A non-phosgene route synthesis of carbamate in continuous fixed bed reactor

Article abstract of DOI:10.1002/jccs.200700128

A non-phosgene route synthesis of carbamate was carried out in a continuous fixed-bed reactor through oxidative carbonylation of aniline using palladium catalysts and sodium iodide as promoter. The activity, selectivity and stability of both carbon and alumina-supported palladium catalysts were evaluated. It was found that the alumina-supported catalyst system exhibited a higher activity and selectivity than that of the carbon-supported system, and an average aniline conversion of 95.6% and carbamate selectivity of 74.6% were achieved for the Se-Pd/Al₂O₃ catalyst after 91 h on stream. Reclamation analysis of the spent Pd/C catalyst suggested that the deactivation was mainly due to the leaching and sintering of palladium metal and the accumulation of insoluble chemicals on catalyst support also aggravated the decline of catalyst activity. When small amounts of selenium were added to the Pd/Al ₂O₃ catalyst, its activity, selectivity and stability were significantly improved which indicated that a promotional effect existed for carbamate formation on a Pd-Se catalyst system.

Full text of DOI:10.1002/jccs.200700128

Journal of the Chinese Chemical Society, 2007, 54, 885-892
885
A Non-phosgene Route Synthesis of Carbamate in Continuous Fixed Bed
Reactor
Mong Liang* (
Chih-Chiao Huang (
), Tzung-Jan Lee (
) and Kun-Yu Lin (
Department of Applied Chemistry, National Chia-Yi University, Chia-Yi 60004, Taiwan, R.O.C.
),
)
A non-phosgene route synthesis of carbamate was carried out in a continuous fixed-bed reactor
through oxidative carbonylation of aniline using palladium catalysts and sodium iodide as promoter. The
activity, selectivity and stability of both carbon and alumina-supported palladium catalysts were evalu-
ated. It was found that the alumina-supported catalyst system exhibited a higher activity and selectivity
than that of the carbon-supported system, and an average aniline conversion of 95.6% and carbamate se-
lectivity of 74.6% were achieved for the Se-Pd/Al O catalyst after 91 h on stream. Reclamation analysis
2 3
of the spent Pd/C catalyst suggested that the deactivation was mainly due to the leaching and sintering of
palladium metal and the accumulation of insoluble chemicals on catalyst support also aggravated the de-
2 3
cline of catalyst activity. When small amounts of selenium were added to the Pd/Al O catalyst, its activ-
ity, selectivity and stability were significantly improved which indicated that a promotional effect existed
for carbamate formation on a Pd-Se catalyst system.
Keywords: Pd-Se bimetallic catalysts; Oxidative carbonylation; Continuous fixed bed reactor.
INTRODUCTION
Oxidative carbonylation of aromatic amines in the
sis via diphenyl urea was carried out using NaI-Pd/C cata-
lyst. The semi-continuous carbamate synthesis proceeded
for only two hours with 80.2% of aniline conversion and
74.8% carbamate yield. To the best of our knowledge, there
are no reports in the literature describing the use of a con-
tinuous fixed-bed reactor for the carbonylation of aromatic
amines. As the annual production of isocyanate has in-
creased dramatically with an estimation of 4.4 millions
metric tons and a growth rate of 5~7% worldwide today,
the continuous fixed-bed reactor has advantages for its con-
venient operation as well as ease of catalyst separation in
industrial production. Thus, the objectives of this study
were to develop a modified palladium catalyst suitable for
continuous preparation of carbamates and to investigate the
catalyst deactivation phenomenon according to reclama-
tion analysis of the spent catalyst.
presence of noble metal catalysts have attracted consider-
able attention in developing a non-phosgene route of iso-
1
cyanate for the past twenty years. The major products of
this reaction, carbamates, are widely used in the pharma-
ceutical and pesticide industries and can be readily con-
verted to isocyanate for polyurethane synthesis via thermo
2
decomposition. A variety of catalyst systems have been in-
vestigated using oxygen as oxidizing agent for the carbon-
3
4
5
ylation reactions. These include: Pd/C, Rh/C, Ru/C, Au/
6
7
8
9
polymer, PdCl
2
, PdCl
2 2 2 2
-CuCl , PdCl -Fe, and PdCl -
1
0
MnCl
such as ZnCl
2
. Other base metals or main group catalyst systems
11
12
2
and SeO
2
have also been proved satisfac-
tory for carbamate formation under different conditions.
Reviewing previous studies, most carbonylation re-
actions have been carried out batchwise; little has been
done for the continuous process, and reports regarding the
recovery of catalysts as well as the deactivation phenome-
EXPERIMENTAL SECTION
1
3
non are also rather rare. Recently, Toochinda and Chuang
Materials and Catalyst Preparations
have reported a gas-solid oxidative carbonylation reaction
in a semibatch fixed-bed reactorwherein a two-step synthe-
All the chemicals including palladium nitrate (Kojima
Chemical Co. Ltd., Japan), sodium iodide, selenium stan-
*
Corresponding author. Tel: 011-886-52717964; Fax: 011-886-52717901; E-mail: mliang@mail.ncyu.edu.tw

8
86 J. Chin. Chem. Soc., Vol. 54, No. 4, 2007
Liang et al.
dard solution (Aldrich, USA), methanol, CO, O
2
(99.9%
wt.%.
purity) were purchased from commercial companies and
used as received. Reagent grade aniline was freshly dis-
tilled before use. Active carbon-supported palladium cata-
lysts (Pd/C) were prepared by impregnating active carbon
The alumina-supported Pd-Se catalyst was prepared
2 3
by sequential impregnation of the Al O support with Pd
first, using the general method described above, followed
by selenium addition. Selenium was introduced to the cata-
lysts system in a mole ratio of Pd:Se = 37:1 by incipient
(
coconut-based carbon, 1/8” extrudated in a 1 mm ´ 5 mm
2
cylindrical, specific surface area 1000 m /g) with Pd(NO
3
)
2
2 3
wetness impregnation of initially prepared 5% Pd/Al O
in aqueous solutions for two hours. After filtration, the cat-
alyst was dried in an air oven for 2 h at 353 K, reduced in
flowing hydrogen at 323 K for 3 h and then cooled to room
temperature under hydrogen flow before being used. The
palladium loading was determined by dissolving a sample
in aqua regia and then analyzing by atomic absorption
spectroscopy.
with the desired amount of selenium standard solution at
ambient temperature for 2 h. Following impregnation, the
resultant mixture was dried at 373 K and then calcined in
air at 873 K for 3 h. After calcination, the catalysts were re-
duced for 3 h in flowing hydrogen at 573 K and then cooled
to room temperature under hydrogen flow before being
used.
The alumina-supported Pd catalyst (Pd/Al
prepared by incipient wetness impregnation technique us-
ing an acetone solution of Pd(NO . g-Alumina (BET: 203
m /g, average pore size: 15.3 nm, pore volume: 0.59 mL/g)
were immersed in a minimum amount of Pd(NO /acetone
2 3
O ) was
Oxidative Carbonylation Reaction
3
)
2
The experimental setup for the fixed-bed reactor is
shown in Fig. 1. The system consists of a reactor, one liquid
feed pump, flow measurement for the liquid and gas, gas-
liquid separator and back pressure regulator for accurate
pressure control. The reactor was made of high-pressure
stainless steel tube (length = 670 mm, i.d. = 18 mm), equipped
with an axial (o.d. = 1.6 mm) thermowell to allow mea-
surement of the temperature profile. Five grams of catalyst
2
3 2
)
solution for 2 h. After impregnation, the samples were
dried at 383 K for 2 h and calcined at 873 K for 3 h, then
cooled to room temperature and reduced at 573 K with a 10
°
C/min linear raise in temperature under flowing hydrogen
at 40 mL/min for 2 h. The nominal palladium loading was 5
Fig. 1. Schematic diagram of continuous fixed-bed reactor set-up for carbamate synthesis.

Synthesis of Carbamate in a Fixed Bed Reactor
J. Chin. Chem. Soc., Vol. 54, No. 4, 2007 887
were placed in the middle portion of the reactor, whereas
the top and bottom portions were filled with ceramic beads
for material distribution and heat exchange. Typical reac-
tion conditions were as follows: 7 wt.% aniline and 0.07
wt.% NaI were dissolved in absolute ethanol and were
pumped by a high pressure metering pump with a flow rate
bonylation of aniline to the desired carbamate can be writ-
ten as follows:
(1)
(2)
(3)
2
of 20 mL/h. Pure CO and O gas provided by gas cylinders
was passed co-currently from the top of the reactor at 150
mL/min and 30 mL/min, respectively, to maintain a total
pressure of 6.6 MPa to maintain the reactor at constant
pressure during a carbonylation experiment. The tempera-
ture of the reactor was controlled by PID-controller and
was maintained at 443 K during the course of reaction. The
pressure in the reactor system was maintained through a
backpressure regulator connected with a gas-liquid separa-
tor. Reactants and the product stream were analyzed by
HPLC (Waters 600) using a UV detector (Waters Model
It was also demonstrated that N,N¢-diphenylurea (DPU),
found as intermediate in the reaction (eq 2), would undergo
solvolysis reaction and yielded carbamates (eq 3). Regard-
less of the type of catalysts used, all the batch processes en-
countered catalyst separation and reusability problems
which were difficult for process development in terms of
operation simplicity and large scale production. Hence, we
intended to explore the feasibility of the continuous pro-
cess and investigate the performance of the catalysts in a
continuous fixed-bed reactor. Three types of Pd/C catalysts
denoted as catalyst A, B, C with Pd loading of 3.0%, 2.34%
and 2.88%, respectively, were prepared in a similar manner
described in the experimental section except that catalyst A
was reduced at 573 K with flowing hydrogen, catalyst B
was reduced at 323 K with flowing hydrogen and catalyst C
was reduced at 318 K with 0.1N HCOONa. The reduction
methods of catalyst precursors highly influenced the parti-
cle size and the dispersion of the precious metal as shown in
Table 1. The reduction with sodium formate resulting in a
lower dispersion than those prepared by hydrogen reduc-
tion suggested that hydrogen appeared to be a more potent
reducing agent than sodium formate in obtaining a smaller
palladium particle size and a higher dispersion on active
carbon support. This result is in good agreement with the
4
86, wavelength: 254 nm) furnished with a Inertsil column
ODS 4.6 ´ 250 mm (Vercopak, USA), mobile phase (v/v)
O/Acetonitrile = 43/57, flow rate = 1 mL/min. BET sur-
H
2
face areas of the catalysts were measured by using a Micro-
meritics Flowsorb II 2300 unit at -196 °C. X-ray diffraction
of the catalyst was carried out with a Scintag ID-3000
diffractometer with Cu Ka radiation (l = 1.5418 Å) for 2q
of 10-70°.
RESULTS AND DISCUSSIONS
Evaluation and Characterization for Pd/C Catalysts
Numerous examples of homogeneous and heteroge-
neous catalysts have been shown to exhibit high selectivi-
ties toward carbamates in oxidative carbonylation of ani-
3
-14
15
16
line or aliphatic amines.
The reaction of oxidative car-
observations made by Pattabirman and Farkas, wherein
Table 1. Physical characteristics of catalysts A, B, C
Fresh Catalyst
Spent Catalyst
a
b
c
a
b
c
Pd loading Particle Size Dispersion Pd loading Particle Size Dispersion
%
nm
%
%
nm
%
Catalyst A
Catalyst B
Catalyst C
a
2.34
3
2.88
08.2
11.2
23.4
10.9
08.1
03.8
1.04
0.81
1.63
20
49.2
45.3
4.5
1.8
2
Analyzed by atomic absorption after digestion with aqua-regia.
b
c
Analyzed by X-ray line broading analysis (XLBA) according to Scherrer equation at 2q = 47°.
Calculated according to semi-emperical formula d = 0.9/D, d is particle size (nm); D is
dispersion.

8
88 J. Chin. Chem. Soc., Vol. 54, No. 4, 2007
Liang et al.
Pd/C catalyst obtained by the hydrogen reduction method
gave higher dispersion than the sodium formate method.
Catalytic conversion and product selectivity on cata-
lysts A, B, and C are plotted in Fig. 2. For all catalysts, de-
activation appeared during the reaction time on stream and
catalyst A exhibited the highest aniline conversion and
ethyl phenyl urethane (EPU, or carbamate) selectivity after
height in radians, l is the wavelength of the X-ray radiation
(Cu K = 0.154 nm), q is the Bragg angle of the XRD peak in
2
0 h. In catalyst A, the selectivity of DPU gradually in-
creased from 7 to 30% (Fig. 2c), whereas the selectivity of
EPU dropped from 63 to 46% (Fig. 2b) after 91 h which
represented the typical decline feature of catalytic activity.
Catalyst B exhibited similar deactivation behavior to Cata-
lyst A except that the conversion and product selectivity
was lower than that on Catalyst A. We presumed that the
poor catalytic performance of catalyst B was related to the
lower reduction temperature which led to the incomplete
reduction of palladium metals and resulted in more severe
leaching and sintering of palladium metals thereafter. The
higher activity was in agreement with the higher metal dis-
persion prepared by different reduction methods. The cata-
lytic reactions were run for approximately 91 and 96 h on
stream for catalysts A and B and thereafter, the fixed-bed
reactor was cooled to room temperature and spent catalyst
samples were unloaded for characterizations. Despite the
highest initial selectivity, catalyst C was tested for only 26
h due to the significant drop of carbamate selectivities and
rapid increase in diphenyl urea selectivity after 20 h on
stream. The reclamation analysis of spent catalysts of A, B
and C was performed by atomic absorption spectroscopy
and power X-ray diffraction. For palladium loading deter-
mination, the Pd metals and active carbon were digested
with concentrated hydrochloric acid and nitric acid in the
form of chlorocomplex [H
2 4
PdCl ] (eq 4) and analyzed by
atomic absorption spectroscopy.
Pd + 4 HCl + 2 HNO
PdCl + 2 H
3
®
H
2
4
2
O + 2 NO
2
(4)
The mean crystallite size of the crystalline palladium on
spent catalyst was measured from X-ray line broadening
analysis (XLBA) using the well-known Scherrer equa-
Fig. 2. Effect of Pd/C catalysts on conversion and
product selectivity of continuous fixed-bed re-
actor (a) aniline conversion (b) carbamate se-
lectivity (c) DPU selectivity. Reaction condi-
tions: aniline, 7 wt.%; NaI, 0.07 wt.%; Solvent:
Ethanol; PCO, 5.5 MPa, PO2, 1.1 MPa; tempera-
1
7
tion:
0
.89l
d =
B(2q)cos q
-
1
where B(2q) is the width of the XRD peak at half peak-
ture, 170 °C; LHSV = 2 h .

Synthesis of Carbamate in a Fixed Bed Reactor
J. Chin. Chem. Soc., Vol. 54, No. 4, 2007 889
degrees, and d is the average crystallite size of the powder
sample in nanometers. The dispersion is calculated accord-
ing to the semi-empirical formula d = 0.9/D, where d is par-
Scheme I Proposed reaction mechanism for the for-
mation of carbamate and diphenyl urea in a
fixed-bed reactor
1
8
ticle size(nm); D is dispersion. The calculated results are
summarized in Table 1. They revealed that the reaction
conditions had brought about drastic changes in the catalyst
structure, especially the Pd particle size after the reaction.
The decrease of activity is mainly caused by the sintering
and leaching of palladium catalysts. For example, approxi-
mately 56, 73 and 43% of palladium are leached out from
the catalyst matrix after catalytic reaction and the crystal-
lite values measured by XRD were also much larger than
that of fresh catalysts. Surface areas and pore volumes of
the active carbon of Catalyst B were measured with nitro-
gen adsorption at -196 °C on a Micromeritics Flowsorb II
1
9,20
2
300 instrument.
The textural properties of catalyst B
showed that the surface area and pore volumes decreased
by about 90% and 13%, respectively, compared to the fresh
catalyst which also explained the gradual loss of activity
during the reaction process. Residual analysis of spent cat-
alyst by HPLC indicated that DPU was the major compo-
nent that blocked the surface, probably due to its poor solu-
bility in the ethanol solvent system.
catalytic mode. The observation that DPU formation in-
creased along with the decline of EPU with time on stream
implied that aminolysis became more favorable when a cat-
alyst was less active. Similar trends could also be found in
the alumina-support catalyst system in this study.
Proposed Reaction Mechanism
Attempts to carry out the reaction in the absence of
sodium iodide resulted in no reaction which indicated that
0
Pd was oxidized to form the active Pd-I species during the
2 3
Catalytic Studies for Pd/Al O
reaction. This phenomenon could cause a leaching of the
metal into the reaction medium and explains the loss of Pd
loading after a period of time. Although the mechanism of
carbamate synthesis is still not clear, a postulated catalytic
cycle of oxidative carbonylation of aniline over supported
palladium is depicted in Scheme I. After impregnation, cal-
To improve efficiency of the fixed-bed process, we
further investigated the alumina-supported catalysts sys-
tem and evaluated catalytic performance of a bimetallic
catalyst. Great interest has developed in recent decades in
2
1-23
the palladium-based bimetallic catalysts.
The incorpo-
0
cination and reduction, the dispersed Pd particle was acti-
Table 2. Textural properties of catalyst B
vated by NaI and CO (i) which underwent oxidative addi-
tion of aniline (ii) and carbonyl insertion (iii) to form the
carbamoyl-palladium species subsequently. It seemed likely
that the carbamoyl-palladium intermediate could undergo
either alcoholysis (iv) or aminolysis (v) to afford the corre-
sponding carbamate or DPU products. Thereafter, the re-
sulting palladium hydride species could be dehydrogenated
by oxygen (vi) to regenerate the catalyst and advance a new
catalytic cycle by association with carbon monoxide (vii).
Another pathway for EPU formation could proceed through
the direct solvolysis of DPU with alcohol (viii) in a non-
Support
1090
Fresh cat.
1040
Spent cat.
102
a
BET surface area
2
(
m /g)
b
Pore volume
(mL/g)
0
.56
.4
166
0.49
Avg. pore diameter
nm)
Pore surface area
4
5.0
(
2
40
(
m /g)
a
Brunauer-Emmett-Teller (BET) surface area of the samples was
measured using the adsorption-desorption isotherms of nitrogen.
Measured by Hg intrusion method.
b

8
90 J. Chin. Chem. Soc., Vol. 54, No. 4, 2007
Liang et al.
ration of a second metal in catalysts has proven to often
provide additional catalytic stability, selectivity and/or ac-
tivity. Various Se-based catalysts have been reported in the
immobilizing a small amount of selenium on Pd/Al
solve the problems described above. The alumina-sup-
ported Pd catalyst (Pd/Al ) were prepared by the incipi-
2 3
O to
2
O
3
2
4-28
carbonylation of nitrobenzene and aniline,
however,
ent wetness impregnation technique with nominal palla-
dium loading of 5 wt.%. The alumina-supported Pd-Se cat-
alysts were prepared by sequential addition of selenium so-
disadvantages arose from the difficulty in catalyst separa-
tion and the malodorousness of toxic selenium species
formed in the reaction mixture. Considering the role of se-
lenium in carbonylation reactions, it was conceivable, but
not yet demonstrated, to modify the catalyst structure by
2 3
lution to the initially prepared 5% Pd/Al O catalysts sys-
tem in a mole ratio of Pd:Se = 37:1 as described in the ex-
perimental section. Both catalysts were tested under simi-
lar reaction conditions, and the results are illustrated in
Figs. 3 and 4. For Se-Pd catalyst, aniline conversion re-
mained almost unchanged with an average of 95.6% during
the 150 h running time, and the average selectivities of
EPU and DPU were maintained at 74.6% and 7.8%, respec-
tively, as shown in Fig. 3. These results were far more supe-
rior to carbon-supported catalysts and were also signifi-
cantly better than the unmodified Pd/Al
2 3
O system. For the
unmodified Pd/Al catalyst, as illustrated in Fig. 4, an
2
O
3
average aniline conversion of 88.1 and average selectivity
of 67.7% for EPU, 10.2% for DPU were obtained under
similar conditions. Comparison of the average selectivities
of EPU observed in Figs. 2, 3 and 4 showed that the type of
support used could significantly affect activity and stability
of catalyst. The higher catalytic activities of alumina sup-
ported catalysts could be in part ascribed to the nature of
the support. The activated carbon support has more micro-
2 3
Fig. 3. Effect of Pd-Se/Al O catalyst on conversion
and product selectivity of a continuous fixed-
bed reactor. Reaction conditions: aniline, 7
wt.%; NaI, 0.07 wt.%; solvent: absolute etha-
nol; PCO, 5.5 MPa, PO2, 1.1 MPa; temperature,
2
pores and a narrower pore diameter (BET:1040 m /g, pore
-
1
1
70 °C; LHSV = 2 h .
volume: 0.56 mL/g, average pore size: 4.4 nm) than that of
2
g-alumina (BET:203 m /g, pore volume: 0.59 mL/g, aver-
age pore size: 15.3 nm), which makes Pd particles in the in-
ner cavities of active carbon inaccessible for catalytic reac-
tion and increases the possibility of pore blockage as well.
2 3
With the addition of a small amount of Se to the Pd/Al O
catalyst, significant activity enhancement was observed
which indicated that a promotional effect existed for car-
bamate formation on Pd-Se catalyst system. The promoting
effect of selenium is not well understood. It is probable that
carbonyl selenide SeCO, formed under high CO pressure
conditions, assisted the carbamate formation. In selenium-
mediated carbonylation of amines, the carbonyl selenide
reacted with amine to form carbamoselenoate (RNHCOSe)
followed by nucleophilic attack (alcohol or amine) to yield
corresponding carbamate or urea derivatives. Sonoda et
2 3
Fig. 4. Effect of Pd/Al O catalyst on conversion and
product selectivity of continuous fixed-bed re-
actor. Reaction conditions: aniline, 7 wt.%;
NaI, 0.07 wt.%; solvent: absolute ethanol; PCO
,
2
4,28
al
isolated the carbamoselenoate intermediates as evi-
5
.5 MPa, PO2, 1.1 MPa; temperature, 170 °C;
-
1
LHSV = 2 h .
dence for such a two-step process.

Synthesis of Carbamate in a Fixed Bed Reactor
Table 3. Comparison of carbamate yields in oxidative
J. Chin. Chem. Soc., Vol. 54, No. 4, 2007 891
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