Polymer-immobilized gold catalysts for the efficient and clean syntheses of carbamates and symmetric ureas by oxidative carbonylation of aniline and its derivatives

Article abstract of DOI:10.1016/S0021-9517(02)93772-3

A series of highly efficient polymer-immobilized gold catalysts were prepared for the oxidative carbonylation of aniline and its derivatives to obtain carbamates and symmetric ureas in a clean, simple, solvent-free, and obtainable product-ready way. TEM showed that the average gold particle size of fresh catalyst was less than 10 nm.

Full text of DOI:10.1016/S0021-9517(02)93772-3

Journal of Catalysis 211, 548–551 (2002)
doi:10.1006/jcat.2002.3772
RESEARCH NOTE
Polymer-Immobilized Gold Catalysts for the Efficient and Clean
Syntheses of Carbamates and Symmetric Ureas by Oxidative
Carbonylation of Aniline and Its Derivatives1
Feng Shi and Youquan Deng2
Centre for Green Chemistry and Catalysis, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences,
Lanzhou, 730000, People’s Republic of China
no need for any organic solvent and a product-ready pure
solid is obtained for urea synthesis.
A series of highly efficient polymer-immobilized gold catalysts
were prepared for the oxidative carbonylation of aniline and
its derivatives to obtain carbamates and symmetric ureas in a
clean, simple, solvent-free, and obtainable product-ready way. TEM
showed that the average gold particle size of fresh catalyst was less
Several polymer-immobilized Au catalysts (or Pd cata-
lysts for the purpose of comparison) were prepared as
follows. Au/poly1 consisted of dried polymer pellets (ca.
φ = 0.5 mm, Ionexchanger IV, Merck) without any pre-
treatment which were impregnated with 0.004 M aque-
ous solution of HAuCl4 · 4H2O and then dried at 60◦C
for 3 h. Au/poly2 consisted of dried polymer which was
pretreated with NaOH, and then the same preparation
procedure as for Au/poly1 was followed. Au/poly3 was
prepared using the same procedure as for Au/poly2 ex-
cept that a 0.004 M solution of HAuCl4 · 4H2O + acetone
was used. The catalyst pellets could be dried more easily
at room temperature using acetone as solvent. Au/poly4
and 5 were prepared using the same preparation proce-
dure as for Au/poly3 except that 0.002 M and 0.001 M
solutions of HAuCl4 · 4H2O + acetone were used respec-
tively. Au-K/poly was prepared using the same procedure
as Au/poly3 but KOH was used instead of NaOH. Pd/poly1
and 2 were prepared using the same procedure as for
Au/poly2, but HAuCl4 · 4H2O was replaced with H2PdCl4
or Pd(phen)2Cl2 (phen: 1,10-phenanthroline monohy-
drate), respectively. The basic properties of polymer-
immobilized Au catalysts are summarized in Table 1. Only
small BET surface areas (Micromeritics ASAP 2010 in-
strument) for the polymer-immobilized Au catalysts were
found. Catalysts with different Au loadings (3520 ICP AES)
were obtained, and high Na contents in Au/poly2–5, were
observed. XPS analysis (VG ESCALAB 210) was con-
ducted over the polymer-immobilized Au catalysts. The re-
sults showed Au+1 in Au/poly1 but Au0 in Au/poly2–5 and
Au-K/poly. No Au+3 species on the polymer could be ob-
served in these catalysts, indicating that redution of Au+3
species occurred during catalyst preparation. TEM (JEOL
JEM-1200EX) images of Au/poly3 confirmed that the av-
erage size of the Au particles was less than 10 nm (Fig. 1).
As shown in Table 2 (entry 1), almost no conversion
could be observed over Au/poly1. High conversion with
c
than 10 nm. ꢀ 2002 Elsevier Science (USA)
Gold, a less expensive and environmentally benign noble
metal in comparison with Pd, Pt, and Rh, has been re-
ceiving increased attention as a novel catalyst material.
Numerous reactions catalyzed with either supported gold
(1–5) or homogeneous gold complexes (6–11) were re-
ported as having excellent performance. Recently, homoge-
neous Au catalysts used for the syntheses of carbamates and
diformamide by carbonylation of amines have also been
reported (12, 13). The former are important intermedi-
ates for the nonphosgene synthesis of isocyanates and have
now been synthesized either by oxidative carbonylation of
amines or by reductive carbonylation of nitro compounds
(14–17).
With our continuing efforts to apply Au as a novel cata-
lyst material and to search for “greener” or clean technolo-
gies for chemical production, polymers (18, 19), as a new
kind of catalyst support, were employed to immobilize Au,
and oxidative carbonylation of aniline and its derivatives
for syntheses of corresponding carbamates and symmetric
ureas was performed in this work (Scheme 1).
In comparison with the homogeneous Au catalysts al-
ready mentioned, the following advantages were found:
(1) catalyst efficiency is greatly enhanced; (2) the catalysts
are reusable; the catalysts are easily separable from the re-
sulting product, (4) the system is simple; and (5) there is
1 This work was supported by the Natural Science Foundation of China
(No. 20173068).
2 To whom correspondence should be addressed. Fax: 86-931-8277088.
E-mail: ydeng@ns.lzb.ac.cn.
548
0021-9517/02 $35.00
c
ꢀ 2002 Elsevier Science (USA)
All rights reserved.

POLYMER-IMMOBILIZED Au CATALYSTS
549
SCHEME 1
excellent selectivity for desired carbamate, however, was
obtained over Au/poly2, and the best catalytic performance
was achieved over Au/poly3, where the corresponding TOF
number reached 530, which is much higher than that of
either homogeneous organometallic gold catalyst (13) or
the palladium catalyst system reported in previous papers
(8, 11). In comparision with Au/poly2, better catalyst activ-
ity was unexpectedly obtained over Au/poly3 when acetone
as a solvent was used during preparation, indicating that
these catalysts are very sensitive to the preparation proce-
dure, although the detailed reason is not clear at this stage.
Au-K/poly catalyst (entry 4), in which potassium hydroxide
instead of sodium hydroxide was used in the catalyst prepa-
ration, was tested in the synthesis of carbamate, and it can
be seen that poor catalyst activity was observed. These re-
sults suggest that sodium in or on the polymer played an
important role in keeping the Au species in a highly ac-
tive state and that the catalyst acitivity is very sensitive to
modification of the polymer since almost no or lower ac-
tivities can be observed over Au/poly1 without sodium and
Au-K/poly containing potassium. In addition to ease of sep-
aration, it is noteworthy that the catalyst activity and selec-
tivity were nearly stable after the catalyst was repeatedly
used five times (entry 5), and 77% of conversion and 98%
of selectivity were maintained even after it was repeatedly
used 12 times (entry 6), showing that recycling of the cata-
lyst is possible. Although not shown here, XRD (Siemens
D/max-RB) analysis of a Au/poly3 sample that was reused
several times indicated the appearance of metal Au peaks
not present in fresh Au/poly3 and that the calculated sizes
FIG. 1. TEM picture of Au/poly3 catalyst.
of Au crystallite in the used Au/poly3 were ca. 12 nm. This
may imply that the particle dimension of Au was not very
stable but grows slightly, and also, this may be the main
reason for the partial deactivation of the catalyst.
The oxidative carbonylation of aniline over the polymer-
immobilized Au catalysts with even lower Au loading (ca.
0.5 wt% for Au/poly4 and 0.2 wt% for Au/poly5) was
also tested. Althogh the conversions were reduced to 71
and 63% (entries 7 and 8), excellent selectivities were
maintained and higher TOF numbers were achieved for
Au/poly4 (513) and Au/poly5 (553).
For the purpose of comparison, Pd catalysts (i.e.,
Pd/poly1 and Pd/poly2 in this work), the most efficient
catalysts for carbonylation reactions, were also examined
(entries 9 and 10), but only 1.8–2% conversions were ob-
tained, although Pd/poly1 and Pd/poly2 were also treated
with sodium hydroxide during preparation as was Au/poly2
or 3.
When p-amino toluene, 4-methoxybenzamine, and m-
nitroaniline over the Au/poly3 catalyst were subjected to
carbonylation (entries 11–13) the catalytic activities de-
creased in turn. The big difference in the conversions is
mainly attributed to the big difference in the electronic
properties of the aniline substituents.
It is worth noting that a new, clean, and highly efficient
method for the synthesis of symmetric diphenyl ureas can
be established when methanol is removed from this car-
bonylation reaction, forming a crystalline product. The re-
sults are listed in Table 3 (entry 1). The corresponding TOF
could reach 1475, which is much higher than that reported
previously (20). The reason that Au/poly4 was chosen for
the synthesis of ureas is that Au/poly4 may be a more
promising catalyst for industrial application because of its
lower Au loading in comparison with Au/poly3 and its high
enough activity in comparison with Au/poly 5. In addition
to ease of prduct recovery, high catalytic performance was
TABLE 1
The Basic Properties of Polymer Immobilized Au Catalysts
BET
Au
(wt%)
Na (K)
(wt%)
Au4f7/2
(eV)
Catalyst
(m2/g)
Pure Polymer
Au/poly1
Au/poly2
Au/poly3
Au/poly3a
Au-K/poly
Au/poly4
Au/poly5
2.82
1.50
1.46
1.69
1.43
1.71
1.48
1.51
—
0.9
1.0
1.0
1.0
1.02
0.51
0.19
—
0
—
85.6
84.4
84.2
83.9
83.9
84.1
84.3
7.5
7.4
7.6
11.8
7.2
7.6
a Au/poly3 used for synthesis of carbamate 12 times.

550
SHI AND DENG
TABLE 2
The Results of Oxidative Carbonylation of Amines to Cabamater
Entry
Substrate
Catalyst
Con. (%)
Sel. (%)
TOF
Product
1
2
aniline
aniline
aniline
aniline
aniline
aniline
aniline
aniline
aniline
aniline
Au/poly1
Au/poly2
Au/poly3
Au-K/poly
Au/poly3
Au/poly3
Au/poly4
Au/poly5
Pd/poly1
Pd/poly2
<1
83
95
18.3
90
77
71
63
1.8
2
>99
>99
>99
>98
>99
>98
>98
>99
>99
>99
∼5
459
530
102
510
510
513
553
3.1
NHCO2CH3
NHCO2CH3
NHCO2CH3
NHCO2CH3
NHCO2CH3
NHCO2CH3
NHCO2CH3
NHCO2CH3
NHCO2CH3
NHCO2CH3
3
4
5a
6b
7c
8d
9
10
22.6
11
12
13
p-amino toluene
Au/poly3
Au/poly3
Au/poly3
73
60
25
>99
>99
>99
330
110
40
NHCO2CH3
p-methoxybenzen-amine
m-nitroaniline
NHCO2CH3
NHCO2CH3
a Used 5 times.
b Used 12 times.
c Reacted for 1.5 h.
d Reacted for 3 h.
still maintained after the catalyst was repeatdly used three also tested (entries 3–5) and high selectivity and TOF num-
times (entry 2), indicating that recycling of the catalyst for bers were still maintained.
urea formation is also possible. With the same reaction con-
Almost the same reaction tendencies as aniline carbony-
ditions and catalyst, aniline derivatives as substrates were lation to carbamate were observed when the contributions
TABLE 3
Results of the Oxidative Carbonylation of Amines to Synthesize Ureas
Entry
Subs.
Catalyst
Con. (%)
Sel. (%)
TOF
Products
NHC(O)NH
NHC(O)NH
NHC(O)NH
NHC(O)NH
NHC(O)NH
1
aniline
aniline
Au/poly4
Au/poly4
Au/poly4
Au/poly4
Au/poly4
70
71
99
99
99
99
—
1475
1493
780
640
—
2a
3
p-amino toluene
4-methoxybenzamine
m-nitroanilne
43
MeO
OMe
4
13
5
<1
NO2
NO2
NHC(O)NH
NHC(O)NH
NHC(O)NH
6
7
8
aniline
aniline
aniline
polyb
−0
<1
<1
—
—
—
—
—
—
Au/poly1
Pd/poly2
a Catalyst Au/poly4 used for the third time.
b Polymer treated with aqueous sodium hydroxide only.

POLYMER-IMMOBILIZED Au CATALYSTS
551
of polymer, polymer-immobilized Au (entries 6 and 7), and were successively introduced into the reactor. The reaction
the polymer-immobilized Pd (entry 8) to aniline transfor- was normally proceeded at 175◦C for 1–3 h. Qualitative
mation to urea were examined.
and quantitative analyses were conducted with a HP
Severalotherpolymers, thoughnotshownhere, werealso 6890/5793 GC-MS. When conversion and selectivity were
tested to immobilize the Au. Lower or even no catalytic ac- high enough, nearly dry solid product mixed with the cata-
tivity for the carbonylation was observed, indicating that lyst pellets could be obtained after reaction. The desired
the polymer itself has a strong impact on the catalytic pure product and catalyst pellets could be easily sepa-
activity.
rated by sieving with a 30 mesh sieve because the size of
Based on the analysis of the results summarized in the catalyst pellets is much larger than that of the solid
Tables 1–3, it can be conjectured that (1) the carbonylation product.
of aniline and its derivatives to carbamates or ureas fol-
lowed similar reaction mechanisms; (2) some kind of syner-
gism among the sodium, gold, and polymer is the key factor
for such high activity toward the carbonylation of aniline
to synthesize the carbamate or ureas; and (3) the reason
for the partial deactivation of catalyst after repeated oper-
ation was related to the size of the gold particles because
no loss of gold was detected after it was repeatedly used for
synthesis of carbamate 12 times.
In conclusion, reusable polymer-immobilized Au cata-
lysts for the syntheses of carbamates and symmetric ureas
by oxidative carbonylation of aniline and its derivatives
have been developed for the first time with a clean, sim-
ple, solvent-free, and obtainable product-ready way. The
synergism, established among Au, Na, and polymer, is the
key factor for such high activity and selectivity, although
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