Enhanced alkenes epoxidation reactivity of discrete bis(8-quinolinol) oxovanadium(IV) or bis(8-quinolinol)dioxomolybdenum(VI) tethered to graphene oxide by a metal-template/metal-exchange method

Article abstract of DOI:10.1016/j.apcata.2013.10.037

A series of imprinted catalysts were obtained by covalent attachment of discrete bis(8-quinolinol)oxovanadium(IV) or bis(8-quinolinol) dioxomolybdenum(VI) complex onto graphene oxide (GO) through the metal-template/metal-exchange method to control the distribution of covalently attached independent ligands, which were examined as catalysts in the epoxidation of alkenes using t-butylhydroperoxide (TBHP) or H₂O ₂ as oxidant in comparison with their homogeneous counterparts and materials synthesized by random grafting of the ligand. The prepared catalysts were characterized by XRD, N₂ adsorption/desorption, TEM, FT-IR, diffusion reflection UV-visible, ICP-AES, TG, Raman, EPR, N element analysis and XPS. FT-IR, diffusion reflection UV-visible, ICP-AES, TG, N element analysis, Raman and XPS results showed that oxovanadium(IV) or dioxomolybdenum(VI) complexes were successfully grafted on GO. XRD, N₂ adsorption/desorption and TEM results indicated that the structures of the samples were well preserved. EPR results revealed that the materials synthesized by the metal-template method had better site-isolation compared with conventional materials. Furthermore, these imprinted oxovanadium(IV) or dioxomolybdenum(VI) complex catalysts are efficient and recyclable, which exhibit higher activity and higher selectivity to target product than either their homogeneous ones or randomly grafted analogues due to better site-isolation.

Full text of DOI:10.1016/j.apcata.2013.10.037

Applied Catalysis A: General 470 (2014) 104–114
Contents lists available at ScienceDirect
Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
Enhanced alkenes epoxidation reactivity of discrete
bis(8-quinolinol)oxovanadium(IV) or
bis(8-quinolinol)dioxomolybdenum(VI) tethered to graphene oxide
by a metal-template/metal-exchange method
Zhifang Lia, Shujie Wua, Dafang Zhengb, Heng Liua, Jing Hua, Hailiang Sua, Jian Suna,
Xu Wangc, Qisheng Huob, Jingqi Guana,∗, Qiubin Kana,∗
a College of Chemistry, Jilin University, Changchun, 130023, PR China
b State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, College of Chemistry, Jilin University, Changchun, 130012, PR China
c State Key Laboratory of Supramolecular Structure and Materials, Jilin University, Changchun, 130012, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
A
series of imprinted catalysts were obtained by covalent attachment of discrete bis(8-
quinolinol)oxovanadium(IV) or bis(8-quinolinol)dioxomolybdenum(VI) complex onto graphene oxide
(GO) through the metal-template/metal-exchange method to control the distribution of covalently
attached independent ligands, which were examined as catalysts in the epoxidation of alkenes using
t-butylhydroperoxide (TBHP) or H2O2 as oxidant in comparison with their homogeneous counterparts
and materials synthesized by random grafting of the ligand. The prepared catalysts were characterized by
XRD, N2 adsorption/desorption, TEM, FT-IR, diffusion reflection UV–visible, ICP-AES, TG, Raman, EPR, N
element analysis and XPS. FT-IR, diffusion reflection UV–visible, ICP-AES, TG, N element analysis, Raman
and XPS results showed that oxovanadium(IV) or dioxomolybdenum(VI) complexes were successfully
grafted on GO. XRD, N2 adsorption/desorption and TEM results indicated that the structures of the sam-
ples were well preserved. EPR results revealed that the materials synthesized by the metal-template
method had better site-isolation compared with conventional materials. Furthermore, these imprinted
oxovanadium(IV) or dioxomolybdenum(VI) complex catalysts are efficient and recyclable, which exhibit
higher activity and higher selectivity to target product than either their homogeneous ones or randomly
grafted analogues due to better site-isolation.
Received 30 June 2013
Received in revised form
27 September 2013
Accepted 21 October 2013
Available online 29 October 2013
Keywords:
Grapheme oxide
Epoxidation
Styrene
Metal-template method
8-Quinolinol complexes
© 2013 Elsevier B.V. All rights reserved.
1. Introduction
olefin oxidation, new method for preparing heterogeneous epoxi-
dation catalysts is encouraged.
The catalytic epoxidation of alkenes plays a major role in syn-
thesis of the corresponding epoxides, which is widely used in the
production of fine chemicals and chemical intermediates [1–4].
Transition metal complexes containing redox active transition
metals (Fe, V and Mo) have been reported as important homoge-
nous catalysts for alkenes oxidation [5–7]. However, most of the
homogeneous catalysts suffer from drawbacks such as deactiva-
tion, difficult recovery and recycling of the catalysts. To overcome
these drawbacks, heterogeneous catalysts prepared by immobiliz-
ing the homogeneous complexes onto insoluble solid supports have
been developed [8,9]. Since the reactivity of many tethered cata-
lysts is less than the corresponding homogeneous counterparts for
For the heterogenization of discrete homogeneous cat-
alysts, recent advances principally adopt immobilization of
single, multidentate ligands such as porphyrin, salen, or 1,4,7-
triazacyclononane onto supports in order to provide metal
coordination for catalytic activity [10,11]. Random grafting
of such complexes onto
a support makes certain that the
homogeneous coordination sphere and catalytic reactivity
are retained. Furthermore, reports of covalent imprint-
ing of coordination sites composed of several independent
ligands, such as bis(8-quinolinol)oxovanadium(IV) and bis(8-
quinolinol)dioxomolybdenum(VI) are very few. The desirable
covalent imprinted coordination sites for a single metal created
from several independent ligands (e.g. 1,10-phenanthroline) was
achieved by Stack etc. [12,13]. The imprinted catalysts possess
better site isolation in consideration of the homogeneity of grafted
metal complexes and show better consistency in the epoxidation
∗
Corresponding authors. Tel.: +86 431 88499140, fax: +86 431 88499140.
E-mail addresses: guanjq@jlu.edu.cn (J. Guan), qkan@mail.jlu.edu.cn (Q. Kan).
0926-860X/$ – see front matter © 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.apcata.2013.10.037

Z. Li et al. / Applied Catalysis A: General 470 (2014) 104–114
105
reaction than the corresponding homogeneous catalysts and
2.2.1. Synthesis of GO
randomly grafted catalysts. Site-isolated bis(1,10-phenanthroline)
complexes on SBA-15 mesoporous silica were prepared via a
metal-template/metal-exchange method. Using this strategy, the
catalysts with the templated ferrous and MnII bisphen sites showed
higher activity and selectivity than the analogous homogeneous
catalyst or immobilized Fe and Mn catalysts prepared by random
grafting of the ligand on SBA-15.
GO was prepared according to the literatures [14,22,26–28].
Typically, graphite powder and concentrated H2SO4 was added to a
250 ml three-necked flask. The mixture was stirred in an ice batch
for 120 min. 6.0 g KMnO4 was then added into the mixture grad-
ually under vigorous stirring and the reaction mixture was stirred
at room temperature for 2 h. After that, the mixture was stirred at
35 ◦C for 30 min and 92 ml deionized water was added under vigor-
ous stirring. Soon afterwards, the mixture was heated to 95 ◦C for
15 min. At last, the mixture was transferred to a 1000 ml beaker and
280 ml deionized water and 5 ml 30% H2O2 solution was added. The
required brown product was obtained by filtering, washing with
a large amount of 5% HCl solution and deionized water and then
drying.
Compared with mesoporous silicates such as MCM-41 and SBA-
15, graphene oxide (GO) has unique nanostructure (monolayer),
ample oxygen carrying functionalities on its basal planes and at
its edges (hydroxyl, epoxide, and carboxylic groups), excellent
mechanical strength and hydrothermal stability, and the prospect
of various applications such as composite materials, catalysis, opto-
electronics, supercapacitors, memory devices and as a drug delivery
agent [14–21]. It is already well known that GO can be synthesized
by the exfoliation of graphitic oxide. For the first time graphite
oxide was synthesized by the oxidation of graphite by William
et al. in 1958 [22]. Monomolecular sheets GO could be obtained
by the exfoliation of this graphitic oxide in organic solvent [23].
Recently, Mungse et al. [14] reported that the prepared graphene-
bound oxo-vanadium Schiff base catalyst was highly efficient and
showed comparable catalytic reactivity for the oxidation of vari-
ous alcohols, diols, and ␣-hydroxyketones to carbonyl compounds
using TBHP as an oxidant. Furthermore, as far as we are aware,
the covalent attachment of a discrete bis(8-quinolinol) oxovana-
dium(IV) and dioxomolybdenum(VI) complexes onto GO by the
metal-template/metal-exchange method has not been reported.
Previously, we found that tethering of transition metals oxo-
vanadium(IV) and dioxomolybdenum(VI) complexes onto SBA-15
material are highly active in the epoxidation of cyclooctene or
styrene [24,25]. However, metal complexes heterogenized by ran-
dom grafting are not very stable during the process of reaction and
the metal species would be partly leached into the solution in the
experimental process. Herein we first report design and charac-
terization of the covalent attachment of discrete bis(8-quinolinol)
oxovanadium(IV) and dioxomolybdenum(VI) complexes onto GO
by the metal-template/metal-exchange method. For comparison,
their homogeneous counterparts and the randomly ligand-grafted
heterogeneous analogues were additionally prepared. Further-
more, their catalytic properties in the epoxidation of alkenes
(cyclooctene, styrene, geraniol or 1-octene) using TBHP or H2O2
as oxidant were investigated.
2.2.2. Synthesis of bis(5-chloromethyl-8-quinolinol)zinc(II)
([ZnII(1C)2] H2O)
5-Chloromethyl-8-quinolinol was prepared and characterized
as described in our early work [29]. In a typical procedure, 11.68 g
8-hydroxyquinoline (8-Q), 12.8 ml HCHO and 1.2 g ZnCl2 were
added to a 250 ml three-necked flask. 100 ml concentrated HCl
was then added into the mixture gradually under vigorous stir-
ring. The reaction mixture was stirred at room temperature for
24 h. After finishing the reaction and keeping reaction product
standing, the required yellow product was obtained by filtering,
washing with acetone and drying. The dried sample was denoted
as 1C·HCl.
Bis(5-chloromethyl-8-quinolinol)zinc(II) was also prepared and
characterized as described in our early work [29]. As a typical run,
2.5 mmol 1C·HCl and 0.275 g Zn(OAc)2·2H2O were dried in advance
at 100 ◦C for 1 h. 1C·HCl was dispersed in a 50 ml two-necked flask
containing 25 ml dried THF. Under stirring, 0.35 ml Et3N was then
added into the mixture. The reaction mixture was stirred at room
temperature for 1 h. After that, 0.275 g Zn(OAc)2·2H2O was added
to the stirred solution. The mixture was stirred under N2 atmo-
sphere for 24 h. The required green solid was obtained by filtering,
washing with tetrahydrofuran and drying. The obtained product
was denoted as [ZnII(1C)2] H2O.
2.2.3. Synthesis of VO(IV) acetylacetonate
VO(IV) acetylacetonate was prepared and characterized as
described in our early work [29]. In a typical procedure, 2.5 g V2O5
and 50 ml acetylacetone were added to flask. The mixture was
refluxed at 80 ◦C for 24 h. The solid was then filtered, washed abun-
dantly with acetone, and dried. The turquoise solid was denoted as
VO(acac)2.
2. Experimental
2.1. Materials
2.2.4. Synthesis of MoO2Cl2(dmf)2
MoO2Cl2(dmf)2 was prepared and characterized as described
in our early work [30]. In a typical synthesis, the mixture of 1.0 g
MoO3 and 10 ml (6 M) HCl was heated to boil until most of MoO3
was dissolved, then the mixture was quickly cooled down to room
temperature. After that, the clear solution was obtained by filtered.
2.5 ml DMF was added to the clear solution and the mixture was
stirred for 2 h. The white solid was obtained by filtered, washed
with acetone, and dried.
Graphite powder, H2SO4, KMnO4, 30% H2O2, V2O5, VOSO4, 8-
hydroxyquinoline, hydrochloric acid, acetylacetone, ZnCl2, HCHO
(W = 38%), Zn(OAc)2·2H2O, 3-mercaptopropyltrimethoxysilane (3-
MPTMS), MoO3, DMF and all organic solvents are A.R. grade.
Na/diphenylketone ketyl was used to dry tetrahydrofuran (THF),
and then THF is distilled under N2 atmosphere, and is stored over
4A molecular sieves.
2.2.5. Synthesis of VOIVT and MoO2VIT by
metal-template/metal-exchange method
2.2. Synthetic procedures (Scheme 1)
ZnIIT was prepared according to our previous work with
slight modification [29]. For ZnIIT preparation, 0.5 mmol pre-dried
[ZnII(1C)2] H2O and 0.18 ml 3-MPTMS were added to a 25 ml two-
necked flask contenting 20 ml dry THF under N2 atmosphere. After-
wards, the mixture was refluxed at 65 ◦C for 15 h. Finally, 10 ml dry
THF were added and then 0.625 g GO was dispersed by ultrasound.
The procedure for the covalent attachment of 8-quinolinol com-
plexes of oxovanadium(IV) and molybdenum dioxide(VI) to GO
(graphene oxide) by random grafting and metal-template/metal-
exchange method to form VOIVT, VOIVG, MoO2VIT and MoO2VIG,
respectively, is depicted in Scheme 1 and described as follows.

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Z. Li et al. / Applied Catalysis A: General 470 (2014) 104–114
Scheme 1. Schematic outlines of the covalent attachment of 1P to the GO via random grafting and metal-templating method to form MG and MT [M = VO(IV) and MoO2(VI)],
respectively: (a) random grafting of 1P; (b) metalation; (c) formation of ZnIIT followed by covalent attachment; (d) demetalation with CH3COOH.
The mixture was refluxed for 1 d under N2 atmosphere. The sam-
ple was filtered, washed repeatedly with a large amount of THF
and dried. Template T was prepared by refluxing 0.5 g of ZnIIT with
10 ml acetic acid at 80 ◦C for 1 d and then filtering and washing with
plentiful water. This process can be repeated twice.
stirred for 24 h. The sample was filtered and washed with abundant
water and extracted with CH2Cl2.
2.2.6. Synthesis of VOIVG and MoO2VIG by random grafting
method
VOIVT was prepared by adding of a certain amount of VO(acac)2
into the methanol solution containing 0.5 g T, stirring at room tem-
perature for 24 h, filtering, washing with CH2Cl2 and extracting
with CH2Cl2.
G was prepared through the reaction of 5-chloromethyl-8-
quinolinol (1C·HCl), 3-MPTMS and GO. Typically, 1.6 mmol 1C·HCl,
1.6 mmol triethylamine and 0.29 ml 3-MPTMS were added into a
25 ml two-necked flask containing 20 ml dry THF under N2 atmo-
sphere. The mixture was refluxed at 65 ◦C for 15 h. Subsequently,
10 ml dry THF was added and then 1.0 g GO was dispersed by
For MoO2VIT preparation, 0.051 g Na2CO3, 20 ml CHCl3, 0.158 g
MoO2Cl2(dmf)2 and 0.6 g T were added to a beaker. The mixture was

Z. Li et al. / Applied Catalysis A: General 470 (2014) 104–114
107
ultrasound. After vigorous stirring for 24 h, the solid was then fil-
tered, washed with CH2Cl2 and dried.
For VOIVG preparation, 0.5 g G and 0.25 mmol VO(acac)2 were
added into the methanol solution. The mixture was stirred at room
temperature for 24 h. The sample was filtered and washed with
methanol and extracted with CH2Cl2.
MoO2VIG was prepared by addition of 0.051 g Na2CO3 and
0.158 g MoO2Cl2(dmf)2 into the CHCl3 solution containing 0.6 g T.
The mixture was stirred at room temperature for 24 h, filtered,
washed with CHCl3 and extracted with CH2Cl2.
2.2.7. Synthesis of the neat complexes VOIVN and MoO2VIN
VOIVN was prepared and characterized as described in our early
work [29]. 1.25 mmol VO(acac)2 was added into the methanol solu-
tion containing 2.5 mmol 8-quinolinol. The mixture was stirred at
room temperature for 24 h. The sample was filtered and washed
with large quantities of methanol.
MoO2VIN was prepared by addition of 0.05 mmol
MoO2Cl2(dmf)2 into the CHCl3 solution containing 0.1 mmol
8-quinolinol. The mixture was stirred at room temperature for
24 h, filtered, and washed with large quantities of CHCl3. FT-IR
(KBr pellets, cm−1):1627 (C=C), 1570, 1491 (py), 898 (Mo=O), 527
(Mo–O), 469 (Mo–N).
2.3. Characterization
The infrared spectra were recorded in KBr disks using a
NICOLET impact 410 spectrometer. Powder X-ray diffractometry
(XRD) measurements were performed on a Shimadzu XRD-6000
diffractometer using a Ni-filtered Cu K␣ radiation scanning from
5◦ to 40◦ (2␪). Samples were dried previously and pulver-
ized. N2 adsorption–desorption isotherms were measured with a
Micromeritics ASAP 2020 system at liquid N2 temperature. Before
measurements, the sample was outgassed at 130 ◦C for 6 h. The
BET surface area was calculated using the Brunauer–Emmett–Teller
(BET) method. Diffuse reflectance UV–vis spectra were recorded
from 200 to 800 nm on a Shimadzu UV-3600 spectrophotome-
ter using BaSO4 as a reference. Metal content was estimated by
inductively coupled plasma atomic emission spectroscopy (ICP-
AES) analysis conducted on a Perkin-Elmer emission spectrometer.
Elemental analyses of nitrogen were obtained with a VarioEL CHN
elemental analyzer. X-ray photoelectron spectroscopy (XPS) was
carried out on an ESCALAB 250 X-ray electron spectrometer using
AlK␣ radiation. EPR spectra were recorded at room temperature on
a JES-FA 200 spectrometer. Morphological features of the materi-
als were characterized by a JEOL JEM-2010 transmission electron
microscope at an accelerating voltage of 200 kV. Raman spectra
were recorded on a Renishaw Raman system model 1000 spectrom-
eter with an excitation wavelength of 514 nm. TG analyses were
performed on a Shimadzu DTA-60 working with the heating rate of
10 ◦C min−1 from 25◦C to 600 ◦C under flowing N2.
Fig. 1. High angle X-ray diffraction patterns of (a) GO, (b) ZnIIT, (c) T, (d) VOIVT, (e)
VOIVG, (f) MoO2VIT and (g) MoO2VIG.
3. Results and discussion
3.1. Structural characteristics of catalysts
The powder XRD patterns of pure support GO and the covalent
attachments of 8-quinolinol complex catalysts are shown in Fig. 1.
For GO (Fig. 1a), the peak at around 11.5◦ can be assigned to the
(0 0 1) reflection of graphite oxide [14,31,32]. It is clear that oxygen
species were successfully inserted into the graphitic layers [33,34].
For ZnIIT, the intensity of the (0 0 1) reflection of graphene oxide
are significantly reduced, which might be due to the 8-quinolinol
complexes distribution in the GO. Moreover, the diffraction peak at
ca. 2␪ = 11.5◦ does not disappear, which clearly indicates that the
structure of graphene oxide was not destroyed during the process of
refluxing of T. A similar pattern is found for all covalent attachment
of 8-quinolinol complexes catalysts (Fig. 1d and e), suggesting that
they are similar in graphene oxide layer. However, the very weak
and broad diffraction peak centered at 21.2◦ is seen for MoO2VIT
and MoO2VIG (Fig. 1f and g), indicating that part of the developed
catalysts is closer to graphene [14].
2.4. Catalytic tests
The catalytic epoxidation of cyclooctene, styrene, geraniol or
1-octene was carried out in a 5 ml flask equipped with a reflux
condenser. Typically, substrate (1 mmol), solvent (MeCN or CHCl3,
1 ml), dodecane (0.5 mmol, internal standard) and catalyst (10 mg)
were mixed. The mixture was heated to 70 ◦C, and then oxidant
(TBHP or H2O2, 5 mmol) was introduced. The reaction products
were analyzed by gas chromatography (GC) with a Shimadzu GC-
8A (Japan) instrument equipped using an HJ-5 capillary column
and FID detector. At the end of the reaction, the catalyst was fil-
tered off, washed with copious solvent, and dried. The catalyst was
then reused in the next reaction without further purification.

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Z. Li et al. / Applied Catalysis A: General 470 (2014) 104–114
Fig. 2. TEM images of (a) VOIVT, (b) VOIVG, (c) MoO2VIT and (d) MoO2VIG.
3.2. Spectroscopic characterization
Fig. 4 shows the FT-IR spectra of the pure support and supported
complexes. The spectra of the pure support GO exhibited char-
acteristic bands at 3386, 1728, 1620, 1368, 1220 and 1054 cm−1
(Fig. 4a), associated with the stretching mode of O–H, C=O, C=C,
=C–H, C–O and C–O–C, respectively [14,26]. Moreover, the spectra
of ZnIIT (Fig. 4b) shows peaks in the region of 1600–1300 cm−1 due
to the C–O, C–N and aromatic ring vibrations of 8-quinolinol, veri-
fying that ZnIIT has been successfully prepared. The characteristic
bands of T (Fig. 4c) still have in common with the ZnIIT in the region
of 1600–1300 cm−1, indicating that the structure of 8-quinolinol
was not destroyed after removing Zn. The characteristic bands of
8-quinolinol can also be found in VOIVT and VOIVG (Fig. 4d and e).
Similar results were obtained for MoO2VIT and MoO2VIG (Fig. 4g
and h).
Fig. 5 exhibits UV–vis spectra of homogeneous analogues,
the pure support and as-prepared tethered 8-quinolinol on GO
nanosheets catalysts. Compared with homogeneous analogues and
the pure support, the absorption bands in the range of 200–400 nm
are observed in all of the tethered catalysts, which are assigned
to n–␲*, ␲–␲* ligand charge transfer [35], suggesting that the 8-
quinolinol ligand has been anchored onto GO. The d–d transitions
at ca. 550 nm are observed for VOIVT and VOIVG, indicating that
oxovanadium(IV) complexes have been incorporated onto the sur-
face of GO. Nonetheless, there is no d–d transition for the supported
molybdenum catalysts, which is in accordance with our early work
[24].
Raman spectroscopy was employed to determine ordered and
disordered crystal structures of carbon materials, such as graphite,
graphene and GO and so on. G band is generally attributed to the E2g
phonon of C sp2 atoms and D band is a breathing mode of ␬-point
phonons of A1g symmetry [36,37]. Raman spectra were carried out
for all of the catalysts as shown in Fig. 6. The spectra of all of the
Fig. 3. N2 adsorption/desorption isotherms profiles of (a) VOIVT, (b) VOIVG, (c)
MoO2VIT and (d) MoO2VIG.
Fig. 2 shows the transmission electron microscopy (TEM)
images of the templated catalysts and randomly ligand-grafted
heterogeneous catalysts. It is obvious that all the samples have
layers structure (monolayers or just
a few layers) and the
crumpled nanosheets are in an agglomerated phase. The N2 adsorp-
tion/desorption isotherms of the templated catalysts and randomly
ligand-grafted heterogeneous catalysts are depicted in Fig. 3.
The supported materials maintain the characteristics of type IV
isotherms. The BET surface area is 27, 36, 26 and 33 m2/g for VOIVT,
VOIVG, MoO2VIT and MoO2VIG, respectively.

Z. Li et al. / Applied Catalysis A: General 470 (2014) 104–114
109
Fig. 5. UV–visible spectra of (A), (a) GO, (b)VOIVN, (c)MoO2VIN, (B), (d) VOIVT, (e)
VOIVG, (f) MoO2VIT and (g) MoO2VIG.
Fig. 4. FT-IR spectra of (a) GO, (b) ZnIIT, (c) T, (d) VOIVT, (e) VOIVG, (f) MoO2VIT and
(g) MoO2VIG.
samples show two strong bands at ca. 1350 and 1588 cm−1 due to
the well-defined D band and G band, respectively. ID/IG (the Raman
intensity ratio of D and G bands) has been regarded as a unique
characteristic tool to investigate the degree of graphitization in
graphitic materials [38]. The values of ID/IG are depicted in Fig. 1S
for all of the catalysts. The values of ID/IG in supported catalysts
decreased notably compared to that in the GO (Fig. 1S), indicating
that the size of the in-plane sp2 domains increases and the covalent
attachment of 8-quinolinol complexes to GO via random grafting
by metal-template method exhibits a comparatively low degree of
defects [39,40].
3.3. TG and EPR studies
TG profiles of the pure support GO, VOIVT, VOIVG, MoO2VIT and
MoO2VIG are presented in Fig. 7. The TG curve of graphene oxide
shows two regions of weight loss (Fig. 7a). The first minor weight
loss below 180 ◦C is due to the loss of the physically adsorbed
Fig. 6. Raman spectra (G and D bands) of (a) GO, (b) VOIVT and (c) VOIVG, (d) MoO2VIT
and (e) MoO2VIG.

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Z. Li et al. / Applied Catalysis A: General 470 (2014) 104–114
Fig. 8. Room temperature EPR spectra of VOIVT and VOIVG.
complex in the templated material has better site-isolation. Similar
results were obtained for the MoO2VIT and MoO2VIG catalysts.
3.4. X-ray photoelectron spectroscopy studies
To investigate the surface constitution of the catalysts, the C1s,
N1s and M2p (M = V or Mo) binding energies are studied, and the
corresponding XPS spectrum of all the samples are presented in
Fig. 9. The C1s spectrum of GO (Fig. 9a) suggests four types of sp2-
hybridized C1s with four peaks centered at ca. 284.3, 285.4, 287.4,
and 289.1 eV due to C=C, C–OH, C–O–C, and O–C=O groups, respec-
tively, indicating that GO has been successfully synthesized [39].
Meanwhile, the C1s XPS spectrum of the other supported cata-
lysts (Fig. 9b–e) also exhibits these same oxygen functionalities,
and the intensity of C–OH signal decreased for these catalysts com-
pared with the starting GO, suggesting that 8-quinolinol ligand
is successfully immobilized onto GO. The binding energy of V2p
for the VOIVT and VOIVG catalysts (Fig. 9 V2p) is 517.1 eV and
524.2 eV, respectively, which can be assigned to vanadium(IV) atom
in the supported vanadium catalysts. The two peaks appeared at
ca. 232.9 eV and 236.1 eV in the 3d orbits are, respectively, due
to Mo 3d5/2 and Mo 3d3/2 in the MoO2VIT and MoO2VIG catalysts
(Fig. 9 Mo3d), indicating that the molybdenum species with such a
binding energy could be assigned to Mo(VI). Correspondingly, the
only peak appeared at ca. 399.1 eV in the 1s orbit is due to N1s
for the MoO2VIT and MoO2VIG catalysts (Fig. 9 N1s), indicating that
8-quinolinol complexes have been prepared. Similar results were
obtained for the vanadium catalysts (Fig. 2S N1s). It is consistent
with the above mentioned conclusion.
Fig. 7. TG curves of (a) GO, (b) VOIVT, (c) VOIVG, (d) MoO2VIT and (e) MoO2VIG.
water, while the second weight loss occurred in the range from
180–210 ◦C is ascribed to the decomposition of oxygen carrying
functionalities [14,41,42]. However, the TG curve for the supported
catalysts shows three step weight losses at temperatures ranged
from 30 ◦C to 600 ◦C under N2 atmosphere. Weight loss below
180 ◦C corresponds to the physically adsorbed water. Weight loss
in the range of 180 to 250 ◦C is probably attributed to the decom-
position of undigested oxygen carrying functionalities. Weight loss
in the temperature range from 250 ◦C to 600 ◦C is allocated to the
decomposition of the ligand. Weight losses of ca. 13.5% and 14.4%
for the VOIVT and VOIVG were observed, respectively. The loadings
of 8-quinolinol are 0.86 and 0.92 mmol/g calculated based on nitro-
gen analysis, which is consistent with the results of TG. Meanwhile
the corresponding metal contents are 0.42 and 0.56 mmol/g esti-
mated by ICP-AES, respectively. Consequently, the molar ratio of
ligand/metal is 2.05 and 1.64 for the VOIVT and VOIVG, respectively,
which is consistent with the increased heterogeneity of the active
sites with a distribution of bis-1 and mono-1 ligated metals. Fig. 8
exhibits EPR spectra of the templated VOIVT and randomly grafted
VOIVG materials. Obviously, compared with VOIVT, a slight signal
broadening is observed in VOIVG, indicating that oxovanadium(IV)
3.5. Catalytic tests
The activity of the derivatized GO materials and the com-
plexes (VOIVT, VOIVG and so on) as catalysts for the epoxidation
of olefins was investigated using cyclooctene, styrene, 1-octene
and geraniol as substrates and t-butylhydroperoxide (TBHP) as oxi-
dant plus (CHCl3 or CH3CN) as the solvent (Table 1). As expected,
all catalysts are active for the epoxidation of olefins. By contrast,
the catalytic results of the homogeneous complexes (VOIVN and
MoO2VIN), ZnIIT, and randomly ligand-grafted heterogeneous ana-
logue (VOIVG and MoO2VIG) as catalysts are also given under the
same conditions. When CH3CN was used as solvent and TBHP was
used as oxidant, the VOIVT catalyst shows 79.4% conversion of
cyclooctene with a TOF of 23.6 and 66.9% yield of the corresponding

Z. Li et al. / Applied Catalysis A: General 470 (2014) 104–114
111
Fig. 9. XPS spectra of C (a) GO, (b) VOIVT, (c) VOIVG, (d) MoO2VIT and (e) MoO2VIG, V2p (b) VOIVT, (c) VOIVG, Mo3d and N1s (d) MoO2VIT and (e) MoO2VIG.
epoxide after 8 h. As for freshly prepared MoO2VIT catalyst, it shows
high activity and good epoxide yield in CHCl3, corresponding to
86.3% conversion of cyclooctene with a TOF of 30.8 and 86.3%
yield of the corresponding epoxide. Nevertheless, homogeneous
complexes and randomly ligand-grafted heterogeneous catalysts
show lower activity, turnover frequency and epoxide yields
than the corresponding templated catalysts, suggesting that site-
isolated bis-8-quinolinol coordination sphere prevents bimolecular

112
Z. Li et al. / Applied Catalysis A: General 470 (2014) 104–114
Table 1
Epoxidation catalyzed by various catalysts.
Catalysts
ZnIIT
Oxidant
Solvent
Substrate
Metal content (mmol g−1
)
Conversiona (%)
Epoxide yield (%)
TOFb (h−1
)
Reference
TBHP
TBHP
H2O2
TBHP
TBHP
H2O2
TBHP
TBHP
TBHP
H2O2
TBHP
H2O2
TBHP
TBHP
TBHP
TBHP
TBHP
H2O2
TBHP
H2O2
TBHP
TBHP
TBHP
H2O2
TBHP
TBHP
CH3CN
CH3CN
CH3CN
CHCl3
Cyclooctene
Cyclooctene
Cyclooctene
Cyclooctene
Geraniol
Geraniol
Styrene
1-Octene
Cyclooctene
Cyclooctene
Geraniol
Geraniol
Styrene
–
3.0
79.4
42.6
40.8
83.7
96.2
50.6
–
64.0
28.1
80.0
93.7
22.9
–
79.0
28.2
86.3
26.0
92.5
93.3
19.5
67.1
82.9
81.2
13.5
75.2
3.0
66.9
42.2
40.4
79.8
43.4
2.3
–
Herein
Herein
Herein
Herein
Herein
Herein
Herein
Herein
Herein
Herein
Herein
Herein
Herein
Herein
[29]
Herein
Herein
Herein
Herein
Herein
Herein
Herein
Herein
Herein
Herein
Herein
VOIV
VOIV
VOIV
VOIV
VOIV
VOIV
VOIV
VOIV
VOIV
VOIV
VOIV
VOIV
VOIV
VOIV
T
T
T
T
T
T
T
G
G
G
G
G
G
N
0.420
0.420
0.420
0.420
0.420
0.420
0.420
0.560
0.560
0.560
0.560
0.560
0.560
–
0.350
0.350
0.350
0.350
0.350
0.350
0.570
0.570
0.570
0.570
0.240
23.6
12.7
12.1
24.9
28.6
15.1
–
14.3
6.3
17.9
20.9
5.1
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CHCl3
CHCl3
CHCl3
CHCl3
CHCl3
CHCl3
CHCl3
CHCl3
CHCl3
–
52.5
28.1
76.3
50.6
0.6
1-Octene
–
–
Cyclooctene
Cyclooctene
Cyclooctene
Cyclooctene
Geraniol
Geraniol
Styrene
Cyclooctene
Geraniol
Geraniol
68.8
27.6
86.3
25.7
82.9
93.2
4.5
65.6
77.9
81.1
0.6
35.1
10.1
30.8
9.3
33.0
33.3
7.0
14.7
18.2
17.8
3.0
MoO2VIT
MoO2VIT
MoO2VIT
MoO2VIT
MoO2VIT
MoO2VIT
MoO2VIG
MoO2VIG
MoO2VIG
MoO2VIG
MoO2VIN
Styrene
Cyclooctene
CHCl3
75.2
39.1
a
Reaction conditions: catalyst 10 mg, substrate 1 mmol, solvent 1 ml, oxidant 1 mmol, duration 8 h, and temperature 70 ◦C.
TOF, h−1: (turnover frequency) moles of substrate converted per mole metal ion per hour.
b
catalyst deactivation. The ZnIIT catalyst shows only 3.0% conversion
of cyclooctene under the same condition, which is almost parallel
to the blank experiment (not show), indicating that the residual Zn
template cannot play a catalytic role.
reaction conditions and the results are listed in Table 2. From what
has been listed in Table 2, we can draw a conclusion that the VOIV
T
catalyst was far more stable than the VOIVG catalyst and could be
successfully recycled for three times without evident loss of activ-
ity and selectivity in the same manner as that of the fresh catalysts,
implying that the imprinted complex catalyst has improved stabil-
ity and recycling due to better site-isolation. The catalytic activity
of the VOIVG catalyst will decline sharply for three times. The recov-
erability results of MoO2VIG catalyst was just the opposite, which
may be due to some uncleaned products on its surface.
Furthermore, the influence of the solvents on the catalytic
properties of various catalysts was also investigated. Table 1 sum-
marizes catalytic results of these control experiments for the
oxidation of cyclooctene. The VOIVT catalyst shows 79.4% conver-
sion of cyclooctene after 8 h when using TBHP as the solvent in
CH3CN, while it only shows 40.8% conversion of cyclooctene after
8 h when using TBHP as the solvent in CHCl3. However, the catalytic
result of MoO2VIT catalyst is almost opposite to that of VOIVT cat-
alyst in CH3CN and CHCl3. It seems clear from this finding that the
oxovanadium(IV) catalyst performs well with TBHP as the oxidant
in CH3CN, while the molybdenum catalyst shows best performance
with TBHP as the oxidant in CHCl3. This result is consistent with our
early work [24,29].
Moreover, the results of the epoxidation of cyclooctene and
geraniol using different oxidants over various catalysts (VOIVT,
VOIVG, MoO2VIT and MoO2VIG) are also investigated (Table 1). It
is obvious that the new heterogeneous catalysts showed compara-
ble or even higher conversion in oxidation of geraniol using H2O2
as oxidant under the similar reaction conditions as compared with
the catalytic results using TBHP as oxidant. However, the catalytic
results for the oxidation of cyclooctene are adverse, which might be
attributed to the decomposing of H2O2 into molecular oxygen and
water [43,44]. The results demonstrate that H2O2 is an appropriate
oxidant in the oxidation of geraniol, while TBHP is an appropriate
oxidant in the oxidation of cyclooctene.
The heterogeneous nature of catalysis over immobilized com-
plexes has been tested in both of the catalytic reactions by leaching
test. Leaching tests were performed for the catalysts VOIVT and
MoO2VIT which were filtered at the reaction temperature (80 ◦C).
Half of the volume was filtered after reacting for 2 h and the result-
ing clear solution was permitted to react for another 6 h. The
leaching results were listed in Fig. 10. This finding shows that
cyclooctene could just be converted at a very low rate after filtrat-
ing VOIVT and MoO2VIT catalysts. The filtrates were confirmed by
ICP-AES. The results of the ICP analysis shows that the contents of
vanadium and molybdenum for VOIVT and MoO2VIT are 0.402 and
0.341 mmol/g, respectively, suggesting that the metal contents in
the recovered catalyst were close to the fresh catalysts. Therefore,
the large majority of the catalysis is carried out by truly heteroge-
neous oxovanadium(IV) and molybdenum(VI) catalysts.
In order to characterize the recovered catalysts VOIVT and
MoO2VIT (after three cycles), FT-IR spectra (Fig. 11) were done for
the recycled catalysts after three catalytic runs. IR spectra of recov-
ered catalysts are found to be nearly identical to those of fresh
catalysts, which indicate the stability and recoverability of the cat-
alysts. These observations are in accordance with the result of the
leaching test.
On the basis of our catalytic experimental data and compar-
ison with the proposed mechanism reported in the literature
of related systems [45,46], the probable reaction mechanism for
the epoxidation of cyclooctene with TBHP over MoO2VIT cata-
lyst is proposed in Scheme 2: (1) the molybdenum(VI) complex
(MoO2VIL-GO 1) was coordinated to TBHP to produce a superoxo
In addition, the epoxidation of various substrates has been
examined using the described reaction conditions (Table 1). The
conversion of geraniol is the highest, followed by cyclooctene, while
the conversion of 1-octene is the worst (no detectable conversion of
1-octene could be observed). It is probably due to electron density
effect and hindrance effect [45].
The recoverability and stability of tethered the oxovanadium(IV)
and molybdenum(VI) catalysts are also investigated by selecting
the epoxidation of cyclooctene as a model reaction under similar

Z. Li et al. / Applied Catalysis A: General 470 (2014) 104–114
113
Table 2
Epoxidation of cyclooctene by various catalysts.
Catalysts
Cycle
Solvent
Metal content (mmol g−1
)
Conversiona (%)
Epoxide yield (%)
TOFb (h−1
)
VOIV
VOIV
VOIV
VOIV
VOIV
VOIV
T
T
T
G
G
G
1
2
3
1
2
3
1
2
3
1
2
3
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CHCl3
CHCl3
CHCl3
CHCl3
CHCl3
CHCl3
0.420
–
0.390
0.560
–
79.4
75.3
73.4
64.0
55.2
25.4
86.3
82.2
81.9
67.1
76.2
72.9
66.9
59.6
59.4
52.5
39.8
17.8
86.3
82.2
81.9
65.6
76.1
72.8
23.6
–
23.5
14.3
–
–
30.8
–
–
MoO2VIT
MoO2VIT
MoO2VIT
MoO2VIG
MoO2VIG
MoO2VIG
0.350
–
0.338
0.570
–
30.3
14.7
–
0.530
17.2
a
Reaction conditions: catalyst 10 mg, cyclooctene 1 mmol, solvent 1 ml, TBHP 1 mmol, duration 8 h, and temperature 70 ◦C.
TOF, h−1: (turnover frequency) moles of substrate converted per mole metal ion per hour.
b
Fig. 10. Kinetic profile of aerobic epoxidation of cyclooctene (ꢀ) and leaching
experiments of (a) VOIVT and (b) MoO2VIT catalysts (ꢁ) (continuing the reaction
after removing the catalyst after 2 h). Reaction conditions are analogous to Table 1.
(a) VOIVT: oxidant (TBHP), solvent (CH3CN); (b) MoO2VIT: oxidant (TBHP), solvent
(CHCl3).
Fig. 11. FT-IR spectra of (a) the fresh catalyst VOIVT, (b) the used catalyst VOIVT for
three cycles, (c) the fresh catalyst MoO2VIT and (d) the used catalyst MoO2VIT for
three cycles.

114
Z. Li et al. / Applied Catalysis A: General 470 (2014) 104–114
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A
synthetic metal-template/metal-exchange method to
imprint the covalent attachment of discrete bis(8-quinolinol)
oxovanadium(IV) or bis(8-quinolinol)dioxomolybdenum(VI) com-
plex onto monolayer graphene oxide (GO) is reported herein
together with their catalytic activities for the epoxidation of
alkenes. The prepared catalysts were characterized by XRD, N2
adsorption/desorption, TEM, FT-IR, diffusion reflection UV–visible,
ICP-AES, TG, Raman, EPR, N element analysis and XPS, suggesting
that oxovanadium(IV) or dioxomolybdenum(VI) catalysts have
been successfully prepared. The templated oxovanadium(IV) or
dioxomolybdenum(VI) catalysts exhibit better selectivity and
more efficient catalytic activity for olefin epoxidation with TBHP
or H2O2 than the analogous homogeneous catalyst or grafted
catalyst under the same conditions. Moreover, these imprinted
complexes catalysts have improved stability and recycling
due to better site-isolation. Future reports will describe the
immobilization of other transition metals catalysts that require
bidentate or multiple ligands in the epoxidation of alkenes by the
metal-template/metal-exchange method.
Acknowledgments
This work was supported by the National Natural Science Foun-
dation of China (21303069), Jilin province (201105006), and Jilin
University (450060445017).
Appendix A. Supplementary data
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Supplementary data associated with this article can be
found, in the online version, at http://dx.doi.org/10.1016/
j.apcata.2013.10.037.