Selective oxidation of arylalkanes with N-Graphitic-Modified cobalt nanoparticles in water

Article abstract of DOI:10.1016/j.catcom.2017.04.038

N-Graphitic-Modified cobalt nanoparticles supported on carbon powder (Co/phen@CB-800-L) were prepared and found to be efficient catalysts for selective oxidation of arylalkanes to aromatic ketones with tert-butyl hydroperoxide (TBHP) in water. A broad sco

Full text of DOI:10.1016/j.catcom.2017.04.038

Accepted Manuscript
Selective oxidation of arylalkanes with N-Graphitic-Modified
cobalt nanoparticles in water
Dong-Huai Tu, Yang Li, Jiangwei Li, Yu-Jie Gu, Bo Wang, Zhao-
Tie Liu, Zhong-Wen Liu, Jian Lu
PII:
DOI:
Reference:
S1566-7367(17)30173-5
doi: 10.1016/j.catcom.2017.04.038
CATCOM 5023
To appear in:
Catalysis Communications
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Revised date:
Accepted date:
26 January 2017
11 April 2017
18 April 2017
Please cite this article as: Dong-Huai Tu, Yang Li, Jiangwei Li, Yu-Jie Gu, Bo Wang,
Zhao-Tie Liu, Zhong-Wen Liu, Jian Lu , Selective oxidation of arylalkanes with N-
Graphitic-Modified cobalt nanoparticles in water. The address for the corresponding
author was captured as affiliation for all authors. Please check if appropriate.
Catcom(2017), doi: 10.1016/j.catcom.2017.04.038
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ACCEPTED MANUSCRIPT
Selective oxidation of arylalkanes with N-Graphitic-Modified cobalt
nanoparticles in water
Dong-Huai Tu,a Yang Li,a Jiangwei Li,a Yu -Jie Gu,a Bo Wang,a Zhao-Tie Liu,*b Zhong-Wen Liu,b
Jian Lu*a
a State Key Laboratory of Fluorine & Nitrogen Chemicals
Xi’an Modern Chemistry Research Institute
Xi’an, 710065, China
b Key Laboratory of Applied Surface and Colloid Chemistry, Ministry of Education
School of Chemistry and Chemical Engineering
Shaanxi Normal University
Xi’an, 710119, China
* Corresponding authors
E-mail addresses: lujian204@263.net (Jian Lu), ztliu@snnu.edu.cn (Zhao-Tie Liu).

ACCEPTED MANUSCRIPT
Abstract
N-Graphitic-Modified cobalt nanoparticles supported on carbon powder
(Co/phen@CB-800-L) were prepared and found to be efficient catalysts for selective
oxidation of arylalkanes to aromatic ketones with tert-butyl hydroperoxide (TBHP) in
water. A broad scope of substrates have been investigated, the desired products were
obtained with yields as high as 99% under the optimized condition in all cases.
Remarkably, this carbon powder supported catalyst could be recovered easily, and
showed high activity within six successive runs.
Keywords:
Cobalt nanoparticles
Nitrogen-doped material
Arylalkanes
Selective oxidation

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1. Introduction
The selective oxidation of arylalkanes (especially sp3 C-H oxidation) is one of
the most important and challenging processes in organic synthesis [1-3]. So far,
various oxidants, such as metal oxides [4], peroxides [5, 6], and halides [7] have been
employed in plenty of studies [8-10]. However, the use of these oxidants usually
produces a large volume of toxic wastes and causes series of environmental issues.
Recently, several methods using homogeneous, and/or heterogeneous catalysts, which
employed noble metals such as Rh, Ru, Pd, Ir [11-14], or non-noble metals such as Fe,
Cu, Mn, V [15-18] have been reported for the oxidation of arylalkanes. Despite these
great advances, the activity, atom economy, and catalysts preparation cost are still less
than satisfactory. Therefore, there is still high demand to explore more efficient
catalyst system for oxidation of arylalkanes under sustainable reaction condition.
As the catalyst is the most important for this process, studies are still ongoing to
obtain appropriate catalysts [19-22]. N-Graphitic-Modified cobalt nanoparticles
supported on carbon materials (Co/N@C) are a kind of potential catalysts due to their
facile preparation, low cost, high activity, and corrosion resistance for
oxidation-reduction reactions [23-28]. In 2014, Beller and co-workers [29, 30] have
reported several redox reactions using this kind of catalysts. Almost at the same time,
Zhang et al. [31, 32] also presented several studies about the active sites of the
catalysts. In addition, there are also a few reports about their application in redox
reactions [33-35]. However, to the best of our knowledge, the application of Co/N@C
as a catalyst in selective oxidation of arylalkanes has not been reported until now.
Herein, we reported an efficient and facile approach for this challenging and
important process catalyzed by Co/phen@CB-800-L using TBHP as an oxidant in
water without a phase transfer catalyst or base. A broad scope of carbonyl compounds
(22 examples) with yields as high as 99% were obtained. Remarkably, the catalyst
Co/phen@CB-800-L could be recovered easily, and showed high activity within six
successive runs.

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2. Experimental and instrumentation
2.1 Preparation of catalysts
Co/phen@CB-800 was prepared according to the literature [29, 31] with some
modifications. Briefly, a mixture of Co(OAc)2·4H2O (1.0 mmol, 248 mg) and
1,10-phenanthroline monohydrate (2.0 mmol, 396 mg) was added to 100 mL of
ethanol and stirred for 2 h at 30 °C. Then, 1.4 g vulcan XC72R carbon powder (CB)
was added and the mixture was stirred at 70 °C for another 6 h. The mixture was
cooled down and ethanol was removed slowly under vacuo. The remaining solid was
dried over night at 80 °C and then transferred to a quartz boat in the oven and fluxed
with N2 for 30 min. Then the solid was heated to 800 °C with a rate of 10 °C/min to
pyrolysis and was held at 800 °C for 2 h in N2. Co/phen@CB-800-L was prepared by
acid leaching of Co/phen@CB-800 in 0.5 mol/L HCl at 80 °C for 6 h, followed by
thoroughly washing with deionized water. Co@CB-800 was prepared with the same
procedure as that used for preparation of Co/phen@CB-800 except that no
1,10-phenanthroline was included. The same method without Co(OAc)2·4H2O or CB
was employed to prepare N@CB-800 or Co/phen-800. Co@CB-800-L was prepared
by acid leaching of Co@CB-800 in 0.5 mol/L HCl at 80 °C for 6 h, followed by
thoroughly washing with deionized water.
2.2 Characterizations
Catalyst powder X-ray diffraction (XRD) was performed by a DX-2500 X-ray
diffractometer with a scanning angel (2θ) of 10o-80o, operated at 40 kV and 40 mA.
The high-resolution transmission electron microscopy (HRTEM) observations were
carried out on a JEM-2100F microscope. Prior to the measurement, catalyst powders
were ultrasonicated in ethanol for 1 h, dispersed onto TEM copper grids, and then
dried at room temperature. X-ray photoelectron spectroscopy (XPS) measurements
were conducted on a Thermo ESCALAB 250Xi system. Energy calibration was
carried out using the C 1s peak of adventitious C at 284.6 eV. Yields of acetophenone
1
13
were determined by GC using bromobenzene as internal reference. H and C NMR
spectra were recorded on 500 MHz spectrometer. BET surface area and pore size

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measurements were performed with N2 adsorption/desorption isotherms at 77 K on a
Micromeritics ASAP 2020 instrument. The concentration of soluble Co species in
collected samples was determined by inductively coupled plasma emission
spectrometer (ICPE-9000, Japan).
2.3. Catalytic oxidation of ethylbenzene using TBHP
In a typical experiment, oxidation of ethylbenzene with TBHP was performed in
a 25 mL round bottom flask equipped with a reflux condenser, filled with
ethylbenzene (1.0 mmol), catalyst (20 mg), solvent (3 mL), and TBHP (2.0 mmol,
70% aqueous solution). Then, the mixture was vigorously stirred at 70 °C for 6 h.
After cooling down, the catalyst was separated by centrifugation and washed with
acetonitrile for three times for the successive run. Then, water (30 mL) was added to
the supernatant and the mixture was extracted with dichloromethane (3  15 mL).
GC-MS was performed to identify the desired product in organic layers, and authentic
sample was also analyzed to compare with the aimed product. The yield was
determined by GC analysis using bromobenzene as an internal standard.
3. Results and discussion
In order to evaluate the activity of Co/phen@CB-800 in selective oxidation of
arylalkanes, we initially chose ethylbenzene 1a as model substrate to optimize the
reaction condition. A solution of Co/phen@CB-800 (20 mg), ethylbenzene 1a (1.0
mmol), TBHP (2.0 mmol), and 1,4-dioxane (3 mL) was stirred at 70 C. Fortunately,
after 6 h, the desired product acetophenone 2a was isolated, although the yield was
only 12% (Table 1, entry 1). Control experiments revealed that both the metal salt and
1,10-phenanthroline were indispensable for the reaction (Table 1, entries 2-3). The use
of support (CB) could improve the catalytic activity conspicuously (Table 1, entry 1
and 4). To our surprise, acid leaching can also improve the catalytic activity (Table 1,
entry 1 and 6), but it had no effect on the activity of Co@CB-800 (Table 1, entry 2
and 5). Then the reaction solvent was screened (Table 1, entries 7-12), and the results
showed that using water, a high yield of 85% was obtained (Table 1, entry 11). Next,
the influence of oxidant was examined, and it could be seen that TBHP was the most

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suitable oxidant for the reaction (Table 1, entries 13-16). Moreover, when the amount
of catalyst reduced to 10 mg (0.1 mol% Co), the conversion of 1a and the yield of 2a
showed no change (Table 1, entry 18). When the reaction temperature was raised to
80 C and the reaction time was extended to 8 h, the reaction system could give 2a
with an excellent yield of 99% (Table 1, entry 21). Hence, the optimized reaction
condition was as follows: Co/phen@CB-800-L (10 mg, 0.1 mol% Co), TBHP (3.0
mmol), water as the solvent, 80 °C for 8 h.
Table 1
Based on the above results, the oxidations of various arylalkanes were explored,
and the results were shown in Table 2. All the tested arylalkanes could be oxidized to
the corresponding aromatic ketones. Arylalkanes with electron-donating groups at the
para- or meta-position are suitable substrates and the corresponding aromatic ketones
were obtained in good to excellent yields (2b, 2c, 2m). However, arylalkanes bearing
electron-donating group at ortho-position resulted in moderate yield of product 2r
(82%), thus revealing
a
strong steric effect. When arylalkanes with
electron-withdrawing groups at the para- and ortho- or para-position were used,
moderate yields of aromatic ketones were obtained (2h-2l, 2n-2q, 2s, 2t).
Interestingly, the trifluoromethyl-substituted arylalkanes were well tolerated in this
transformation (2l, 2p). To our delight, substrate fluorene 1v could be oxidized to
fluorenone (2v) in an excellent yield of 98%. The above results indicated that the
as-prepared Co/phen@CB-800-L was an efficient catalyst for oxidation reaction and
was able to oxidize various arylalkanes to aromatic ketones with high to excellent
yields under a mild condition.
Table 2
In order to probe the catalyst structure that is responsible for the high activity and
good stability of the Co/phen@CB-800-L, we deliberately designed control
experiments and performed extensive characterizations using XRD, BET, XPS and
HRTEM techniques.
It is obvious that either Co@CB-800 or phen@CB-800 was inactive (Table 1,
entries 2-3), suggesting that all of the Co, N, and C elements were indispensable for

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the overall catalytic performances. Therefore, the XRD analysis was used to identify
the composition of Co/phen@CB-800 and Co/phen@CB-800-L firstly. The peaks of
Co3O4 could be detected in Co/phen@CB-800 (Figure S1). However, in
Co/phen@CB-800-L, these peaks disappeared suggesting that the washable cobalt
was mainly particulate Co3O4. It should be noted that no diffraction peaks of Co
nanoparticles were observed in Co/phen@CB-800-L due to the small size, low
loading of cobalt nanoparticles and high dispersion on the support. In addition, the
catalytic activity of Co/phen@CB-800-L was higher than that of Co/phen@CB-800
(Table 1, entry 1 and 6), indicating that the Co3O4 might not be the catalytic sites.
Meanwhile, the specific surface area, pores volume and average pores diameter of
Co/phen-800 and Co/phen@CB-800-Lwere analyzed by BET. As shown in Table S1,
the surface area of Co/phen@CB-800-L is larger, which is beneficial for promoting
the active component uniform dispersion, than that of Co/phen-800. This may be the
reason that the catalytic activity of Co/phen@CB-800-L was higher than that of
Co/phen-800 (Table 1, entry 4 and 6). Figure S2 showed that the adsorption isotherms
were characteristic of mesoporous material of type IV (as per the IUPAC
classification).
The surface compositions and elemental analysis of various materials were
measured by XPS and summarized in Table S2. Materials prepared without Co(OAc)2,
1,10-phenanthroline or support showed low activity, while the Co/phen@CB-800-L
that contained the lowest amounts of Co and nitrogen showed the highest activity.
These results clearly showed that the nitrogen-doped graphitic layers with Co on the
surface of CB are crucial for the activity in selective oxidation of arylalkanes [27].
The chemical states of Co and N were also measured with XPS. The N 1s spectrum of
the materials disclosed graphitic, oxidized nitrogen, pyridinic nitrogen or nitrogen
species bound to Co [27, 31] (Figure S3). The less active catalyst prepared without
Co(OAc)2 or support mainly contained oxidized nitrogen and pyridinic nitrogen. And
the Co/phen@CB-800-L contained the most graphitic nitrogen. Notably, the presence
of N-doped graphitic layers on the support promotes the selective oxidation of
arylalkanes. In addition, The Co 2p spectrum of Co/phen@CB-800 and

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Co/phen@CB-800-L revealed 780.3 eV for Co 2p3/2 (Figure S4), it indicated that
cobalt (Ⅱ) exist on the surface [27].
The most active catalysts, Co/phen@CB-800 and Co/phen@CB-800-L, were
investigated by high-resolution transmission electron microscopy (HRTEM). As
shown in Figure S5, there were at least two types of Co-containing species. Those
were Co oxide particles and cobalt nanoparticles. Most of the species are closely
attached to the support CB. While, Co oxide particles disappeared after acid leaching,
the remaining cobalt nanoparticles dispersed uniformly on the surface of CB with
N-doped graphitic layers in some complex graphitic structures (Figure S6). This
assumption is also supported by the XRD and elemental analysis results.
As is well known, the reusability and stability of a heterogeneous catalyst are of
great importance from the views of industry, economy and green chemistry. Therefore,
the reusability of the catalyst Co/phen@CB-800-L was studied by the model
oxidation reaction using 1a (1 mmol) under the standard condition. After the reaction,
the catalyst was separated from the reaction mixture by simple centrifugation, washed
with acetonitrile for three times and used again for the oxidation of 1a under the same
condition. As shown in Figure 1, the Co/phen@CB-800-L showed high catalytic
activity within six successive runs. The conversion of ethylbenzene and the yield of
acetophenone were all higher than 90%. At the same time, ICP analyses were
performed for each experiment to establish the concentration of soluble Co species in
aqueous/organic phases. As shown in Table S3, no soluble Co species was detected in
either aqueous or organic phase except micro-amount in aqueous phase in the first
process. These results indicated that Co/phen@CB-800-L is a highly efficient,
recyclable and environmentally friendly catalyst for selective oxidation of arylalkanes
under the mild condition.
Fig. 1
4. Conclusions
In conclusion, we have demonstrated that the arylalkanes could be selectively
oxidized to the corresponding aromatic ketones via prepared Co/phen@CB-800-L

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catalyst with TBHP in water. A broad scope of substituted arylalkanes have been
explored. The desired aromatic ketones were obtained in good to excellent yields.
Remarkably, the Co/phen@CB-800-L catalyst could be recovered easily, and showed
high activity within six successive runs. This work might provide some guides for the
future application in organic synthesis and industry production.
Acknowledgements
The authors gratefully acknowledge the financial support from the National Natural
Science Foundation of China (21327011, 21503610), the Program for Changjiang
Scholars and Innovative Research Team in University (IRT-14R33), and the Post-
Doctoral Research Project of Shaanxi Province.
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Figs
Fig.1 Reusability of Co/phen@CB-800-L for oxidation of ethylbenzene to acetophenone
Fig.1

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Tables
Table 1 Optimization of reaction condition.a
Table 2 Oxidation of arylalkanes with Co/phen@CB-800-L in water.a
Table 1
Entry
Catalyst
Solvent
Oxidant
Conv. (%) b
Sel. (%)
Yield (%) b
1
Co/phen@CB-800
Co@CB-800
1,4-dioxane
1,4-dioxane
1,4-dioxane
1,4-dioxane
1,4-dioxane
1,4-dioxane
MeCN
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
K2S2O8
2Na₂CO3·3H₂O₂
H2O2
38
6
32
0
12
N. d.
N. d.
4
2
3
phen@CB-800
<1
18
8
0
4
Co/phen-800
22
<1
51
79
46
65
85
94
95
69
81
<1
43
96
97
95
96
>99
5
Co@CB-800-L
trace
23
6
Co/phen@CB-800-L
Co/phen@CB-800-L
Co/phen@CB-800-L
Co/phen@CB-800-L
Co/phen@CB-800-L
Co/phen@CB-800-L
Co/phen@CB-800-L
Co/phen@CB-800-L
Co/phen@CB-800-L
Co/phen@CB-800-L
Co/phen@CB-800-L
Co/phen@CB-800-L
Co/phen@CB-800-L
Co/phen@CB-800-L
Co/phen@CB-800-L
Co/phen@CB-800-L
45
33
69
65
81
90
87
52
32
16
28
92
91
82
97
>99
7
26
8
DMF
32
9
i-PrOH
t-BuOH
H2O
42
10
11
12
13
14
15
16c
17d,e
18f,e
19g,e
20h,f,e
21i,h,f,e
69
85
t-BuOH:H2O(1:1)
H2O
83
36
H2O
26
H2O
trace
12
H2O
O2
H2O
TBHP
TBHP
TBHP
TBHP
TBHP
88
H2O
88
H2O
78
H2O
93
H2O
>99
b
a Reaction condition: ethylbenzene (1.0 mmol), catalyst (20 mg), oxidant (2.0 mmol), solvent (3 mL), 70 C for 6 h.
e
f
Determined by GC using bromobenzene as internal reference. c O2 balloon. d Catalyst (30 mg). TBHP (3.0 mmol).
h
Catalyst (10 mg). g Catalyst (5 mg). Reaction at 80 C. i Reaction for 8 h.

ACCEPTED MANUSCRIPT
Table 2
Entry
1
Substrate
Product
Yield (%)b
99
2
3
4
5
99
93
83
86
6
7
96
90

ACCEPTED MANUSCRIPT
8
78
81
83
73
76
90
81
85
81
9
10
11
12
13
14
15
16

ACCEPTED MANUSCRIPT
17
18
19
20
21
83
82
77
79
93
22
98
a Reaction condition: arylalkane (1 mmol), catalyst (10 mg), TBHP (3 mmol), H2O (3 mL), 80
C, 8 h. b Isolated yield.

ACCEPTED MANUSCRIPT
Highlights


Co/phen@CB-800-L catalyst was synthesized.
The catalyst show high catalytic activity, selectivity in sp3 C-H
oxidation.


Selective oxidation of alkylarenes were performed in H2O.
A broad scope of carbonyl compounds could be obtained in high
yields.

The catalyst could be recycled up to six times without obvious loss
in activity.