Silica gel confined ionic liquid + metal complexes for oxygen-free carbonylation of amines and nitrobenzene to ureas

Article abstract of DOI:10.1002/adsc.200404242

A new kind of silica gel confined ionic liquid containing a metal complex as heterogenized catalysts was prepared for the carbonylation of amines and nitrobenzene without molecular oxygen to afford the corresponding ureas with greatly enhanced catalytic activity (TOF exceeded 11000 mol·mol ^-1· h^-1), with a much lower amount of ionic liquids being needed, with easy catalyst separation and possible reusability, and avoidance of the using of explosive CO + O₂ gas mixtures. Such an enhancement in catalytic activity may be derived from the effect of a high concentration of ionic liquid containing a metal complex due to the confinement into the nanopores or cavities of the silica gel matrix.

Full text of DOI:10.1002/adsc.200404242

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Silica Gel Confined Ionic LiquidþMetal Complexes for
Oxygen-Free Carbonylation of Amines and Nitrobenzene to
Ureas
Feng Shi, Qinghua Zhang, Yanlong Gu, Youquan Deng*
Centre for Green Chemistry and Catalysis, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences,
Lanzhou, 730000, Peopleꢀs Republic of China
Fax: (þ86)-931-4968116, e-mail: ydeng@ns.lzb.ac.cn
Received: August 1, 2004; Accepted: December 17, 2004
Abstract: A new kind of silica gel confined ionic liq-
uid containing a metal complex as heterogenized cat-
alysts was prepared for the carbonylation of amines
and nitrobenzene without molecular oxygen to af-
ford the corresponding ureas with greatly enhanced
such oxidative carbonylation processes, which may be
dangerous during its application in industry. In the light
of a catalytic reaction process under environmental and
economic impacts, the synthesis of disubstituted ureas
by “oxidative” carbonylation without using dioxygen
has also been paid more and more attention. Although
many catalysts have been developed for this process in
recent years,[11] the catalytic efficiency was still very
low and separation of catalysts from the products has
not yet been solved successfully. Therefore, the develop-
ment of a highly efficient, easily separable catalyst sys-
tem for the syntheses of disubstituted symmetrical or
unsymmetrical ureas, especially those in which aromatic
groups are contained, for an oxidative carbonylation
process without molecular oxygen, was highly desired.
Room temperature ionic liquids or molten salts, espe-
cially those derived from the combination of quaternary
ammonium salts and weakly coordinating anions, have
been demonstrated to be ideal immobilizing agents for
various “classical” transition-metal catalyst precursors
with excellent catalytic performance or easy product
catalytic activity (TOF exceeded 11000 mol·molꢀ1
·
hꢀ1), with a much lower amount of ionic liquids be-
ing needed, with easy catalyst separation and possi-
ble reusability, and avoidance of the using of explo-
sive COþO2 gas mixtures. Such an enhancement in
catalytic activity may be derived from the effect of
a high concentration of ionic liquid containing a met-
al complex due to the confinement into the nano-
pores or cavities of the silica gel matrix.
Keywords: amines; carbonylation; ionic liquids; ni-
trobenzene; phosgene-free
Since the discovery of the carbon monoxide-induced re- separation for many reactions.[12] More recently, it was
duction of nitro groups 30 years ago, there has been a exemplified that the carbonylation of aniline for the syn-
widespread increase of interest in the application of car- thesis of the corresponding carbamates could proceed in
bon monoxide in the carbonylation of nitrogen-contain- ionic liquids with much higher efficiency than in tradi-
ing compounds because the subject is of great signifi- tional organic solvents using selenium or a palladium
cance both in academia and in the chemical industry.[1] complex as catalyst.[13] However, large amounts of ionic
Needless to say, the syntheses of disubstituted ureas liquid would be used for this homogeneous catalytic
with carbon monoxide was one of the most important as- process and the ionic liquids, so far, are still so expensive
pects due to their extensive applications such as agricul- that they cannot be used and consumed as ordinary or-
tural chemicals, resin precursors, dyes, polymers, drug ganic solvents. At the same time, a heterogeneous cata-
candidates including HIV protease inhibitors, FKBP12 lytic process, due to the ease of separation and the pos-
inhibitors, CCK-B receptor antagonists and endothelin sibility to be used in a fixed-bed reactor, would be
antagonists etc.[2] Furthermore, they could be intermedi- more preferred in the chemical industry. Therefore,
ates for the phosgene-free synthesis of isocyanates.[2] In the immobilization of an ionic liquid should be an ideal
previous studies, various of catalysts including Ru,[3] method to minimize the amount of utilized ionic liquid
Rh,[4] Co,[5] Mn,[6] Se,[7] Au,[8] W[9] and most commonly and facilitate product separation. A typical process for
and especially reported recently by Gabriele et al., the support or immobilization of ionic liquid catalysts
Pd,[10] have been examined for the syntheses of ureas was reported recently,[14] in which the ionic liquid frag-
through oxidative carbonylation of amines with good ment such as the dialkylimidazolium cation was cova-
to excellent catalytic performance. However, the explo- lently bonded to the silica surface. Chemical bonding
sive COþO2 gas mixture has been inevitably used for of the dialkylimidazolium cation to a solid surface, how-
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DOI: 10.1002/adsc.200404242
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Feng Shi et al.
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ever, may limit the free degree of dialkylimidazolium (entries 1–4). The corresponding TOF exceeded
cation and even change the physicochemical properties 11000 mol·molꢀ1 ·hꢀ1, which should be unprecedented
of the ionic liquids. Another method for immobilization in comparison with previous homogeneous ionic liquid
of ionic liquids was by dipping the porous support in an carbonylation systems.[13b] It could be conjectured that
ionic liquid containing the metal complex.[15] Due to better conversion would be achieved if, for example,
such a physical deposition, leaching of the ionic liquid the amount ofnitrobenzene added was stoichiometrical-
and catalyst was unavoidable if used under rigorous re- ly in excess of that of aniline. After the reaction, the de-
action conditions.
sired dry and crystallized product mixed with the cata-
In this work, we report a new concept for synthesizing
silica gel-immobilized ionic liquids containing a metal
complex as heterogenized catalysts for the carbonyla-
tion of amines and nitrobenzene to afford ureas with un-
expectedly high catalytic activity.
The key innovation of such a catalyst involves the
physical confinement of a dialkylimidazolium ionic liq-
uid containing a metal complex into the pores or cavities
of silica gel through one-pot assembly of the ionic liquid,
the metal complex and the silica gel by a modified tradi-
tional sol-gel process. This process is different from the
so-called “ship in a bottle” method,[16] because it uses
ready-made “ships” and then tailor-making of a “bottle”
(Figure 1).
Two ionic liquids, DMImBF4 (1-decyl-3-methylimida-
zolium tetrafluoroborate, relatively larger in size) and
EMImBF4 (1-ethyl-3-methylimidazolium tetrafluoro-
borate), and four metal complexes, including
HRu(PPh3)2Cl2, Rh(PPh3)3Cl, Pd(PPh3)2Cl2 and
Co(PPh3)3Cl2, were employed. A series of silica gel-con-
fined ionic liquid-metal complexes were prepared (de-
noted as M-ionic liquid/silica gel) and dry, solid and po-
rous samples were obtained (Figure 2).
Firstly, the synthesis of symmetrical diphenylureas
from nitrobenzene and aniline over these catalysts
were tested, see Table 1. The carbonylations of aniline
and nitrobenzene proceeded efficiently, and the best re-
sult was obtained over the catalyst Rh-DMImBF4/silica
gel with a conversion of 93% and a selectivity of 92%
Figure 2. Picture of the prepared solid sample (Rh-
DMImBF4/silica gel (top) and HRTEM (JEOL JEM 2010)
picture of the sample (bottom).
Scheme 1.
Figure 1. One-pot assembly of silica gel confined ionic liquid-metal complexes with a modified sol-gel process.
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Oxygen-Free Carbonylation of Amines and Nitrobenzene to Ureas
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Table 1. Results of the synthesis of disubstituted ureas from nitrobenzene and amines.
Entry
Amine
Catalyst
Conversion [%]
Selectivity [%]
TOF[a]
NB
AM
1
2
3
4
Co-DMImBF4/silica gel
Ru-DMImBF4/silica gel
Rh-DMImBF4/silica gel
Pd-DMImBF4/silica gel
Rh-EMImBF4/silica gel
Rh-DMImBF4/silica gel
Rh-EMImBF4/silica gel
Rh(PPh3)3Cl
19
37
93
81
85
83
16
8
18
39
92
79
84
81
16
9
>98
>98
>98
>98
>98
>98
>98
>95
–
2140
1760
11548
7440
10550
10800
9987
1586
–
5
6[b]
7[b]
8[c]
9[d]
10[e]
silica gelþDMImBF4
–
41
–
39
silica gelþDMImBF4 þRh(PPh3)3Cl
>98
7463
11
12
Rh-DMImBF4/silica gel
Rh-DMImBF4/silica gel
86
85
94
97
>94
>96
11818
11943
13
14
15
Rh-DMImBF4/silica gel
Rh-DMImBF4/silica gel
Rh-DMImBF4/silica gel
87
78
88
93
81
87
>92
>98
>98
11818
9890
10884
16
Rh-DMImBF4/silica gel
–
–
–
–
[a]
TOF¼mol substrates converted per mol metal complexes per hour.
[b]
[c]
[d]
[e]
The catalyst system had been recovered and reused for the second time.
1 mg of Rh(PPh3)3Cl was used.
0.1 g of silica gel and 0.2 g of DMImBF4 were used.
1 mg of Rh(PPh3)3Cl, 0.1 g of silica gel and 0.2 g of DMImBF4 were used.
lyst was obtained, and the supported catalyst could be similar peak of DMImBF4 (1581 cmꢀ1) could be ob-
separated from the product after methanol had been served clearly even without any weakening after the cat-
added to the resultant solid mixture (catalyst deposits alysts were reused (Figure 3).[17] New peaks for the di-
on the bottom of the reactor). An 81% aniline conver- phenylurea, 1701 cmꢀ1 and 1699 cmꢀ1, were also ob-
sion and>98% selectivity were maintained when Rh- served in the spectra of used catalysts (Figures 3c and
DMImBF4/silica gel was reused for the second time, en- f). These results suggested that the deactivation of Rh-
try 5. Over Rh-EMImBF4/silica gel, a slightly lower ac- EMImBF4/silica gel was mainly attributable to
tivity was observed and the catalytic activity was greatly EMImBF4 leaching from silica gel, probably due to its
reduced when it was reused for the second time, entries 6 relatively smaller molecular in size, while DMImBF4
and 7. Analyses of rhodium contents in Rh-DMImBF4/ could be confined into the pores of silica gel in a relative-
silica gel and Rh-EMImBF4/silica gel before and after ly more stable manner due to its larger molecular size.
reaction were 0.11 wt %, 0.13 wt % and 0.09 wt %,
In order to understand the contribution of each metal
0.10 wt %. DRIFTS (diffuse reflectance infrared Fouri- complex, ionic liquid and silica gel to the reaction, the
er transform spectroscopy, Nexus 870) characterization metal complex only or physical mixtures of the ionic liq-
of the bulk ionic liquids and silica gel-confined ionic liq- uidþpure silica gel, and metal complexþionic liquidþ
uid catalysts indicated that the characteristic peak of silica gel were also examined and showed poor catalytic
EMImBF4 (1580 cmꢀ1) almost disappeared while the performance, entries 8–10, and even more metal com-
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Feng Shi et al.
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enhancement in catalytic activity was exhibited when
the ionic liquid containing the metal complex was physi-
cally confined into the pores or cavities of the silica gel
matrix, and such an enhancement in catalytic activity
may be derived from the effect of the high concentration
of ionic liquid containing the metal complex due to this
confinement (Figure 4a), and such an effect would dis-
appear when the bulk ionic liquid and metal complexes
were completely diluted with large amounts of the reac-
tant molecules, thus resulting in poor catalytic perform-
ance (Figure 4b).
Syntheses of asymmetric ureas were also tested when
aniline was replaced with n-butylamine, n-hexylamine
and cyclohexylamine, entries 11–13. The corresponding
asymmetrical ureas were also formed with high conver-
sion and selectivity, and relatively higher conversions of
alkylamines (93–97%) were observed compared with
that of nitrobenzene (85–87%). This means that direct
carbonylation between alkylamines occurred since
some dialkylureas (2–3%) were also detected on GC/
MS. Finally, several other aromatic amines, i.e., p-meth-
ylaniline and p-methoxyaniline, were subjected to the
reaction, entries 14 and 15, with sufficient high catalytic
performance. No reaction occurred on using o-nitroani-
line as substrate, entry 16, suggesting that the presence
of the electron-attracting NO2 severely weakened the
activity of the NH2.
In summary, a new kind of silica gel-confined ionic liq-
uid containing a metal complex as heterogenized cata-
lysts for the carbonylation of amines and nitrobenzene
without molecular oxygen to afford the corresponding
ureas has successfully been developed with the follow-
ing advantages: (1) greatly enhanced catalytic activity
with TOF exceeded 11000 mol·molꢀ1 ·hꢀ1; (2) much
lower amount of ionic liquids needed; (3) easy separa-
tion and reusability of catalyst are possible; (4) the use
of explosive COþO2 gas mixtures is avoided. Such an
enhancement in catalytic activity may be attributed to
the effect of the high concentration of ionic liquid con-
taining the metal complex due to its confinement in
the pores or cavities of the silica gel matrix. The concept
Figure 3. DRIFTS spectra of (a) pure bulk EMImBF4, (b)
Rh-EMImBF4/silica gel, (c) Rh-EMImBF4/silica gel after re-
acting for one time, (d) pure bulk DMImBF4, (e) Rh-
DMImBF4/silica gel, and (f) Rh-DMImBF4/silica gel after
use.
plex or ionic liquid were charged in comparison with of such a catalyst system may be universal, and could be
that present in the silica gel-confined ionic liquid con- expanded to other catalysis fields.
taining metal complex catalyst. This means that a major
Figure 4. Reaction environment for the carbonylation of aniline and nitrobenzene in immobilized (left) and bulk (middle) ionic
liquids.
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Oxygen-Free Carbonylation of Amines and Nitrobenzene to Ureas
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Experimental Section
Synthesis of Ionic Liquids
The ionic liquids and metal complexes HRu(PPh3)2Cl2,
Rh(PPh3)3Cl, Pd(PPh3)2Cl2 and Co(PPh3)3Cl2 were synthe-
sized according to procedures reported previously.[18]
Synthesis of Supported Ionic Liquids Containing Metal
Complexes
Ten mL of tetraethyl silicate (TEOS), 1 mL of ionic liquid con-
taining the metal complexes [HRu(PPh3)2Cl2, Rh(PPh3)3Cl,
Pd(PPh3)2Cl2 or Co(PPh3)3Cl2], 7 mL of ethanol and 5 mL of
hydrochloric acid (5 M) were added into a 100-mL of conical
flask. After being reacted and aged at 608C for 12 hours, the re-
sultant solid mixture was for 3 hours under ca. 1–5 mmHg vac-
uum at 1508C and the five catalysts Ru-DMImBF4/silica gel,
Rh-DMImBF4/silica gel, Pd-DMImBF4/silica gel, Co-
DMImBF4/silica gel and Rh-EMImBF4/silica gel were ob-
tained. The contents of DMImBF4 ionic liquid were 17–19
wt % in the different catalysts. AES analysis (ARL 3520
ICP) showed that the metal contents of the catalysts Ru-
DMImBF4/silica gel, Rh-DMImBF4/silica gel, Pd-DMImBF4/
silica gel, Co-DMImBF4/silica gel and Rh-EMImBF4/silica
gel were 0.14 wt %, 0.11 wt %, 0.15 wt %, 0.07 wt % and 0.13
wt % which indicated that almost all of the metal complexes
were entrapped into the silica gel.
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Process for the Synthesis of Ureas
All carbonylation reactions of amines were conducted in a 90-
mL autoclave with a glass tube inside equipped with magnetic
stirring. In each reaction, 0.1 g of catalysts, 10 mmol of nitro-
benzene, 10 mmol of amines and 5.0 MPa of carbon monoxide
were charged successively. The reaction proceeded at 1808C
for 1.5 hours, and after the reaction 20 mL of methanol were
added to the resulting mixture which was qualitatively ana-
lyzed with HP 6890/5973 GC/MS, and quantitatively analyzed
with Agilent (Shanghai) 1790 GC (FID detector) by the exter-
nal standard method.
Acknowledgement
This work was financially supported by the National Science
Foundation of China (No. 20225309).
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