Aluminium based binary catalytic system for the solvent free conversion of CO2 to carbonates with high activity and selectivity

Article abstract of DOI:10.1039/c8ra00835c

An easily prepared and low-cost aluminium based metal complex catalyst was prepared using kojic acid (Hkoj) as a ligand, and this developed oxo-coordinated Al(koj)₃ complex showed high activity and selectivity for the CO₂ fixation reaction with epoxides under mild conditions without any organic solvents. Various cyclic carbonates were obtained in excellent yields (up to 99%). This stable catalytically active Al(koj)₃ has strong Lewis acidity for the activation of epoxides, and meanwhile the hydroxy group in Al(koj)₃ may play a role in boosting the catalytic activity through possible hydrogen bonding interactions with the epoxide.

Full text of DOI:10.1039/c8ra00835c

Aluminium based binary catalytic system for the
solvent free conversion of CO2 to carbonates with
high activity and selectivity†
*
Ran Ma,
Haojie Sun and Yuanzhi Cui
An easily prepared and low-cost aluminium based metal complex catalyst was prepared using kojic acid
(Hkoj) as a ligand, and this developed oxo-coordinated Al(koj)3 complex showed high activity and
selectivity for the CO2 fixation reaction with epoxides under mild conditions without any organic
solvents. Various cyclic carbonates were obtained in excellent yields (up to 99%). This stable catalytically
active Al(koj)3 has strong Lewis acidity for the activation of epoxides, and meanwhile the hydroxy group
in Al(koj)3 may play a role in boosting the catalytic activity through possible hydrogen bonding
interactions with the epoxide.
Introduction
Recent daily and weekly values of CO2 concentration measured
at Mauna Loa Observatory have remained above 400 parts per
million.1 This is a signicant symbolic event in the global
warming process, because part of global warming is attributed
to increasing carbon dioxide levels in atmosphere, which is
mainly caused by the combustion of huge amounts of fossil
fuels (such as coal, petroleum, and natural gas). The rising CO2
concentration may cause
a pronounced change to our
ecosystem because plant species are very sensitive to the
increasing CO2 concentration, as this will inuence

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Table 1 Catalyst screening for the synthesis of styrene carbonate from
a
styrene oxide with CO2
Scheme 1 Possible mechanism for the catalytic formation of cyclic
carbonates.
Entry
Cata./mol%
TBAI/mol%
Yieldb/%
1
2
3
4
5
6
7
Zn(koj)2/0.25
Mn(koj)2/0.25
Fe(koj)3/0.25
Al(koj)3/0.25
—
0.25
0.25
0.25
0.25
0.25
0.25
—
40
52
50
69
21
43
0
Hkoj/0.75
—
a
The reaction was carried out on a 5.0 mmol scale of styrene oxide at
b
100 ꢀC under
1 MPa CO2 (initial). NMR yield using 1,1,2,2-
Scheme 2 Multifunctional metal complexes used for the activation of
tetrachloroethane as the internal standard.
epoxides in this work.
radioprotective agents,12a moderately promoted the reaction
(40% and 52% yields, respectively, entries 1–2). An uncommon
iron complex, Fe(koj)3, also showed comparable reactivity (entry
3). Note that the reaction using Al(koj)3 as a catalyst gave a good
yield of 69% with the assistance of TBAI (entry 4). In the absence
of Al(koj)3, the yield dropped from 69% to 21% (entry 4 vs. 5). To
clarify the role of the hydroxy group in Al(koj)3, we also exam-
ined the reaction using Hkoj and a slightly higher yield of
carbonate was obtained (entry 5 vs. 6). This result may indicate
that the hydroxy group in Al(koj)3 plays a role in boosting the
catalytic activity through the possible hydrogen bonding inter-
actions with the epoxide.8d No reaction occurred without any
catalyst (entry 7).
Lewis acidities.9 In addition, oxo-coordinated metal complexes
also displayed high catalytic activity in the cycloaddition reac-
tion of CO2 with epoxides.10 In our previous work, a tetraoxo-
coordinated zinc complex, i.e. 2-hydroxypyridine N-oxide zin-
c(II), Zn(OPO)2, was used and demonstrated remarkable turn-
over frequencies (up to 22 000 hÀ1).11 Therefore, we continued
to consider highly active oxo-coordinated metal complexes for
the synthesis of cyclic carbonates.
Kojic acid (5-hydroxy-2-(hydroxymethyl)-4-pyrone, Hkoj) is
considered to be one of the safest food products and is widely
used as a food additive to prevent enzymatic browning.12 Hkoj
has chelating activity and its metal complexes have reasonable
hydrolytic stability, neutral charge and signicant lipophilicity.
Thus these kojic acid based metal complexes, e.g. Al(koj)3, may
have strong Lewis acidities for the activation of epoxides, and
meanwhile the hydroxy group in Al(koj)3 may play a role in
boosting the catalytic activity through the possible hydrogen
bonding interactions with the epoxide (Scheme 2). Hence, in
this manuscript, we conducted a thorough investigation to
explore the catalytic activity of these kojic acid based metal
complexes, which are easy to prepare, low cost, multifunctional
for the cycloaddition of CO2 with epoxides.
We continued our investigations by optimizing the reaction
parameters using Al(koj)3 as the catalyst (Table 2). Remarkably,
with increasing temperature from 100 ꢀC to 120 ꢀC, the yield of
styrene carbonate increased signicantly from 69% to 93%
(entry 1 vs. 2). The yield decreased sharply when the reaction
ꢀ
ꢀ
was conducted at 80 C, which thus suggested that 120 C was
the appropriate temperature for the cycloaddition reaction
(entry 2 vs. 3). From a thermodynamics point of view, the
cycloaddition of epoxides with CO2 is an exothermic reaction
and 120 ꢀC is an easy operation temperature for industrial
Results and discussion
Table 2 Further optimization reaction conditions for the cycloaddi-
tion reactiona
First, different kojic acid based metal complexes were prepared
according to methods reported in the literatures13 and the
structures were conrmed using FT-IR and elemental analysis.
The cycloaddition of styrene oxide with CO2 was chosen as
a benchmark reaction (Table 1). TBAI (tetrabutylammonium
iodide) was used as a co-catalyst for this transformation due to
its excellent nucleophilicity. The initial screening experiments
were conducted at 100 ꢀC, with 1 MPa CO2 for 10 h with
a catalyst loading of 0.25 mol%. The effect of different metal
centers in the complexes was evaluated for the given reaction.
Zn(koj)2 and Mn(koj)2, which have been reported as potential
Entry
Cata./mol%
T/ꢀC
Yieldb/%
1
2
3
4
5
6
Al(koj)3/0.25
Al(koj)3/0.25
Al(koj)3/0.25
Al(koj)3/0.15
Al(koj)3/0.1
Al(koj)3/0.05
120
100
80
120
120
120
95
69
13
94
72
66
a
Reaction conditions: styrene oxide (5.0 mmol), TBAI (0.25 mol%), and
b
1 MPa CO2 (initial). NMR yield with 1,1,2,2-tetrachloroethane as the
internal standard.
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production. Then, the effect of catalyst loading on the reaction
was studied. A quantitative yield was obtained even aer
decreasing the catalyst loading from 0.25 mol% to 0.15 mol%
(entry 4). Even at a loading of 0.05 mol%, a yield of 66% was
achieved (entries 5–6). Thus, we dened the optimal reaction
conditions for the cycloaddition as 0.15 mol% Al(koj)3 at 120 ꢀC
with 1 MPa CO2 under solvent-free conditions.
Table 3 Conversion of CO2 to functionalized carbonate catalyzed
with Al(koj)3/TBAIa
In order to evaluate the efficiency and general applicability of
this oxo-coordinated Al(koj)3 complex, a series of epoxides
(terminal and internal) was examined for the synthesis of
carbonates using CO2 in the presence of Al(koj)3 (Table 3).
Propylene carbonate is widely used as an electrolyte and alter-
native green solvent in organic syntheses. An excellent yield of
up to 99% was obtained using this developed catalyst system
(entry 1). Using TBAB (tetrabutylammonium bromide) instead
of TBAI also gave a quantitative yield of propylene carbonate.
Furthermore, the reusability of the Al(koj)3/TBAI system was
examined for the synthesis of propylene carbonate, and no
signicant loss in its catalytic activity was observed aer three
successive runs (entry 1). The results achieved mark this
protocol as an attractive method for potential future industrial
applications. Other terminal epoxides smoothly converted to
the corresponding cyclic carbonates in high yields and with
excellent selectivities (entries 2–6). As for disubstituted epox-
ides, e.g. isobutylene oxide and cyclohexene oxide, these showed
inferior activity even under harsh conditions (longer reaction
times and increased amount of catalyst) compared with
terminal counterparts, due to the increased steric hindrance
(entries 7–8).8c
Entry
1
Substrate
Yieldb/%
99(94), 94(91)c, 96(95)d
2
95(91)
3
4
91(89)
90(88)
5
94(91)
95(90)
42(40)
6
7e
8e
37(34)
a
Conditions: epoxide (5 mmol), Al(koj)3 (0.15 mol%), TBAI
Based on previous studies14 and the above experiments,
a plausible mechanism for this Al(koj)3 catalyzed coupling of
CO2 with epoxides was proposed and depicted in Scheme 3.
Lewis acid metal catalyst activates the epoxide, followed by
nucleophilic attack of the iodine anion from TBAI, to form the
metal-coordinated iodo alkoxide. Then, the addition of CO2
b
(0.25 mol%), 1 MPa CO2, 120 ꢀC, and 10 h. NMR yield with
1,1,2,2-tetrachloroethane as the internal standard. The yield in
parentheses is the isolated yield. TBAB instead of TBAI. Aer
three cycles using Al(koj)3/TBAI. Al(koj)3 (0.25 mol%), 2 MPa CO2,
and 15 h.
c
d
e
Scheme 3 Plausible reaction pathway for the coupling of epoxides with CO2 catalyzed by Al(koj)3/TBAI.
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leads to the formation of the cyclic carbonate and regeneration determined using NMR analysis with 1,1,2,2-tetrachloroethane
of the catalyst. Notably, the hydroxy group in Al(koj)3 may also as the internal standard. The products were puried using
play a role in boosting the catalytic activity through possible column chromatography with ethyl acetate–petroleum ether as
hydrogen bonding interactions with the epoxide.
the eluent. The precipitated catalyst was washed with diethyl
ether ve times and dried using a pressure blowing concen-
trator ready to be used directly for the next reaction.
Conclusions
In summary, oxo-coordinated Al(koj)3 was developed as an
efficient catalyst for the synthesis of carbonates from epoxides
with CO2 under mild conditions without any solvent. A series of
terminal epoxides were converted to the corresponding
carbonates in yields of up to 99%. Moreover, this developed
catalyst system can be reused at least three times with no
signicant decrease in the propylene carbonate yield. This
developed catalyst system shows great potential for industrial-
ization due to the fact that it is easily prepared, inexpensive, has
stable catalytic activity and requires reaction conditions that are
easily realized.
Conflicts of interest
There are no conicts to declare.
Acknowledgements
We thank Shaanxi University of Science and Technology for the
nancial support of this research (2016GBJ-20).
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General information
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General procedure for the cycloaddition reaction of epoxides
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