Aerobic epoxidation of styrene over Zr-based metal-organic framework encapsulated transition metal substituted phosphomolybdic acid

Article abstract of DOI:10.1016/j.mcat.2021.111552

Catalytic epoxidation of styrene with molecular oxygen is regarded as an eco-friendly alternative to producing industrially important chemical of styrene oxide (STO). Recent efforts have been focused on developing highly active and stable heterogeneous catalysts with high STO selectivity for the aerobic epoxidation of styrene. Herein, a series of transition metal monosubstituted heteropolyacid compounds (TM-HPAs), such as Fe, Co, Ni or Cu-monosubstituted HPA, were encapsulated in UiO-66 frameworks (denoted as TM-HPA@UiO-66) by direct solvothermal method, and their catalytic properties were investigated for the aerobic epoxidation of styrene with aldehydes as co-reductants. Among them, Co-HPA@UiO-66 showed relatively high catalytic activity, stability and epoxidation selectivity at very mild conditions (313 K, ambient pressure), that can achieve 82 % selectivity to STO under a styrene conversion of 96 % with air as oxidant and pivalaldehyde (PIA) as co-reductant. In addition, the hybrid composite catalyst can also efficiently catalyze the aerobic epoxidation of a variety of styrene derivatives. The monosubstituted Co atoms in Co-HPA@UiO-66 are the main active sites for the aerobic epoxidation of styrene with O₂/PIA, which can efficiently converting styrene to the corresponding epoxide through the activation of the in-situ generated acylperoxy radical intermediate.

Full text of DOI:10.1016/j.mcat.2021.111552

Molecular Catalysis 506 (2021) 111552
Contents lists available at ScienceDirect
Molecular Catalysis
journal homepage: www.journals.elsevier.com/molecular-catalysis
Aerobic epoxidation of styrene over Zr-based metal-organic framework
encapsulated transition metal substituted phosphomolybdic acid
a,
1
b,
1
a
a
a
a,
Dianwen Hu , Xiaojing Song , Hao Zhang , Xinyu Chang , Chen Zhao , Mingjun Jia *
a
Key Laboratory of Surface and Interface Chemistry of Jilin Province, College of Chemistry, Jilin University, Changchun, 130012, PR China
b
College of Biological and Chemical Engineering, Nanyang Institute of Technology, Nanyang, 473004, PR China
A R T I C L E I N F O
A B S T R A C T
Keywords:
Catalytic epoxidation of styrene with molecular oxygen is regarded as an eco-friendly alternative to producing
industrially important chemical of styrene oxide (STO). Recent efforts have been focused on developing highly
active and stable heterogeneous catalysts with high STO selectivity for the aerobic epoxidation of styrene.
Herein, a series of transition metal monosubstituted heteropolyacid compounds (TM-HPAs), such as Fe, Co, Ni or
Cu-monosubstituted HPA, were encapsulated in UiO-66 frameworks (denoted as TM-HPA@UiO-66) by direct
solvothermal method, and their catalytic properties were investigated for the aerobic epoxidation of styrene with
aldehydes as co-reductants. Among them, Co-HPA@UiO-66 showed relatively high catalytic activity, stability
and epoxidation selectivity at very mild conditions (313 K, ambient pressure), that can achieve 82 % selectivity
to STO under a styrene conversion of 96 % with air as oxidant and pivalaldehyde (PIA) as co-reductant. In
addition, the hybrid composite catalyst can also efficiently catalyze the aerobic epoxidation of a variety of
styrene derivatives. The monosubstituted Co atoms in Co-HPA@UiO-66 are the main active sites for the aerobic
Phosphomolybdic acid
Styrene
Epoxidation
Oxygen activation
Heterogeneous catalysis
2
epoxidation of styrene with O /PIA, which can efficiently converting styrene to the corresponding epoxide
through the activation of the in-situ generated acylperoxy radical intermediate.
1
. Introduction
Fortunately, these problems may be solved by introducing a
co-reductant such as aldehydes or amide-type solvents like DMF (N,
′
Styrene oxide (STO) is an essential organic intermediate for
N -dimethylformamide), that can enforce styrene epoxidation through
manufacturing pharmaceuticals and fine chemicals [1–16]. Recently,
interest is growing in developing an environmentally friendly route to
produce STO in order to replace the unfavorable styrene bromohydrin
route or styrene peracid oxidation route [5–9]. Catalytic epoxidation of
styrene with oxidants like tert-butyl hydroperoxide (t-BuOOH) [1,3,9],
an intermediate active oxygen species in presence of a suitable catalyst
[12,15,16]. Among the commonly used co-reductants, aliphatic alde-
hydes such as isobutyraldehyde (IBA) and pivalaldehyde (PIA) have
shown great advantages in facilitating the activation of molecular oxy-
gen at very mild conditions. Such oxygen/aldehyde epoxidation route,
the so-called “Mukaiyama” method, may turn to more significant
considering the fact that the byproduct of carboxylic acid (co-oxidation
of aldehyde) is also valuable in plastic and polymer industry [18]. So far,
some relatively active and selective heterogeneous catalysts for the
aerobic epoxidation of styrene with aldehyde as co-reductant have
already been reported, including mesoporous SBA-15 immobilized Mn
(III) porphyrin, mixed-valence {V16} clusters confined in a Ni-based
metal-organic framework [12,19]. Despite these progresses, more ef-
forts are still required for getting highly efficient and readily available
heterogeneous catalysts for the aerobic epoxidation of styrene.
hydrogen peroxide (H
2
O
2
) [4–8,10,11], or molecular oxygen (O
2
) is
thought to have great potential in green production of STO [2,12–16] .
Especially, direct utilization of molecular oxygen at the mild condition
would be more attractive if a highly efficient heterogeneous catalyst is
available.
Early attempts demonstrated that to achieve high STO selectivity is
very difficult due mainly to the highly active nature of the olefinic group
in the terminal position of styrene, which can be readily oxidized to
benzaldehyde or other byproducts [8]. Besides, molecular oxygen is
usually unreactive towards organic molecules at low temperature due
mainly to the spin-forbidden process (from triplet to singlet state) [17].
As an important catalyst system, heteropoly acid compounds (HPAs)
*
Corresponding author.
E-mail address: jiamj@jlu.edu.cn (M. Jia).
The two authors contributed equally to this work.
1
https://doi.org/10.1016/j.mcat.2021.111552
Received 9 January 2021; Received in revised form 18 March 2021; Accepted 25 March 2021
Available online 7 April 2021
2
468-8231/© 2021 Elsevier B.V. All rights reserved.

D. Hu et al.
Molecular Catalysis 506 (2021) 111552
have been widely used for various catalytic oxidation processes
including olefin epoxidation owning to their unique redox and acid
properties [20]. More interestingly, HPAs have shown great advantage
in fabricating highly active and stable single-site catalysts, such as
transition metal (TM) monosubstituted HPAs (TM-HPAs) and HPAs
stabilized noble metal (Pt, Rh) single-atom catalysts [21,22]. By
immobilizing various HPAs-based compounds onto some porous sup-
ports like silica, porous polymers, metal oxide, active carbon and
metal-organic frameworks (MOFs), a variety of active and selective
heterogeneous catalysts have been obtained for olefin epoxidation with
(60 mL). After stirring for 1 h, the mixture was transferred to a
Teflon-lined stainless-steel reactor and heated in an oven at 393 K for 24
h. After cooling down to room temperature, the blue precipitates were
filtered, then extracted with methanol in a Soxhlet apparatus for 24 h
and dried in vacuum at 363 K overnight. The obtained composites are
denoted as HPA@UiO-66 and TM-HPA@UiO-66 (TM = Fe, Co, Ni, and
Cu), respectively. The loading amount of HPA or TM-HPA in the hybrid
composites was determined by ICP-OES spectrometer as shown in
Table S1.
H
2
O
2
or tert-butyl hydroperoxide as the oxidant [23–27]. Notably, a few
2.3. Catalytic test
recent works demonstrated that supported HPAs catalysts may also have
the capability of activating molecular oxygen in the presence of a
In a typical catalytic reaction procedure, 2.0 mmol of styrene (or
styrene derivatives), 5.0 mmol of branched aliphatic aldehyde (PIA or
IBA), 5 mL of acetonitrile (solvent), and 25 mg of catalyst were added to
a 25 mL two-necked bottle equipped with a stirring bar and a reflux
co-reductant. For instance,
amino-modified MCM-41 could catalyze the oxidation of styrene with
/IBA, although the main product is benzaldehyde rather than STO
28]. Our previous work also suggested that amine-modified SBA-15
5 2
H PV Mo10O40 immobilized onto
O
2
[
condenser. O or air was provided using a balloon filled with specified
2
supported transition metal (Fe, Co or Cu) monosubstituted phospho-
molybdic acids are the efficient catalysts for the aerobic epoxidation of
olefins like cyclooctene, cyclohexene, and 1-octene in the presence of
IBA. However, when styrene is used as a reagent, the selectivity of
epoxide is very low (7%), and phenylacetaldehyde (77 %) turns to the
main product under the test condition [29].
gas. The mixture was stirred for the required time at a specific tem-
perature. The progress of the reaction was monitored by Shimadzu GC-
14C gas chromatograph equipped with a flame ionization detector (FID)
using a SE-54 capillary column (30 m ×0.25 mm ×0.25
μ
m). After the
reaction, the spent catalyst was isolated by centrifugation and washed
with acetonitrile and methanol repeatedly, dried overnight at 363 K
under vacuum.
Recently, the appearance of structural/chemical stable Zr-based
MOFs provides a great opportunity for developing novel hybrid com-
posites based on HPA and Zr-MOFs [30]. Significantly, some Zr-MOFs (e.
g., UiO-67 and UiO-66) encapsulated HPA composites with enhanced
structural stability and catalytic activity could be synthesized by
adopting direct solvothermal synthesis method. By using the similar
synthesis strategy, our group recently obtained a hybrid composite
based on polyoxomolybdic cobalt and Zr-MOF (UiO-bpy), which could
efficiently catalyze the aerobic epoxidation of olefins with t-BuOOH as
initiator, and a relatively high STO selectivity of 59 % could be achieved
for styrene epoxidation [31]. Lately, another hybrid catalyst composed
of Co-monosubstituted phosphomolybdic acid and UiO-66 was also
synthesize, which exhibit high catalytic activity and stability for the
epoxidation of various olefins with t-BuOOH as the oxidant [32].
Concerning the great advantages of UiO-66 in fabricating structur-
ally stable and catalytically active supported HPAs-based catalysts, we
here attempted to enlarge the synthesis and application scope of UiO-66
by encapsulating a series of TM (TM = Fe, Co, Ni and Cu) mono-
substituted phosphomolybdic acids inside its framework, hopefully may
generate more efficient and stable HPA-based heterogeneous catalysts
for the aerobic epoxidation of styrene with aliphatic aldehydes as co-
reductants. In addition, we also tried to clarify the nature of the cata-
lytically active sites, and to reveal the aerobic epoxidation mechanism
over the UiO-66 encapsulated TM monosubstituted HPA catalysts by
combining a variety of characterization results and the comparative
experiments on free radical scavenging activities.
3. Results and discussion
3.1. Catalyst characterization
3.1.1. Chemical composition, structure and morphology analysis
The XRD patterns of HPA@UiO-66 and TM-HPA@UiO-66 compos-
ites are illustrated in Fig. 1 The powder X-ray diffraction pattern of the
parent UiO-66 is in good agreement with the simulated one, indicating
the phase purity of this sample. It is found that the XRD patterns of the
HPA@UiO-66 and TM-HPA@UiO-66 composites are almost indistin-
guishable from that of pure UiO-66, suggesting that the introduction of
heterpolyacids does not affect the basic structure of the framework of
UiO-66. This result may also provide an initial evidence that the intro-
duced HPA or TM-HPA should be uniformly dispersed throughout the
hybrid composites since no evident reflections related to the HPA or TM-
HPA crystalline could be detected [27,31].
The SEM images depicted in Fig. 2 show that the morphologies of the
HPA@UiO-66 and TM-HPA@UiO-66 composites are quite identical with
that of UiO-66, appertaining to relatively uniform particles with an
irregular shape. These results confirm further that the structure and
morphology of the hybrid HPA@UiO-66 and TM-HPA@UiO-66 com-
posites maintain well, which is identified with the XRD results [27,32].
Fig. 3 shows the TEM images and the EDS mapping images of the
representative sample of Co-HPA@UiO-66. Clearly, the hybrid com-
posite is composed of irregular aggregates of nanoparticles, while no
obvious Co-HPA crystalline can be found in the images. In addition, the
energy-dispersive X-ray spectroscopy (EDX) mappings reveal that all the
elements (Zr, O, P, Mo, and Co) are uniformly distributed throughout the
hybrid composites, thus providing a direct evidence of the high disper-
sion of HPA or TM-HPA clusters in the UiO-66 frameworks (Fig. 3 c–g)
[27,31,32].
2
. Experimental
2
.1. Materials and instrumentation information
The full list of chemicals, reagents and instrumentation information
are presented in the Electronic Supporting Information (ESI).
Fig. 4 shows the nitrogen adsorption-desorption isotherms and the
pore size distributions of UiO-66 and Co-HPA@UiO-66. Both samples
exhibit the typical type I isotherms, demonstrating the presence of mi-
cropores. As listed in Table 1, the calculated BET specific surface area of
2
.2. Catalyst preparation
UiO-66 was synthesized according to the procedure reported in lit-
2
ꢀ 1
eratures [33,34]. The hybrid composites of TM-HPA encapsulated in
UiO-66 were synthesized following the previous literature procedure
UiO-66 is 1717 m
g
, more extensive than that of Co-HPA@UiO-66
). Besides, the pore volumes of these two ma-
composite (1598 m2
g
ꢀ 1
with some modification [32–34]. Typically, 0.46 g of ZrCl
4
, 0.33 g of
terials follow a similar trend, decreasing somewhat (from 0.85 to 0.79
3
ꢀ 1
terephthalic acid (BDC), 0.08 g of phosphomolybdic acid (HPA), or
metal-monosubstituted phosphomolybdic acid (TM-HPA, TM = Fe, Co,
Ni, and Cu) and 37 wt % hydrochloric acid (5 mL) were dissolved in DMF
cm g ) after introducing Co-HPA into the UiO-66 framework. The pore
size distributions calculated by non-local density functional theory
(NLDFT) method show that the hybrid composite has similar pore size
2

D. Hu et al.
Molecular Catalysis 506 (2021) 111552
Fig. 1. XRD patterns of (A) simulated UiO-66 and UiO-66; (B) HPA@UiO-66 and TM-HPA@UiO-66 composites.
Fig. 2. SEM images of (a) UiO-66, (b) HPA@UiO-66 composite, (c) Fe-HPA@UiO-66 composite, (d) Co-HPA@UiO-66 composite, (e) Ni-HPA@UiO-66 composite and
f) Cu-HPA@UiO-66 composite.
(
Fig. 3. (a,b) TEM images of Co-HPA @UiO-66 composite, (c‒g) EDS mapping images of Co-HPA@UiO-66 composite.
distribution to the parent UiO-66, suggesting that most of the cages/
channels of the MOFs are still open and accessible after the addition of a
portion of HPA and TM-HPA clusters (Fig. 4 inset).
present in Fig. 5. As shown in Fig. 5A, the spectrum of HPA reveals the
ꢀ 1
characteristic bands of phosphomolybdic acids, appearing at 1055 cm
ꢀ
1
ꢀ 1
(P–O
a
), 958 cm (Mo–O
d
, terminal oxygen), 866 cm (Mo–O
b
–Mo,
ꢀ 1
bridging oxygen), and 786 cm
(Mo–O –Mo, bridging oxygen),
c
3
.1.2. Spectral analysis
respectively [27,31]. The corresponding vibration bands of TM-HPAs
Fourier transform infrared (FT-IR) spectra of various samples are
(TM = Fe, Co, Ni, and Cu) shift slightly due to the distortion induced
3

D. Hu et al.
Molecular Catalysis 506 (2021) 111552
2
Fig. 4. N adsorption-desorption isotherms and pore size distribution (inset) of materials (A) UiO-66, (B) Co-HPA@UiO-66 composite.
by the partial substitution of Mo with the transition metal cations [26].
Fig. 5B shows the spectra of pure UiO-66, HPA@UiO-66 and
TM-HPA@UiO-66 composites. The prominent vibration bands appeared
Table 1
The textural parameters derived from N
6 and Co-HPA@UiO-66 composites.
2
adsorption-desorption profiles of UiO-
6
ꢀ 1
between 1300 and 1750 cm
are mainly attributed to the
Sample
BET surface area
Pore volume
Pore diameter
(nm)
–
C
-
–
Ostretching vibration and the COO symmetric stretch of the organic
2
ꢀ 1
3
ꢀ 1
(
m
g
)
(cm
g )
ꢀ 1
linkers, while the absorption peaks at 763 and 685 cm correspond to
the Zr-O bond in Zr-MOFs [35]. By comparing the spectrum of
Co-HPA@UiO-66 with the precursor of Co-HPA, it could be seen that the
UiO-66
1717
1598
0.85
0.79
2.0
2.0
PMo11Co@UiO-66
Fig. 5. FT-IR spectra of (A) HPAs and (B)HPA@UiO-66 composites.
Fig. 6. (A) UV–vis spectra of HPAs in CH
3
CN; (B) DR-UV–vis spectra of UiO-66 and HPA@UiO-66 composites, inset are the DR-UV–vis spectra of UiO-66 and Co-
HPA@UiO-66 composite.
4

D. Hu et al.
Molecular Catalysis 506 (2021) 111552
characteristic peaks belonged to the Zr-MOF in the spectrum of
Co-HPA@UiO-66 composite are more prominent (Fig. S1). However, the
peaks corresponding to HPA could hardly be distinguished in the spec-
trum of the composite, possibly caused by the superimposition with the
framework, possibly originating from the multiple coordination in-
teractions between the oxygen atoms residing outside of the HPA anions
and the coordinatively unsaturated Zr4 located in the defect sites of
UiO-66 [21,22,30]. The fact that no significant Raman signals belonged
to HPAs in the spectrum of Co-HPA@UiO-66 composite reflects the
relatively low HPAs loading in this hybrid composite (Fig. S2).
Fig. 8 displays the 31P MAS NMR spectra of Co-HPA, Co-HPA@UiO-
66 and the spent sample of Co-HPA@UiO-66. A sharp peak can be
observed at ꢀ 3.3 ppm for Co-HPA, which is the P atom’s characteristic
signal in HPA with Keggin structure [42]. For Co-HPA@UiO-66 com-
posite, the signal of P atom turns to broader, and shifts slightly to ꢀ 2.0
ppm, implying the Keggin structure of the HPAs remains well after the
formation of the hybrid composite. The downfield shift and the peak
broadening of 31P resonance reveal further the presence of interaction
between the HPAs units and the framework of UiO-66 [43].
+
ꢀ 1
vibration bands of the organic linkers in the region 700–1200 cm [35].
Fig. 6A shows the UV–vis absorption spectra of the precursors of HPA
and TM-HPAs dissolved in MeCN. For HPA, the strong absorption band
at ~215 nm should be attributed to the charge transfer from oxygen to
molybdenum of the Mo = O
d
bonds, and the weak band at ~309 nm
–Mo and Mo–O –Mo bonds in HPAs
should be assigned to the Mo–O
b
c
with the Keggin structure [36]. The samples of TM-HPAs (TM = Fe, Co,
Ni, and Cu) also exhibit similar spectra to that of HPA. A slight blue-shift
of the Keggin structural bands in the spectra could also be related to the
incorporation of transition metal into the primary structure of HPA,
which may result in a subtle change in the chemical environment of the
Mo species [37]. In the UV–vis spectra of UiO-66 and HPA@UiO-66
composites, two bands around 234 and 260 nm presented in the
3.1.3. Chemical state and surface analysis
spectra of all the samples are attributed to the adsorption of
π
-
π
* tran-
The surface composition and chemical states of the precursor of
HPAs, UiO-66 support, and the hybrid composites are investigated by X-
ray photoelectron spectroscopy (XPS) analyses (Fig. 9). As shown in
Fig. 9A, the Mo 3d XPS spectrum of HPA shows two significant signals at
sition of organic ligand and the ligand to metal charge transfer effect,
respectively (Fig. 6B). By carefully comparing the spectrum of UiO-66
with the spectra of the hybrid composites, using Co-HPA@UiO-66
composite as the representative sample (Fig. 6B inset), it can be seen
that an extra shoulder appears at around 300–400 nm in the spectrum of
Co-HPA@UiO-66, which could be attributed to the presence
Mo-containing species in the hybrid composites [38]. Moreover, the
broad absorption in the range of 600–800 nm may be corresponded to
the d-d transitions of MoV species, indicating that a portion of MoV
species exist in the hybrid HPA@UiO-66 and TM-HPA@UiO-66 com-
posites [33,34].
VI
VI
236.5 and 233.4 eV, which can be attributed to the Mo 3d3/2 and Mo
3d5/2, respectively [24,25]. The P 2p signal appeared at 134.4 eV is
The Raman spectra of HPAs and the hybrid composites are shown in
Fig. 7. In the case of HPA (Fig. 7A), the typical bands at 1003, 980, 916,
ꢀ
1
6
12, and 259 cm
could be assigned to
ν
s
(Mo–O
d
),
ν
as(Mo–O
d
),
ν
(Mo–O –Mo), (Mo–O
b
ν
s
c
–Mo), and (Mo–O
ν
s
a
), respectively [39]. The
corresponding vibration bands of TM-HPA (TM = Fe, Co, Ni, and Cu)
shift slightly compared to that of HPA, due mainly to the incorporation
of a partial of transition metal cations. For the support of UiO-66
(
Fig. 7B), the most prominent Raman band located at 1616 cm-1 is
attributed to the C–
–
C aromatic stretch in the BDC linker, while the
vibrational manifestation of the aromatic-ring in-plane bending
-
1
-1
appeares at 634 cm . The signal at 1144 cm is belonged to the
Csymmetric ring breathing, while the signal at 866 cm-1 is assigned
to the OH bending with some contribution from C Csymmetric
C
–
–
-
1
breathing. The doublet peaks observed at 1437 and 1454 cm are
related to the carboxylate group’s symmetric stretching in the BDC
linker [40,41]. The Raman bands of HPA@UiO-66 and
TM-HPA@UiO-66 composites show a slightly blue shift compared with
the corresponding precursor of UiO-66, indicating the existence of
relatively strong interaction between the HPA units and the UiO-66
3
1
Fig. 8.
P NMR spectra of Co-HPA, Co-HPA@UiO-66, and spent Co-
HPA@UiO-66 composites.
Fig. 7. Raman spectra of (A) HPAs and (B)HPA@UiO-66 composites.
5

D. Hu et al.
Molecular Catalysis 506 (2021) 111552
Fig. 9. (A) XPS profiles for the binding energies
of Mo 3d in HPA, Co-HPA, Co-HPA@UiO-66
and spent Co-HPA@UiO-66 composites; (B) XPS
profiles for the binding energies of P 2p in HPA,
Co-HPA, Co-HPA@UiO-66 and spent Co-
HPA@UiO-66 composites; (C) XPS profiles for
the binding energies of Co 2p in Co-HPA, Co-
HPA@UiO-66 and spent Co-HPA@UiO-66
composites; (D) XPS profiles for the binding
energies of Zr 3d in UiO-66, Co-HPA@UiO-66
and spent Co-HPA@UiO-66 composites.
attributed to the P species in HPA (Fig. 9B) [33,34]. For Co-HPA, no
significant changes are observed in the spectra of Mo 3d and P 2p,
indicating that the substitution of a small portion of Mo atoms with Co
atoms has no noticeable effect on the oxidation state and coordination
environment of Mo and P atoms in HPA. To further investigate the
chemical state of the representative sample of Co-HPA@UiO-66, the EPR
measurements were also carried out. As shown in Fig. S3, a significant
to HPAs units [33,34].
3
.2. Aerobic epoxidation of styrene
The catalytic properties of HPA@UiO-66 and TM-HPA@UiO-66 (TM
Fe, Co, Ni, and Cu) composites were firstly investigated for the
=
epoxidation of styrene with O
2
/PIA (Scheme 1). It should be mentioned
to
or air. Besides, the usage of co-reductants like
peak with the
g
=
1.90 was observed in the spectrum of
firstly that the epoxidation of styrene could not proceed if using N
replace the molecular O
2
Co-HPA@UiO-66 composite, suggesting that the partial reduction of
VI
V
Mo
to Mo indeed occurs during the synthesis process of
2
PIA is also essential in this process. Without adding PIA into the catalytic
system, almost no oxidant products could be detected after several hours
reaction (Fig. 10). The additional blank experiment shows that the sty-
rene could be converted to the corresponding epoxide with a 28 %
Co-HPA@UiO-66 composite, although most of the Mo species still exist
VI
in the state of Mo [34,44]. A similar phenomenon was also reported in
previous literature on studying the solvothermal synthesized
HPA@UiO-66 composite [33,34]. In Fig. 9C, the XPS spectrum of Co 2p
for the Co-HPA presents two prominent signals at around 795.5 and
2
conversion and 47 % selectivity of STO in 2 h in the presence of O /PIA
II
II
even without the addition of catalyst (Table 2 and Fig. S4). This should
be an indication that PIA could be easily oxidized by molecular oxygen
to generate corresponding active oxygen species under the test condi-
tions. Moreover, the support of UiO-66 is also catalytically active for the
reaction, 69 % conversion of styrene and 53 % STO selectivity could be
achieved after 2 h reaction. Previously, both computational and exper-
imental studies revealed that a large number of structure defects, which
are originated from the missing linker molecules in the frameworks of
Zr-MOFs, may also act as the catalytically active sites for activating
7
80.3 eV, which could be ascribed to Co 2p1/2 and Co 2p3/2, respec-
tively [9,45]. The characteristic signals appeared at 184.4 and 182.0 eV
Fig. 9D) can be mainly attributed to Zr 3d3/2 and Zr 3d5/2 in UiO-66
(
framework [33,34]. Compared with the spectra of Co-HPA and
UiO-66, the hybrid composite of Co-HPA@UiO-66 exhibits a slight
decrease in the binding energies of P 2p, Mo 3d, and Co 2p, accompa-
nying with a small increase in the binding energy of Zr 3d. These results
provide an strong evidence that multiple interactions should be exist
between the framework of UiO-66 and the encapsulated HPAs clusters,
which can lead to the partial electron transfer from UiO-66 frameworks
molecular O
2
and aldehyde [40,41]. This might be the main reason why
the support of UiO-66 shows a moderate catalytic activity for the
Scheme 1. Oxidation products distribution of Styrene.
6

D. Hu et al.
Molecular Catalysis 506 (2021) 111552
exhibit higher STO selectivity than that of UiO-66 support and the
HPA@UiO-66 when a similar styrene conversion (85 %ꢀ 87 %) is
reached through adjusting the reaction time. The above results suggest
that the substitution of a small amount of Mo in HPA with transition
metal atoms like Co, Ni, Cu may be beneficial for the improvement of
both the catalytic activity and the selectivity to STO in the aerobic
epoxidation of styrene.
It should be mentioned that the unsupported Co-HPA exhibit a
higher activity than Co-HPA@UiO-66, as also given in Table 2. This
should be due mainly to the fact that the unsupported Co-HPA is
essentially a homogeneous catalyst for the epoxidation reaction, and the
dissolved active sites are more easily accessible to the reagents.
Apparently, the confinement of Co-HPA inside the size-matched cage of
UiO-66 could prominently decrease the accessibility of Co-HPA clusters
to the reagents due to the diffusion limitation, thus leading to the
decrease in catalytic activity to a certain extent. Besides, the epoxide
selectivity of Co-HPA is 71 %, just slightly higher than that of Co-
HPA@UiO-66 (65 %), suggesting that the confinement of Co-HPA
units in UiO-66 does not bring obvious effect on the selectivity of the
catalyst.
Fig. 10. Styrene oxidation in different atmosphere. Reaction condition: styrene
Styrene oxidation was also carried out over Co-HPA@UiO-66 by
2
.0 mmol, PIA 5.0 mmol, Co-HPA@UiO-66 composite 25 mg, acetonitrile 5 mL,
balloon, reaction temperature 313 K.
O
2
2
using air to replace molecular O . As shown in Fig. 10, when using air as
the oxygen source, it requires about 9 h to get a 96 % conversion of
styrene, indicating the reaction rate decreases obviously with the
reduction of oxygen concentration. Notably, the selectivity of STO in-
creases by 15 % from 67 % to 82 % after replacing molecular oxygen
with air, suggesting that decreasing oxygen concentration may generate
a positive effect on improving the STO selectivity (Table 3 and Table S3).
The resultant 82 % selectivity of STO is comparable with the most se-
lective heterogeneous Mukaiyama epoxidation catalysts like Cu@UiO-
Table 2
Styrene oxidation performance with different catalysts using PIA as co-reductant
a
.
Sel. (%)
Entry
Catalyst
Con. (%)
STO
Bza
Pha
1
2
3
4
5
6
7
Blank
28
69
70
80
77
76
54
99
47
53
54
65
66
62
60
71
53
44
42
31
30
34
31
24
0
3
4
4
4
4
9
5
6
6(2COOH) and LDH-[Mn(Cl)salen] ever reported in literatures [48,
49].
Besides PIA, the reagent of IBA is also a kind of common branched
aliphatic aldehyde, which has been widely used in Mukaiyama epoxi-
dation reactions [46,47]. By using O /IBA, a relatively high activity
could also be achieved over Co-HPA@UiO-66 under the test conditions
Table 3 Entry 6–8). With increasing the reaction temperature, both the
UiO-66
HPA@UiO-66
Co-HPA@UiO-66
Ni-HPA@UiO-66
Cu-HPA@UiO-66
Fe-HPA@UiO-66
Co-HPA
2
b,c
8
(
a
Reaction condition: styrene 2.0 mmol, PIA 5.0 mmol, catalyst 25 mg,
balloon, reaction temperature 313 K, and reaction time 2 h.
Reaction condition: styrene 10.0 mmol, PIA 25.0 mmol, catalyst 8 mg,
reaction rate and the selectivity of STO increase considerably, and a 71
% selectivity of STO could be obtained at 338 K after 4 h reaction, which
is much higher than the previously reported amino-modified SBA-15
supported Co-HPA (Table 3) [29]. Under the similar reaction conditions,
the SBA-15 supported Co-HPA catalyst exhibited very high selectivity to
Pha (77 %), implying that the presence of a number of basic amino
groups on the surface of SBA-15 support should be responsible for the
formation of isomerization product of Pha. Previously, it was reported
that the epoxides could be easily transferred to the corresponding al-
dehydes, ketones, alcohols and ethers over various acidic or basic cat-
alysts, such as silica-alumina gels, Nafion-H and zeolites [53–55].
Therefore, it seems reasonable to propose here that the surface
acidic-basic property of support is also a very important factor in
influencing the product selectivity of the supported Co-HPA catalyst for
the aerobic epoxidation of olefins.
acetonitrile 5 mL, O
b
2
2
acetonitrile 25 mL, O balloon reaction temperature 313 K, and reaction time 2
h.
c
Homogeneous state.
2
epoxidation of styrene with O /PIA.
As shown in Table 2 and Fig. S4, the hybrid composite of HPA@UiO-
6 exhibits a similar catalytic activity to the support of UiO-66, implying
6
that the introduced HPA units are nearly inactive for this process.
Notably, the three transition-metal monosubstituted HPA encapsulated
catalysts of TM-HPA@UiO-66 (M = Co, Ni, and Cu) exhibit relatively
high catalytic activity and STO selectivity. An 80 % conversion of sty-
rene and a 65 % selectivity of STO could be obtained after 2 h reaction
when using Co-HPA@UiO-66 as the catalyst. With further extending the
reaction time to 3 h, a 99 % styrene conversion could be reached with a
similar STO selectivity (67 %). Compared with Co-HPA@UiO-66 com-
posite, the hybrid composites of Ni-HPA@UiO-66 and Cu-HPA@UiO-66
show a slightly lower conversion of styrene with similar epoxide selec-
tivities (66 % and 62 %). It is unexpected to see that Fe-HPA@UiO-66
composite shows lower catalytic activity than other TM-HPA@UiO-66
catalysts, even lower than the support of UiO-66. This might indicate
that Fe species can strongly coordinate with acyl peroxy acid to form
relatively stable complexes, thus generating obvious inhibition role in
the further oxidation of olefins [29]. In order to eliminate the influence
of reaction equilibrium, the product selectivities of the representative
catalysts were also compared under a similar conversion of styrene. As
shown in Table S2, all the composite catalysts containing TM-HPAs still
The recyclability of Co-HPA@UiO-66 was examined through several
recycling runs. As shown in Fig. S5, the catalytic activity and the STO
selectivity remain constant for ten catalytic cycles, indicating the su-
perior recyclability of the composite catalyst. The structure stability of
Co-HPA@UiO-66 could be further confirmed by additional character-
ization results of the spent catalyst. As shown in Fig. S6–S9 and Fig. 9,
the results of XRD, FT-IR, DRUV-vis, Raman and XPS demonstrate no
significant difference in the spectra between the fresh and the spent Co-
ꢀ 1
HPA@UiO-66 catalysts. The appearance of weak vibration at 2974 cm
in the FT-IR spectrum of the spent catalyst should be attributed to the
existence of a small number of reactants or products adsorbed on the
surface of the spent Co-HPA@UiO-66 catalyst.
Based on the above characterization results and the related literature
[
30], the excellent stability of Co-HPA@UiO-66 composite might be
7

D. Hu et al.
Molecular Catalysis 506 (2021) 111552
Table 3
a
Styrene oxidation performance of various catalyst samples with different aldehyde
.
Entry
Catalyst
Atmosphere
Aldehyde
n(Aldehyde)/
Temperature
K)
Time
(h)
Con.
(%)
Sel.
(%)
Related
Work
n(Styrene)
b
(
1
2
Co-HPA@UiO-66
Co-HPA@UiO-66
Cu@UiO-66(2COOH)
Cu@UiO-66(2COOH)
LDH-[Mn(Cl)salen]
Co-HPA@UiO-66
Co-HPA@UiO-66
Co-HPA@UiO-66
PMo11Co/SBA-15
Oxygen
Air
PIA
PIA
PIA
PIA
PIA
IBA
IBA
IBA
IBA
2.5
2.5
2.0
2.0
2.0
2.5
2.5
2.5
2.0
313
313
313
323
298
318
328
338
308
3.0
9.0
6.0
5.0
6.0
8.0
6.0
4.0
1.0
99
96
99
99
71
43
91
94
89
67
82
50
80
88
44
48
71
7
This work
This work
[46]
c
3
Oxygen
Oxygen
Oxygen
Oxygen
Oxygen
Oxygen
Oxygen
c
4
c
[46]
5
[27]
6
7
8
This work
This work
This work
[27]
c
9
a
Reaction condition: styrene 2.0 mmol, Co-HPA@UiO-66 composite 25 mg, acetonitrile 5 mL, O
Selectivity of STO.
2
/ Air balloon.
b
c
Specific experimental conditions see related work.
Table 4
Styrene derivative oxidation performance of Co-HPA@UiO-66 composite using PIA as co-reductant
a
.
Sel. of
(%)
substrate
Time
(h)
Con. of
Entry
1
1 (%)
2
3 (%)
4 (%)
5 (%)
b
1
3.0
99
52
19
29
—
1
1
1
1
1
1
1
a
b
c
d
e
f
b
2
3.0
3.0
3.0
97
96
98
89
28
16
11
32
24
—
21
18
—
19
42
c
3
4
d
5
6
3.0
4.0
54
86
50
51
42
41
8
8
—
—
7
8
5.0
3.0
98
95
51
47
41
50
8
3
—
—
9
4.0
3.0
4.0
98
93
98
46
53
53
49
39
40
5
8
7
—
—
—
1
1
0
1
g
1
1
2
3
4.0
4.0
99
98
50
36
46
64
4
—
—
1
1
h
i
—
a
b
c
Reaction condition: styrene derivative 2.0 mmol, PIA 5.0 mmol, Co-HPA@UiO-66 composite 25 mg, acetonitrile 5 mL, O
2
balloon, reaction temperature 313 K.
Oxidation products distribution of 1a and 1b see Scheme S1.
Other byproducts include 4-methylbenzoic and 4-methylphenylacetic.
d
Other byproducts include 4-tert-butylbenzoic and 4-tert-butylphenylacetic.
8

D. Hu et al.
Molecular Catalysis 506 (2021) 111552
mainly attributes to the following reasons: First, substitution of Co
atoms with the Mo atoms in HPA anions could provide a relatively stable
coordination environment for the monsubstitued Co cations; Second, the
support of UiO-66 is composed of a kind of octahedral cage (1.1 nm in
diameter) connected with a smaller triangular window (0.8 nm), which
can fit well the size of the HPA anions (about 1.0 nm), thus could provide
an effective protection to the encapsulated Co-HPA clusters against
leaching; Moreover, the existence of relatively strong metal-support
interaction between the Co-HPA clusters and the framework of UiO-66
should also play critical role in stabilizing the incorporated Co-HPA
anions [21,22,30].
3
.3. Aerobic epoxidation of styrene derivatives
The catalytic properties of Co-HPA@UiO-66 catalyst were further
investigated for various styrene derivatives under the similar reaction
conditions. As shown in Table 4, the styrene derivatives could be
2
oxidized to epoxide and other side-products with O /PIA over the
composite catalyst. For 4-methyl styrene, -methyl styrene, trans-
α
β-methyl styrene, the presence of electron-donating methyl has no
obvious effect on the converting rate of the reactions (Table 4 Entry
Scheme 2. The possible reaction path of the selective styrene oxidation over
TM-HPA@UiO-66 catalysts. For simplicity, the benzene ring has been omitted
in in the part of the mechanism.
1
–3). However, the epoxidation selectivity changes considerably with
the change of the substituted position of the methyl group. The highest
epoxide selectivity of 89 % could be obtained for the oxidation of trans-
β-methyl styrene, while the side products of aldehydes (2-phenyl-
propanal) and ketone (acetophenone) are more easily formed for the
scavenger in control experiments. As shown in Fig. 11A, the addition of
t-BuOH or PBQ has no obvious inhibition effect on the reaction process.
When TEMPO was added to the reaction system at the beginning, no
significant conversion of styrene was detected (Fig. 11B). Besides, when
TEMPO was added during the reaction term, the reaction almost stopped
immediately. These results clearly demonstrate that acyl free radical is
indeed present in the reaction system.
oxidation of -methyl styrene. Besides, deep oxidation is preferred to
α
occur when 4-methyl styrene is used as substrate, showing the formation
of the carboxylic acid products such as 4-methyl benzoic acid. When
using the more substantial electron-donating group of tert-butyl to
replace the methyl group on the aromatic ring, 4-methyl phenyl acetic
acid turns to the main product of the reaction (Table 4 Entry 4) [50].
Styrene derivatives with electron-withdrawing group were also
tested for the aerobic epoxidation reaction. Compared with styrene,
relatively low reaction rate and epoxide selectivity are detected for 4-
fluoro styrene, 4-chloro styrene, and 4-bromo styrene (Table 4 Entry
Besides, when epoxides were observed to be created as the major
2
products in the reactions of O /aldehyde in the presence of transition
metal monosubstituted heteropolyacid compounds, high-valent metal
oxo complexes might be formed, which should act as an active inter-
mediate for oxygen atom transfer. Based on our present results and the
related literatures [46,47,51,52], it can be concluded that the acylper-
oxy path should be the main reaction route for the epoxidation of sty-
5
–11), which might be explained by the fact the electron-withdrawing
group can reduce the electron density of the olefinic group and retard
the formation of epoxide product. By comparing the oxidation proper-
ties of 4-chloro styrene, 3-chloro styrene, and 2-chloro styrene (Table 4
Entry 912,13 and Fig. S10), it was found that the oxidation product
distributions change obviously with the change of the substituted posi-
tion of Cl. For the oxidation of 2-chloro styrene, the main oxidant
product is chloro-substituted benzaldehyde rather than epoxide, which
is different from that of 3-chloro styrene and 4-chloro styrene (with
relatively high epoxide selectivity). Considering the fact that the chlo-
rene with
mechanism is proposed as described in Scheme 3. First, acyl radical
generated via an initiation step was easily oxidized by O to form
2
O /PIA over Co-HPA@UiO-66 catalyst. A plausible
2
acylperoxy radical. Then, the acylperoxy radical was activated by
Co-HPA@UiO-66 to generate Co-oxo complexes I. Subsequently, styrene
molecule attacked the complex to form the intermediate II. Finally, the
unstable intermediate II could either result in the epoxide of STO, or was
attacked by another acylperoxy radical to generate the byproduct of
benzaldehyde.
–
rine atom in 2-chloro styrene is closer to C C double bond, a steric
–
effect of the substituent might be also play a key role in influencing the
epoxidation selectivity of the styrene derivatives [50].
The above results reveal that the monosubstituted Co species in Co-
HPA@UiO-66 composite should be the main active sites for the aerobic
2
epoxidation of styrene with O /PIA, which is quite favorable for the
3
.4. Recognition of active sites and understanding of proposed
formation of epoxide of STO. Concerning the fact that using air to
replace molecular oxygen is more advantageous for improving the
selectivity of STO, it seems reasonable to propose that decreasing oxy-
gen concentration to a certain extent may considerably decrease the
concentration of acylperoxy radicals, thus reducing the possibility to
generate deeply oxidized product of benzaldehyde, meanwhile,
increasing the selectivity to styrene oxide.
mechanisms
For transition-metal-catalyzed Mukaiyama epoxidation reactions,
two main mechanistic pathways have been proposed in the related lit-
eratures [46,47,51,52]. As shown in Scheme 2, the first pathway reveals
a radical chain mechanism involving acylperoxy radicals as the inter-
mediate species of oxidant (path 1). The second pathway consists of the
formation of peroxy acid intermediate (path 2). Additionally, some
•
•
ꢀ
4. Conclusion
other types of radicals such as OH and O have also been suggested as
2
possible active intermediates for the aerobic epoxidation of olefins [19].
To further understand the reaction mechanism of the present catalysis
system, several free radical scavengers, such as tert-butyl alcohol
In summary, a series of transition metal monosubstituted hetero-
ployacid compounds encapsulated in UiO-66 frameworks were synthe-
sized for the catalytic application in the aerobic epoxidation of styrene
with aldehyde as co-reductant. Among them, Co-substituted composite
showed exceptional catalytic activity, stability and epoxidation
(
t-BuOH), 1,4-benzoquinone (PBQ) and 2,2,6,6-tetramethyl-1-piperidi-
nyloxy (TEMPO) were separately introduced into the reaction system
•
•
ꢀ
to specifically inhibit the generation of OH, O and acyl free radical
2
9

D. Hu et al.
Molecular Catalysis 506 (2021) 111552
Fig. 11. Time-resolved studies of Co-HPA@UiO-66 composite catalyzing performance in the presence of different scavengers. Reaction condition: styrene 2.0 mmol,
PIA 5.0 mmol, Co-HPA@UiO-66 composite 25 mg, acetonitrile 5 mL, reaction temperature 313 K. The ratio of scavengers: styrene is 1: 50.
Acknowledgments
Financial support from the National Natural Science Foundation of
China (No. 21173100) is gratefully acknowledged.
Appendix A. Supplementary data
Supplementary material related to this article can be found, in the
online version, at doi:https://doi.org/10.1016/j.mcat.2021.111552.
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