Binuclear molybdenum Schiff-base complex: An efficient catalyst for the epoxidation of alkenes

Article abstract of DOI:10.1016/j.mcat.2019.110498

Dimolybdenum Schiff-base complex (DMSBC) which contain O,N-bidentate ligand was synthesized and characterized. The complex can be used as a promising catalyst in the epoxidation of olefins with tert-butyl-hydroperoxide (TBHP) as oxidant. The catalytic activity of the DMSBC was optimized by adjusting various parameters. Epoxidation of olefins by DMSBC indicated that the catalyst exhibited superb catalytic activity with high conversions up to 99.6%, high selectivity up to 100%, and high turnover frequency (TOF) of 208 h^?1. Kinetic analysis provided that epoxidation of cyclooctene by DMSBC possessed moderate activation energy (95.3 ± 2 kJ·mol^?1). Furthermore, the calculated pre-exponential factor (A) proved that strong collisions probability take place in the reaction. Additionally, the recycle experiments demonstrated that DMSBC could be recovered and repeatedly applied.

Full text of DOI:10.1016/j.mcat.2019.110498

MolecularCatalysis475(2019)110498
Contents lists available at ScienceDirect
Molecular Catalysis
journal homepage: www.elsevier.com/locate/mcat
Binuclear molybdenum Schiff-base complex: An efficient catalyst for the
epoxidation of alkenes
Yingxiong Guo1, Longqiang Xiao1, Pan Li, Wenhong Zou, Wenzhe Zhang, Linxi Hou⁎
College of Chemical Engineering, Fuzhou University, Xueyuan Road No. 2, Fuzhou 350116, China
A R T I C L E I N F O
A B S T R A C T
Keywords:
Dimolybdenum Schiff-base complex (DMSBC) which contain O,N-bidentate ligand was synthesized and char-
acterized. The complex can be used as a promising catalyst in the epoxidation of olefins with tert-butyl-hy-
droperoxide (TBHP) as oxidant. The catalytic activity of the DMSBC was optimized by adjusting various para-
meters. Epoxidation of olefins by DMSBC indicated that the catalyst exhibited superb catalytic activity with high
conversions up to 99.6%, high selectivity up to 100%, and high turnover frequency (TOF) of 208 h−1. Kinetic
analysis provided that epoxidation of cyclooctene by DMSBC possessed moderate activation energy
Dimolybdenum complex
Olefin epoxidation catalyst
Binuclear Schiff-base
Recyclable catalyst
(95.3
2 kJ·mol−1). Furthermore, the calculated pre-exponential factor (A) proved that strong collisions
probability take place in the reaction. Additionally, the recycle experiments demonstrated that DMSBC could be
recovered and repeatedly applied.
1. Introduction
used for the epoxidation of olefins and presented excellent catalytic
activity with high yields and high selectivity [19]. It is noteworthy that
Epoxide is one of the worthiest chemical intermediates for produ-
cing various important industrial products, such as surfactants, epoxy
resins, lubricating oils, textiles, cosmetics, surface coatings, corrosion
protection agents, pesticides, detergents, sweeteners, chiral pharma-
ceuticals, perfumes, and so on [1,2]. One of the most efficient method
to produce epoxides is the epoxidation of alkenes. Epoxides were ob-
tained with high yields and high selectivity by the assistant of hetero-
geneous/homogeneous catalysts or stoichiometric amounts of per-acids
[3–6]. However, per-acids can induce environmental pollution and re-
quire high cost. In order to shrink the extravagant use of large amounts
of oxidants, such as per-acids and sodium hypochlorite, the catalysts
based on transition metal have been extensively developed [7–12].
As a sustainable world development, the awareness of environ-
mental protection is increasing. The use of nontoxic, and resource-rich
metals, such as molybdenum, has attracted more and more attentions.
In the industry, promising economic effect has been obtained by the
production of propylene oxide with molybdenum as well as alkyl hydro-
peroxides as oxidants (named Halcon-Arco process) [13]. Furthermore,
various kinds of molybdenum-base catalysts have been developed as
heterogeneous catalysts in epoxidation reaction. One typical example is
the usage of [Mo(CO)6] and alkyl peroxide [14–18]. Besides, the cat-
alyst consisted of molybdenum and various kinds of ligands has been
the complexes consisted of Schiff-base ligands and molybdenum are
highly efficient homogeneous/heterogeneous catalysts [20–24]. Re-
cently, the epoxidation of terminal olefins are accomplished by mo-
lybdenum Schiff-base complexes. According to previous works, most
molybdenum Schiff-base complexes are efficient homogeneous cata-
lysts. Generally, homogeneous catalysts could be used under mild re-
action conditions [25–27]. However, the drawbacks of homogeneous
catalyst, including unrecyclable ability and poor stability, urgently
need to be solved. Heterogeneous catalysts are gained much attention
compared to homogeneous catalysts since they could be easily sepa-
rated and recycled from reaction mixture, which makes them play a
critical role in the development of environmentally friendly and sus-
tainable processes [28–31]. Mahdi Mirzaee al. used boehmite nano-
particles (BNPs) anchor Schiff base to generate molybdenum com-
plexes, which could transform unreacted cis-cyclooctene to epoxy
cyclooctane with good catalytic performance [32]. Maryam Zare al.
immobilized molybdenum (VI) complex onto Fe3O4/SiO2 as hetero-
geneous catalyst, and had realized the catalyst recycle from the mixture
[33]. Behnam Babaei al. had prepared the catalyst by tethering a mo-
lybdenum (VI) Schiff-base complex via post-synthesis modification of
MnFe2O4 nanoparticles and the catalyst was recycled and reused sev-
eral times [34]. Nevertheless, those methods may lead to the loss of
⁎ Corresponding author.
E-mail address: lxhou@fzu.edu.cn (L. Hou).
1 These authors contributed equally to this work and should be regarded as co-first authors.
https://doi.org/10.1016/j.mcat.2019.110498
Received 30 April 2019; Received in revised form 24 June 2019; Accepted 1 July 2019
2468-8231/©2019ElsevierB.V.Allrightsreserved.

Y. Guo, et al.
MolecularCatalysis475(2019)110498
catalyst activity. Therefore, it is eagerly desirable but still highly
challenging to develop novel heterogeneous catalysts with high cata-
lytic efficiency.
mixture of 3,3-diaminobenzidine (1 mmol) and MeOH (25 mL) under
continuous stirring at 80 °C, and then the brown mixture was refluxed
for 24 h until a deep orange to red mixture was obtained. The rude
product was collected by centrifugal separation. Subsequently, the or-
ange product was obtained and recrystallized from MeOH for several
times, then the product was collected by centrifugal separation and
dried (Yield:87%). 1HNMR (500 MHz, CDCl3, TMS) = 1.23 (t, 24H,
CH3), 3.44 (m, 16H, CH2), 6.26 (t, 8H, CH), 7.02 (m, 4H, CH), 7.07 (d,
2H, CH), 7.22 (t, 4H, CH), 8.50 (s,4H, CH), 13.48 (s, 4H, OH). 13C NMR
(151 MHz, CDCl3) δ 164.70 (d, J = 16.9 Hz), 160.96 (s), 160.26 (s),
151.98 (d, J = 10.4 Hz), 142.02 (d, J = 133.0 Hz), 138.31 (s), 133.98
(d, J =27.7 Hz), 124.42 (s), 119.34 (s), 117.49 (s), 109.61 (d, J
=7.5 Hz), 103.76 (s), 98.22 (s), 44.59 (s), 12.78 (s). MS(ESI): m/
z = 915.528 [M+H]+. C56H66N8O4 (914.52): calcd. Anal calcd for
In this work, a novel dimolybdenum Schiff-base complex (DMSBC)
was obtained by the reaction of MoO2(acac)2 and a binuclear Schiff-
base ligand (H2L). The DMSBC acted as heterogeneous catalyst in the
epoxidation of some cyclic and terminal olefins. The result revealed
that DMSBC exhibited excellent catalytic efficiency in the epoxidation
of various linear and cyclic alkenes. Moreover, the kinetic analysis of
catalytic oxidation was investigated to determine the activation energy
of catalyst. Besides, DMSBC exhibited poor solubility at ambient tem-
perature in some organic solvents. In addition, it could be separated
from the reaction mixture mainly by filtration and centrifugation, and
successfully reutilized at least three times without significantly redu-
cing the catalytic efficiency, and could be use six times still have the
catalytic efficiency.
C
56H66N8O4: C, 73.49; H, 7.27; N, 12.24. Found: C, 73.35; H, 7.28; N,
12.26.
2.3. Synthesis of symmetrical dimolybdenum Schiff-base complex (DMSBC)
2. Materials and methods
MoO2(acac)2 (2.2 mmol) and binuclear Schiff-base ligand (H2L)
(2 mmol) were mixed in MeOH (80 mL) under nitrogen atmosphere
(Scheme 1b). The solution rapidly turned to crimson and then was
stirred at 80 °C for 24 h. The MeOH was evaporated under vacuum and
crimson solid was obtained. The crimson solid was collected by cen-
trifugal separation and washed by MeOH for several times. The crimson
solid was obtained after dried in vacuum (Yield:82%). 1H NMR
(500 MHz, DMSO, TMS) = 1.10 (t, 12H, CH3), 3.42 (m, 8H, CH2), 6.04
(s, 2H, CH), 6.37 (d, 2H, CH), 6.84 (d, 1H, CH), 7.43 (t, 4H, CH), 8.81
(d,1H, CH), 9.22 (d, 1H, CH). 13C NMR (151 MHz, DMSO) δ 165.47-
165.08 (m), 164.33 (s), 163.87 (s), 154.16 (d, J =34.2 Hz), 152.14 (s),
136.92-136.65 (m), 134.54-134.26 (m), 115.12-114.69 (m), 112.23 (s),
111.68 (s), 99.36 (s), 97.39 (s), 96.38 (s), 44.55 (t, J = 17.7 Hz), 19.04
(s). Anal Calcd for C56H62Mo2N8O8: C, 57.63; H, 5.35; N, 9.60. Found:
C, 57.21; H, 5.23; N, 9.48.
2.1. Synthesis materials
All commercial grade solvents and chemical reagents were pur-
chased and used without further purification. Infrared spectra of solid
samples were recorded with a Nicolet iS50 fourier transform infrared
(FT-IR)spectrometer in the range of 4000–400 cm−1 using KBr-disk
method. 1H NMR spectra were observed on a Bruker Avance III
(500 MHz). The mass spectra were performed on Agilent LC-QTOF mass
spectrometer. Gas chromatography (GC) measurements were carried
out on a ShimadzuGC-2010 gas chromatograph. Elemental analysis was
performed on Vario EL cube. The X-ray photoelectron spectroscopy was
performed on ESCALAB 250.
2.2. Synthesis of O,N-bidentate Schiff-base ligand (H2L)
A typical process for the synthesis of symmetrical Schiff-base ligand
by the condensation of the amino group with the aldehyde group was
described (Scheme 1a) [35]. A mixture of 4-(diethylamino)salicy-
laldehyde (6 mmol) and MeOH (25 mL) was added drop-wise into a
2.4. General procedure for the epoxidation of alkenes
In a typical procedure, a mixture of catalyst (50 mg 0.04 mmol),
substrate (10 mmol), 1,2-dichloroethane (10 ml) and TBHP (30 mmol)
(Table 2) was stirred and heated to the appropriate temperature in oil
bath for pre-determined time. At the end of the reaction, the catalyst
was removed by centrifugation and the product was analyzed by gas
chromatography.
3. Results and discussion
3.1. The characterizations of free H2L and DMSBC
Fourier-transform infrared (FT-IR) spectroscopy curves of five
samples were shown in Fig. 1. The characteristic C–O stretching vi-
brations and C–N asymmetric stretching vibrations could be found in
the range of 1236 cm−1 and 1330 cm−1. A meaningful characteristic
band at 1620 cm−1 belonged to the vibration of C]N group, which
proved the successful reaction of imines groups with aldehydegroups
(Schiff base) [32]. As shown in Fig. 1d, a new peak at 904 cm−1 was
assigned to Mo = O stretching vibration of DMSBC comparing to
Fig. 1c, which may due to coordination of molybdenum to Schiff-base
ligand [36,37].
3.2. The X-ray photoelectron spectroscopy (XPS) of DMSBC
The XPS spectrum of DMSBC is shown in Fig. 2. From the XPS
survey spectrum (Fig. 2a), bands at 232.4 eV (Mo3d), 285 eV (C1s),
400.5 (N1s), 410 eV (Mo3p), and 529 (O1s) could be used to determine
the chemical composition of catalyst. The Mo3d core level spectrum
(Fig. 2b) shows the dominant Mo3d3/2 and Mo3d5/2 peaks at 232.6 eV
Scheme 1. Procedure for the synthesis of a: binuclear salophen ligand and b
dimolybdenum Schiff-base complex (DMSBC).
2

Y. Guo, et al.
MolecularCatalysis475(2019)110498
Scheme 2. Epoxidation of cyclooctene with DMSBC.
was illustrated in Scheme 2. The result (Fig. 3) described 92.0% con-
version and almost 100% selectivity within 1 h, revealing the catalyst
has high catalytic activity.
3.3.1. The influence of solvent on catalytic activity of DMSBC
The catalytic activity of DMSBC was investigated in non-polar, polar
aprotic, polar protic solvents and the solvents ordered by increasing
polarity. The highest activity of DMSBC was obtained in 1,2-di-
chloroethane (C2H4Cl2) (Table 1), which could be explained by the
theory of hard and soft acid base (HSAB) [36]. Coordinating solvents
such as acetonitrile (CH3CN), dimethylformamide (DMF) and ethanol
(EtOH) may compete with tert-butyl hydroperoxide (TBHP) to occupy
the coordination sites on the hard Mo(VI) ion, leading to the remark-
able reduction of the conversion. On the other side, the higher con-
version and selectivity were obtained in the non-coordinative solvents
such as 1,2-dichloroethane (C2H4Cl2) and chloroform (CHCl3), which
may be attributed to the coordination of TBHP to Mo (VI) center fol-
lowed by transfer of oxygen to alkenes [35].
Fig. 1. FT-IR spectra of (a) 4-(Diethylamino)salicylaldehyde (b) 3,3-
Diaminobenzidine (c) Binuclear Schiff-base ligand (H2L) (d) Fresh catalyst
(DMSBC) (e) Used catalyst (DMSBC) after six runs.
and 235.7 eV, which indicated that the Mo species are retained in the
+6 oxidation state in DMSBC. The major role of the molybdenum (VI)
ion is to withdraw electrons from the peroxidic-oxygens, making them
more susceptible to attack by nucleophiles such as olefins. The O1s core
level spectrum (Fig. 2c) could be deconvoluted into two bands: 530.79
for oxo-oxygen, and 532.17 for peroxo-oxygen respectively.
3.3.2. Proposed mechanism of the epoxidation of cyclooctene in the
presence of DMSBC
3.3. Study of the catalytic activity of DMSBC in the epoxidation of
cyclooctene
Although the mechanism of epoxidation of olefin is still con-
troversial, the mechanism about the coordination of TBHP to Mo (VI)
center resulted in the formation of MoO-H and MoO2-tBu moieties,
which was more possible [32,36,38]. The mechanism is shown in
The action scheme of the catalyst in the epoxidation of cyclooctene
Fig. 2. XPS survey spectra of (a) DMSBC (b) Core level spectra of Mo3d,and (c) Core level spectra of O1s.
3

Y. Guo, et al.
MolecularCatalysis475(2019)110498
Fig. 3. The catalytic activity of DMSBC in the epoxidation of cyclooctene.
Reaction conditions: catalyst (50 mg 0.04 mmol), cyclooctene (10 mmol), C2H4Cl2 (10 ml), TBHP(30 mmol) and 80°C reflux. Selectivity and yields were determined
by GC, Selectivity toward the corresponding epoxide.
Table 1
Table 2
The influence of solvent on catalytic activity of DMSBC (cyclooctene as sub-
The result of the epodxidation of cyclooctene by DMSBC with different amount
strate)a.
of TBHPa.
Entry Cat.(mg) solvent
T(oC) Time(h) Conversion (%) Selectivity (%)
Entry
Cat.(mg)
solvent
TBHP (mM)
Conversion (%)
Selectivity (%)
1
2
3
4
5
50
50
50
50
50
DMF
80
80
80
80
12
12
12
12
1
7.0
94.4
93.5
93.0
94.8
99.6
1
2
3
4
50
50
50
50
C2H4Cl2
C2H4Cl2
C2H4Cl2
C2H4Cl2
10
20
30
40
37.9
71.7
92.0
98.9
77.4
77.4
99.6
94.7
CH3CN
EtOH
CHCl3
44.5
64.3
75.6
92.0
C2H4Cl2 80
a
Reaction conditions: catalyst (50 mg 0.04 mmol), 10 mmol cyclooctene,
1,2-dichloroethane (10 ml), time (1 h) and reflux. Selectivity and yields were
determined by GC, Selectivity toward the corresponding epoxide.
a
Reaction conditions: catalyst (50 mg 0.04 mmol), cyclooctene (10 mmol),
solvent (10 ml), TBHP(30 mmol) and reflux. Selectivity and yields were de-
termined by GC, Selectivity toward the corresponding epoxide.
Scheme 3. Proposed mechanism of the epoxidation of cyclooctene in the presence of DMSBC and TBHP.
4

Y. Guo, et al.
MolecularCatalysis475(2019)110498
Table 3
Epoxidation of different alkenes in the presence of DMSBC.
Entry
Cat. (mg)
Time (h)
Alkenes
T (oC)
Conversion (%)
Selectivity (%)
TOF
1
2
3
4
5
6
7
none
50
50
50
50
10
12
1
12
5
cyclooctene
styrene
cyclooctene
octylene
cyclohexene
hexene
Norbornene
80
80
80
80
80
80
80
3.9
49.9
85.0
4.6
7.4
208
7.0
33.0
6.3
29.0
92.0(99.*)a
63.9
99.6
100
100
100
90.4
91.0(99.*)b
90.0
50
50
12
10
98.1
a
Complete conversion after 3 h, > 99%.
Complete conversion after 4 h, > 99%.
b
Fig. 4. (a) The conversion profiles of cyclooctene obtained at the indicated reaction temperature by using DMSBC. (b) Arrhenius plot of the epoxidation of cy-
clooctene with DMSBC and TBHP.
Scheme 3. The progress of reaction has been monitored by electronic
absorption spectroscopy (Fig S6, Supporting Information). A absorption
band observed at 230 nm is probably due to n to π transition of the
-C = N- and -C = O- of the ligand. The spectra of DMSBC sho wa band
at 410 nm, which assign to the ligand to metal(molybdenum atom)
charge transfer (LMCT) [39–42]. The addition of TBHP causes the de-
crease in intensities of the 410 band. The change along with the gen-
eration of an isosbestic point at 410 nm indicate the interaction of
catalyst with TBHP and the plausible formation of the corresponding
peroxido complex. Although the isolation of the real intermediates has
not been successfully achieved, the formation of relevant dioxidomo-
lybdenum (VI) complexes with TBHP to generate [Mo(VI)O(O2)]2+
complexes has been reported [43,44]. However, the addition of cy-
clooctene into the above solution does not obviously change the wa-
Fig. 5. Catalyst reusability in the epoxidation of cyclooctene with TBHP and
velengths of observable absorption peaks. Based on the obtained oxi-
dation products and experiments described above, a reaction pathway
including oxido-peroxido intermediates can be rationally proposed. The
catalyst is responsible for receiving and transferring oxygen atoms.
Firstly, the TBHP proton transfer oxy-complex resulting in the co-
ordination of TBHP to the molybdenum center. The α-O atom was then
transferred to the olefin to produce the epoxide (the major product)
DMSBC.
Reaction conditions: cyclooctene (10 mmol), TBHP (30 mmol), 1,2-di-
chloroethane (10 ml) refluxing 1 h. Selectivity and yields were determined by
GC, Selectivity toward the corresponding epoxide
Table 4
Catalytic activity of various catalysts for the epoxidation of cyclooctene.
Entry
1
Catalyst
Substrate
Solvent
C2H4Cl2
Time (h)
1
Conv. (%)
TOF (h−1
)
References
This work
[MoO2L]
Cyclooctene
92.0
(99.*a
100
97.0
94.0
208
2
3
4
[MoO2(Sal-Tryp)]
MoO2(acac)2@ UiO-66-NH2-SA
MoO2(acac)2@
Cyclooctene
Cyclooctene
Cyclooctene
CCl4
C2H4Cl2
C2H4Cl2
8
0.75
1
30.5
61.0
39.2
[36]
[37]
[37]
UiO-66-NH2-TC
5
6
Mo-Im-BNPs
Mo-A-BNPs
Cyclooctene
Cyclooctene
CCl4
CCl4
1
3.5
97.0
97.0
126
89.0
[31]
[31]
a
Complete conversion after 3 h, > 99%.
5

Y. Guo, et al.
MolecularCatalysis475(2019)110498
meanwhile releasing tert-butylalcohol (byproduct), yielding the initial
catalyst. The mechanism could interpret the high reaction speed of
electron-rich alkenes compared with electron-poor alkenes. Besides,
this mechanism suggest the addition of TBHP increase the conversion of
olefin, which agrees to our experiment results (Tables 2 and 3). Theo-
retical studies proved that the formation of hydrogen bond was ex-
tremely important during the reaction. Thus, not only the transfer of
oxygen, but also the transfer of hydrogen has the barrier. The produc-
tion of epoxy compounds has almost no barrier. In contrast, there is an
obvious energy barrier on the release of tert-butylalcohol (Table 2)
[38].
operation. The catalyst prepared in the epoxidation reaction of cy-
clooctene gained high conversion rate after 60 min of the first three
runs. The catalytic activity of the catalyst in the fourth to sixth run
showed suitable catalytic performance, but lower than the former three
runs. One conclusion could be drawn is that the DMSBC was recycled
used for the epoxidation of cyclooctene for three times without sig-
nificantly reducing the catalytic efficiency. On the other hand, very low
molybdenum species was detected in the filtrate for former two runs,
which was confirmed by ICP analysis (Fig S7, Supporting Information).
Moreover, no significant new peak can be found in FT-IR curve (Fig. 1e)
when compared to that of fresh catalyst (Fig. 1d), proving the catalyst
exhibited excellent stability. Small changes in signal intensity and po-
sition were probably due to overlapping or shifting of the band when
tert-BuOH, and cyclooctene oxide coexisted [37]. Therefore, these
stretching vibrations strongly confirm the retrievability and the stabi-
lity of the catalyst.
3.3.3. Epoxidation of alkenes with DMSBC
The DMSBC could also be used for the epoxidation of a wide range
of alkenes. The control experiment shown that the yields still extremely
low after ten hours reflux without catalyst (Table 3, Entry 1). On the
contrary, some cyclic and terminal olefins could be effectively and se-
lectively converted to epoxies in the presence of catalyst, which re-
vealed the catalyst has good catalytic activity. In the epoxidation of
endocyclic alkenes, the selectivity of epoxy compounds is almost 100%,
which is due to the high activity of the double bonds and the relative
high stability of epoxy products. Linear aliphatic olefins such as 1-
hexene and 1-octene converted to their corresponding 1,2-epoxy al-
kanes as an exclusive product. However, the conversion of 1-hexene
(90.0%) was higher than 1-octene (63.9%) which suggested that cata-
lytic activity decreases along with chain length of olefins. Larger hexyl
group of 1-octene connected to double bond sterically hinders its ap-
proach to the active site compared to 1-hexene whose double bond
carries a smaller butyl group. Due to the conjugative effect of benzene
ring, styrene showed lower activity. Basing on the mechanism sug-
gested above, higher electron donating olefins that have inner double
bonds shows splendid activity comparing to terminal olefins. The pro-
duct of the epoxidation reaction were collected at different time in-
tervals and then identified and quantified by gas chromatography. The
turnover frequency (TOF) was calculated as follows:
3.5. Comparison with other catalysts
Asgharpour al. testified that the [MoO2(Sal-Tryp)] catalyst could be
used for the epoxidation of cyclooctene. They performed the reaction in
CCl4 and obtained 100% yield (Table 4, entry 2) [46]. Additional,
Kardanpour et al. performed the epoxidation of cyclooctene by
MoO2(acac)2@UiO-66-NH2-SA and MoO2(acac)2@UiO-66-NH2-TC in
C2H4Cl2 (Table 4, entry 3,4), which showed that those catalysts have
good catalytic performances for epoxidation of alkenes [47]. Further-
more, Mirzaee et al. reported that Mo-Im-BNPs and Mo-A-BNPs could
be reused several times without significant loss of activity and se-
lectivity (Table 4, entry 5,6) [36]. In this work, 92.0% conversion and
high turnover frequency is 208 h−1 (Table 4, entries 1). Comparing
with the reported catalysts (Table 4), this catalyst exhibited better
catalytic property for epoxidation of turnover frequency.
4. Conclusion
A heterogeneous dimolybdenum Schiff-base catalyst was success-
fully prepared. The catalyst showed remarkable catalytic activity in the
epoxidation of cyclic and terminal olefins. Based on the experiments,
the results indicated that solvent strongly effect the reaction speed,
selectivity and conversion on the epoxidation of olefins. Moreover, the
highest yield (99%) and selectivity (99.6%) were obtained in 1,2-di-
chloroethane (C2H4Cl2). Kinetic analysis indicated that dimolybdenum
moles converted
TOF=
moles of Mo(active site )taken for reaction × reaction time
3.3.4. Dynamics of the epoxidation of cyclooctene by DMSBC
To explore the source and activity of the catalyst for aerobic oxi-
dation of olefins to form epoxy compounds, it is necessary to carry out
kinetic analysis. In this present work, some key kinetic parameters were
studied by using the oxidation of cyclooctene as a model reaction.
Firstly, the effect of temperature on catalytic activity of catalyst was
investigated [45]. Considering the selectivity of the reaction, we per-
formed the reaction 323 K, 333 K, 343 K and 353 K (Fig. 4). As the result
shown in Fig. 4, the conversion of cyclooctene increased gradually from
30.8% to 99.2% along with the reaction temperature increase from
323 K to 353 K. The reaction was carried out at different temperatures,
and the initial k was obtained by the linear fitting of the time versus
conversion curve. According to the Arrhenius equation of ln k= ln
A−Ea/RT, we plotted the relationship between ln k and 1/T, which has
excellent linear relationship (Fig. 4B R2 = 1). Activation energy (Ea)
and pre-exponential factors (A) obtained by the Arrhenius equation
catalyst had moderate activation energy (95.3
2 kJ·mol−1 for cy-
clooctene oxidation). The large pre-exponential factor (A) meant that
there was a high probability of collision to account for the high reaction
rate. Furthermore, the catalyst could be easily recovered by centrifugal.
It could be reused at least six times with suitable catalytic performance.
Acknowledgment
We are grateful for the financial support from the National Natural
Science Foundation of China (No 21676057).
Appendix A. Supplementary data
were 95.3
2 kJ·mol−1 and 7.9 × 1010 mol−1 dm3·s−1, respectively.
The result indicated that the epoxidation of cyclooctene has high con-
version speed and high probability of collision.
Supplementary material related to this article can be found, in the
online version, at doi:https://doi.org/10.1016/j.mcat.2019.110498.
References
3.4. Recycling experiments of DMSBC for epoxidation of cyclooctene under
optimized conditions
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