Atom-economy synthesis of N-substituted carbamate from urea derivative and dimethyl carbonate catalyzed by La/SiO2: Characterization and activity

Article abstract of DOI:10.1002/cjoc.201090048

A series of silica gel immobilized lanthanum catalysts were prepared for the atom-economy synthesis of N-substituted carbamates from urea derivatives and dimethyl carbonate. The La/SiO₂ catalysts with lanthanum loadings varied from 1.3 wt% to 8.5 wt% were characterized by AES, BET, XRD, TEM, FT-IR, XPS and TPD. According to the characterization, lanthanum species with particle sizes of 5-10 nm on the surface of silica gel were formed. The catalysts were all amorphous and the surface areas were 336.5-530.2 m²/g. NH₃-TPD analysis showed that all samples exhibited similar acid strength with different acid amounts. FT-IR measurement indicated that the component of lanthanum species on the catalyst surface were La(OH)₃, LaOOH and hydrated La₂O₃. Also, the peak value of the absolute amount of LaOOH was obtained with 4.3 wt% lanthanum loading. The BET surface area decreased dramatically when the lanthanum loading was above 4.3 wt%. In consideration of the results obtained from the catalytic reactions, it could be concluded that LaOOH was the possible active species and high surface area was important for the high catalytic activity.

Full text of DOI:10.1002/cjoc.201090048

FULL PAPER
Atom-economy Synthesis of N-Substituted Carbamate from
Urea Derivative and Dimethyl Carbonate Catalyzed by La/SiO2:
Characterization and Activity
Guo, Xiaoguanga,b(郭晓光)
Wang, Liguoa,b(王利国)
Shang, Jianpenga,b(尚建鹏)
Li, Jiana,b(李健)
Ma, Yuboa,b(马昱博)
Shi, Feng*,a(石峰)
Deng, Youquan*,a(邓友全)
a Centre for Green Chemistry and Catalysis, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences,
Lanzhou, Gansu 730000, China
b Graduate School of Chinese Academy of Sciences, Beijing 100039, China
A series of silica gel immobilized lanthanum catalysts were prepared for the atom-economy synthesis of
N-substituted carbamates from urea derivatives and dimethyl carbonate. The La/SiO2 catalysts with lanthanum
loadings varied from 1.3 wt% to 8.5 wt% were characterized by AES, BET, XRD, TEM, FT-IR, XPS and TPD.
According to the characterization, lanthanum species with particle sizes of 5— 10 nm on the surface of silica gel
were formed. The catalysts were all amorphous and the surface areas were 336.5— 530.2 m2/g. NH3-TPD analysis
showed that all samples exhibited similar acid strength with different acid amounts. FT-IR measurement indicated
that the component of lanthanum species on the catalyst surface were La(OH)3, LaOOH and hydrated La2O3. Also,
the peak value of the absolute amount of LaOOH was obtained with 4.3 wt% lanthanum loading. The BET surface
area decreased dramatically when the lanthanum loading was above 4.3 wt%. In consideration of the results ob-
tained from the catalytic reactions, it could be concluded that LaOOH was the possible active species and high sur-
face area was important for the high catalytic activity.
Keywords N-substituted carbamate, urea derivative, dimethyl carbonate, lanthanum, green chemistry
Introduction
could be reduced if carbon dioxide as carbonyl source
can be realized. The difficulty in realization of this target
is the inert property of carbon dioxide. It has been re-
ported that carbamates could be synthesized with amine
and alkyl halide as starting materials and carbon dioxide
as carbonyl source.22,23 Unfortunately, the use of alkyl
halide would produce a large amount of inorganic waste.
It has been reported that disubstituted ureas could be
synthesized from amine and carbon dioxide.24-26 As it is
known, organic carbamate could be obtained through the
alcoholysis of urea derivatives and the amine byproduct
from the disubstituted urea could be recycled for the next
run.27,28 Undoubtedly, it is gratifying if all the fractions
inside disubstituted urea could be transferred into a car-
bamate molecule. The synthesis of N-substituted car-
bamate using urea and dimethyl carbonate as starting
materials offered a nice method to solve this problem.29-31
The indirect employment of carbon dioxide as carbonyl
source therefore is also realized. Here we present the new
results about N-substituted carbamate synthesis from urea
and dimethyl carbonate using silica gel immobilized nano
lanthanum as catalyst, as shown in Scheme 1.
Organic carbamates are key intermediates for the
synthesis of pharmaceuticals, pesticides, fungicides and
herbicides.1-8 Moreover, it has become the potential
compound for the non-phosgene synthesis of isocyanate
via thermal decomposition. Unfortunately, till today, the
most convenient method for the synthesis of these prod-
ucts is the aminolysis of chloroformate, which is pro-
duced from phosgene and alcohols.9,10 Although it is a
versatile technique for carbamate synthesis, the toxicity
and corrosion properties of phosgene and hydrogen chlo-
ride have caused severe environmental problems.11 In the
last decades, great efforts have been made in order to
explore an environmentally benign method for carbamate
synthesis. The classical research work in this area is the
carbonylation of nitroaromatics and amines catalyzed by
various transitional metal complexes.12-17 Also, dimethyl
carbonate as a green carbonylation reagent has attracted
extensive attention.18-21 Apart from carbon monoxide and
dimethyl carbonate, carbon dioxide should be an ideal
carbonyl source in chemical industry. In addition to its
nontoxic character, the release of “green house” gas
*
E-mail: fshi@licp.cas.cn; ydeng@licp.cas.cn; Tel./Fax: 0086-0931-4968116
Received April 14, 2009; revised September 11, 2009; accepted October 16, 2009.
Project supported by the National Natural Science Foundation of China (No. 20503037).
164
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Chin. J. Chem. 2010, 28, 164— 170

Atom-economy Synthesis of N-Substituted Carbamate
Scheme Indirect use of carbon dioxide in organic
1
for 1 h to remove physisorbed NH3, TPD analysis was
carried out from 100 to 500 ℃ at a heating rate of 10
℃/min under argon flow of 50 mL/min.
N-substituted carbamate synthesis
X-ray photoelectron spectroscopy (XPS) was per-
formed with a VG ESCALAB 210 instrument. Mg Kα
radiation was used with a pass energy of 150 eV at an
energy scale calibrated versus C 1s peak at 285.00 eV
arising from adventitious carbon.
X-ray diffraction (XRD) was measured on a Siemens
D/max-RB powder X-ray diffractometer. Diffraction
patterns were recorded with Cu Kα radiation (30 mA,
40 kV) over a 2θ range of 10° to 50° with a position-
sentient detector using a step size of 0.017°.
High resolution transmission electron microscopic
(HR-TEM) investigations were carried out using a JEOL
JEM-2010 Electron Microscope. The powder of catalysts
was suspended in ethanol by ultrasonic dispersion for 5—
10 min and then the resulting solution was dropped on a
carbon film supported by a 300 mesh copper grid.
FT-IR analysis was conducted with a Nicolet 910
FT-IR spectrometer. Samples with different lanthanum
loadings were loaded on the sample holder (KBr) to
form a uniform thin layer.
Experimental
Catalyst preparation
Lanthanum nitrate [La(NO3)3•6H2O, ca. 0.66 g] and
urea (ca. 0.15 g) were dissolved in ca. 25 mL of distilled
water. The molar ratio of lanthanum to urea was ca. 1∶
1.7. 5.0 g of silica gel (200—300 mesh, BET surface
area＝601.1 m2/g, pore volume＝0.74 cm3/g, average
pore diameter＝4.93 nm. It was pretreated at 600 ℃ for
4 h before use) was dropped into the solution under ul-
trasound irradiation. The suspension was further mag-
netically stirred at 90 ℃ for 3 h and the final pH value
of the system was ca. 8. Then it was filtrated and dried in
air at 100 ℃ for 4 h. It was further calcined at 600 ℃
for 4 h and ca. 5.2 g of 4.3 wt% La/SiO2 catalyst was
obtained. By varying the quantity of La(NO3)3•6H2O
added, catalysts with different lanthanum loadings were
obtained with the same procedure.
Qualitative and quantitative analyse were carried out
with GC-MS (HP 6890/5973), GC-FID (Agilent 6820,
1
biphenyl was used as an internal standard), H NMR
(Bruker AMX FT 400-MHz) and 13C NMR (Bruker
AMX FT 100-MHz).
General procedure
All reactions were conducted in a 90 mL stainless
steel autoclave with a glass tube inside equipped with
magnetic stirring. In each reaction, urea derivative (5
mmol), dimethyl carbonate (15—75 mmol) and 50 mg
of La/SiO2 catalyst were charged respectively into the
autoclave. After being flushed with N2, it was heated up
to 150 ℃ and reacted for 6—12 h. Then, the autoclave
was cooled to room temperature and opened to air. The
catalyst was separated by centrifugation and filtration.
The pure compounds were obtained with different pro-
cedures. For compound 1, the solvent was removed and
pure product could be obtained after the solid was fur-
ther washed by hexane/diethyl ether and then vacuum
dried. For compounds 2, 3 and 4, the products were pu-
rified by column chromatography [silica gel,
V(petroleum ether)∶V(methylene dichloride)＝7.5∶
2.5]. For compounds 5 and 6, the solvent and DMC
were removed and pure products could be obtained after
the solids were further washed by hexane/acetone and
vacuum dried. For compounds 7, 8 and 9, the solvent
and DMC were removed and the carbamates could be
achieved after the solid was further washed by etha-
nol/diethyl ether or ethanol/hexane and vacuum dried.
Characterization of the catalysts
The loadings of La were determined on an induc-
tively coupled plasma-atomic emission spectrometer
(ICP-AES) (ThermoElemental Company in the USA)
by dissolving the samples in aqueous nitric acid.
The BET surface area, pore volume and average
pore diameter of catalyst were measured by physisorp-
tion of N2 at 76 K using a Micromeritics ASAP 2010
instrument. Before the measurement, the samples were
degassed at 200 ℃ for 12 h to remove adsorbed gases
from the catalyst surface. The isotherms were elaborated
according to the BET method for surface area calcula-
tion, with the Horwarth-Kavazoe and BJH methods used
for micropore and mesopore evaluation, respectively.
TPD analysis with NH3 was carried out on an im-
proved GC112 instrument to study the acidity of the
catalyst surface. In a typical experiment, 250 mg of
dried sample (dried at 120 ℃ for 5 h) was taken in a
U-shaped quartz sample tube. The sample was pre-
treated under argon flow (50 mL/min) at 500 ℃ for 1 h.
After pretreatment, the sample was cooled to ambient
temperature under argon and then treated by a 10%
NH3-Ar mixture (75 mL/min) for 1 h at 40 ℃. After
being flushed with pure argon (50 mL/min) at 100 ℃
Results and discussion
Characterization of catalysts
BET analyse of pure silica gel and the silica gel im-
Chin. J. Chem. 2010, 28, 164— 170
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Guo et al.
FULL PAPER
mobilized La samples are given in Table 1. Clearly, the
BET surface area and pore volume decreased after the
immobilization of lanthanum. For the average pore di-
ameter, it kept constant at the initial stage but dropped
dramatically when the lanthanum loading was higher
than 4.3 wt%. This turning point was also observed on
the surface area and pore volume measurement. The
BET surface area was ca. 500 m2/g and the pore volume
was between 0.59—0.74 cm3/g when lanthanum load-
ings were lower than 4.3 wt%. However, the BET sur-
face area was only 336.5 m2/g and the pore volume was
0.44 cm3/g when the lanthanum loading reached 8.5
wt%. Therefore, 4.3 wt% La/SiO2 might be a good
composition for this catalyst based on the BET meas-
urement.
disturbance of electron binding energy between La 4d
and Si 2p. The La 3d signals consist of the 3d3/2 and
3d5/2 spin-orbit doublets, and the La 3d5/2 peaks are cen-
tered at 835.8, 835.8, 835.9 and 836.0 eV for catalysts
with different lanthanum loadings. The binding energy
(B.E.) value for current La features is in agreement with
＋
the standard XPS data, i.e. La3 ion.32 So t＋he chemical
state of the surface lanthanum species is La3 .
Table 1 BET parameters of the catalysts
Areaa/
Vol.b/
d c/
nm
Entry Catalyst
(m2•g－
601.1
530.2
496.9
447.3
388.5
336.5
)
(cm3•g－
)
1
1
1
2
3
4
5
6
SiO2
0.74
0.67
0.63
0.59
0.47
0.44
4.9
5.0
4.8
5.3
4.1
3.8
1.3 wt% La/SiO2
3 wt% La/SiO2
4.3 wt% La/SiO2
6.4 wt% La/SiO2
8.5 wt% La/SiO2
Figure 2 XPS of La 3d.
Meantime, the atom ratio of La 3d/Si 2s of 4.3 wt%
La/SiO2 is 0.48. The theoretical value for a monolayer
dispersion according to models proposed in former re-
ports is 0.49.33,34 So it can be assumed that the lantha-
num species is monolayer dispersed on the silica gel
surface.
Typical XRD patterns obtained from pure SiO2, 4.3
wt% La/SiO2 and 8.5 wt% La/SiO2 were presented in
Figure 3. Clearly, pure silica gel, 4.3 wt% La/SiO2 and
8.5 wt% La/SiO2 exhibited similar patterns. The immo-
bilization of La on silica gel has resulted in negligible
effect on the crystal structure of catalyst samples. The
silica gel itself is amorphous and highly dispersed lan-
thanum is formed. According to the XRD measurement,
it could be concluded that the increment of lanthanum
loading did not cause aggregation and crystallization of
lanthanum.
a BET surface area. b Volume of pore. c Average pore diameter.
Through the NH3-TPD measurement, similar acid
strength with different acid amounts was observed in all
samples (Figure 1). The acidity of the catalysts is possi-
bly originated from the silica gel surface. The incorpo-
ration of lanthanum species caused slight weakness of
the surface acidity. Furthermore, the acid amount was
decreased dramatically when the lanthanum loading was
above 4.3 wt%. This is consistent with the observation
of the BET analysis. The remarkable decreasing in sur-
face area leads to less acid amount.
Figure 1 NH3-TPD profile of La/SiO2 sample. The loadings are
0 wt%, 1.3 wt%, 4.3 wt%, 6.4 wt% and 8.5 wt% respectively
from the topside.
Figure 3 XRD patterns of pure SiO2, 4.3 wt% and 8.5 wt%
La/SiO2 samples.
XPS analysis has been performed in order to under-
stand the surface structure of silica gel immobilized
lanthanum catalysts well (Figure 2). The spectrum of Si
2s was served as the preferable choice to reduce the
Typical HR-TEM pictures of samples 4.3 wt%
La/SiO2 and 8.5 wt% La/SiO2 are shown in Figure 4,
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Chin. J. Chem. 2010, 28, 164— 170

Atom-economy Synthesis of N-Substituted Carbamate
which confirmed the suggestions of XRD measurement.
Similar lanthanum species particle size was observed
over different samples. The particle size of lanthanum
species is ca. 5—10 nm in all the samples. Also, from
the TEM analysis, all the samples showed uniform
structures.
3 wt%, 4.3 wt% and 8.5 wt%, were investigated by
FT-IR (Figure 5). From the IR spectra, the most signifi-
cant change for different loading catalyst occurs in the
region of OH vibrations. As it can be seen from Figure
5a, sample of lanthanum oxide under air included four
－1
typical peaks (3608, 3449, 1636 and 641 cm ). For the
FT-IR spectra of immobilized lanthanum catalysts, the
－1
peak at 641 cm disappeared completely, and the peak at
－1
－1
3608 cm [La(OH)3], 3449 cm (LaOOH) and 1636
－1
cm (possibly absorbed water) still existed.35-37 The re-
sults showed that there was no lanthanum oxide main-
tained in the La/SiO2 catalysts. Samples of 3—8.5 wt%
La/SiO2 catalysts presented a broad band from 3700 to
－1
3200 cm , and it was further fitted with three peaks
－1
(3608, 3449 and 3246 cm ), which could originate from
three different types of surface OH groups [i.e. La(OH)3
(I), LaOOH (II) and hydrated La2O3 (III)]35 (Figure 5b—
5d).
Figure 4 HR-TEM images of the 4.3 wt% (left) and 8.5 wt%
(right) La/SiO2 samples.
Additionally, the relative contents of La(OH)3, La-
OOH and hydrated La2O3 for 3—8.5 wt% La/SiO2
catalyst were listed in Table 2. With increasing the
To further investigate the composition of lanthanum,
a series of catalysts with varied lanthanum loadings, i.e.
Figure 5 FT-IR spectra of La/SiO2 catalysts. (a) 1: La2O3, 2: 3 wt% La/SiO2, 3: 4.3 wt% La/SiO2, 4: 8.5 wt% La/SiO2; (b) 1: OH group
of La2O3, 2: OH group of 3 wt% La/SiO2; (c) OH group of 4.3 wt% La/SiO2; (d) OH group of 8.5 wt% La/SiO2; all the samples were
calcined at 600 ℃ under air for 5 h. The peaks in b, c and d were fitted with the I, II and III peaks.
Table 2 Relative content of OH compound on differently loaded catalysts surfacea
Loading
3 wt%
La(OH)3
8.2%
LaOOH
79.4%
77.1%
33.5%
Hydrated La2O3
12.4%
Absolute amount of LaOOHb
2.8%
3.9%
3.4%
4.3 wt%
8.5 wt%
8.7%
14.2%
20.2%
46.3%
a Each peak area was calculate by integrating the fitted Gaussian equation, and then using a normalization method, the relative contents
were obtained. b The content of LaOOH on SiO2 surface＝Loading×Relative content of LaOOH.
Chin. J. Chem. 2010, 28, 164— 170
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167

Guo et al.
FULL PAPER
lanthanum loadings, the relative contents of La(OH)3
and hydrated La2O3 increased correspondingly. How-
ever, the relative content of LaOOH increased initially
and reached a maximum (absolute amount ca. 3.9 wt%)
with 4.3 wt% lanthanum loading, and then decreased.
unchanged with increasing the lanthanum loading,
which suggests that the lower activity is not due to the
aggregation of catalytically active species. The XRD
characterization showed there was no crystallization
occurring with increasing the lanthanum loading. Simi-
lar information was obtained from XPS analysis. How-
ever, the surface area, pore volume and average pore
diameter all decreased severely based on the BET
analysis when the loading was higher than 4.3 wt%.
FT-IR measurement revealed the highest LaOOH abso-
lute amount in 4.3 wt% La/SiO2 catalyst. Therefore, the
possible active catalyst species is LaOOH and high sur-
face area is also important.
Although a good result was obtained under reaction
conditions as mentioned above, the usage of a large ex-
cess of DMC made the practical application difficult.
Other organic solvent was therefore chosen to improve
the reaction with less DMC as reactant. Three typical
organic solvents, i.e. DMF, THF and toluene, were used
to explore the possibility (Entries 8—11). Interestingly,
the non-polar toluene exhibited a more positive result
than the polar DMF and THF. The yield to MCC
reached 74% with 85% conversion of DCU when the
molar ratio of DCU to DMC was 1∶3. If the molar ra-
tio of DCU to DMC was increased to 1∶5, the yield to
MCC was 88%. This result was better than the
self-solvent system, in which the molar ratio of DCU to
DMC was 1∶15. Undoubtedly, the application of sol-
vent to this reaction can efficiently promote the forma-
tion of carbamate.
Results for reaction condition optimization and dis-
cussion
The catalyst screening and reaction condition opti-
mization were carried out over the reaction of
N,N'-dicyclohexyl urea (DCU) and dimethyl carbonate
(DMC) (Table 3). The blank reaction was tested in the
absence of catalyst and the yield to methyl N-cyclohexyl
carbamate (MCC) was ＜1% (Entry 1). As it can be
imagined, 28% MCC formed if silica gel itself was used
as catalyst (Entry 2). According to the NH3-TPD analy-
sis, there was a large amount of acid upon silica gel sur-
face. This is helpful for this transesterification. Excellent
yield could be obtained when silica gel immobilized
lanthanum was used as catalyst (Entries 3—7). Accord-
ing to the isolated yield, the optimal composition of the
catalyst was 4.3 wt% La/SiO2. The isolated yield to
MCC reached 90% together with 94% selectivity.
Higher or lower lanthanum loading led to lower conver-
sion of starting material and lower yield of the desired
product, which means both the activity and selectivity of
the catalyst decreased.
The characterization of the catalyst samples gives a
good explanation to this phenomenon. By the HR-TEM
measurement we can know that the particle size is
Table 3 Catalyst screening and reaction condition optimizationa
Entry
1
2e
Catalyst
None
Conv.b/%
Sel.c/%
Yieldd/%
＜5%
—
82
92
92
94
91
91
60
80
92
94
—
SiO2
41
70
28
3
4
1.3 wt% La/SiO2
3 wt% La/SiO2
4.3 wt% La/SiO2
6.4 wt% La/SiO2
8.5 wt% La/SiO2
4.3 wt% La/SiO2
4.3 wt% La/SiO2
4.3 wt% La/SiO2
4.3 wt% La/SiO2
59
75
90
85
83
15
50
74
88
88
5
＞99
98
6
7
96
8f1
9f2
10f3
11g
35
68
85
97
a 5 mmol (1.12 g) of DCU, 75 mmol (8 mL) of DMC, 50 mg of (0.1—0.5 mol% La) catalyst, 150 ℃, 6 h. b Conversion of DCU deter-
mined by GC-FID. c Chemoselectivity to methyl N-cyclohexyl carbamate determined by GC-FID. d Isolated yield. All the compounds
1
e
were characterized by H NMR, 13C NMR and MS analysis. Commercial compound (200—300 mesh, BET surface area＝601.1 m2/g,
pore volume＝0.74 cm3/g, average pore diameter＝4.93 nm). It was pretreated at 600 ℃ for 4 h and then cooled in a drier before use.
—
f 3 5 mmol (1.12 g) of DCU, 15 mmol (1.6 mL) of DMC, 5 mL of solvent, 50 mg of 4.3 wt% La/SiO2, 150 ℃, 6 h. The solvents were
DMF, THF and toluene from f 1 to f 3, respectively. g 5 mmol (1.12 g) of DCU, 25 mmol (2.7 mL) of DMC, 5 mL of toluene, 50 mg of 4.3
wt% La/SiO2, 150 ℃, 6 h.
f 1
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Chin. J. Chem. 2010, 28, 164— 170

Atom-economy Synthesis of N-Substituted Carbamate
Generality investigation
Conclusion
By applying the optimized reaction condition, the
generality of the silica gel immobilized lanthanum in
carbamate synthesis from urea derivatives and dimethyl
carbonate was investigated (Table 4). Aliphatic urea
derivatives with different structures were firstly studied.
The isolated yields to corresponding carbamates were
81%—90% (Entries 1—5). The structure of side chains
of the urea derivatives has less effect on the formation
of carbamates although the presence of benzyl group
needs a little bit longer reaction time. Excellent yields
were also obtained when aromatic urea derivatives were
used as starting materials (Entries 6—9). The isolated
yields were 72%—85% for three typical aromatic urea
derivatives, i.e. N,N'-diphenyl urea, N,N'-di-4-tolyl urea
and N,N'-di-4-chlorophenyl urea.
In conclusion, a series of silica gel immobilized lan-
thanum catalysts were successfully prepared for the
syntheses of N-substituted carbamates from urea deriva-
tives and dimethyl carbonate. This method is simple,
easily operable and general. And different methyl car-
bamates with various structures were synthesized with
high isolated yields.
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