Epoxidation of cyclohexene with H2O2 over efficient water-tolerant heterogeneous catalysts composed of mono-substituted phosphotungstic acid on co-functionalized SBA-15

Article abstract of DOI:10.1002/aoc.5115

A series of Keggin-type heteropolyacid-based heterogeneous catalysts (Co-/Fe-/Cu-POM-octyl-NH₃-SBA-15) were synthesized via immobilized transition metal mono- substituted phosphotungstic acids (Co-/Fe-/Cu-POM) on octyl-amino-co-functionalized mesoporous silica SBA-15 (octyl-NH₂-SBA-15). Characterization results indicated that Co-/Fe-/Cu-POM units were highly dispersed in mesochannels of SBA-15, and both types of Br?nsted and Lewis acid sites existed in Co-/Fe-/Cu-POM-octyl-NH₃-SBA-15 catalysts. Co-POM-octyl-NH₃-SBA-15 catalyst showed excellent catalytic performance in H₂O₂-mediated cyclohexene epoxidation with 83.8% of cyclohexene conversion, 92.8% of cyclohexene oxide selectivity, and 98/2 of epoxidation/allylic oxidation selectivity. The order of catalytic activity was Co-POM-octyl-NH₃-SBA-15?>?Fe-POM-octyl-NH₃-SBA-15?>?Cu-POM-octyl-NH₃-SBA-15. In order to obtain insights into the role of -octyl moieties during catalysis, an octyl-free catalyst (Co-POM-NH₃-SBA-15) was also synthesized. In comparison with Co-POM-NH₃-SBA-15, Co-POM-octyl-NH₃-SBA-15 showed enhanced catalytic properties (viz. activity and selectivity) in cyclohexene epoxidation. Strong chemical bonding between -NH₃ ⁺ anchored on the surface of SBA-15 and heteropolyanions resulted in excellent stability of Co-POM-octyl-NH₃-SBA-15 catalyst, and it could be reused six times without considerable loss of activity.

Full text of DOI:10.1002/aoc.5115

Received: 13 December 2018
DOI: 10.1002/aoc.5115
Revised: 19 June 2019
Accepted: 21 June 2019
F U L L P A P E R
Epoxidation of cyclohexene with H O over efficient
2
2
water‐tolerant heterogeneous catalysts composed of
mono‐substituted phosphotungstic acid on
co‐functionalized SBA‐15
1
,2
2
3
1
Manman Jin | Qingtao Niu | Zhenmei Guo | Zhiguo Lv
1
State Key Laboratory Base for Eco‐
A series of Keggin‐type heteropolyacid‐based heterogeneous catalysts (Co‐/Fe‐/
chemical Engineering, College of
Chemical Engineering, Qingdao
Cu‐POM‐octyl‐NH ‐SBA‐15) were synthesized via immobilized transition
3
University of Science and Technology, No.
metal mono‐ substituted phosphotungstic acids (Co‐/Fe‐/Cu‐POM) on octyl‐
53 Zhengzhou Road, Qingdao 266042,
China
amino‐co‐functionalized mesoporous silica SBA‐15 (octyl‐NH ‐SBA‐15).
2
2
Department of Chemistry and Chemical
Characterization results indicated that Co‐/Fe‐/Cu‐POM units were highly dis-
persed in mesochannels of SBA‐15, and both types of Brønsted and Lewis acid
Engineering, Jining University, Qufu,
Shandong 273155, China
sites existed in Co‐/Fe‐/Cu‐POM‐octyl‐NH ‐SBA‐15 catalysts. Co‐POM‐octyl‐
3
3
College of Marine Science and Biological
NH ‐SBA‐15 catalyst showed excellent catalytic performance in H O ‐mediated
Engineering, Qingdao University of
Science and Technology, No.53
3
2 2
cyclohexene epoxidation with 83.8% of cyclohexene conversion, 92.8% of cyclo-
hexene oxide selectivity, and 98/2 of epoxidation/allylic oxidation selectivity.
Zhengzhou Road, Qingdao 266042, China
Correspondence
The order of catalytic activity was Co‐POM‐octyl‐NH
3
‐SBA‐15 > Fe‐POM‐
Zhiguo Lv, State Key Laboratory Base for
Eco‐chemical Engineering, College of
Chemical Engineering, Qingdao
University of Science and Technology, No.
octyl‐NH ‐SBA‐15 > Cu‐POM‐octyl‐NH ‐SBA‐15. In order to obtain insights
3
3
into the role of ‐octyl moieties during catalysis, an octyl‐free catalyst (Co‐
POM‐NH ‐SBA‐15) was also synthesized. In comparison with Co‐POM‐NH ‐
3
3
53 Zhengzhou Road, Qingdao, 266042,
SBA‐15, Co‐POM‐octyl‐NH ‐SBA‐15 showed enhanced catalytic properties
3
China.
(
viz. activity and selectivity) in cyclohexene epoxidation. Strong chemical
Email: lvzhiguo@qust.edu.cn
+
bonding between ‐NH3
anchored on the surface of SBA‐15 and
Funding information
heteropolyanions resulted in excellent stability of Co‐POM‐octyl‐NH ‐SBA‐15
3
National Natural Science Foundation of
China, Grant/Award Number:
NSFC21376128; Major Basic Research
Projects of Shandong Provincial Natural
Science Foundation, Grant/Award Num-
ber: ZR2017ZC0632
catalyst, and it could be reused six times without considerable loss of activity.
KEYWORDS
co‐functionalized SBA‐15, cyclohexene, epoxidation, heteropolyacid, mono‐substituted
1
| INTRODUCTION
active and selective catalysts for cyclohexene/olefins epox-
idation to circumvent the above‐mentioned side reactions
[4–9]
Cyclohexene oxide (viz. epoxide) synthesized via epoxida-
and facilitate the formation of the desired product.
tion of cyclohexene is indispensable for the fine‐chemical
Polyoxometalates (POMs) have already been promising
catalysts for plenty of oxidation reactions due to their acid
[1]
industry. Nevertheless, two major side reactions, allylic
oxidation and epoxide ring‐opening, can considerably
occur in epoxidation of cyclohexene, resulting in cyclo-
[
10]
property.
Incorporation of transition metal into POM
units (M‐POMs) has aroused considerable interest as an
[
2,3]
[11–14]
hexene epoxidation to epoxide becoming difficult.
Great efforts have been made in the development of new
efficient method to develop POM‐derived catalysts.
A
primary disadvantage that impedes the actual
Appl Organometal Chem. 2019;e5115.
wileyonlinelibrary.com/journal/aoc
© 2019 John Wiley & Sons, Ltd.
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https://doi.org/10.1002/aoc.5115

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JIN ET AL.
+
application of M‐POMs in synthetic chemistry is that they
are difficult to recycle. Heterogenization of M‐POMs/
POMs can offer materials that are much easier to separate,
chemically bonded to ‐NH3 and surrounded with hydro-
phobic ‐octyl moieties in the mesochannels of SBA‐15.
For the first time, we synthesized new and efficient
Co‐/Fe‐/Cu‐POM‐based epoxidation catalysts using
octyl‐amino‐co‐functionalized mesoporous silica SBA‐15
[
15]
and may show improved selectivity and activities.
This
[
16]
concept has been demonstrated in several systems,
[17]
[18]
including V ‐POM/Support,
GO/MNPs/PW,
POM‐
(octyl‐NH ‐SBA‐15) as a support. Si‐OH groups that
existed in SBA‐15 structure were used as anchoring points
2
2
[19]
[20]
poly
and POM‐ZrO2.
Despite interesting features of
M‐POMs‐based heterogeneous catalysts, and recent prog-
ress in design and synthesis of these efficient catalysts,
clean separation and recycling still present a considerable
for immobilization of ‐octyl and ‐NH moieties via a post‐
synthetic strategy. In addition, ‐NH3 cations grafted on
2
+
octyl‐NH ‐SBA‐15 were used for immobilization of
2
[21,22]
challenge.
Concerning the diversity of M‐POMs in
Keggin‐type Co‐/Fe‐/Cu‐POM anions through electro-
composition, property and structure, there will be consid-
erable scope for developing and designing more efficient
heterogeneous M‐POMs‐based catalysts for olefins
epoxidation reactions. It is interesting to note that the
use of heterogeneous M‐POMs‐loaded materials that acted
as catalysts in liquid (olefin)/liquid (H O )/solid (catalyst)
static attraction. Co‐/Fe‐/Cu‐POM‐octyl‐NH ‐SBA‐15
3
catalysts with both Brønsted and Lewis acid sites showed
excellent catalytic performance in cyclohexene epoxida-
tion, and the order of catalytic activity was Co‐POM‐
octyl‐NH ‐SBA‐15 > Fe‐POM‐octyl‐NH ‐SBA‐15 > Cu‐
3
3
POM‐octyl‐NH ‐SBA‐15. Besides, Co‐POM‐octyl‐NH3‐
2
2
3
triphase catalysis system is challenging, as mass transfer
resistance exists between reactants. A crucial factor to
improve catalytic activity is to control or regulate chemi-
cal features of the surface of solid supports, as the reaction
rates of heterogeneous reactions should be determined via
such surface processes as diffusion of reaction substrates
and products, access of reactants to active center, as well
as via microscopic environments around the catalysis cen-
ter. It was worth mentioning that M‐POMs could be
chemically immobilized on some materials via forming
positive charges on the support by surface modification
SBA‐15 catalyst showed enhanced catalytic performance
in cyclohexene epoxidation in comparison with
immobilized Co‐POM anions on octyl‐free mesoporous
SBA‐15 (Co‐POM‐NH ‐SBA‐15). The presence of ‐octyl
3
moieties grafted in SBA‐15 led to a more hydrophobic
surface, which resulted in higher catalytic activity of
synthesized materials.
2 | EXPERIMENTAL
to reduce the leaching of active species, and ‐NH moieties
2.1 | Catalysts preparation
2
could provide sites for the immobilization of charge
matching components (viz. M‐POMs). Further surface
silylation with organic groups (−octyl, etc.) could improve
the catalytic activity of as‐synthesized catalyst via chang-
ing the surface properties to decrease mass transfer resis-
tance between oil and water phases. To the best of our
knowledge, there is no report for immobilization of
The synthesis process of hybrid catalysts with nanostruc-
ture was illustrated in Scheme 1. Two different organic
groups, −octyl and ‐NH , were successively grafted on
2
Si‐OH groups of SBA‐15, respectively. [PW Co/Fe/
1
1
n‐
CuO ·yH O] polyoxometalate anions were immobilized
on acidulated amino groups (‐NH ) via electrostatic
40
2
+
3
M‐POMs on ‐octyl and ‐NH co‐functionalized mesopo-
binding, while hydrophobic regions around the
2
rous silica SBA‐15. In this case, heteropolyanions were
polyanions were formed by anchored alkyl groups.
SCHEME 1 Schematic illustration of the synthesis concept of hybrid catalysts

JIN ET AL.
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2.1.1 | Synthesis of octyl‐SBA‐15
Co (2.05%); Fe‐POM: P (1.07%), W (69.75%), Fe (2.13%);
Cu‐POM: P (1.08%), W (70.40%), Cu (2.23%). Anal. found
SBA‐15 material was synthesized according to a previ-
in dehydrated Co‐POM:
dehydrated Fe‐POM: H (0.23%), O (24.34%); dehydrated
Cu‐POM: H (0.24%), O (22.26%).
H
(0.24%),
O
(22.09%);
[23]
ously reported procedure (Scheme 1).
Octyl‐SBA‐15
(
(
Scheme 1) was prepared by adding octyl‐trimethoxysilane
‐octyl, 1.0 mmol) to a suspension of SBA‐15 (1.0 g) in
anhydrous toluene (35 ml). The mixture was stirred and
2.1.4 | Preparation of hybrid catalysts
refluxed for 24 hr under N protection. Subsequently, solid
2
octyl‐SBA‐15 was collected by filtration, extracted
with toluene using Soxhlet for 12 hr, and dried under
Typically, octyl‐NH ‐SBA‐15 (0.5 g) or NH ‐SBA‐15
2
2
(
(
(
0.5 g) was mixed with trifluoromethanesulfonic acid
CF SO H, 2 mmol) while being stirred in CH Cl
2
35 ml) for 8 hr. Subsequently, the solid was washed with
vacuum. The synthetic process of NH ‐SBA‐15 was similar
2
3
3
2
to that of octyl‐SBA‐15, except that in this case 3‐
aminopropyltriethoxysilane was added instead of ‐octyl.
ethyl alcohol and dried under vacuum to obtain the
Anal. found in NH ‐SBA‐15: C, 8.55%; H, 2.02%; N, 1.63%.
2
resultant material octyl‐NH ‐SBA‐15 or NH ‐SBA‐15.
3
3
Octyl‐NH ‐SBA‐15 (0.5 g) or NH ‐SBA‐15 (0.5 g) was
3
3
2
.1.2 | Preparation of octyl‐NH ‐SBA‐15
added to aqueous solution (30 ml) containing an appro-
priate amount of Co‐/Fe‐/or Cu‐POM while being stirred
and refluxed for 6 hr. The resulting solid was filtrated,
2
Typically, 3‐aminopropyltriethoxysilane (1.0 mmol)
was added to anhydrous toluene (35 ml) containing
octyl‐SBA‐15 (1.0 g) while being stirred and refluxed for
washed with warm H O, and dried under vacuum to pro-
2
duce Co‐/Fe‐/or Cu‐POM‐octyl‐NH ‐SBA‐15 (Scheme 1)
3
2
4 hr under N2 protection. Afterwards, the resulting
or Co‐POM‐NH ‐SBA‐15 (viz. octyl‐free catalyst). Anal.
3
octyl‐NH ‐SBA‐15 (Scheme 1) was filtered, extracted with
found in Co‐POM‐octyl‐NH ‐SBA‐15: C, 8.92%; H,
2
3
toluene using Soxhlet for 12 hr, and dried under vacuum.
2.47%; N, 1.17%. Anal. found in Fe‐POM‐octyl‐NH3‐
SBA‐15: C, 8.53%; H, 2.37%; N, 1.22%. Anal. found in
Anal. found in octyl‐NH ‐SBA‐15: C, 10.94%; H, 2.56%; N,
2
1
.57%.
Cu‐POM‐octyl‐NH ‐SBA‐15: C, 9.77%; H, 2.38%; N,
3
1
.31%. Anal. found in Co‐POM‐NH ‐SBA‐15: C, 5.27%;
3
H, 1.92%; N, 1.21%.
2
.1.3 | Synthesis of H PW Co/Fe/
x 11
CuO ·yH O
4
0
2
2.2 | Characterization
Co, Fe and Cu mono‐substituted phosphotungstic acids
were synthesized according to a modified procedure
N adsorption isotherms were performed on ASAP 2020
2
[24]
described by Tsigdinos et al.
Typically, aqueous
V4.01 (V 4.01 H; Micromeritics) sorptometer at −196°C,
and the samples were outgassed at 100°C for 8 hr before
the measurement. X‐ray diffraction (XRD) patterns were
collected on a Rint 2000 vertical goniometer (Rigaku)
using Cu Kα radiation (λ = 0.154 nm) at 2°(2θ)/min
and operated at 30 mA and 40 kV. UV–Vis DRS spectra
were gathered from 200 nm to 700 nm on a Cary 500
(8.01) spectrophotometer. Fourier transform‐infrared
(FT‐IR) spectra were performed on a HGCS spectrometer
solution (20 ml) of Na HPO ·12H O (0.01 mol) was mixed
2
4
2
with aqueous solution (20 ml) of Co (NO ) ·6H O
3
2
2
(0.01 mol). Concentrated sulfuric acid (about 1 ml) was
added to the cooled mixture to obtain a bright red
solution. Subsequently, aqueous solution (40 ml) of
Na WO ·2H O (0.11 mol) was added to the bright red
2
4
2
solution and abundant flocculent precipitate immediately
appeared. Afterwards, concentrated sulfuric acid (about
−
1
−1
5
ml) was slowly added to the mixture with vigorous
from 4000 cm to 400 cm using material diluted with
KBr. Thermal gravimetric analysis‐difference thermogra-
vimetry measurements were performed on a METTLER
TGA/DSC1. The temperature was increased from 30°C
to 700°C at a heating rate of 10°C/min under air atmo-
sphere. Transmission electron microscopy (TEM) pictures
were obtained by JEM‐2100 electron microscope. The sur-
stirring to obtain a clear red solution. After the solution
was cooled, heteropolyacid was extracted with 80 ml of
ethyl ether and heteropoly etherate was located in the
bottom layer. After separation and removing ether, the
remaining red solid was dissolved in H O (50 ml) and
2
crystallized to obtain the final H PW CoO ·yH O (Co‐
7
11
40
2
POM). The above‐mentioned synthetic process was
repeated to prepare H PW FeO ·yH O (Fe‐POM) or
face acidity of Co‐/Fe‐/Cu‐POM‐octyl‐NH ‐SBA‐15 was
3
monitored from FT‐IR spectra collected after pyridine
adsorption. Typically, 100 mg of sample was degassed at
200°C under vacuum for 3 hr followed via absorbing
pyridine. Then, excess pyridine was removed via heating
6
11
40
2
H PW CuO ·yH O (Cu‐POM), except that in these cases
7
11
40
2
Fe (NO ) ·9H O or Cu (NO ) ·3H O was added, respec-
3
3
2
3 2
2
tively. Anal. found in Co‐POM: P (1.07%), W (70.05%),

4
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JIN ET AL.
the sample at 120°C for 1 hr. Temperature‐programmed
desorption of adsorbed pyridine starting at 120°C was
investigated by stepwise heating of the sample to
characterize acid sites type and strength under vacuum.
P‐NMR spectrum of Co‐POM was collected on an
Aligent 600 MHz DD2 spectrometer.
H1‐type hysteresis loops, which was characteristic of a
[26]
mesoporous structure with uniform pore size.
This
indicated that the ordered mesostructure was still main-
tained after introduction of organic moieties (viz. ‐octyl
31
and ‐NH ) and heteropolyanions into mesochannels of
2
SBA‐15. Compared with pure SBA‐15, hysteresis loops
of these hybrid materials shifted to lower relative pres-
sure and became less vertical, illustrating that pore
2.3 | Catalytic tests
diameters (D ) were decreased. Pore size distributions of
P
SBA‐15 and hybrid materials were depicted in Figure 1
The catalytic performance was assessed via cyclohexene
epoxidation with H O . In a typical run, cyclohexene
(
c) and (d). All samples exhibited monomodal distribu-
2
2
tions centered at 6.14–9.10 nm, suggesting the presence
of uniform pore size and the existence of mesopores.
The texture parameters of these materials were sum-
marized in Table 1. Compared with SBA‐15, the specific
surface area (SBET), total pore volume (V ) and D of
(5 mmol), MeCN (5 ml) and catalyst (20 mg) were
successively added to a round‐bottomed flask while being
stirred and heated at 65°C, and then 50 wt% H O2
2
(18 mmol) was added drop by drop. The reaction was
T
P
continued for 3 hr at 65°C with stirring. The details of
cyclohexene epoxide acquisition and analysis were
silylated SBA‐15 (i.e. octyl‐SBA‐15, NH ‐SBA‐15 and
2
octyl‐NH ‐SBA‐15) sharply decreased. As expected, Co‐/
[25]
2
observed elsewhere.
Fe‐/Cu‐POM‐octyl‐NH ‐SBA‐15 and Co‐POM‐NH ‐SBA‐
3
3
1
5 presented reduced SBET, V and D compared with
T P
3
3
| RESULTS AND DISCUSSION
.1 | Catalyst characterization
octyl‐NH
2
‐SBA‐15 and NH ‐SBA‐15, respectively. This
2
phenomenon confirmed the accommodation of Co‐/Fe‐/
Cu‐POM anions in the mesochannels of SBA‐15. From
inductively coupled plasma analysis, heteropolyanions
contents in the solid catalysts were 0.108, 0.125, 0.141
The N sorption experiment was carried out to investigate
2
the mesoscopic quality and textural properties of synthe-
sized materials. As shown in Figure 1(a) and (b), all sorp-
tion isotherms exhibited typical IV‐type curves with clear
and 0.117 mmol/g for Co‐POM‐octyl‐NH ‐SBA‐15,
3
Fe‐POM‐octyl‐NH ‐SBA‐15, Cu‐POM‐octyl‐NH ‐SBA‐15
3
3
and Co‐POM‐NH ‐SBA‐15, respectively.
3
FIGURE 1
2
N sorption isotherms (a and b) and pore size distributions (c and d) of synthesized materials

JIN ET AL.
5 of 12
TABLE 1 Texture parameters of synthesized materials
2
3
Samples
S
BET (m /g)
V
T
(cm /g)
D
p
(nm)
POM loading (mmol/g)
SBA‐15
708
552
393
350
298
324
283
312
1.09
0.85
0.84
0.56
0.53
0.51
0.48
0.60
9.10
7.51
8.77
7.01
6.46
6.20
6.14
7.68
–
octyl‐SBA‐15
–
NH
octyl‐NH
Co‐POM‐octyl‐NH
2
‐SBA‐15
–
2
‐SBA‐15
–
3
‐SBA‐15
‐SBA‐15
‐SBA‐15
0.108
0.125
0.141
0.117
Fe‐POM‐octyl‐NH
3
Cu‐POM‐octyl‐NH
3
3
Co‐POM‐ NH ‐SBA‐15
The wide‐angle XRD patterns of Co‐/Fe‐/Cu‐POM and
phosphotungstic acid with Keggin type were detected in
these spectra. Characteristic bands of H PW O
12 40
H PW O between 10° and 60° were displayed in
3
12 40
3
Figure 2(a). H PW O showed characteristic XRD peaks
appeared at 1080 (P‐O in central PO4 tetrahedron
3
12 40
located at 2θ = 8, 16–23, 25–30 and 31–38°. In comparison
stretching frequency), 973 (W=O in exterior WO octahe-
6
with H PW O , partial substitution of W by Co/Fe/or
dron terminal bands), 889 (vibration of W‐O ‐W bridge)
3
12 40
b
−
1
Cu resulted in slight alteration in 2θ values of these
characteristic peaks. For hybrid catalysts (Figure 2b), they
all displayed a broad diffraction peak in 2θ = 15–30°,
which was the typical characterization of mesoporous
silica. No characteristic peaks assigned to H PW O
and 810 cm (band of W‐O ‐W bridge), respectively. For
c
Co‐/Fe‐/Cu‐POM samples, corresponding bands (viz.
−
1
1080, 973, 889 and 810 cm ) slightly shifted, which was
a description of distortion that resulted from partial
replacement of W by Co/Fe/or Cu. The P‐O band at
3
12 40
−
1
were observed, which suggested homogeneous distribu-
tion of Co‐/Fe‐/Cu‐POM units in the mesochannels of
mesoporous SBA‐15.
1080 cm of H PW O usually splits up into two bands
3 12 40
n‐
for (PW ) (formed via substituting one W atom from
11
[
28]
H PW O ),
and the disappearance of the splitting of
3
12 40
The small‐angle XRD patterns of silylated SBA‐15 and
hybrid catalysts were exhibited in Figure 2(c). Silylated
SBA‐15 displayed an intense peak that corresponded to
P‐O absorption peaks in the FT‐IR spectra of Co‐/Fe‐/
Cu‐POM was attributed to the introduction of Co, Fe or
n‐
Cu into the quasi‐octahedron vacancy of (PW ) , the
1
1
n‐
(100) reflection, and two relative weak peaks attributed
Co, Fe or Cu unit made up for the vacancy of (PW )
11
[
29] 31
to (110) and (200) reflections, which were indexed to
and restored the molecular symmetry.
P‐MAS NMR
[27]
highly ordered hexagonal lattice as reported.
Immobi-
spectroscopy was performed to further identify if there is
any change in the environment around the phosphorus
lization of heteropolyanions on mesochannels of silylated
SBA‐15 resulted in slightly reduced intensities of these
three reflection peaks with slight shifts to higher 2θ
values compared with silylated SBA‐15. Nevertheless,
the mesostructure of SBA‐15 still remained after
silylation and heteropolyanions immobilization.
3
1
moiety. The P‐MAS NMR for H PW O exhibited a
3
12 40
[
30]
chemical shift at −15.62 ppm,
while that of Co‐POM
displayed a chemical shift at −14.01 ppm (Figure 2a,
inset), indicating one W atom had been substituted by
[
31]
one transition metal atom.
As seen in Figure 3(b),
The FT‐IR spectra of Co‐/Fe‐/Cu‐POM and H PW O
SBA‐15 showed anti‐symmetric and symmetric stretching
3
12 40
were displayed in Figure 3(a). Typical IR bands ascribed to
vibration bands located at 1100 (νas Si‐O‐Si), 810 (ν Si‐O‐
s
FIGURE 2 X‐ray diffraction (XRD) patterns of synthesized materials

6
of 12
JIN ET AL.
3
FIGURE 3 Fourier transform‐infrared (FT‐IR) spectra of H PW12O40 and synthesized materials
−
1
Si) and 460 (δ Si‐O‐Si) cm . The bands at 3453, 1632 and
15. One peak at 265 nm corresponded to O → W charge
transfer, and two broad bands centered at 340 and
−
1
9
60 cm
were attributed to stretching vibrations of
adsorbed H O molecules (δ H O) and Si‐OH groups,
550 nm attributed to the existence of Co (II) were
2
2
[
32]
[36]
respectively. For octyl‐SBA‐15, in addition to the bands
originated from SBA‐15, asymmetric and symmetric
detected.
Particularly, the spectrum of Co‐POM‐octyl‐
NH ‐SBA‐15 differed from that reported about Co‐POM
3
−
1
[37]
stretching of ‐CH ‐ at 2929 and 2851 cm were observed,
datively linked to NH ‐silica.
Furthermore, no peaks
2
2
which suggested the alkyl groups were chemically grafted
and shoulders were detected between 628 nm and
640 nm, suggesting the dative bond between the amino
[33]
−1
on SBA‐15. New bands at 1595 cm attributed to ‐N‐H
bending vibration of ‐NH were detected in octyl‐NH ‐
[
38]
group and cobalt was not formed.
This manifested
2
2
[34]
SBA‐15 and NH ‐SBA‐15, respectively,
suggesting ‐
electrostatic binding between Co‐POM and octyl‐NH ‐
2
3
[
39]
NH2 groups were successfully anchored in SBA‐15.
SBA‐15.
−
1
Stretching of ‐N‐H that appeared at 3380 and 3310 cm
was not detected, as it overlaid with broad band of surface
The thermal behavior of representative samples (i.e.
octyl‐NH ‐SBA‐15 and Co‐POM‐octyl‐NH ‐SBA‐15) was
2
3
−
1 [35]
Si‐OH in the range of 3500–3000 cm . Compared with
investigated via TG‐DTA analyses (Figure 6). Co‐POM‐
NH ‐SBA‐15, the intensities of the other two bands of
octyl‐NH ‐SBA‐15 exhibited three obvious steps of mass
2
3
octyl‐NH ‐SBA15 that corresponded to ‐CH ‐ stretching
loss between 30°C and 700°C. The first mass loss in
2
2
−
1
at 2935 and 2867 cm increased. For Co‐/Fe‐/Cu‐POM‐
octyl‐NH ‐SBA‐15 and Co‐POM‐NH ‐SBA‐15 materials,
30–100°C was ascribed to loss of adsorbed H O. The second
2
mass loss occurred at 200°C due to removal of H O
3
3
2
[
40]
an IR band that corresponded to vibration of W‐O‐W
molecules per Keggin unit.
The third mass loss in
−
1
bridge was detected (898 cm ). The peak of W=O
300–530°C was logical to ascribe to organic part decompo-
sition in Co‐POM‐octyl‐NH ‐SBA‐15, as octyl‐NH ‐SBA‐15
−
1
overlaid with the band of Si‐OH in 960 cm , and
3
2
increased intensity of this band was clearly observed.
The bands (i.e. 898 and 960 cm ) detected in FT‐IR
exhibited similar major mass loss between 300°C and
530°C. The end of decomposition appeared at about
600–700°C, and constituent inorganic oxides were formed.
Thermal analysis further confirmed Co‐POM anions
−
1
spectra of Co‐/Fe‐/Cu‐POM‐octyl‐NH ‐SBA‐15 and Co‐
3
POM‐NH ‐SBA‐15, regardless of the slight shift of bands
3
positions, suggested the Keggin structure of phosphotung-
were successfully introduced to octyl‐NH ‐SBA‐15 and
2
stic acid was well reserved.
Co‐POM‐octyl‐NH ‐SBA‐15 catalyst was stable at the
3
The mesostructure and morphology of synthesized
employed reaction temperature (65°C).
octyl‐NH ‐SBA‐15 (Figure 4a and b) and Co‐POM‐octyl‐
The pyridine adsorbed FT‐IR analysis was carried out
to evaluate the strength and types of acid sites of Co‐/
2
NH ‐SBA‐15 (Figure 4c and d) were revealed via
3
TEM analysis. As seen in Figure 4, highly uniformed
mesostructure and narrow pore distribution were clearly
observed. This indicated that the mesostructure of SBA‐
Fe‐/Cu‐POM‐octyl‐NH ‐SBA‐15 catalysts. Figure 7 exhib-
3
ited pyridine FT‐IR (Py‐FT‐IR) spectra of Co‐POM‐octyl‐
NH ‐SBA‐15 (Figure 7a), Fe‐POM‐octyl‐NH ‐SBA‐15
3
3
1
5 remained intact after introduction of organic moieties
(Figure 7b) and Cu‐POM‐octyl‐NH ‐SBA‐15 (Figure 7c)
3
and Co‐/Fe‐/Cu‐POM units.
catalysts recorded after adsorption of pyridine and subse-
Figure 5 displayed UV–Vis spectra of initial SBA‐15,
quent evaluation at 120°C and 350°C. According to the
[
41]
Co‐POM and hybrid Co‐POM‐octyl‐NH ‐SBA‐15 catalyst.
literature,
SBA‐15 showed weak surface Lewis (L) acid
3
SBA‐15 showed no peaks in 200–700 nm. Similar spectra
sites after adsorption of pyridine at room temperature
were detected in Co‐POM and Co‐POM‐octyl‐NH ‐SBA‐
(RT), which vanished completely after outgassing at
3

JIN ET AL.
7 of 12
FIGURE 4 Transmission electron microscopy (TEM) images of octyl‐NH
2
‐SBA‐15 (a and b) and Co‐POM‐octyl‐NH
3
‐SBA‐15 (c and d).
(
a and c: along the [001] direction; b and d: along the [110] direction)
FIGURE 6 Thermogravimetric (TG)‐differential thermal analysis
(DTA) curves of octyl‐NH ‐SBA‐15 and Co‐POM‐octyl‐NH ‐SBA‐15
catalyst
FIGURE 5 UV–Vis spectra of SBA‐15, Co‐POM and Co‐POM‐
octyl‐NH ‐SBA‐15 catalyst
2
3
3
−
1
2
00°C. Py‐FT‐IR spectra of Co‐/Fe‐/Cu‐POM‐octyl‐NH3‐
In addition to the bands located at 1445 and 1595 cm ,
SBA‐15 catalysts collected after adsorption of pyridine at
Co‐/Fe‐/Cu‐POM‐octyl‐NH ‐SBA‐15 catalysts also exhib-
ited bands at 1488, 1545 and 1635 cm ascribed to pro-
tonated pyridine bonded to surface Brønsted (B) acid
3
−
1
−1
RT showed bands at 1445 and 1595 cm , assigned to pyr-
[
42]
idine coordinately bonded to weak surface L acid sites.

8
of 12
JIN ET AL.
3
FIGURE 7 Pyridine adsorbed Fourier transform‐infrared (FT‐IR) spectra of Co‐/Fe‐/Cu‐POM‐octyl‐NH ‐SBA‐15 catalysts
sites, suggesting the existence of Co‐/Fe‐/Cu‐POM anions
led to the development of surface B acid sites. The inten-
sities of these bands (i.e. 1445, 1488, 1545, 1595 and
H O as oxidant. The results of catalytic performance of
2 2
all obtained hybrid catalysts were listed in Table 3.
Detectable products were cyclohexene oxide (epoxide),
1,2‐cyclohexanediol (1,2‐diol), 2‐cyclohexene‐1‐ol (2‐ol),
2‐cyclohexene‐1‐one (2‐one) and cyclohexenyl hydroper-
oxide (peroxide). Two different mechanisms, radical
mechanism and direct epoxidation, existed in cyclohex-
ene epoxidation. The existence of 2‐ol originated from
peroxide indicated partial reaction occurred via allylic
oxidation, as depicted in Scheme 2; 2‐one detected by
gas chromatography (GC) essentially resulted from perox-
−
1
1
635 cm ) decreased after outgassing at rising tempera-
ture, but they were still recorded even after outgassing
at 350°C, indicating that B and L acid sites in Co‐/Fe‐
POM‐octyl‐NH ‐SBA‐15 catalysts were rather strong.
3
Nevertheless, for Cu‐POM‐octyl‐NH ‐SBA‐15 catalyst, all
3
of these bands almost disappeared after outgassing at
3
50°C. As seen from Py‐FT‐IR spectra, the order of
relative acid strength was Co‐POM‐octyl‐NH ‐SBA‐
3
[
43]
1
5 > Fe‐POM‐octyl‐NH ‐SBA‐15 > Cu‐POM‐octyl‐NH ‐
ide decomposition during GC analysis.
Epoxide
3
3
SBA‐15. The relative amounts of B and L acid sites of
amount was far more than that obtained by radical mech-
anism, indicating direct epoxidation also took place.
Cyclohexene conversion was as low as 6.9% (No. 1) in
Co‐/Fe‐/Cu‐POM‐octyl‐NH ‐SBA‐15 catalysts were sum-
3
marized in Table 2. As seen from Table 2, the order of
relative acid sites amounts was similar to that of acid
strength. Based on Py‐FT‐IR analysis, it was concluded
that both B and L acid sites existed in Co‐/Fe‐/Cu‐POM‐
the absence of a catalyst. Co‐POM‐octyl‐NH ‐SBA‐15
3
(No. 2) exhibited high catalytic activity with cyclohexene
conversion of 83.8%, epoxide selectivity of 92.8% and
epoxidation/allylic selectivity of 98/2. When Fe‐/Cu‐
octyl‐NH ‐SBA‐15 catalysts. The strong B and L acid sites
3
were essential to cyclohexene epoxidation.
POM‐octyl‐NH ‐SBA‐15 acted as catalysts (No. 3–4),
3
respectively, cyclohexene conversion of 72.5% and
58.5%, epoxide selectivity of 80.2% and 67.4%, and
3
.2 | Catalytic epoxidation of cyclohexene
epoxidation/allylic selectivity of 91/9 and 87/13 were
obtained. The order of catalytic activity was Co‐POM‐
octyl‐NH ‐SBA‐15 > Fe‐POM‐octyl‐NH ‐SBA‐15 > Cu‐
Catalytic activity of synthesized Co‐/Fe‐/Cu‐POM‐octyl‐
NH ‐SBA‐15 and Co‐POM‐NH ‐SBA‐15 catalysts was
evaluated in cyclohexene epoxidation to epoxide using
3
3
POM‐octyl‐NH ‐SBA‐15. This was explained by the differ-
3
3
3
[
44]
ent acidic strengths.
The order of acidic strength was
3
TABLE 2 The relative acid sites amounts of Co‐/Fe‐/Cu‐POM‐octyl‐NH ‐SBA‐15 catalysts
B amount
(mmol/g)
L amount
(mmol/g)
Total acidity
(mmol/g)
Samples
Temperature (°C)
B/L
Co‐POM‐octyl‐NH
3
‐SBA‐15
RT
190.34
106.27
79.14
493.40
210.37
155.65
683.74
316.65
234.79
0.39
0.51
0.51
1
3
20
50
Fe‐POM‐octyl‐NH
3
‐SBA‐15
RT
145.60
91.27
52.76
402.76
149.99
63.69
548.36
241.26
116.45
0.36
0.61
0.83
1
3
20
50
Cu‐POM‐octyl‐NH
3
‐SBA‐15
RT
58.81
28.56
12.05
352.82
133.99
37.03
411.63
162.55
49.08
0.17
0.21
0.33
1
3
20
50
RT, room temperature.

JIN ET AL.
9 of 12
a
2 2
TABLE 3 Catalytic performance of as‐prepared catalysts in cyclohexene epoxidation with H O
Selectivity (%)
Cyclohexene
Epoxide/
1,2‐diol
Epoxidation/
Allylic ox.
b
No.
Catalysts
Con. (%)
epoxide
1,2‐diol
2‐ol
2‐one
others
1
2
3
4
5
No
6.9
83.8
72.5
58.5
79.4
41.6
92.8
80.2
67.4
82.1
4.5
3.7
7.3
6.1
5.8
27.3
0.8
3.2
5.8
2.1
8.2
1.6
5.6
5.4
5.5
18.4
1.1
90/10
96/4
92/8
92/8
93/7
56/44
98/2
Co‐POM‐octyl‐NH
3
‐SBA‐15
‐SBA‐15
‐SBA‐15
Fe‐POM‐octyl‐NH
3
3.7
91/9
Cu‐POM‐octyl‐NH
3
15.3
4.5
87/13
92/8
3
Co‐POM ‐NH ‐SBA‐15
a
2 2
Reaction conditions: m (catalyst) = 20 mg, n (cyclohexene) = 5 mmol, V (MeCN) = 5 ml, n(H O ): n (cyclohexene) = 1.8:1, T = 65°C, t = 3 hr.
b
(
Epoxide + diol)/(2‐ol + 2‐one).
[2]
2 2
SCHEME 2 Mechanism of epoxidation of cyclohexene with H O
Co‐POM‐octyl‐NH ‐SBA‐15 > Fe‐POM‐octyl‐NH ‐SBA‐
thus an efficient diffusion of reactant molecules to active
sites was realized. Higher epoxide selectivity (92.8%) and
epoxidation/allylic oxidation selectivity (98/2) were
3
3
1
5 > Cu‐POM‐octyl‐NH ‐SBA‐15. Besides, Co‐POM‐
3
octyl‐NH ‐SBA‐15 showed better catalytic performance
3
than that of Co‐POM‐NH ‐SBA‐15. As there were no dis-
obtained over Co‐POM‐octyl‐NH ‐SBA‐15 catalyst, hence,
3
3
tinct differences in their textural properties, therefore, this
Co‐POM‐octyl‐NH ‐SBA‐15 was selected as the preferable
3
phenomenon indicated that the activity of Co‐POM‐octyl‐
catalyst to optimize the reaction conditions.
NH ‐SBA‐15 was obviously enhanced by a decrease of the
3
number of Si‐OH groups on the surface using octyl as the
modified agent. Si‐OH groups on the surface offered
centers for adsorption of organic compounds like epoxide
via hydrogen bonding. The diffusion to and away from the
active sites in the mesochannels was impeded, which
would decrease the overall catalytic activity of the
3.2.1 | Comparing catalytic efficiency of
different heteropolyacid‐based supported
catalysts
[45]
catalyst.
Converting partial Si‐OH into Si‐O‐Si‐octyl
Cyclohexene epoxidation to epoxide was selected as a
probe reaction to compare catalytic efficiency of different
heteropolyacid‐based supported catalysts, and the com-
parison was in terms of reaction condition, reaction time
and epoxide yield. As seen in Table 4, although other M‐
POM/POM‐based supported materials catalyzed the
reaction, they required higher reaction temperature and
increased the hydrophobicity and the adsorption effect
was reduced. The nanostructure of Co‐POM‐octyl‐NH3‐
SBA‐15 provided a suitable environment for highly active
epoxidation reaction, including a large specific interface
area, active center on the hydrophobic interface, as well
as the residual nanospaces in SBA‐15 mesochannels,

10 of 12
JIN ET AL.
3
TABLE 4 Comparison of catalytic efficiency of Co‐POM‐octyl‐NH ‐SBA‐15 with some different catalysts reported for cyclohexene
epoxidation
Reaction
time (hr)
Reaction
condition
Epoxide
yield (%)
No.
Catalyst
References
1
2
3
4
5
6
7
8
9
Co‐POM‐octyl‐NH
PW12/AS‐0.8‐400
Ti‐POM/MIL‐101
3
‐SBA‐15
3
4
MeCN/65 °C
MeCN/80 °C
MeCN/70 °C
MeCN/50 °C
t‐BuOH/80 °C
MeCN/50 °C
77.8
90
This work
[
[
[
46]
47]
38]
6
< 8.58
7.5
PW12/NH
2
‐SBA
3
i
[48]
Aerosil‐Ti (O Pr)
5% PW12/MIL‐101
PVMo‐MCM‐41
24
3
44.3
67.2
19.8
9.45
70
[
[
[
[
[
47]
49]
49]
50]
51]
15
15
12
6
C
C
2
2
H
H
4
Cl
Cl
2
2
/refluxed
/refluxed
PVMo‐MCM‐41‐NH
PVMo‐TiO
PTA/Si‐imid@Si‐MNPs
2
4
2
MeCN/refluxed
Cl /70 °C
1
0
C
2
H
4
2
85.5
longer reaction time to reach comparable catalytic perfor-
conversion and epoxide selectivity were 83% and 91%,
respectively, which were similar to those of a fresh one.
mance to Co‐POM‐octyl‐NH ‐SBA‐15 catalyst.
3
3
.3.1 | Hot filtration test
3
.3 | Reusability of Co‐POM‐octyl‐NH3‐
SBA‐15 catalyst
In order to check for any leaching of Co‐POM anions into
the solution during reaction, a hot‐filtration test was car-
ried out for the epoxidation reaction. Typically, cyclohex-
ene (5 mmol) was allowed to react with H O in MeCN
The reusability of Co‐POM‐octyl‐NH ‐SBA‐15 catalyst in
the cyclohexene epoxidation reaction was examined.
3
2
2
After six consecutive reaction runs, consistent catalytic
medium with 20 mg of Co‐POM‐octyl‐NH ‐SBA‐15. The
3
activity over Co‐POM‐octyl‐NH ‐SBA‐15 had been
reaction was carried out at 65°C for 3 hr, and then the
reaction mixture was immediately filtered under hot
conditions. At this stage, epoxide yield was 77.8%. Subse-
quently, the reaction was continued at 65°C for a further
3
detected, establishing the fact that Co‐POM‐octyl‐NH3‐
SBA‐15 could be recycled without any considerable loss
of activity (Figure 8). The average values of cyclohexene
3
7
hr. Nevertheless, no increase of epoxide yield beyond
7.8% was detected. This result clearly reconfirmed the
heterogeneous nature of Co‐POM‐octyl‐NH ‐SBA‐15.
3
After six reaction runs, Co‐POM content in Co‐POM‐
octyl‐NH ‐SBA‐15 was marginally decreased to
3
0.101 mmol/g, which was 6.48% lower than that of the
fresh one. These data revealed the fact that Co‐POM units
bonded to ‐NH groups in the mesochannels of SBA‐15
3
were purely heterogeneous in cyclohexene epoxidation.
Reused Co‐POM‐octyl‐NH ‐SBA‐15 catalyst (after the
3
sixth run) was further characterized via UV–Vis and
wide‐angle XRD techniques to study if any further
change occurred in Co‐POM‐octyl‐NH ‐SBA‐15 after
3
reaction. Co‐POM‐octyl‐NH ‐SBA‐15 exhibited similar
3
UV–Vis spectrum to the fresh one (Figure 9), suggesting
Co‐POM was still immobilized in the mesochannels of
SBA‐15 even after six reaction runs. Wide‐angle XRD
FIGURE 8 Reusability of Co‐POM‐octyl‐NH
Reaction conditions: m (Co‐POM‐octyl‐NH ‐SBA‐15) = 20 mg, n
): n (cyclohexene) = 1.8:1, V
MeCN) = 5 ml, T = 65°C, t = 3 hr
3
‐SBA‐15 catalyst.
pattern of the recovered Co‐POM‐octyl‐NH ‐SBA‐15
3
3
(
(
cyclohexene) = 5 mmol, n(H
2
O
2
(Figure 10) showed no characteristic peaks of Co‐POM,
indicating the Co‐POM anions were still highly dispersed

JIN ET AL.
11 of 12
selectivity of 92.8% and epoxidation/allylic oxidation
selectivity of 98/2. The order of catalytic activity was
Co‐POM‐octyl‐NH ‐SBA‐15 > Fe‐POM‐octyl‐NH ‐SBA‐
3
3
1
5 > Cu‐POM‐octyl‐NH ‐SBA‐15. Co‐POM‐octyl‐NH ‐
3
3
SBA‐15 containing active species surrounded via
hydrophobic alkyl groups in the mesochannels of SBA‐
15 showed better catalytic activity than that of octyl‐free
Co‐POM‐NH ‐SBA‐15 catalyst. The more hydrophobic
3
interface of SBA‐15 might accelerate diffusion of reac-
tants to catalysis center. Particularly, average values of
83% of cyclohexene conversion and 91% of epoxide selec-
tivity were obtained over Co‐POM‐octyl‐NH ‐SBA‐15 for
3
six reaction cycles. The superiority of the present study
lies in achieving higher selectivity for the desired
product epoxide, and suggests a prospective strategy for
novel modified materials to develop an organic–inorganic
cooperative function on the base of the nanostructure of
mesoporous silica.
FIGURE 9 UV–Vis spectra of fresh and reused Co‐POM‐octyl‐
NH ‐SBA‐15 catalyst
3
ACKNOWLEDGEMENTS
This work was supported by grants from Major Basic
Research Projects of Shandong Provincial Natural Science
Foundation (ZR2017ZC0632); National Natural Science
Foundation of China (NSFC21376128).
ORCID
Zhiguo Lv https://orcid.org/0000-0001-6792-0088
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