-
quently facilitate the mechanistic studies in the formation of
the core−shell structure, the cavity not only enables to expose
more active sites for the reactants14,15 but also provides a
composite materials and their applications. However, to the
special microenvironment for heterogeneous catalysis.7,8,16
best of our knowledge, no YS-COF@MOF composite has
been reported yet. This is mainly due to the complexity of the
Generally, the properties of functional materials very much
depend on their compositions and structures.17,18 Although the
synthetic procedure, especially, the use and removal of the
attractive performance can be realized by yolk−shell
composites,19−21 it is always difficult to further systematically
Received: April 21, 2021
Revised: July 3, 2021
Published: July 14, 2021
optimize their performance, as it is infrequent to use tailorable
materials to serve as functional cores and shells. Therefore, it is
highly desirable to construct novel yolk−shell composites with
tailorable properties of both core and shell for diverse
applications.
https://doi.org/10.1021/acs.chemmater.1c01383
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Figure 1. Schematic illustration of the synthesis of yolk−shell COFs@MOF nanocomposites (a) and the in situ transformation of the COF shell
(b); TEM images of UiO-66-(COOH)2 (c), Am-TpPa@UiO-66-(COOH)2 (d), and YS-TpPa@UiO-66-(COOH)2 (inset is an enlarged TEM
image of a single particle. Red: TpPa, yellow: UiO-66-(COOH)2, and blue: cavity).
sacrificial template in traditional methods to build other yolk−
shell materials.14,42,43
UiO-66-(COOH)2 was successfully obtained. The formation
of YS-TpPa@UiO-66-(COOH)2 from the core−shell Am-
TpPa@UiO-66-(COOH)2 precursor was confirmed by trans-
mission electron microscopy (TEM) analysis (Figure 1b−d).
As shown in TEM images, after solvothermal treatment, an
obvious gap between the core and the shell emerged in every
individual YS-TpPa@UiO-66-(COOH)2 particle, indicating
the formation of yolk−shell structures. The continuous and
smooth shell in Am-TpPa@UiO-66-(COOH)2 evolved into a
rough surface containing many small grains in YS-TpPa@UiO-
66-(COOH)2.
The successful formation of the COF outside of the UiO-66-
(COOH)2 particle was proven by Fourier transform infrared
spectroscopy (FTIR), solid-state 13C NMR, and X-ray
photoelectron spectroscopy (XPS) spectra. The FTIR
spectrum of YS-TpPa@UiO-66-(COOH)2 matches well with
that of UiO-66-(COOH)2 and TpPa (Figures 2a and S1). The
ketoenamine units in TpPa were formed by reversible
condensation of Tp with Pa forming an enol−imine isomer,
followed by a rapid irreversible enol-to-keto tautomerization.45
The presence of the stretching band at 1585 cm−1 (CC)
reveals the formation of the ketoenamine group in the TpPa
shell,46 while the main bands at 730, 1380, and 1722 cm−1
correspond to the Zr−O, carboxylate group (COO−), and
noncoordinating carboxyl groups (COOH),47 respectively,
showing the presence of UiO-66-(COOH)2 in YS-TpPa@
UiO-66-(COOH)2. The solid-state 13C NMR spectrum of YS-
TpPa@UiO-66-(COOH)2 further verifies the coexistence of
TpPa and UiO-66-(COOH)2 (Figure 2b), also indicating the
structural integrity of the UiO-66-(COOH)2 core and the
TpPa shell during the synthetic process. After Ar+ ion etching
the YS-TpPa@UiO-66-(COOH)2 for about 30 s, the increase
in the intensity of the Zr 3d XPS peak further confirmed the
presence of the UiO-66-(COOH)2 core in the COF shell
(Figure 2c).
Herein, we developed an efficient template-free strategy to
fabricate YS-COF@MOF nanocomposites via a general
solvothermal method. The resultant composites featured a
combination of tailorable cores and shells and multifunction-
alities. This desired outcome was realized by an in situ
amorphous-to-crystalline transformation and a shrinkage of the
external amorphous polyimine shell in the parent core−shell
homologues. A representative yolk−shell TpPa@UiO-66-
(COOH)2 nanocomposite with the unique spatially distributed
acid−base sites exhibited enhanced catalytic performance in
one-pot deacetalization−Knoevenagel (D−K) cascade reac-
tions compared with its core−shell counterparts and the
random mixture of UiO-66-(COOH)2 and TpPa. These results
combined the advantages of MOFs and COFs and lead the
way toward the construction of yolk−shell composites with
adjustable functions.
RESULTS AND DISCUSSION
■
The synthetic strategy of yolk−shell-structured COF@MOF
nanocomposites is shown in Figure 1a. In a typical synthesis,
the presynthesized UiO-66-(COOH)2 as a core was first
44
mixed with organic building blocks [1,3,5-triformylphloroglu-
cinol (Tp) and p-phenylenediamine (Pa)] to form a core−shell
nanostructure with an amorphous polyimine network shell
(designated as Am-TpPa@UiO-66-(COOH)2) by a Schiff-
base reaction. The subsequent solvothermal treatment resulted
in the formation of a yolk−shell-structured nanocomposite
(designated as YS-TpPa@UiO-66-(COOH)2). Various syn-
thetic conditions were screened to transfer the amorphous
polyimine shell in the core−shell precursor to the crystalline
shell and simultaneously generate a void between the MOF
core and the shell (Table S1). To our delight, under the
pyrrolidine-catalyzed conditions, yolk−shell-structured TpPa@
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Figure 2. FTIR spectra of UiO-66-(COOH)2, TpPa, and YS-TpPa@UiO-66-(COOH)2 (a); 13C solid-state NMR of UiO-66-(COOH)2, TpPa,
and YS-TpPa@UiO-66-(COOH)2 (b); Zr 3d XPS upon Ar+ sputtering of YS-TpPa@UiO-66-(COOH)2 (c); PXRD patterns of TpPa, UiO-66-
(COOH)2, Am-TpPa@UiO-66-(COOH)2, and YS-TpPa@UiO-66-(COOH)2 (d); N2 sorption isotherms of UiO-66-(COOH)2, TpPa, and YS-
TpPa@UiO-66-(COOH)2 (e); and pore size distribution of TpPa, UiO-66-(COOH)2, and YS-TpPa@UiO-66-(COOH)2 (f).
Crystalline structures of the YS-TpPa@UiO-66-(COOH)2
were investigated by powder X-ray diffraction (PXRD) analysis
(Figure 2d). The characteristic diffraction peaks of UiO-66-
(COOH)2 were all preserved in the core−shell precursors and
the yolk−shell composites. The characteristic diffraction peak
at 4.8° related to the reflection from (100) plane of TpPa45 in
Am-TpPa@UiO-66-(COOH)2 was quite weak, suggesting that
the polyimine was mainly composed of amorphous structures.
The solvothermal treatments significantly increased the
crystallinity of the shell, although the peak was relatively
weak compared to the strong diffraction peak of UiO-66-
(COOH)2.48
(Figure 2e). YS-TpPa@UiO-66-(COOH)2 exhibits a com-
bined type-I/IV isotherm. The porous structure is anticipated
from UiO-66-(COOH)2 and TpPa. The pore size distribution
of yolk−shell YS-TpPa@UiO-66-(COOH)2 revealed two
major nanopores of around 5.0 and 16.9 Å, which are
consistent with the respective pores in UiO-66-(COOH)2 and
TpPa (Figure 2f). The BET surface areas of yolk−shell YS-
TpPa@UiO-66-(COOH)2 were calculated to be 496 m2 g−1,
which is higher than those of UiO-66-(COOH)2 (453 m2 g−1)
due to the presence of TpPa with a higher surface area of 848
m2 g−1. The thermal stability of YS-TpPa@UiO-66-(COOH)2
was analyzed using thermogravimetric analysis (TGA),
revealing that its thermal stability is similar to that of UiO-
66-(COOH)2 (Figure S2).
The permanent porosity of YS-TpPa@UiO-66-(COOH)2
was assessed by measuring N2 sorption isotherms at 77 K
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Figure 3. TEM images and PXRD patterns of YS-TpPa@ZIF-8 (a,b); YS-TpPa@ZIF-67 (c,d); and YS-ZIF-67@TpBD (e,f).
Under identical experimental conditions, yolk−shell YS-
TpPa@UiO-66 can also be obtained using UiO-66 as the
core44,49 (Figure S3). ZIFs,50,51 the other typical kind of
MOFs, can also be employed as the core in the yolk−shell
nanocomposites. As shown in Figure 3a−d, when ZIF-8 and
ZIF-67 with different metal ions were used, the gaps with a
rhombic dodecahedral shape between the core and shell in YS-
TpPa@ZIF-8 and YS-TpPa@ZIF-67 were clearly observed,
while the TpPa shell preserved the well-defined rhombic
dodecahedral shape of ZIFs. We further adjusted the TpPa
shell to TpBD45 by considering its longer linker. TEM images
and PXRD patterns indicated the formation of the yolk−shell-
structured YS-TpBD@UiO-66-(COOH)2 (Figure S4) and YS-
TpBD@ZIF-67 (Figure 3e,f). In a word, these results proved
that the universality of the formation of yolk−shell-structured
COF@MOF and consequently, the compositions and
structures of the yolk−shell COF@MOF can be tuned
through the varying types of MOFs and COFs.
To investigate the formation mechanism of the yolk−shell-
structured COF@MOF, we carried out the time-dependent
experiments of the formation of YS-TpPa@UiO-66-(COOH)2
and YS-TpPa@ZIF-8. The YS-COF@MOF samples isolated
after 24, 48, and 72 h reaction time were analyzed using PXRD
and TEM analyses to characterize the crystallinity and
morphology of YS-COF@MOF at different time intervals.
TEM images of the 24 h samples showed that the core−shell
structure remains almost identical to that of the precursor. In
the 72 h sample for YS-TpPa@UiO-66-(COOH)2 and 48 h
sample for YS-ZIF-8@TpPa, an obvious gap between the core
and the shell was observed, showing the generation of the
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Figure 4. Time-dependent experiments of YS-TpPa@UiO-66-(COOH)2 (a−c); PXRD patterns of YS-TpPa@UiO-66-(COOH)2 (d) and H-TpPa
from TpPa@ZIF-8 (e) at different time intervals; the thickness of the TpPa shell in TpPa@ZIF-8 at different time intervals (f); and proposed
mechanism of the formation of yolk−shell-structured COF@MOF (g).
yolk−shell structure (Figures 4a−c and S5). The diffraction
peak assigned to the (100) facet of TpPa appeared in the
PXRD patterns of the 24 h samples (Figure 4d), suggesting the
transformation of the amorphous polyimine to the crystalline
framework.52 In the continuous solvothermal treatment, the
PXRD peak intensity of TpPa increased by virtue of the self-
correction of defects. Hollow TpPa (designated as H-TpPa)
could be obtained after the removal of ZIF-8 from YS-TpPa@
ZIF-8 (Figure S6). PXRD patterns of the H-TpPa at different
time intervals showed that the peak intensity gradually
increased with increasing solvothermal treatment time (Figure
4e), which was in agreement with the decrease in the full width
at half-maximum (fwhm) of (100) diffraction peaks (Figure
S7). The (100) peak slightly shifts to a lower diffraction angle
with the increase in crystallinity mainly due to the optimization
of the planarity and the resulting expansion of the lattice. It is
noteworthy that the size and morphology of MOF and MOF@
COF particles in the core−shell and yolk−shell samples are
identical (Figures S8 and S9). The thermal stability test
showed that the fwhm of the solvothermally treated UiO-66-
(COOH)2 was similar to that of the fresh sample (Figure S10),
indicating that the crystalline size of UiO-66-(COOH)2
particles almost unchanged in the thermal treatments.
Energy-dispersive X-ray spectrometry revealed that the
contents of Zr in Am-TpPa@UiO-66-(COOH)2 and YS-
TpPa@UiO-66-(COOH)2 were identical (Figure S11),
showing negligible mass loss of COF after the solvothermal
process. Therefore, the formation of yolk−shell structures was
mainly related to the changes in the shell materials. As shown
in Figure 4f, the shell thickness decreases with the
solvothermal reaction time going by. The average thickness
of the TpPa is reduced from 106 nm (0 h) to 82 nm (24 h), 74
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Figure 5. D−K reaction (a); time-dependent catalytic performance of YS-TpPa@UiO-66-(COOH)2 for the D−K condensation reaction (b);
control experiments with different catalysts (c); reusability of YS-TpPa@UiO-66-(COOH)2 for the D−K condensation reaction (d); and proposed
catalytic mechanism of YS-TpPa@UiO-66-(COOH)2 for the D−K condensation reaction (e).
nm (48 h), and 60 nm (72 h). Considering the simultaneous
increase in the crystalline intensity and decrease in the shell
thickness and the maintenance of the MOF core, we speculate
that the cavity between the MOF core and the COF shell
mainly originated from the outward shrinkage of the shell in
the process of the transformation of the amorphous polyimine
to the crystalline framework. However, when NH2-UiO-66 was
employed as the core, only core−shell-structured TpPa@NH2-
UiO-66 was obtained (Figure S12), since the amine groups in
the ligand of NH2-UiO-66 could undergo a Schiff base reaction
leading to a strong covalent interface between the MOF core
and COF shell,38 inhibiting the outward shrinkage of the COF
shell. When the transformation was carried out in the presence
of acetic acid as a catalyst, just core−shell TpPa@UiO-66-
(COOH)2 was obtained (Figure S13). We speculate that acetic
acid could both strongly interact with the MOF surface and the
adsorbed amino compounds via electrostatic interactions,
imposing restrictions on the cleavage of amino groups from
the MOF surface. However, pyrrolidine could replace the
adsorbed amino compounds on the MOF surface. Besides,
pyrrolidine can react to some extent with the residual aldehyde
groups in the shell to form an iminium ion,53 contributing to
the attack of the amino groups on the highly electrophilic
iminium carbons and thus leading to the efficient reformation
of the polyimine networks into COFs.54
line transformation and the simultaneous shrinkage of the shell
under the base-catalyzed conditions. The shrinkage of the shell
may originate from the disorder-to-order transition of the shell
and/or inside-out Ostwald ripening process.55 However, we
believe that the Ostwald ripening effect should also occur
during the synthesis of the yolk−shell structures,56 but it will
be inhibited owing to the diffusion control resulting from the
irreversible enol-to-keto tautomerization. The detailed for-
mation mechanism of the yolk−shell structure is depicted in
Figure 4g. First, owing to the weak interactions between the
surface of MOF and the monomers, the subsequent
condensation reactions yield the core−shell-structured poly-
imine@MOF.32 Under solvothermal conditions, the disor-
dered polyimine reformed and shrunk to the ordered
framework,52,57 forming the crystalline shell. During the
shrinkage process, the shell separated from the MOF core
under the pyrrolidine-catalyzed condition, leading to the gap
between the core and the shell,40 producing the YS-COF@
MOF.
Yolk−shell-structured materials have attracted remarkable
attention in catalysis.58 The hollow shell offers a confined
space to keep the active yolk, while the inner void ensures the
pathway for mass transfer. Specially, the tunability of the
designed yolk−shell COFs@MOFs endow them tailorable
functions.59 Because the carboxylic groups in UiO-66-
(COOH)2 and ketoenamine groups in TpPa are physically
separated, YS-TpPa@UiO-66-(COOH)2 could be employed as
an acid−base bifunctional catalyst. The acid−base character-
Based on the analyses of experimental results and previous
mechanistic studies of COF growth, the formation of the YS-
COF@MOF possibly results from the amorphous-to-crystal-
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istics were determined by NH3- and CO2-temperature-
programmed desorption (TPD), respectively. As shown in
Figure S14, YS-TpPa@UiO-66-(COOH)2 exhibited the CO2
desorption peak within the range of 150−300 °C. It can
therefore be inferred that basic sites existed in the TpPa. The
adsorbed CO2 could be assigned to the type of CO2 that has
interacted with the NH species in the TpPa. Two main NH3
desorption peaks were observed in the NH3-TPD of YS-
TpPa@UiO-66-(COOH)2, which are caused by the combina-
tion of physical adsorption and chemical adsorption on the
acid sites. The TpPa shell with periodic pores and the cavity
between the shell and core did not hinder the exposure of acid
sites in UiO-66-(COOH)2. YS-TpPa@UiO-66-(COOH)2 can
effectively catalyze the one-pot tandem D−K condensation of
benzaldehyde dimethyl acetal and malononitrile (Figure 5a).
The tandem D−K condensation involves two reaction steps.
The first step is the deacetalization of benzaldehyde dimethyl
acetal with water to generate benzaldehyde over the acid sites.
In the second step, Knoevenagel condensation of benzalde-
hyde with malononitrile proceeds over the basic sites, yielding
the unsaturated product, which is an important intermediate in
the synthesis of natural products, pharmaceuticals, and
functional polymers. The conversion of the reactant and the
selectivity of the target product were monitored by gas
chromatography and mass spectrometry. As expected, the
result showed that the tandem reaction can be efficiently
catalyzed by YS-TpPa@UiO-66-(COOH)2 with 99% yield of
the benzylidene malononitrile product (Figure 5b). The
catalytic performance is among the best compared to other
heterogeneous catalytic systems that have been reported so far
(Table S2). To better understand the catalytic activity of yolk−
shell TpPa@UiO-66-(COOH)2, several control experiments
were investigated. As shown in Figure 5c, no product was
detected without the addition of YS-TpPa@UiO-66-
(COOH)2. When UiO-66-(COOH)2 was added into the
reaction system, the deacetalization took place smoothly while
the Knoevenagel reaction almost did not occur. When YS-
TpPa@UiO-66, core−shell TpPa@NH2-UiO-66, and TpPa
were employed in the catalytic reaction, respectively, very low
conversion of benzaldehyde dimethyl acetal was observed,
owing to the absence of acid sites in these materials, implying
the importance of the cooperativity between acid and base sites
for the D−K one-pot tandem reaction. The core−shell-
structured Am-TpPa@UiO-66-(COOH)2 showed a relatively
low ability to fulfill the one-pot reaction with 99% conversion
of benzaldehyde dimethyl acetal, 75% yield of benzaldehyde,
and 24% yield of benzylidene malononitrile under identical
reaction conditions, indicating the fact that the crystalline COF
shell with ordered pores and the hollow cavity to facilitate the
mass transfer and inhibit the annihilation of acid−base sites is
crucial for the D−K condensation reaction. The use of a
random mixture of UiO-66-(COOH)2 and TpPa leads to a
99% conversion of benzaldehyde dimethyl acetal and only 52%
yield of benzylidene malononitrile mainly because of the
annihilation of the opposite groups on the surface, thus
resulting in destruction of cooperative catalysis.60 These results
confirmed that the separation of the acidic and basic sites
within the yolk−shell-structured COF@MOF is quite
important to achieve the high catalytic performance. The
catalyst can be used at least 5 times without obvious loss of
catalytic activity (Figure 5d). The PXRD peaks and FTIR of
the recovered YS-TpPa@UiO-66-(COOH)2 match well with
those of the as-synthesized sample (Figure S15), indicating the
intact integrity of the structure.
The possible catalytic mechanism of YS-TpPa@UiO-66-
(COOH)2 was proposed (Figure 5e). First, the starting
material of benzaldehyde dimethyl acetal was passed through
the ordered pores in the COF shell. Then, the deacetalization
of benzaldehyde dimethyl acetal catalyzed by carboxylic groups
in the UiO-66-(COOH)2 core afforded a benzaldehyde
intermediate. Finally, the Knoevenagel condensation of
benzaldehyde and malononitrile took place in the TpPa
shell, yielding the benzylidene malononitrile product. The
Knoevenagel condensation is supposed to be the rate-
determining step.61 On account of the diversity and
tailorability of COFs, yolk−shell COF@MOF containing
stronger base sites with higher efficiency could be expected
to be developed.
CONCLUSIONS
■
Series of yolk−shell-structured COF@MOF nanocomposites
were successfully constructed through a template-free
solvothermal method. Time-dependent experiments and
characterization findings showed that the formation of a
hollow cavity between the core and the shell resulted from the
transformation of an amorphous external shell with a
disordered structure to a crystalline network with an ordered
structure along with the in situ shrinkage of the shell. The
representative yolk−shell-structured YS-TpPa@UiO-66-
(COOH)2 nanocages possessing spatially distributed acidic
and basic sites showed synergistically enhanced catalytic
performances in the one-pot D−K tandem reaction compared
to that of its homologues. Importantly, benefiting from the
diversity and adjustable nature of MOFs and COFs, the
systematic design of functionality of COFs@MOFs can be
facilely targeted. We envision that the future research on
combinations of MOFs and COFs in yolk−shell composites
will allow for various novel functions.
EXPERIMENTAL SECTION
■
Synthesis of YS-UiO-66-(COOH)2@TpPa. The YS-TpPa@UiO-
66-(COOH)2 was prepared using the two-step method. In the first
step, core−shell microspheres Am-TpPa@UiO-66-(COOH)2 consist-
ing of UiO-66-(COOH)2 in the core and an amorphous polyimine
network in the shell were prepared. Typically, the dispersion of UiO-
66-(COOH)2 (16 mg) in tetrahydrofuran (11 mL) was mixed with Pa
(9 mg) under ultrasonication for 30 min, resulting in a milk-white
homogeneous solution. The obtained solution was kept at 50 °C for
30 min under vigorous stirring allowing for the interaction between Pa
and UiO-66-(COOH)2. Subsequently, a solution of 1,3,5-triformyl-
phloroglucinol (Tp, 12 mg) in tetrahydrofuran (4 mL) was added to
the abovementioned dispersion at a feeding rate of 0.4 mL/min, and
the reaction was proceeded for 12 h. The solvent was removed by
rotary evaporation to obtain the core−shell Am-TpPa@UiO-66-
(COOH)2 as red powder. In the second step, the Am-TpPa@UiO-66-
(COOH)2 microspheres were dispersed in a mixed solvent of 1,2-
dichlorobenzene and n-butanol (1.35 mL/0.15 mL) in a Pyrex tube
(body length of 20 cm). Pyrrolidine (0.15 mL) as a catalyst was added
into the Pyrex tube. The tube was then flash-frozen at 77 K (liquid N2
bath) and degassed by three freeze−pump−thaw cycles, before
evacuating to a pressure of 0.5 mbar. The tube was flame-sealed,
reducing the total length by ca. 15 cm. After gradual warming up to
room temperature, the tube was heated at 120 °C for 3 days. The
solid was collected by filtration, washed with acetone, and dried at 60
°C under vacuum for 12 h to yield YS-TpPa@UiO-66-(COOH)2 as
crimson powder.
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Synthesis of Yolk−Shell-Structured TpPa@UiO-66. YS-
TpPa@UiO-66 was prepared by the abovementioned method except
that UiO-66-(COOH)2 was replaced by UiO-66.
Synthesis of Yolk−Shell-Structured TpPa@ZIF-67 and
TpPa@ZIF-8. YS-TpPa@ZIF-67 and YS-TpPa@ZIF-8 materials
were prepared by the abovementioned method except that UiO-66-
(COOH)2 was replaced by ZIF-67 and ZIF-8 and the amounts of Tp
and Pa were changed to 9.6 and 7.2 mg, respectively.
Synthesis of Yolk−Shell-Structured TpBD@UiO-66-(COOH)2
and TpBD@ZIF-67. YS-TpBD@UiO-66-(COOH)2 and YS-TpBD@
ZIF-67 materials were prepared by the abovementioned method
except that Pa was replaced by BD and the amounts of Tp and BD
were changed to 11.25 and 15 mg, respectively.
Synthesis of H-TpPa. The as-synthesized YS-TpPa@ZIF-8 (30
mg) was placed in a solution of 1,4-dioxane (2 mL) and acetic acid
(200 μL) for 1 h at room temperature. The residual powder was
collected by filtration, washed with water and ethanol, and dried at 60
°C under vacuum for 12 h to afford H-TpPa.
Catalytic Cascade Reactions. The catalytic performance of YS-
TpPa@UiO-66-(COOH)2 was studied in the cascade reaction of
benzaldehyde dimethyl acetal with malononitrile. The catalysts were
activated at 373 K overnight before being employed for the catalytic
studies. In a typical procedure, benzaldehyde dimethyl acetal (22.4
μL, 0.15 mmol), malononitrile (12.6 μL, 0.2 mmol), and 25 μL of
superpure water were well-dispersed in 0.5 mL of acetonitrile followed
by adding 5 mg of catalysts. After sonication for 5 min, the obtained
solution was stirred at 353 K for a period to finish the D−K reactions.
The conversion of the substrate and the selectivity of target products
were measured by gas chromatography and mass spectrometry (GC−
MS, Agilent Technologies 5977B GC/MSD). For recycling tests, the
catalyst was isolated and washed with acetone after each reaction and
subsequently reused in the next run with the same reaction
conditions. The percentage and the yield were calculated by the
area normalization method based on GC−MS data.
Fuzhou University, Fuzhou 350108, P. R. China; Beijing
National Laboratory for Molecular Sciences, College of
Chemistry and Molecular Engineering, Peking University,
Beijing 100871, P. R. China
Xiaolin Lyu − Key Laboratory of Eco-materials-Advanced
Technology, College of Materials Science and Engineering,
Fuzhou University, Fuzhou 350108, P. R. China;
orcid.org/0000-0003-4221-6806
Shenghong Zhong − Key Laboratory of Eco-materials-
Advanced Technology, College of Materials Science and
Engineering, Fuzhou University, Fuzhou 350108, P. R. China
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.chemmater.1c01383
Author Contributions
Q.D. and H.H. contributed equally to this work. The
manuscript was written through contributions of all authors.
All authors have given approval to the final version of the
manuscript.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported primarily by the National Key
Research and Development Program of China
(2020YFA0710303), the National Natural Science Foundation
of China (nos. U1905215 and 52072076), and the Natural
Science Foundation of Fujian Province of China (no.
2019J01226).
REFERENCES
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.chemmater.1c01383.
■
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AUTHOR INFORMATION
Corresponding Authors
■
Liuyi Li − Key Laboratory of Eco-materials-Advanced
Technology, College of Materials Science and Engineering,
Fuzhou University, Fuzhou 350108, P. R. China;
orcid.org/0000-0002-3066-2991; Email: lyli@fzu.edu.cn
Yan Yu − Key Laboratory of Eco-materials-Advanced
Technology, College of Materials Science and Engineering,
Fuzhou University, Fuzhou 350108, P. R. China;
orcid.org/0000-0001-6748-2620; Email: yuyan@
fzu.edu.cn
Dongsheng Xu − Beijing National Laboratory for Molecular
Sciences, College of Chemistry and Molecular Engineering,
Peking University, Beijing 100871, P. R. China;
orcid.org/0000-0003-1646-905X; Email: dsxu@
pku.edu.cn
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Authors
Qiang Dang − Key Laboratory of Eco-materials-Advanced
Technology, College of Materials Science and Engineering,
Fuzhou University, Fuzhou 350108, P. R. China
Hanlin Huang − Key Laboratory of Eco-materials-Advanced
Technology, College of Materials Science and Engineering,
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