Oxidative carbonylation of amine using water-soluble palladium catalysts in biphasic media

Article abstract of DOI:10.1016/j.molcata.2010.10.018

Application of water-soluble palladium catalysts for oxidative carbonylation of aniline to N,N′ diphenyl urea (DPU) has been reported. The water-soluble palladium catalysts prepared from sulfonated N-containing ligands were found to be highly stable under reaction conditions and easily recyclable due to insoluble urea product in the reaction medium. This is in contrast to the sulfonated phosphine ligands, which are vulnerable to oxidation under reaction conditions, showing poor activity and stability. Commercially available as well as laboratory synthesized ligands were used for preparing water-soluble palladium catalysts, for oxidative carbonylation of aniline. The best activity was obtained for Pd complex with disodium 2,2′-bipyridine-4, 4′-disulfonate (Bipy-DS) ligand. Under optimized conditions Pd(BipyDS)Pd(OAc)₂ catalyst gave TOF of ～210 h^-1 with aniline conversion of ～97% with ～91% selectivity for N,N′-diphenyl urea. It was found that the catalyst was easily reusable up to five times, with negligible loss in the catalytic activity. The effect of reaction parameters was investigated and a plausible reaction mechanism has been proposed.

Full text of DOI:10.1016/j.molcata.2010.10.018

Journal of Molecular Catalysis A: Chemical 334 (2011) 20–28
Contents lists available at ScienceDirect
Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
Oxidative carbonylation of amine using water-soluble palladium catalysts in
biphasic media
a
a
a,∗
a
b,1
Mahesh R. Didgikar , Sunil S. Joshi , Sunil P. Gupte , Makarand M. Diwakar , Raj M. Deshpande
,
c,1
Raghunath V. Chaudhari
Chemical Engineering & Process Development Division, National Chemical Laboratory, Dr. Homi Bhabha Road, Pune 411 008, India
Dow Chemicals International Pvt. Ltd., 6th Floor, Tower ‘C’, Panchshil Tech. Park, Pune 411008, India
Chemical & Petroleum Engineering Department, Center for Environmentally Beneficial Catalysis, The University of Kansas, Lawrence, KS 66047, USA
a
b
c
a r t i c l e i n f o
a b s t r a c t
ꢀ
Article history:
Received 13 April 2010
Received in revised form 21 October 2010
Accepted 25 October 2010
Available online 31 October 2010
Application of water-soluble palladium catalysts for oxidative carbonylation of aniline to N,N diphenyl
urea (DPU) has been reported. The water-soluble palladium catalysts prepared from sulfonated N-
containing ligands were found to be highly stable under reaction conditions and easily recyclable
due to insoluble urea product in the reaction medium. This is in contrast to the sulfonated phos-
phine ligands, which are vulnerable to oxidation under reaction conditions, showing poor activity and
stability. Commercially available as well as laboratory synthesized ligands were used for preparing water-
soluble palladium catalysts, for oxidative carbonylation of aniline. The best activity was obtained for Pd
Keywords:
Carbonylation
Amine
Urea
Water-soluble palladium catalyst
Biphasic catalysis
ꢀ
ꢀ
complex with disodium 2,2 -bipyridine-4,4 -disulfonate (Bipy-DS) ligand. Under optimized conditions
−1
Pd(BipyDS)Pd(OAc)2 catalyst gave TOF of ∼210 h with aniline conversion of ∼97% with ∼91% selec-
ꢀ
tivity for N,N -diphenyl urea. It was found that the catalyst was easily reusable up to five times, with
negligible loss in the catalytic activity. The effect of reaction parameters was investigated and a plausible
reaction mechanism has been proposed.
©
2010 Elsevier B.V. All rights reserved.
1
. Introduction
complex catalysts in combination with iodide promoters have also
been shown to be effective for the reaction. McCusker et al. [9]
have shown that oxidative carbonylation of amines to urea deriva-
tives can be efficiently carried out using W(CO)6 catalyst in a
water–dichloromethane biphasic media. They have shown excep-
tionally efficient catalyst–product separation as well as excellent
product purity. However, iodine promoter was required in sub-
strate to iodide ratio of two and was not recyclable. In order to
develop a practically useful catalyst system, molecular oxygen
needs to be employed in place of iodine as a re-oxidant. In this
connection, several supported metal catalysts, mostly consisting of
palladium, have been investigated in detail [4,6,10]. However, the
major problem in both homogeneous and heterogeneous catalysis
is the sparing solubility of the aromatic urea even in highly polar
solvents. Catalyst separation methods like sieving have been used
in such cases, which may lead to loss of expensive catalyst [11]. Shi
et al. have developed a homogeneous catalyst system consisting
of a palladium complex and ionic liquid [5]. The reaction occurs in
homogeneous phase and at the end of the reaction, water is added
creating biphasic conditions wherein the catalyst components sep-
arate into the aqueous phase, while the product (and unconverted
amine byproducts, etc.) remains in the organic phase. Water is then
distilled off to recover the catalytic phase for recycle, while purified
product can be obtained by separation.
Synthesis of substituted ureas by oxidative carbonylation of
amines is an attractive alternative to the conventional phosgena-
tion route; however, it has limitations due to processing difficulties
such as poor solubility of inorganic halide salt promoters (e.g. NaI,
KI, etc.) in the organic solvents, inefficient catalyst recovery from
reaction mixture and contamination of product by halide promoter
used. Often polar solvents such as dimethyl formamide, N-methyl
pyrrolidone and dimethyl sulfoxides are used either as solvents or
co-solvents to enhance the activity of the catalysts [1]. However,
the leaching of active components from supported noble metal
catalysts (e.g. Pd/C) in highly polar and corrosive halide contain-
ing medium further complicates the issue of recovery and recycle
of the catalyst (and of product urea). In many cases, water (prod-
uct of reaction) has become detrimental to the catalytic system
and water quenching agents such as molecular sieves, orthofor-
mates, acetals and enol ethers have been employed to improve
the yields [2]. Homogeneous Pd [3–5], Rh [6,7] and Ru [8] metal
∗ Corresponding author. Tel.: +91 20 2590 2169; fax: +91 20 2590 2621.
E-mail address: sp.gupte@ncl.res.in (S.P. Gupte).
Present address.
1
1
381-1169/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.molcata.2010.10.018

M.R. Didgikar et al. / Journal of Molecular Catalysis A: Chemical 334 (2011) 20–28
21
In the present work we demonstrated that aqueous–organic
biphasic catalytic system involving a water-soluble catalyst can be
of a great advantage in terms of catalyst and product separation
and at the same time easy recycling of homogeneous catalyst.
Transition metal catalyst
2
+ H O
2
HN
CO+O2/I-
NH2
HN
O
2
. Experimental
ꢀ
Scheme 1. Oxidative carbonylation of aniline to N,N diphenyl urea.
2.1. Synthesis of water-soluble ligands
Isoquinoline-5-sulfonic acid sodium salt (IQS-Na) was synthe-
sized by simple neutralization of isoquinoline-5-sulfonic acid (IQS)
using NaOH. To an aqueous solution of IQS (1.0 g; 4.779 mmol in
(± 1 K) and a magnetically driven stirrer having variable agitation
speed adjustment and indicator. In a typical experiment (these
conditions are henceforth referred to as standard conditions),
weighed amounts of aniline (74 mmol), toluene (50 ml), water
(10 ml), sodium iodide (0.90 mmol) and the water-soluble cata-
5
g deionized water), a cold aqueous solution of sodium hydrox-
ide (0.172 g; 4.779 mmol, 20% w/v) was added drop wise and kept
under continuous stirring for 2 h at room temperature under inert
atmosphere. This solution was concentrated on rota-evaporator
lyst Pd(OAc) BipyDS (0.078 mmol), were charged and the autoclave
2
was sealed. The autoclave was purged once with nitrogen and
thrice with carbon monoxide; and then pressurized with the carbon
monoxide under constant stirring. Oxygen was then pressurized
into the autoclave, so as to make the ratio of carbon monoxide
to oxygen 13:1. Pressure at 423 K reached up-to 68.9 bar. It was
observed that reproducibility of standard reaction (measured as
TOF values) was sensitive to slight variations in the amount of
oxygen introduced at the initial stage (a typical average devia-
tion in TOF value of a standard reaction was 4.8). The heating was
started, and an agitation speed of 16.66 Hz (1000 R.P.M.) was set as
the temperature reached 423 K. The progress of the reaction was
monitored by observing the pressure drop in the autoclave and by
◦
at 60 C, resulting in a white solid which was washed with 90%
methanol several times and vacuum dried. The water-soluble
ligand IQS-Na was analyzed by elemental analysis. Triphenylphos-
phine trisulfonate sodium salt (TPPTS) was synthesized according
to the literature procedure by sulfonation of triphenylphosphine
[
12]. The solid TPPTS obtained was recrystallized from ethanol,
weighed and stored under argon atmosphere. The yield was found
to be 80–85%. TPPTS was characterized by 31P NMR analysis (sin-
glet at ı = −5.15 (TPPTS) and ı = 35.22 (OTPPTS)) which is consistent
with that reported in the literature [12], showing approximately
9
5% TPPTS and 5% OTPPTS formation. For the synthesis of water-
soluble Pd complex, TPPTS was used as synthesized without further
purification. Sulfonated N-containing ligands were synthesized
according to literature procedure involving five steps [13]. These
ligands were characterized by IR, NMR and elemental analysis.
feeding a 2:1 mixture of CO and O from a reservoir vessel in order
2
to keep the reactor pressure constant at 68–69 bar. (Caution! The
reaction conditions employed in the work were safe from point of
view of explosion limits; however, special precautions should be
taken when reactions of CO and O2 mixture under high-pressure
are performed. Proper safety precautions against explosion haz-
ards such as performing the experiments in well ventilated and
isolated cubicle having equipment control operable from out side
were employed during this work.) The reaction was carried out for
2 h time duration and at the end of which, the contents were cooled
2
.2. Synthesis of Pd(OAc) (Bipy)
2
A solution of palladium acetate (100 mg, 0.4455 mmol) was pre-
ꢀ
pared in 10 ml chloroform, and a solution of 2,2 -bipyridine (84 mg,
0.5378 mmol; 1.2 equivalent of Pd) in 5 ml chloroform was added
to it dropwise, the complex started precipitating out gradually, and
the solution was further stirred at room temperature for 1 h. The
solvent was removed by filtration, and the yellow solid obtained
was dried using vacuum at room temperature. A yellow crystalline
to room temperature. Gas phase was analyzed for CO, O and CO2
2
using ORSAT apparatus before venting off in a hood under contin-
uous vacuum, and then the autoclave was opened. The solid urea
(DPU) formed in the reaction was filtered off, washed with cold
petroleum ether, dried and weighed. The two phases in the filtrate
were separated using a separating funnel. The organic phase was
then analyzed by gas chromatography for estimating the unreacted
aniline. The organic phase was concentrated using rotary evapora-
tor at 323 K to remove the solvent. The solid and the concentrate
from organic layer of the filtrate were then dissolved in weighed
quantity of dimethyl formamide, and the solution was analyzed
by HPLC to determine the amount of substituted urea formed in
the reaction. Crystals of urea recovered at the end of the reac-
solid of Pd(OAc) (Bipy) which was obtained was characterized by
2
elemental analysis.
2.3. Synthesis of water soluble palladium complexes
Water-soluble palladium complexes of sulfonated N-containing
ligands were prepared by stirring the ligand and palladium precur-
sor overnight at room temperature. In typical experiment water
soluble ligand BipyDS (192.6 mg, 0.5345 mmol; 1.2 equivalent of
Pd) was dissolved in 5 ml degassed distilled water in a round-
bottomed flask with the help of a magnetic needle. The solution
appeared faint pink in color. To this solution 100 mg (0.4454 mmol)
of palladium acetate was added under stirring. The solution was
kept under vigorous stirring at room temperature for 12 h. At the
end of 12 h, a clear, homogeneous, brownish solution was obtained.
This solution was diluted to 10 ml using distilled water. For the
synthesis of water-soluble complexes of rhodium and ruthenium
similar procedure shown above was followed.
1
13
tion were analyzed by IR, H and C NMR and elemental analysis
(see supplementary materials for details). For recycle experiments,
water soluble catalyst was recycled simply by recycling the aque-
ous phase containing catalyst. Fresh lot of 0.68 mmol of NaI was
also added during each recycle to compensate for NaI lost in organic
phase.
3. Results and discussion
Few preliminary reactions were carried out to establish catalytic
2
.4. Oxidative carbonylation of aniline, isolation and
−
1
activity (based on turnover frequency, TOF (h )) of conventional
Pd/C–NaI catalyst system for oxidative carbonylation of aniline in
dimethyl formamide as the solvent [1]. As expected, excellent ani-
characterization of products
The reactions were carried out in a 600 ml Parr Hastelloy-C-
76 autoclave; provided with gas inlet, gas outlet, a safety rupture
−
1
line conversion (∼99%) and yield (∼97%) of DPU with TOF of 37 h
2
were observed according to stoichiometry shown in Scheme 1.
disc (gold faced, 140 bar), pressure, digital temperature, control

2
2
M.R. Didgikar et al. / Journal of Molecular Catalysis A: Chemical 334 (2011) 20–28
NaSO3
using Bipy-DS ligand. No palladium precipitation was observed
under these conditions (entry 2), and the product N,N -diphenyl
NaSO3
NaSO3
ꢀ
urea was found to be precipitated in the reaction medium, and
could be easily separated by filtration. The aqueous layer consist-
ing of the water-soluble palladium complex could be conveniently
separated by phase separation.
N
N
N
Bipy-DS
IQS-Na
3.3. Development of biphasic system
NaSO3
NaSO3
NaSO3
NaSO3
For the development of a biphasic system, the activity of homo-
geneous catalyst Pd(OAc) (Bipy) was considered as a benchmark,
2
against which the present biphasic system was compared. The
activity of Pd(OAc) (Bipy) catalyst in toluene (see Table 3, entry 1,
2
−
1
TOF 210 h ) was significantly higher comparable to Pd–NaI–DMF
N
N
−
1
system (TOF 37 h ) [1]. This was unlike 5% Pd/C–NaI–toluene
system, wherein even poorer conversions to urea were realized
N
N
−
1
BathP-DS
BathC-DS
in toluene as a solvent (TOF 7 h ) [1]. Pd(OAc) (Bipy)–toluene
2
system under 5:1, toluene:water biphasic condition gave tarry
material with lowering the selectivity of DPU (entry 2) and with
Scheme 2. Water-soluble sulfonated ligands screened for biphasic oxidative car-
bonylation of aniline.
−
1
water as solvent only ∼65% of DPU selectivity with TOF of 86 h
was observed (entry 3). Several water-soluble Pd complexes were
evaluated under biphasic conditions, the palladium complex pre-
pared from water-soluble sulfonated phosphine ligand e.g. TPPTS
gave very poor activity for urea formation under biphasic condi-
3
.1. Catalyst screening
The water-soluble palladium, rhodium and ruthenium salts
−
1
were compared for their catalytic activity for oxidative car-
bonylation of aniline under standard aqueous-biphasic reaction
conditions and in the presence of excess water-soluble BipyDS lig-
and. Under the experimental conditions, the corresponding metal
complexes with BipyDS ligand are presumably formed which are
believed to be the active catalysts. The results obtained are shown
in Table 1. The water-soluble palladium complex formed from pal-
ladium acetate and BipyDS ligand was found to be the most active
with TOF of 221 h and the yield of DPU 88.6% (see entry 1). There-
fore, it was decided to screen palladium complexes synthesized
from palladium acetate and other water-soluble ligands for oxida-
tive carbonylation of aniline as the substrate.
tions due to metal precipitation (TOF 56 h , entry 8). While, it
was also observed that in water medium the water-soluble com-
plex Pd(OAc)(BipyDS) was less efficient compared to its organic
analogue, i.e., Pd(OAc) (Bipy) (see Table 3, entries 1 and 6). The
2
lower activity of Pd(OAc) (Bipy) catalyst in water (entry 3) medium
2
could be due to poor solubility of CO and O2 in the aqueous cat-
alytic phase. The activity of water-soluble catalyst Pd(OAc)(BipyDS)
could be improved by performing the reaction in a biphasic
toluene–water phase (see entries 4 and 5). Under optimized bipha-
sic conditions (toluene:water; 5:1), the activity of water soluble
−1
Pd(OAc) (BipyDS) complex catalyst was comparable to that of
2
Pd(OAc) (Bipy) in the homogeneous phase with toluene as a sol-
2
vent (entries 1 and 5) but significantly higher than the literature
−
1
3.2. Screening of water soluble N-containing ligands
bench mark with supported Pd catalysts (TOF 37 h ). Thus, it has
been observed that for the water-soluble catalysts, conversion and
the selectivity depend on the ratio of the aqueous and organic
phase hold-ups (entries 4, 5 and 6). It can be seen that most of
the reaction takes place at the L–L interface as increase in organic
phase increase the conversion of aniline (entries 6, 5 and 4). This
result is justified by the fact that the reaction rates depend largely
on the mass transfer of the substrate from organic phase into the
aqueous phase, which in turn is based on the L–L interface value
of organic phase and water, as well as concentration gradient of
aniline in two phases. It may be noted that, in water alone as a
solvent catalyst system shows considerable activity (entry 6), indi-
cating that second liquid phase (omega phase) e.g. aniline and
quaternary ammonium salt as a combined organic phase might
contribute to observed activity (this aspect is further discussed in
Section 3.12 under mechanism). An experiment was carried out
in the absence of iodide promoter, which resulted in no aniline
conversion; confirming to the literature reports that oxidative car-
bonylation of amine essentially requires an iodide promoter [2,4]
(entry 7).
It is known that water soluble sulfonated phosphine ligands
such as TPPTS are prone to oxidation, but their N-analogues are
resistant to oxidation in the presence of oxygen or air [14]. There-
fore, N-containing heterocyclic ligands along with TPPTS were
chosen for the screening purpose in the present study. Commer-
cially available as well as laboratory synthesized ligands from
published literature were used for preparing water-soluble palla-
dium catalysts (from Pd(OAc) precursor). These were evaluated for
2
oxidative carbonylation of aniline, using a toluene–water biphasic
medium and sodium iodide as a halide promoter. The N-containing
ligands screened were: isoquinoline sulfonic acid sodium salt
ꢀ
(
IQS-Na) as a monodentate ligand, disodium 2,2 -bipyridine disul-
fonate (Bipy-DS), bathophenanthroline disulfonic acid disodium
salt (BathP-DS) and bathocuproine disulfonic acid disodium salt
BathC-DS) as bidentate ligands (see Scheme 2). The effect of lig-
(
ands on conversion of aniline was studied under standard reaction
conditions and results are presented in Table 2. Catalysts prepared
from bidentate N-containing ligands were more active (TOF in the
−1
range of 513–543 h , see Table 2, entries 2–4 respectively) com-
pared to those with monodentate N-containing ligand (entry 1). The
selectivity towards DPU however remained high in all the cases.
For bidentate ligands, the aniline conversion levels achieved were
more or less the same and no significant effect of increasing steric
hindrance of bidentate N-containing ligand was observed on the
reaction (see entries 2–4); this is contrast with the observations
made by Ishii et al. in the oxidative carbonylation of phenol [15].
Further optimization of biphasic system was therefore carried out
3.4. Solvent screening
Several organic solvents were screened for the aqueous-
biphasic reactions using the optimized solvent composition
(organic to aqueous volume ratio of 5). The performance of
organic–aqueous biphasic systems usually depends on solvent
properties such as (a) polarity of the organic solvent, (b) solubility
of the liquid phase substrate (aniline in this case) in organic phase,

M.R. Didgikar et al. / Journal of Molecular Catalysis A: Chemical 334 (2011) 20–28
23
Table 1
Results on catalytic performance of different precursors with water soluble Bipy-DS ligand.
a
Catalyst precursorb
Aniline conversion (%)
DPU (%)
−1
TOF (h )
Sr. No.
Selectivity
Yield
1
2
3
Pd(OAc)2
97.2
86.5
77.1
95.6
92.8
90.5
88.6
80.2
69.8
221
190
166
RhCl3·xH2O
RuCl3·xH2O
a
Aniline 74 mmol, catalyst precursor 0.078 mmol, Bipy-DS 0.156 mmol, water 10 ml, toluene 50 ml, NaI 0.90 mmol, T 423 K, pressure 68.9 bar, CO:O2 13:1, agitation 16.66 Hz,
time 2 h.
b
The precursors and the ligand were stirred at room temperature in deionized water overnight to make the water-soluble complex, which was charged in to the reaction.
Table 2
Results on effect of water-soluble (WS) ligands on catalytic performance using Pd(OAc)2 precursor (time 0.5 h).
TOF (h 1
−
)
Sr. No.
WS-N-ligand (L/Pd ratio)
Aniline conversion (%)
DPU selectivity (%)
1
2
3
4
IQS-Na (2:4)
32.7
58.7
59.1
55.9
96.5
97.5
94.8
96.7
300
543
531
513
Bipy-DS (1:2)
BathP-DS (1:2)
BathC-DS (1:2)
Table 3
Screening of water-soluble Pd complexes (Pd; 0.078 mmol, time 2.0 h).
Aniline conversiona (%)
DPU selectivityb (%)
TOF (h
−1
)
Sr. No.
Catalyst
Solvent composition (ratio, v/v)
1
2
3
4
5
6
Pd(OAc)2(Bipy)
Pd(OAc)2(Bipy)
Pd(OAc)2(Bipy)
Pd(OAc)2(BipyDS)
Pd(OAc)2(BipyDS)
Pd(OAc)2(BipyDS)
Pd(OAc)2(BipyDS)
Pd(TPPTS)2
Toluene
Toluene + water (5:1)
Water
Toluene + water (1:1)
Toluene + water (5:1)
Water
Toluene + water (5:1)
Toluene + water (5:1)
97.1
78.3
56.3
81.7
94.5
61.3
0
91.2
77.1
64.5
89.7
95.9
81.4
0
210
143
86
174
215
119
0
c
7
d
8
30.0
78.1
56
a
Determined by GC versus standards.
Determined by HPLC versus standards.
Reaction in absence of NaI.
b
c
d
Pd precipitation was observed.
Table 5
Screening of iodide promoters.
(
c) solubility of gases as reactants in the organic phase, (d) miscibil-
ity or solubility of the organic solvent in water, and (e) solubility of
products in the organic and aqueous phases. The results along with
physical properties of solvents are presented in Table 4. The solu-
bility and diffusivity of reactants depend on solvent properties such
as polarity, dielectric constant and viscosity, which ultimately con-
trol the reaction rates. It was observed that the conversion of aniline
was excellent (>85%) in all the cases except p-xylene, wherein lower
Iodide promoter
Aniline conversion (%)
DPU selectivity (%)
TOF (h 1
−
)
EtI
I2
HI
61.3
68.2
83.1
89.9
95.1
96.2
95.8
96.8
93.6
92.6
95.6
91.8
96.4
95.3
94.1
93.6
136
150
188
196
217
217
214
215
KI
NaI
TBAI
CsI
LiI
−1
TOF value (168 h ) was observed.
TBAI = tetrabutyl ammonium iodide.
3.5. Screening of iodide promoters
Several iodide-containing compounds were also screened for
−
2
their activity as promoters under biphasic conditions. The results
are presented in Table 5. From the iodide-screening results, it was
quite evident that iodide promoters having good solubility (>10%
w/v solubility) in water showed excellent activity (HI, KI, NaI, TBAI,
solubility in water (solubility of iodine in water is only 3.3 × 10
g
◦
per 100 g water at 25 C) [16] results in a lower yield of DPU. Accord-
ingly, ethyl iodide is sparingly soluble in water, and the conversion
of aniline was much lower, although the selectivity to DPU was
not affected in all cases (>90%). It was observed that all the alkali
metal iodides as well as tetraethylammonium iodide were excel-
−1
CsI, LiI, TOF ∼ 200 h ). In case of iodine, for example, the lack of
required concentration of iodide in aqueous phase due to sparing
Table 4
Effect of solvents on diphenyl urea yield.
Wma (%)
Conversion (%)
DPU selectivity (%)
−1
TOF (h )
Solvent
Cyclohexane
Toluene
Diethylether
Benzene
Chlorobenzene
Heptane
p-Xylene
0.01
0.05
6.89
0.18
N.A.
0.0003
0.018
89.5
96.8
88.4
87.0
93.4
93.0
73.0
96.0
98.1
97.2
96.3
96.0
93.1
96.8
204
225
204
199
213
205
168
a
Water miscibility (w/w).

2
4
M.R. Didgikar et al. / Journal of Molecular Catalysis A: Chemical 334 (2011) 20–28
7
6
5
4
3
2
1
0
0
0
0
0
0
0
900
Table 7
Aniline conversion
DPU Yield
PdTOF
Effect of aniline amount on catalyst activity and selectivity (time 0.5 h).
TOF (h 1
−
)
Aniline (mmol)
Aniline conversion (%)
DPU selectivity (%)
60
74
91
43.1
52.7
61.0
66.6
94.9
93.6
99.6
97.9
388
468
577
619
6
3
0
00
00
100
Table 8
Effect of CO partial pressure on activity of Pd(OAc)2BipyDS catalyst (time 0.5 h).
TOF (h 1
−
)
CO pressure (bar)
Aniline conversion (%)
DPU selectivity (%)
3
4
5
7
6
5
24.6
36.9
45.2
53.6
65.1
96.5
96.4
97.5
95.7
97.4
225
337
417
487
601
64
72
0
of aniline and TOF increased with approximately first order depen-
dence on aniline amount. The results obtained are shown in Table 7.
In order to investigate the effect of partial pressure of carbon
monoxide (PCO), partial pressure of oxygen was kept constant at
0
5
10
15
20
25
NaIaq/Pd
Fig. 1. Effect of iodide loading (reaction time 0.33 h).
4
.83 bar and PCO was varied in the range of 37–72 bar. The purpose
of study was to check for the oxygen starvation effect, i.e., increase
in the demand for oxygen with increase in PCO. Under such condi-
tions reaction rates are expected to be influenced by mass-transfer,
resulting in the lowering of TOF. Table 8 shows the effect of PCO
on conversion of aniline and on selectivity of DPU. The effect was
found to be linear, and the TOF values increased with increase in
PCO indicating that the system is free of oxygen starvation.
The effect of partial pressure of oxygen was studied in the PO2
range of 4.14–6.89 bar, while keeping the CO partial pressure at
64.12 bar (see Table 9). The partial pressure variation showed a
consistent rise in the conversion of aniline as well as TOF values
as the PO2 was increased; the PO2 effect on TOF was much pro-
nounced compared to PCO implying that the reaction is sensitive
to PO2 . This result confirmed that the reaction is in kinetically con-
trolled regime. It may be noted here that the side reaction of CO
oxidation to CO2 is negligible (discussed later) and this does not
seem to result in starvation of reactants during CO and O2 partial
pressure variation.
lent promoters for the reaction. These observations indicate that
the catalytic reaction takes place at the interface phase, and not
in the bulk liquid phase. NaI was used as the iodide promoter for
further studies as it was convenient and cost effective.
3.6. Effect of sodium iodide promoter
Effect of NaI/Pd ratio was investigated and the results are shown
in Fig. 1. The optimum NaI/Pd ratio for which the highest conver-
sion of aniline and TOF were obtained, was found to be ∼5 which
is higher than that reported earlier by Gupte and Chaudhari [1].
The higher value of NaI/Pd ratio observed in the present study may
be attributed to the partitioning of iodide in organic and aqueous
phases, which decreases the effective concentration of iodide in the
reaction phase and hence ultimately needs higher amount of NaI
for optimum activity of the catalyst.
3.7. Effect of reaction parameters
3.8. Catalyst recycle study
Effects of temperature, aniline concentration and partial pres-
sures of carbon monoxide and oxygen were investigated under
biphasic conditions. Reactions were carried out using aniline as a
The recovery and reusability of the catalyst are obviously the
major advantages of a two-phase catalytic reaction, even as the cat-
alyst used can be tailor-made by manipulating the ligands. In the
present study also, the reusability of the water-soluble palladium
complex was critically checked. Table 10 shows the performance
substrate and Pd(OAc) (BipyDS)–NaI as the catalyst system with
2
typically 30 min of reaction time. The effect of temperature on
oxidative carbonylation of aniline was studied in the range of
3
73–443 K (see Table 6). The results show that the catalyst system
of Pd(OAc) (BipyDS) for oxidative carbonylation of aniline up to
2
is robust in the temperature range investigated. Catalyst activity
was found to increase with increase in temperature. DPU selectivity
was poor at lower temperature (373 K) due to possibility of aniline
5 recycles. In each recycle, the aqueous phase containing catalyst
was separated and used as such for the next reaction. The loss in the
activity of the catalyst was negligible (see Table 10, TOF values in
−
1
remaining trapped as quaternary aniline salt (e.g. [RNH ]I) [9] but
the range 210–191 h ). The aqueous solution of Pd(OAc) (BipyDS)
3
2
above 393 K DPU selectivity remains high at ∼98%. As expected the
TOF was also found to increase with increase in temperature.
Effect of amount of aniline on activity of catalyst was investi-
gated in the range of 60–100 mmol. It was found that the conversion
was found to be stable for several days from the observation that no
discoloration or palladium precipitation was observed during this
period. The organic phase recovered after the reaction was analyzed
by AAS for the palladium content, and was found to contain <1 ppm
Table 6
Table 9
Temperature effect (time 0.5 h).
Effect of oxygen partial pressure (time 0.5 h).
TOF (h−1)
O2 pressure (bar)
Aniline conversion (%)
DPU selectivity (%)
−1
TOF (h )
Temperature (K)
Aniline conversion (%)
DPU selectivity (%)
3
3
4
4
73
93
23
43
6.6
24.7
49.2
61.9
88.2
97.0
97.2
96.8
56
228
454
568
4.14
4.83
5.52
6.21
45.6
54.8
59.6
63.3
96.5
417
501
551
575
96.36
97.48
95.68

M.R. Didgikar et al. / Journal of Molecular Catalysis A: Chemical 334 (2011) 20–28
25
Table 10
Recycle of water soluble catalyst.
presence of diphenyl urea [7] resulting in increase in CO2 forma-
tion with increase in reaction times, and such behavior was also
observed in the present study. The CO2 selectivity based on total
CO consumed in the reaction was calculated, and it was found to
be 6.9% (accounting dissolved CO2 present in liquid phase).
Recycle number
Aniline conversion (%)
DPU selectivity (%)
TOF (h−1)
0
1
2
3
4
5
97.1
93.7
92.8
91.5
90.5
87.3
91.2
92.8
90.5
89.7
90.3
92.2
210
206
199
195
194
191
3.10. Screening of amines
The applicability of the biphasic catalytic system was investi-
gated for a range of amines, under standard reaction conditions (for
details Table 11). Excellent activity and selectivity was observed
when aniline, p-toluidine, p-anisidine, ˇ-napthylamine and o-
phenylene diamine (OPDA) were used as substrates (see Table 11,
entries 1a, 2b, 3c, 5e, and 6f respectively). The aromatic diamine,
OPDA was found to be an excellent substrate for the present study
palladium. Since the water-soluble complex was completely recov-
ered after the reaction, it was expected that the sodium iodide
promoter could be recovered as well at the end of the reaction.
Therefore, a comparative account of the concentration of iodide in
the initially charged as well as recovered aqueous phase was car-
ried out using potentiometric estimation. It was observed, however,
that the iodide was lost up to ∼75% of its original loading from the
aqueous phase at the end of second recycle. The loss was attributed
to the iodine dissolved into the organic phase. The recycle reactions
also indicated decline in aniline conversions when no fresh iodide
was replenished into recycle experiments (data not shown).
(see entry 6f). Primary aliphatic amines, such as benzylamine and
butylamine, also gave excellent results (entries 7g, and 8h, TOF
−
1
1
92 and 218 h respectively). Moderate activity of catalyst was
observed when chloroaniline was used as a substrate (entry 4d,
−
1
4
8% yield of urea, TOF 114 h ). It was observed that in both the
reactions, the aqueous layer containing the water-soluble catalyst
retained a clear yellow color after the reaction. Also, no palladium
precipitation was observed in these reactions. Primary diamines
were also screened, to check for the formation of cyclic substi-
tuted ureas. Ethylene diamine and piperidine did not show any
conversion under the similar experimental conditions (entries 9i
and 12m). In order to understand whether the reason for the fail-
ure of these amines to react was a limitation of the biphasic catalyst
system or the inherent low reactivity of the aliphatic diamine, a
similar experiment was carried out using the conventional 5% Pd/C,
NaI catalyst system, with toluene as the solvent and in absence
of water in the reaction medium. In this case no gas absorption
or conversion of amine was observed in the experiment, indicat-
ing that ethylene diamine, under the present set of experimental
conditions, is not reactive for oxidative carbonylation (entries 9i
and 10j). One of the possibilities is that, the bidentate ethylene-
diamine can strongly bind to electrophilic Pd(II) complex catalyst
precursor forming a chelate thereby deactivating the catalyst. For
dibenzylamine as the substrates, no reaction occurred under bipha-
sic conditions (see entry 11k). From the amine screening studies,
it was evident that the oxidative carbonylation of primary amines
3.9. CO oxidation
Carbon monoxide oxidation to carbon dioxide was found to take
place under the conditions of our studies. To assess the forma-
tion of CO2 side reaction, reaction was performed under standard
conditions. A typical concentration–time profile under standard
conditions for the aqueous-biphasic oxidative carbonylation of ani-
line for selective synthesis of 1,3-diphenylurea is shown in Fig. 2.
The consumption of aniline throughout the reaction profile was
found to be commensurate with the formation of the disubstituted
urea, and no aniline derived by-products like oxamides or quina-
zolines were observed. However, the gas phase shows formation of
carbon dioxide, which is well known for Pd metal catalyzed reac-
tions involving CO, O and water [15]. Separate experiments were
2
carried out for a fixed reaction time to quantify the CO formation in
2
a biphasic system. Gas samples from the autoclave were analyzed
at the end of each experiment. The formation of CO2 by oxidation
of CO takes place due to the nucleophilic attack of water molecule
on the CO molecule coordinated to the transition metal centre [17].
It has also been reported that the formation of CO2 increases in
(both aliphatic as well as aromatic) results in satisfactory yields
of corresponding urea in biphasic medium using a water-soluble
catalyst, although secondary amines do not react under these con-
ditions.
0
0
0
0
.08
.06
.04
.02
Aniline
DPU
3.11. XPS analysis of fresh and spent catalyst
The X-ray photoelectron spectroscopy (XPS) was used for the
analysis of fresh [Pd(OAc) (BipyDS)] and used catalyst. The X-
2
ray actually removes the core orbital electrons of the elements
at specific binding energies, which are fingerprints for particular
oxidation states of the element. The XPS data also delivers some
information about the immediate local electronic environment of
the selected element incorporating the perturbation of the electron
density due to the neighbouring functional groups and/or ligands.
For obtaining XPS analysis of spent catalyst, aqueous phase con-
taining water-soluble catalyst was recovered at the end of the 5th
recycle experiment and was concentrated to dryness under vacuum
on a rota-evaporator to obtain catalyst powder having amorphous
nature. The freshly prepared Pd(OAc) (BipyDS) as well as the recov-
2
0
ered catalyst were characterized by XPS for sulfur, nitrogen, carbon
and palladium atoms for their respective binding energies (B.E.)
and oxidation states. The B.E. values of the elements present in
0
20
40
60
80
100
120
Time, min
Pd(OAc) (BipyDS) (see Table 12 and B.E. values therein [18]; Fig. 3)
2
Fig. 2. Concentration–time profile in biphasic conditions.
catalyst compared well with literature values obtained from simi-

2
6
M.R. Didgikar et al. / Journal of Molecular Catalysis A: Chemical 334 (2011) 20–28
Table 11
Screening of amines.
a
−1
−1
␯CO (cm )
Sr. No.
Amine (1)
Urea (2)
Amine conversionb (%)
94.5
Urea yieldc (%)
87.8
TOF (h
)
1
2
3
4
a
b
c
208
1649
96.8
98.3
54.1
94.4
79.6
48.0
224
189
114
1640
1634
1633
d
5
6
e
f
95.6
96.6
81.8
84.4
194
200
1632
1742
7
8
9
g
h
i
96.8
97.4
N.R.
81.1
92.1
–
192
218
–
1627
1623
–
1
0jd
N.R.
–
–
–
1
1
1k
N.R.
N.R.
–
–
–
–
–
–
2m
a
b
c
Amine 74 mmol, Pd(OAc)2(BipyDS) 0.078 mmol, water 10 ml, toluene 50 ml, NaI 0.90 mmol, T 423 K, pressure 68.9 bar, CO:O2 13:1, agitation 16.66 Hz, time 2 h.
Determined by GC versus standards.
Isolated yields, products characterized by IR, NMR and elemental analysis.
Catalyst 5% Pd/C, solvent toluene; N.R.: no reaction.
d
lar Pd compounds [19,20]. The data for each element was corrected
with respect to signal of the adventitious carbon at 285 eV. The
binding energies of the other elements like N 1s, O 1s and S 2p also
matched very well with the electronic configuration of the water-
soluble complex. For example, O 1s state occurs at 533.1 eV, which
is similar to the O in an anion, e.g. carboxylate, or sulfonate; bind-
ing energy value for N 1s is 400.5 eV, which is slightly higher than
the binding energy for N in pyridine (∼398.7 eV) due to the chelat-
ing through the N atom in the complex and consequent depletion
of electron density. The binding energy value of sulfur 2p, which
was found to be 169.3 eV was also matched closely with the S 2p
value in sulfate group like in Na SO , which is 169.6 eV. All these
2 4
binding energy values indirectly support the molecular composi-
tion of the water-soluble Pd(OAc) (BipyDS) complex. A comparison
2
of the XPS pattern of the recovered and fresh complex also confirms
that the molecular nature of the complex and more particularly the
oxidation state of palladium remains unchanged after the reaction.
−2
For pure Pd(OAc) the 3d state occurs at around 338.5 eV, while
2
5/2
the palladium is in the 2+ oxidation state. The soft and resonat-
ing character of the OAc anion imparts lesser electron density to
the Pd centre and hence Pd binding energy is slightly higher for
Pd(OAc) . While in the case of other complexes of palladium coor-
2
Table 12
XPS data for Pd(OAc)2(BipyDS) (values in eV).
B.E.
Elements
C
N 1s
O 1s
S 2p3/2
Na 1s
Pd 3d5/2
Uncorrected
Corrected
288.9
285.0
285.0
404.4
400.5
400.0
537.0
533.1
532.1
173.2
169.3
169.6
1075.8
1071.9
1071.5
342.3
338.4
338.6
Literaturea
a
B.E. values for comparison for elements are based on literature values [18]. However there is no XPS analysis report for Pd(OAc)2(BipyDS) or analogous molecules for
comparison.

M.R. Didgikar et al. / Journal of Molecular Catalysis A: Chemical 334 (2011) 20–28
27
NH2
NH2
Organic phase
L-L interphase
NCO
HI
Step V1
N
N
I
NH3
Pd
+ 2NaOAc
Step II
I
I
[II]
GAS
N
Step V
NHPh
DPU
solid
Pd
Step I
L (org.)
N
I
NCO
Step IV N
[III]
N
N
L-L (interphase)
L (aq.)
S (urea)
CO
OAc
Step III
Pd
+ 2NaI
HI
OAc
[
O ]
2
Step VII
CONHPh
I
[I]
-
HI
Pd
H O
2
I2
N
COL
O2L
CO
2L
Step VIII
COg
N
carbamoyl species
IV]
O
Pd
2g
[
N
CO2g
[V]
L-L interphase
SO Na
3
Aqueous phase
N
OAc
OAc
N
N
OAc
Pd
Pd
N
OAc
SO3Na
Scheme 3. Proposed mechanism of urea synthesis in a biphasic system.
ꢀ
dinated by 2,2 bipyridine like in PdCl (Bipy), 3d
state occurs at
5/2
3.12. Plausible reaction mechanism
2
lower values e.g. 338.0 eV. The 3d5/2 state in the Pd(OAc) (BipyDS)
2
occurs at 338.4 eV, indicating that the OAc group in the complex is
intact, while the sulfonated bipyridine is coordinated to palladium.
A characteristic band gap of 5.1 eV was also observed between the
The chemistry and catalysis of oxidative carbonylation of amine
to urea and carbamate has been recently reviewed by Gabriele et al.
[2], McElwee-White and co-workers [21] and by Ragaini [22]. It
is generally believed that, Pd catalyzed oxidative carbonylation of
amine proceed via reduction of Pd(II) catalyst precursor in presence
of CO by amine which in turned is oxidized. The reduced catalytic
species is then re-oxidized in presence of oxidizing agents such as
halide containing promoters and molecular oxygen regenerating
the starting catalytic species [2,4,23,24]. When molecular oxygen
is employed for abstraction of hydrogen of amine, water is gen-
erated [2,4,23–25]. The generated water competes with substrate
amine giving rise to a side reaction of carbon monoxide oxidation
to carbon dioxide which decreases the efficiency of the catalyst.
Therefore, designing a catalytic system involving water as a sol-
vent and at the same time minimizing CO oxidation is challenging.
In order to minimize the CO oxidation, amine should be preferably
activated rather than water by the catalyst. It is also known that in
oxidation of CO to CO2, presence of amine plays an important role,
and in absence of amine it has been reported that CO2 formation
is particularly low [7]. The water soluble palladium catalyst pre-
sumably is more efficient towards amine carbonylation, compared
to CO oxidation to carbon dioxide, particularly when sparingly
water soluble aromatic amines are used. Also, small amount of qua-
ternary amine (viz. monophenyl ammonium iodide in this case)
formed during catalysis (see Scheme 3) is readily soluble in aqueous
medium and it seems likely that this amine salt is more susceptible
to be attacked by water-soluble Pd catalyst. Further under our reac-
tion conditions of biphasic catalysis, we have not noticed any metal
precipitation indicating that water soluble Bipy-DS ligand stabi-
lize the Pd complex and Pd precipitation is prevented. Also, XPS
analysis of spent catalyst does not indicate the presence of metal-
3
d5/2 and 3d3/2 signals of palladium for the palladium complex.
Also, since no peak is observed at B.E. value of 335.7 eV, which is
the characteristic B.E. value for metallic Pd 3d5/2 level, confirming
that palladium is not present in zero oxidation state in both fresh
and recovered catalyst [20].
2
2
2
2
1
1
1
1
1
3000
2000
1000
0000
9000
8000
7000
6000
5000
3
d5/2
A
3
d3/2
B
325
330
335
340
345
350
355
Binding energy, eV
Fig. 3. XPS spectra of (A) fresh Pd(OAc)2BipyDS and (B) Pd(OAc)2BipyDS recovered.

2
8
M.R. Didgikar et al. / Journal of Molecular Catalysis A: Chemical 334 (2011) 20–28
lic Pd, therefore in the proposed mechanism formation of metallic
Pd is not shown. The mechanism speculated in Scheme 3 is based
on the formation carbamoyl species (species IV) as a key inter-
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Acknowledgement
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MRD would like to acknowledge the CSIR, India for providing
the research fellowship.
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Appendix A. Supplementary data
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Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.molcata.2010.10.018.