Two inorganic–organic hybrids based on a polyoxometalate: Structures, characterizations, and epoxidation of olefins

Article abstract of DOI:10.1080/00958972.2018.1468026

Two new inorganic–organic hybrids, [Co₃(bpdo)₆(H₂O)₄Cl₂][SiW₁₂O₄₀]·H₂O (Co-SiW) and [Ni₃(bpdo)₆(H₂O)₄Cl₂][SiW₁₂O₄₀]·H₂O (Ni-SiW) (bpdo?=?4,4′-bis(pyridine-N-oxide)), were synthesized from the Keggin-type [SiW₁₂O₄₀] anion and Co(II) and Ni(II) under hydrothermal conditions, respectively, and characterized by elemental analyses, powder XRD, IR spectra, and single-crystal X-ray diffraction. The structural analysis indicates that the 1D chain is constructed from a POM-based half-cage as a secondary building block linked by bridging bpdo ligands. The zigzag chains further stack into a three-dimensional body with channels. The 3D network structure with amphiphilic cavities is shaped by electrostatic interactions through the planes, which has potential to allow molecules such as styrene and H₂O₂ ingress and egress. Both of the hybrids demonstrated catalytic activity for epoxidation of olefins, which was examined using styrene and aqueous hydrogen peroxide (30%) as oxidant in acetonitrile, along with Co-SiW and Ni-SiW in a heterogeneous manner at 60?°C. Moreover, the conversion of epoxidation reaction in a heterogeneous manner is close to that of homogeneous catalysis, while being conveniently recovered and steadily reused without change of catalyst structure after epoxidation reactions.

Full text of DOI:10.1080/00958972.2018.1468026

Journal of Coordination Chemistry
ISSN: 0095-8972 (Print) 1029-0389 (Online) Journal homepage: http://www.tandfonline.com/loi/gcoo20
Two inorganic–organic hybrids based on a
polyoxometalate: Structures, characterizations,
and epoxidation of olefins
Zhuolin Shi, Chongzhen Mei, Guiqin Niu & Qiuxia Han
To cite this article: Zhuolin Shi, Chongzhen Mei, Guiqin Niu & Qiuxia Han (2018): Two
inorganic–organic hybrids based on a polyoxometalate: Structures, characterizations, and
epoxidation of olefins, Journal of Coordination Chemistry, DOI: 10.1080/00958972.2018.1468026
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Journal of Coordination Chemistry, 2018
https://doi.org/10.1080/00958972.2018.1468026
Two inorganic–organic hybrids based on a polyoxometalate:
Structures, characterizations, and epoxidation of olefins
Zhuolin Shia, Chongzhen Meib, Guiqin Niua and Qiuxia Hana
ahenan Key laboratory of Polyoxometalate Chemistry, institute of molecular and Crystal engineering, school
b
of Chemistry and Chemical engineering, henan university, Kaifeng, China; institute of environmental and
municipal engineering, north China university of Water Conservancy and electric Power, Zhengzhou, China
ABSTRACT
ARTICLE HISTORY
received 29 december 2017
accepted 15 march 2018
Two new inorganic–organic hybrids, [Co3(bpdo)6(H2O)4Cl2]
[SiW12O40]·H2O (Co-SiW) and [Ni3(bpdo)6(H2O)4Cl2][SiW12O40]·H2O (Ni-
SiW) (bpdo = 4,4′-bis(pyridine-N-oxide)), were synthesized from the
Keggin-type [SiW12O40] anion and Co(II) and Ni(II) under hydrothermal
conditions, respectively, and characterized by elemental analyses,
powder XRD, IR spectra, and single-crystal X-ray diffraction. The
structural analysis indicates that the 1D chain is constructed from a
POM-based half-cage as a secondary building block linked by bridging
bpdo ligands. The zigzag chains further stack into a three-dimensional
body with channels. The 3D network structure with amphiphilic
cavities is shaped by electrostatic interactions through the planes,
which has potential to allow molecules such as styrene and H2O2
ingress and egress. Both of the hybrids demonstrated catalytic activity
for epoxidation of olefins, which was examined using styrene and
aqueous hydrogen peroxide (30%) as oxidant in acetonitrile, along
with Co-SiW and Ni-SiW in a heterogeneous manner at 60 °C. Moreover,
the conversion of epoxidation reaction in a heterogeneous manner
is close to that of homogeneous catalysis, while being conveniently
recovered and steadily reused without change of catalyst structure
after epoxidation reactions.
KEYWORDS
Polyoxometalate; hybrid;
olefins; epoxidation;
heterogeneous catalyst
CONTACT Qiuxia han
qiuxia_han@163.com
© 2018 informa uK limited, trading as taylor & francis Group

2
Z. SHI ET AL.
1. Introduction
Epoxides are important intermediates for organic synthesis both in industry and academia
[1–5]. For instance, phenyl epoxyethane is a highly useful intermediate in synthetic drugs,
such as levamisole hydrochloride, which has been widely used as an antihelmintic agent
and immunostimulants in medicine [6, 7]. Although a number of epoxidation processes use
various catalysts and oxidants [8–11], the Halcon process and chlorohydrination based on
organic peroxides and peracids are still used extensively. However, the chlorohydrin process
has disadvantages due to the equipment corroded badly and the large output of chloride
laden sewage. Meanwhile, the Halcon process utilizes hydroperoxide epoxidation reagents
and produces a large quantity of coproducts (such as t-butylalcohol and styrene), whose
commercial values are lower than the chlorohydrination but environmentally acceptable
[12–15]. Therefore, it is a pressing need for the development of new epoxidation catalysts
with low cost and little waste [16].
Among various epoxidation catalysts, polyoxometalates (POMs) are the most promising
candidates due to their excellent thermal and oxidative stability and high efficiency in homo
-
geneous conditions [17–20]. For example, Ishii et al. found effective epoxidation of terminal
olefins catalyzed by H3PW12O40 combined with cetyl-pyridinium chloride as a phase transfer
agent [21]. Mizuno et al. reported a silicotungstate compound, [γ-SiW10O34(H2O)2]4−, which
exhibits high catalytic activity in the epoxidation of various olefins including non-reactive
terminal olefins [22]. Although POMs as homogeneous catalysts generally exhibit high activ-
ity and selectivity in synthetic reactions, their practical applications remain limited due to
catalyst instability and difficulty in separation and recycling [23]. Heterogeneous catalysts
exhibit significant potential advantages over homogeneous catalysts, such as no leaching
of POMs, high specific surface area, ease of separation, efficient recycling, and minimization
of metal traces in the product [24]. Introduction of POMs into the hierarchical frameworks
provided a possibility to avoid losing of POMs from the framework and increase the stability
and the corresponding functions [25]. Many heterogeneous POMOF catalysts have been
reviewed. For example, in 2016 Duan et al. reported photosensitizing decatungstate-based
MOFs with 1D channels via in situ synthesis under solvothermal conditions which exhibits
remarkable photocatalytic activities for the β- or γ-site C-H alkylation of aliphatic nitriles [26].
In 2016, Lin et al. encapsulated tetranickel-containing [Ni4(H2O)2(PW9O34)2]10− into the pores
of phosphorescent MOFs. The POM@MOF assemblies provide a tunable platform for design-
ing highly effective photocatalytic HER catalysts [27]. In 2015, Lin et al. developed a simple
and effective strategy to prepare a multifunctional molecular material based on a highly
stable MOF bearing the [Ru(bpy)3]2+ moiety and a POM, leading to efficient visible-light-driven
hydrogen production [28]. In 2015, Zhou et al. devoted their efforts to isolating a Zn-ε-Keggin
fragment and obtained two POM-based materials, [TBA]3[ε-PMoV MoVI4O36(OH)4Zn4][BTB]4/3
8
(NENU-500) and [TBA]3[ε-PMoV MoVI4O37(OH)3Zn4][BPT] (NENU-501). The facile preparation
8
of these POM-based materials, along with their long-term aqueous stability, low overpoten-
tial and high activity, offers promising features for potential use in hydrogen generation [29].
In this paper, two inorganic–organic hybrids, [Co3(bpdo)6(H2O)4Cl2][SiW12O40]·H2O (Co-SiW)
and [Ni3(bpdo)6(H2O)4Cl2][SiW12O40]·H2O (Ni-SiW), were synthesized from Keggin-type
[SiW12O40]4− and the transition metal ions Co(II) and Ni(II) under hydrothermal conditions
and explored these species for catalysis of phenylethylene. The results demonstrated that

JOURNAL OF COORDINATION CHEMISTRY
3
they are highly effective as a selective catalyst for phenylethylene epoxidation. Meanwhile,
the framework confinement enhances catalyst stability and permits catalyst separation and
reuse; the catalytic activity of the oxidation reaction mainly comes from the POMs.
2. Experimental
2.1. Materials and physical measurements
All reagents were used as purchased without purification. [SiW12O40]4− [30] and bpdo [31]
were prepared according to the literature methods and characterized by IR. Elemental anal-
yses (C, H, and N) were performed on a Perkin Elmer 240C elemental analyzer. Inductively
Coupled Plasma (ICP) analyses (Co, Ni, and W) were performed on a Jarrel-AshJ-A1100 spec-
trometer. IR spectra were recorded from a sample powder palletized with KBr on a Nicolet170
SXFT-IR spectrometer from 4000–400 cm−1.
2.2. Preparation of Co-SiW and Ni-SiW
[Co3(bpdo)6(H2O)4Cl2][SiW12O40]·H2O (Co-SiW).
A
mixture of [SiW12O40]4− (53.67 mg,
0.015 mmol) dissolved in acetonitrile (3 mL), a solution of CoCl2·6H2O (14.27 mg, 0.06 mmol)
in water (3 mL) and bpdo (37.61 mg, 0.2 mmol) in water (2.5 mL) were added together with
stirring. The pH value of the mixture was carefully adjusted with a dilute HCl solution (1 mol
L−1) and the mixture stirred overnight. The resulting suspension was sealed in a 25 mLTeflon-
lined reactor and kept at 110 °C for 3 days. After cooling the autoclave to room temperature,
red single crystals of Co-SiW were separated, washed with water and air-dried (55% yield
based on [SiW12O40]4−). Anal. Calcd. for C60H58Cl2Co3N12O57SiW12 (4318.97) (%): Co, 4.12; Cl,
1.65; W, 51.08; Si, 0.65; C, 16.80; N, 3.90; H, 1.22. Found (%): Co, 4.09; Cl, 1.64; W, 51.07; Si, 0.65;
C, 16.68; N, 3.89; H, 1.21.
[Ni3(bpdo)6(H2O)4Cl2][SiW12O40]·H2O (Ni-SiW). The preparation of Ni-SiW was similar to
that of Co-SiW, except that NiCl2·6H2O was used in place of CoCl2·6H2O. Similarly, crystals
were obtained after 3 days and the red crystals were deposited at the bottom of the solution
(48% yield based on [SiW12O40]4−). Anal. Calcd for C60H58Cl2Ni3N12O57SiW12 (3958.16) (%): Ni,
4.45; Cl, 1.79; W, 55.74; Si, 0.71; C, 9.20; N, 4.24; H, 1.30. Found (%): Ni, 4.44; Cl, 1.79; W, 55.73;
Si, 0.709; C, 9.10; N, 4.25; H, 1.32.
2.3. Phenylethylene epoxidation
The catalytic of phenylethylene epoxidation reaction was examined using a heterogeneous
mixture of 43.2 mg (0.1 mmol) of crystalline samples of Co-SiW in 5.0 mL acetonitrile, then
adding 5.5 mL (50 mmol) phenylethylene and 8.5 mL 30% (75 mmol) hydrogen peroxide
solution, and stirring in an oil bath at 60 °C. In order to prevent the light decomposition
reaction of hydrogen peroxide, the round bottom flask was wrapped with aluminum foil
and the epoxidation products were determined by an Agilent 7890 GC-MS with a DB-FFAP
column (30 m × 250 × 0.25 μm) and the injector temperature of 280 °C.

4
Z. SHI ET AL.
Table 1. Crystallographic data and structural refinements for Co-siW and ni-siW.
Co-SiW
Ni-SiW
empirical formula
temperature/K
C60h58Cl2Co3n12o57siW12
C60h58Cl2ni3n12o57siW12
296(2)
296(2)
Wavelength (Å)
formula weight
0.71073
4319.2
0.71073
3958.16
Crystal system
space group
monoclinic
C2/c
23.6070(3)
15.8220(2)
27.4707(4)
90.00
109.0990(5)
90.00
9695.8(2)
14.840
monoclinic
C2/c
23.5506(10)
15.7770(7)
27.4505(13)
90.00
108.983(2)
90.00
9644.7(7)
14.972
a/Å
b/Å
c/Å
α/deg
β/deg
γ/deg
V/Å3
absorption coefficient (mm−1)
Z
4
7860
4
7136
f (000)
Crystal size (mm)
ɵ range for data collection (°)
index range
0.23 × 0.18 × 0.15
1.83 - 25.10
−28 ≤ h ≤ 28, −18 ≤ k ≤ 18, −30 ≤ l ≤ 32
0.24 × 0.22 × 0.14
1.58 - 25.00
−28 ≤ h ≤ 27, −18 ≤ k ≤ 18,
−32 ≤ l ≤ 32
26,953
8488 [R(int) = 0.0366]
full-matrix least-squares on F2
2.726
reflections collected
reflections unique
refinement method
29,191
8596 [R(int) = 0.0578]
full-matrix least-squares on f2
2.967
ρ
calc/g·cm−3
μ/mm−1
14.840
8596/654
1.094
14.972
8488/688
1.043
data/parameters
Goof
R [I > 2σ(I)]
R = 0.0819, wR = 0.2121
R11 = 0.0865, wR22 = 0.2157
3.394 and −3.425
R = 0.0544, wR = 0.1485
R11 = 0.0636, wR22 = 0.1532
3.946 and −2.654
R1indices (all data)
largest diff. peak and hole (eÅ−3)
2.4. Crystallographic data collection and refinement
Intensity data for Co-SiW and Ni-SiW were collected on a Bruker CCD Apex-II diffractometer
with Mo Kα radiation (λ = 0.71073 Å) at 296 K. The structures were resolved by direct methods
using the SHELXTL-97 program package [32]. The remaining atoms were found from suc-
cessive full-matrix least-squares refinements on F2 and Fourier syntheses. Lorentz polarization
and multi-scan absorption corrections were applied. No hydrogens associated with the water
molecules were located from the difference Fourier map. Positions of the hydrogens attached
to carbon were geometrically placed. All hydrogens were refined isotropically as a riding
mode using the default SHELXTL parameters. Crystal data, data collection parameters and
details of the structure refinement are given in Table 1. The relevant bond lengths and angles
are listed in Table 2.
3. Results and discussion
3.1. Spectroscopic study
IR spectra of Co-SiW display the characteristic vibration patterns derived from the Keggin
framework. Four characteristic vibration bands are observed at 1069, 922, 886, and 796 cm−1,
which are assigned to ν(P–Oa), ν(W–Ot), ν(W–Ob), and ν(W–Oc), respectively [33]. Four vibra-
tions resulting from bpdo molecules, ν(N–O) (1228 cm−1), ν(ring) (1472 cm−1), δ(C–H, in plane)

JOURNAL OF COORDINATION CHEMISTRY
5
Table 2. selected bond distances (Å) and angles (°) for Co-siW.
si(1)–o(22)#1
si(1)–o(22)
si(1)–o(23)#1
si(1)–o(23)
si(1)–o(24)#1
si(1)–o(24)
si(1)–o(21)#1
Co(1)–Cl(1)#2
Co(1)–Cl(1)
Co(1)–o(34)
Co(1)–o(34)#2
Co(1)–o(31)
Co(1)–o(31)#2
Cl(1)#2–Co(1)–Cl(1)
Cl(1)#2–Co(1)–o(34)
Cl(1)–Co(1)–o(34)
Cl(1)–Co(1)–o(31)
o(36)–Co(2)–o(33)
o(36)–Co(2)–o(14)
o(33)–Co(2)–o(14)
o(36)–Co(2)–o(2 W)
Co(2)–o(36)
1.50(2)
1.50(2)
1.62(2)
1.62(2)
1.63(2)
1.63(2)
1.64(2)
2.077(12)
2.077(12)
2.078(13)
2.078(13)
2.108(14)
2.108(14)
179.998(3)
90.1(6)
89.9(6)
95.1(5)
92.2(6)
93.1(6)
Co(2)–o(33)
Co(2)–o(14)
Co(2)–o(2 W)
Co(2)–o(1 W)
2.083(15)
2.086(12)
2.087(16)
2.099(16)
2.164(16)
69.2(11)
123.1(15)
70.3(12)
108.5(12)
108.4(11)
125.8(11)
108.5(12)
70.3(12)
179.998(3)
92.3(5)
Co(2)–Cl(2)
o(22)–si(1)–o(23)
o(23)#1–si(1)–o(23)
o(22)#1–si(1)–o(24)#1
o(22)–si(1)–o(24)#1
o(23)#1–si(1)–o(24)#1
o(23)–si(1)–o(24)#1
o(22)#1–si(1)–o(24)
o(22)–si(1)–o(24)
o(34)–Co(1)–o(34)#2
o(34)–Co(1)–o(31)
o(34)#2–Co(1)–o(31)
o(31)–Co(1)–o(31)#2
o(33)–Co(2)–Cl(2)
o(14)–Co(2)–Cl(2)
o(2 W)–Co(2)–Cl(2)
o(1 W)–Co(2)–Cl(2)
87.7(5)
179.996(2)
170.7(6)
88.9(6)
89.1(7)
88.8(7)
92.8(6)
87.0(7)
2.014(16)
(1180 cm−1), and δ (N–O) (841 cm−1), are also observed. The results of IR spectra are consistent
with those of X-ray diffraction structural analysis.
3.2. Description of the crystal structure of [Co3(bpdo)6(H2O)4Cl2][SiW12O40]·H2O
POMs, especially Keggin-type heteropolyanions, display special advantages for forming
coordination bonds or hydrogen bonds with organic or inorganic moieties to form a wide
range of intriguing topologies and structures [34–37]. Single-crystal X-ray diffraction analyses
reveal that Co-SiW and Ni-SiW are isostructural. Therefore, Co-SiW is described as an example
below.
The reaction system including [SiW12O40]4−, CoCl2·6H2O and bpdo in the mixed solution
of H2O and C2H5OH under hydrothermal conditions led to a new inorganic–organic POM
incorporated hybrid. Structural analysis displays that Co-SiW crystallizes in a monoclinic space
group C2 (Figure 1). The polyoxoanion [SiW12O40]4− exhibits a classical Keggin-type structure.
In the skeleton, the tetrahedral {SiO4} group resides in the center of the [SiW12O40]4− Keggin
cluster and shares four oxygens with four {W3O13} groups. The BVS (bond valence sum) values
indicate that all the oxidation states of tungsten are +6. The results are consistent with the
formula of the title compound given by X-ray structure determination. Relevant W–O bond
distances of the [SiW12O40]4− can be grouped into three sets: W–Ot (terminal) 1.627(17)–
1.731(12) Å; W–Ob,c (bridge) 1.83(2)–1.931(9) Å and W–Oa (central) 2.32(2)–2.48(2) Å.
Three Co(II) ions connected by six bridged bpdo ligands, alternatively, consolidate the
main skeleton of the half-cage [Co3(bpdo)6(H2O)4Cl2]4+. Both crystallographic independent
Co(1) and Co(2) centers are coordinated in distorted octahedral geometries, in which Co(1)
is coordinated by four oxygens from bpdo ligands building the equatorial plane (with Co(1)
– O bond distances from 2.078(13) to 2.108(14) Å) and another two chlorides on the axial
sites of the octahedron (with Co(1)–Cl bond 2.077(12) Å). Whereas, the six-coordinate Co(2)
is coordinated by two oxygens from coordinated water, two oxygens from bpdo ligands,

6
Z. SHI ET AL.
Figure 1. (a) the coordination environment of two crystallographically independent Co(1) and Co(2). (b)
the coordination environment of [siW12o40]4− in Co-siW. (c) the 1-d chain-like structure of Co-siW. (d)
the 2-d plane formed by 1-d chains with weak interaction of Co-siW. Color code: Co (plum), n (blue), o
(red), Cl (green), C (gray); hydrogens are omitted for clarity.
one terminal oxygen and one chloride (with Co(2)–O bond distances from 2.014(16) to
2.099(16) Å).
Surprisingly, the two vertices of [SiW12O40]4− connect two Co(2) ions. The 1D chains are
constructed from a POM-based cobalt half-cage as a secondary building block linked by
bridging bpdo ligands. The whole one-dimensional chain exhibits an interesting zigzag
shape that can be seen from Figure 1(c). Through strong π…π–stacking interactions between
the adjacent pyridine rings (Figure 1(d)), a 2D planar structure is formed via the interactions
between π…π–stacking and hydrogen bonds; these hybrids are stacked parallel giving rise
to the 1D channels through interactions between [SiW12O40]4−. Calculations using PLATON
show that the average distance of π…π–stacking interactions between adjacent pyridine
rings is 3.7333(7) Å and the accessible pores are about 886.0 Å3 [38], occupying approximately
9.1% of the volume of the whole crystal, suggesting the possibility to increase contact
between reactants. A 3D network structure (Figure 2) with amphiphilic cavities is shaped by
electrostatic interactions through the planes, which has potential to allow molecules such
as styrene and H2O2 ingress and egress.

JOURNAL OF COORDINATION CHEMISTRY
7
Figure 2. the 3d network composited by 2d sheets and polyanions from different axis. (a) View of the
Co-siW along the b axis. (b) View of the Co-siW along the c axis.
Table 3. the reaction effect of Co-siW on the catalytic oxidation of styrene at different times.
Reaction time (h)
Styrene oxide (%)
Phenylacetaldehyde (%)
Product yield (%)
4
8
12
48.12
34.83
26.55
4.9
43.67
59.28
53.02
78.50
86.23
Table 4. the oxidation effect of styrene by different catalysts after 4 h.
Catalysts
Styrene oxide (%)
Phenylacetaldehyde (%)
Product yield (%)
Selectivity (%)
[(C4h9)4 n]4[siW12o40]
Co-siW
ni-siW
87.12
48.12
48.33
<5
0.88
4.9
4.6
0
88
99
90.76
91.31
–
53.02
52.93
<5
CoCl2·6h2o
3.3. Phenylethylene epoxidation
The epoxidation of phenylethylene was examined initially using styrene and aqueous hydro-
gen peroxide (30%) as oxidant in acetonitrile (5 mL), along with Co-SiW and Ni-SiW in a
heterogeneous manner at 60 °C. The experiment results indicate that the yield of styrene
epoxidation catalyzed by Co-SiW is 48.12% in four hours. As time goes on the total yield of
epoxidation increases and the rate of styrene oxide decreases with the content of pheny-
lacetaldehyde increasing, as shown in Table 3.
Comparison among the catalysts (Table 4) reveal that catalytic performance in styrene
epoxidation differed little between Co-SiW and Ni-SiW. In addition, epoxidation of styrene
with corresponding metal salts have little catalytic activity. The catalyst was separated by
centrifuge after the reaction. The supernatant liquid was separated and tested by UV. The
result reflects that no characteristic peaks of POMs in the spectrogram showing the POMs
were retained in the framework. The conclusion can be inferred from the structure of Co-SiW,
the bonding action between the POM and framework comes from coordination of the central
cobalt to the terminal oxygen of POM, which is much stronger than electrostatic force and
hydrogen bonding. The above analysis comes to the conclusion that the catalytic reaction
of epoxidation is purely heterogeneous. Hitherto, several efficient catalyst systems with the

8
Z. SHI ET AL.
Figure 3. PXrd patterns of Co-siW, a-simulated, b-experimental, c-recovered catalyst after three runs.
epoxidation of olefins have been reported. For instance, the polyoxomolybdate nanocluster
(NH4)12[Mo36(NO)4O108(H2O)16] supported on zeolite as catalyst for the epoxidation of alkenes
with tert-butyl hydroperoxide (TBHP) in water [39]. Although the exploration above has
been performed excellent for the formation of epoxidation of olefins, it requires higher
concentration of oxidant, as well as higher temperatures.
Subsequently, the stability of Co-SiW and Ni-SiW were characterized. The structure of
Co-SiW and Ni-SiW crystals were retained intact in the condition of air after a few months.
Furthermore, the crystals are insoluble in water and common organic solvents including
ethanol, acetonitrile, acetone, DMF, DMSO, etc. After 10 days of soaking in the condition of
catalytic reaction, the catalyst sample was separated and characterized by IR, which showed
that the framework of the catalyst is not decomposed. For proving the framework remains
intact, the catalyst also been investigated by XRD (Figure 3). The XRD measurements showed
that the Co-SiW retains its framework structure.
4. Conclusion
We obtained two new inorganic–organic hybrids, which were constructed from [SiW12O40]4−
and [M3(dpdo)6(H2O)4Cl2]4+ with three-dimensional structure through hydrothermal reaction
and explored as catalysts of epoxidation. They have great potential as heterogeneous cata-
lysts, possessing some excellent characteristics, such as unambiguous X-ray crystallography,
good dispersion of POMs at the molecular level, mild catalytic environment, highly stable
crystalline framework, and repeated catalyst recycling. The results demonstrated that both
Co-SiW and Ni-SiW conform to green chemistry and benign synthetic method for producing
useful heterogeneous catalyst, which showed promise in oxidation reactions.

JOURNAL OF COORDINATION CHEMISTRY
9
Disclosure statement
No potential conflict of interest was reported by the authors.
Funding
This work was supported by the China Postdoctoral Science Foundation [grant number 5872]; National
Natural Science Foundation of China [grant numbers 21601048, 21671055].
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