Highly efficient ionic liquid-mediated palladium complex catalyst system for the oxidative carbonylation of amines

Article abstract of DOI:10.1016/S0021-9517(03)00198-2

A highly efficient catalyst system, palladium-ionic liquid, was developed for the carbonylation of amines to give carbamate or urea. The desired products could be precipitated by adding water into the resulting mixture and the catalyst system could be reused with slight loss of catalytic activity. BMImBF₄ ionic liquid + Pd(phen)Cl₂ was an effective catalytic system for the oxidative carbonylation of aniline to give corresponding carbamate and for the cyclohexyl amine to give urea. The diversity of the ionic liquids could form an optimal homogeneous catalyst system with a specific organometallic toward specific reactions. There existed strong interactions between the ionic liquid and the Pd complex, which might enhance and stabilize the Pd complex catalyst.

Full text of DOI:10.1016/S0021-9517(03)00198-2

Journal of Catalysis 219 (2003) 372–375
www.elsevier.com/locate/jcat
Highly efficient ionic liquid-mediated palladium complex catalyst system
for the oxidative carbonylation of amines
Feng Shi, Jiajian Peng, and Youquan Deng ∗
Center for Green Chemistry and Catalysis, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou, 730000, China
Received 13 February 2003; revised 3 April 2003; accepted 22 April 2003
Abstract
A highly efficient catalyst system, palladium complex–ionic liquid, was developed for the carbonylation of amines to give carbamate or
urea. The desired products could be precipitated by adding water into the resulting mixture and the catalyst system could be reused with
slight loss of catalytic activity.
2003 Elsevier Inc. All rights reserved.
1. Introduction
Recently, room temperature ionic liquids, a new kind of
reaction media with very low vapor pressure and high dis-
solvability toward many organic and inorganic substances,
have been attracting growing interests, and many catalytic
reactions proceeded in ionic liquids as reaction media were
reported with excellent performance [9–16]. It can be as-
sumed that the diversity of ionic liquids may form an optimal
homogeneous catalyst system with a specific organometallic
complex toward a specific reaction.
In this work, the ionic liquid-mediated palladium (1,10-
phenanthroline)2+ complex catalyst system for the oxidative
carbonylation of amines to give corresponding carbamates
or ureas was investigated without using any additional or-
ganic solvents. The aim is to establish an effective catalytic
system for the carbonylation of amines through the selective
dissolvability and special mediation of ionic liquid.
Isocyanates, e.g., toluene diisocyanate and 4,4ꢀ-diphenyl-
methyldiisocyanate, are currently manufactured by phos-
genation of the corresponding amines with toxic phosgene
in industry, which may cause serious environmental pollu-
tion and equipment corrosion. Therefore, produced either
by oxidative carbonylation of amines or by reductive car-
bonylation of nitro compounds in the presence of an alcohol,
carbamates as intermediates for the synthesis of isocyanates
with nonphosgene routes have been extensively studied for
the past two decades. Pd, Ru, Rh, Au, and other transitional
metal complexes were employed as the catalysts [1–7], in
which complexes of Pd coordinated with N-containing com-
pounds are the most effective catalyst systems [8]. The cor-
responding catalytic turnover frequency (TOF), however, is
still not high enough for industrial application, and large
amounts of volatile organic solvents were inevitably used in
most of these processes. Furthermore, palladium complexes
coordinated with N-containing compounds are usually in-
soluble in most conventional organic solvents, and it is well
known that reaction media play an important role in homo-
geneous catalysis, although, so far, such mediation is still
not very clear. Moreover, organic solvents, for establishing a
suitable homogeneous catalyst system for the carbonylation
of N-containing compounds, are very limited.
2. Experimental
The ionic liquids, BMImBF4 (BMIm = 1-butyl-3-meth-
ylimidazolium), BMImPF6, BMImFeCl4, and BMImCl,
and palladium (1,10-phenanthroline)2+ complexes were re-
spectively synthesized according to previously published
papers [17,18] with slight modifications. Pd(OAc)2 and
Pd(acac)2 were of AR grade. Characterizations of ionic
liquids and palladium complexes were conducted with FT
Raman (Nicolet 910) and FTIR (Bruker H210).
*
Corresponding author.
E-mail address: ydeng@ns.lzb.ac.cn (Y. Deng).
0021-9517/$ – see front matter
2003 Elsevier Inc. All rights reserved.
doi:10.1016/S0021-9517(03)00198-2

F. Shi et al. / Journal of Catalysis 219 (2003) 372–375
373
Fig. 1. Step I: Carbonylation of aniline with CO, O , and MeOH in the presence of Pd(phen)Cl /(BMImBF ) and a liquid mixture containing the desired
2
2
4
product were obtained after reaction. Step II: About 10 ml water was added into the resulting liquid mixture, the resulting product would precipitate. Step III:
Desired solid product and corresponding yield could be recovered and obtained after filtration and dryness. Step IV: Ionic liquid containing Pd(phen)Cl could
2
also be recovered and reused after the percolate was distilled to remove water.
All reactions were conducted in a 90-ml autoclave with
a glass tube inside equipped with magnetic stirring. In each
reaction, 1 mg Pd complex catalyst, 1 ml ionic liquid, 1 ml
amines, 10 ml methanol, and 5.0 MPa of mixture gases
(CO purity 99.99% 4.5 MPa and O2 99.99% 0.5 MPa) were
successively introduced without any additional organic sol-
vent. The reaction proceeded at 175 ◦C for 1 h since low
conversion was obtained at lower temperatures. When the
conversion and selectivity were high enough, the resulting
product “precipitated” when ca. 10 ml water was added into
the resulting liquid mixture after reaction because carbamate
is insoluble in water, while the Pd(phen)Cl2 was still “dis-
solved” into the ionic liquid. Therefore, the desired solid
product and corresponding yield could be recovered and
obtained after filtration and dryness. Ionic liquid contain-
ing Pd(phen)Cl2 could also be recovered and reused after
the percolate was distilled to remove water and residual
methanol. A schematic diagram of such reaction and sep-
aration processes is given in Fig. 1. Qualitative analyses
were conducted with a HP 6890/5793 GC-MS. Quantitative
analysis was conducted over a HP 1790 GC and the conver-
sions and selectivities were determined by external standard,
and were calculated according the chromatographic peak ar-
eas of the resulting products given by the GC chemstation.
Table 1
Solubility of Pd(phen)Cl in various liquid media (25 C)
◦
2
Solvent
Solubility (mg/100 ml)
BMImBF4
110
120
6000
< 2.5
< 2.5
< 2.5
BMImPF
6
BMImCl
H O
2
MeOH
Chlorobenzene
BMImCl, indicating that the Pd(phen)Cl2 solubility in ionic
liquid can be adjusted through the tuning of cation or an-
ion of ionic liquids. It is noteworthy that no Pd(phen)Cl2
precipitate could be observed when ionic liquid containing
Pd(phen)Cl2 with soluble amounts was further mixed with a
large amount of methanol or water, suggesting that some in-
teraction between Pd(phen)Cl2 and ionic liquid was formed,
which modified the Pd(phen)Cl2 solubility in methanol or
water.
With BMIm+ as cation and in the presence of Pd
(phen)Cl2, the effect of anions, e.g., BF4−, PF6−, Cl−, and
FeCl4− on the aniline carbonylation was examined (Table 2,
entries 1–4). Moderate and high conversions with excellent
selectivities were achieved in BMImBF4 and BMImFeCl4.
The corresponding TOF reached to ca. 4540 mol mol−1 h−1
,
which is much higher than that reported in previous litera-
3. Results and discussion
−
tures [1]. BF4 was slightly better than FeCl4−, although
FeCl3 was one of the most effective cocatalysts in the car-
bonylation reactions of N-containing compounds. Poor cat-
alytic performances were observed for PF6−. This may be
related to the instability of BMImPF6 ionic liquids under
the reaction conditions employed in this work, because re-
markable degradation of this ionic liquid was observed after
reaction. Such degradation, however, was not observed with
BMImBF4. On the other hand, poor catalytic activity was
First, the solubility of Pd(phen)Cl2 in water, methanol,
and a series of ionic liquids was examined at room tem-
perature (Table 1). Pd(phen)Cl2 is completely insoluble
in water or methanol, while ca. 0.11–0.12 g/100 ml of
Pd(phen)Cl2 solubility was observed in BMImBF4 and
BMImPF6, which is high enough to establish a homoge-
neous Pd(phen)Cl2 + ionic liquid catalyst system. About
6 g/100 ml of Pd(phen)Cl2 solubility was achieved in

374
F. Shi et al. / Journal of Catalysis 219 (2003) 372–375
Table 2
Carbonylation of amines catalyzed with a palladium complex in ionic liquids
Entry
Ionic liquid
Conversion (%)
Selectivity (%)
TOF
Desired product
a
1
BMImBF
99
5
98
99
4540
228
4
2
3
4
5
BMImPF
BMImCl
6
58
96
2.1
71.4
99
41
96
99
2680
4420
104
BMImFeCl
–
> 99
99
4
b
6
BMImBF
BMImBF
BMImBF
BMImBF
99
3250
4540
980
4
4
4
4
c
7
98
d
8
> 99
< 1
e
9
3560
a
Isolated yield, 91%.
b
c
d
e
The catalyst system reused for the second time.
0.3 mg of additional Pd(phen)Cl was added.
2
Pd(OAc) was used as catalyst.
2
Cyclohexyl amine was used as substrate and dicyclohexyl urea with 90% selectivity was obtained.
observed if reaction proceeded in the absence of ionic liquid
(entry 5).
most effective Pd(phen)Cl2/BMImBF4 catalyst system, the
oxidative carbonylation of aliphatic amines, i.e., cyclohexyl,
was also tested (entry 9). High conversion of cyclohexyl
amine was achieved, but the selectivity for carbamate was
less than 1% while dicyclohexyl urea was the main product
with > 90% selectivity. At the same time, since the solubil-
ity of dicyclohexyl urea in such reaction medium was very
low, crystallized product could be obtained by simple filtra-
tion. These results showed that BMImBF4 ionic liquid +
Pd(phen)Cl2 is an effective catalytic system not only for the
oxidative carbonylation of aniline to give corresponding car-
bamate but also for the cyclohexyl amine to give urea.
The recycling of the BMImBF4 + Pd(phen)Cl2 catalyst
system was also examined. Since carbamate is insoluble
with water, the resulting product “precipitated” when ca.
10 ml water was added into the resulting liquid mixture after
reaction, while the Pd(phen)Cl2 could be still “dissolved”
into the ionic liquid (Fig. 1). Therefore, the desired solid
product could be recovered after filtration to separate the
liquid mixture containing BMImBF4, Pd(phen)Cl2, residual
methanol, and water, and 91% of yield was achieved after
dryness (entry 1). Ionic liquid containing Pd(phen)Cl2 could
also be recovered and reused after the percolate was dis-
tilled to remove residual methanol and water, and 70% of
conversion was observed (entry 6). However, 98% of con-
version was regained if 0.2–0.3 mg of extra Pd(phen)Cl2 was
added into the recovered BMImBF4 ionic liquid containing
Pd(phen)Cl2 (entry 7), indicating that part of Pd(phen)Cl2
may leach during the separation and recovery.
The experimental results also confirmed that Pd(phen)Cl2
was the most effective catalyst for the reaction since
Pd(OAc)2 or Pd(acac)2 (though not shown here) showed
poor catalytic performance under similar conditions (en-
try 8), indicating that phenanthroline is also indispensable
and it was unlikely that in situ formation of the Pd–carbene
complex was the catalytic species for the carbonylation. The
synergism among palladium, phenanthroline, and ionic liq-
uids resulted in high catalytic activity.
In order to gain insight into the ionic liquid-mediated
Pd(phen)Cl2 catalyst system, characterizations of FT Raman
(Fig. 2) and FTIR (Fig. 3) were conducted. BMImCl was se-
lected because it possessed higher solubility for Pd(phen)Cl2
(ca. wt 6%) but < 0.2 wt% toward other ionic liquids. In
comparison with FT Raman spectra of pure Pd(phen)Cl2 and
BMImCl, strong or new peaks at 704 cm−1 (γC–H of im-
idazolium), 1399 cm−1 (imidazolium ring stretching), and
1588 cm−1 (imidazolium ring stretching) [19–21] appeared,
while strong adsorption between 700 and 900 cm−1 (δC–H of
o-phenanthroline) [22,23] of pure Pd(phen)Cl2 disappeared
when Pd(phen)Cl2 was added into BMImCl. A strong and
new peak was also observed at 1517 cm−1 (imidazolium
ring stretching) [19–21] in FTIR spectra. These changes sug-
gested that there exist strong interactions between imidazole
cations and Pd(phen)2+
.
The main by-product detected was N-phenylamide for
the oxidative carbonylation of aniline. Since no species re-
sulting from the oxidized imidazole was detected with GC-
MS, the imidazolium cations could be stable enough under
the reaction conditions employed in this work. Using the
4. Conclusion
In summary, this is the first demonstration that ionic
liquid-mediated palladium complexes could be a very effec-

F. Shi et al. / Journal of Catalysis 219 (2003) 372–375
375
tive catalyst system for oxidative carbonylation of amines to
give either carbamate or urea. This work also demonstrated
that the diversity of the ionic liquids could form an optimal
homogeneous catalyst system with a specific organometal-
lic toward specific reactions. There exist strong interactions
between the ionic liquid and the palladium complex, which
may not only enhance but also stabilize the palladium com-
plex catalyst. The nonvolatility of ionic liquid, interaction
between the ionic liquid and the palladium complex, and the
insolubility of carbamate or urea in water make such a cata-
lyst system recyclable.
Acknowledgment
This work was financially supported by the Natural Sci-
ence Foundation of China (No. 20225309).
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2