CoFe2O4 Nanocrystals Mediated Crystallization Strategy for Magnetic Functioned ZSM-5 Catalysts

Article abstract of DOI:10.1002/adfm.201802088

Zeolites have many applications in the petrochemical and fine chemical industry and their functionalization does expand the spectrum of potentials. However, the integration of functional nanocrystals into zeolite frameworks with controlled size, dispersion, and crystallization behavior still remains a significant challenge. Here, a new synthesis of magnetic functioned ZSM-5 zeolite catalysts via a CoFe₂O₄ nanocrystal mediated crystallization strategy is reported. It is found that high crystallinity of CoFe₂O₄ nanocrystals results in a well-dispersed encapsulation of them into a single-crystal of ZSM-5 due to non-further-grown nanocrystals during the fast ZSM-5 growth. On the contrary, low crystallinity of CoFe₂O₄ nanocrystals leads to the polycrystalline zeolite growth due to the secondary growth of nanocrystals accompanied by the zeolite crystallization and large lattice mismatch between them. The successful encapsulation of small CoFe₂O₄ nanocrystals (≈4 nm) into single crystals lies on the preattachment of them into solid silica gel. During the growth of ZSM-5 crystals, no secondary growth of nanocrystals happens and its motion is restricted. The encapsulation of magnetic CoFe₂O₄ nanocrystals not only endows magnetic function into zeolites for the first time, but also does not impact catalytic performance of ZSM-5 in acetalization of cyclohexanone with methanol, which is highly promising in catalytic industries.

Full text of DOI:10.1002/adfm.201802088

FULL PAPER
Magnetic Zeolites
www.afm-journal.de
CoFe O Nanocrystals Mediated Crystallization Strategy
2
4
for Magnetic Functioned ZSM-5 Catalysts
Bin Li, Erol Yildirim, Wei Li, Dianpeng Qi, Jiancan Yu, Jiaqi Wei, Zhiyuan Liu,
Zhenkun Sun, Yong Liu, Biao Kong, Zhaoteng Xue, Zhuangjian Liu, Shuo-Wang Yang,*
Xiaodong Chen,* and Dongyuan Zhao*
research into the controlled crystal nuclea-
[
1–4]
Zeolites have many applications in the petrochemical and fine chemical
industry and their functionalization does expand the spectrum of potentials.
However, the integration of functional nanocrystals into zeolite frameworks
with controlled size, dispersion, and crystallization behavior still remains a
significant challenge. Here, a new synthesis of magnetic functioned ZSM-5
zeolite catalysts via a CoFe O nanocrystal mediated crystallization strategy
tion and growth.
Among them, the
crystallization of one host phase with
the coalescence of another guest species
e.g., ion complexes, nanocrystals)
been recognized as a widespread method
to achieve multifunctional materials with
tunable structure and properties.
[
5,6]
(
has
[
7–9]
2
4
is reported. It is found that high crystallinity of CoFe O nanocrystals results
Zeolites, a kind of microporous crystal-
line aluminosilicates, have been widely
applied in the fields of adsorption, sepa-
2
4
in a well-dispersed encapsulation of them into a single-crystal of ZSM-5 due
to non-further-grown nanocrystals during the fast ZSM-5 growth. On the
contrary, low crystallinity of CoFe O nanocrystals leads to the polycrystalline
ration, and catalysis in petrochemical and
2
4
[10–18]
chemical industry.
Aiming to the
zeolite growth due to the secondary growth of nanocrystals accompanied
by the zeolite crystallization and large lattice mismatch between them. The
successful encapsulation of small CoFe O nanocrystals (≈4 nm) into single
crystals lies on the preattachment of them into solid silica gel. During the
growth of ZSM-5 crystals, no secondary growth of nanocrystals happens and
functionalization of host zeolite matrixes,
various guest species are added into the
precursor of zeolites, in which the nuclea-
tion and growth of zeolite crystals occur
2
4
[
19–26]
together with guest species.
Chen and
[
27]
co-workers have successfully introduced
Pd nanoparticles inside mesoporous MFI
its motion is restricted. The encapsulation of magnetic CoFe O nanocrystals
2
4
not only endows magnetic function into zeolites for the first time, but also
does not impact catalytic performance of ZSM-5 in acetalization of cyclohex-
anone with methanol, which is highly promising in catalytic industries.
zeolite nanocrystals through
a modi-
fied Kirkendall growth process. Yu and
[
28]
co-workers
reported the synthesis of
silicalite-1 with ultrasmall Pd clusters
confined in the intersectional channels
combining a modified Pd ion and the
1
. Introduction
precursor in a hydrothermal process. However, based on these
strategies reported previously, the ion complexes are usually
trapped into the channels of zeolite frameworks which might
result in a poor mass transfer for the applications involving the
bulk reactants. Furthermore, the composition of guest species
on the zeolite crystallization behaviors is beyond well under-
stood, resulting in little satisfactory control over the assembly
of zeolite building blocks. The main reason may be due to
the rigidity of zeolite crystals. During the crystallization of the
zeolites from the traditional homogeneous precursors, guest
species cannot be easily involved into a thermodynamically
controlled crystal growth. Adding guest species (e.g., ions or
nanocrystals) directly into the homogeneous precursors of zeo-
lites usually leads to large and nonuniform nanocrystals as well
as poor dispersion and unsatisfactory functional properties.
Therefore, exploring new strategies to control zeolite crystalli-
zation and endow it useful functions with the coalescence of
adjustable nanocrystal properties is a great challenge to achieve
the diversity and complexity of nanocomposites.
The significance of crystallization process for the design and
synthesis of functional nanocomposites has driven multilevel
Dr. B. Li, Dr. D. Qi. Dr. J. Yu, J. Wei, Dr. Z. Liu, Prof. X. Chen
School of Materials Science and Engineering
Nanyang Technological University
5
0 Nanyang Avenue, Singapore 639798, Singapore
E-mail: chenxd@ntu.edu.sg
Dr. E. Yildirim, Dr. Z. Liu, Dr. S.-W. Yang
Institute of High Performance Computing
Agency for Science
Technology and Research (A*STAR)
Singapore 138632, Singapore
E-mail: yangsw@ihpc.a-star.edu.sg
Dr. W. Li, Dr. Z. Sun, Dr. Y. Liu, Dr. B. Kong, Dr. Z. Xue, Prof. D. Zhao
Laboratory of Advanced Materials
Department of Chemistry
Fudan University
Shanghai 200438, China
E-mail: dyzhao@fudan.edu.cn
Here, we report a magnetic functioned ZSM-5 composite
from the mediation of cobalt iron oxide (CoFe O ) nanocrystals
DOI: 10.1002/adfm.201802088
2
4
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Figure 1. Scheme of ZSM-5 crystallization mediated by attached CoFe O nanocrystals with different relative crystallinity.
2
4
over the zeolite crystallization.[29,30] A mesoporous silica solid
gel was selected as a precursor, which acted as a matrix to con-
at the temperature of 180 °C, the CoFe O @ZSM-5 hybrids are
2
4
obtained (denoted as CoFe O @ZSM-5-600). The X-ray diffrac-
2
4
fine the magnetic CoFe O4 nanocrystals growth. We found
tion (XRD) pattern (Figure S3a, Supporting Information) of
CoFe O @ZSM-5-600 shows typical diffraction peaks, which
2
that high relative crystallinity of the nanocrystals could result
in a successful encapsulation of them into the single crystals
of ZSM-5 and endow the zeolites magnetic function, because
no further growth of nanocrystals happens during the fast
growth of ZSM-5 single-crystal. On the contrary, the nanocrys-
tals with a low relative crystallinity lead the polycrystalline
zeolite growth to form microspheres composed of the assem-
bled ZSM-5 nanorods and well-dispersed CoFe O at the voids
2
4
can be well indexed to zeolite MFI structure, indicating that
ZSM-5 phase with a high crystallinity is formed. The SEM
image (Figure 2a) of the sample CoFe O @ZSM-5-600 demon-
2
4
strates the coffin-shaped and twin crystal morphology which is
commonly formed for zeolites with MFI structure. The particle
(inset of Figure 2a) exhibits clear edges, indicating the single-
crystal structure of ZSM-5. The transmission electron micros-
copy (TEM) image (Figure 2d) clearly shows that numerous tiny
nanocrystals are well dispersed in the matrix of zeolite crystals.
The size distribution curve of CoFe O nanocrystals (Figure 2g)
2
4
between the zeolite crystals (Figure 1) because the secondary
growth of nanocrystals accompanied by the zeolite crystalliza-
tion and large lattice mismatch between them. The successful
encapsulation of the small CoFe O nanocrystals (≈4 nm) into
2
4
displays a mean diameter centered at ≈4 nm. The corre-
sponding selected-area electron diffraction (SAED, Figure 2d
inset) pattern exhibits a clear single-crystal diffraction charac-
teristic of zeolite ZSM-5 together with polycrystalline diffraction
ring of CoFe O nanocrystals. Due to the small lattice space
2
4
ZSM-5 matrixes lies on the preattachment of the high crystal-
line nanocrystals on the pore surface of solid silica gels. During
the growth of ZSM-5, no secondary growth of nanocrystals
happens and its motion is restricted. The obtained magnetic
single-crystal zeolites encapsulated with CoFe O nanocrystals
2
4
of CoFe O nanocrystals, its diffraction rings mainly appear at
2
4
2
4
−1
possess high magnetization (4.7 emu g ) with superparamag-
the outer position which is attributed to the lattice plane (511)
with a d-spacing of ≈0.16 nm. Furthermore, the high-resolution
TEM (HRTEM) image (Figure S4, Supporting Information)
also confirms that the tiny CoFe O nanocrystals are encapsu-
netic property, a uniform size (≈9 µm), and high surface area
2
−1
(
330 m g ). These magnetic single-crystal ZSM-5 catalysts
exhibit an excellent conversion, selectivity in the acetalization of
cyclohexanone with methanol. The encapsulation of magnetic
CoFe O nanocrystals does not impact the catalytic perfor-
2
4
lated within the zeolite matrix. These results indicate that the
involvement of CoFe O nanocrystals does not disturb the crys-
2
4
2
4
mance of ZSM-5 zeolites, which is highly promising in the
chemical and catalytic industries.
tallinity of the host ZSM-5 single crystals. Nitrogen isotherms
of the CoFe O @ZSM-5-600 hybrids (Figure S5a, Supporting
2
4
Information) show a sharp uptake at a low relative pressure
P/P ), indicating the characteristic micropores. At a higher
(
0
relative pressure, the nearly H2-type hysteresis loop is observed
which is ascribed to the intercrystalline mesoporosity stemmed
2
. Results and Discussion
[22]
The scanning electron microscopy (SEM) image (Figure S1, Sup-
porting Information) shows that the commercial mesoporous
silica solid gels as a precursor have a polyhedral morphology
with a uniform size of ≈45 µm. Through an impregnation
strategy, CoFe O nanocrystals can be incorporated into the
from the introduction of nanocrystals.
Brunauer-Emmett-Teller (BET) surface areas and pore volumes
The corresponded
2
−1
3
−1
for the samples are calculated to be 330 m g and 0.20 cm g ,
respectively.
The CoFe O @SiO composites after being calcined at
2
4
2
4
2
mesopores of the solid silica gels (Figure S2, Supporting Infor-
mation). The obtained CoFe O @SiO composites after being
400 and 200 °C are also used to prepare the precursors for
the growth of zeolite ZSM-5 crystals (donated as CoFe O @
2
4
2
2
4
calcined at 600 °C are used to prepare the precursors for the
growth of zeolite ZSM-5 crystals. After a hydrothermal treatment
ZSM-5-400 and CoFe O @ZSM-5-200, respectively) under
the same condition with the samples at 600 °C. XRD patterns
2 4
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Figure 2. a–c) SEM, d–f) TEM images, and g–i) CoFe O -based particle size distributions of the obtained CoFe O @ZSM-5-600, CoFe O @ZSM-5-400,
2
4
2
4
2
4
and CoFe O @ZSM-5-200 composites (Inset in (d): SAED pattern, scale bar: 5 1/nm).
2
4
(Figure S3b,c, Supporting Information) show that the samples
samples CoFe O @ZSM-5-400 and CoFe O @ZSM-5-200 are
2 4 2 4
also possess a high crystallinity of ZSM-5 phase. SEM images
of the sample CoFe O @ZSM-5-400 (Figure 2b) and CoFe O @
calculated to be 367 and 379, and the pore volumes are 0.22
3
−1
and 0.23 cm g , respectively (Figure S5b,c, Supporting Infor-
mation). The surface area and pore volume of them are a little
higher than that of sample CoFe O @ZSM-5-600. It indicates
2
4
2
4
ZSM-5-200 (Figure 2c) show a spherical morphology with a rel-
atively uniform size of ≈9 µm in diameter. Moreover, the SEM
images of the cracked microspheres (the inset in Figure 2b,c)
reveal that the microsphere is composed of numerous assem-
bled nanorods. TEM images of the CoFe O @ZSM-5-400
2
4
that the single-crystal ZSM-5 is denser than the polycrystalline
microspheres with both microporosity and moderate mesopo-
rosity ascribed to the agglomeration of nanorods.
2
4
(
Figure 2e) and CoFe O @ZSM-5-200 (Figure 2f) show that
The composition of precursors for zeolites is important for
2
4
the ZSM-5 nanorods pack together and many nanocrystals are
dispersed on these nanorods. The size distribution curve of
the CoFe O nanocrystals displays a mean diameter centered
the crystallization of ZSM-5. The CoFe O4 nanocrystals can
2
be successfully incorporated into the mesopores of the solid
silica gel (Figures S6 and S7, Supporting Information). The
nanocrystals size distribution varies from 12.5 to 4.2 nm as
the calcination temperature increased (Figure S6c,f,i, Sup-
porting Information) due to the incomplete decomposition
of metal nitrate precursor and restricted solid diffusion in the
2
4
at ≈8 and ≈12 nm for CoFe O @ZSM-5-400 (Figure 2h) and
2
4
CoFe O @ZSM-5-200 (Figure 2i), respectively. The nanocrys-
2
4
tals size change is related to the starting particle crystallinity.
The HRTEM images (inset of Figure 2e,f) of the cracked micro-
spheres show that lattice fringes of CoFe O nanocrystals and
[
31]
disordered mesopores of silica gel. The XRD pattern of the
2
4
ZSM-5 lattice. These results clearly suggest that the CoFe O4
sample (Figure 3B) calcined at 600 °C exhibits well-resolved
2
nanocrystals are dispersed in the voids among the ZSM-5
nanorods. These results significantly indicate that the structure
of the hybrid CoFe O @ZSM-5 is strongly related to the size
diffraction peaks indexed to CoFe O4 phase (JCPDS No.
2
22-1086), indicating a high crystallinity. The relative crystal-
linity of the CoFe O nanocrystals can be decreased to 78 and
2
4
2
4
of guest nanocrystals which stem from the different calcination
for the crystallization of CoFe O . The BET surface areas for the
39% by reducing the calcination temperature from 400 to 200
°C, respectively (Figure 3A), suggesting the mixed metal oxides
2
4
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Figure 3. A) The relative crystallinity functioned with the calcination temperature of the CoFe O nanocrystals attached in the mesoporous silica gel;
2
4
B) XRD patterns, C) TPR profiles of CoFe O @SiO calcined at different temperature: (a) 200, (b) 400, and (c) 600 °C.
2
4
2
are existed. The X-ray photoelectron spectroscopy (XPS) spectra
ZSM-5 crystals. To qualitatively verify this process, chemical
hardness of metal ions (Fe3 and Co ), amorphous metal oxide
+
2+
of CoFe O @SiO calcined at 600 °C (Figure S8a,b, Sup-
2
4
2
porting Information) show typical peaks for Fe2p3/2 (724.0 eV),
(Fe O , CoO, and SiO ), and crystalline CoFe O nanocrys-
2
3
2
2
4
Fe2p1/2 (710.0 eV), Co2p3/2 (793.8 eV), and Co2p (779 eV)
tals were calculated based on density functional theory (DFT)
1/2
[
32,33]
[36]
with satellite peaks for the CoFe O materials.
As compa-
(Table S1, Supporting Information). The crystalline CoFe O
2
4
2
4
rable, for the samples after the hydrothermal treatment, the
nanocrystals with the lowest chemical hardness are stable in
an alkaline solution. On the other hand, the metal ions and
amorphous metal oxides with a higher chemical hardness are
characteristic peaks for Fe3 and Co from CoFe O4 mate-
+
2+
2
rials (Figure S8c,d, Supporting Information) suggest that no
valence change of metal species for CoFe O nanocrystals with
[
37]
not stable and may further react to form crystalline CoFe O
2
4
2
4
high crystallinity. For the samples of CoFe O @SiO calcined
nanocrystals in an alkaline condition, which may impact the
crystallization behavior of ZSM-5.
2
4
2
at 400 and 200 °C, XPS spectra indicate that the iron and cobalt
species from mixed oxide. H -temperature-programmed-
Based on the above observations, we propose a CoFe O
2
2
4
reduction (TPR) profile (Figure 3C) of the CoFe O @SiO
nanocrystal mediated crystallization strategy for ZSM-5 zeolite
composites. In our work, CoFe O nanocrystals with different
2
4
2
composites sample calcined at 200, 400, and 600 °C all show
the characteristic peaks for the reduction of CoO to metallic
cobalt (340–363 °C), Fe O to Fe O (430–460 °C), and Fe O
2
4
crystallinity that are preattached in the mesopores of the silica
solid gel precursors lead to the different crystallization behav-
iors of the zeolites during the hydrothermal process. Generally,
conventional coffin-shaped ZSM-5 crystals (Figure S9, Sup-
porting Information) are formed if the same silica gel without
the CoFe O nanocrystals is used. When the guest CoFe O
2
3
3
4
3
4
[
32,34]
to FeO and metallic Fe (above 600 °C).
that the peaks at the low temperature centered at 180 and
It should be noted
3
+
2+
2
30 °C are ascribed to the reduction of Fe and Co , respec-
[
34]
tively.
For the sample calcined at 600 °C, the peaks are
2
4
2
4
shifted to higher temperature compared with those of samples
calcined at 200 and 400 °C, suggesting that the metal oxides
become difficult to be reduced, which is due to the increased
crystallinity of nanocrystals and interaction between metal
oxides and silica.[ In the growth process of ZSM-5 crystals,
the metal ions or amorphous metal oxides are not stable and
nanocrystals are incorporated into the silica gels, the metal
ions or the amorphous metal oxides which exist around the
CoFe O nanocrystals with a lower relative crystallinity provide
2
4
extra active sites for further CoFe O nanocrystals growth via
2
4
35]
Ostwald ripping process in the alkaline condition. This pro-
cess makes the preattached CoFe O nanocrystals further grow
2
4
may act as the resource for the secondary growth of CoFe O4
nanocrystals, which is competitive with the crystallization of
during the crystallization process of zeolites. While the crys-
tallization of two phases happens simultaneously, the lattice
2
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match between the interfaces normally determine the size
and position of incorporated guest nanocrystals.[38] The lattice
mismatch between the two majority lattice planes of ZSM-5
inert CoFe O nanocrystals are supposed to be involved into
2
4
the fast crystallization process of ZSM-5 and no phase separa-
tion happens. In such systems, the structural differences may
be accommodated even though a distinct mismatch between
(
plane 101 with d-spacing of 1.1 nm) and CoFe O4 (plane
2
[
44]
3
11 with d-spacing of 0.25 nm) is up to 77% (Table S2, Sup-
the lattice planes of the two phases exists.
the small CoFe O nanocrystals can be encapsulated into the
Consequently,
porting Information). Such large lattice mismatch can result
2
4
in either host polycrystalline or anisotropic structures with
zeolite singel crystals. In summary, through controlling the
relative crystallinity of preattached guest nanocrystals in
the mesoporous solid silica gels, the ZSM-5 crystallization
behaviors can be tuned from the polycrystalline to single
crystalline with desired function.
segregation of the host and guest species.[
39,40]
In our case, the
further-grown CoFe O4 nanocrystals cannot be encapsulated
2
in the ZSM-5 single crystals, resulting in the phase separation
from the crystallization of ZSM-5.[ Conversely, the CoFe O4
41]
2
nanocrystals with high crystallinity are so stable and inert
that no addition active sites for further growth. Therefore,
during the hydrothermal process, only the crystallization of
ZSM-5 happens and the lattice mismatch between two phases
is not the dominated factor. Due to the initial attachment of
small nanocrystals (≈4 nm) on the pore surface of the solid
silica gels and interaction between them, the dynamics of the
nanocrystal motion is restricted.[ Meanwhile, solid silica gel
for heterogeneous precursors and high hydrothermal tempera-
ture (180 °C) for zeolite growth are the critical conditions for
The saturation magnetization values at room temperature
for the samples CoFe O @ZSM-5-600, CoFe O @ZSM-5-400,
2
4
2
4
and CoFe O @ZSM-5-200 are measured to be 4.7, 1.2, and
2
4
−
1
0.6 emu g , respectively (Figure 4A and Figure S10, Supporting
Information). During the hydrothermal treatment, the metal
ions/amorphous metal oxide further grow into crystalline metal
oxide. However, their crystallinity of nanocrystals is still lower
than that of CoFe O @ZSM-5-600, leading to the decreased
1,5]
2
4
[
45,46]
magnetization values.
The hysteresis loops recorded at 5 K
(Figure 4B) show that the coercivity of the samples CoFe O @
2
4
allowing kinetic-controlled aggregation of zeolite nucleus and
ZSM-5-600, CoFe O @ZSM-5-400, and CoFe O @ZSM-5-200
2 4 2 4
fast ZSM-5 crystal growth.[
12,42]
Fast ZSM-5 frameworks growth
is 12 232, 2516, and 1305 Oe and the remanence is 5.2, 1.06,
−
1
is prior to the motion of preattached and non-further-grown
small CoFe O nanocrystals. As a result, ZSM-5 growth is
and 0.56 emu g , respectively. These values are increased with
the enforcement of crystallinity. The zero field cooling (ZFC)
curve (Figure 4C) for the sample CoFe O @ZSM-5-600 com-
2
4
inclined to kinetically circumvent small CoFe O nanocrystals
2
4
2
4
with decreased internal strain[ rather than excluding them
43]
posites shows a main plateau around 285 K, suggesting that
the blocking temperature is in the room temperature range. It
from the ZSM-5 single-crystal frameworks. The rigid and
Figure 4. Hysteresis loops recorded at A) 300 and B) 5 K of the samples CoFe O @ZSM-5-200 (a), CoFe O @ZSM-5-400 (b), and CoFe O @ZSM-5-
2
4
2
4
2
4
6
00 (c) composites; ZFC and FC curves of C) CoFe O @ZSM-5-600 and D) pure CoFe O nanocrystals prepared through adding NaOH solution into
2
4
2
4
iron and cobalt ion aqueous solution.
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should be noticed that a peak at 10.5 K is observed, indicating
The excellent magnetic properties of the CoFe O @ZSM-5
2
4
[
47]
that some magnetic ordering transition happens.
The
composites make them excellent candidates for various
applications such as adsorption and catalysis due to its easy
manipulation. The key feature is whether the encapsulation
of magnetic CoFe O nanocrystals could impact the catalytic
ordering transition temperature (T ) is also related to the par-
O
ticle size of the CoFe O nanocrystals by Equation (1)
2
4
2
4
T
O
= KV /25k
B
(1)
performance. Here, we apply the acetalization of cyclohex-
anone with methanol (Figure 5A) to validate the catalytic
performance of the composite CoFe O @ZSM-5-600. The
−
16
−1
where k is the Boltzmann constant (1.38 × 10 erg K ), K is
B
2
4
6
−3
the anisotropy constant (1.2 × 10 erg cm ), and V is volume
acetalization reaction between ketone and alcohol is an impor-
tant organic synthetic route which has been widely employed
in industrial process for the production of drug intermediate
of the particle.[ The diameter of the CoFe O4 particles is
48]
2
calculated to be ≈4 nm, which is close to the data from the
TEM images. As a comparison, there is no peak in ZFC curve
[
56]
and target fine chemicals.
lite CoFe O @ZSM-5-600 composite catalyst shows a high
The magnetic functional zeo-
(Figure 4D) of pure CoFe O materials (Figures S11 and S12,
2 4
2
4
Supporting Information). It is well known that CoFe O mate-
rials are widely used in magnetic fluids, magnetic resonance
cyclohexanone conversion of ≈96% after the reaction is sus-
tained for 60 min (Figure 5B). The selectivity is 100% to the
dimethoxycyclohexane and the yield of dimethoxycyclohexane
achieves 90% after the reaction for 1 h. Various solid-acid cata-
lysts have shown good activities in the acetalization of cyclohex-
anone with methanol, such as mesoporous silica, zeolites, and
2
4
imaging (MRI) contrast, etc., due to its intrinsic coercivity other
than the superparamagnetic property.[
49–55]
Here, for the first
time, a crystalline framework zeolite ZSM-5 is used to encap-
sulate the magnetic CoFe O nanocrystals with defined size and
2
4
[
57–60]
shape, achieving the superparamagnetic property.
metal oxides.
Here, the conversion of cyclohexanone on
Figure 5. A) The scheme for the acetalization of cyclohexanone with methanol; B) the conversion of cyclohexanone functioned with reaction time;
C) the reusability measurement of CoFe O @ZSM-5-600 catalysts; D) the XRD patterns; and E) hysteresis loops recorded at 300 K of the CoFe O @
2
4
2
4
ZSM-5-600 before and after catalytic evaluation.
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our catalyst is above 90% after 1 h reaction at 50 °C, which is Acknowledgements
much better than that of the commercial ZSM-5 (≈80%) at the
This work was supported by Singapore Ministry of Education
same condition. In our catalysts, the CoFe O nanocrystals do
2
4
(MOE2014-T2-2-140
and
MOE2015-T2-2-060),
the
National
not occupy the micropores of ZSM-5 and are surrounded by
the ZSM-5 frameworks. Thus, the encapsulation of magnetic
CoFe O nanocrystals does not impact the diffusion of mole-
Research Foundation, Prime Minister’s Office, Singapore under
NRF Investigatorship (NRF2016NRF-NRFI001-21). The authors also
acknowledge the support from the State Key Basic Research Program
of the PRC (2018YFA0209401 and 2016YFA0204000), Key Basic
Research Program of Science and Technology Commission of Shanghai
Municipality (17JC1400100), and NSF of China (Grant Nos. U1463206,
2
4
cules and catalytic performance of ZSM-5. Meanwhile, due to
the unique magnetic property, the CoFe O @ZSM-5 compos-
2
4
ites can be easily separated by a magnet from the solution and
exhibit an identical performance even after more than six cycles
of usage (Figure 5C). More importantly, the structure and mag-
netic property are well retained during the catalyst regeneration
process in terms of calcination in air (Figure 5D,E). In previous
2
1210004, and 21733003).
Conflict of Interest
reports, the magnetic function of Fe O or γ-Fe O is not stable
3
4
2
3
The authors declare no conflict of interest.
in air condition. They are easily oxidized into α-Fe O , thus
2
3
losing the magnetic function.[
61–64]
Furthermore, single-crystal
structure is also more stable mechanically than the polycrys-
Keywords
talline aggregates.[
12,65]
In our work, the CoFe O nanocrystals
2 4
possess a stable magnetic property due to their synthesis by the
air calcination. And the structure is also stable due to the single
crystal feature, which is beneficial for the harsh requirement in
the catalytic application.
catalysis, crystallization, functionalization, nanocrystals, zeolites
Received: March 23, 2018
Revised: May 1, 2018
Published online:
3
. Conclusions
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Supporting Information
Supporting Information is available from the Wiley Online Library or
from the author.
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