Synthesis of core-shell Ce-modified mixed metal oxides derived from P123-templated layered double hydroxides

Article abstract of DOI:10.1039/d1ra00227a

Layered double hydroxides are a promising platform material which can be combined with a variety of active species based on their characteristic features. Silicon@P123-templated Ce-doped layered double hydroxide (SiO₂@CeMgAl-LDH(P123)) composites were synthesizedviaa facilein situco-precipitation method, and characterized by TEM, X-ray diffraction, FTIR, XPS, CO₂-,etc.in detail. Meanwhile, the calcined powder (SiO₂@CeMgAl-LDO(P123)) possessed an excellent core-shell structure and a high surface area inherited from the LDH structure, which led to an outstanding catalytic activity (99.7% conversion of propylene oxide, 92.4% selectivity of propylene glycol methyl ether) under mild reaction conditions (120 °C). Cerium oxide provides a large number of oxygen vacancies and significantly improves the medium basic strength of the material, which facilitates the selective ring-opening of PO. Furthermore, the introduction and removal of P123 make the cerium oxide uniformly dispersed on the LDH layers, providing more reaction sites for the reaction of methanol and propylene oxide. The core-shell structure prepared by thein situco-precipitation method could solve the shortcomings of agglomeration of layered double hydroxides and prolong the catalytic life evidently.

Full text of DOI:10.1039/d1ra00227a

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Synthesis of core–shell Ce-modified mixed metal
oxides derived from P123-templated layered
double hydroxides†
Cite this: RSC Adv., 2021, 11, 8375
Kaijun Wang,
Qifan Mao, Weimin Fei, Lingxin Kong, Xiaoyan Cao and Zhenggui Gu*
Layered double hydroxides are a promising platform material which can be combined with a variety of
active species based on their characteristic features. Silicon@P123-templated Ce-doped layered double
hydroxide (SiO
method, and characterized by TEM, X-ray diffraction, FTIR, XPS, CO
calcined powder (SiO @CeMgAl-LDO(P123)) possessed an excellent core–shell structure and a high
2
@CeMgAl-LDH(P123)) composites were synthesized via a facile in situ co-precipitation
2
-, etc. in detail. Meanwhile, the
2
surface area inherited from the LDH structure, which led to an outstanding catalytic activity (99.7%
conversion of propylene oxide, 92.4% selectivity of propylene glycol methyl ether) under mild reaction
ꢀ
conditions (120 C). Cerium oxide provides a large number of oxygen vacancies and significantly
improves the medium basic strength of the material, which facilitates the selective ring-opening of PO.
Furthermore, the introduction and removal of P123 make the cerium oxide uniformly dispersed on the
LDH layers, providing more reaction sites for the reaction of methanol and propylene oxide. The core–
shell structure prepared by the in situ co-precipitation method could solve the shortcomings of
agglomeration of layered double hydroxides and prolong the catalytic life evidently.
Received 11th January 2021
Accepted 1st February 2021
DOI: 10.1039/d1ra00227a
rsc.li/rsc-advances
Although traditional solid base catalysts such as NaOH and
KOH have good catalytic activity in similar alkoxylation reac-
Introduction
Low toxicity, high efficiency, and a stable solvent are indis- tions, such homogeneous and highly corrosive substances do
pensable features in a green chemical process. Propylene glycol great harm to industrial equipment and increase the difficulty
ethers are important industrial derivatives of epoxides by of product rening. Recent researches of alcohol ether synthesis
5
6
etherication. Their molecular structure has two strong soluble took ionic liquids and zeolitic imidazolate framework (ZIF-8)
functional groups: a hydroxyl group and ether bond group. The as important research objects. These new types of catalysts can
hydroxyl group is hydrophilic while the ether bond is lipophilic, achieve a high PO conversion rate with a relatively small
1–3
so this kind of material is also called a “universal solvent”.
amount of use, but they become a stumbling block in the
Among the many synthetic routes of propylene glycol ether, the product purication process. When used in the actual produc-
propylene oxide method has a high atom utilization rate and tion process, the above catalysts are difficult to achieve the ex-
low operation risk, so it has become the only green synthesis pected results. Heterogeneous catalysts are widely used in the
4
route to realize industrialization. However, due to the steric synthesis of chemical products. They reduce the corrosion of
hindrance effect of propylene oxide, different products will be equipment and the difficulty of product rening. Moreover,
produced under the action of acid and base in the reaction most of the heterogeneous catalysts have the advantages of
process of propylene oxide and lower fatty alcohol. The primary simple preparation, controllable morphology and long catalytic
hydroxy ether is the main product in the presence of acid-type life. It is the main trend of the development of chemical
catalysts. At the same time, basic catalysts favor the formation industry and one of the important ways to realize green chem-
of secondary hydroxy ether. Some studies have shown that ical process.
secondary alcohols have lower reproduction and developmental
In recent ten years, there have been a lot of reports on the
toxicity than primary alcohols. Therefore, it is of great signi- synthesis of alcohol ether with heterogeneous solid base cata-
cance to improve the selectivity of secondary alcohols.
lysts. Timofeeva prepared Al-pillared interlayered clays (Al-
PILCs) and Zr,Al-pillared clays through the intercalation of Al-
7
and Zr-polyoxocations into layered aluminosilicates with
8
various textural properties and chemical compositions to
explore the synthesis process of alcohol ethers. The catalytic
Jiangsu Provincial Key Laboratory of Materials Cycling and Pollution Control, Nanjing
Normal Univesity, Nanjing, Jiangsu 210023, China. E-mail: 07160@njnu.edu.cn
performance of these materials was investigated in the
†
Electronic supplementary information (ESI) available. See DOI:
0.1039/d1ra00227a
K
synthesis of PGME. 0.4% Zr Al-MMNO3
ꢁ
got a certain catalytic
1
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ꢀ
activity at 60 C that conversion of PO was 71% and selectivity structure and certainly at surface, the interaction forces
20,21
towards to PGME was 66%. The research found that the between the surfaces and the reactants would be different.
increase of Zr content was benecial to decrease the selectivity The specic surface area, dispersion of active species, oxygen
of PGME, which was the result of a change in the nature of acid– cavity defect and lattice oxygen uidity of CeO can be changed
2
base sites. The highest conversion and selectivity toward PGME by metal doped CeO
were observed in the presence of materials with medium- CeO . Therefore, cerium based composite metal oxides have
strength basic sites, because high-strength basic sites can lead been widely used in the eld of catalysis.
2
and the interaction between metal and
2
22–24
Recently, incor-
to a strong stabilization of methoxide and propylene-like poration of cerium oxide based nanostructured materials into
9
species on the surface of the solid. Liu combined KF with layered double hydroxide to form composite material is another
25,26
La O -SBA-15 at low temperature to form ordered microstruc- burning topic of research interest.
Core–shell nanoparticles
2
3
ture on the solid surface and generate superbase sites. The (CSNs) are a class of nanostructured materials that have recently
conversion of propylene oxide was 93%, and the selectivity of 1- received increased attention owing to their interesting proper-
methoxy-2-propanol was 93%. For the solid base catalyst used in ties and broad range of applications in catalysis, biology,
2
7
this system, the higher specic surface area and the medium materials chemistry and sensors.
10
strong base strength can promote the yield of PGME obviously.
In our previous work, we have explored how to increase the
Recently, for another piece of research, Timofeeva found that the specic surface area of the catalyst while ensuring the basic
catalytic properties of AM-4 could be adjusted by treatment with strength. We proved that the core–shell functional materials
11
28
0
.0625–0.25 M HNO . Increasing the concentration of HNO led to with specic morphology can be prepared by certain methods.
3
3
a decrease in basicity, which played a critical role in the reaction We found that MgAl-LDH supported on SiO surface by in situ
2
rate and the selectivity towards PGME. Their activity depended on coprecipitation method can form core–shell materials with
the type of active sites, which is controlled by acid concentration. composite shell distribution in transverse and longitudinal
In a word, the more active sites with moderate and strong base directions. Aer calcination, the composite structure still
strength and larger specic surface area are more conducive to the maintains a certain integrity, and solves the problem that LDH
formation of secondary hydroxy alcohol ether.
The LDH structure is based on M(OH)
sharing edges in order to build M(OH) brucite-like layer. LDH
is easy to agglomerate. At the same time, it shows ideal catalytic
octahedral units activity in xed bed reactor.
6
12
Taking into consideration the above discussion, we report
2
plays an important role in the eld of industrial catalysis. This herein a novel SiO @CeMgAl-LDH nanocatalyst, which was
2
is not only its special layer structure and ion exchange proper- easily fabricated by the in situ coprecipitation of the CeMgAl-
ties, but also it can be used as a carrier or an active component. LDH anchored on the surface of amorphous SiO sphere. The
2
This feature greatly expands the possibility of material topog- template agent P123 is introduced in the material and removed
raphy modication. Therefore, in recent years, LDH-based by calcination for further treatment. The obtained material
multi-level structural composite materials have become the (SiO
2
@CeMgAl-LDO(P123)) was fully characterized and its
13–16
focus of attention.
The metal components in the hydro- catalytic activity was evaluated in the synthesis of PGME (Fig. 1).
talcite laminate can be exibly adjusted, so a wide variety of The catalytic performances of the as-prepared samples were
hydrotalcite-like catalysts can be designed for different reaction correlated with the observed structural evolution of the cata-
17–19
systems.
abundance. The crystal of CeO
uorite with primary characteristic surfaces of (111), (110) and structure catalysts.
100). Due to the differences in the packing of atoms in bulk
Cerium is a rare earth element with the highest lysts. More importantly, this work could provide some direc-
2
has the structure of cubic tions and perspectives for the preparation of stable core–shell

(
2
Fig. 1 Schematic illustration of formation of SiO @CeMgAl-LDO(P123) nanocomposites.
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located in the interlayer gallery of LDH as agglomerated cerium
oxide species due to its high ionic radius (0.102 nm). Aer
calcination, CeMgAl-LDO is mainly composed of amorphous
Results and discussion
Structural and morphology characterization
ꢀ ꢀ ꢀ ꢀ
O , MgO and CeO . Peaks at 28.7 , 33.2 , 47.7 and 56.6 are
2 3 2
The TEM images of the synthesized CeMgAl-LDO(P123), Al
SiO
2
@CeMgAl-LDH(P123), and SiO
2
@CeMgAl-LDO(P123) are corresponding to the (111), (200), (220), (311) planes of Fluorite
(JCPDS PDF#78-0694). MgO may gradually enter the CeO
shown in Fig. 2. Fig. 2a (CeMgAl-LDO) shows a well-developed CeO
2
2
layered and platelet structure. Due to the introduction of P123 lattice to form a new solid solution structure. Moreover, there is
and its removal at a high temperature, it can be seen that small no obvious diffraction peak of heterophase species in the whole
CeO grains are scattered on the LDO surface, and their average XRD spectrum of CeMgAl-LDO, which shows that the metal
2
particle sizes are 4 and 4.5 nm, respectively. The LDH platelets oxides prepared by nucleation crystallization isolation method
grow vertically and randomly on the surface of the sphere SiO
2
have pure crystal phase composition. The XRD patterns of
to form a hierarchical layer with thickness of around 50 nm SiO
@CeMgAl-LDH(P123) and SiO @CeMgAl-LDO(P123) both
2
2
ꢀ
(
Fig. 2c). Aer calcination at 550 C, the shell layer is trans- show an LDH/LDO-originated diffraction pattern which
formed into multi-component metallic oxides, but the core– conrmed the formation of the LDH/LDO on the silica sphere.
shell structure still maintained, indicating that the composite The intensity of LDH/LDO originated peaks decreased due to
prepared by coprecipitation method have good structural the coexistence of amorphous SiO
stability (Fig. 2e). Through the further analysis of the shell Mg and Si–O–Al bonds on the SiO
morphology, different kinds of crystal lattice can be observed. reduction of the crystallite size not only in the stacking direc-
2
. It was reported that Si–O–
2
surface deeply affected
2
9
These correspond to the (111) and (311) planes of CeO
200) planes of MgO, respectively. These are both found in the
shell layer of CeMgAl-LDO and SiO @CeMgAl-LDO(P123).
2
and tion but also in the plane direction.
The FT-IR spectra are shown in Fig. 4. The peaks located in
(
ꢁ
1
the 3000–3500 cm region can be ascribed to the characteristic
2
The crystal structure of the as-prepared materials was absorption bands of O–H attached to the metal cation bonds in
conrmed by XRD analysis (Fig. 3). The CeMgAl-LDH and the hydroxide layer and stretching vibration of interlayer water
CeMgAl-LDH(P123) samples exhibit the characteristic diffrac- molecules. The width and relative strength of –OH in LDO
tion peak of LDHs, indicating that Ce-doping and P123 addition decreased signicantly due to hydroxyl removal caused by the
does not destroy the layered structure of LDHs. Ce prefers to be calcination process. The bending vibration of interlayer water
0
0
0
2 2
Fig. 2 TEM images of samples (a, b and b ) CeMgAl-LDO, (c, d and d ) SiO @CeMgAl-LDH(P123), (e, f and f ) SiO @CeMgAl-LDO(P123).
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32
catalytic applications. The TGA-DTG curves of samples are
shown in Fig. 5. Compared to Mg–Al LDH, all the samples show
a similar thermal behavior as follow: (1) in the temperature
ꢀ
range 70–190 C, loosely held interlayer water is lost; (2) in the
ꢀ
3+
temperature range 190–280 C, the OH– group bonded with Al
ꢀ
is lost; (3) in the temperature range 280–450 C, the OH– group
bonded with Mg is lost; (4) the weight of the three composites
tends to be stable aer 500 C. The introduction of P123 makes
2
+
ꢀ
the three weightlessness temperatures shi to high tempera-
ture. This is due to the hydrogen bonding between P123 and
ꢁ
OH or H O, so higher energy requirements put forward in the
2
process of chemical bond transformation.
The X-ray photoelectron spectroscopy (XPS) was used to
study the oxidation state of the surface elements in SiO
2
@-
CeMgAl-LDO(P123), and SiO @CeMgAl-LDH(P123). The Ce 3d,
2
Mg 1s, and O 1s regions of the spectra are shown in Fig. 6b–d.
The binding energy peaks at 1301 and 117 eV can be attributed
to Mg 1s and Al 2s, respectively. The HXPS spectra of Ce 3d
before and aer samples being calcined indicate the presence of
Fig.
2
3 XRD patterns of CeMgAl-LDH, SiO @CeMgAl-LDH, and
calcined samples.
3+
4+
3+
the mixed valence Ce and Ce states. The main peaks of Ce
3
+
3
8
3
d3/2 and Ce 3d5/2 are shown at the bending energy of 990 eV and
82 eV. One additional peak at 907.6 eV is also attributed to the Ce
p3d3/2 states. The peaks of Ce 4p are found at the binding ener-
gies of 916.5 eV.
32–34
It shows that cerium exists in similar form in
4+
both materials, but the relative content of some Ce increases
obviously aer calcination. More importantly, it is benecial for
3
+
4+
Ce to lose an electron to be oxidized to Ce , improving the lattice
3+
oxygen defects and the redox transformation between Ce and
4+
35
Ce . The O 1s binding energy peak appeared at 529 eV in
SiO @CeMgAl-LDO(P123) was assigned to Ce–O, peak at 531 eV
with oxygen vacancies of Ce in CeO
in the layered structure of LDH/LDO.
2
3+
2
and surface oxygen species
The specic BET surface area and pore volumes of CeMgAl-
LDO, and SiO @CeMgAl-LDO(P123) are shown in Fig. 7, S2†
2
and Table 1. It can be easily found that SiO @CeMgAl-
2
LDO(P123) showed much higher specic BET surface area
2
ꢁ1
3
ꢁ1
(
358.71 m
g
) and larger pore volumes (1.13 cm
g )
2
ꢁ1
3
ꢁ1
Fig.
2
4 FT-IR profile of LDH, SiO @CeMgAl-LDH, and calcined
compared with CeMgAl-LDO (144.94 m g ; 0.72 cm g ). As
samples.
ꢁ
1
molecules are found at 1650 cm . Similar to the change in the
OH vibration peak, the absorption peak of LDO here basically
disappears as water is removed between layers of the roasting
–
ꢁ1
process. A new peak appears at around 1092 cm which could
be supposed to the stretching vibrations of Si–O–Si and Si–O–H,
indicating that SiO spheres are coated by the LDH platelets
2
successfully in core–shell composites. The peak between 500
ꢁ1
and 750 cm can be assigned to the stretching vibrational
30
ꢁ1
mode of the Ce–O bonds. The peaks at 2932 and 2878 cm are
attributed to the alkyl stretching vibrations, both of which
31
indicate the presence of P123 in as-synthesized materials. The
FT-IR spectra indicated that the CeMgAl-LDHs/LDOs were
involved on the surface of SiO2.
It is known that calcined LDHs, layered double oxides
(LDOs), are useful intermediates for functionalised LDHs and
that their tuneable acid/base properties can be utilised in
Fig. 5 TG–DTA profile of samples.
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Fig. 6 (a) The XPS survey and high resolution scans of samples, (b) the Ce 3d high resolution spectrum of SiO
2
@CeMgAl-LDH/LDO(P123), (c)
2 2
the mg 1s high resolution spectrum of SiO @CeMgAl-LDH/LDO(P123), (d) the O 1s high resolution spectrum of SiO @CeMgAl-LDH/LDO(P123).
can be seen from Fig. 7, nitrogen adsorption–desorption that all the catalysts displayed CO desorption peaks at a similar
2
ꢀ
ꢀ
ꢀ
isotherms of CeMgAl-LDO, and SiO @CeMgAl-LDO(P123) both temperature range of 200–400 C and 500–600 C. The low-
2
exhibit intermediate isotherms between type II (absence of temperature desorption peaks centered at 270 C could be
a plateau at high P/P
0
) and type IV (low N
2
adsorption at low P/ contributed to weakly CO
2
chemisorbed by aluminum hydroxy
37
ꢀ
P
0
), with a hysteresis loop in the relative pressure range of 0.4 to and MgO. The peaks located at about 325 C could be assigned
1.0, indicating the presence of capillary condensation in the to the medium basic sites derived from the oxygen vacancies
mesopore structure. According to the IUPAC classication, formed during the doping of cerium in MgAl-LDO based
interestingly, these two samples present a mixture hysteresis composite. Combined with the results of specic surface area
loop of H2 and H3 types with an abrupt closure at a relative measurement, the core–shell structure has more complete
pressure of 0.45 due to the tensile strength effect, suggesting the stereoscopic structure and provides more alkaline sites at the
presence of slit-shaped and ink-bottle pores in the mesopore same time. Moreover, the material with P123 template has the
36
network associated with the aggregates of plate-like particles.
In the adsorption desorption curve of SiO @CeMgAl-
strongest total alkali content and strong alkali center strength.
2
LDO(P123), the hysteresis loop is larger, which shows that
there are more complex hole structures. During the formation
of the nuclear shell structure, the shell structure is relatively
complex, with different forms such as horizontal t, vertical
growth and mixed construction. Due to the pore forming
process of P123, the pore structure becomes more complex.
The CO -TPD was used to analyze the effect of the strength
2
and number of basic sites on the surfaces of the catalysts and
the results were presented in Fig. 8 and Table 1. It could be seen
Catalytic synthesis of Propylene glycol methyl ether
Propylene glycol methyl ether (PGME) is used as an “alkahest”
solvent in industrial and consumer product. It is generally
produced by the reaction of propylene oxide (PO) with meth-
anol, whose mechanism depends on the type of catalyst. As
shown in Table 2, the epoxide ring of PO may open at the least
sterically hindered position in basic catalysis, leading to the
main formation of propylene glycol methyl ether (a).
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ions were appropriate to dissociate methanol to methoxide and
had the ability to abstract a proton from propylene oxide.
In the present work, the catalytic performance of the as-
prepared core–shell composites was tested in the synthesis of
propylene glycol methyl ether from methanol and PO. Table 2
shows that the conversion of PO in the presence of SiO
2
@-
Ce0.33Mg
2
1
Al -LDO(P123) was 99.7%, a relatively high level
among the prepared and reported catalysts. Although ZIF-8 has
the best selectivity for propylene glycol monomethyl ether, it is
difficult to apply ZIF-8 in the actual production process
considering the stability and cost of the catalyst. The selectivity
towards PGME was higher in the presence of SiO @Ce0.33Mg2-
2
1
Al -LDO(P123) (92.4%) than other heterogeneous catalysts.
Probably, this phenomenon is related to the difference in the
strength of the moderate basic sites. The activity of CeMgAl-
LDO is greatly higher than those of the MgAl-LDO or pure
2 3
Al O sample, indicating that the introduction of Ce ions into
the Mg–Al catalysts can signicantly enhance the catalytic
Fig.
SiO
7
N
2
adsorption/desorption isotherms of CeMgAl-LDO and
performance. The CeO grains were uniformly distributed on
2
2
@CeMgAl-LDO(P123).
the LDO structure and interacted strongly with Mg–O–Al bonds.
This strong interaction leads to the formation of coordinately
3
+
unsaturated surface Ce cations. Besides, the introduction of
Ce cations in Mg–Al mixed oxides can modify the acid–base
properties of the mixed oxides. Some studies have shown that
a
Table 1 BET surface and base sites of samples
CO
2
3
uptake
ꢁ1
(
cm g STP)
2
the aggregation of CeO grains can weaken the catalytic
BET surface
performance of the catalyst. Therefore, the specic gravity of Ce
in the composite should not be too large, and the agglomeration
2
ꢁ1
Catalyst
(m g
)
W
M
S
of CeO should be avoided. According to TEM analysis, when
MgAl-LDO
CeMgAl-LDO
CeMgAl-LDO(P123)
167.89
136.59
144.94
342.11
358.71
5.24
2.75
4.03
4.27
3.66
40.81
0.36
2
7.44 P123 was introduced, CeO was well dispersed, which was
2
103.43
137.87
129.08
145.62
12.99 consistent with the phenomenon that the catalytic performance
SiO
SiO
2
@CeMgAl-LDO
13.05
13.59
of P123 modied materials was signicantly better than that of
2
@CeMgAl-LDO(P123)
41,42
unmodied materials. According to the literature,
the
a
3
W, weak basic site; M, medium basic sites; S, strong basic sites, cm
STP.
strength of basic sites determines the reaction rate and the
isomer selectivity toward PGME. Our experimental data also
point out that strong basic sites lead to a decrease in the
selectivity of PGME. It is reported that a high strength of strong
basic sites can lead to a strong stabilization of methoxide and
propylene-like species which formed as intermediates on the
surface of the solid and, in this way, hinder the process of
ꢁ1
g
The mechanism of the reaction on different single compo-
nent basic catalysts has been discussed.
sites of Lewis-acid–Bronsted-base pairs and isolated surface O
3
8,39
The moderate basic
2ꢁ
8
reaction.
The element ratio of the catalyst and the reaction conditions
were also studied. In Table 2, we investigated the element ratio
of the catalyst and found that SiO @Ce Mg A -LDO(P123)
2
0.33
2 1
has the best catalytic activity. When the amount of Ce increased,
although the conversion of PO did not decrease, the selectivity
of PGME decreased signicantly. In the TEM image analysis
(
Fig. S1†) of Ce
and crystal structure can be found, and the agglomeration of
CeO is serious. When the specic gravity of the crystal plane
1 3 1
Mg A -LDO(P123), more complex lattice fringes
2
which plays the main catalytic role decreases, the selectivity will
be affected negatively. Then in the exploration of reaction
conditions, too long reaction time and high temperature will
affect the selectivity of PGME. This is because that under
a certain proportion or environmental conditions, PGME and
PO will polymerize to form polypropylene glycol monomethyl
ether with different degrees of polymerization, which will seri-
ously affect the quality of the product. Aer single factor
2
Fig. 8 CO -TPD profile of all as-prepared composite materials.
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Table 2 Catalytic activity for the synthesis of PGME on different catalysts
Experimental conditions
Selectivity of
PGME (%)
ꢁ1
ꢀ
Catalyst
Catalyst (wt%)
Me/PO (mol mol
)
T ( C)
Time (h)
Conversion of PO (%)
—
—
1
1
1
3
4
4
4
4
8
5
8
8
5
5
4
4
4
4
4
4
4
4
120
120
120
120
60
8
8
8
8
6
4
5
5
5
5
8
8
8
8
8
8
8
8
21.3
23.8
60.3
58.5
98.7
93.0
55.4
90.3
88.4
84.1
96.7
97.6
97.8
97.8
99.7
90.3
94.6
88.3
58.2
74.2
70.6
83.8
61.2
91.0
93.8
92.6
92.3
86.3
89.6
89.8
86.3
91.6
92.4
86.5
90.4
89.6
SiO
Al
Mg
2
2
O
3
3
Al-LDO
7
R
*
*
*
*
*
*
Al-MM
12%KF/La
9
4
4
1
6
2
O
3
-SBA-15
1
125
120
110
110
120
120
120
120
120
120
120
120
120
0
ZIF-8
MAF-5
AM-4
1.85
1.85
1.8
1.6
2
2
2
2
2
0
1
2.9%ZIF-8/Nafen
Ce0.33Mg
Ce0.33Mg
Ce
SiO
SiO
SiO
SiO
SiO
2
Al
Al
1
-LDO
-LDO(P123)
-LDO(P123)
2
1
1
Mg
3
Al
1
2
2
2
2
2
@Ce0.33Mg
@Ce0.33Mg
@Ce
@Ce0.3Mg
@Ce0.05Mg
2
2
1
Al
1
-LDO
-LDO(P123)
-LDO(P123)
-LDO(P123)
Al -LDO(P123)
A
1
1
Mg
3
A
2
2
2
3
Al
1
3
1
optimization (Fig. S3†), the conversion of PO reaches 99.7% and
Experimental section
Materials
selectivity of PGME reaches 92.4% at a molar ration Me/PO 4 : 1
ꢀ
and 120 C for 8 h via 1 wt% SiO
2
@Ce0.33Mg
2
A
1
-LDO(P123).
The catalyst was separated by ltration aer the rst test, All chemicals employed in this study were analytical grade, from
ꢀ
washed with methanol, dried in air and activated at 550 C for commercial sources. Magnesium nitrate hexahydrate
6
h, and then reused for the next run under the same condi- (Mg(NO ) $6H O), cerium nitrate hexahydrate (Ce(NO3)3-
3 2 2
tions. The results are depicted in Fig. 9. As shown in the gure, $6H O), and aluminum nitrate hexahydrate (Al(NO ) $9H O)
2
3 3
2
the catalytic activity of the reused catalyst was slightly affected were purchased from Zhenxin Reagent Co., Ltd. Sodium
at the h run, exhibiting >93.0% conversion and retaining hydroxide (NaOH), sodium carbonate (NaCO ), and tetraethyl
3
8
7.0% selectivity to PGME. These results imply that the catalyst orthosilicate (Si(OC H ) , TEOS) were purchased from sino-
2 5 4
can be efficiently recovered and recycled.
pharm chemical reagent Co., Ltd. Epoxy propane (C H O),
3 6
ethanol (C H OH), and methyl alcohol (CH OH) were
2
5
3
purchased from Shanghai Lingfeng Chemical Regent Co., Ltd.
Poly(ethylene glycol)-block-poly(propylene glycol)-block-poly(-
ethylene glycol)(P123) was purchased from Aladdin.
Preparation of SiO spheres
2
43
Spherical SiO was prepared according to the previous report.
2
13.7 ml TEOS, 15 ml ammonia, 30 ml deionized water and 50 ml
ethanol were combined in a 250 ml eggplant ask. The white
ꢀ
suspension was stirred vigorously at 25 C for 18 h. The obtained
solid was centrifuged for 5 min at 3000 rpm then washed with
ethanol thoroughly followed by drying under vacuum overnight.
Preparation of CeMgAl-LDH(P123) and SiO
LDH(P123)
2
@CeMgAl-
The SiO
2
was covered with P123-templated CeMgAl-LDH by in
spheres (0.5 g)
Fig. 9 Recycling test in reaction between propylene oxide and
2
methanol in the presence of SiO @Ce0.33Mg
2
Al
1
-LDO(P123).
situ growth of the metal cations. Typically, SiO
2
©
2021 The Author(s). Published by the Royal Society of Chemistry
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2 3
and Na CO (20 mmol) were dispersed in distilled water by template make more active sites appear, which can improve the
ultrasonication for 30 min. Another solution of Ce(NO ) $6H O selectivity of propylene glycol monomethyl ether. On the other
3
3
2
(
4 mmol), Mg(NO ) $6H O (24 mmol), Al(NO ) $9H O(12 mmol) hand, the formation of core–shell structure improves the
3 2 2 3 3 2
were prepared in distilled water (100 ml, in the presence of 1.0 g dispersion of the catalyst, and overcomes the disadvantage of
P123), then were added in the above solution drop-by-drop at easy agglomeration of hydrotalcite. The coprecipitation method
ꢀ
vigorous stirring at 40 C for 18 h. During the reaction process, ensures the relatively stable structure of the core and shell, and
the pH was kept at 10 by the addition of 2 M NaOH. The improves the service life of the catalyst. This work provides
ꢀ
acquired mixture was washed with ethanol and drying at 60 C a new perspective on core–shell layered double hydroxide and
2
in a vacuum oven overnight. (SiO @CeMgAl-LDH(P123)). To contributes to the design of high performance heterogeneous
obtain the mixed oxides (SiO @CeMgAl-LDO(P123)), the catalyst.
2
ꢀ
precursor was calcined at 550 C for 6 h in a muffle furnace in
ꢀ
ꢁ1
air at a heating rate of 5 C min . The synthetic steps of the
composite materials were shown in Fig. 1. The pure
Conflicts of interest
LDH(CeMgAl-LDH(P123)) and LDO(CeMgAl-LDO(P123)) were There are no conicts to declare.
prepared by same co-precipitation method without the addition
of SiO2.
Acknowledgements
The authors acknowledge nancial support from China Petro-
leum & Chemical Corporation (J418013-3), and Jiangsu Scien-
tic and Technological Transformative Project
SBA2018030374). The related measure and analysis instrument
for this work were supported by Centre for Analysis, Nanjing
Normal University.
Characterizations
The powder X-ray diffraction patterns (XRD) measurements
were performed on a X-ray diffractometer (Rigaku Co., Japan, D/
max 2500VL/PC) using Cu Ka radiation (40 kV, 100 mA), and
a 2q angle ranging from 2 to 90 , respectively. The HRTEM
images were obtained on JEM-2100, ZEOL-Japan in which the
samples were prepared by dispersing the powdered samples in
ethanol by sonication for 15 min and then dropdrying on
(
ꢀ
ꢀ
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