Building N-Heterocyclic Carbene into Triazine-Linked Polymer for Multiple CO2 Utilization

Article abstract of DOI:10.1002/cssc.202002154

The development of new CO₂ detection technologies and CO₂ “capture-conversion” materials is of great significance due to the growing environmental crisis. Here, multifunctional triazine-linked polymers with built-in N-heterocyclic carbene (NHC) sites (designated as NHC-triazine@polymer) are presented for simultaneous CO₂ detection, capture, activation, and catalytic conversion. NHC-triazine@polymer were readily obtained through polymerization of cyanophenyl-substituted NHC. The obtained film-like polymers exhibited interesting CO₂-triggered fluorescence “turn-on” response and CO₂-sensitive reversible color change. Both NHC and triazine sites could act as efficient binding sites for CO₂, and the CO₂ uptake of NHC and triazine reached 1.52 and 1.36 mmol g^?1, respectively. Notably, after being captured by NHC, CO₂ was activated into a zwitterionic adduct NHC?CO₂ that could be easily transformed into cyclic carbonate in the presence of epoxides. Moreover, NHC-triazine@polymer were stable and active catalysts for the conversion of low-concentration CO₂ in a gas mixture (7 vol %) into cyclic carbonates as well as for hydrosilylation of CO₂ to formamides.

Full text of DOI:10.1002/cssc.202002154

Chemistry–Sustainability–Energy–Materials
Accepted Article
Title: Building N-Heterocyclic Carbene into Triazine-Linked Polymer for
Multiple CO2 Utilization
Authors: Chengtao Yue, Wenlong Wang, and Fuwei Li
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To be cited as: ChemSusChem 10.1002/cssc.202002154
Link to VoR: https://doi.org/10.1002/cssc.202002154
01/2020

10.1002/cssc.202002154
ChemSusChem
FULL PAPER
Building N-Heterocyclic Carbene into Triazine-Linked Polymer for
Multiple CO2 Utilization
Chengtao Yue, †[a, b] Wenlong Wang†[c] and Fuwei Li*[a]
Abstract: The development of new CO2 detection technology
and CO2 “capture-conversion” material is of great significance
due to the growing environmental crisis. Here, we present the
multifunctional triazine-linked polymers with built-in N-
heterocyclic carbene (NHC) sites (designated as NHC-
triazine@polymer) for simultaneous CO2 detection, capture,
activation and catalytic conversion. NHC-triazine@polymer were
readily obtained through polymerization of cyanophenyl-
substituted NHC. The obtained film-like polymers exhibit
interesting CO2-triggered fluorescence “turn-on” response and
CO2-sensitive reversible color change. Both NHC and triazine
sites can act as efficient binding sites for CO2, and the CO2
uptake of NHC and triazine reached 1.52 mmol/g and 1.36
mmol/g, respectively. Notably, after being captured by NHC, the
CO2 was activated into zwitterionic adduct NHC-CO2 that could
be easily transformed into cyclic carbonate in the presence of
epoxides. Moreover, NHC-triazine@polymer were stable and
active catalyst for the conversion of low concentration CO2 in gas
mixture (7 vol%) into cyclic carbonates as well as for
hydrosilylation of CO2 to formamides.
Introduction
For the utilization of CO2, the cycloaddition of epoxides with
CO2 to form cyclic carbonates has attracted great interest due to
the 100% atomic economy and the broad application of cyclic
carbonates in the pharmaceutical and fine chemical industries.6
Many metal-based and metal-free catalysts have been developed
for conversion of CO2 into cyclic carbonates.27 Particularly, Lu and
his colleagues reported their pioneering work on using NHCs as
efficient metal-free catalysts to catalyze this reaction.28 Due to
their electron-rich nature, NHC can capture and activate CO2 to
form the zwitterionic adduct NHC-CO2, resulting in an effective
organocatalyst for the cycloaddition reaction of CO2 and
epoxide.28 Moreover, the reaction of NHC with CO2 is an attractive
CO2-sensing system which has been reported for CO2 detection.29
Based on this, we envision to develop a new multifunctional NHC-
based material through utilizing its different responses to CO2
beyond the small molecular NHC. In this sense, the triazine-linked
polymeric material with built-in NHC functionality is a kind of
promising candidate, considering the triazine linkage can be
readily produced via trimerization of cyano-group and it also
shows outstanding CO2 capturing capability due to its intrinsic
high nitrogen content.30-32
In this contribution, we present the synthesis of triazine linked
polymers with built-in NHC (NHC-Triazine@polymer) through
superacid-catalyzed trimerization of cyanophenyl-substituted
NHC precursors. The obtained polymers show a good film-
forming property, and more importantly, the introduction of NHC
endows these polymers with multi-functions to meet the diverse
needs in CO2 utilization. The absorption and desorption of CO2 in
NHC sites induced a unique fluorescent response and color
change in the film-like polymer. Both NHC and triazine groups
were effective binding sites for CO2, and after being captured by
NHC, the CO2 molecules were in-situ activated into zwitterionic
adduct NHC-CO2 which were readily transformed into cyclic
carbonate with epoxides. Moreover, NHC-Triazine@polymer
exhibit high activity and stability toward the cycloaddition reaction
of CO2 with epoxide and hydrosilylation of CO2 to formamides.
CO2 is noted as one of the primary greenhouse gases and its
significant increase in the atmosphere has been considered to be
a main reason for global warming and ocean acidification.1
Meanwhile, CO2 is also known as a green and renewable C1
building block which has been widely used in synthesis of
valuable chemicals.2 To alleviate the environmental crisis and
make fully use of CO2, considerable efforts have been devoted to
exploring new absorbent material for efficient capture of CO2,3-5
as well as developing new catalytic systems for effective
activation and conversion of CO2.6-11 However, for the practical
utilization of CO2 in a CO2-containing gas mixture, the prior
separation of CO2 is usually necessary, which is time- and
energy-consuming.12 Alternatively, the use of multifunctional
materials which are capable of both capture and conversion of
CO2, offers an ideal way to enhance the efficiency of CO2
utilization.13,14 Although significant advances have been achieved
over bifunctional materials such as carbon structures,15,16 metal
oxides,17 MOFs,18-21 and organic polymers,22-26 the development
of new multifunctional materials that can realize CO2 “capture-
conversion” is still highly desired.
[a]
Dr. C. Yue, Prof. F. Li
State Key Laboratory for Oxo Synthesis and Selective Oxidation,
Suzhou Research Institute of LICP
Lanzhou Institute of Chemical Physics (LICP), Chinese Academy of
Sciences
Lanzhou 730000, China
E-mail: fuweili@licp.cas.cn
[b]
[c]
Dr. C. Yue
University of Chinese Academy of Sciences
Beijing 10049, China
Dr. W. Wang
School of Materials Science and Engineering
Dongguan University of Technology
Dongguan 523808, China.
[†]
These authors contributed equally to the article.
Supporting information and the ORCID identification number(s) for the
author(s) of this article can be found under:
1
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10.1002/cssc.202002154
ChemSusChem
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Scheme 1. Synthetic route for triazine-linked polymers with built-in NHC (NHC-triazine@polymer).
Figure 1. Spectroscopic and structure characterization. (a) FTIR and (b) The 13C CP-MAS NMR spectra of NHC-HCl-1 and NHC-Triazine@polymer1. (c) The
thermal stabilities of different NHC-Triazine@polymer. (d) CO2 TPD profiles of NHC-CO2-Triazine@polymer and the corresponding monomers.
These features promise NHC-Triazine@polymer to be multi-
functional materials, which act not only as new CO2 detection
materials, but also as important CO2 “capture-conversion”
materials.
(amorphous) structure (Figure S1 and Figure S2).33 In addition,
they showed good swelling property in many organic solvents
such as DMF, DMSO, chloroform, toluene, tetrahydrofuran and
various epoxides (Figure S3).
The formation of NHC-functionalized triazine-linked polymer
was initially detected by FTIR analysis. The absence of a peak at
2226 cm-1 belonging to the nitrile group in NHC-HCl and the
presence of two strong absorption bands at 1509 and 1367 cm-1
which are assignable to the aromatic C-N stretching and
“breathing” modes, indicate the formation of triazine units in NHC-
HCl-Triazine@polymer1 and NHC-CO2-Triazine@polymer1
Results and Discussion
Spectroscopic and structure characterization. Unlike the
covalent triazine-based frameworks (CTFs) which are usually in
the form of power, it is found that the present NHC-
Triazine@polymers can form flexible film with no crystalline
2
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(Figure 1a and Figure S4).34-37 The stretching band at 1637 cm-1
belonging to asymmetric ν(CO2) vibration suggests the generation
of zwitterionic adduct NHC-CO2 in the NHC-CO2-
Triazine@polymer1.38 The 13C CP/MS NMR spectra provide more
information about the NHC-Triazine@polymer1 (Figure 1b and
Figure S5). Signals at about 25 and 29 ppm are attributed to the
aliphatic methyl and isopropyl moietr, respectively.39,40 The
absence of nitrile signal in NHC monomer at 111 ppm and the
presence of aromatic carbons in the triazine (∼172 ppm) verify
the transformation of the nitrile group into triazine linkage.37
Signals at about 124, 130, 135 and 145 ppm belong to the
aromatic carbons of the aromatic subunits and the carbons in
imidazolium ring, indicating the successful integration of NHC into
the triazine-linked polymer. Besides, a signal at 162 ppm, which
is assigned to carboxylate in NHC-CO2, further confirms the
formation of NHC-CO2-Triazine@polymer1. Moreover, the XPS
spectrum of NHC-Triazine@polymer1 shows the signal of the
nitrogen atoms in triazine linkages at 399.3 eV, which also reveals
the formation of triazine (Figure S6).41 The above results clearly
proved the successful synthesis of the triazine-linked polymers
with built-in NHC species.
The thermal stability of the polymer was assessed by TGA
(Figure 1c), which revealed that the overall weight of the NHC-
HCl-Triazine@polymer were well-maintained up to 370 °C. Such
high thermal stability is possibly attributed to the formation of rigid
triazine-frameworks.42 Notably, an obvious weight loss between
150~220 °C is observed in the process of heat treatment of the
NHC-CO2-Triazine@polymer, which is resulted from the
decarboxylation of zwitterionic adduct NHC-CO2.43. Base on this
unique property, CO2-TPD is used to investigate the temperature-
condition initiated at about 100 °C and the peak temperatures
were found to be 178 °C for NHC-CO2-Triazine@polymer1 and
197 °C for NHC-CO2-Triazine@polymer2, respectively. In
comparison to NHC-CO2-Triazine@polymer1, the saturated
imidazolium NHC-CO2-Triazine@polymer2 exhibits higher
thermal stability. Besides, NHC-CO2-Triazine@polymer showed a
higher peak temperature than that of the molecular NHC-CO2
species, indicating the restricted diffusion of CO2 in the triazine-
linked polymers.
CO2 detection. Based on the unique molecular structure of this
series of NHC-Triazine@polymer and their reactivity toward CO2,
we explored the fluorescence characteristic of NHC-
Triazine@polymers as well as their potential application in CO2
gas detection.44 As shown in Figure 2a and Figure 2b, NHC-HCl-
Triazine@polymer show good florescent property and a strong
emission peak at about 432 nm was observed when it is excited
by the UV light. However, the fluorescence is quenched when
NHC-HCl-Triazine@polymer is treated with potassium
bis(trimethylsilyl)amide (KHMDS) and the free carbene
incorporated polymer NHC-Triazine@polymer is formed. This is
because the presence of the amino groups in free carbene
effectively quench the fluorescence of biphenyl through a
photoinduced intramolecular electron transfer (PET) process.45,46
Interestingly, the strong fluorescence recovered again after
bubbling CO2 for a few seconds, then leading to the formation of
a
positively
charged
zwitterionic
adduct
NHC-CO2-
Triazine@polymer through the capture of CO2 by the carbene unit,
which results in suspension of the PET process and induces an
interesting turn-on fluorescent response (Figure 2d). Tests on the
“turn-on” fluorescent response behavior to other gases
demonstrate that NHC-Triazine@polymer displays very high
response selectivity toward CO2 (Figure 2c).
dependent
CO2
releasing
property
of
NHC-CO2-
Triazine@polymer. As can be seen from Figure 1d, the
decarboxylation of NHC-CO2-Triazine@polymer under such
Figure 2. CO2 detection. (a) and (b) Photoluminescence spectrum of different NHC-Triazine@polymer. (c) Photoluminescence spectrum of NHC-Triazine@polymer
for different gas. (d) Schematic representation of CO2-triggered fluorescent response of NHC-Triazine@polymer. (e) CO2-sensitive reversible color change.
3
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Moreover, NHC-Triazine@polymer also shows an interesting
CO2 sensitive reversible color-change performance. As shown in
Figure 2e, after heating in DMF at 100 °C for 10 minutes under
nitrogen, the yellow transparent NHC-CO2-Triazine@polymer
changes to brown. When CO2 was reintroduced into the system,
the film quickly returns to the original color. The interesting color
change proceeded along with the CO2 desorption and adsorption
process, during which the desorption of CO2 in NHC-CO2-
Triazine@polymer (transparent yellow) first gives a free carbene
film NHC-Triazine@polymer (opaque black) and binding of CO2
to NHC-Triazine@polymer further leads to NHC-CO2-
Triazine@polymer (transparent yellow). Moreover, this color
change performance can be well maintained after several
adsorption and desorption cycles of CO2, which confirms the CO2-
the calculated CO2 uptake by NHC sites reached 6.7 wt% and 6.9
wt% (1.52 mmol/g and 1.56 mmol/g), respectively, indicating that
CO2 gas could quantitatively react with the polymeric carbene
sites.
The CO2 adsorption capacity of triazine group was investigated
using temperature dependent CO2 uptake measurements under
pressures up to 1 bar at 273 K and 295 K, respectively. As can be
seen in Figure S8 and Table S1, all the films show moderate CO2
uptake (0.59-1.36 mmol/g) and high CO2 adsorption selectivity.
Take NHC-CO2-Triazine@polymer1 as an example, the CO2
uptake achieves 1.36 mmol/g while the N2 and CH4 uptake only
reach 0.09 and 0.04 mmol/g at 273 K (Figure 3a and Table S1).
The CO2/N2 and CO2/CH4 adsorption selectivity calculated from
initial slope of adsorption curves at 0.01 bar are 21 and 47,49-52
respectively (Figure 3b and Figure S8), which verifies its high
adsorption selectivity for CO2 over N2 and CH4. Note that the N2
adsorption isotherm of these polymers at 77 K display restricted
adsorption behavior, with the Brunauer-Emmett-Teller (BET)
surface areas calculated from N2 adsorption isotherm at 77 K
being zero. The restricted N2 uptake indicates that NHC-
Triazine@polymer are compactly piled in solid state showing
nonporous structures. However, the CO2 isotherms recorded at
273 K give the expected BET surface areas, suggesting the
presence of small pores that only allow the diffusion of the
smallest CO2 molecule. This unique size selectivity in the
polymers accounts for their high CO2 gas adsorption selectivity.
Given the BET surface area of different polymers and their
corresponding CO2 uptakes, it can also be found that high BET
surface areas benefit the increase in the gas uptake, and
therefore NHC-CO2-Triazine@polymer1 with the largest BET
surface area (1035 m2/g) shows the highest CO2 uptakes (1.36
mmol/g at 273 K and 0.78 mmol/g at 295 K). Moreover, the CO2
adsorption data measured at 273 K and 295 K were fitted by the
Clausius-Claperyron equation to estimate the enthalpy of
adsorption. The isosteric heats of adsorption (Qst) were found to
be 22-46 kJ mol-1, at zero coverage (Figure S8f).53,54 The high Qst
for CO2 indicates a strong binding affinity between triazine and
CO2 molecules.
sensitive
reversible
color-change
property
of
NHC-
Triazine@polymer. The CO2-triggered fluorescence “turn-on”
response and the CO2-sensitive reversible color change property
indicate the potential application of NHC-Triazine@polymer in
CO2 detection.
CO2 conversion. Having explored the CO2 capture property of
NHC-Triazine@polymer, we next investigated their catalytic
performance in cycloaddition reaction of CO2 with epichlorohydrin
(ClPO) as a model epoxide. As shown in Table 1, 0.5 mol% of the
unsaturated imidazolium NHC-CO2-Triazine@polymer1 provided
almost quantitative yield of (chloromethyl)-1,3-dioxolan-2-one
(ClPC) (Entry 4), which is comparable to that of the homogeneous
NHC-CO2 under identical reaction conditions (Entry 1). A slightly
lower yield (66%) was obtained when using NHC-CO2-
Triazine@polymer2 as catalyst (Entry 5), indicating that the
stronger electron donation of NHC-2 to CO2 may produce a more
stable NHC-CO2 with a relative lower activity. On the other hand,
both Triazine@polymer1 and NHC-HCl-Triazine@polymer1
showed low activity with 9% and 37% yield of ClPC (Entry 2 and
3), respectively. These results indicate that the NHC in NHC-
Triazine@polymer can effectively activate and transform CO2 into
cyclic carbonate. Theoretically, the purity of CO2 gas will affect the
reactivity of some metal catalyzed cycloaddition of epoxide with
CO2 because some gas components will possibly deactivate the
metal. Herein, the representative NHC-Triazine@polymer could
selectively capture the CO2 in gas mixture to produce cyclic
carbonates in the presence of other gas components such as N2,
CH4 and CO (Entries 6-10). Notably, even when the CO2 volume
Figure 3. CO2 capture. (a) CO2, N2 and CH4 adsorption isotherms of NHC-CO2-
Triazine@polymer1. (b) Initial slope calculation for CO2, N2 and CH4 isotherms
collected at 273 K.
CO2 capture. Considering that both NHC and triazine sites in
the NHC-Triazine@polymer can capture CO2,30-33,47,48 the CO2
adsorption capacity of NHC and triazine were investigated by TG
analysis and temperature dependent CO2 uptake measurements,
respectively (Figure 1c and Figure 3a). As shown in Figure 1c, the
NHC-CO2-Triazine@polymer exhibit a temperature-dependent
release of CO2. Both NHC-CO2-Triazine@polymer1 and NHC-
CO2-Triazine@polymer2 have a weight loss at 157~212 °C and
4
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10.1002/cssc.202002154
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percentage was down to 7.1%, the yield of ClPC could be as high
as 94% over NHC-CO2-Triazine@polymer1 (Entry 10), indicating
its promising capture, activation and catalytic conversion of the
low concentration CO2.
epoxide with CO2 (Figure 4a, Condition B).57-59 All these results
under condition A and B indicate that present cycloaddition
possibly proceeds via initial activation of CO2 by NHC and then
ring-opening of epoxide by the resultant NHC-CO2 before ring
cyclization to give cyclic carbonate, and the NHC can slowly
catalyze the ring-opening of epoxide followed by homo-
polymerization rather than the cross-coupling with CO2. Based on
these findings and the previous knowledge,28 a possible catalytic
Table 1. Catalytic activity of NHC-Triazine@polymer for the conversion of
CO2 to cyclic carbonates.[a]
mechanism
of
NHC-CO2-Triazine@polymer
catalyzed
cycloaddition of CO2 with epoxides was proposed (Figure 4b).
The nucleophilic attack of epoxide by the zwitterionic NHC-CO2
CO2/N2/CH4/CO
(MPa)
Yield
(%)
Entry
Catalyst
adduct in NHC-CO2-Triazine@polymer generates
a
new
1
2
NHC-CO2-1
0.5/0/0/0
0.5/0/0/0
99
9
zwitterion which contains an alkoxy anion. Then the
intramolecular nucleophilic attack of the alkoxy anion toward the
carbon atom of carbonyl group produces cyclic carbonate and the
free NHC. Finally, NHC-Triazine@polymer reacts with CO2 to
regenerate the NHC-CO2-Triazine@polymer.
Triazine@polymer1
3
NHC-HCl-Triazine@polymer1
NHC-CO2-Triazine@polymer1
NHC-CO2-Triazine@polymer2
NHC-CO2-Triazine@polymer1
NHC-CO2-Triazine@polymer1
NHC-CO2-Triazine@polymer1
NHC-CO2-Triazine@polymer1
NHC-CO2-Triazine@polymer1
0.5/0/0/0
37
97
66
96
94
95
96
94
4
0.5/0/0/0
5
0.5/0/0/0
Taking the advantage of facile recyclability of the polymeric
catalyst, their catalytic stability in the cycloaddition reaction
between epoxide and CO2 was further investigated by
recyclability and long-term stability test with NHC-CO2-
Triazine@polymer1 as the optimized catalyst. As shown in Figure
5a and Figure S9, NHC-CO2-Triazine@polymer1 could be used
for eight times without significant loss of catalytic activity,
indicating a good reusability. The FT-IR spectrum of the recycled
catalyst after eight cycles shows characteristic vibrations at 1637,
1509 and 1367 cm-1 similar to the fresh one, and no obvious
changes can be found (Figure 1a). The long-term stability was
further tested in a continuous-flow reactor using CO2-N2 mixture.
Initially, as displayed in Figure 5b, the polymeric NHC-CO2-
Triazine@polymer worked stably at about 70% yield of ClPC for
200 hours, after that the yield could be enhanced to over 90% by
decreasing the flow rate of ClPO and this excellent yield could be
kept for another 150 hours without any reactivity change observed.
The catalyst was collected after the reaction and washed with
methyl tert-butyl ether (TBME) to remove the reactant and product
before 13C CP-MAS NMR characterization again (Figure 1b). It is
found that the NMR spectrum of the spent catalyst is also highly
consistent with the fresh one. The above results verify the good
stability of NHC-CO2-Triazine@polymer1 in the cycloaddition
reaction between CO2 and epoxides.
6
0.5/1.5/0/0
0.5/1.5/2.0/0
0.5/0/3.5/0
0.5/0/0/1.5
0.5/0/6.5/0
7
8
9
10
[a] Reaction conditions: epichlorohydrin (5 mmol), catalyst (0.5 mmol%),
100 °C, 6 h, isolated yield.
It has been previously reported that both NHC and NHC-CO2
can be the active sites for the ring-opening of epoxide.55,56 To
figure out whether NHC or NHC-CO2 adduct is the catalytic active
component, their corresponding catalytic behaviors were studied
in the model cycloaddition using in-situ NMR. It is found that the
NHC-CO2 could directly react with ClPO to produce ClPC without
additional CO2 (Figure 4a, Condition A), and stoichiometric
reaction between the NHC-CO2 and ClPO also gave almost 100%
conversion into ClPC (Scheme S1), suggesting the efficient
reactivity of NHC-CO2 with epoxide to produce cyclic carbonate.
On the contrary, no ClPC product was observed upon treating the
preheated mixture of NHC and ClPO with CO2 for 60 min, and the
formation of polyepichlorohydrin evidenced by the NMR peaks at
3.66-3.71 ppm indicates that free NHC can catalyze the ring-
opening polymerization of ClPO rather than the cycloaddition of
Figure 4 Mechanism investigation. (a) Control experiments to study the reaction mechanism of cycloaddition reaction. Condition A: NHC-CO2 (0.05 mmol),
epichlorohydrin (0.05 mmol), CDCl3 (0.6 mL). Condition B: NHC (0.05 mmol), epichlorohydrin (0.06 mmol), CDCl3 (0.6 mL). (b) Possible reaction mechanism.
5
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To study the general applicability of this catalytic system, the
cycloadditions of CO2 with different epoxides were performed. It
has been previously reported that the heterogeneous catalysts in
cycloaddition reaction between CO2 and epoxides usually
suffered from limited substrate scope due to the size effect when
their pore diameter is less than that of the molecular dimension of
epoxides.34,51,53,60-63 Herein, the good swelling property of NHC-
CO2-Triazine@polymer1 is expected to eliminate the mass
transfer limitation of the large-sized substrate, providing a benefit
to expand the substrate scope.64-67 Based on this, we tested the
swelling degrees of NHC-CO2-Triazine@polymer1 in five
representative epoxides using equilibrium swelling method (Eq.
S1) and explored its catalytic activity in catalytic synthesis of
representative cyclic carbonates. As shown in Table 2, NHC-CO2-
Triazine@polymer1 displayed a good swelling property in these
epoxides, and all epoxides were effectively converted into the
corresponding cyclic carbonates in high yields (89-98%). Notably,
even for the large-sized phenyl glycidyl ether and styrene oxide
which have been found to be inactive in many heterogeneous
catalytic system,34,68 NHC-CO2-Triazine@polymer1 could also
effectively catalyzed their cycloadditions with CO2. These results
demonstrate that the swelling property of NHC-CO2-
Triazine@polymer1 enables this polymeric catalyst work like a
quashi-homogeneous catalyst and benefits the access of large-
sized reactants to the catalytic sites within the polymer, promoting
its catalytic activity in the cycloaddition reaction of CO2 with
various epoxides.
Figure 5. Catalyst stability. (a) Reusability of NHC-CO2-Triazine@polymer1.
Reaction conditions: epichlorohydrin (5 mmol), catalyst (0.5 mmol%), CO2 (1
bar), 100 °C, 8 h. (b) Long-term stability of NHC-CO2-Triazine@polymer1 during
a continuous flow reaction with a fixed-bed reactor. Reaction conditions:
epichlorohydrin (30 μL/min for the first 200 hours and 20 μL/min for 200~390 h),
catalyst (1.0 g), 100 °C, CO2 (20 mL/min), and N2 (180 mL/min), total pressure
(3.0 MPa).
Table 3. Hydrosilylation of CO2 to formamides.[a]
Entry
1
Amines
Formamides
Yield (%)
96
Table 2. Cycloaddition of CO2 with different Epoxides.[a]
2
70
Swelling
Yield
(%)
Entry
1
Epoxides
Carbonates
degree[b] (%)
3
4
5
61
90
92
205
1208
387
98[c]
97
2
3
92[c]
[a] Reaction conditions: amine (0.5 mmol), NHC-CO2-Triazine@polymer1 (5
mmol%), THF (2 mL), room temperature (r.t.), 24 h, isolated yield.
4
5
622
457
98
To verify the generality of this catalyst in multiply CO2 utilization,
hydrosilylation of CO2 to formamide was also investigated. As
shown in Table 3, the representative amines could be effectively
transformed into the corresponding formamides in moderate to
good yields (61-96%) under the catalysis of NHC-CO2-
Triazine@polymer1 at room temperature. Particularly, only
89[c]
[a] Reaction conditions: epoxide (5 mmol), NHC-CO2-Triazine@polymer1
(0.5 mmol%), 100 °C, 6 h, isolated yield. [b] Swelling degrees of NHC-CO2-
Triazine@polymer1 in epoxides are calculated based on equilibrium
swelling method. [c] 120 °C.
monoformylation of aniline derivatives occurred, and
the
formation of N, N-bisformylanilines via double N-formylation
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100 oC. After reaction for 6 h, the reactor was cooled down to room
temperature and the excess CO2 was discharged. The resultant mixture
was filtered through a filter paper. After removing the volatiles in vacuo,
the crude products were further purified by chromatography on silica gel
using petroleum ether / ethyl acetate as eluent.
For the reusability of NHC-Triazine@polymer, the collected catalyst was
washed thoroughly with TBME and dried at 60 oC under vacuum overnight,
then charged into the next run without further treatment.
which have been found in homogeneous catalytic systems, was
not observed possibly due to the spatial confinement effect
enabled by this swelling polymer catalyst (Entries 3-5, Table 3).69
This catalytic application further highlights the potential
application of NHC-CO2-Triazine@polymer in CO2 utilization.
Long-term stability of NHC-Triazine@polymer was tested in a fixed-bed
Conclusion
continuous-flow
stainless
microreactor.
Reaction
conditions:
epichlorohydrin (30 μL/min for the first 200 hours and 20 μL/min for
246~390 h), catalyst (1.0 g), 100 oC, CO2 (20 mL/min), and N2 (180
mL/min), total pressure (3.0 MPa). The effluent passed through a gas-
liquid separator in an ice-water bath to collect the liquid product. After the
initial reaction time of 6 h, the liquid product in the separator was emptied
and then the samples were taken every 6 or 12 hours. The resulting
samples were analyzed by GC with n-dodecane as the internal standard.
Triazine-linked polymers with built-in NHC sites (NHC-
Triazine@polymer) have been facilely synthesized by superacid-
catalyzed trimerization of “task-specific” cyanophenyl-substituted
NHCs. Benefiting from the good film-forming property and the
versatility of NHC and the triazine linker, the NHC-
Triazine@polymer show multiple applications in CO2 detection,
capture, activation and catalytic conversion. These film-like
polymers exhibit interesting CO2-triggered fluorescence “turn-on”
response and CO2-sensitive reversible color change. Both NHC
and triazine moieties are efficient binding sites for CO2, and their
CO2 uptake reached 1.52 and 1.36 mmol/g, respectively. More
importantly, the CO2 could be in-situ activated by NHC sites
through the formation of zwitterionic adduct NHC-CO2, and the
resulting NHC-CO2-Triazine@polymer were highly active and
stable catalysts for the conversion of low concentration CO2 (low
to 7 vol%) into cyclic carbonates in the presence of other gas
mixture, such as CO and CH4. Moreover, this catalyst can also
effectively catalyzed the hydrosilylation of CO2 to formamides. All
the results highlight the outstanding potential of NHC-
Triazine@polymer in multiple CO2 utilization and also pose more
hints on development of triazine-linked bifunctional polymers and
their other applications through introducing molecular metal
complexes or metal nanoparticles.
Hydrosilylation of CO2 to formamides
In a typical experiment, amine (0.5 mmol), phenylsilane (0.75 mmol),
catalyst (5 mol%) and dry THF (2 mL) were loaded in a 5 mL ampoule vial
equipped with a magnetic stir bar. Then, the vial was transferred to a 100
mL stainless steel autoclave, sealed, and purged with CO2 for three times
before it was finally pressurized with 5 atm CO2.Then, the reactor was
stirred at room temperature for 24 h. After the reaction, the excess CO2
was discharged and the reaction mixture was filtered through a filter paper.
After removing the volatiles in vacuo, the crude products were further
purified by chromatography on silica gel using petroleum ether / ethyl
acetate as eluent.
Characterization
The prepared polymers were fully characterized by Fourier transform
infrared spectroscopy (FT-IR), 13C cross-polarization magic angle spinning
solid-state NMR spectroscopy (13C CP/MS NMR), X-ray photoelectron
spectroscopy (XPS), thermogravimetric analysis (TGA), CO2 temperature
programmed desorption (CO2-TPD), Powder X-ray diffraction (PXRD),
Scanning electron microscopy (SEM) and Photoluminescence
Spectroscopy (PL). CO2/CH4/N2 sorption analyses were performed on
Micromeritics ASAP 2020 Physisorption analyzer under two different
temperatures (273 and 295 K) using a water bath. The CO2/CH4 and
CO2/N2 selectivities were estimated using the ratios of the Henry law
constant calculated from the initial slopes of the single component gas
adsorption isotherms at low pressure coverage (<0.10 bar). Heats of
absorption (Qst) values were calculated by using the standard calculation
routine based on Clausius-Clapeyron equation. Fluorescence
Experimental Section
Synthesis of NHC-Triazine@polymer
NHC-Triazine@polymer were prepared by self-polymerization of
cyanophenyl-substituted NHCs using CF3SO3H as both reaction medium
and catalyst (Scheme 1). Typically, 2.5 mL of CF3SO3H was quickly added
to 0.5 g of cyanophenyl-substituted NHCs at 0 °C under nitrogen. The
viscous red solution was stirred for 1 h at room temperature and then
spread into a thin layer. After standing for 48 h, the coating film was
washed with water, diluted ammonia and ethanol, respectively. The
obtained slightly yellow transparent film (NHC-HCl-Triazine@polymer)
was dried under vacuum at 60 °C for 24 h. Then, it was immersed into 20
mL THF, and 1.0 M solution of KHMDS in THF (1.6 mL) was added
dropwise under nitrogen in about 10 min. The resulting mixture was stirred
at room temperature for 2 h during which a black opaque film (NHC-
Triazine@polymer) was formed. Further bubbling CO2 into this mixture for
1 h, the color of the film quickly changed into slightly yellow transparent.
After being washed with THF (3 x 20 mL) and ethanol (3 x 20 mL)
respectively, the NHC-CO2-Triazine@polymer was obtained in about 90%
yield after dried under vacuum at room temperature overnight.
luminescence spectra were measured by
a F97Pro Fluorescence
Spectrophotometer (LengGuang Tech.) under 360 nm wavelength
excitation.
Acknowledgements
This work was supported by National Key R&D Program of China
(2018YFB1501600), Natural Science Foundation of China
(21972151), Light of West China of Chinese Academy of
Sciences (CAS), DNL Cooperation Fund, CAS (DNL180303), and
Key Research Program of Frontier Sciences of CAS
(QYZDJSSW-SLH051).
Cycloaddition of CO2 with epoxides
The cycloaddition reaction was conducted in a 100 mL Teflon-lined
stainless steel autoclave reactor. A mixture of epichlorohydrin (10 mmol)
and catalyst (0.05 mmol) was added in the reactor and the closed reactor
was purged with CO2 for three times before it was finally pressurized with
5 atm CO2. Then it was immersed in an oil bath which was preheated at
Conflict of interest
The authors declare no conflict of interest.
7
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10.1002/cssc.202002154
ChemSusChem
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Keywords: carbon dioxide • cyclic carbonates • N-heterocyclic
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