Gold-catalyzed phosgene-free synthesis of polyurethane precursors

Article abstract of DOI:10.1002/anie.200905160

(Figure Presented) Golden catalyst: In a two-step one-pot catalytic process gold nanoparticles supported on CeO₂ are able to convert nitroaromatics into aromatic carbamates, thereby providing an alternative phosgene-free route towards aromatic polyurethanes (see figure).

Full text of DOI:10.1002/anie.200905160

Communications
DOI: 10.1002/anie.200905160
Supported Catalysts
Gold-Catalyzed Phosgene-Free Synthesis of Polyurethane
Precursors**
Raquel Juꢀrez, Patricia Concepciꢁn, Avelino Corma,* Vicente Fornꢂs, and
Hermenegildo Garcꢃa*
Aromatic polyurethanes are manufactured by a chemical
route which involves the use of phosgene as a reactant. An
alternative catalytic green process that avoids phosgene by
using dimethyl carbonate (DMC) instead, is presented
herein. Typically DMC reacts with aromatic amines to yield
N-methylation products, but with the catalyst developed
herein selective N-carbamoylation takes place with a greater
than 99% selectivity at full conversion. The process involves a
reusable solid catalyst derived from gold on nanocrystalline
ceria. Besides aromatic amines, the catalyst also allows the
reaction to proceed directly from nitroaromatics through a
one-pot cascade reaction. This finding opens the possibility
for an environmentally friendly route for the carbamoylation
catalysts remain in the reaction mixture and purification is
needed to separate the resulting inorganic salts from the
[7]
products. In addition, these catalysts cannot be reused in
subsequent runs. In this regard the use of organic carbonates
for the N-carbamoylation of aromatic amines could represent
a “green” alternative to phosgene (Scheme 1), since the only
by-product will be an alcohol, which can be recycled to form
the dialkyl carbonate. If carbamates can be selectively formed
by N-carbamoylation of aromatic amines, then they can be
easily transformed into the corresponding polyurethanes
[
8–10]
(Scheme 1).
An overall process based on DMC will be highly favorable
from an environmental point of view not only because of the
phosgene replacement, but also because the carbonyl groups
of aromatic amines that could be based on CO as a primary
2
feedstock.
in the polyurethanes will be derived from CO through DMC
2
[
8–12]
The current industrial route for manufacturing polyur-
ethanes and polycarbonates is based on the use of phosgene
(Scheme 1).
Although the current industrial production of
DMC still uses oxidative carbonylation of methanol, novel
[
1,2]
(
Scheme 1).
However, owing to the high toxicity of
routes based on the use of CO as a feedstock are ready to be
2
[13,14]
phosgene there is an urgent need to develop alternative
reactants. Several phosgene-free syntheses of polyurethane
implemented.
CO can be obtained from air or, more
2
favorably, it can be generated from combustion or fermenta-
tion processes. Therefore, fermentation can simultaneously
provide the alcohol and the CO needed for the synthesis of
2
organic carbonates, making the whole route CO -neutral with
2
doubtless advantages from the point of view of sustainability
[15]
and environmental considerations.
In contrast to the
catalytic CO -neutral route presented herein, other alterna-
2
tives based on CO, such as the reductive carbonylation of
nitroarenes and oxidative carbonylation of amines, are based
on the use of CO coming from nonrenewable fossil fuels and
Scheme 1. Synthetic routes to polyurethanes involving either phosgene
or organic carbonates.
may generate two equivalents of CO in the process. There-
2
fore, processes based on carbonylation contribute to the
precursors have been reported, but they either generate waste
increase of CO in the environment, and considering the large
production scale of aromatic polyurethanes and the Kyoto
2
[
3]
[4,5]
such as halide salts or CO2,
or they require more than a
[6]
stoichiometric amount of a strong base; one of these
methods employs a reusable heterogeneous catalyst. The
reaction of aniline with O-methyl carbamate in methanol can
agreement regarding the reduction on CO , these processes
2
are unfavorable compared to the one reported herein.
Notably, N-carbamoylation of aliphatic amines with DMC
to form the corresponding carbamates is already well-known.
However, the production of aromatic polyurethanes, which
account for about 85% of the market, still requires the use of
phosgene. The reason for this is that the industrially relevant
aromatic diamines react with DMC to preferentially afford
the N-methylation product with some minor amount of the
N-carbamoylation product.
be carried out with 6% ZnCl2 or Zn(AcO) , but these
2
[*] R. Juꢀrez, Dr. P. Concepciꢁn, Prof. A. Corma, Prof. V. Fornꢂs,
Prof. H. Garcꢃa
Instituto Universitario de Tecnologꢃa Quꢃmica UPV-CSIC
Universidad Politꢂcnica de Valencia
Av. De los Naranjos s/n, 46022 Valencia (Spain)
Fax: (+34)963-877-809
Herein we show that gold nanoparticles supported on
nanoparticulated ceria is an efficient and reusable solid
catalyst for the selective N-carbamoylation of aromatic
amines, and more specifically for the desired dicarbamoyla-
tion of 2,4-diaminotoluene (DAT), which is the most impor-
tant aromatic amine for polyurethane production. Moreover,
E-mail: acorma@itq.upv.es
hgarcia@qim.upv.es
[
**] Financial support by the EU Commission (TopCombi Project) and
the Spanish MICINN (CTQ2009-11065) is gratefully acknowledged.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.200905160.
1
286
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Angewandte
Chemie
we show that by using a single gold catalyst it is possible to
combine the hydrogenation of nitroaromatics with the
N-carbamoylation reaction in a one-pot catalytic process;
this reaction requires only hydrogen and DMC as the reagents
and produces methanol as the only by-product. Methanol can
be reused for the DMC synthesis. When DAT is reacted with
DMC, two reactions can in principle occur: The desired
N,N’-dicarbamoylation and the formation of the undesired
N-methyl derivatives. Since there are two amino groups in the
DAT molecule, this can lead to a complex mixture of products
as indicated in Scheme 2.
The reactions were carried out using an excess of DMC as
the solvent. When the reaction of DATwith DMC was carried
out in the presence of the conventional Lewis acid Zn-
[
16]
(
OAc)2, we found a high conversion of DAT with a modest
yield for the desired dicarbamoylated product 2, which
resulted form the formation of significant amounts of mono-
carbamoylated products para-1 and ortho-1, and N-methy-
lated products (Table 1, entry 1). Furthermore when the
Scheme 2. Possible products derived from DAT as it undergoes
N-methylation and N-carbamoylation reactions.
Zn(OAc) catalyst was recovered from the reaction media
2
and reused, the yield for the N-carbamoylated products was
even lower because of the decomposition of Zn(OAc) and
2
the formation of inactive ZnO (entries 2 and 3). Organo-
This result is remarkable, and becomes more relevant as the
catalyst can be reused, at least three times, while maintaining
a 99% conversion and a yield of up to 99% (entries 4 and 5).
Importantly, when using the commercially available CeO2
support, which has larger particles (40 nm) relative to the
[17]
[18]
catalysts
and transition-metal exchanged zeolites
also
promote N-methylation of DAT with either none or very
minor amounts of N-carbamoylation occurring.
As a result of conventional solid Lewis acids not being
able to promote the required N-carbamoylation, and consid-
ering the ability of gold, specifically supported gold, to
interact and activate CO as well as
nanocrystalline CeO , the catalyst activity and product yields
2
were much lower (entry 6). Gold-free nanocrystalline CeO2
[19,20–23]
other reactants,
we consid-
[
a]
Table 1: Results for the reaction of DAT with DMC in the presence of a series of catalysts.
ered the possibility of using gold to
activate DMC. If such an activation Entry Catalyst
of the CO moiety promoted the
Mass
DAT Conv.
[%]
ortho-1
+
para-1 [%]
Yield of
2 [%]
N-Methylated
product
[%]
[
b]
[b]
balance
[
b]
[b]
[b]
[%]
transfer of
a
methoxycarbonyl
group rather than the transfer of
the methyl group, the preferred
process would be the formation of
carbamates. We first studied the
reaction of DAT with DMC over a
1
2
Zn(OAc)
Fresh
Zn(OAc)2
After one use
ZnO (40 nm)
Au/CeO2
2
95Æ3
95Æ3
99
32
36
3
25
1
39
96
3
4
97Æ3
99Æ2
16
99
1
4
–
96
99
–
catalyst
derived
from
gold
(0.44%) Fresh
(
0.44 wt%) supported on nanocrys-
5
6
Au/CeO2
(0.44%) 3rd reuse
Au/CeO2
98Æ2
99Æ3
99
65
–
100
–
–
talline CeO2 (5 nm diameter).
Transmission electron microscopy
(
of the gold particles in this catalyst
were within 2–5 nm in diameter
(
73
27
(0.44%; 40 nm)
TEM) showed that a large fraction
7
8
9
CeO (5 nm)
97Æ3
97Æ3
99Æ3
92
–
58
48
–
10
52
–
–
–
–
90
2
CeO (40 nm)
2
Au/TiO2
see Figure S1 in the Supporting
(0.44%)
Information). The time/conversion 10
Au/Fe203
(0.44%)
Pd/CeO2
95Æ3
96Æ3
95Æ3
98Æ2
99Æ2
27
87
10
8
–
34
4
–
65
–
100
1
plot for the reaction of DAT with
1
1
1
1
2
3
DMC in the presence of Au/CeO2
0.44 wt%) is given in Figure 1, and
(
0.44%)
Pd/TiO2
1.5%)
(
96
100
20
the results obtained after 7 hours of
reaction time are compared with
those obtained using other catalysts
(
Pt/TiO2
(1.5%)
Au(core)–Pd(shell)/TiO
–
–
(
Table 1). Interestingly, by using the 14
2
40
77
3
Au/CeO catalyst the yield of the
dicarbamoylated 2 was higher than
2
[a] Reaction conditions: DAT (0,98 mmol, 120 mg), DMC (29,69 mmol, 2,67 g), catalyst: Au, Pd, Pt, Zn
0,5% mol respect to DAT), CeO2 and ZnO (100 mg), 7 h, 1408C. [b] Determined using GC/MS
(
9
5% at a 99% conversion of DAT. methods.
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Communications
Supporting Information, show that DMC adsorbs dissocia-
tively onto the CeO surface, giving methoxy (IR bands at
2
À1 [26]
1
1
104 and 1044 cm ) and carbonate species (IR band at
588 cm ).
À1 [27]
Desorption at increasing temperatures (see
Figure S3 in the Supporting Information) indicate an easier
desorption (potentially higher reactivity) of carbonate than
the methoxy species, the latter remaining practically
unchanged up to 393 K under vacuum. We have seen that
these strongly adsorbed methoxy species are exactly the same
as those formed when adsorbing pure methanol onto the
nanocrystalline CeO2.
À1
When aniline (1606 cm ) was introduced into the IR cell
after adsorbing DMC, and the wafer was heated at 303 K, the
band corresponding to the carbonates slowly starts to
disappear. At 393 K the band associated with the carbonate
Figure 1. Product selectivity for the reaction of DAT and DMC in the
presence of Au/CeO (0.44%; DAT/DMC mol ratio 30, T=423 K).
2
À1
ortho-1 (^), para-1 (&
), 2 (~).
(1588 cm ) has mostly disappeared, whereas the methoxy
groups have only partially disappeared; higher temperatures
are required to achieve their full release (Figure S4a in the
Supporting Information).
also gives high selectivity for N-carbamoylation, but lower
activities than Au/CeO (entries 7 and 8).
In the case of the Au/CeO sample, the same bands and
2
2
At this point we were able to conclude that: a) in our
qualitative behavior as that of CeO is observed, however, the
2
hands, nanocrystalline CeO is a selective carbamoylation
most remarkable difference being that during the in situ
reaction with aniline, the band belonging to the surface
carbonates disappears faster at room temperature than with
2
catalyst; b) the presence of gold on the nanocrystalline
catalyst strongly boosts activity to the desired dicarbamoyla-
tion product; and c) gold on nanocrystalline ceria is a reusable
catalyst.
CeO and has mostly disappeared at 343 K. (Figure S4b).
2
À1
Meanwhile on Au/CeO the presence of a band at 1656 cm ,
2
[13]
Since it has been shown that the nature of the support
which can be attributed to the carbonyl group of the
carbamate, is observed. On the contrary, for the nonselective
[
24]
plays an important role in gold catalysis, we also used gold
supported on TiO and Fe O , and the resultant catalysts form
Au/TiO , DMC also adsorbs dissociatively but the methoxy
2
2
3
2
À1
the N-methylated products in high yield (entries 9 and 10).
Meanwhile when gold was supported on carbon, an almost
equal amount of N-methylation and monocarbamoylation
occurred without formation of the desired product 2. Palla-
dium supported on ceria gives much lower yields of the
desired product 2 (entry 11), relative to that obtained using
gold, but the yield of the undesired N-methylation product is
still very low. In contrast, palladium or platinum supported on
titania yields almost exclusively N-methylation products
groups (1157, 1121, and 1054 cm ) are much easily desorbed
than with Au/CeO (see Figure S5 in the Supporting Infor-
2
À1
mation). Furthermore, the carbonate species (1566 cm ) are
very strongly adsorbed. In this case, when aniline was
introduced, the methoxy band starts to disappear already at
room temperature and completely disappears at 343 K,
À1
whereas the bands at 3415, 1307, and 1287 cm , associated
[28]
with methylaniline, are formed. In contrast, a broader IR
À1
À1
band at 1574 cm , together with bands at 2956 and 2856 cm
(
entries 12 and 13). However, core(Au)–shell(Pd) nanoalloys
associated to the formation of some formate species, are also
observed. Bands representing methylaniline and the for-
[
25]
[29]
supported on titania exhibit an intermediate product yield
entry 14). This indicates that both the support and the noble
(
mate species increase in intensity with increasing reaction
temperature (see Figure S6 in the Supporting Information).
The in situ results obtained with Au/CeO and Au/TiO nicely
metal play a role in the catalysis and opens the possibility to
develop other systems with even higher activity than our
current Au/CeO2 catalyst. Among the different supports
2
2
explain why N-methylation was preferentially observed with
studied here nanocrystalline CeO gives, by far, the best
Au/TiO whereas carbamoylation was the preferred reaction
2
2
catalytic performance.
product during the previous batch reactor experiments.
The results obtained with DAT can be extended to other
aromatic amines and organic carbonates. We have observed
Supported gold, particularly Au/TiO , is a selective
2
[30,31]
catalyst for the hydrogenation of aromatics.
Thus, a
that Au/CeO can selectively catalyze the N-carbamoylation
two-step process in which Au/TiO effects the hydrogenation
2
2
of various para-substituted anilines with essentially complete
conversions and yields to the N-carbamoylation product (see
Table S1 in the Supporting Information). Au/CeO2 also
catalyzes the selective and complete N-dicarbamoylation of
DAT using diethyl carbonate.
and subsequently Au/CeO promotes the N-carbamoylation
2
appears to be a general route to access aromatic carbamates.
We have explored the possibility of combining these two
reactions in a single-pot process to produce the dicarbamoy-
lation of DAT, which is by far the most relevant aromatic
N-carbamoylated product from the industrial point of view.
The two-step, one-pot process requires the treatment of
2,4-dinitrotoluene with DMC under H and using Au/CeO as
With the aim of gaining a better understanding of what
occurs at the molecular level on the surfaces of CeO , Au/
2
CeO , and Au/TiO , and to explain why the two latter
2
2
2
2
catalysts behave so differently, we carried out an in situ FTIR
spectroscopy study. The results given in Figure S2 in the
catalyst (Scheme 3). The results presented show that it is
possible to achieve high conversion of 2,4-dinitrotoluene with
1
288
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Following this procedure, 3.5 nm gold nanoparticles supported on
TiO are obtained.
2
Core–shell Au(core)–Pd(shell)-TiO was prepared by stirring a
2
solution of Au/CeO (2 g; gold content 0.44 wt%) in water and an
2
À4
acetone dissolution of [PdCl (PhCN) ] (150 mL, 2.5 ꢀ 10 m). The
2
2
slurry was vigorously stirred at room temperature for 4 h. The solid
was then filtrated, exhaustively washed with distilled water, and
then dried at 373 K overnight. The solid was then reduced with
1
-phenylethanol at 433 K for 2 h. The catalyst was then washed,
Scheme 3. One-pot reaction of 2,4-dinitrotoluene with DMC.
filtered, and dried at room temperature for 12 h. The final Pd content
was found to be 0.5 wt% by atomic absorption analysis.
Pd/TiO and Pt/TiO catalysts were prepared by impregnation of
2
2
excellent yields of the product 2 by means of the one-pot
procedure. The carbamoylation also occurs at 1208C with a
2
TiO (2 g; Degussa P25, 10 g, S = 55 m g) with a solution of PdCl2
2
BET
(
0.345 g; Aldrich, 60% purity) or H PtCl ·6H O (0.28 g; Aldrich),
2 6 2
9
9% yield and 89% conversion after a 14 hour reaction time.
In conclusion, an active, selective, and reusable catalyst
respectively, in H O (7 mL; milliQ). The slurry was stirred for 2 h at
2
room temperature, then all the liquid was evaporated and the solid
was dried at 373 K overnight and then reduced with 1-phenylethanol
at 433 K for 2 h. The catalyst was then washed, filtered, and dried at
room temperature for 12 h. The final Pd or Pt content was found to be
wt% by atomic absorption analysis.
Reaction procedure: Experiments were performed in reinforced
glass reactors equipped with temperature and pressure controllers.
The reactions were carried out using an excess of DMC as the solvent.
For each reaction, a mixture of DAT (1 mmol) and DMC (30 mmol)
was placed into the reactor (3 mL capacity) together with an
appropriate amount of catalyst. The DAT and DMC used in this
study are commercially available from Sigma–Aldrich with purities
higher than 95%. n-Dodecane was used as an internal standard for
the determination of the conversion and product yields. The reactors
based on Au/CeO has been found for reacting aromatic
2
amines with DMC to produce a dicarbamoylated product
required for the synthesis of aromatic polyurethanes. This
process opens a new route for polyurethane production that
5
avoids the use of phosgene and CO while fixing CO through
2
the reactant DMC without generating any by-product except
recyclable methanol or ethanol. Furthermore, through a one-
pot process requiring only hydrogen, DMC, and a single
catalyst, it is possible to convert 2,4-dinitrotoluene directly
into its corresponding dicarbamate.
were sealed and H was introduced (5 bar). The reactor was then
2
placed in a preheated silicone bath at 1408C. During the experiment,
the stirring rate was fixed at 1000 rpm (magnetic stirring). Aliquots
were taken from the reactor at different reaction times, and the
catalyst particles were removed from the solution by centrifugation at
Experimental Section
Catalyst preparation: Zn(OAc) and ZnO were commercial samples
2
supplied by Sigma–Aldrich.
CeO2 nanoparticles: Nanoparticulated ceria was prepared by
adding an aqueous ammonia solution (1.12 L, 0.8m) to 375 mL of a
12000 rpm. The crude reaction mixture was then analyzed by GC/MS
methods and also by comparison to pure samples of the desired
products which were prepared by reacting commercially available
toluenediisocianate with methanol. Figure S7 in the Supporting
Information shows some chromatograms illustrating the time con-
version as well as the mass spectra of ortho-1, para-1, and 2. The
reactions were carried out at least in triplicate. No significant
deviations in the conversions and yields among the runs were
observed. Only experiments with mass balances ꢀ 95% were
considered. Table 1 includes the average values of mass balance,
conversion, and yield.
Ce(NO3) (0.8m) at ambient temperature with continuous stirring.
4
The colloidal dispersion of CeO2 nanoparticles was heated in a
polyethylene terephthalate vessel at 373 K for 24 h. The resulting
yellow precipitate was filtered and dried under vacuum overnight.
The cerium oxide synthesized has, owing to the small size of the
2
À1
nanoparticles, a very large surface area (180 m g ).
Au/CeO : A solution of HAuCl ·3H O (200 mg) in deionized
2
4
2
water (40 mL) was brought to pH 10 by addition of a solution of
NaOH (0.2m). Once the pH value was stable the solution was added
to a slurry containing colloidal CeO (10 g) in H O (50 mL). After
adjusting the pH to 10 with NaOH (0.2m), the slurry was vigorously
FTIR procedure: FTIR spectra were collected on a BioRad FTS-
40 A spectrometer. The infrared cell, connected to a dosing system,
was designed to treat the samples in situ under controlled atmos-
2
2
stirred for 18 h at room temperature. The AuCeO solid was then
À5
2
pheres and temperatures. The samples were evacuated at 10 mbar
filtered and exhaustively washed with distilled water until no traces of
chlorides were detected by the AgNO3 test. These washes are
and 373 K for 1 h prior to the adsorption experiments. DMC (2 mbar)
and aniline (0.5 mbar) were coadsorbed onto the support surface at
room temperature. After reactant adsorption the sample was
evacuated to remove the excess of both reactants. Spectra were
collected at different temperatures.
À
important as traces of Cl remain strongly bonded to gold and are
highly detrimental for the overall activity. The catalyst was dried at
room temperature under vacuum. The total Au content of the final
catalyst was 0.44% as determined by chemical analysis. Under this
procedure, 3–4 nm gold nanoparticles (average size) supported on
CeO were obtained. Another Au/CeO sample containing a higher
Received: September 15, 2009
Revised: December 7, 2009
Published online: January 18, 2010
2
2
Au loading (1.60 wt%) was prepared following an analogous
procedure but instead using 60 mg of HAuCl ·3H O for 1.0 g of CeO .
4
2
2
The Au/TiO catalyst consists of 1.5 wt% gold on TiO and was
2
2
Keywords: nanoparticles · gold · green chemistry ·
heterogeneous catalysis
supplied by the World Gold Council (reference catalysts, Type A). It
can also be prepared by depositing the gold from an aqueous solution
of HAuCl (Alfa Aesar) onto a sample of TiO (P25 Degussa). The
.
4
2
deposition precipitation procedure is done at 343 K and pH 9, using
NaOH (0.2m) to maintain a constant pH over 2 h. Under these
conditions, gold deposition occurs with 80% efficiency. The catalyst is
then recovered, filtered, washed with deionized water, and dried at
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