Nano-sized polydopamine-based biomimetic catalyst for the efficient synthesis of cyclic carbonates

Article abstract of DOI:10.1016/j.tetlet.2014.04.033

Polydopamine (PDA) is a biocompatible and biomimetic material. Herein, nano-sized PDA sphere was prepared and the combination of alkali metal halide and PDA was investigated as a catalyst for the synthesis of cyclic carbonates from epoxide and carbon dioxide. It was found that the activity of PDA could be obviously enhanced in the presence of alkali metal salts. After reaction, the catalyst and the products could be separated easily, and the catalyst was reusable. The origin of the high catalytic efficiency and the reaction mechanism were also discussed.

Full text of DOI:10.1016/j.tetlet.2014.04.033

Tetrahedron Letters 55 (2014) 3239–3243
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Nano-sized polydopamine-based biomimetic catalyst for the efficient
synthesis of cyclic carbonates
a,
Zifeng Yang a,b, Jian Sun a, Xiaomin Liu a, Qian Su a, Ying Liu a, Qian Li b, , Suojiang Zhang
⇑
⇑
a Beijing Key Laboratory of Ionic Liquids Clean Process, Key Laboratory of Green Process and Engineering, State Key Laboratory of Multiphase Complex Systems, Institute of
Process Engineering, Chinese Academy of Sciences, Beijing 100190, PR China
b School of Chemistry and Chemical Engineering, Henan University, Kaifeng 475001, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Polydopamine (PDA) is a biocompatible and biomimetic material. Herein, nano-sized PDA sphere was
prepared and the combination of alkali metal halide and PDA was investigated as a catalyst for the syn-
thesis of cyclic carbonates from epoxide and carbon dioxide. It was found that the activity of PDA could be
obviously enhanced in the presence of alkali metal salts. After reaction, the catalyst and the products
could be separated easily, and the catalyst was reusable. The origin of the high catalytic efficiency and
the reaction mechanism were also discussed.
Received 17 January 2014
Revised 26 March 2014
Accepted 10 April 2014
Available online 20 April 2014
Keywords:
Polydopamine
Ó 2014 Published by Elsevier Ltd.
Carbon dioxide
Cyclic carbonates
Synergistic catalysis
Alkali metal halide
Recently, the increasing pressure of CO2 released by human
being promotes a high requirement of C1 conversion. As one of
the most important branches of C1 chemistry, the synthesis of cyc-
lic carbonates via the reaction between CO2 and epoxides
(Scheme 1) was investigated intensively,1 and thus various cata-
lysts have been developed so far including alkali metal halides
based binary catalysts,2–7 and biomass supported catalysts.8 It is
worth mentioning that the combinations of low cost, stable, and
nucleophilic alkali metal halides with promoters such as b-CD,2
cellulose,3 formic acid,4 lignin,5 amino acids,6 and H2O7 provide a
good chance in developing cheap and environmentally benign cat-
alysts. It is believed that continuous efforts on the development of
new promoters would achieve further enhancement in efficiency
under moderate conditions.
Inspired by mussels marine animals which can be glued on the
rocks by their mussel adhesive proteins, most recently, Lee et al.
introduced a distinctive approach to surface modification in which
self-polymerization of dopamine (Fig. 1c) produced an adherent
polydopamine (PDA) coating on a wide variety of materials by a
simple dip-coating with dopamine solution.9 The formed surface
modification method has been explored for many applications,
including improving the hemocompatibility of biomaterials and
tuning the cell behaviors on their surfaces,10,11 Li-ion batteries,12
enzyme immobilization,13 controlled drug release, and water treat-
ment.14,15 Thus, it becomes a popular method to confer multi-func-
tionality to solid–liquid interfaces through the formed
biocompatible PDA thin films.9 Moreover, PDA could be considered
as a good promoter for the synthesis of cyclic carbonates because
of abundant hydroxyl groups and active N (ANH, AN@) in the
structure (Fig. 1b).4,7
Development of cheap, efficient, and environmentally benign
catalysts for the synthesis of cyclic carbonates under solvent-free
conditions is still an attractive topic. Encouraged by the advanta-
geous properties of PDA, herein, we conducted the reactions in
the presence of potassium halide and PDA under solvent-free con-
ditions for the first time, and found that the catalyst showed excel-
lent synergetic effect on the reactions, by which high yields could
be obtained for mono- and di-substituted epoxides. We believe
that this efficient, stable, and ecologically safe route to synthesize
cyclic carbonates has great potential in industrial application.
O
Cat.
O
+
O
CO2
O
T, P
R
R
R=H, CH2,CH2Cl,C4H9,Ph, etc
⇑
Corresponding authors. Tel.: +86 378 3881589 (Q.L.); tel./fax: +86 10 82627080
(S.Z.).
Scheme 1. Cyclic carbonates synthesis.
E-mail addresses: liq@henu.edu.cn (Q. Li), sjzhang@home.ipe.ac.cn (S. Zhang).
http://dx.doi.org/10.1016/j.tetlet.2014.04.033
0040-4039/Ó 2014 Published by Elsevier Ltd.

3240
Z. Yang et al. / Tetrahedron Letters 55 (2014) 3239–3243
The PDA was synthesized according to the method described in
more, the effects of halide anion and alkali metal cation on the
reaction in the presence of PDA were tested. It was found that
the catalytic activity improved with the increase in leaving ability
of halide anion and alkali metal cation (entries 3–8). For example,
with the leaving ability increasing from ClÀ, BrÀ to IÀ, the corre-
sponding activity order of anion is ClÀ < BrÀ < IÀ (entries 3–8). Also
the activity order of cation is Na+ < K+, which is in accordance with
their leaving abilities (entries 3–8) (see SI, Table S1). Therefore, it
can be deduced that the leaving ability is dominant for the activity.
In addition, the performance of other bases, such as Na2CO3, K2CO3,
and K3PO4, combined with PDA is provided in Table 1, entries 11–
13. The results showed that their activities are unsatisfactory.
Thereafter, the influence of the molar ratio of KI to PDA on the yield
of PC was also investigated (Table 1, entries 5, 9, 10) with the total
catalyst amount kept constant. Maximum result was obtained at
the molar ratio of KI to PDA 1:1. Based on the result, KI/PDA (1:1,
molar ratio) was selected for further investigation.
Figure 1a.16 The corresponding results were list as follows. From
Figure 2, it can be seen that the size of nano-PDA could be varied
from 180 to 630 nm by controlling the dropping rate of ammonia
to dopamine. The Fourier transform infrared (FTIR) spectrum of
PDA in Figure 3a showed the characteristic spectral peaks of sev-
eral functional groups, such as NAH, AOH, CAO, C@N, and CAN.
XPS results in Figure 3b–d showed that groups such as CAN,
C@N, CAO, and NAH could be found in the structure of the corre-
sponding PDA nano-particles. The above results confirmed the
structure of synthesized PDA material.
After the structure characterization, the activity of different
PDA based catalysts was tested using the reaction of propylene
oxide (PO) and CO2, and the results are summarized in Table 1.
PDA alone showed a little activity for this reaction, and almost
no product was detected (entry 1). When using KI alone, the activ-
ity is still low (entry 2). It is surprising that the presence of alkali
metal halides could result in an obvious increase in the activity
of PDA (entries 3–5), and a highest PC yield of 72% as well as
99% selectivity could be realized with KI/PDA (entry 5). Further-
Thereafter, the effect of various conditions on the reaction for
the synthesis of PC was carried out using KI/PDA as the catalyst.
As can be seen in Table 2, the reaction temperature has an obvious
OH
HO
a
b
NH
n
N
HN
or
n
H
Room temperature
OH
OH
N
HO
OH
OH
HO
+
NH4OH
Water and ethanol
HO
HO
NH2
c
Polydopamine
Dopamine
=
Figure 1. (a) Schematic illustration of the synthesis of PDA. (b) The suggested structure of PDA. (c) The chemical structure of dopamine.
Figure 2. SEM images of PDA with different diameters (from a to d: 630, 450, 320, and 180 nm).

Z. Yang et al. / Tetrahedron Letters 55 (2014) 3239–3243
3241
a
b
80
60
40
20
O 1s
C 1s
N 1s
C-N
C=N
N-H O-H
C=C
100
200
300
400
500
600
700
800
500
1000
1500
2000
2500
3000
3500
Wavenumbers (cm-1
)
Binding energy (ev)
c
d
Sp2 C
C-N
-N=
N-H
C-O
C=N
396
398
402
404
Binding en4e0rg0y (ev)
280
282
284
286
288
290
292
294
Binding energy (ev)
Figure 3. Characterization of PDA, (a) FTIR spectrum, (b) XPS spectrum, (c) C 1s XPS spectrum and (d) N 1s XPS spectrum of PDA.
Table 1
Screening of catalysts for the synthesis of propylene carbonatea
Table 2
Optimization of reaction conditionsa
Entry
Catalyst
PO conv.b (%)
PC selec.b (%)
Yield (%)
Entry
t (h)
T (°C)
P (MPa)
Yieldb (%)
1
2
3
4
5
6
7
8
PDA
KI
Trace
5
10
13
73
9
10
39
42
53
4
99
99
99
98
99
99
99
98
98
99
99
99
97
Trace
5
10
13
72
9
10
38
41
52
4
1
2
3
4
5
6
7
8
9
1
1
1
1
0.66
1.5
1.5
1.5
1.5
2
110
130
140
120
120
120
120
120
120
120
130
2
2
2
2
2
2
1
3
4
1
2
30
91
96
72
47
99
74
97
98
92
65
KCl/PDA
KBr/PDA
KI/PDA
NaCl/PDA
NaBr/PDA
NaI/PDA
KI/PDA
9c
10d
11
12
13
KI/PDA
10
11
K2CO3/PDA
Na2CO3/PDA
K3PO4/PDA
0.66
2
5
2
5
a
Reaction conditions: PO (10 mmol), PDA (10 mg), KI (10 mg).
Determined by GC.
b
a
Reaction conditions: PO (10 mmol), PDA (10 mg, 0.06 mmol), KI (10 mg,
0.06 mmol), other metal halide (0.06 mmol), 120 °C, 2 MPa, 1 h.
b
Determined by GC.
PDA (6.67 mg), KI (13.33 mg).
PDA (13.33 mg), KI (6.67 mg).
c
because PO was also a reactant. Such an effect of CO2 pressure on
catalytic activity has been observed in other catalytic systems.3–6
With PO as the substrate, experiments were carried out to
examine the recyclability of KI/PDA at 120 °C and 2 MPa in 1.5 h.
As shown in Figure 4, no decrease in PC yields for six repeated runs
indicates the high stability of the catalyst. In addition, the SEM and
FT-IR spectrum proved the stability of the catalyst (Figs. 5 and S1).
The above results indicated that KI/PDA was an effective cata-
lyst for the synthesis of PC from PO and CO2. Next, examinations
were carried out in order to screen the applicability of the catalyst
to various epoxides, and the results are summarized in Table 3.
Among the mono-substituted terminal epoxides (1a–1f), it was
found that with the growth of the substituent chain, the activity
order of epoxides is 1a > 1b > 1c > 1d. On the other hand, 1f was
d
effect on the reaction. With temperature increasing from 110 to
140 °C, PC yield increased sharply from 30% to 96% (entries 1–4).
Also, prolonging reaction time from 0.66 to 2 h could enhance PC
yield from 47 to 99% (entries 4–6, 10, 11). Table 2 also illustrates
the effect of pressure on this reaction in the range of 1–4 MPa.
The results suggested that the optimized reaction pressure was
2 MPa (entries 6–9). The decrease of PC yield in the high pressure
of 4 MPa might be because that too high pressure would reduce PO
conversion because of lowered PO concentration in the vicinity of
the catalyst, a condition which was not favorable to the reaction

3242
Z. Yang et al. / Tetrahedron Letters 55 (2014) 3239–3243
Table 3
PC yield
PC selectivity
Cycloaddition of CO2 to terminal epoxides catalyzed by KI/DPAa
100
80
60
40
20
0
Entry Epoxide
Product
Time
(h)
Conv./
selec.b
(%/%)
O
O
1a
O
O
1
1
99/99
99/99
2a
O
O
1b
O
O
2
3
4
5
1.5
O
2b
O
O
O
O
CH2Cl
1c
O
5
5
5
95/98
71/98
35/99
1
2
3
4
5
6
7
CH2Cl
O
2c
Recycled times
Bu
1d
O
O
Figure 4. Recycling experiment for KI/PDA. Reaction conditions: PO (10 mmol),
PDA (10 mg), KI (10 mg), 120 °C, 2 MPa CO2, 1 h.
Bu
O
2d
Ph
O
1e
good substrates to give cyclic carbonates in good yield and with
excellent selectivity. However, in the case of 1e, the reaction was
carried out with a relatively low yield. To further exemplify the
catalytic potential of KI/PDA, disubstituted epoxides were also
tested (Table 3, entries 7–9). Disubstituted epoxides such as 1g
showed lower activity than mono-substituted terminal epoxides
1a–1f (entries 7 vs 1–6), and required a longer time to give 2g.
In particular for 1h, only 9% cyclic carbonate yield was achieved
after 10 h, which is probably due to its higher steric hindrance.17
As proposed by previous reports, hydroxyl groups in the cata-
lyst play a possible role in accelerating the ring-opening of epoxide
by hydrogen bonding.4,18 Also it was reported that the signal of the
H atom in the hydroxyl group shifts to the downfield in 1H NMR
spectra after hydrogen bonding with epoxide (e.g., PO).18 In order
to make an understanding on the interaction of KI/PDA catalyst
and epoxide, NMR detection was carried out using dopamine and
PO as model compounds. The 1H NMR spectra of dopamine deter-
mined indicated that PO can form hydrogen bond with the –OH
groups of dopamine (see SI, Fig. S2). For further confirmation, the
corresponding interactions between dopamine and PO were simu-
lated through the density functional theory (DFT) study (see SI,
Scheme S1), which was in accordance with the NMR result.
Based on the discussions above, a possible mechanism for the
formation of cyclic carbonates was proposed (Scheme 2). Firstly,
the AOH groups of PDA facilitate possibly an electrophilic interac-
tion with the oxygen of the epoxide via hydrogen bonds (a). And
the anion of KI gives a nucleophilic interaction to the b-carbon of
O
O
Ph
2e
O
O
O
O
6
5
99/98
O
1f
2f
O
O
1g
O
7
8
O
10
10
30/98
9/99
2g
O
O
O
1h
O
2h
a
Reaction conditions: epoxide (10 mmol), PDA (10 mg), KI (10 mg) 120 °C, CO2
2 MPa.
b
Determined by GC–MS.
the epoxide (a). The synergistic catalysis roles played by KI and
PDA make an easy ring-opening of epoxide. Due to the potential
function of PDA on CO2 activation19, an alkyl carbonate salt formed
(c) in parallel would react with ring-opened intermediate (b) to
afford another alkyl carbonate salt (d). Finally, the cyclic carbonate
× 10, 000
a
b
× 10, 000
1µm
1µm
Figure 5. SEM results for PDA before (a) and after 6th use (b).

Z. Yang et al. / Tetrahedron Letters 55 (2014) 3239–3243
3243
HO
OH
HO
HO
HO
OH
n
HO
OH
N
H
HN
NH
H
N
HN
OH
OH
HN
HO
OH
NH
O
O
+
KI
O
HO
OH
O
R
NH
O
HO
OH
NH
O
(a)
H H
HN
O
O
O
K+
I-
(d)
CO2
H H
R
K+
O
O-
C
I
N
HO
O-
O
+H
R
NH
O
C
HO
(c)
H
O-
O
O
H
R
K+
(b)
I
Scheme 2. A plausible reaction mechanism for the cycloaddition of CO2 with epoxide catalyzed by KI and PDA.
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is formed by subsequent intermolecular ring-closure and the cata-
lyst is regenerated.
In conclusion, KI/PDA binary catalytic system was used for the
synthesis of cyclic carbonates from epoxide and CO2. The size of
PDA could be easily controlled and showed a remarkable effect
on the reaction due to the excellent synergetic effect of PDA and
KI. Especially, the hydrogen bonding between O atom in PO and
hydroxyl groups of PDA can promote the reactions effectively. In
addition, the catalyst could also be applicable to other terminal
or disubstituted epoxides. The process represents a simple, ecolog-
ically safer, and cost-effective route to synthesize cyclic carbonate
with high product quality, as well as easy catalyst recycling.
Acknowledgements
The authors are grateful to the support by the National Basic
Research Program of China (2009CB219901), and National Science
Fund of China (21006117, 21006112 and 21306098).
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Supplementary data
Supplementary data associated with this article can be found, in
theonline version, at http://dx.doi.org/10.1016/j.tetlet.2014.04.033.
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