Co-N-doped carbon nanotubes supported on diatomite for highly efficient catalysis oxidative carbonylation of amines with CO and air

Article abstract of DOI:10.1016/j.apcata.2017.09.030

Cobalt-nitrogen-doped carbon nanotubes stably supported on diatomite were obtained by employing Co(OAc)₂/phenanthroline. The resulting material was found to be excellent catalysts for the carbonylation of a variety of amines with CO other than phosgene. Both high activity and selectivity were achieved in this carbonylation process, and it allows air as a cheap oxidizing agent. Moreover the catalyst could be recycled for several times with relatively higher activity compared to homogeneous catalyst palladium acetate.

Full text of DOI:10.1016/j.apcata.2017.09.030

Accepted Manuscript
Title: Co-N-doped carbon nanotubes supported on diatomite
for highly efficient catalysis oxidative carbonylation of amines
with CO and Air
Authors: Jiangwei Li, Dong-huai Tu, Yani Li, Weiqiang
Wang, Qinwei Yu, Jianming Yang, Jian Lu
PII:
S0926-860X(17)30471-4
DOI:
Reference:
https://doi.org/10.1016/j.apcata.2017.09.030
APCATA 16424
To appear in:
Applied Catalysis A: General
Received date:
Revised date:
Accepted date:
15-5-2017
25-8-2017
25-9-2017
Please cite this article as: Jiangwei Li, Dong-huai Tu, Yani Li, Weiqiang Wang, Qinwei
Yu, Jianming Yang, Jian Lu, Co-N-doped carbon nanotubes supported on diatomite for
highly efficient catalysis oxidative carbonylation of amines with CO and Air, Applied
Catalysis A, General https://doi.org/10.1016/j.apcata.2017.09.030
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Co-N-doped carbon nanotubes supported on diatomite for highly
efficient catalysis oxidative carbonylation of amines with CO and Air
Jiangwei Lia, Dong-huai Tu, Yani Li, Weiqiang Wang, Qinwei Yu, Jianming Yang a* and Jian Lu a*.
a.State Key Laboratory of Fluorine & Nitrogen Chemicals, Xi’an Modern Chemistry Research Institute, Xi’an, 710065, China
Graphical abstract
Highlights


Cobalt oxide based nitrogen-doped carbon nanotubes on diatomite were obtained;
The resulting material was fund to be excellent catalysts for the oxidative carbonylation of amines
with CO and air;


The catalyst showed higher activity than Pd(OAc)2 under the same condition;
The catalyst could be easily recovered and reused for several times with high activity.
ABSTRACT
Cobalt-nitrogen-doped carbon nanotubes stably supported on diatomite were obtained by employing
Co(OAc)2/phenanthroline. The resulting material was found to be excellent catalysts for the
carbonylation of a variety of amines with CO other than phosgene. Both high activity and selectivity
were achieved in this carbonylation process, and it allows air as a cheap oxidizing agent. Moreover the
catalyst could be recycled for several times with relatively higher activity compared to homogeneous
catalyst palladium acetate.
1. Introduction
Ureas are important carbonyl derivatives in agricultural chemicals, pharmaceutical intermediates
and resin precursors[1-3]. The classical synthetic route for ureas are based on the use of phosgene and
its derivatives, which are poisonous[4]，and therefore cause environmental problems. In recent years
alternative routes have been developed by employing carbonyl derivatives, CO or CO2.[5-7] Particularly,

the method using CO, amines and oxide as the raw materials is very attractive from the atom economic
point and the only by-product is water. Several homogeneous catalysts have been proposed for this
reaction showing high yields to the desired urea products.[8-10] However, homogeneous catalytic
systems have the common drawback of difficult separation of catalyst and product, leading to limited
industrial applications. To overcome these problems, variety of heterogeneous catalysts for has been
reported.[8, 11-13] However, most of these catalysts have employed noble metal like Pd and Au.[2, 4-6]
Herein,we developed a new heterogeneous catalyst of high activity and selectivity for carbonylation of
amines through pyrolysis of cobalt acetate and 1, 10-phenanthroline with diatomite. In the past, the
metal/N doped graphene (M/NGr) catalyst has been mainly supported on carbon or graphene, the
preparation process of which are usually energy consumble. While the supporter diatomite is cheap and
environment-friendly, and our system can be easily recovered and recycled for several times. In addition,
unlike the traditional metal/N doped carbon material, the morphology of the resultant carbon were found
to be nanotubes attached on surface of diatomite.
Recently, the incorporation of dopants into the matrix of the parent support has become an attractive
area in material sciences. More specifically, N-doped carbon materials generated major interest. So far,
these Cobalt oxide /NGr has been applied in the selective hydrogenation of nitroarenes, imines, and the
selective oxidation of alcohols to esters.[14-22] As far as we know, this is the first time for this kind of
catalysis used in carbonylation of amines.
2. Experimental
2.1 Catalyst preparation
In a similar procedure to prepare the Co/N-doped graphene, dispersible and uniform nanotubes were
obtained by using cobalt acetate as metal agent and 1,10-phenanthroline as nitrogen-rich ligand. Cobalt
(II) acetate (1g, 2.5 mmol) and 1, 10-phenanthroline (990 mg, 5.5 mmol) were stirred in ethanol (90 mL)
for 3h at room temperature. Then, diatomite powder (5g) was added and the whole mixture was kept
stirring at room temperature for 6h. Ethanol was removed by vacuum. The obtained pink solid sample
was dried at 80 °C for 24h, after that it was grinded to a fine powder in an agate mortar. Then, the grinded
powder was transferred into a corundum crucible and placed in the oven. The oven was heated to 800 °C
at the rate of 10 °C per minute, and held at certain temperature (600，800 and 1000°C) for 2 hours under
flow of nitrogen gas. The oven was allowed to cool to room temperature naturally in a nitrogen
atmosphere. The resultant black material was grinded again so as to obtain a fine powder.
For comparison, the identical procedure was applied to the preparation of Ni-based or metal free N
doped carbon catalyst using the same starting materials except the nickel acetate. For Co/diatomite the
detailed procedure could be seen in the supporting information.
2.2 Catalytic activity tests
The standard reaction was carried out in a Teflon-lined stainless-steel reactor 35 mL in capacity
with a magnetic stirrer.107 mg of benzylamine (1 mmol), 20 mg of catalyst and 2 mL of 1, 4-dioxane
was first added, after that the reactor was charged with 1.5 MPa CO and 0.1 MPa air. The mixture was
stirred for 8h at desired temperature. Then, the reaction mixture was cooled to room temperature and
centrifugated to get rid of catalyst. The resultant mixture was then analyzed by GC and GC-MS. All GC-
yields are averages from at least 3 runs, the starting materials as well as products were calibrated from
the commercially available materials. Then the solvent was removed, and the residue was purified by
recrystallization from the appropriate solvent to give the corresponding urea.

2.3 Catalyst recycling experiments
For the catalyst recycling experiments, reactions were carried on a 1 mmol benzylamine in the
presence of 20 mg of the catalyst. All reactions were conducted in a 35 mL high-pressure reactor
according to procedures described above. The catalyst was easily separated from the reaction mixture by
centrifugation at 9000 RPM for 15 min, and washed with ethanol for 3 times. Before being resubmitted
to the reaction condition, the collected catalysts were dried under a high vacuum for 3 h.
3. Results and discussion
In order to investigate the structure of the catalyst, several characterization were taken out. The
scanning electron microscopy (SEM) image showed that the active part of the catalyst was mainly in the
form of nanotubes with the mean exterior diameter of about 50 nm and length of several micrometers.
As it’s shown in Figure 1(b~c), the carbon nanotubes were uniformly distributed on surface of diatomite.
Catalyst GN-Co-600, GN-Co-1000, GN-800 and GN-Ni-800 have also been prepared with the same
procedure, however, no carbon nanotubes was found in all of them(see Supporting Information).
The nature of cobalt, carbon and nitrogen species on the surface of catalyst were analysed by X-ray
photoelectron spectroscopy (XPS). Fig. 2A shows resolution Co 2p spectrum of the catalyst GN-Co-800.
As indicated by the bending energy of 780.5 and the presence of satellite peak, the Co species can be
ascribed to Co2+ bonded with N in the graphene.[20, 23] The results of XPS showed that the graphitic
layers contain different types of carbon as well as nitrogen atoms. After pyrolysis, there appear four
chemical states of N at 398.6, 400.5, and 402.3 eV (Figure 3B), which can be attributed to pyridinic N,
pyrrolic-N and graphitic-N respectively.[24] The percentage of pyridine-type nitrogen decreased with
o
the increase of pyrolysis temperature and when the pyrolysis temperature increased to 1000 C, the
pyridine-type nitrogen almost disappeared (Figure 3, a-e). The surface composition and element analysis
of the materials are summarized in Table 1. Pyrolysis at 600oC, 800oC and 1000 oC led to active catalysts,
which contain 18.46%, 6.15% and 13.41% carbon by elemental analysis (EA) measurements (Table 1).
However the catalyst GN-Co-600 and GN-Co-1000 showed much weaker activity than catalyst GN-Co-
800 because of low graphitization degree or doped nitrogen element. In addition, catalyst GN-800
prepared without metal salt and catalyst GN-Ni-800 prepared with nickel acetate showed almost no
activity for the carbonylation of amines even that they contained the same nitrogen species and amounts
(Table 1). These results clearly showed that the doped nitrogen element and coexisted cobalt are crucial
for the activity in carbonylation of amines. Compared with nickel acetate, cobalt acetate showed higher
catalytic activity in turning 1, 10-phenanthroline into carbon. Furthermore, the C1s spectra for GN-Co-
800 showed 3 peaks (Figure S2): the sharp peak at 284.6 eV confirmed the presence of graphitic carbon,
corresponding to the energy of sp2 C=C bond of pyrolytic graphite. Further the small signals at higher
binding energies corresponded to C-O (285.5 eV) and C-N (287.7 eV) species in N-CNTs,
respectively.[25]
Raman spectra of the catalysts were demonstrated in Figure 2D. The G band (~1593 cm-1) was
observed for graphitic structures due to the E2g vibrational mode present in the sp2-bonded graphitic
carbon and the D-band (~1339 cm-1) was associated with disorder-induced amorphous carbon.[26]27
Normally, the intensity ratio of the D to G peak (ID/IG) is employed to evaluate the graphitization degree
of carbon based materials. The ID/IG ratio for GN-Co-600, GN-Co-800 and GN-Co-1000, are 3.23, 2.63
and 1.65, respectively, indicating presence of less defects and active sites which can be ascribed to doped
nitrogen. In comparison, the ID/IG ratio is 3.28 for GN-800 and 2.58 for GN-Ni-800, which is much higher

than that of GN-Co-800, meaning more defects and doped nitrogen. These results was agreed to the XPS
and EA data, showing that high pyrolysis temperature can reduce the content of the doped nitrogen and
increase graphitization degree. Moreover, the cobalt and nickel element can increase graphitization
degree without sacrificing the ratio of doped nitrogen.
Initial carbonylation experiments were carried out with benzylamine (1a) as model substrate (1
mmol). Applying the GN-Co-800 as catalyst the desired product 1,3-dibenzylurea (2a) was obtained in
94% yield under 1.5 MPa CO and 0.1MPa air at 110 oC for 8h in 1,4-dioxane. Under the same condition,
Cat.2 and Cat.3 made without cobalt acetate and ligand, respectively, showed no activity for this
transformation. The Cat. 4 made by employing nickel acetate as the metal center also showed no activity.
As expected, applying a homogeneous catalyst mixture of Co(OAc)2·4H2O (2 mol%) with 1,10-
phenanthroline (4 mol%) did not result in any desired product (2a). Similarly, the heterogenized catalyst
without pyrolysis (Cat. 5) revealed no activity of carbonylation in the model reaction (Table 2, entry 5).
Applying the homogeneous catalyst palladium acetate, the reaction could complete with a 32 % yield
and 98% selectivity. The addition of copper(II) chloride could enhance the yield further to 82% (Table 2,
entries 7 and 8). Variation of solvents such as DMF, MeCN, THF, DMSO and toluene were tested in the
presence of Cat.1, but no improvement was observed (Table 2, entries 11~15). At lower CO pressure (0.8
MPa) the reaction was retarded, whereas an enhanced product yield (94% of 1, 3-dibenzylurea) was
obtained under 1.5 MPa CO, but no further improvement was achieved by increasing the CO pressure to
2.4 MPa. Increasing the reaction temperature, the product yield was improved from 41% (80 oC) to 94%
(110oC). The selectivity of the model reaction decreased obviously when the temperature was 140 oC.
The optimized condition achieved in above screening reactions was applied to various amines. To
our delight, all these carbonylation for amines showed excellent conversion and selectivity. As shown in
Scheme 2, different halo-substituted amines were selectively carbonylated and obtained the
corresponding ureas in good yields. The reaction also worked nicely with substrates substituted both with
electron-withdrawing groups such as the fluorine substituent, and with electron-donating groups like
methoxyl and alkyl. It is worth to notice that, the reaction selectivity of the substrates substituted with
electron-donating groups is higher than these with electron-withdrawing groups. The steric hindrance
might have some negative effect on the reaction yield as indicated by steric effect.
To demonstrate the stability and reusability the catalyst, six more recycles of the model reaction
were conducted. The catalyst was easily separated from the reaction mixture by centrifugation at 9000
RPM for 15 min, and then washed with ethanol for three times. As shown in Figure 4, a slight decline of
conversion was observed, but the selectivity and conversion were both still remained above 80% after 3
run. The decrease of catalytic activity might mainly be attributed to the loss of catalyst after
centrifugation.
4. Conclusions
In conclusion, a heterogeneous catalyst of Cobalt based N-doped carbon nanotubes on diatomite
were obtained by pyrolysis of Co(OAc)2/phenanthroline on diatomite. The resulting catalyst exhibit
excellent catalytic performance for carbonylation of amines with 0.1 MPa of air as an environmentally
friendly oxidant. This new material was also characterized by several techniques and allows for the
carbonylation of amines to give the corresponding urea in good yields. Moreover, the catalyst could be

easily recycled for several times without loss of catalytic activity. Further applications of these Co-
containing carbon nanotubes on other supports are currently under way in our laboratory.
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Figure 1．(a) SEM image of diatomite before loading the catalyst (b) SEM image of Co/N-CN @ Diatomite showing

finely distributed carbon nanotubes on the surface of diatomite (c, d) SEM image of carbon nanotube
Figure 2．XPS spectrum: (A) Co 2p XPS spectra for GN-Co-800, (B) N 1s XPS spectra for catalysts; a) GN-Co-
600; b) GN-Co-800; c) GN-Co-1000; d) GN-800 e) GN-Ni-800
Figure 3．Raman spectra for a) GN-Co-600; b) GN-Co-800; c) GN-Co-1000; d) GN-800 e) GN-Ni-800

Figure 4. Recyclability test of GN-Co-800, Reaction conditions: 1mmol of benzylamine, 2 mL of 1,4-dioxane, 20
mg of GN-Co-800, 1.5MPa CO, 0.1MPa air, 110oC, 8h. Conversions and Yields were determined by GC using
dodecane as an internal standard.
Scheme 1. Preparation of Co/N-CN @ Diatomite.
Scheme 2. Substrate scope for the carbonylation of amines catalyzed by heterogeneous catalyst GN-Co-800;

aReaction conditions: 1 mmol of amine, 20mg catalyst, 1.5MPa CO, 0.1 MPa air. Substrate concentration was 0.5
mmol per mL of 1,4-dioxane. bDetermined by isolation of unreacted amines. cIsolated yield.
Table 1. XPS measurements EA of nitrogen and carbon content for the catalysts
H content
N content (%)
Carbon (%)
(%)
EA
0.66
0.01
--
Catalyst
XPS
3.64
1.32
0.61
0.64
0.53
EA
2.2
XPS
EA
18.46
6.15
13.41
2.45
4.72
GN-Co-600
GN-Co-800
GN-Co-1000
GN-800
31.25
15.33
20.46
6.72
0.62
0.19
0.21
0.45
--
GN-Ni-800
--
9.91
Table 2. Catalytic carbonylation of 1a with CO under varied conditions.
Entry
1
Catalyst
Cat. 1
Cat. 2
Cat. 3
Cat. 4
Cat. 5
Cat. 6
Pd(OAc)2
Pd(OAc)2
Cat. 1
Cat. 1
Cat. 1
Cat. 1
Cat. 1
Cat. 1
Cat. 1
Cat. 1
Cat. 1
Solvent
1,4-dioxane
1,4-dioxane
1,4-dioxane
1,4-dioxane
1,4-dioxane
1,4-dioxane
1,4-dioxane
1,4-dioxane
1,4-dioxane
1,4-dioxane
DMF
T (°C)
Conv.(Yield)
110
110
110
110
110
110
110
110
110
110
110
110
110
110
110
80
98% (94%)
-- (--)
2
3
-- (--)
4
43% (--)
5
-- (--)
6
-- (--)
7
36% (32%)
85% (82%)
70% (66%)
99% (92%)
98% (87%)
78% (73%)
95% (71%)
99% (62%)
66% (39%)
47% (41%)
98% (83%)
8a
9b
10c
11
12
13
14
15
16
17
MeCN
THF
DMSO
toluene
1,4-dioxane
1,4-dioxane
140
Reaction conditions: 1 mmol of benzylamine, 20 mg catalyst, 1.5 MPa CO, 0.1 MPa air, 2 mL of solvent; a) 2 mmol
of CuCl2 was added; b) 0.8 MPa CO; c) 2.4 MPa CO; Cat. 1: GN-Co-800, Cat. 2: L/ diatomite, Cat. 3: Co/ diatomite,
Cat. 4: GN-Ni-800, Cat. 5: Co/L-diatomite, Cat. 6: GN-800.