N-Substituted carbamate synthesis using urea as carbonyl source over TiO2-Cr2O3/SiO2 catalyst

Article abstract of DOI:10.1039/c5gc01007a

The use of urea as an active form of carbon dioxide is a feasible way to substitute phosgene in the chemical industry. This paper reports an effective route for the synthesis of N-substituted carbamates from amines, urea and alcohols. Under the optimized reaction conditions, several important N-substituted carbamates were successfully synthesized in 95-98% yields over a TiO₂-Cr₂O₃/SiO₂ catalyst. The catalyst could be reused for several runs without deactivation. The catalysts were characterized by BET, XPS, XRD, and TPD, which suggested that the strength and amount of the acidic and basic sites might be the major reason for the high catalytic activity of TiO₂-Cr₂O₃/SiO₂.

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Paper
N-substituted carbamates syntheses using urea as carbonyl source over
TiO2-Cr2O3/SiO2 catalyst
Peixue Wang,a,b Yubo Ma, a Shimin Liu, a Feng Zhou,a,b Benqun Yanga,b and Youquan Deng,*a
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
5
DOI: 10.1039/b000000x
The use of urea as an active form of carbon dioxide is a feasible way to substitute phosgene in the
chemical industry. This paper reports an effective route for the syntheses of N-substituted carbamates
from amines, urea and alcohols. Under the optimized reaction conditions, several important N-substituted
carbamates were successfully synthesized with 95-98% yields over TiO2-Cr2O3/SiO2 catalyst. The
10 catalyst could be reused for several runs without deactivation. The catalysts were characterized with BET,
XPS, XRD, and TPD, which suggested that the strength and amount of the acidic-basic sites might be the
major reason for the highly catalytic activity of TiO2-Cr2O3/SiO2.
phosgenation of amines,29-30 reductive carbonylation of nitro
1. Introduction
compounds,31-32 oxidative carbonylation of amines,33-34
50 methoxycarbonylation of amines,35-36 alcoholysis of substituted
As an abundant, renewable and economical carbon resource,
15 transformation of CO2 to useful carbon compounds has attracted
significant interest.1-5 Clearly, the direct usage of CO2 as building
block, e.g. carbonyl source, in chemical industry should be an
ideal choice.6-7 However, CO2 is the most oxidized state of
carbon and acts as a relatively inert molecule, therefore, the
20 activation of CO2 normally needs the usage of a highly active
starting material or to shift the equilibrium to the target product
by removing a particular compound.8-10 New methods such as
CO2 hydrogenation to produce HCOOH, CH3OH, CH4, etc, have
been extensively studied.11-12 But this technique is severely
25 restricted by the current hydrogen supply. In this context,
exploration of alternative ways for CO2 activation and utilization
in large scale are highly desired. Until now, manufacturing urea is
very mature13-15 and currently global supply of urea is exceeding
demand, especially in China. In this sense, using urea as
30 feedstock to produce higher valuable chemicals opens a new
window for CO2 utilization and alternative carbonyl sources to
substitute phosgene, e.g., manufacturing of carbonates and N-
substituted carbamates via direct CO2 activation are very difficult,
while using urea to produce carbonates16-22 and N-substituted
35 carbamates are very efficient and low cost.
N-substituted carbamates represent an important class of
compounds with various interesting properties and have wide
utility in various areas including pharmaceuticals, agrochemicals,
protection of amino group in peptide chemistry, and as linkers in
40 combinational chemistry.23-25 Moreover, they have become the
potential raw material for the non-phosgene synthese of
isocyanates through thermal decomposition,26-28 such as 1,6-
hexamethylene diisocyanate, isophorone diisocyanate, 4,4'-
dicyclohexylmethane diisocyanate and 4,4'-diphenylmethane
45 diisocyanate. According to the type of carbonyl sources (COCl2,
CO, CO2, dimethyl carbonate and ureas), the processes for the
synthesis of N-substituted carbamates (Scheme 1) mainly include
39-
ureas37-38 and direct syntheses from CO2, amines and alcohols.
41
Scheme 1 The processes for synthesis of N-substituted carbamates with
55 different carbonyl sources.
The phosgenation process generally involves environmentally
hazardous phosgene and produces highly corrosive hydrochloride
as a side product. The carbonylation of nitroaromatics or amines
with carbon monoxide catalyzed by transitional metal complexes
60 suffers from some problems such as poisonous carbon monoxide,
higher cost of transitional metal complexes. Thus, great efforts
have been made to explore environmentally benign methods for
carbamates production. For example, dimethyl carbonate (DMC)
could be used as an attractive ecofriendly alternative to phosgene,
65 however, the higher price of DMC limits its practical application.
Besides, the alcoholysis of substituted ureas result in a poor atom
economy and in each case amine is produced as byproducts,
which decrease the efficiency of functional group of the reagents.
To improve the atom economy, N-substituted carbamates have
70 been synthesized from urea derivatives and dialkyl carbonates,
which is also restricted by the high cost.42-45 Another approach is
the direct synthese of carbamates from CO2, amines and alcohols,
but only monocarbamates could be generated and the direct N-
substituted dicarbamates syntheses from CO2, diamines and
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alcohols were not reported. Otherwise, alkyl carbamates as a new
green carbonyl sources were uesd for the syntheses of N-
substituted carbamates and good results were reported.46-50
However, the lower efficiency, separation, and recovery problems
of the catalysts are still maintained as the major problems.
Urea could also be employed as a phosgene substitute for the
syntheses of N-substituted carbamates via one-pot reaction. In
such process, urea can be used as the nontoxic carbonylation
agent, which is abundant resource with lower price. Meanwhile,
500 °C in air for 4 h.
TiO2-Cr2O3/SiO2: 10mL 26-27 wt% HNO3 and 7.62 g
Cr(NO3)3·9H2O was successively added into a 100 mL beaker
containing 4 mL Ti(OC4H9)4 (HNO3 was used to adjust pH value
55 to 2-3). Subsequently, upper butanol formed during the
hydrolysis of tetrabutyl titanate was carefully separated by a
separatory funnel. Then 10 g SiO2 pellets was added and
impregnated. The precursor was dried at 90 °C and calcined at
500 °C in air for 4 h.
5
10 NH3 co-produced in the reaction can be easily recycled to urea,
since the urea synthesis from CO2 and NH3 has been established.
Therefore, this route could be considered as indirect utilization of
CO2 to produce the green chemicals of carbamates. However,
only few references described the reaction of amines with urea
15 and alcohols, and the route underwent relatively high temperature
(>200 °C).51-52 Herein, we report the preparation of TiO2-
Cr2O3/SiO2 as highly effective and recoverable catalyst for N-
substituted carbamates syntheses with amines and urea, Scheme
2. XRD, BET, XPS, and CO2 (NH3)-TPD characterizations were
20 conducted to explore the relationship between structure and
performance. Moreover, the influence of the different substrates,
and reaction variables (such as temperature, time) were also
studied in a batch reactor system.
60 2.2 Carbamates syntheses from amines, urea and alcohols
All reactions were carried out in a 90 mL stainless steel autoclave
with a glass tube inside and a magnetic stirrer. As an example, 10
mmol of amines, 10-30 mmol of urea, 50-200 mmol alcohols and
8 wt% catalyst (based on the mass of charged amines) were
o
65 charged in the reactor. The reaction proceeded at 170-210 C for
4-16 h. After the reaction, the autoclave was cooled down to the
room temperature. The catalyst was recovered by centrifugation
and then qualitatively identified by HP-6890 GC-MS equipped
with a SE-54 capillary column. The quantitative analysis of the
70 product was determined by Agilent 7890 GC equipped with a SE-
54 capillary column and a FID detector (biphenyl was used as
internal standard). The gas chromatograph applied a temperature
ramp for the analysis: the column temperature was started at 80
oC, and then the temperature was raised 15 oC per minute until a
75 temperature of 260 oC was reached. The temperature was
maintained for five minutes before the column oven was cooled
to 80 for the next analysis.
3. Results and discussions
25 Scheme 2 The syntheses of N-substituted carbamates from amines, urea
and alcohols over TiO2-Cr2O3/SiO2 catalyst.
3.1 Catalysts for the synthesis of carbamates
80 A series of metal oxide catalysts supported on SiO2, including
TiO2/SiO2, Cr2O3/SiO2, NiO/SiO2, NiO-Cr2O3/SiO2 and TiO2-
Cr2O3/SiO2, were employed for the synthesis of carbamates.. In
the preliminary study, 1,6-hexanediamine (HDA) was selected as
a model substrate to test this protocol as shown in Table 1. In the
85 blank test (entry 1), HDA conversion reached 99% while the
yield of diethyl hexamethylenedicarbamate (EHDC) was only
12%. Since, we found that a large amount of white solid was
formed and could not dissolve in the resulting liquid. The IR
spectra (Fig. S1) of solid products exhibited the peaks at 1618
90 cm-1 and 1577 cm-1, which contributes to the characteristic
absorption peaks of carbonyl C=O and CO–N–H, respectively,
suggesting ureas derivatives were generated in the products.53-54
Besides, ethyl carbamate (EC) and diethyl carbonate (DEC) were
obtained in the liquid products, which may derived from the
95 alcoholysis of excess urea. Furthermore, trace amount of
alkylated HDA were also detected by GC-MS, which probably
formed via the side reaction between HDA and DEC or the direct
reaction between the amine and alcohol. In order to understand
the formation of alkylated HDA, the reaction using amine and
100 ethanol as reactants was carried out, and no alkylated by-product
was observed, indacating that the alkylated by-product is from
side reaction between HDA and DEC instead of direct reaction
between the amine and alcohol. Although the reaction between
HDA and urea could proceed in the absence of catalyst, catalyst
105 is essential for higher yield of EHDC. As pure SiO2 used as
catalyst, poor catalytic activity was obtained (only 30% EHDC
2. Experimental
2.1 Catalysts preparation
NiO/SiO2: 4.0 g Ni(NO3)2·6H2O was added into a 100 mL
30 beaker containing 10 mL H2O (pH value 3-3.5). Then 10 g SiO2
pellets was added and impregnated. The precursor was dried at 90
°C and calcined at 500 °C in air for 4 h.
TiO2/SiO2: 10 mL 26-27 wt% HNO3 aqueous solution was
added into a 100 mL beaker containing 4 mL tetrabutyl titanate
35 (HNO3 was used to adjust the pH value to 1-2) under vigorous
stirring. Subsequently, upper butanol formed during the
hydrolysis of tetrabutyl titanate was carefully separated by a
separatory funnel. Then 10 g SiO2 pellets (purchased from
Qingdao Hai Yang Chemical Co., Ltd.), which was pretreated at
40 600 °C for 4 h, was used for wet impregnation with the solution
aforementioned. The precursor was dried at 90 °C for 12 h, and
then calcined at 500 °C in air for 4 h.
Cr2O3/SiO2: 7.69 g Cr(NO3)3·9H2O was added into a 100 mL
beaker containing 10 mL H2O (pH value 3-3.5). Then 10 g SiO2
45 pellets was added and impregnated. The precursor was dried at 90
°C and calcined at 500 °C in air for 4 h.
NiO-Cr2O3/SiO2: 4.0
g
Cr(NO3)3·9H2O and 2.9
g
Ni(NO3)2·6H2O were added into a 100 mL beaker containing 10
mL H2O (pH value 3-3.5). Then 10 g SiO2 pellets was added and
50 impregnated. The precursor was dried at 90 °C and calcined at
2
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yield) (entry 2). NiO/SiO2 gave higher yield (58%) for EHDC
reaction times, the yield of EHDC increased smoothly and
reached a maximum yield after 8 h (entries 4-6). When the
reaction time was 4 h, the conversion of HDA could reach 95%,
while the yield of EHDC was only 30% and the main products
compared to the pure SiO2 (entry 3), however, ureas derivatives
were also detected in the by-products. The yield of EHDC could
be greatly improved when Ti or Cr oxides was supported on SiO2
5
(entries 4-7). Thereinto, TiO2-Cr2O3/SiO2 catalyst displayed the 45 were substituted urea. The amounts of substituted urea were
highest catalytic activity and complete conversion of HDA could
reach with 98% yield of EHDC. These results indicated that the
substituted urea may be the intermediate for EHDC synthesis
from HDA with urea, which could be easily prepared even
decreased with an increase of reaction time, which further
confirmed that the substituted urea was the intermediate for this
reaction. It was also found that the EHDC yield was sensitive to
the molar ratio of MHDA:MROH (entries 7-9). A significant
10 without catalyst, but the further alcoholysis of the substituted urea 50 increase in EHDC yield was observed when the ethanol/HDA
to EHDC is difficult and catalyst has to be introduced in this step
for the high yield (Scheme 3).
molar ratio increased from 5 to 15, and reached the maximum
when the ratio was 15. Generally, in a substitution reaction, a
strong nucleophilic group is prone to substituting a weak one. As
The reusability of solid catalysts is a key parameter in
determining their suitability in practical applications. Therefore,
−
NH2 is a stronger nucleophile than CH3CH2O−, alcoholysis of
15 attempts were made to recycle the TiO2-Cr2O3/SiO2 catalyst by 55 the substituted urea reaction is thermodynamically unfavorable.55
simple filtration, and reused without any processing. >99 % HDA
conversion with 96% EHDC yield was obtained after 5 times
recycle (entry 8), suggesting the catalyst was stable and the
catalytic activity could be essentially preserved. It realized the
As the molar ratio of ethanol/HDA is 5, the decomposition of the
urea was observed, since we found that some white solid was
formed on the pipelines at the top of the autoclave. Theoretically,
higher ethanol/HDA molar ratio favors to increase of the EHDC
20 synthesis of EHDC from amines, urea and alcohols is an effective 60 yield, nevertheless, further increasing the ethanol/urea molar ratio
and practical pathway.
to 20, the yield of EHDC was slightly declined. At this stage,
although the detailed reason is still not clear, it can be conjectured
that insufficient availability of catalyst species may be one of the
reasons due to the remarkable increase of reaction volume. When
65 excessive ethanol was added, most of the activity sites on the
catalyst surface have been covered by alcohol molecules and few
substituted urea molecules could be activated. It is also
uneconomical for the EHDC synthesis. Besides, as the molar
ratio of Urea/HDA was 2, 80% yield of EHDC was achieved,
70 however, as the molar ratio of Urea/HDA increased to 3, no
remarkable increase of EHDC yield was observed, which
suggested that 2.4 times urea might be sufficient for this process
(entries 10-11). The influence of the structure of alcohol on the
reaction was also investigated (entries 12-13). Methanol gave
75 64% yield towards dimethyl hexamethylenedicarbamate (MHDC)
and a large quantity of N-methyl HDA and monocarbamate were
formed. The reaction of butanol, HDA and urea under 190 °C
produces a large quantity of butyl carbamate (BC), however, only
58% yield of dibutyl hexamethylenedicarbamate (BHDC) was
80 obtained after 8 h reaction. Since we note that butanol is less
reactive than other alcohols, the temperature was increased from
190 °C to 210 °C, and the yield of BHDC could be reached to
95%. Additionally, the possibility of the one-pot reaction of
diamine, CO2 and alcohol to afford the corresponding N-
85 substituted carbamate was investigated (entry 15). The results
showed that the corresponding carbamate was not detected and
the main product was the ureas derivatives, which indicated that
the one-pot reaction of diamines, CO2 and alcohol to the
corresponding dicarbamates was unfeasible under present
90 conditions.
Table 1 Results of EHDC synthesis from HDA and urea over different
catalysts a
Entry
Catalyst
none
SiO2
NiO/SiO2
TiO2/SiO2
Conversion (%)
Yield (%)b
1
2
3
4
5
6
7
8
99
96
98
99
99
~100
~100
99
12
30
58
80
62
78
98
96
Cr2O3/SiO2
NiO-Cr2O3/SiO2
TiO2-Cr2O3/SiO2
Used TiO2-Cr2O3/SiO2
25 a reaction conditions: 10 mmol HDA; 24 mmol urea; 150 mmol ethanol;
8wt% catalyst (based on the mass of charged HDA); 190 °C; 8 h. b GC
yield based on charged HDA.
O
H2N
+
NH2
H2N
NH2
O
+C2H5OH
+
Step 1
O
O
O
O
H
Side reaction
Side reaction
N
N
H2N
O
n
H
H2N
Catalyst
N
+
H
C2H5OH
O
+
Step 2
+
C2H5OH
H
N
O
H
N
Et
N
O
O
H
O
N
Et
O
O
H
Scheme 3 Possible reaction pathways for the reaction of HDA, urea and
30 ethanol.
3.2 Effect of reaction conditions
In order to optimize the reaction conditions, experiments
focusing on the effects of the reaction temperature, time, molar
ratio of HDA:Urea:ROH and alcohol structure were tested and
35 the obtained results are listed in Table 2. The EHDC yield
increased rapidly with increasing the reaction temperature, and
the yield reached a maximum (98% yield) at 190 °C, (entries 1-3).
Whereas, further increase of the reaction temperature up to 210
°C resulted in the decrease of yield, presumable due to the
40 decomposition of urea at high temperature. By increasing the
Nevertheless, we must recognize that reaction conditions (15
equivalents alcohol, 8 h reaction time, 8 wt% catalyst, i.e.,
MHDA:MUrea:MROH=1:2.4:16) are difficult to achieve industrial
application now. A possible strategy is half of the raw materials
95 (i.e., MHDA:MUrea:MROH= 0.5:1.2:8) added to the reactor at first,
after 2~3
h
reaction, the remaining half reactants
(MHDA:MUrea=0.5:1.2, dissolved into a small amount of alcohols)
were heated to the desired temperature in the pre-heater and then
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continuously fed into the reactor by a high-pressure pump. In this 40 successfully synthesized from the aliphatic amines, urea and
way, we can not only keep the original ratio of raw materials, and
reduce the excess of reagents/catalyst theoretically.
alcohols.
Then, the reaction of various aromatic amines, urea and
ethanol was also studied (entries 11-16). Under the optimized
reaction conditions, 73% conversion of aniline was obtained
45 (entry 11), which could be ascribed to the weaker basicity of the
aromatic amine. For other aromatic amines, the obtained results
showed that the conversion (80-99%) of aromatic amines and the
isolated yields (70-97%) of the corresponding N-substituted
carbamates were lower than that of aliphatic amines. Furthermore,
50 the amount of N-ethylated aromatic amines was increased when
the aromatic amines especially toluene diamine (TDA) used as
substrate. The formed N-ethylated aromatic amine should be
related to the reaction between aromatic amine and diethyl
carbonate formed via urea alcoholysis. All those results suggested
55 that the reaction of urea with aromatic amines was harder than
with aliphatic amines, which should be attributed to the
nucleophilicity difference in the two types of amines. An
aliphatic amine has no aromatic ring attached directly to the N
atom, while aromatic amines have the N atom connected to an
60 aromatic ring as in the various anilines. The aromatic ring
decreases the electron cloud density of the N atom and reduces
the ability of N atom combined with proton, i.e. basicity
decreased. So, aromatic amines were found to be less reactive
than aliphatic amines. Aside from their basicity, the dominant
65 reactivity of amines is their nucleophilicity. The conversion of
aromatic amines was lower than aliphatic amines, which may be
ascribed to the weaker nucleophilicity of the aromatic amine.
Table 2 Optimization of the reaction conditions for the N-substituted
5 carbamates synthesis from urea and HDA a
Temperature
Entry
Alcohols MHDA:Murea:MROH
Time (h) Yield (%)b
(oC)
170
190
210
190
190
190
190
190
190
190
190
190
190
210
190
1
2
C2H5OH
C2H5OH
C2H5OH
C2H5OH
C2H5OH
C2H5OH
C2H5OH
C2H5OH
C2H5OH
C2H5OH
C2H5OH
CH3OH
C4H9OH
C4H9OH
C2H5OH
1:2.4:15
1:2.4:15
1:2.4:15
1:2.4:15
1:2.4:15
1:2.4:15
1:2.4:5
8
8
20
98
90
30
71
97
20
96
94
80
98
64
58
95
0
3
8
4
4
5
6
6
10
8
7
8
1:2.4:10
1:2.4:20
1:2:15
8
9
8
10
11
12
13
14
15c
8
1:3:15
8
1:2.4:15
1:2.4:15
1:2.4:15
1:0:15
8
8
8
8
a reaction conditions: 10 mmol HDA, 0.1 g TiO2-Cr2O3/SiO2 catalyst, 90
mL autoclave. b GC yield based on charged HDA. c 2 MPa CO2.
3.3 N-substituted carbamates syntheses from various amines,
urea and alcohols
10
To investigate the limitation and scope of this protocol, the
reaction between several different aliphatic amines, such as
butylamine, cyclohexylamine, isophorone diamine, 4,4'-
diaminodicyclohexyl methane, and 1,4 butanediamine with urea
for corresponding carbamates were further tested in the optimized
Table 3 Syntheses of N-substituted carbamates with different amines over
TiO2-Cr2O3/SiO2 catalyst a
Conversion Yield
Alcohols
Entry
Amines
(%)
(%) b
80
CH3OH
C2H5OH
C4H9OH
98
1
2
3
15 conditions, and the results are given in Table 3. First, various
alcohols on the butylamine reaction for corresponding N-
substituted carbamates was tested (entries 1-3). 80%, 98% and
93% yields of the corresponding N-substituted carbamates were
obtained. When the cyclohexylamine used as other substrate
20 (entries 4-6), the reactivity order was consistent with the
butylamine used as substrate. The slightly decrease of N-
substituted carbamate yield when methanol was used may be due
to the formation of N-methylated amines byproducts. Based on
the experimental results, the conversions of amines decreased
25 with increasing the length of alcohol from methanol to butanol,
whlie the higher yield of corresponding carbamate could be
obtained using ethanol as substrate. It has been reported that the
alcoholysis of urea was a typical nucleophilic substitution
reaction, which occurred on the acid and base sites of the
30 catalyst.55 The possible explanation may be that the
nucleophilicity of alkoxyl anion RO- decreased with increase of
the alkyl chain length. Furthermore, various aliphatic amines and
ethanol as substrates were further investigated, entries 7-10.
Although the conversions of various aliphatic amines were all
35 nearly 100%, the yields of the corresponding N-substituted
carbamates of cyclic amines were slightly lower than those of
chain amines, which could be ascribed to the high steric
hindrance of diamines caused by the cyclic structure. All those
results suggested that the N-substituted carbamates could be
99
96
98
93
CH3OH
C2H5OH
C4H9OH
98
99
90
78
97
84
4
5
6
C2H5OH
C2H5OH
99
99
96
95
7
8
C2H5OH
99
96
9
C2H5OH
96
94
10
C2H5OH
C2H5OH
73
96
71
92
11
12
C2H5OH
80
78
13
C2H5OH
C2H5OH
99
99
95
97
14
15
C2H5OH
99
70
16
4
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a reaction conditions: 10 mmol amines, 12 mmol or 24 mmol urea for
monamine and diamine, respectively, 0.1 g TiO2-Cr2O3/SiO2 catalyst, 150
mmol alcohols, 190 oC, 8 h, 90 mL autoclave. b GC yield based on
charged amine.
45 amount of weak basic sites and strong basic sites was about 0.602
mmol/g and 0.638 mmol/g, respectively. Similar NH3-TPD
characterization was also carried out over the obtained samples
(Fig. 1B). Generally, a peak of NH3 desorption appeared at
temperatures range of 100-120 oC was observed over all the
50 tested samples, which were derived from the weak strength acidic
sites. TiO2-Cr2O3/SiO2 showed the relatively stronger acidic
strength, while TiO2/SiO2 showed the relatively larger amount of
acidic sites and the specific acidity was 3.68 mmol/g, the amounts
of acidic sites for other catalysts were only about 1.8 mmol/g.
5
3.4 Results of the catalysts characterization
The physical-chemical properties of the prepared catalysts are
summarized in Table 4. The SiO2 support possesses BET surface
as large as 601 m2/g, as well as pore size 5.0 nm and pore volume
0.74 cm3/g. As expected, the decrease of BET surface area and
10 pore volume was observed when Ti, Ni or Cr were loaded. By
contrast, the pore diameter was retained at about 5.0 nm without
remarkable decline. Meanwhile, the BET surface area of the used
TiO2-Cr2O3/SiO2 catalyst was almost not changed after five times
recycle. The variation of the BET surface area of those catalysts
15 combined with their catalytic activities for the N-substituted
carbamates syntheses from urea and amines suggested that the
BET surface area has little effect on the catalytic performance for
such catalytic system.
Table 4 Physicochemical properties of the catalysts
Total acidic Total
SBET
dp
Vp
Catalyst a
sites
basic sites
(m2/g) (nm) b (cm3/g) c
(mmol/g) (mmol/g)
SiO2
601
554
370
511
5.0
5.2
5.1
5.1
0.74
0.69
0.51
0.68
-
-
2.9wt%TiO2/SiO2
8.5wt%Cr2O3/SiO2
5wt%NiO/SiO2
3.1wt%NiO-
3.68
1.55
1.53
0.75
0.50
0.44
55
469
434
463
5.0
5.0
5.1
0.47
0.49
0.46
1.83
1.87
-
0.64
1.24
-
3.5wt%Cr2O3/SiO2
1.4wt%TiO2-
6.1wt%Cr2O3/SiO2
Reused-1.4wt%TiO2-
6.1wt%Cr2O3/SiO2
20 a The contents of metal were detected by ICP-AES, baverage pore
diameter, c average pore volume
The XPS analyses confirmed that the bond valence of Ti 2p3/2
and Cr 2p3/2 are 458.7 eV and 577.2 eV respectively in TiO2-
Cr2O3/SiO2 catalyst (Fig. S2), suggesting that the chemical states
25 of Ti and Cr species on the catalyst surface were mainly Ti+4 and
Cr3+. As shown in table S1, higher contents of surface Cr and Ti
species were detected than the bulk TiO2-Cr2O3/SiO2, which
means that the Cr-Ti enrich on the surface of catalyst. XRD
patterns of the TiO2-Cr2O3/SiO2 (Fig. S3) showed that only weak
30 diffraction peaks of rhomb-centered rhombohedral Cr2O3 were
observed, which suggested that Cr2O3 in the catalyst is poorly
crystallized and TiO2 in the catalyst is amorphous. All these
results indicated that Cr2O3 and TiO2 species are highly dispersed
on the surface of SiO2.
Fig. 1 TPD-CO2 (A) and TPD-NH3 (B) profiles of (a) NiO/SiO2 (b)
Cr2O3/SiO2 (c) TiO2/SiO2 and (d) TiO2- Cr2O3/SiO2.
According to the results of CO2 (NH3)-TPD and catalytic
60 performance of those catalysts, the yields of N-substituted
carbamates were mainly impacted by the strong of acidic-basic
sites. At this stage, much work is needed before clearly
discerning the role of metals in TiO2-Cr2O3/SiO2 in catalysis,
however, previous studies showed that the reaction between urea
65 and alcohols promoted with basic catalysts proceeded via a
nucleophilic substitution mechanism. TiO2-Cr2O3/SiO2 exhibited
relatively stronger basic strengths in comparison with TiO2/SiO2
and Cr2O3/SiO2, and showed the best catalytic performance. The
presence of strong basic sites in TiO2-Cr2O3/SiO2 probably due to
70 strong synergy between TiO2 and Cr2O3 species on SiO2, which
was consistent with XRD results. Moreover, the role of the Lewis
acid site of M+ (M = Ti and Cr, synergistic effect between Ti and
35
To elucidate the difference between the catalysts, acid and base
properties of these catalysts were studied by TPD of adsorbed
NH3 and CO2, respectively. The CO2-TPD results of TiO2/SiO2,
Cr2O3/SiO2, NiO/SiO2 and TiO2-Cr2O3/SiO2 are shown in Fig. 1A.
All the catalysts showed the first desorption peak at about 60 oC,
40 which can be attributed to the interaction of CO2 with sites of
weak basic strengths. Only TiO2-Cr2O3/SiO2 catalyst showed a
second desorption peak at higher temperatures (460 oC), which
can be attributed to strong basic sites. The total basicity for the
catalyst TiO2-Cr2O3/SiO2 was 1.24 mmol/g (Table 4), and the
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DOI: 10.1039/C5GC01007A
Green Chemistry
Page 6 of 7
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catalytic activity.
4. Conclusions
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In summary, an effective route for the syntheses of N-substituted
carbamates from amines, urea and alcohols over TiO2-Cr2O3/SiO2
catalyst was developed. Under the optimized reaction conditions,
several important N-substituted carbamates were successfully
synthesized with >90% yields. The characterization results of the
catalysts suggested that the relatively stronger basic and acidic
10 strengths might be the major reason for the highly catalytic
activity of TiO2-Cr2O3/SiO2. In this way, the carbamates
synthesis involving urea would be one of the preferable
alternatives to meet the environmentally benign quest.
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Green Chemistry
View Article Online
DOI: 10.1039/C5GC01007A
The syntheses of N-substituted carbamates from amines, urea and alcohols over TiO2-Cr2O3/SiO2
catalyst.