Oxidative carbonylation of aromatic amines by selenium compounds

Article abstract of DOI:10.1006/jcat.1999.2458

Various factors affecting the oxidative carbonylation of aromatic amines were investigated in the presence of a catalyst system comprising SeO₂ and alkali metal carbonate. An active catalyst, K₂SeO₃ was obtained from the reaction of SeO₂ and K₂CO₃ in methanol and the activity was tested. The effects of added alkali metal salts, solvents, aromatic amine substrates, molar ratio of aniline to catalyst, temperature, and pressure on the catalytic activity were studied. The activation energy of the carbonylation of aniline was evaluated as 17.0 kcal/mol. The highest turnover frequency was found to be 2082/h at aniline/catalyst = 12,800. A plausible mechanism for the carbonylation of aniline to diphenyl urea and phenyl carbamate has also been proposed.

Full text of DOI:10.1006/jcat.1999.2458

Journal of Catalysis 184, 526–534 (1999)
Article ID jcat.1999.2458, available online at http://www.idealibrary.com on
Oxidative Carbonylation of Aromatic Amines by Selenium Compounds
Hoon Sik Kim,�,1
�
�
�
Yong Jin Kim, Hyunjoo Lee, Sang Deuk Lee, and Chong Shik Chin†
�
CFC Alternatives Research Center, Korea Institute of Science and Technology, P.O. Box 131, Cheongrayng,
Seoul 130-650, Korea; and †Department of Chemistry, Sogang University, Seoul 121-742, Korea
E-mail: khs@kist.re.kr
Received November 30, 1998; revised February 8, 1999; accepted February 14, 1999
In the previous communication, we reported that a cata-
Various factors affecting the oxidative carbonylation of aromatic lyst comprising SeO2 and alkali metal carbonate was highly
amines were investigated in the presence of a catalyst system com- effective for the oxidative carbonylation of aniline to pro-
prising SeO2 and alkali metal carbonate. An active catalyst, K2SeO3
was obtained from the reaction of SeO2 and K2CO3 in methanol and
the activity was tested. The effects of added alkali metal salts, sol-
vents, aromatic amine substrates, molar ratio of aniline to catalyst,
temperature, and pressure on the catalytic activity were studied.
duce phenyl carbamate and diphenyl urea as illustrated in
Eqs. [1–3] (19).
1
2
RNH2 + CO + O2 → RNHC( == O)NHR + H2O,
The activation energy of the carbonylation of aniline was evaluated
as 17.0 kcal/mol. The highest turnover frequency was found to be
2
[1]
2
082/h at aniline/catalyst = 12,800. A plausible mechanism for the
carbonylation of aniline to diphenyl urea and phenyl carbamate has
also been proposed.
0
0
RNHC( == O)NHR +R OH → RNH2 +RNHC( == O)OR ,
2]
[
�c 1999 Academic Press
1
0
0
RNH2 +CO + O2 +R OH → RNHC( == O)OR + H2O,
2
1
. INTRODUCTION
[3]
Carbamates are important materials as precursors for
preparing isocyanates since they produce isocyanates (R = phenyl, R = C1–C4).
and alcohols in good yields by thermal cracking (1, 2).
The synthesis of substituted ureas has also been exten-
0
Carbamates are commercially manufactured by reacting sively studied due to their importance in the chemical in-
phosgene with the corresponding primary amine (3). dustry as precursors of carbamates, pigments, and resins,
Unfortunately, this conventional process possesses several and the non-phosgene methods for converting substituted
disadvantages such as the use of highly toxic phosgene, ureas to the corresponding carbamates are well established
the formation of large quantities of corrosive hydrogen in the literature (20–23).
chloride as a secondary product, and, furthermore, the
In this paper, various aspects of the oxidative carbony-
presence of difficult-to-remove hydrolyzable chlorine com- lations of aromatic amines are investigated by using a new
pounds. Accordingly, there have been numerous attempts catalyst system comprising SeO2 and alkali metal carbon-
to produce carbamates by using non-phosgene methods ate, and a plausible mechanism is presented.
(
4–10). One of the approaches is the catalytic oxidative car-
bonylation of an amine with carbon monoxide and oxygen
in the presence of an alcohol and an appropriate catalyst.
A number of effective catalyst systems have been reported
for preparing carbamates via oxidative carbonylation of
amines (11–13). Group 8–10 transition metals and copper
are commonly used catalysts for this purpose (14–16).
Selenium and selenium compounds have also been demon-
strated as catalysts in the carbonylation of aliphatic and
alicyclic amines, but the catalysts show extremely low activ-
ity toward the carbonylation of aromatic amines (17–18).
2
. EXPERIMENTALS
2
.1. Chemicals
Amines, selenium metal, selenium dioxide, and alkali
metal carbonates were purchased from Aldrich Chemical
Co. and used without further purification. Solvents were all
reagent grade and distilled from appropriate drying agents
under a nitrogen atmosphere prior to use. Over 99% pu-
rity of carbon monoxide and oxygen were obtained from
Samsung Fine Chemical Co. and Daesung Oxygen Co., re-
spectively.
1
To whom correspondence should be addressed.
526
0021-9517/99 $30.00
Copyright �c 1999 by Academic Press
All rights of reproduction in any form reserved.

CARBONYLATION OF ANILINE
527
2
.2. Instrumentation
methanol, 1 g toluene as an internal standard, and cata-
lyst (SeO2/K2CO3). The reactor was purged with nitrogen
and then with O2/CO mixture gas and was then pressur-
ized to about 4.08 MPa with O2/CO mixture (molar ratio of
O2/CO = 20/80). The bomb was then heated with agitation
Gas chromatographic analyses were made for both gas
and liquid samples. Gas samples were analyzed using a
Gow-Mac gas chromatography equipped with a thermal
conductivity detector and a Carbosphere column (6 ft). Liq-
uid samples were analyzed using a Young-In 680D gas chro-
matography equipped with a flame ionization detector and
a HP-1 capillary column (0.32 �m � 50 m). Mass spectral
analyses were carried out employing a HP 5890A GC/HP
�
to a specified reaction temperature (130 or 170 C) with the
addition of O2/CO mixture gas from a reservoir tank. After
the reaction, the bomb was cooled to room temperature and
vented, and the product mixture was removed for analysis.
5
917A MS detector. Urea analysis was carried out on a
2.5. Synthesis of K2SeO3. In a 100 ml 3-neck flask,
Waters HPLC using 4.6 � 150 mm Hyersil ODS column K2CO3 (40 mmol) was reacted with SeO2 (40 mmol) dis-
and UV detector (254 nm). Shimadzu (XRD 6000) diffrac- solved in 25 ml methanol at room temperature for 1 h.
tometer with a nickel-filtered Cu K� excitation source was As soon as the K2CO3 was added to the methanolic so-
employed to obtain XRD patterns for all the catalyst sam- lution of SeO2, CO2 started to evolve. Removal of the sol-
ples. The X-ray source was operated at 30 kV and 40 mA vent in vacuum gave a white solid. The white compound
�
with scanning rate 2 /min. Compound identification was ac- was characterized as K2SeO3 by elemental analysis [found,
complished by the comparison of measured spectra of the Se (38.1% ), K (37.8% ); calculated, Se (38.5% ), K (38.5% )]
samples with those of reference samples or JCPDS powder and XRD results after several recrystallizations from di-
diffraction file data.
ethyl ether/methanol solvent (yield: 87% ).
2
.3. Carbonylation Reaction
3
. RESULTS AND DISCUSSION
All carbonylation reactions were conducted in a Parr 100
or 300 ml stainless steel bomb with a magnetic drive stir-
rer and an electrical heater. Reactions were carried out at
SeO2-catalyzed oxidative carbonylations of aromatic
amines were performed in various solvents in the presence
of an alkali metal carbonate promoter. The effects of pro-
moter, substrate, temperature, and pressure of O2/CO on
the carbonylation of aniline were evaluated.
In order to ensure that the catalyst retains its original ac-
tivity, several experiments on the reusability of the catalyst
were carried out.
1
000 rpm stirring rates to minimize mass transfer of O2/CO
mixture gas as a limitation to the reaction rates. Toluene
was used as an internal standard due to the presence of high
boiling insoluble materials in the reaction mixture, such as
urea. The reactor was purged with nitrogen followed with
O2/CO mixture gas and then pressurized to about 4.08 MPa
with O2/CO mixture (molar ratio of O2/CO = 20/80). The
bomb was then heated with agitation to a specified reaction
temperature with the addition of O2/CO mixture gas from
a reservoir tank (ballast tank) to maintain a specified pres-
sure. The drop in pressure in the ballast tank was recorded
by means of a pressure transducer and a recorder, which
A plausible reaction mechanism of the oxidative car-
bonylation of aniline catalyzed by K2SeO3 was also pro-
posed.
3
.1. Effect of Added Alkali Metal Salt
SeO -catalyzed oxidative carbonylation of aniline was
2
provides a convenient method for observing the progress carried out in the presence of various alkali metal salts.
�
of the reaction. After the reaction, the bomb was cooled to The reactions were performed in methanol at 130 C and
room temperature. Product mixtures were analyzed by GC, 6.80 MPa (O /CO = 20/80) and the results are listed in
2
HPLC, and GC–MS. Solid catalyst mixtures were charac- Table 1. The addition of alkali metal halide, nitrite, ni-
terized by XRD.
trate, or sulfate was found to be ineffective for the car-
bonylation of aniline. Unlike other alkali metal salts, the
presence of the alkali metal carbonate enabled the car-
bonylation of aniline to proceed very rapidly. Among the
alkali metal carbonate, the activity was found in the or-
0
2
.4. Alcoholysis of N,N -Diphenyl Urea
2.4.1. Under N2 atmosphere. A 100 ml stainless Parr
reactor was charged with 20 mmol diphenyl urea, 25 ml
methanol, 1 g toluene as an internal standard, and catalyst
�
der SeO2/Cs2CO3 = SeO2/Rb2CO3 > SeO2/K2CO3 > SeO2/
Na2CO3 > SeO2/Li2CO3, suggesting that the solubility and
basicity of alkali metal carbonate might play an important
role (19). It is interesting to note that only alkali metal car-
bonate is the active promoter among the alkali metal salt
tested. It is presumed that the reaction of SeO2 with al-
(
SeO2/K2CO3). The reactor was purged with nitrogen and
�
then heated to 130 or 170 C with agitation. After 2 h of
reaction, the bomb was cooled to room temperature and
the product mixture was analyzed by GC and HPLC.
2.4.2. Under (O2/CO) pressure. A 100 ml stainless Parr kali metal salts such as halide, nitrite, nitrate, and sulfate
reactor was charged with 20 mmol diphenyl urea, 25 ml may not produce active alkali metal containing selenium

5
28
KIM ET AL.
TABLE 1
2 3 2 2 5
lation of aniline, implying that both K SeO and K Se O
may act as active species. The role of alkali metal carbon-
ate and the reason for the activity of alkali metal containing
selenium compound in the oxidative carbonylation of aro-
matic amine is described in the previous communication
Effect of Alkali Metal Salts on the Oxidative
a
Carbonylation of Aniline
Selectivity (% )
Alkali metal
salts
(
19).
Catalyst
Conversion (% )
Carbamateb
Ureac
3
.3. Effect of Solvent
SeO2
SeO2
SeO2
SeO2
SeO2
SeO2
SeO2
K2Se2O5
K2SeO3
SeO2
None
KBr
KI
KNO3
KNO2
K2SO4
K2CO3
None
None
N.R.
N.R.
N.R.
N.R.
N.R.
N.R.
97.5
95.4
95.0
67.1
63.4
Table 2 shows the effect of solvent on the conversion of
aniline. In nonpolar solvents like n-hexane, benzene, and
toluene, aniline was not carbonylated at all in the presence
of a catalyst consisting of SeO2 and K2CO3 (K2CO3/SeO2 =
49.2
27.2
30.5
9.5
50.3
72.0
68.8
80.4
78.6
5
), while in polar solvents like methanol, ethanol, isopropyl
alcohol, n-propanol, sec-butyl alcohol, THF, and DMF, high
conversion of aniline was obtained. It is surprising to note
that K2Se2O5 and K2SeO3, ionic products obtained from
the reactions of SeO2 and K2CO3 in methanol, were able
to carbonylate aniline even in nonpolar solvents such as
n-hexane, benzene, and toluene. These results strongly in-
dicate that the active species are ionic and the formation of
such ionic active species is favored in polar solvent. Surpris-
ingly, unlike in other C1–C4 alcohol, the catalyst comprising
SeO2 and K2CO3 showed very little activity in tert-butyl al-
cohol solvent, and it is believed that the steric hindrance of
d
K2CO3/KI
SeO2
K2CO3/K2SO4e
8.4
a
Conditions: aniline 40 mmol, SeO2 0.2 mmol, K2CO3 1 mmol,
�
methanol 25 ml, toluene 1 g (internal standard), temperature 130 C, pres-
sure 6.80 MPa (O2/CO = 20/80), reaction time 2 h.
b
Carbamate: methyl-N-phenyl carbamate.
Urea: N,N -diphenyl urea.
KI 0.2 mmol.
K2SO4 0.2 mmol.
c
d
e
0
compound. Another possibility is that halide, nitrite, ni- tert-butyl alcohol prohibits the formation of active species
trate, or sulfate anion left in the catalyst or reaction mix- such as K2Se2O5 and K2SeO3. This is supported by the ex-
ture may act as a detrimental poison. In fact, the activity perimental result that the reaction by K2Se2O5 and K2SeO3
of a catalyst composed of SeO2 and K2CO3 was decreased
significantly by the addition of K2SO4 or KI.
3
.2. Alkali Metal Containing Selenium Compounds
We reported that the reaction of K2CO3 with 2 equiv of
SeO2 in methanol produced KSeO2(OCH3) and K2Se2O5
19). K2SeO3 was similarly prepared by the 1 : 1 reaction of
(
K2CO3 with SeO2 in methanol and the compound was char-
acterized by XRD and elemental analysis. As shown in
Fig. 1, the XRD pattern is in good agreement with that of
K2SeO3 in the JCPDS Handbook. SeO2 reacts with meth-
anol to give (CH3O)Se( == O)(OH) and (CH3O)2Se(OH)2
(
19), and the subsequent reaction of K2CO3 with equiva-
lent of (CH3O)2Se(OH)2 would produce K2SeO3 as in
Eqs. [4–7]:
SeO2 + CH3OH → (CH3O)Se( == O)(OH),
[4]
(
(
(
CH3O)Se( == O)(OH) + CH OH
3
→
(CH3O)2Se(OH)2,
CH3O)2Se(OH)2 + K2CO3
(CH3O)2Se(OK)2 + H2O + CO2,
CH3O)2Se(OK)2 → K2SeO3 + (CH3)2O.
[5]
→
[6]
[7]
Like K2Se2O5, K2SeO3 also showed the similar activity to
the catalyst composed of SeO2 and K2CO3 in the carbony-
FIG. 1. XRD patterns of A–D.

CARBONYLATION OF ANILINE
529
TABLE 2
toward the reductive and oxidative carbonylations of ni-
trobenzene that were carried out in methanol at 130 C for
�
Effect of Solvents on the Oxidative Carbonylation of Anilinea
2
h.
The carbonylation of aromatic diamine such as 2,4-
Selectivity (% )
0
diaminotoluene and 4,4 -methylenedianiline is important
in the chemical industry because the resulting carbonylated
products, diurea and/or dicarbamate, can be directly used
as intermediates of TDI (toluene diisocyanate) and MDI
Catalyst
Solvent
Conversion (% ) Carbamateb Ureac
SeO2/K2CO3 Methanol
SeO2/K2CO3 Ethanol
97.5
95.7
94.6
92.8
91.4
96.7
95.4
1.1
N.R.
N.R.
N.R.
97.8
80.4
85.6
49.2
35.2
15.3
9.1
2.1
—
50.3
63.4
83.7
89.7
96.8
98.8
98.6
—
SeO2/K2CO3 n-Propanol
SeO2/K2CO3 Isopropyl alcohol
SeO2/K2CO3 sec-Butyl alcohol
SeO2/K2CO3 THF
(
methylenediphenyl diisocyanate).
3
.5. Effect of Molar Ratio of Aniline/SeO2
SeO2/K2CO3 DMF
—
tr.
The effect of molar ratio of aniline/SeO2 on the carbony-
SeO2/K2CO3 tert-Butyl alcohol
SeO2/K2CO3 n-Hexane
SeO2/K2CO3 Benzene
SeO2/K2CO3 Toluene
lation of aniline was studied in the presence of SeO2/K2CO3
�
at 130 C (K2CO3/SeO2 = 5) and 6.80 MPa (O2/CO = 20/80).
The molar ratio of aniline to SeO2 was varied in the range
50–25,600, and the results are shown in Fig. 2. The conver-
sion of aniline remained almost constant at around 95% in
the molar ratio range between 50 and 1600, and thereafter
K2SeO3
K2SeO3
K2SeO3
tert-Butyl alcohol
Toluene
0.9
—
—
98.1
98.6
98.5
Benzene
a
Conditions: aniline 40 mmol, SeO2 0.2 mmol, K2CO3 1 mmol, sol- sharp decrease in conversion was observed. The selectiv-
�
vent 25 ml, toluene 1 g (internal standard), temperature 130 C, pres- ity of carbamate decreased as the molar ratio increased,
sure 6.80 MPa (O2/CO = 20/80), reaction time 2 h.
whereas the selectivity of diphenyl urea increased with the
increasing molar ratio. The maximum turnover frequency
was found to be 2082/h at molar ratio of 12,800, indicating
that our catalyst system comprising SeO2 and K2CO3 is as
b
Carbamate: methyl-N-phenyl carbamate.
c
0
Urea: N,N -diphenyl urea.
in tert-butyl alcohol proceeded as fast as in other C1–C4 al- active as Pd system reported by Fukuoka et al. (14).
cohols. In separate experiments, the reaction of SeO2 with
3
.6. Effect of Temperature and Pressure
K2CO3 in THF and DMF at room temperature also pro-
duced K-containing selenium compounds by elemental and
XRD analyses, but the full characterization of these com-
pounds was not successful. Like K2Se2O5 and K2SeO3, these
unidentified compounds also show high catalytic activity for
the carbonylation of aniline in nonpolar solvents, again sup-
porting the assumption that the nature of active species is
ionic and such ionic active species are favorably formed in
polar solvents.
Figures 3 and 4 show the effects of temperature and
pressure on the oxidative carbonylation of aniline to give
phenyl carbamate and diphenyl urea in the temperature
range 90–170 C at various pressure of O2/CO (molar ratio
of O2/CO = 20/80) ranging from 2.04 to 6.80 MPa. A plot of
�
TABLE 3
Oxidative Carbonylation of Various Aromatic Aminesa
3
.4. Effect of Substrate
Selectivity (% )
SeO2-catalyzed oxidative carbonylations of aniline and
substituted anilines were performed in methanol at 130 C
Amine
Conversion (% )
Carbamateb
Ureac
�
in the presence of a promoter, K2CO3. As shown in Table 3, Aniline
97.5
96.4
95.2
94.1
98.9
99.1
97.6
N.R.
N.R.
N.R.
N.R.
49.2
23.4
0.5
0.4
tr.
50.3
75.3
98.1
98.7
98.6
98.1
99.1
o-Toluidine
aniline, o-toluidine, dimethyl anilines, 2,4,6-trimethylani-
line, 2,4-diaminotoluene, and 4,4 -methylenedianiline were
carbonylated to give high yields of corresponding carba-
mates and/or ureas while p-nitroaniline was not carbony-
0
0
4,4 -Methylenedianiline
2
2
2
,4-Diaminotoluene
,4-Dimethylaniline
,6-Dimethylaniline
tr.
tr.
lated at all under the same experimental condition. Rel- 2,4,6-Trimethylaniline
p-Nitroaniline
p-Nitroaniline
Nitrobenzene
atively higher yield of carbamate was obtained in the
carbonylation of aniline, but ureas were predominantly
d
formed in the case of substituted aniline, possibly due to
the electronic effect of substituents. It is not clear at the mo-
ment why the carbonylation of p-nitroaniline did not take
place. However, it is likely that nitro group in p-nitroaniline
played a certain role in quenching the catalytic activity
of SeO2/K2CO3. Experimental results also show that the
catalyst comprising SeO2 and K2CO3 catalyst was inactive
d
Nitrobenzene
a
Conditions: substrate 40 mmol, SeO2 0.2 mmol, K2CO3 1 mmol,
�
methanol 25 ml, toluene 1 g (internal standard), temperature 130 C, pres-
sure 6.80 MPa (O2/CO = 20/80), reaction time 2 h.
b
Carbamate: methyl-N-phenyl carbamate.
Urea: N,N -diphenyl urea.
Reactions carried out in the absence of O2, pressure 6.80 MPa.
c
0
d

5
30
KIM ET AL.
FIG. 2. Effect of molar ratio (aniline/SeO2) on the oxidative carbonylation of aniline. Conditions: aniline 40 mmol, molar ratio (K2CO3/SeO2 = 5),
�
methanol 25 ml, toluene 1 g, temperature 130 C, pressure 6.80 MPa (O2/CO = 20/80), reaction time 2 h. Carbamate, methyl-N-phenyl carbamate; urea,
0
N,N -diphenyl urea.
FIG. 3. Effect of temperature on the oxidative carbonylation of aniline. Conditions: aniline 40 mmol, SeO2 0.2 mmol, K2CO3 1 mmol, meth-
0
anol 25 ml, toluene 1 g, pressure 6.80 MPa, reaction time 2 h. Carbamate, methyl-N-phenyl carbamate; urea, N,N -diphenyl urea.

CARBONYLATION OF ANILINE
531
FIG. 4. Effect of pressure on the oxidative carbonylation of aniline. Conditions: aniline 40 mmol, SeO2 0.2 mmol, K2CO3 1 mmol, methanol 25 ml,
�
0
toluene 1 g, temperature 130 C, reaction time 2 h. Carbamate, methyl-N-phenyl carbamate; urea, N,N -diphenyl urea.
�
log rate vs 1/T is shown in Fig. 5, from which the value of increase in temperature. The decrease in conversion at
�
activation energy, Ea, was evaluated as 17.0 kcal/mol.
temperatures higher than 130 C can be rationalized by
The conversion of aniline increased with increasing tem- assuming that the rate of alcoholysis of diphenyl urea is
�
perature up to 130 C and then decreased with further higher than that of carbonylation of aniline, i.e., the rate
FIG. 5. Arrhenius plot.

5
32
KIM ET AL.
TABLE 4
the oxidation of CO to CO . The oxidative carbonylation
2
0
a
of aniline (40 mmol) was performed in methanol (25 ml) at
Alcoholysis of N,N -Diphenyl Urea
�
1
30 C and 6.80 MPa (O2/CO = 20/80) for 2 h. GC analysis of
Selectivity (% )
Pressure (MPa) Temp. ( C) Conversion (% ) Carbamateb Aniline
gas sample showed that 7.7% of CO consumed was oxidized
to CO2. Conversion of aniline and selectivity to methyl-N-
phenyl carbamate were 97.5% and 49.2% , respectively. In
�
0
.1 (N2)
130
170
130
170
60.5
96.2
25.5
67.5
49.2
49.2
82.8
75.9
50.4
50.5
16.7
23.7
order to find out the effect of CO formation on the car-
bonylation of aniline, the carbonylation reaction was con-
ducted in the presence of added CO2 using 6.80 MPa of
2
0
6
6
.1 (N2)
.80 (O2/CO)
.80 (O2/CO)
CO2/O2/CO mixture gas (CO2/O2/CO = 12/18/70) instead
a
0
Conditions: N,N -diphenyl urea 20 mmol, SeO2 0.2 mmol, K2CO3 of O /CO (O /CO = 20/80). Gas chromatographic analy-
2
2
1
mmol, methanol 25 ml, toluene 1 g (internal standard), O2/CO = 20/80,
sis of liquid sample gave 98.1% of conversion and 48.3%
of selectivity, which were similar to those obtained from
the above experiment performed without the presence of
added CO2.
reaction time 2 h.
b
Carbamate: methyl-N-phenyl carbamate.
of formation of aniline from diphenyl urea is higher than
From these results, it is concluded that the oxidation of
that of aniline disappearance by carbonylation at higher CO to CO2 occurs in significant yield, but the effect of CO2
temperature (see Eqs. [1–3]). The results on the alcoholysis formation on the carbonylation of aniline can be negle-
�
�
of diphenyl urea at 130 C and 170 C are listed in Table 4. cted.
Table 4 shows that under atmospheric pressure of N2 at
�
1
30 C, the 60.5% of diphenyl urea was converted into ani- 3.8. Proposed Mechanism
line and phenyl carbamate. When the reaction was con-
ducted at 6.80 MPa of O2/CO, however, the conversion
dropped significantly from 60.5% to 25.5% . Such a pressure
effect was also observed by Macho et al. (4). The selectivity
of phenyl carbamate increased with the increase of tem-
perature while the selectivity of diphenyl urea decreased,
strongly indicating that diphenyl urea is an intermediate to
the formation of phenyl carbamate. The effect of pressure
on the conversion of aniline as shown in Fig. 4 seems to be
less pronounced than that of temperature because the con-
version reached over 85% as long as the pressure was main-
tained above 2.04 MPa of O2/CO (O2/CO = 20/80). How-
ever, the selectivity of phenyl carbamate was found to be
more strongly dependent on the pressure as going from 8%
at 2.04 MPa to 49% at 6.80 MPa even though the depen-
dence was much less pronounced compared with that of
temperature. In order to test the catalyst reusuability, the
carbonylation reaction was performed for 2 h in the pres-
Based on the experimental results, the mechanism of the
oxidative carbonylation of aniline catalyzed by K2SeO3 is
proposed in Scheme 1. As described in the experimental
section, the active species, K2SeO3 is formed by the reac-
tion of SeO2 with K2CO3. In a second step, aniline reacts
with K2SeO3 to give amido complex II. Amido species II
could take up one molecule of CO to form selenium car-
bonyl species, Se(OK)2(CO)(OH)(NHPh), and the subse-
quent insertion of CO into Se–N bond would produce car-
bamoyl complex III. The formation of carbamoyl species
III seems to be a key step in the proposed mechanism. Ad-
dition of one molecule of another aniline to species III and
the subsequent elimination of diphenyl urea and H2O give
rise to selenium species IV (path A). IV can also be pro-
duced by interaction of species III with one molecule of
methanol and subsequent elimination of phenyl carbamate
and H2O (path B). Liberated diphenyl urea can be added
to I, forming Se(OK)2(OH)(NPhCONHPh), which, in turn,
reacts with methanol to give phenyl carbamate, simultane-
ously regenerating species II (path C). The final step is the
regeneration of K2SeO3 (I) by the interaction of species IV
with molecular O2. Even though diphenyl urea was found to
be easily converted into phenyl carbamate by reacting with
methanol in the presence of SeO2/K2CO3 catalyst, it cannot
be concluded that diphenyl urea is the sole intermediate to
the formation of phenyl carbamate. Phenyl carbamate also
�
ence of a catalyst, SeO2/K2CO3 at 130 C and 6.80 MPa of
O2/CO in methanol. After the reaction, the reaction solu-
tion was filtered to remove insoluble diphenyl urea. The
resulting solution containing the catalyst was reused with
a fresh charge of aniline. It was found that the catalyst re-
tained its original activity even after four reuses.
3
.7. Effect of CO2
The catalytic activity of SeO2/K2CO3 was tested for the can be produced directly via a separate route such as path
oxidation of CO to CO2, and the effect of CO2 on the ox- B other than the diphenyl urea route.
idative carbonylation of aniline was investigated.
It isreported that oxidation ofCO to CO2 takesplace dur-
ing the oxidative carbonylation of amines (24). As most of
the catalysts proposed in the literature, our catalyst system,
4
. CONCLUSIONS
Various factors affecting the oxidative carbonylation of
SeO2/K2CO3, also showed some catalytic activity toward aromatic amines were investigated in the presence of a new

CARBONYLATION OF ANILINE
533
SCHEME 1. A plausible reaction mechanism of oxidative carbonylation of aniline catalyzed by K2SeO3.
catalyst system comprising SeO2 and alkali metal carbon-
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1
ACKNOWLEDGMENT
(
The authors thank the Ministry of Science and Technology for the fi-
nancial support of this study.
(

5
34
KIM ET AL.
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