Increased selectivity for allylic oxidation of cyclohexene using TiO2 modified V2O5/MoO3 catalysts

Article abstract of DOI:10.1016/j.catcom.2017.05.023

V₂O₅ and MoO₃ mixed oxides supported by SiO₂ were studied as the catalysts in the aqueous phase allylic oxidation of cyclohexene to unsaturated alcohol 2-cyclohexene-1-ol and unsaturated ketone 2-cyclohexene-1-one. The additional layer of TiO₂ deposited by atomic layer deposition significantly suppresses the epoxidation pathway and the formation of cyclohexane oxide and cyclohexane-1, 2-diol. X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and transmission electron microscopy (TEM) were used to study the relationship between the structure of the catalysts and their catalytic performance. The effects on the catalytic activity of different solvent, hydrogen peroxide to cyclohexene ratio, and reusability were investigated.

Full text of DOI:10.1016/j.catcom.2017.05.023

Accepted Manuscript
Increased selectivity for allylic oxidation of cyclohexene using
TiO2 modified V2O5/MoO3 catalysts
Guoqiang Yang, Matthew D. Huff, Huiyuan Du, Zhibing Zhang,
Yu Lei
PII:
DOI:
Reference:
S1566-7367(17)30217-0
doi: 10.1016/j.catcom.2017.05.023
CATCOM 5058
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Catalysis Communications
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Revised date:
Accepted date:
28 March 2017
21 May 2017
22 May 2017
Please cite this article as: Guoqiang Yang, Matthew D. Huff, Huiyuan Du, Zhibing
Zhang, Yu Lei , Increased selectivity for allylic oxidation of cyclohexene using TiO2
modified V2O5/MoO3 catalysts. The address for the corresponding author was captured
as affiliation for all authors. Please check if appropriate. Catcom(2017), doi: 10.1016/
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ACCEPTED MANUSCRIPT
Increased Selectivity for Allylic Oxidation of Cyclohexene Using TiO2
Modified V2O5/MoO3 Catalysts
Guoqiang Yang1, Matthew D. Huff2, Huiyuan Du1, Zhibing Zhang1,*, Yu Lei2,*
1School of Chemistry and Chemical Engineering, Nanjing University, Nanjing, Jiangsu, 210093,
China
2Department of Chemical and Materials Engineering, University of Alabama in Huntsville,
Huntsville, Alabama 35806, USA
*Corresponding Authors
E-mail addresses: zbzhang@nju.edu.cn (Z.Z.); yu.lei@uah.edu (Y. L.)

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1. Introduction
The addition of oxygen to hydrocarbons is one of the simplest methods to manufacture
value-added chemicals. Oxidation of hydrocarbons to epoxides, alcohols, aldehydes, and acids is
a large category of catalytic organic reactions. For example, the oxidation of cyclic olefin, such
as cyclohexene (C6H10, (1)), produces several oxygenated derivatives of great industrial interests,
as shown in Scheme 1.[1] As a model compound, cyclohexene (1) is particularly interesting as it
has two active groups in the molecule, namely the C-H bond at the allylic site and the C=C bond.
In the epoxidation pathway, the C=C bond is oxidized leading to formation of cyclohexane oxide
(2) and a subsequent hydration product cyclohexane-1,2-diol (3). In the allylic oxidation pathway,
the C-H bond is attacked, resulting in formation of α,β unsaturated alcohol and ketone, namely 2-
cyclohexene-1-ol (4) and 2-cyclohexene-1-one (5). Both pathways are industrially important.
The epoxidation pathway could eventually lead to the formation of adipic acid ((CH2)4(COOH)2),
a chemical intermediate with high market demand.[2, 3] The α, β unsaturated alcohol and ketone
generated via allylic oxidation pathway are important intermediates in the fragrance industry and
industrial organic synthesis.[4] Besides the industrial significance, understandings of the
selective oxidation of cyclohexene could potentially help to design next generation catalysts for
direct oxidation of hydrocarbons with two or more active groups.
Scheme 1. Catalytic oxidation of cyclohexene
MoO3-based catalysts have been regarded as active catalysis for selective oxidation of
hydrocarbons.[5, 6] Particularly, Mo6+/Mo5+ pairs are considered catalytic centers where Mo6+
oxo-peroxo species catalyzes the oxidation reaction and is simultaneously reduced to Mo5+.[5, 7]
The Mo5+ can be subsequently oxidized back to Mo6+ oxo-peroxo species by oxygen sources
such as O2 and H2O2.[8] The replenishing process can potentially benefit from the presence of
vanadium oxides. When heated, the surface vanadium oxides can be re-dispersed to isolated VO4
active sites.[9] These site isolated VO4 species is known to help to replenish the oxygen
vacancies. The synergistic effects between MoO3 and V2O5 enable highly active catalysts for
selective oxidation reactions.[10, 11]

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Titanium silicalite-1 (TS-1) is commercially used to catalyze propylene to propylene
oxide.[12] The presence of site-isolated Ti sites is believed important to improve the propylene
epoxidation selectivity. Similarly, studies show that the presence of Ti sites favors the direct
epoxidation pathway for selective oxidation of cyclohexene.[13] However, the selectivity could
be affected by composition of materials and operation conditions. For example, high local
concentration of H2O2 near Ti sites could lead to allylic oxidation pathway.[14] In other studies,
Ti-grafted mesoporous silica catalysts favored allylic oxidation pathway. This study will focus at
the effect of Ti in promoting V2O5-MoO3/SiO2 for selective oxidation of cyclohexene.
Atomic layer deposition (ALD) is a thin film deposition technique.[15] Taking
advantages of surface self-limiting reactions, it enables conformal coatings of nanoparticles and
thin films on high surface area supports with atomic precision. This feature makes it a promising
tool to precisely synthesize advanced heterogeneous catalysts with well-controlled structure.[16,
17] In this work, we used TiO2 ALD to modify the surface the SiO2 support. The presence of
TiO2 significantly suppressed the formation of cyclohexene oxide, leading to its selectivity
decreased from 21% to 1.7%. This results in an increase in the selectivity to unsaturated alcohol
and ketone from 67% to 79% at similar cyclohexene conversion.
2. Experimental
2.1. Catalyst Preparation
Silicycle S10040M SiO2 with a surface area of ~ 100 m2/g was used as the catalyst
support. TiO2 ALD was performed by a viscous flow benchtop reactor (Gemstar-6, Arradiance)
as described previously.[18] One ALD cycle of TiO2 was deposited at temperatures of 200 °C
using alternating exposure to titanium isopropoxide (TTIP, Sigma-Aldrich, 99.999%) and

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deionized water. In this work, the resulting TiO2 modified SiO2 is represented as 1cTiO2/SiO2.
The growth rate of TiO2 ALD is ~0.3 Å/cycle which corresponds to an estimated ~4 wt% loading
of TiO2 on SiO2.[19]
Deposition of V2O5-MoO3 composite oxide nanomaterials were carried out by
hydrothermal deposition on SiO2 and 1cTiO2/SiO2 as supports, respectively. 1.7421 g (1.410
mmol) (NH4)6Mo7O24·4H2O (Sigma-Aldrich, 99.98%) and 0.1027 g (0.878 mmol) NH4VO3
(Sigma-Aldrich, 99.996%) were added to 50 mL of deionized water and the suspension was
dissolved under stirring. After dissolved, stirring was continued for additional 15 minutes.
Afterward, all 50 mL of the aqueous solution was added into 3 g of either the 1cTiO2/SiO2 or
SiO2, and mixed again for 15 minutes in order to make the support is very well covered with
aqueous solution. As a result, the as-prepared catalysts contain 1 wt% V2O5 and 24 wt% MoO3.
Finally, the suspension was put in furnace at 100 °C for 19.5 hours, and the solid powder was
immediately calcined at 600 °C for 3 hours. The as-prepared sample was sealed in a capped glass
vail in the dark for further analysis or use.
2.2. Catalyst Characterization
Transmission electron microscope (TEM) images and energy dispersive x-ray
spectroscopy (EDX) were obtained with a JEM-200CX microscope operated at 100 kV. The
specimens for TEM were prepared by ultrasonically suspending the sample in acetone and
depositing a drop of the suspension onto a grid. X-ray photoelectron spectroscopy (XPS) was
performed in a PHI 5000 VersaProbe spectrometer equipped with an Al Kα X-ray source in
ultrahigh vacuum (UHV) (<10−10 Torr). Binding energies (±0.2 eV) were referenced to the C1s
peak at 284.6 eV as graphite. X-ray diffraction (XRD) patterns were recorded on a Rigaku

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MiniFlex 600 powder X-ray diffractometer equipped with a rotating anode using Cu Kα radiation
(40KV, 15 mA). The Fourier transform infrared (FT-IR) spectra of the samples were recorded on
−1
a ThennoNicolet NEXUS870 FT-IR instrument (KBr discs) in the 4000–400 cm region.
2.3 Catalyst Activity Test
The liquid phase catalytic oxidation of cyclohexene was carried out in a magnetically
stirred round-bottom flask (25 mL) connected to a reflux condenser. 5 mmol of cyclohexene, 10
mL of acetonitrile, 0.5 mmol of o-dichlorobenzene (o-DCB) as internal standard, and 10 mg of
catalyst were placed into the glass flask. The reaction was initiated by adding 30 wt% H2O2
solution with vigorous stirring. The quantitative analysis of reactants and products was
performed on a Shimadzu GC2014 gas chromatograph equipped with a WondaCAP-5 capillary
column (5% Diphenyl 95% Dimethylpolysiloxane 30m×0.32mm×0.25μm) with a flame-
ionization detector. A Shimadzu GCMS-QP2010 was used to identify substrates and their
oxidation products resulting from catalysis. Cyclohexane oxide, cyclohexane-1,2-diol, 2-
cyclohexene-1-ol and 2-cyclohexene-1-one are identified as major oxidation products. Other
byproducts include adipaldehyde, 2,3-epoxycyclohexanol and 2,3-epoxycyclohexanone. Pristine
SiO2, TiO2/SiO2, and titanium silicalite (TS-1) were tested under reaction conditions as control
experiments. The cyclohexene conversion was negligible. In the reusability measurements, the
spent catalysts were recycled using high-speed centrifugation, washed by diethyl ether, and dried
at 60 °C overnight.
3. Results and Discussion
3.1. Catalyst Characterization

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Figure 1 shows the HR-TEM images of V2O5-MoO3@SiO2 and V2O5-MoO3@(TiO/SiO2)
catalysts, respectively. For V2O5-MoO3@SiO2 catalyst, spherical shape MoO3 nanoparticles of
~13.2 nm in diameter are supported by SiO2 gel. Due to the higher Z-number, MoO3 appears to
be darker spheres. The HR-TEM images show clear crystalline structure of MoO3 with a plain
distance of 3.35 Å, consistent with the (021) facet of MoO3. Interestingly, for V2O5-
MoO3@(TiO/SiO2) catalysts, the MoO3 appears in the shape of nanorod with a length of ~20.8
nm and width of ~6.4 nm. The surface of the nanorod also shows a fringe of ~3.35 Å, similar to
what was observed to MoO3 in the V2O5-MoO3@SiO2 catalysts. EDX measurements were
carried out on both catalysts to qualitatively determine the purity of the samples (please see
Figure S1 in the Supplemental Information (SI)). While there were V, Mo, Ti and Si in V2O5-
MoO3@(TiO/SiO2) catalysts, only V, Mo and Si were detected in V2O5-MoO3@SiO2 catalysts.
There was no other impurity presence in the as-prepared catalysts.
Figure 1. TEM (up, left) and HRTEM (down, left) images of V2O5-MoO3@SiO2. TEM (up,

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right) and HRTEM (down, right) images of V2O5-MoO3@(1cTiO2/SiO2).
The purity of the sample was further confirmed using XPS survey scans (please see
Figure S2 in the SI). The elements observed in each sample were consistent with the EDX results.
Figure 2 and Table 1 compared the Mo 3d spectra obtained from the as-prepared and spent
samples. Mo 3d5/2 and 3d3/2 spin-orbit splitting of doublets were observed in Mo 3d XPS spectra.
The difference in binding energy between the doublets is 3.1 eV which matches well with the
literature values for Mo oxides.[20] In the as-prepared catalysts, only one component appears at
binding energies of Mo 3d5/2 peak at 232.9 eV and commonly assigned to Mo6+ species in
octahedral coordination. The used V2O5-MoO3@SiO2 catalysts have three components which can
be ascribed to Mo5+ species (232.3 eV), Mo6+ species in an octahedral coordination (233.0 eV),
and Mo6+ species in tetrahedral coordination (233.6 eV). With the additional of TiO2, the used
V2O5-MoO3@(TiO/SiO2) catalysts only have two components which are Mo5+ species (232.3
eV) and Mo6+ species in the octahedral coordination (233.0 eV). During the allylic oxidation
reaction, both catalysts were partially reduced from Mo6+ to Mo5+. The Mo5+ components
accounts for 68.3% and 71.3% of the molybdenum species in the used V2O5-MoO3@SiO2 and
V2O5-MoO3@(TiO/SiO2), respectively. The presence of Mo6+/Mo5+ pairs is consistent with the
literature where the pairs are considered as catalytic centers.
V 2p3/2 and 2p1/2 spin-orbit splitting of doublets were observed with a difference in
binding energy of 7.6 eV (please see Figure S3 in SI). The as-prepared catalysts have two
components of vanadium, i.e., V5+ (518.0 eV) and V4+ (517.0 eV).[21] The V5+ : V4+ ratio is
about 1 : 1 in the as-prepared catalysts. Due to the low loading of vanadium and possible

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leaching during the allylic oxidation reaction, the signal-to-noise of vanadium XPS spectra is not
sufficient to provide meaningful discussion, and therefore is not presented.
Table 1. The binding energies of Mo 3d5/2 and V 2p3/2 excitation.
VMo@SiO2 VMo@(TiO2/SiO2)
VMo@SiO2 VMo@(TiO2/SiO2)
used
used
BE (eV)
BE (eV)
BE (eV)
BE (eV)
Mo 3d5/2
V 2p3/2
-
232.9
-
-
232.9
-
232.3
233.0
233.6
232.4
233.1
-
Mo5+
Octahedral Mo6+
Tetrahedral Mo6+
518.0
517.0
518.0
517.0
-
-
-
-
V5+
V4+

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Figure 2. Mo 3d XPS spectra of the fresh and used catalysts.
XRD was used to probe the structure of the as-prepared catalysts. The as-prepared
catalysts contain α-MoO3 structure, as shown in Figure 3a, which matches well with the XPS
data fittings. Figure 3b shows several features determined using FT-IR spectroscopy. Two

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features can be ascribed to the presence of SiO2, including the Si-O-Si stretch vibration mode
-1
-1
-1
(1103 cm ) and Si-O-Si rocking mode (471 cm ). [22] The feature at 868 cm is likely the
-1
symmetric stretching mode of Mo-O-Mo and the Si-O-Si bending mode. The feature at 819 cm
is attributed to the stretching mode of vibration of asymmetric bridging oxygen in Mo-O-Mo. [23]
-1
The feature at 990 cm is assigned to the Mo=O characteristic stretching vibration of α-
MoO3.[24] The loading of vanadium and titanium were probably too low to be detected by IR
spectroscopy.
(a)
(b)
Figure 3. (a) XRD patterns and (b) FT-IR spectrums of the catalysts. Red line: V2O5-
MoO3@(1cTiO2/SiO2). Black line: V2O5-MoO3@ SiO2.
3.2. Catalyst Performance Evaluation

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Reaction conditions such as the choice of solvent, reaction temperature, amount of
catalysts in use, hydrogen peroxide to cyclohexene ration were optimized by carrying a series of
controlled experiments. Table S1 shows the results obtained by testing nine different solvents,
including acetone (CH3COCH3), methanol (CH3OH), ethanol (CH3CH2OH), acetonitrile
(CH3CN), dichloromethane (CH2Cl2), chloroform (CHCl3), tetrahydrofuran ((CH2)4O, THF),
benzonitrile (C6H5CN), and water (H2O). The non-polar solvent chloroform favored the
formation of cyclohexene oxide. The polar protic solvents (methanol, ethanol, and water)
resulted in high selectivity to α, β unsaturated alcohol or ketone and the selectivity to
cyclohexene oxide decreased with increasing solvent dielectric constant (DC). However, neither
non-polar nor polar protic solvents were ideal solvents for selective oxidation of cyclohexene as
they suffered from lower cyclohexene conversion (1% - 2%). Polar aprotic solvents in general
offered higher cyclohexene conversion as compared to non-polar and polar protic solvents.
However, the reaction pathway seemed independent of dielectric constant of the polar aprotic
solvents. While dichloromethane (DC = 9.1)and benzonitrile (DC = 26.0) favored the formation
of cyclohexene oxide, other polar aprotic solvents (DC = 7.5 – 37.5) resulted in the formation of
α, β unsaturated alcohol or ketone. In terms of reaction activity, highest conversion (11%) was
achieved when acetonitrile was used as the solvent. Therefore, acetonitrile was chosen as the
solvent for the rest of the study due to its 94% combined selectivity towards α, β unsaturated
alcohol and ketone and a relative high conversion.
The proper reaction temperature was determined by evaluating the catalysts in
temperature range from 35 °C to 75 °C with 10 °C interval (please see Figure S4). The
conversion increases with increasing reaction temperature. After 10 h, the conversion of the
reaction at 75 °C almost reaches 100% while the conversion at 35 °C is only 24%. In terms of

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selectivity, high temperature favors epoxidation pathway forming epoxy and diol and low
temperature favors allylic oxidation pathway forming α, β unsaturated alcohol and ketone. With
increasing reaction temperature, the combined selectivity to 2-cyclohexene-1-ol and 2-
cyclohexene-1-one gradually decreased from 83% at 35 °C to 23% at 75 °C. Particularly, at
75 °C the selectivity to cyclohexane-1,2-diol increases significantly to 58% with minimal
cyclohexane oxide. It implies that high temperature favors epoxidation pathway and significantly
improves the reaction rate of the subsequent hydration to form cyclohexane-1,2-diol. As a result,
a reaction temperature of 55 °C was chosen to carry out future catalyst evaluations due to its
relatively high conversion and combined selectivity to 2-cyclohexene-1-ol and 2-cyclohexene-1-
one.
The proper amount of catalyst used in the reaction was evaluated 55 °C, as shown in
Figure S5. The conversion of cyclohexene is negligible when no catalyst is added. The
conversion increases with increasing the amount of catalyst from 0 mg to 20 mg. However, the
conversion decreases when the amount of catalyst further increases. The conversion decrease is
probably because the catalyst starts to effectively accelerate the disproportionation of H2O2 into
O2 and H2O. In addition, the selectivity to cyclohexane-1,2-diol increases from 10% to 34%
when the amount of catalyst increases from 5 mg to 40 mg. As a consequence, 10 mg - 20 mg
catalysts were determined to be the proper quantity of catalysts to be used in the reaction.
The effect of H2O2 was studied by varying the H2O2/cyclohexene ratio from 0.5, 1, 1.5 to
2.0 (please see Figure S6). Under H2O2 poor condition (H2O2 : cyclohexene = 0.5 : 1), the
combined selectivity to 2-cyclohexene-1-ol and 2-cyclohexene-1-one are 91% without any
production of cyclohexane-1,2-diol. The selectivity to 2-cyclohexene-1-one decreased with

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increasing H2O2 molar ratio of hydrogen peroxide to cyclohexene. Excess amount of H2O2 did
not significantly cause formation of diol and other byproducts.
V2O5-MoO3@SiO2 and V2O5-MoO3@(TiO/SiO2) catalysts were then evaluated using the
optimized reaction conditions determined by the controlled experiments. The results are shown
in Figure 4. The presence of TiO2 significantly suppressed the epoxidation reaction and the
formation of cyclohexene oxide, leading to decrease in selectivity from 21% to 1.7%. The
lowering of cyclohexene oxide selectivity results in an increase in the selectivity to unsaturated
alcohol and ketone from 48% to 79% at similar cyclohexene conversion. The presence of TiO2
also decreases the selectivity to diol from 22% to 15% and selectivity to other byproduct from
9.2% to 4.0%.
The reusability of the V2O5-MoO3@(TiO/SiO2) catalysts were tested for five times. The
conversion of cyclohexene gradually decreases from 63% at the initial entry to 46% at the fifth
recycle, and the selectivity to cyclohexane oxide increases from 2% to 13%. This is probably due
to the leaching of active components such as TiO2 during the reaction and recycling process,
which was observed in previous studies. [14, 25] Because of the possible TiO2 leaching during
each entries, the composition of V2O5-MoO3@(1cTiO2/SiO2) catalyst gradually became similar
to the unmodified V2O5-MoO3@SiO2 catalyst, causing the reaction pathway shifted from allylic
oxidation to epoxidation.
The behavior of V2O5-MoO3@SiO2 catalyst with and without TiO2 ALD coating is very
different. According to these preliminary results, it appears that the TiO2-modified V2O5-
MoO3@SiO2 catalyst does not alter the reactivity but significantly suppresses the epoxidation

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reaction pathway. Work is in progress to elucidate these differences and overcome the challenge
of materials leaching during the reaction.
Figure 4. Catalysts performance (left) and reusability (right). (Reaction conditions: Cyclohexene
5mmol, the molar ratio of hydrogen peroxide to cyclohexene is 1.5, catalyst 10 mg, acetonitrile
10 mL, 55 °C, 10 h. The catalyst was separated by centrifugation after the previous run, dried at
100 °C under vacuum and then subjected to the following run under the same conditions.
4. Conclusions
Selective oxidation of cyclohexene was investigated for SiO2 supported V2O5-MoO3
mixed oxide catalysts. It is feasible to tune the reaction pathway between epoxidation and allylic
oxidation by the choice of solvents. Under the conditions studied in this work, low temperature,
low H2O2/cyclohexene ratio, and small amount of catalyst in use favors allylic oxidation pathway.
Upon modification of the SiO2 surface with one ALD cycle of TiO2, the V2O5-
MoO3@(1cTiO2/SiO2) catalyst showed good activity in allylic oxidation of cyclohexene
compared with its unmodified counterpart V2O5-MoO3@SiO2. The epoxidation pathway is

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largely suppressed with the presence of TiO2 on the SiO2 surface, resulting in increased
selectivity to the unsaturated alcohol and ketone, namely 2-cyclohexene-1-ol and 2-cyclohexene-
1-one. Catalyst leaching from V2O5 and TiO2decreases both the conversion and the selectivity to
the allylic oxidation products.
Acknowledgements
G. Yang, H. Du, and Z. Zhang gratefully acknowledge the grants from the National Natural
Science Foundation of China (No.21476105). M. Huff and Y. Lei acknowledge the National
Science Foundation (Grant# CBET-1511820) for the financial support.
Appendix A. Supplementary Data
Supplementary
data
to
this
article
can
be
found
online
at
http://dx.doi.org/xx.xxxx/j.catcom.2017.xx.xxx.

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ACCEPTED MANUSCRIPT
Graphical abstract
Keywords: cyclohexene, allylic oxidation, epoxidation, atomic layer deposition, mixed metal
oxides, zeolite

ACCEPTED MANUSCRIPT
Highlights
 V2O5-MoO3 mixed oxides are effective catalysts for selective oxidation of cyclohexene
 Atomic layer deposition was used to precisely deposit a uniform layer of TiO2
 The presence of TiO2 significantly suppressed the formation of byproducts
 The reaction pathways can be controlled by tuning reaction conditions