Selective aerobic oxidation of toluene to benzaldehyde on immobilized CoOx on SiO2 catalyst in the presence of N-hydroxyphthalimide and hexafluoropropan-2-ol

Article abstract of DOI:10.1016/j.catcom.2019.01.017

A cost-effective method for the selective oxidation of toluene to benzaldehyde was developed based on immobilized CoO_x on SiO₂ catalyst with predominating cobaltous ions in the presence of N-hydroxyphthalimide (NHPI) and hexafluoropr

Full text of DOI:10.1016/j.catcom.2019.01.017

Accepted Manuscript
Selective aerobic oxidation of toluene to benzaldehyde on
immobilized CoOx on SiO2 catalyst in the presence of N-
hydroxyphthalimide and hexafluoropropan-2-ol
Guojun Shi, Sihao Xu, Yan Bao, Jinyang Xu, Yuxin Liang
PII:
S1566-7367(19)30020-2
DOI:
Reference:
https://doi.org/10.1016/j.catcom.2019.01.017
CATCOM 5601
To appear in:
Catalysis Communications
Received date:
Revised date:
Accepted date:
2 August 2018
15 January 2019
23 January 2019
Please cite this article as: G. Shi, S. Xu, Y. Bao, et al., Selective aerobic oxidation
of toluene to benzaldehyde on immobilized CoOx on SiO2 catalyst in the presence
of N-hydroxyphthalimide and hexafluoropropan-2-ol, Catalysis Communications,
https://doi.org/10.1016/j.catcom.2019.01.017
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ACCEPTED MANUSCRIPT
Selective aerobic oxidation of toluene to benzaldehyde on immobilized CoOx on
SiO2 catalyst in the presence of N-hydroxyphthalimide and
hexafluoropropan-2-ol
Guojun Shi* gjshi@yzu.edu.cn, Sihao Xu, Yan Bao, Jinyang Xu, Yuxin Liang
School of Chemistry and Chemical Engineering, Yangzhou University, Yangzhou 225002,
Jiangsu Province, People’s Republic of China
*Corresponding author at: School of Chemistry and Chemical Engineering, Yangzhou
University, Yangzhou 225002, Jiangsu Province, People’s Republic of China.
Abstract
A cost-effective method for the selective oxidation of toluene to benzaldehyde was developed
based on immobilized CoOx on SiO2 catalyst with predominating cobaltous ions in the
presence of N-hydroxyphthalimide (NHPI) and hexafluoropropan-2-ol (HFIP) using ambient
molecular oxygen at room temperature. A toluene conversion of 91% and a selectivity to
benzaldehyde of 68% were realized. HFIP was confirmed to work as a restraining agent
against the further oxidation of benzaldehyde to benzoic acid on the immobilized CoOx/SiO2
catalyst in the presence of NHPI and HFIP.
Keywords: Toluene; Molecular oxygen; Selective oxidation; Benzaldehyde; Cobalt oxide;
Immobilization.
1. Introduction
Benzaldehyde presents increasing and versatile applications in perfumery,
pharmaceutical and agricultural industries, and it is generally produced by chlorination
of toluene followed by saponification. Benzaldehyde is also obtained as a by-product
of benzoic acid produced industrially, which proceeds by a liquid-phase oxidation of
toluene catalysed by cobalt ions using molecular oxygen and acetic acid as oxidant and

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solvent, respectively [1-3]. However, some long-standing problems remain unsolved
in the present mass production of benzaldehyde, such as chlorine contamination of the
oxidative end-point, serious corrosion of the equipment, environmental pollution, and
the poor space-time yield, which have motivated the academic and industrial
perspectives to seek a simple, elegant, cost-effective and benign method for the
production of benzaldehyde [1,3].
Vapor-phase oxidation of methylbenzene to benzaldehyde is favourable in a
continuous operation with a high production capacity and a waiver of separation of
catalyst, which is encountered in the homogeneous reaction [1,4-7]. However, the
substrate conversion is generally controlled at a low level to avoid overoxidation to
benzoic acid and deep oxidation products, such as COx [1,4]. The heterogeneous
photocatalysis was used to probe aerobic oxidation of toluene to benzaldehyde in
consideration of its environmentally benign characteristics, but its transformation
efficiency must be markedly enhanced till the practical application can be considered
[8, 9]. Liquid-phase oxidation of toluene can prevent the production of COx and other
small molecular products. Unfortunately, benzoic acid is found as the predominating
product because the desired benzaldehyde is much more easily oxidized than toluene
[3]. An important progress was recently made by Pappo’s group in the highly selective
formation of benzaldehyde from toluene by aerobic oxidation using a cobalt
acetate/N-hydroxyphthalimide
(NHPI)
in
the
presence
of
1,1,1,3,3,3-hexafluoropropan-2-ol (HFIP), where a yield of more than 95% for
benzaldehyde was realized using molecular oxygen at room temperature [10, 11]. It
was found that the spontaneous oxidation of benzaldehyde at room temperature can be
completely suppressed by HFIP. However, benzaldehyde was totally converted into
benzoic acid in acetonitrile or acetic acid under the same conditions. Furthermore, the
H-bond donor-acceptor interaction between benzaldehyde and HFIP was detected by
13C NMR spectra, and the strong H-bonding was supported by their computational
studies [10, 11]. Based on well-established chemistry of Ishii and co-workers [10] for
the conversion of toluene into benzoic acid (Scheme S1), the authors contributed the
highly efficient transformation from toluene to benzaldehyde to the role of HFIP,
which prevents the subsequent oxidation of benzaldehyde to benzoic acid by inhibiting
the cleavage of the aldehydic C-H bonds. In spite of the unprecedented achievement of

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Pappo’s group in the transformation from methylbenzene to benzaldehyde, there are
still some barriers that need to be removed if this transformation has to be applied
industrially, including the separation of cobalt acetate and NHPI and the usage of the
expensive solvent HFIP, which will be inevitably consumed during its use and
recycling.
In this work, cations of cobalt, manganese and cerium were supported on porous
silica, and the resulting transition mixed metal oxide catalysts were used to catalyze
the aerobic oxidation of toluene to benzaldehyde in the presence of NHPI and the
solvent HFIP. The role of HFIP was compared to that of acetic acid, acetonitrile and
ethanol, and its inhibiting mechanism in the benzaldehyde oxidation to benzoic acid
was investigated based on the immobilized cobalt oxide on SiO2 catalyst.
2. Experimental section
2.1. Catalysts preparation
CoOx/SiO2 catalyst was prepared by the incipient wetness impregnation method.
Typically, the calculated cobaltous acetate (>99.5%, SinoPharm) was dissolved in deionized
water, and 2 g of dried SiO2 (370 m2/g, SinoPharm) was added to the above solution with
continuous stirring. The obtained mixture was dried at 393 K for 12 h followed by calcination
for 4 h at 673 K in static air. MnOx/SiO2 and CeOx/SiO2 catalysts were prepared by a similar
method using manganese and cerium acetate, respectively, as the corresponding precursor.
2.2. Sample characterization
Atomic absorption spectroscopy (AAS, PerkinElmer, PE-2100) was used to determine the
actual Co loading in the prepared CoOx/SiO2 sample. An accurate amount of sample was
grinded and dissolved in a solution of hydrofluoric acid, and then diluted roughly to 5 ppm
for analysis.
N2 adsorption–desorption was conducted at 77 K on a Micromeritics ASAP 2010
micropore analysis system. About 100 mg of the sample was degassed in vacuum at 393 K
for 2 h before adsorption. The BET equation was used to determine the specific surface area
(m2/g) of the samples, and the BJH method to calculate the pore volume (cm3/g) and
average pore diameter (nm).
Powder XRD spectra were acquired by a Bruker D8 Advance X-ray diffractometer using
Cu Kα monochromatic radiation source (λ=0.15406 nm). The 2θ degree scans covered the

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range 10 - 80º with a step of 0.02 º. The applied voltage and current were 40 kV and 40 mA,
respectively.
TEM images were obtained using a Tecnai 12 electron microscope to determine the
morphology and mean Pt particle size of the catalyst samples. The latter were dispersed in
deionized water by means of ultrasonic bath for 15 min after careful grinding. The HRTEM
analysis was carried out using a Tecnai G2 F30 S-TWIN electron microscope.
FT-IR spectra were recorded on a TENSOR 27 spectrometer with a resolution of 4 cm-1
in the 400~2000 cm-1 range. The self-supporting wafer diluted by KBr was prepared with a
sample concentration of 1%.
The laser Raman spectra (LRS) of the samples were obtained at room temperature in air
on an invia Raman microscope spectrometer (RENISHAW) equipped with a CCD detector.
The 532 nm line of an Ar+ laser was used as the excitation source with an intensity of 10
mW.
The UV-Vis/DRS spectra were recorded on a Cintra10e UV-Vis spectrometer equipped
with a diffuse reflectance attachment. BaSO4 was used as a reference. All spectra were
recorded at room temperature under ambient gas atmosphere. A sample holder was used to
support the wafer of a sample of  100 mg.
XPS measurements were performed on an ESCALAB250Xi electron spectrometer.
Monochromatic Al Kα (1486.6 eV, 15 kV) was used as incident radiation. The obtained
binding energies were determined with an overall resolution better than 0.05 eV. The C 1s
binding energy from adventitious hydrocarbon was used as a charge reference, and this was
fixed at 284.8 eV.
2.3. Catalytic activity measurements
The catalytic performance of the synthesized immobilized transitional metal oxides was
evaluated towards the aerobic oxidation of toluene. The reaction was carried out in a Schenk
tube (10 ml) at room temperature under an O2 atmosphere (>99.9%, 1 atm). In a typical
reaction, 0.50 mmol of toluene, 0.005 g of catalyst, 0.050 mmol of NHPI and 1 ml of HFIP
were introduced into the Schenk tube, and the rest of the tube was repeatedly purged by O2
(99.9%) before measurements. The resulting mixture after 4 h of reaction was analyzed by
off-line GC (SHIMADZU GC-2014) equipped with a SGE AC-10 capillary column and a
flame ionization detector.
In a scale oxidation, 100 mmol of toluene, 1.0 g of catalyst (2 mmol/g), 10 mmol of NHPI

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and 200 ml of HFIP were added into a three-necked flask. The reaction was carried out at
room temperature for 4 h under continuous magnetic stirring after a careful displacement of
the gaseous place of the flask using O2 (99.9%). The reaction mixture was filtered to recycle
the immobilized catalyst, and the liquid part was analyzed by GC. The recycled catalyst was
repeatedly washed using anhydrous ethanol followed by drying in static air at 120 oC for 10 h.
The solvent HFIP was recycled by distillation.
3. Results and discussion
3.1. Textural and structural properties
The SiO2 (silica gel, 370 m2/g, 7.5 nm) carrier was used to deposit the cobalt
acetate by the incipient wetness impregnation, and the resulting CoOx/SiO2 catalyst (2
mmol/g) exhibited excellent textural properties and a high degree of dispersion of
cobalt oxide on the support (Fig. S1, Table S1 in ESI). The content of cobalt ions in
the catalysts was determined by ICP-OES, and this was found well agreeing with the
nominal Co loading (Table S2).
1100
 CoO JCPDS 71-1178
B
904
572
A
670
CoOx/SiO
1231
964
468
1636
1450
2


CoOx/SiO2
SiO
2
SiO2
10
20
30
40
50
60
70
80
2000 1800 1600 1400 1200 1000 800 600 400
Wavenumber(cm-1
)
2 (Degree)
Fig. 1. Powder XRD patterns (A) and FT-IR spectra (B) of support SiO2 and of the synthesized CoOx/SiO2
catalyst (2 mmol/g).
Powder XRD, FT-IR, Raman and UV-Vis/DRS were adopted to investigate the
phase of cobalt oxide in the synthesized CoOx/SiO2 catalyst, and results are shown in
Fig. 1 and Fig. S3 (ESI), respectively. Very weak XRD characteristic peaks at ca. 2θ
degree of 36o and 42o were detected on the CoOx/SiO2 catalyst, which can be assigned
to CoO with face-centered cubic CoO structure (JCPDS 71-1178), also suggesting a
high dispersion degree of cobalt oxide on the support (Fig. 1a). A weak absorption
peak located at 
1450 cm-1 in the FT-IR spectrum of CoOx/SiO2 catalyst recorded can

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be assigned to CoO species (Fig. 1B). However, the absorption peaks at 572 and 670
cm-1 can be attributed to the presence of a spinel Co3O4 phase [12,13]. The other
absorption IR bands seen in the FT-IR spectrum are due to the support and its surface
hydroxyls [14]. In consideration of a weak signal of CoO in FT-IR [12], this suggests
that the prepared CoOx/SiO2 catalyst contained a considerable amount of CoO besides
Co3O4.
In the Raman spectrum of CoOx/SiO2 catalyst (Fig. S2A, ESI), the characteristic
bands observed at 478, 515, 616 and 685 cm-1 can be ascribed to the presence of the
crystalline Co3O4 [15]. However, the bands centred at 460 and 670 cm-1 should be
assigned to cobalt oxide, CoO [16]. The diffuse reflectance spectrum of the CoOx/SiO2
catalyst presented absorption bands at 521, 591 and 657 nm (Fig. S3B, ESI), which is
an indication of the presence of Co2+ ions [13]. A weak shoulder at 765 nm might
suggest the presence of Co3O4 spinel [13].
3.2. Surface oxidation states of cobalt
Fig. 2 compares the Co 2p XP spectrum of the CoOx/SiO2 catalyst with that of the
bulk of Co3O4, which was prepared by thermal decomposition of cobalt acetate at 673
K in static air, and confirmed by XRD as a pure phase of Co3O4. The XP spectrum of
the bulk Co3O4 was in line with the characteristics reported in the literature [19]. The
Co 2p XP spectrum of the CoOx/SiO2 catalyst exhibits two main peaks at 781.6 and
797.5 eV, corresponding to the Co 2p3/2 and Co 2p1/2, respectively, with a
spin-energy separation of 16.1 eV, which is in agreement with the XP spectra of CoO
reported [19,20]. In addition, two shake-up peaks at 6.0 eV higher binding energy
further confirmed the domination of the cobaltous ions on the surface of the prepared
CoOx/SiO2 catalyst [20].

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779.9
794.8
Co3O4
781.6
787.6
797.5
803.5
CoOx/SiO2
810 805 800 795 790 785 780 775
Binding energy (eV)
Fig. 2 Core level Co 2p XP spectra of the bulk Co3O4 and the synthesized CoOx/SiO2
catalyst.
3.3. Morphologies and elemental dispersion
The TEM image and elemental mapping of the CoOx/SiO2 catalyst are shown in
Fig. 3 and Fig. S4 (ESI), respectively. It was found that CoOx appears as nanoparticles
of less than 5 nm in size, and homogeneously dispersed on the SiO2 carrier, result
which agrees with the XRD pattern shown in Fig. 3a. Fig. 3B presents a clear lattice
spacing of ca. 0.25 nm, which can be assigned to the (111) crystal plane of CoO
(JCPDS 71-1178). The energy dispersive spectrum of the CoOx/SiO2 catalyst
confirmed cobalt oxide(s) as the sole species besides SiO2 (Fig. S5, ESI). The STEM
image and the elemental mapping images, further demonstrated that cobalt, oxygen
and silicon were highly dispersed (Fig. S4A, B, ESI). In addition, the TEM images of
CoOx/SiO2 catalyst with different loadings were compared and the results are shown in
Fig. S6 (ESI). It was found that the mean particle size of CoOx increased with
increasing loading. A superfluous loading of CoOx (4 mmol/g) led to a sharp increase
in the size of cobalt oxide particle and an obvious aggregation.

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B
A
0.24 nm
Fig. 3 TEM image (A) and HRTEM image (B) of CoOx/SiO2 catalyst.
3.4. Catalytic performance
Aerobic oxidation of toluene to benzaldehyde over the prepared catalysts was carried out
at room temperature in the presence of NHPI and HFIP, and the results are shown in Table 1.
No conversion of toluene is observed in the absence of CoOx and NHPI (entry 1), indicating
that autoxidation of toluene is negligible under the applied reaction conditions. Only
CoOx/SiO2 catalyst was found inert for toluene oxidation under the conditions applied (entry
2). There is a toluene conversion of 0.6% detected when only NHPI was used as catalyst
(entry 3), which suggests that NHPI itself can catalyze the oxidation of toluene to
benzaldehyde, benzyl alcohol and benzoic acid, with a very low reactivity.
The supported CoOx/SiO2 catalyst (entry 4) exhibits a high toluene conversion (91.2%) in
the presence of HFIP, which is similar to that observed in a homogeneous reaction by Pappo’s
group [10]. This shows that the immobilized cobalt oxide on silica and the homogeneous
NHPI can synergistically catalyze the selective oxidation of toluene to benzaldehyde in the
HFIP. However, ca. 68% of selectivity to benzaldehyde was detected, and this is lower than
that observed by Pappo et. al. [10], indicating that the aerobic oxidation of toluene over the
immobilized CoOx catalyst could proceed in a different way than the homogeneous catalytic
reaction. With respect to the CoOx/SiO2 catalyst, the MnOx/SiO2 presents a much lower
reactivity (entry 5), and the supported cerium oxide catalyst shows a poor activity for toluene
oxidation (entry 6). These results indicate that the immobilized cobalt oxide catalyst is more
active for the selective oxidation of toluene to benzaldehyde under the applied reaction

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conditions. The immobilization of cobalt ions is favorable for the separation of the supported
cobalt catalyst from the reaction mixture and the recycling of the solvent HFIP.
The effect of different solvents used on the selective oxidation of toluene to
benzaldehyde in the presence of CoOx/SiO2, NHPI and dioxygen was investigated
(entry 7-10). There is a much lower toluene conversion observed in
2,2,2-trifluoroethanol than that in HFIP (entry 7). With acetic acid as the solvent, the
reaction proceeded to a relatively high toluene conversion, but a severely low
selectivity to benzaldehyde is observed with benzoic acid as the predominating
product (entry 8). Compared with the reactions in HFIP, 1,1,1-trifluoroethanol and
acetic acid, lower toluene conversion (22%) was found in acetonitrile (entry 9).
However, toluene was nearly converted in ethanol under the same conditions (entry
10). It is clear that, over the immobilized cobalt oxide catalyst, HFIP markedly
promoted toluene conversion and the selective formation of the desired benzaldehyde.
The dramatic enhancement in the selectivity to benzaldehyde was ascribed to the
formation of a H-bond adduct between HFIP and benzaldehyde, which largely slowed
down the abstract of the aldehydic C-H bond, and restrained the production of benzoic
acid from benzaldehyde via a Ph(CO)• intermediate [10]. Certainly, the positive role
of HFIP was also embodied in an obvious improvement of toluene conversion. The
role of HFIP might be attributed to its promotional role on the activation of
molecular oxygen by forming hydrogen bonds between them [21-23].
The SiO2 support presented no reactivity under the applied reaction conditions
(entry 11), and the non-supported Co3O4 gave a poor reactivity (entry 12). The
prepared CoOx/SiO2 catalysts with loading between 0.5 and 4 mmol/g exhibit high
toluene conversions and benzaldehyde selectivities (entry 4, 13-15), and the optimized
reactivity was found at the loading of 2 mmol/g, which might be attributed to the
chemical state (Fig. 2) and high dispersion degree of the cobalt ions on SiO2 carrier
(Fig. S6, ESI). The reaction was also performed in static air atmosphere on the
optimized CoOx/SiO2 catalyst (entry 16), and a much lower reactivity was observed,
suggesting that a high concentration of dioxygen will facilitate the transformation of
toluene to benzaldehyde.

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Table 1. Aerobic oxidation of toluene on supported CoOx catalysts with NHPI in solvents at room
temperature. a
Selectivity (%)
Loading
(mmol/g)
Starting
material
Conversion
(%)
Entry
Catalyst
Solvent
Benzyl
alcohol
Benzoic
acid
Benzaldehyde
1
2
3
4
5
6
Blank b
CoOx/SiO2
Blank d
toluene
toluene
toluene
toluene
toluene
toluene
HFIP
HFIP
HFIP
HFIP
HFIP
0
-
-
-
c
0
-
-
-
0.6
91.2
26.0
1.0
41.7
68.8
62.3
36.4
14
44.3
20.3
1.7
15.5
CoOx/SiO2
MnOx/SiO2
CeOx/SiO2
2
2
2
8.4
30.7
14.4
HFIP
2,2,2-trifluoroethan
ol
7
CoOx/SiO2
2
toluene
30.4
75.2
53.9
3.2
16.6
0.5
21.6
93.5
8
CoOx/SiO2
CoOx/SiO2
CoOx/SiO2
SiO2
2
2
toluene
toluene
toluene
toluene
toluene
toluene
toluene
acetic acid
9
acetonitrile
ethanol
HFIP
23.8
1.3
0
21.5
2.2
-
2.2
0.0
-
71.8
97.8
-
10
11
12
13
14
15
16
2
-
Co3O4
-
HFIP
7.0
62.1
58.9
63.9
23.4
34.0
27.2
1.9
3.2
6.2
CoOx/SiO2
CoOx/SiO2
CoOx/SiO2
0.5
1
HFIP
47.3
69.2
HFIP
4
toluene
toluene
HFIP
HFIP
81.4
48.4
69.0
61.5
16.7
34.0
11.7
2.5
e
CoOx/SiO2
2
benzyl
alcohol
17
CoOx/SiO2
2
HFIP
99.8
19.2
-
80.6
31.8
92.2
55.5
-
0
99.1
19.2
3.6
18
19
20
CoOx/SiO2
CoOx/SiO2
2
2
2
benzaldehyde
HFIP
HFIP
HFIP
f
toluene
69.7
65.1
9.5
30.3
g
CoOx/SiO2
toluene
a Reaction conditions: toluene (0.50 mmol), supported oxide catalyst (2 mmol/g, 0.005 g), NHPI (0.05
mmol), O2 (99.9%, 0.1 MPa), solvent (1 ml), 4 h. The composition of reaction mixture was analyzed by
GC-FID.
b Without CoOx/SiO2 and NHPI.
c Without NHPI.
d Only with NHPI.
e Tested under air gas atmosphere.
f toluene (100 mmol), catalyst (2 mmol/g, 1.0 g), NHPI 10 mmol), O2 (99.9%, 0.1 MPa), solvent (200 ml).
g evaluated using the catalyst recycled from entry 19.
The role of HFIP was further investigated by using benzaldehyde and benzyl
alcohol as the raw materials, and the reactions were carried out under the same
experimental conditions (entry 17, 18). The CoOx/SiO2 catalyst presented a high
conversion of benzyl alcohol, up to 99.8% under the adopted experimental conditions,
and the predominating product is benzoic acid besides 19.2% of selectivity to
benzaldehyde. In the case of benzaldehyde, only 31.8% of conversion is observed and
the benzoic acid is the exclusive resultant, indicating that HFIP can restrain the
benzaldehyde transformation towards benzoic acid. However, the fact that part of

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benzaldehyde can be converted into benzoic acid under the same reaction conditions
adopted in toluene transformation, clarified that HFIP cannot prevent the conversion
of benzaldehyde to benzoic acid under the reaction conditions. This result can also
explain why the observed selectivity to benzaldehyde on CoOx/SiO2 catalyst was 20%
lower than that reported in homogeneous catalysis [10].
Benzoic acid was the sole product when benzaldehyde was used as the raw
material under the adopted experimental conditions, suggesting that benzaldehyde can
be directly converted into benzoic acid and the disproportionation of benzaldehyde
into benzyl alcohol and benzoic acid (Cannizzaro reaction) could not occur. A low
selectivity to benzaldehyde (19.2%), which is lower than benzaldehyde conversion
(31.8%) when it is the starting material (entry 18), was observed when benzyl alcohol
was used as the raw material under the same experimental conditions (entry 17),
suggesting that benzyl alcohol can be directly converted into benzoic acid besides
benzaldehyde. There was no toluene detected in the product mixture, indicating the
absence of the disproportionation reaction of benzyl alcohol.
The effect of reaction time on toluene conversion and selectivity to benzaldehyde,
benzyl alcohol and benzoic acid were investigated, and results are depicted in Fig. 4.
At the beginning of reaction (0.5 h), benzaldehyde and benzyl alcohol are the
predominating products, and there is little benzoic acid produced, which suggests that
benzoic acid is not directly generated from toluene. There is a higher selectivity to
benzaldehyde than benzyl alcohol at the initial stage of reaction, but 80% of benzyl
alcohol is converted to benzoic acid when it is used as starting material (entry 17),
which suggests that the desired benzaldehyde can be directly generated from toluene
besides benzyl alcohol.
A competitive side reaction from toluene to benzyl alcohol can be suggested
because ca. 40% of selectivity to benzyl alcohol was observed at the initial stage of
reaction. However, research results reported by the groups of Pappo and Ishii clarified
that the autoxidation of toluene does not proceed via benzyl alcohol in homogeneous
catalysis [2, 10]. In fact, benzyl alcohol was found as an intermediate in the production
of benzaldehyde from toluene by heterogeneous catalysis over transition metal oxides
[24-27]. It was suggested the benzyl radical can react with the Co(III) hydroperoxide

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to produce the intermediate benzyl alcohol under the adopted experimental conditions,
which is shown in Eq. (1):
LnCoIIIOOCoIIILn
LnCoIIIOOH
CH2OH
LnCoIIIOO
CH2
CH3
(1)
The selectivity to benzyl alcohol monotonously decreased with reaction time,
which can be ascribed to the competition between its generation from toluene and its
further oxidation to benzaldehyde with reaction time. The generation and consumption
of benzaldehyde also proceeded at the same time and an increasing trend was observed
before 3.5 h of reaction, and the observed relatively high selectivity to benzaldehyde
should be attributed to the inhibiting role of HFIP on benzaldehyde transformation to
benzoic acid [10].
toluene coversion
100
benzaldehyde selectivity
benzyl alcohol selectivity
benzoic acid selectivity
80
60
40
20
0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
Reaction time (h)
Fig. 4 Effect of reaction time on toluene oxidation at room temperature.
An enlarge-scaled reaction was carried out to probe the possibility of the further
application of the catalyst, with 1 g of catalyst, 200 ml of HFIP and 10 mmol of NHPI,
and catalytic results can be found in entry 19 of Table 1. A toluene conversion of 92.2%
and a selectivity to benzyl alcohol of 69.7% are observed, which are similar to those
observed in the Schenk tube over the same catalyst (1 ml of HFIP). About 91% of the
solvent and 94% of the immobilized catalyst (CoOx/SiO2) were recycled. The results
suggest that the recycling of the solvent and the immobilized catalyst are possible
under the experimental conditions. However, toluene conversion was found a
decrease when the used catalyst was evaluated in spite of an increased combined

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selectivity to benzaldehyde and benzyl alcohol (entry 20). The loss of catalytic activity
can be partially ascribed to the loss of cobalt species in the used CoOx/SiO2 catalyst,
since part of cobalt species was leached during the catalytic test (Table S2, ESI).
Therefore, further investigations are required to decrease the loss of cobalt species
and thus enhance reusability of the immobilized cobalt ions on SiO2 catalysts. An
immobilization of cobalt ions by coprecipitation or ion exchange instead of
impregnation appears possible due to stronger interactions between the cobalt ions
and the substrate.
4. Conclusions
CoOx/SiO2 catalysts were prepared by the incipient wetness impregnation method
and the cobaltous oxide was found as the predominating phase on SiO2. Toluene was
selectively oxygenated to benzaldehyde on the prepared CoOx/SiO2 catalysts in the
presence of NHPI and HFIP using molecular oxygen at room temperature. A high
toluene conversion and a benzaldehyde selectivity up to 91.2% and 68.8%,
respectively, were acquired. HFIP was observed to play an outstanding promotional
role on the conversion of toluene to benzaldehyde and an obvious restraining role on
the further oxidation of benzaldehyde to benzoic acid on the immobilized CoOx/SiO2
catalyst.
Acknowledgements
The authors gratefully acknowledge the funding of project by the Priority
Academic Program Development of Jiangsu Higher Education Institutions.
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Graphical abstract
toluene coversion
100
benzaldehyde selectivity
benzyl alcohol selectivity
benzoic acid selectivity
80
60
40
20
0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
Reaction time (h)

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Highlights

Immobilized CoOx catalysts were prepared with predominating cobaltous ions
on SiO2.


The immobilized Co ions were active for toluene oxidation with NHPI in HFIP.
A toluene conversion of 91.2% and a benzaldehyde selectivity of 68.8% were
found.

HFPI promoted the selective formation from toluene on CoOx/SiO2 with NHPI.