Syntheses of polycarbonate and polyurethane precursors utilizing CO2 over highly efficient, solid as-synthesized MCM-41 catalyst

Article abstract of DOI:10.1016/j.tetlet.2006.04.057

As-synthesized MCM-41 was used as a reusable, heterogeneous catalyst for the eco-friendly synthesis of cyclic carbonate precursors of polycarbonates via a cycloaddition reaction of CO₂ with epoxides. This catalyst is also efficient for the synt

Full text of DOI:10.1016/j.tetlet.2006.04.057

Tetrahedron Letters 47 (2006) 4213–4217
Syntheses of polycarbonate and polyurethane
precursors utilizing CO over highly efficient,
solid as-synthesized MCM-41 catalyst
2
R. Srivastava, D. Srinivas and P. Ratnasamy*
*
National Chemical Laboratory, Pune 411 008, India
Received 21 February 2006; revised 5 April 2006; accepted 10 April 2006
Available online 9 May 2006
Abstract—As-synthesized MCM-41 was used as a reusable, heterogeneous catalyst for the eco-friendly synthesis of cyclic carbonate
precursors of polycarbonates via a cycloaddition reaction of CO
2
with epoxides. This catalyst is also efficient for the synthesis of
alkyl and aryl carbamate precursors of polyurethanes via the reaction of amines, CO and alkyl halides. Both these reactions were
2
carried out under mild conditions and without using any solvent or co-catalyst. CO
toxic phosgene in the conventional synthesis of these chemicals.
Ó 2006 Elsevier Ltd. All rights reserved.
2
is utilized as a raw material replacement for
1
. Introduction
sure and/or temperature. Commercial production of
1
4
cyclic carbonates by BASF (Ludwigshafen, Germany)
1
5
Cycloaddition of CO to epoxides producing cyclic car-
and Chimei-Asahi Corporation (Taiwan) involves the
use of quaternary ammonium salt-based catalysts in
2
bonates is one of the few industrial processes that utilize
CO as a raw material. Cyclic carbonates are widely
used as electrolyte components in lithium batteries,
polar solvents and chemical intermediates.1–3 They are
also precursors for polycarbonates (Scheme 1). They
are conventionally manufactured using toxic chemicals
1
5
the homogeneous phase. The commercial process is
conducted at 453–473 K and at 50–80 bar pressure.
The development of a highly efficient solid catalyst
system for chemical fixation of carbon dioxide under
2
mild conditions still remains a challenge. Recently, we
4
16–20
like phosgene. A large number of homogeneous
reported
the application of zeolite-encapsulated
catalyst systems including quaternary ammonium salts,
metal halides, metal complexes and ionic liquids have
been reported to be effective for the cycloaddition reac-
metal phthalocyanine complexes, titanosilicate molecular
sieves, zeolite-beta and adenine-modified mesoporous
Ti/Al-SBA-15, for this reaction. We now report a supe-
rior solid catalyst, as-synthesized MCM-41, for the same
application.
tion.5
–10
Metal oxides catalyze this reaction,
11–13
but
their catalytic activity and selectivity are unsatisfactory
and, moreover, require a polar organic solvent such as
N,N-dimethylformamide (DMF) to achieve high yields
of carbonates. Other heterogeneous catalysts suffer from
one or more of the following disadvantages: (1) low
stability, (2) a requirement of an additional co-cata-
lyst/promoter (4-(N,N-dimethylamino)pyridine, e.g.),
1
2
Organic carbamates (R NHCO R ) are another class of
2
important compounds widely used for pharmaceuticals,
pesticides and herbicides, and more generally, for the
production of intermediates for fine and commodity
2
1
chemicals. Carbamates are precursors for polyure-
thanes. Commercially, carbamates are synthesized by
aminolysis of chloroformate esters, derived from phos-
(
3) poor catalyst reusability and (4) a need for high pres-
2
2
gene and an alcohol. Many alternative routes, such
as catalytic carbonylation of nitroaromatics and oxida-
tive carbonylation of amines, have also been devel-
2
Keywords: Carbon dioxide utilization; CO fixation; Cyclic carbonates;
Carbamates; Polycarbonates; Polyurethanes; MCM-41; Eco-friendly
processes.
2
3
*
Corresponding authors. Tel.: +91 020 2590 2018; fax: +91 020 2590
633 (D.S.); e-mail addresses: d.srinivas@ncl.res.in; p.ratnasamy@
oped. The reaction of amines, CO and alkyl halides
2
2
is an eco-friendly method for the production of carb-
amates (Scheme 1). This reaction is catalyzed by onium
ncl.res.in
0040-4039/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2006.04.057

4214
R. Srivastava et al. / Tetrahedron Letters 47 (2006) 4213–4217
(
1) Synthesis of cyclic and polycarbonates: R = -CH Cl, -CH or -C H
2
3
6
5
O
O
Polymerization
+
CO2
POLYCARBONATES
O
O
R
R
Cyclic carbonate
(
2) Synthesis of carbamates and polyurethanes: R' = alkyl or aryl
O
-
HBr
R'NH + CO2 + n-BuBr
R'NH
C
On-Bu
2
POLYURETHANES
Carbamate
2
Scheme 1. Chemical synthesis utilizing CO as a raw material instead of phosgene.
salts, basic catalysts, sterically hindered organic bases,
(5–8 h) (entries 6–10). Various heterogeneous catalysts
reported in the literature such as MgO and Mg–Al
mixed oxides are active only in the presence of solvents
such as DMF. In the industrial process for the manufac-
2
4
crown ethers or solid cesium carbonate. Earlier, we
2
5
found that titanosilicates and zeolite-encapsulated
metal complexes also catalyze this reaction. However,
2
4,25
4
in all these cases
the reaction requires a strong
ture of organic carbonates, a large excess of the solvent
donor solvent like DMF. We report in this study that
the as-synthesized form of MCM-41, as a mesoporous,
ordered silicate, is highly efficient for the synthesis of
carbamates even in the absence of any solvent.
dichloromethane is required. It may be noted that the
reaction proceeds over the present catalyst even without
solvent (Table 1, entries 1, 6 and 8). However, the cyclic
carbonate selectivity was slightly lower. This selectivity
was improved in solvents like DMF and CH CN, but
3
CH Cl and CH OH lowered the selectivity (Table 1,
2
2
3
2
. Results and discussion
entries 3 and 4). Without a catalyst, ECH conversion
was very low (only 14 mol %), moreover the product
did not contain any cyclic carbonate. When the organic
template was removed by calcination, both the catalytic
activity and selectivity for cyclic carbonates were drasti-
cally reduced (conversion = 15.3 mol %; selectiv-
ity = 73.2 mol %). Adsorption of epoxide (ECH) was
higher (by 1.6 times) on the as-synthesized form of
MCM-41 than on the calcined form.
2
6
As-synthesized MCM-41 was prepared as reported.
Cycloaddition of CO to an epoxide in the presence
of as-synthesized MCM-41 yielded a cyclic carbonate
as the main product and diol and ethers as minor prod-
ucts (Table 1). In the cycloaddition of epichlorohydrin
2
7
2
(
3
ECH) and CO , complete conversion was achieved in
2
h (Table 1, entries 1–5). Propene oxide (PO), styrene
oxide (SO) and butene oxide (BO) required longer times
The reaction did not take place at 323 K even after 12 h.
The epoxide (ECH) conversion and carbonate yield
increased with an increase in reaction temperature and
pressure (Fig. 1). A maximum yield of carbonate was
Table 1. Catalytic activity of as-synthesized MCM-41 in the cyclo-
addition of CO to epoxides
2
a
obtained at 393 K and 6.9 bar CO pressure. Most of
Entry Epoxide Solvent Run Epoxide
Cyclic
TOF
2
À1 c
the reported heterogeneous catalysts required a large
amount of catalyst. The present catalyst is highly effi-
cient and only 50 mg of the catalyst was required for
time conversion carbonate (h
h) (mol %) selectivity
)
(
b
(
%)
1
8 mmol of substrate.
1
2
3
4
5
6
7
8
9
0
ECH
ECH
ECH
ECH
ECH
PO
PO
SO
SO
BO
Nil
CH
CH
CH
DMF
Nil
3
3
3
3
3
5
5
8
8
5
99.0
98.6
99.6
99.8
80.0
89.0
83.2
62.7
100
93.7
100
95.2
64
64
64
64
64
36
35
23
22
37
3
2
3
CN
Cl
OH
The catalyst could be recycled several times, by centrifu-
gation, air-drying and then reuse without any further
treatment. MCM-41 was recycled eight times (Fig. 2).
Catalytic activity decreased after the fifth recycle. The
intensity of the XRD peaks also decreased after the fifth
recycle (Fig. 3). Hence, the reduced activity of MCM-41
after the fifth recycle is possibly due to the partial loss in
crystallinity/long-range ordering of the catalyst.
2
100
91.9
90.1
93.7
88.4
94.0
CH
Nil
CH
CH
3
CN
3
3
CN
CN
98.2
91.8
1
a
Reaction conditions: Epoxide, 18 mmol; solvent, 0 or 10 ml; catalyst,
0 mg; CO pressure, 6.9 bar; temperature, 393 K. ECH = epichloro-
hydrin; PO = propene oxide; SO = styrene oxide; BO = n-butene
5
2
Quaternary ammonium halides are used as catalysts in
the commercial process for synthesis of cyclic carbon-
ates.
oxide.
1
4,15
b
c
We compared these homogeneous catalysts
The remaining % selectivity includes the minor products, diols and
with our solid catalyst by performing catalytic activity
studies under our reaction conditions (393 K, 6.9 bar,
4 h). Catalytic activity (turnover frequency) increases
ethers.
Turnover frequency (TOF) = moles of epoxide converted per mole of
quaternary ammonium ion (template) in as-synthesized MCM-41 per
hour.
+
+
in the following order: Me N (12.4) < Et N (29.9) <
4
4

R. Srivastava et al. / Tetrahedron Letters 47 (2006) 4213–4217
4215
ECH conversion
Cyclic carbonate selectivity
ECH conversion
Cyclic carbonate selectivity
1
00
(
a)
(a) With solvent - CH3CN
1
00
8
6
4
2
0
0
0
0
0
8
6
4
2
0
0
0
0
0
3
33
353
373
393
423/T(K)
(
b) Without solvent
1
00
1
00
(
b)
8
6
4
2
0
0
0
0
0
8
6
4
2
0
0
0
0
0
0
1
2
3
4
5
6
7
8
1
.7 3.5 5.2 6.9
13.8/P(bar)
Number of recycles
Figure 1. Influence of temperature and pressure on the reaction of
Figure 2. Catalyst recyclability tests: synthesis of chloropropene
epichlorohydrin (ECH) with CO over as-synthesized MCM-41
catalyst. Reaction conditions: catalyst, 50 mg; ECH, 18 mmol;
CH CN, 10 ml; reaction time, 3 h. (a) CO pressure, 6.9 bar. (b)
2
2
carbonate from epichlorohydrin (ECH) and CO .
3
2
Temperature, 393 K.
+
+
Pr N
4
(42.9) < Bu N
(46.3) < cetyltrimethylammo-
4
2
0
nium ion (52.8). Since these salts are soluble in mix-
tures of reactants and products, special methods have
to be adopted for their separation and purification.
The MCM-41 catalyst could be separated simply by
filtration. As-synthesized MCM-41 contains cetyltri-
methylammonium cations as the template. The apparent
turnover frequency (TOF) is more for the solid catalyst
Fresh
(
TOF = 64) (Table 1, entry 2) than for the homogeneous
quaternary ammonium salt (TOF = 52). The structural
features of the solid catalyst play some role in the cata-
lytic activity. When the reaction was carried out over as-
synthesized zeolite-beta catalyst, we had to use three
times the amount of the catalyst than that used in the
present study.
th
4
recycle
recycle
th
6
2
0
th
8 recycle
2
3
4
5
6
7
8
9
10
2
θ
The coupling reaction of aniline, CO and n-butyl
bromide yielded two products: n-butyl N-phenylcarb-
amate as the major product and N,N-di-n-butylaniline
2
2
7
Figure 3. XRD of as-synthesized MCM-41: fresh catalyst and used
catalyst.
(
N-alkylation) as the minor product (Table 2). Various
N-alkyl and N-aryl carbamates could be synthesized
under mild conditions from the corresponding amines,
CO2 and n-butyl bromide (n-BuBr) over MCM-41
catalysts. Both aliphatic and aromatic amines could be
converted into their carbamates by this method. With
different amines, the carbamate yields varied in the order:
n-dodecylamine > cyclohexylamine > n-hexylamine >
benzylamine > aniline > 2,4,6-trimethylaniline (Table 2).
Aliphatic amines were more easily converted into
their corresponding carbamates compared to aromatic
amines. With all the known catalyst systems, the reaction
has to be carried out in a solvent medium (DMF); in
the absence of solvent, the N-alkylated compound
was formed as the main product. Interestingly, with as-
synthesized MCM-41, carbamates can be synthesized
with high selectivity (84–95%) even without using the
solvent.

4216
R. Srivastava et al. / Tetrahedron Letters 47 (2006) 4213–4217
a
Table 2. Solvent-free synthesis of carbamates over as-synthesized MCM-41
À1 b
Entry
1
Amines
Amine conversion (mol %)
100
Carbamate selectivity (%)
90.1
TOF (h
36.2
)
NH2
2
3
NH2
90.4
94.0
94.5
80.6
32.7
34.0
NH2
NH2
4
5
76.0
51.0
91.0
83.7
27.5
18.5
NH2
NH2
6
30.0
94.8
10.9
a
Reaction conditions: Amine, 10 mmol; n-butyl bromide, 10 mmol; catalyst, 150 mg; CO
Turnover frequency (TOF) = moles of amine converted per mole of quaternary ammonium ion (template) in as-synthesized MCM-41 per hour.
2
pressure, 3.4 bar; run time, 3 h; temperature, 353 K.
b
3
. Conclusions
J. Org. Chem. 2003, 68, 1559–1562; (c) Lu, X.-B.; Liang,
B.; Zhang, Y.-J.; Tian, Y.-Z.; Wang, Y.-M.; Bai, C.-X.;
Wang, H.; Zhang, R. J. Am. Chem. Soc. 2004, 126, 3732–
Cyclic carbonates and N-alkyl and N-aryl carbamates
were synthesized utilizing CO in excellent yields over
3
733.
2
8
. (a) Peng, J.; Deng, Y. New J. Chem. 2001, 25, 639–641; (b)
Kawanami, H.; Sasaki, A.; Matsui, K.; Ikushima, Y.
Chem. Commun. 2003, 896–897.
as-synthesized MCM-41, as a solid, mesoporous silicate
catalyst. The reactions occurred under mild conditions
and without using an additional co-catalyst or solvent.
The catalyst could be recovered and reused for up to five
recycles. This catalyst is superior to both the homo-
geneous and polymer resin-based quaternary ammo-
nium salts used hitherto for this reaction.
9
. Cal o´ , V.; Nacci, A.; Monopoli, A.; Fanizzi, A. Org. Lett.
2
002, 4, 2561–2563.
10. Yano, T.; Matsui, H.; Koike, T.; Ishiguro, H.; Fujihara,
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1
1
1
Acknowledgements
R.S. acknowledges the Council of Scientific and Indus-
trial Research (CSIR), New Delhi, for a fellowship. This
work forms a part of the Network project ‘Catalysis and
Catalysts’ (P23-CMM005.3) sponsored by CSIR, New
Delhi.
14. BASF starts alkylene carbonate plant (product review),
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2
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(Shimadzu QP-5000; with a 30 m long, 0.25 mm i.d. and
0.25 mm thick capillary column DB-1), GC-IR (Perkin–
Elmer 2000; BP-1 column; with a 25 m long and 0.32 mm
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manner using homogeneous quaternary ammonium halide
catalysts.
Synthesis of alkyl and aryl carbamates: In a typical
reaction, an amine (10 mmol), n-butyl bromide (10 mmol)
and as-synthesized MCM-41 (50 mg) were charged into a
300 ml Parr reactor. The reactor was then pressurized with
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2
004, 96, 127–133, Textural properties of MCM-41—as-
CO (3.4 bar) and the temperature was raised to 353 K.
Each reaction was conducted for 4 h. At the end, the
reactor was cooled to 298 K and unutilized CO was
2
2
2
synthesized form:
S
BET = 83 m /g; calcined form—
BET = 972 m /g, pore volume = 0.75 cc/g, average pore
diameter = 2.7 nm..
2
S
vented out. The catalyst was recovered from the reaction
mixture by filtration. The products were analyzed by thin
layer chromatography (TLC) and gas chromatography
2
7. Synthesis of cyclic carbonates: In a typical cycloaddition
reaction, epoxide (18 mmol), MCM-41 (50 mg) and sol-
vent (0 or 10 ml) were taken in a 300 ml Parr pressure
reactor. The catalysts were used in the as-synthesized and
dried (373 K) form and contained the organic template in
1
and identified by GC–MS, FT-IR and H NMR, as
described above. In some cases, the products were isolated
by column chromatography (silica gel 60–120 mesh; 98:2
petroleum ether:ethyl acetate mixture as eluent) and a
mass balance was established.
the cavities. The reactor was pressurized with CO
6.9 bar), then the temperature was raised to 393 K and
2
(