Metal acetylacetonates as highly efficient and cost effective catalysts for the synthesis of cyclic carbonates from CO2 and epoxides

Article abstract of DOI:10.1007/s10562-012-0803-7

Metal acetylacetonates were found to be efficient and cost effective catalysts for the formation of cyclic carbonates by cycloaddition of carbon dioxide with epoxides, providing high to excellent yields of the corresponding carbonates. Among the various c

Full text of DOI:10.1007/s10562-012-0803-7

Catal Lett (2012) 142:615–618
DOI 10.1007/s10562-012-0803-7
Metal Acetylacetonates as Highly Efficient and Cost Effective
Catalysts for the Synthesis of Cyclic Carbonates from CO2
and Epoxides
Subodh Kumar
•
Suman L. Jain Bir Sain
•
Received: 19 September 2011 / Accepted: 10 March 2012 / Published online: 27 March 2012
Ó Springer Science+Business Media, LLC 2012
Abstract Metal acetylacetonates were found to be effi-
cient and cost effective catalysts for the formation of cyclic
carbonates by cycloaddition of carbon dioxide with epox-
ides, providing high to excellent yields of the corre-
sponding carbonates. Among the various catalysts such as
acetylacetonates of Co, Ni, Cu, Zn, Fe, Cr and VO studied,
Ni(acac)2 was found to be promising catalyst for this
reaction. The present methodology was found to be supe-
rior due to the easy accessibility and comparatively inex-
pensive nature of metal acetylacetonates than salen
complexes.
greenhouse gas leading to global warming [1, 2]. It is mainly
released by burning fossil fuels/biomass as a fuel and by
certain industrial and resource extraction processes.
As a solution for this serious problem, the capture of
CO and its utilization for the value added chemicals is
2
particularly important rather than just dumping it [3–8].
The coupling of CO with epoxides to form cyclic car-
2
bonates is an important reaction as they are valuable
compounds that have applications in various fields. Owing
to their high solubility, high boiling, low toxicities, and
biodegradability, cyclic carbonates are utilized as aprotic
polar solvents in degreasing, paint stripping, and cleaning
[9, 10]. Methodologies for the formation of cyclic car-
Keywords Carbon dioxide Á Epoxide Á Cyclic carbonate Á
Acetylacetonate Á Cycloaddition
bonates utilizing CO as a chemical feedstock are well
2
established [11]. Many catalysts such as alkali metal salts
[
12], metal oxides [13], transition metal complexes [14,
1
Introduction
15], ion-exchange resins [16], functional polymers [17],
quaternary ammonium and phosphonium salts [17], ionic
liquids [18], lanthanide oxychloride [19, 20], cellulose/KI
[21] have been developed for the have been developed for
Climate change has attained significant prominence in last
two decades as atmospheric CO concentrations have indeed
2
increased by almost 100 ppm from pre-industrial level to the
present scenario. More than 80 % of our energy comes from
the combustion of fossil fuels and it is well-accepted that
carbon dioxide is the most prominent anthropogenic
the reactions of CO and epoxides to produce cyclic car-
2
bonates. Prominent among these, the most successful and
popular approach is using metal salens of Al [22, 23], Co
[24], Cr [25–27] and other metal derivatives [28, 29].
Nevertheless, concerning an industrial scale synthesis of
cyclic carbonates, development of low toxic, cost-effective
and high active catalytic system is particularly desired.
Metal acetylacetonates, easily accessible and relatively
inexpensive as compared to the salen complexes are sur-
prisingly remained unexplored as catalysts for the forma-
tion of cyclic carbonates.
This work has received best paper award in International Conference
on Unconventional Sources of Fossil Fuels and Carbon Management,
ICUSFFCM-2011.
S. Kumar Á S. L. Jain (&) Á B. Sain
Chemical Sciences Division, Indian Institute of Petroleum,
Council of Scientific and Industrial Research, Dehradun 248005,
India
In the present paper we describe a systematic study of
the catalytic activity of various metal acetylacetonates for
the synthesis of cyclic carbonates from the epoxides and
carbon dioxide under mild reaction conditions (Scheme 1).
e-mail: suman@iip.res.in
B. Sain
e-mail: birsain@iip.res.in
123

6
16
S. Kumar et al.
2
Results and Discussion
axial positions are free to bind with CO and epoxide in
2
situ during the reaction. In contrast, Co(III) has octahedral
geometry in which axial position are occupied and further
bonding of substrates to metal will be difficult and may
cause steric hindrance as well. Besides these experiments,
we also investigated the effect of temperature and pressure
on the cycloaddition of styrene oxide and carbon dioxide
under otherwise same reaction conditions (Table 1, entry
11–15). It was observed that the temperature and pressure
also played an important role on the yield of cyclic car-
bonate. At ambient temperature (25 °C), the reaction rate
At first, to evaluate the activity of various metal acetyl-
acetonates such as Co, Ni, Cu, Zn, Cr, VO and Fe, we
studied the cycloaddition of CO and styrene epoxide under
2
solvent free conditions. The results of these experiments
are presented in Table 1. The catalytic efficiency was
found to be mainly depended on the nature of the transition
metal and its oxidation state. Among the tested catalysts,
Ni(acac) was found to be best catalyst in terms of con-
2
version and selectivity of the desired compound. However,
Co(acac) , Zn(acac) and Cr(acac)2 were found to be
was found to be slow and depended on the CO pressure
2
2
2
moderate catalysts for this transformation. All the reactions
were carried out at in the presence of co-catalyst tetrae-
applied. The observed conversion was found to increase as
the CO pressure increased. At higher reaction tempera-
2
thylammonium bromide (NEt Br) at 50 °C and 50 bar
tures (60–80 °C), the selectivity of the reaction towards the
formation of carbonate was continuously decreased with
4
pressure for 4.5 h under solvent less condition in a 100 mL
stainless steel high pressure reactor (Fig. 1).
increasing the CO pressure. This is due to the formation of
2
Among the metal derivatives of Co, Cr and Ni, the
catalysts having the oxidation state II were found to be
more reactive as compared to the catalysts having III oxi-
dation state of the transition metal (Table 1, entry 1–4, 7,
corresponding polycarbonate by the accelerated polymeri-
8
). This is probably due to the geometrical and structural
difference of the ?2 and ?3 complexes. For example,
Co(acac) has square planner geometry and therefore its
2
O
O
M(acac)2
+
CO2
O
O
0
C H
50 C
6
5
5
0 bar
C H
6
5
M=Co, Cu, Cr, VO, Zn, Fe
Scheme 1 Synthesis of cyclic carbonates
Fig. 1 High pressure autoclave
Table 1 Reaction of styrene
a
b
Entry
Catalyst
pCO
2
/bar
T/°C
Reaction
time (h)
Conv. (%)
Yield (%)
oxide with CO
M(acac) catalysts and Et
at various conditions
2
using different
2
4
NBr
1
2
3
4
5
6
7
8
9
1
Ni(acac)
Ni(acac)
2
3
50
50
50
50
50
50
50
50
50
50
50
10
20
40
50
50
50
50
50
50
50
50
50
50
25
80
50
50
4.5
4.5
4.5
4.5
4.5
4.5
4.5
4.5
4.5
4.5
8.0
2.0
4.5
4.5
100
22.5
70.2
Trace
50
97
12
60
–
Co(acac)
Co(acac)
2
3
Zn(acac)
Fe(acac)
VO(acac)
2
37
–
2
Trace
Trace
21.7
80
2
2
–
Reaction conditions: styrene
oxide (10 mmol), catalyst
Cr(acac)
Cr(acac)
3
2
20
75
–
(
(
4
1 mol%, 0.1 mmol), n-Et NBr
0.1 mmol) without using
0
Mn(acac)
Ni(acac)2
Ni(acac)2
20
additional solvent for 4.5 h
a
Calculated based on the
remaining styrene oxide
11
12
50
35
25
60
75
50
1
1
3
4
Ni(acac)
Ni(acac)
2
2
70
b
Isolated yields of the cyclic
carbonate
85
1
23

Metal Acetylacetonates as Highly Efficient and Cost Effective Catalysts
617
zation between carbonates and CO under high pressure. It
2
Table 2 Ni(acac) catalyzed cycloaddition of various epoxides with
2
CO
was found that efficient and selective conversion into sty-
2
rene carbonate could be achieved at only 50 °C and 50 bar
Entry Epoxide
Product
Reaction
Time (h)
4.5
Yield
of CO under otherwise similar reaction conditions. When
2
b
(%)
the reaction of styrene oxide with CO was performed in
2
O
O
O
1
97
92
the presence of tetraethylammonium bromide (NEt Br)
4
O
alone without metal catalyst, the much lower conversions
2
0.2 % of the styrene carbonate was obtained under similar
O
O
O
O
2
4.0
reaction conditions. This finding established that the metal
catalyst is essentially required for the efficient conversion
in the present transformation. However, in a blank reaction
without tetraethylammonium bromide and metal catalyst,
there was no reaction occurred between styrene oxide and
O
O
3
3.5
2.5
90
98
O
O
O
O
4
O
O
CO under similar reaction conditions.
2
Next, we extended the scope of the reaction with a
O
O
5
3.0
6.0
94
65
variety of epoxides, we used Ni(acac) as catalyst and
2
Cl
O
O
O
Cl
studied the cycloaddition in the presence of co-catalyst
O
O
tetraethylammonium bromide at 50 °C under the 50 bar
6
O
pressure of CO without using any reducing agent under
2
solvent less conditions (Scheme 2). The results of these
experiments are summarized in Table 2. All the epoxides
were efficiently and selectively converted to the corre-
sponding carbonates in high to excellent yields.
O
O
7
2.0
2.0
98
98
n-Bu
O
O
n-Bu
O
O
8
It is worthy to mention that Ni-based catalytic systems
have been less explored for the formation of carbonates by
cycloaddition of epoxides and carbon dioxide. In this con-
text, Decortes et al. [30] reported a variety of Ni-based
catalytic systems to be effective for the synthesis of cyclic
carbonates from epoxides and carbon dioxide. However the
need of volatile organic solvents and longer reaction times
are the major drawbacks of these methods. Recently, De
Pasquale et al. [31] reported an efficient Ni (II) catalyst
system for the coupling of carbon dioxide and epoxides
under solvent-less conditions. Despite of the inexpensive
nature, high stability and higher catalytic reactivity, the need
of promoters such as triphenylphosphene and Zn powder as
a reducing agent, higher reaction temperature make this
method of less applicable. Thus, the method described here
is advantageous in many ways over the previous ones like,
O
O
Reaction conditions: substrate (10 mmol), catalyst (1 mol%,
.1 mmol), n-Et NBr (0.1 mmol) at 50 °C under pressure 50 bar
without using additional solvent
0
4
a
Isolated yields
unaffected and we could obtain the similar conversion and
yield of the cyclic carbonate during this course, indicating
that the catalytic cycle did not involve the insitu reduction
of Ni(II) to Ni (0) as described by De Pasquale et al. [31]
Thus, we assume that the reaction probably initiates by the
coordinating the metal complex with epoxide followed by
the attack of a nucleophilic group (chloride ion from
TEAB), leading to epoxide ring-opening and formation of a
metal-bound alkoxide [33]. In the subsequent step, inser-
easy availability and facile synthesis of Ni(acac) , mild
2
reaction conditions and solvent-less protocol.
tion of CO in M–O bond to form a metal-bound carbox-
2
The metal catalyst and co-catalyst tetraethylammonium
bromide both are essential for the present reaction. In the
presence of organic solvent such as acetonitrile the reaction
gave comparable results but solvent-less conditions were
found to be best and make this procedure more desirable
from environmental viewpoints.
ylate, followed by the production of cyclic carbonate as
shown in Scheme 3.
O
O
Ni(acac)2
The mechanism of the cycloaddition of epoxides with
carbon dioxide in presence of metal catalyst and quaternary
salts is well documented in the literature [32]. To under-
stand the mechanistic pathway of the reaction we also
performed the experiment by using Zn as reducing agent
under described reaction conditions. The reaction remained
+
CO2
O
O
R
Et NBr
4
0
50 C
0 bar
R
5
2 4
R=CH3, Ph, ClCH , -(CH ) -
2
2
Scheme 2 Ni(acac) -catalyzed synthesis of cyclic carbonates
123

6
18
S. Kumar et al.
-
Br-
Delhi is kindly acknowledged for financial support. Mr. Subodh
Thanks to CSIR, New Delhi for his research fellowship.
M
O
Et
4
NBr
M
Br
R
Br
R
O
O=C=O
O
O
M
O
O
O
-M
O
R
R
Et
4
N+
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3
Conclusion
4
5
6
In summary we have described a highly efficient and cost
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4
(
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3
588
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Acknowledgments We thank to Director IIP, for his kind permis-
sion to publish these results. Dr. Basant Kumar and his group is
acknowledged for providing GCMS analysis. Dr. G. M. Bahuguna is
acknowledged for providing IR spectra of the compounds. DST, New
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1
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