Efficient fixation of CO2 by a zinc-coordinated conjugated microporous polymer

Article abstract of DOI:10.1002/cssc.201402162

Zinc-coordinated conjugated microporous polymers (Zn-CMPs), prepared by linking salen zinc and 1,3,5-triethynylbenzene, exhibit extraordinary activities (turnover frequencies of up to 11600 h^-1), broad substrate scope, and group tolerance for the synthesis of functional organic carbonates by coupling epoxides with CO₂ at 120 °C and 3.0 MPa without the use of additional solvents. The catalytic activity of Zn-CMP is comparable to those of homogeneous catalysts and superior to those of other heterogeneous catalysts. This catalyst could be reused more than ten times without a significant decrease in performance.

Full text of DOI:10.1002/cssc.201402162

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DOI: 10.1002/cssc.201402162
Efficient Fixation of CO2 by a Zinc-Coordinated Conjugated
Microporous Polymer
Yong Xie,[a, b] Ting-Ting Wang,[b] Rui-Xia Yang,[b] Nian-Yu Huang,[a] Kun Zou,*[a] and Wei-
Qiao Deng*[b]
Zinc-coordinated conjugated microporous polymers (Zn-CMPs),
prepared by linking salen zinc and 1,3,5-triethynylbenzene, ex-
hibit extraordinary activities (turnover frequencies of up to
11600 hꢀ1), broad substrate scope, and group tolerance for the
synthesis of functional organic carbonates by coupling epox-
ides with CO2 at 1208C and 3.0 MPa without the use of addi-
tional solvents. The catalytic activity of Zn-CMP is comparable
to those of homogeneous catalysts and superior to those of
other heterogeneous catalysts. This catalyst could be reused
more than ten times without a significant decrease in per-
formance.
Recently, several efficient homogeneous catalysts have been
reported in this area. North and co-workers used a bimetallic
aluminum (salen) complex, which could catalyze the reaction
of CO2 with propylene oxide (PO) at room temperature and at-
mospheric pressure.[4] The Sakai group developed a magnesi-
um-based porphyrin (Mg-porphyrin) that catalyzed a terminal
epoxide at 1208C and 1.7 MPa with TOFs at as high as
12000 hꢀ1 [5]
. A more recent work reported by Kleij et al. identi-
fied an aluminum complex that showed high activity (TOFs up
to 36000 hꢀ1) and broad substrate scope even for internal ep-
oxides.[6] Although significant progress has been made, two
major challenges remain in this area. First, the efficient homo-
geneous catalysts suffer from separation problems whereas
the heterogeneous catalysts have low catalytic activities for
this reaction; thus, an efficient heterogeneous catalyst is highly
demanded. Second, to the best of our knowledge, the report-
ed heterogeneous catalysts were only effective for terminal ep-
oxides; therefore, it is a big challenge to develop a catalyst
that can transform internal epoxides.
Conjugated microporous polymers (CMPs)[7] are highly flexi-
ble porous materials that may be tailored for various applica-
tions such as catalysis,[8] gas storage,[9] light harvesting[10] lumi-
nescence,[11] sensors,[12] superhydrophobic separations,[13] and
supercapacitors.[14] On combining these various properties, we
expect to obtain new functionalities for CMPs. For example,
we have developed a cobalt-functionalized CMP that can cap-
ture CO2 and catalyze its conversion at room temperature and
atmospheric pressure.[15]
In recent years, concerns about global warming have boosted
interest in the chemical fixation of CO2, which can be regarded
as the most environmentally abundant C1 building block avail-
able.[1] More specifically, the cycloaddition of CO2 to epoxides,
which yields useful cyclic or polycarbonate products,[2] is of
particular interest as it represents a much greener alternative
to the use of phosgene. Cyclic carbonates are an important
class of chemical products used for various purposes, for exam-
ple, as electrolytes in lithium-ion batteries, as raw material for
plastics, as valuable raw materials and intermediates in the
production of pharmaceutical and fine chemicals, and as envi-
ronmentally friendly nontoxic solvents; therefore, a variety of
industrial processes for the production of these carbonates
have been developed. However, their preparation often suffer
from low catalyst reactivity with turnover frequencies (TOFs)
rarely >200 hꢀ1 even at elevated temperatures (>1308C), air
sensitivity, and the use of additional solvents, high pressure,
and high temperature conditions.[3]
Herein, we report a highly efficient heterogeneous catalyst,
a zinc-coordinated conjugated microporous polymer (hereafter
referred to as Zn-CMP), which is synthesized by linking salen
zinc with 1,3,5-triethynylbenzene. This Zn-CMP exhibits extraor-
dinary catalytic activity (TOFs up to 11600 hꢀ1) towards the
coupling of CO2 and epoxides, including terminal and internal
[a] Y. Xie, Dr. N.-Y. Huang, Prof. Dr. K. Zou
Hubei Key Laboratory of Natural Products Research
College of Chemistry and Life Sciences,
China Three Gorges University
Yichang 443002 (PR China)
E-mail: kzou@ctgu.edu.cn
[b] Y. Xie, T.-T. Wang, R.-X. Yang, Prof. Dr. W.-Q. Deng
State Key Laboratory of Molecular Reaction Dynamics
Dalian National Laboratory for Clean Energy
Dalian Institute of Chemical Physics
Chinese Academy of Science
457 Zhongshan Road, Dalian 116023 (PR China)
Fax: (+86)411-84675584
E-mail: dengwq@dicp.ac.cn
Supporting Information for this article is available on the WWW under
http://dx.doi.org/10.1002/cssc.201402162.
Scheme 1. Synthesis of functional organic carbonates from CO2 and epox-
ides.
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sp1-hybridized ꢀCꢁCꢀ sites. The peaks at 40.0–
60.0 ppm were ascribed to sp3-hybridized ꢀCH2 sites
and ꢀCH sites in the cyclohexane moiety. The peak
with the lowest intensity, at 10.0–20.0 ppm, was as-
cribed to ꢀCH3 groups.
Zn-CMP also exhibits excellent material properties,
and the results are summarized in Table S2. The po-
rosity in the polymer was investigated by N2 adsorp-
tion analyses at 77.4 K (See Figure S4). The Brunauer–
Emmett–Teller (BET) surface area determined from
the N2 sorption data is 791 m2 gꢀ1, and the total pore
volume of Zn-CMP is 1.81 cm3 gꢀ1, which is substan-
tially larger than that of many high-pore-volume
metal organic frameworks (MOFs), including
NOTT-112
([Cu3(L)-(H2O)3]·8DMSO·15DMF·3H2O)
(1.62 cm3 gꢀ1)[16a] and Mg-MOF-74 (0.62 cm3 gꢀ1).[16b]
Because of its high BET surface area and pore
volume, Zn-CMP exhibits an excellent CO2 adsorption
capacity as shown in Table S2. According to adsorp-
tion isotherms, the uptake of CO2 reached
58.4 mggꢀ1 at room temperature and atmospheric
pressure (Figure S5). The CO2 adsorption capacity of
Zn-CMP is comparable to those of the CMPs reported
(59.4, 36.1, 71.3, 66.0, and 55.4 mggꢀ1 for 1, 2, 3, 4
and 5,[17] respectively; detailed structures are shown
in Figure S6). The comparison indicates that Zn-CMP
is a good CO2 adsorption material among CMPs. We
expect that the CO2 adsorption ability of this polymer
would enhance the local concentration of CO2
Scheme 2. Synthesis route for Zn-CMP.
derivatives and is the most efficient heterogeneous catalyst re-
ported thus far (Scheme 1). The catalytic activity remains high
even after more than ten reuses.
To prepare Zn-CMP, we first synthesized the dibromo-func-
tionalized precursor monomers salen zinc by a complexation
reaction of (R,R)-N’N’-bis(5-bromo-3-tert-butylsalicylidene)-1,2-
cyclohexanediamine (salen) and ZnEt2 (1.0m solution in
hexane). The resulting precursor was co-polymerized with
1,3,5-triethynylbenzene in toluene under reflux to produce Zn-
CMP (Scheme 2). The synthetic details are given in the Sup-
porting Information. Zn-CMP is insoluble in almost all of organ-
ic solvents and stable in strong acids and bases (Supporting In-
formation, Table S1). Its pyrolysis temperature is higher than
3008C (Figure S1).
Figure 1. (a) SEM image of Zn-CMP (scale bar: 2.0 mm). (b) HR-TEM image of
Zn-CMP (scale bar: 2.0 nm).
around the catalytic centers inside the polymer and thus im-
prove its catalytic activity. The content of zinc in the polymer is
6.60 wt%, which is less than the theoretical value 9.56 wt%
(Table S2). The presence of OꢀH vibrations (3280 and
1440 cmꢀ1) in FTIR spectra of Zn-CMP indicates that some
parts of the salen units in the polymer are not coordinated to
the zinc ions (Figure S7), which may be caused by metal leak-
ing during the synthesis procedure.
Scanning electron microscopy (SEM) and high-resolution
transmission electron microscopy (HR-TEM) were used to
record the morphological structure of Zn-CMP. The SEM image
shown in Figure 1a reveals that Zn-CMP consists of submicron
spheres, and HR-TEM picture (Figure 1b) shows that the pores
on the polymer surface are roughly 0.5 nm in diameter. Ac-
cording to these images (additional images are shown in Fig-
1
ure S2), the polymer was featureless at those length scales. H–
Zn-CMP was initially used as a solid catalyst for the cycload-
dition of CO2 to PO with n-tetrabutylammonium bromide
(TBAB) as co-catalyst. At a Zn-CMP loading of 0.2 mol% and
a TBAB loading of 1.8 mol%, a 94.1% yield of propylene car-
bonate was achieved with TOF=470 hꢀ1 (entry 5, Table 1),
which is higher than that achieved by using the corresponding
13C cross-polymerization/magic angle spinning (CP/MAS) solid-
state nuclear magnetic resonance (NMR) was used to identify
the structures of Zn-CMP (Figure S3). The peaks at 120.0–
150.0 ppm corresponded to the CꢀC sites of the polymer’s
benzene rings. The peaks at 70.0–100.0 ppm were ascribed to
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phosphine) iminium] exhibited the highest TOF (36000 hꢀ1) for
4-butyl-1,3-dioxolan-2-one formation from 1,2-epoxyhexane at
908C and 1.0 MPa CO2 pressure. For the same reaction, the
TOF for using Mg-porphyrin as catalyst could reach 12000 hꢀ1
at 1208C and 1.7 MPa CO2 pressure. The TOF using Mg-porphy-
rin as catalyst was only 3300 hꢀ1 for the reaction of PO with
CO2, which indicated that that the reaction of 1,2-epoxyhexane
with CO2 is easier than that with PO. The homogeneous cata-
lyst Zn-complex (Ni(PPh3)Cl2/PPh3/Zn)[19a] and zinc-containing
ionic liquid ([EMIm]2[Br2Zn(Et2PO4)2])[19b] were also good cata-
lysts for the synthesis of propylene carbonate with the TOFs
up to 3544 and 2362 hꢀ1, respectively. The ZnBr2/hexabutylgua-
nidinium bromide catalyst[19c] exhibited an activity (TOFs up to
8670 hꢀ1) for the coupling reaction of CO2 with PO. We found
that Zn-CMP exhibited an activity (TOFs up to 11600 hꢀ1) com-
parable to that of most reported homogeneous catalysts for
this reaction.
Table 1. Synthesis of propylene carbonate from CO2 and PO.[a]
Entry
Amount added [mol%]
Zn-CMP TBAB
Yield[b]
[%]
TOF[c]
[hꢀ1
]
1[d]
2
3[e]
4[f]
5
6
7
8
9
10
11
12
13[g]
salen zinc (0.2) 1.8
72.5
10.2
11.5
30.9
94.1
74.8
55.3
47.1
43.4
38.1
35.7
29.0
76.1
362
51
13
0.2
0
0
0.9
1.8
1.8
1.8
1.25
1.25
1.25
1.25
0.9
0.9
1.8
CMP
0.2
0.1
470
748
0.05
0.02
0.01
0.0075
0.005
0.0025
0.2
1100
2350
4340
5080
7140
11600
8
[a] Reaction conditions: PO (50.0 mmol, 2.90 g), 1208C, 3.0 MPa initial CO2
pressure, 1 h, 40 mL autoclave. [b] Yields of isolated product obtained
after column chromatography. [c] TOF=mol of PC produced per mol of
Zn catalytic center per hour. [d] Zn-CMP was replaced with [(R,R)-N’N’-
Table S5 shows the comparison between Zn-CMP and the
representative heterogeneous catalysts. These catalyst systems,
including zeolites and MOFs, showed good catalytic activities
for the cycloaddition of CO2 with PO or styrene oxide. Zeolites
such as beta[20a] and MCM-41[20b] were reported to have decent
catalytic activities in the cycloaddition reaction of CO2 with PO
to form propylene carbonate at 1208C and 0.69 MPa CO2. A
long reaction time (5 h) was needed to obtained decent yields
of propylene carbonates (>90%). Several MOFs, such as Mg-
MOF-74[16b] and Co-MOF-74,[20c] also exhibited good catalytic
activities in the cycloaddition of CO2 with styrene oxide to
form styrene carbonate at 2.0 MPa CO2 and 1008C with
>95.0% yields. To compare Zn-CMP and Mg-MOF-74, we car-
ried out experiments under the same experimental conditions
and found that the reaction catalyzed by Zn-CMP required 3 h
whereas Mg-MOF-74 needed 4 h to achieve the same yields.
To investigate the recycling stability of Zn-CMP, the experi-
ments were repeated with a large number of iterations. We
found that Zn-CMP could be reused more than ten times with-
out significant decrease of catalytic activity (yields decreased
slightly from 98.2% to 93.1%) at 1208C and 3.0 MPa CO2 (Fig-
ure S8).
bis(3-tert-butyl-salicylidene)-1,2-cyclohexanediamine]
(salen
zinc).
[e] TOF=mol of PC produced per mol of Brꢀ per hour. [f] 100 mg CMP.
[g] Reaction conditions: PO (50.0 mmol, 2.90 g), 258C, 0.1 MPa initial CO2
pressure, 48 h.
homogeneous catalyst salen-zinc (362 hꢀ1; entry 1, Table 1).
The possible reason for such enhancement of catalytic activity
is that the nanopores inside the polymer absorb CO2 and
enrich the local concentration of CO2 near the catalytic centers
resulting in a high catalytic performance of Zn-CMP. The en-
hancements of catalytic activities caused by nanopore confine-
ment have been reported in other systems.[18] In the absence
of a co-catalyst, only a yield of 10.2% propylene carbonate
was obtained (entry 2, Table 1). In the absence of Zn-CMP, the
cycloaddition catalyzed by the co-catalyst TBAB alone gave rise
to a yield of 11.5%–17.6% (entry 3, Table 1 and entries S1–S3,
Table S3). The salen unit without zinc showed a weak catalytic
activity (entry 4, Table 1). With the use of reduced amounts of
Zn-CMP and co-catalyst (entries 6–12, Table 1), considerable
conversions could still be achieved, resulting in high TOFs of
up to 11600 hꢀ1 per mole of zinc loading (entry 12, Table 1).
Moreover, at a Zn-CMP loading of 0.2 mol% and a TBAB load-
ing of 1.8 mol%, a 76.1% yield of propylene carbonate was
achieved with TOF 8 hꢀ1 at room temperature and atmospheric
pressure. (Entry 13, Table 1). This TOF is comparable to that of
the homogeneous bimetallic aluminum (salen) catalyst, which
was 2–10 hꢀ1 calculated from experimental data in Ref [8b]
under the same conditions.
After establishing the high catalytic activity of Zn-CMP for
propylene carbonate formation (Table 1), we then investigated
its reactivity for the cycloaddition of CO2 into various highly
functionalized terminal epoxides to form the organic carbo-
nates 2b–2k (Scheme 3). Generally, a low loading of Zn-CMP
(0.1–0.2 mol%) with co-catalyst TBAB (3.6 mol%) was used to
catalyze the reaction at 1208C and 3.0 MPa CO2. All corre-
sponding carbonates (2b–2k) could be obtained with decent
yields (>90%) and high selectivity (>99%). This result indi-
cates that Zn-CMP has a high versatility towards cycloaddition
of CO2. In addition to alkanes, alkenes, alkynes, alkyl halides, al-
cohols, ethers, and aryl groups (2b–2j), Zn-CMP could even
mediate the formation of the cyclic carbonate product 2k.
From these combined results we could clearly conclude that
Zn-CMP is an excellent catalyst for the conversion of highly
functional terminal epoxides.
To the best of our knowledge, Zn-CMP is the most active
heterogeneous catalyst for the reaction of CO2 and epoxides
reported thus far. To make a comparison to the reported cata-
lysts, we summarized representative homogeneous catalysts in
Table S4 and heterogeneous catalysts in Table S4. As shown in
Table S3, the catalytic activity of Zn-CMP is even comparable to
the most active homogeneous catalyst systems, such as Mg-
porphyrin,[5] aluminum-based complexes (Al-complex),[6] zinc-
based complexes (Zn-complex),[19a] or zinc-containing ionic liq-
uids.[19b] The catalyst Al-complex/PPN-Br [PPN=bis-(triphenyl-
Zn-CMP was further investigated as catalyst for catalyzing
the cycloaddition of CO2 with internal epoxides (Scheme 4,
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Scheme 4. Reaction scope for internal epoxides. Cyclic carbonates (2l–2o)
obtained from their corresponding epoxides using Zn-CMP/TBAB as catalytic
system. General conditions are epoxides (50.0 mmol), Zn-CMP (quantity indi-
cated), TBAB (3.6 mol%), 1208C, 3.0 MPa initial CO2 pressure, 40 mL auto-
clave. Yields of the isolated product obtained after column chromatography.
Further details: for 2o 3.0 mL methyl ethyl ketone (MEK) was used in reac-
tion; for 2o the corresponding epoxide was 25 mmol.
Scheme 3. Reaction Scope for terminal epoxides. Cyclic (bis)-carbonates
(2b–2k) obtained from their corresponding epoxides using Zn-CMP/TBAB as
catalytic system. General conditions are epoxides (50.0 mmol), Zn-CMP
(quantity indicated), TBAB (3.6 mol%), 1208C, 3.0 MPa initial CO2 pressure,
reaction times 1 h for 2b–2e and 2 h for 2 f–2k, 40 mL autoclave. Yields of
isolated product obtained after column chromatography.
ecule was coordinated to the zinc ion and an intramolecular
rearrangement occurred to form the zinc carbonate (PC1),
which could cyclize to form the internal carbonate and regen-
erate the salen zinc center and co-catalyst.
Density functional theory (DFT) calculations were performed
to investigate this mechanism. Based on the proposed mecha-
nism, five intermediates and three transition states were opti-
mized as listed in Tables S6–S13. The energy profiles (in kcal
molꢀ1, 1 cal=4.2 J) of these structures (in ꢁ) are depicted in
Scheme 5. The reaction was presumed to occur in four steps,
starting from the adsorption of trans-2,3-epoxybutane to the
salen zinc to form an intermediate (IN1 in Scheme 5) as an exo-
synthesis of 2l–2o), which are widely considered as difficult
substrates with few achieved successful conversions. The corre-
sponding organic carbonates (2l–2o) have been successfully
produced by increasing the reaction time at 1208C and
3.0 MPa CO2. To obtain reasonable yields of product 2l, a pro-
longed reaction time of 12 h was required (90.6%). A decent
yield of 93.5% was achieved for 2m after 24 h. When epoxy
cyclohexane was used as a sub-
strate, the corresponding car-
bonate 2n was obtained with
a 66.3% yield after 12 h. The car-
bonate 2o has been rarely syn-
thesized through catalytic cyclo-
addition of CO2 with stilbene
oxide.[6] However, Zn-CMP can
provide a reasonable yield of
56.1% after 40 h. The trans-dis-
position of both substituents in
carbonates 2m and 2o can be
determined by 1H NMR and
13C NMR spectroscopy (Figur-
es S22 and S23).
The salen-zinc center played
a key role for the reaction, thus
a possible catalytic cycle was
proposed in Figure S24. The
function of co-catalyst, quaterna-
ry ammonium salt (nBu4NBr), is
Scheme 5. Calculated energy profiles (in kcalmolꢀ1) and optimized structures (in ꢁ) of species for the catalytic
to open the ring of the internal
epoxide. Consequently, CO2 mol- mechanism.
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¼
with the highest activation energy (TS1, DE =20.49 kcal
molꢀ1). The third step was the insertion of the CO2 molecule,
which is an intramolecular rearrangement process with a mini-
¼
mum activation energy (TS2, DE =0.42 kcalmolꢀ1). The last
step was the formation of the product, which is a backbiting
¼
reaction (TS3, DE =8.04 kcalmolꢀ1). The calculation results
confirmed that the catalytic procedure occurred easily as the
highest activation energy of the entire catalytic reaction was
only 20.49 kcalmolꢀ1.[21]
In summary, we demonstrated the use of a zinc-conjugated
microporous polymer (Zn-CMP) as a highly active and versatile
heterogeneous catalyst for organic carbonate formation. In the
presence of n-tetrabutylammonium bromide (TBAB), the turn-
over frequency (TOF) of the catalytic reaction can reach
11600 hꢀ1 at 1208C and 3.0 MPa CO2. The catalytic activity of
Zn-CMP is comparable to that of the homogeneous catalysts
and better than that of the best heterogeneous catalyst. More-
over, Zn-CMP is so far the only efficient heterogeneous catalyst
suitable for internal epoxides. This polymer can be reused
more than ten times without significant loss of its catalytic ac-
tivity.
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Experimental Section
The Supporting Information contains experimental details as well
as Tables and Figures referred to in the text.
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This work was supported by NSFC 21373202, 21173209,
91333116, “talent 100” program of Chinese Academy of Sciences,
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Received: March 11, 2014
Published online on && &&, 0000
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ChemSusChem 0000, 00, 1 – 5
&
5
&
ÞÞ
These are not the final page numbers!

COMMUNICATIONS
Y. Xie, T.-T. Wang, R.-X. Yang, N.-Y. Huang,
K. Zou,* W.-Q. Deng*
&& – &&
Efficient Fixation of CO2 by a Zinc-
Coordinated Conjugated Microporous
Polymer
CO2 cycles go round: Zinc-coordinated
conjugated microporous polymers have
been used as highly efficient heteroge-
neous catalysts for CO2 fixation. This
polymer can be used to efficiently cata-
lyze the reaction of internal epoxides
with CO2 to form functional organic car-
bonates. Excellent catalytic activities
and improved recyclability of this poly-
mer have been achieved.
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ChemSusChem 0000, 00, 1 – 5
&
6
&
ÞÞ
These are not the final page numbers!