Showcasing research from Lin-Bing Sun’s Research Laboratory,
State Key Laboratory of Materials-Oriented Chemical
Engineering, College of Chemistry and Chemical Engineering,
Nanjing Tech University, China.
As featured in:
Title: Facile fabrication of cost-effective porous polymer
networks for highly selective CO2 capture
Mesoporous polymers were fabricated via facile nucleophilic
substitution reactions under the direction of a template. The
obtained materials, which consist of abundant secondary amines,
are highly active in selective CO2 capture and can be readily
regenerated.
See Lin-Bing Sun,
Hong-Cai Zhou et al.,
J. Mater. Chem. A, 2015, 3, 3252.
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Cite this: J. Mater. Chem. A, 2015, 3,
3252
Received 8th November 2014
Accepted 7th January 2015
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Facile fabrication of cost-effective porous polymer
networks for highly selective CO2 capture†
Lin-Bing Sun,*a Ai-Guo Li,a Xiao-Dan Liu,a Xiao-Qin Liu,a Dawei Feng,b Weigang Lu,b
Daqiang Yuanb and Hong-Cai Zhou*b
DOI: 10.1039/c4ta06039c
www.rsc.org/MaterialsA
Due to their synthetic diversification, low skeletal density, and high
physicochemical stability, porous polymer networks (PPNs) are highly
promising in a variety of applications such as carbon capture. Nevertheless, complicated monomers and/or expensive catalysts are normally utilized for their synthesis, which makes the process tedious,
costly, and hard to scale up. In this study, a facile nucleophilic
substitution reaction was designed to fabricate PPNs from low-cost
monomers, namely chloromethyl benzene and ethylene diamine. A
surfactant template was also used to direct the assembly, leading to
the formation of PPN with enhanced porosity. It is fascinating that the
polymerization reactions can occur at the low temperature of 63 C in
the absence of any catalyst. The obtained PPNs contain abundant
secondary amines, which offer appropriate adsorbate–adsorbent
interactions from the viewpoints of selective CO2 capture and energyefficient regeneration of the adsorbents. Hence, these PPNs are highly
active in selective adsorption of CO2, and unusually high CO2/N2 and
CO2/CH4 selectivity was obtained. Moreover, the PPN adsorbents can
be completely regenerated under mild conditions.
The worsening climatic situation due to global warming has
become an urgent environmental concern nowadays, and
excessive CO2 emission from fossil fuel combustion is considered to be a major anthropogenic source of greenhouse gases in
the atmosphere. CO2 is also a detrimental component of natural
gas. The presence of CO2 can cause corrosion of relevant pipelines and equipment as well as reduction in both transport
capacity of the pipeline and heat capacity of the gas. As a
result, separation of CO2 from N2 (post-combustion for ue gas)
and from CH4 (pre-combustion for natural gas) has attracted
increasing attention.
a
State Key Laboratory of Materials-Oriented Chemical Engineering, College of
Chemistry and Chemical Engineering, Nanjing Tech University, Nanjing 210009,
China. E-mail: lbsun@njtech.edu.cn
b
Department of Chemistry, Texas A&M University, College Station, Texas 77842-3012,
USA. E-mail: zhou@chem.tamu.edu
† Electronic supplementary
10.1039/c4ta06039c
information
(ESI)
3252 | J. Mater. Chem. A, 2015, 3, 3252–3256
available.
See
DOI:
The traditional technique to remove CO2 is “wet scrubbing”
by using aqueous amine (e.g. monoethanolamine, MEA) solutions. Nevertheless, this process shows several inherent shortcomings including high regeneration costs and erosion of
equipment. Among currently available CO2 capture technologies, adsorption using porous materials is regarded as the most
promising alternative.1,2 The porous solids have specic heat
capacities that are substantially less than those of aqueous
solutions. Furthermore, they are easier to handle and free of
corrosion problems. In the past decades, some metal–organic
frameworks (MOFs) have been reported to possess excellent
capacity for CO2 capture because of their large surface areas and
high pore volumes. However, most MOFs are unstable in high
temperatures, moisture, and other rigorous environments, and
thus cannot meet the harsh industrial demands.3–6 Fortunately,
porous polymer networks (PPNs, also known as covalent
organic frameworks,7 hyper-crosslinked polymers,8 conjugated
microporous polymers,9 polymer of intrinsic microporosity,10
covalent triazine-based frameworks,11 porous aromatic frameworks,12 etc.) constructed from lightweight elements through
strong covalent bonding have become a focus of attention.13–15
These materials generally possess low skeletal density, can be
synthesized in diverse ways, and display high physicochemical
stability, and thus they are highly competitive in CO2 capture.
However, complex monomers, expensive catalysts, or high
reaction temperatures are commonly required for the synthesis
of most reported PPNs, which make the synthetic process
complicated, costly, and hard to scale up.11,12,16 A case in point is
the synthesis of porous aromatic frameworks reported by Ben
et al.,17 for which the expensive catalyst bis(1,5-cyclooctadiene)
nickel(0), that is Ni(COD)2, has to be employed for the Yamamoto-type Ullmann polymerization reaction of this synthesis. In
the preparation of triazine-based porous polyimide polymer
networks reported by Senker's group,11 the complex monomer
2,4,6-tris(4-aminophenyl)-1,3,5-triazine was used; this monomer was synthesized from the quite preliminary precursor
4-bromobenzonitrile via a series of tedious organic reactions.
Despite great efforts, fabrication of PPNs from low-cost
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monomers via a catalyst-free and facile polymerization reaction
has been a great challenge.
Amine groups are well-known active sites that can create
strong interactions between adsorbents and CO2.18–21 Various
types of amines have been introduced to the frameworks of PPNs.
Primary amines can capture CO2 efficiently by forming stable
complexes, but are difficult to regenerate. Tertiary amines can be
regenerated easily under mild conditions, but at the expense of
adsorption capacity and selectivity. Secondary amines, however,
are ideal building blocks for PPNs since they can strike an
appropriate balance between adsorption performance and
energy-efficient regeneration.18,22–25
Herein, we developed a strategy to construct a new porous
polymer network, PPN-80, by a facile nucleophilic substitution
reaction of 2,4,6-tris(chloromethyl)mesitylene (M1) and ethylene
diamine (M2) (Scheme 1). Aiming to enhance the porosity of PPN,
a surfactant template was also used to direct the assembly,
leading to formation of PPN-81 with mesoporosity. Due to the
reactivity of benzyl chloride in M1, the polymerization reactions
were shown to take place at a temperature as low as 63 C in
the absence of any catalyst. Furthermore, both monomers are
inexpensive and readily available. The resultant PPNs contain
abundant secondary amines, and thus provide an appropriate
adsorbate–adsorbent interaction, which is benecial to both
selective CO2 capture and energy-saving regeneration of the
adsorbents. In our experiments, our materials efficiently adsorbed CO2, while N2 and CH4 were scarcely adsorbed, revealing an
extremely high selectivity of CO2 over N2 and CH4. Moreover, the
PPNs could be completely regenerated under mild conditions.
For the synthesis of PPN-80, the monomers M1 and M2 were
initially dissolved in tetrahydrofuran (THF), leading to the
formation of a clear colorless solution. Aer heating at 63 C for
about 0.5 h, white powders as the target products began to be
generated, and the yield increased with the reaction time. In a
similar process, PPN-81 was synthesized by the addition of
template (triblock copolymer P123) to the initial solution containing monomers. No catalysts were used for the synthesis of
both PPN-80 and PPN-81. The synthesis of the polymer was also
tested at other temperatures, varying from 50 to 63 C. The
Scheme 1 (A) Polymerization of monomers to form PPN-80 in the
absence of template and PPN-81 in the presence of template. (B)
Proposed interaction between template molecules and amine groups.
This journal is © The Royal Society of Chemistry 2015
Journal of Materials Chemistry A
results show that the polymerization reactions can also take
place at 60 C, although the yield of the polymer was 90% of that
synthesized at 63 C. Similarly, the polymer can be produced at
55 C, with the yield at about 40% of that synthesized at 63 C.
Upon further decreasing the temperature to 50 C, no polymer
was obtained at all.
The infrared (IR) spectra of PPN-80 and PPN-81 are similar,
as shown in Fig. 1A. Both spectra have bands at 3330 and
1110 cm 1, which can be attributed to N–H and C–N stretching
vibrations, respectively. The presence of these bands indicates
the successful introduction of amine groups into the frameworks. There is also a band at 1650 cm 1, which originated from
the –NH2 bending vibration, indicating some –NH2 groups
remained free. Indeed, both –NH2 groups in most diamines can
react with the monomer M1, leading to the formation of –NH–
linkages that assemble the polymer into a 3D organic network.
Only a few pendant –NH2 groups formed in the polymer; these
groups derived from the reaction of one of the –NH2 groups of
the diamine with M1. It is thus reasonable to conclude that a
lower –NH2 band intensity reects a higher degree of PPN
polymerization. The band intensity of –NH2 was also quantied
by comparing it to the C–C vibration band of benzene rings at
about 1570 cm 1, which was selected as the reference band. The
relative intensity of –NH2 to C–C was calculated to be 4.2 and 2.1
for PPN-80 and PPN-81, respectively. The amount of –NH2 in
PPN-80 is thus about twice as high as that in PPN-81. On the
other hand, the more intense the band at 1110 cm 1 (C–N), the
greater should be the degree of polymerization. Based on
combining the information about the –NH2 and C–N bands, it is
Fig. 1 (A) IR spectra and (B) solid-state
and PPN-81.
13
C NMR spectra of PPN-80
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safe to say that PPN-81, with a more intense C–N band and
weaker –NH2 band, achieved a higher degree of polymerization
than did PPN-80.26 Our solid-state 13C nuclear magnetic resonance (NMR) experiments yielded spectra that display three
peaks for PPN-80 and PPN-81 (Fig. 1B). The two sharp peaks at
132 and 14 ppm can be assigned to the sp2 C of the benzene rings
and to the carbon atoms of the methyl groups directly connected
to the benzene rings, respectively. The broad peak at 47 ppm
appears to be a combination of the peaks of carbon atoms connected to alkyl carbon and nitrogen. The results of elemental
analysis show that the PPNs mainly consist of three elements,
namely C, H, and N, with a tiny amount of Cl (Table S1†). The
measured elemental contents are in line with the theoretical
values, and this agreement indicates that the polymerization
reactions proceeded as designed. The lower Cl content and
higher N content of PPN-81 conrms the IR results, implying the
higher polymerization degree of PPN-81. The IR spectra, along
with NMR and elemental analysis results, demonstrate the
successful construction of PPNs via nucleophilic substitution
reactions.
The pore structure of PPNs was evaluated by N2 adsorption at
77 K. The N2 uptake of PPN-80 is generally low, as shown in
Fig. S1,† which is indicative of the sizes of the pores being quite
small, hence restricting entry of N2. This is benecial to the
selective adsorption of CO2 rather than N2 as described later.
Due to the use of a template in the synthetic process, PPN-81
showed an apparently higher N2 uptake than did PPN-80.
Moreover, the uptake increased gradually with the relative
pressure, and the isotherm presents a hysteresis loop. Pore size
distributions were further calculated, and are shown in Fig. S2.†
PPN-81 has an obvious pore size at about 4 nm, which is absent
in PPN-80. These results provide evidence for the directing role
of the template, which leads to the formation of mesopores in
PPN-81. Thermogravimetric (TG) analysis yielded comparable
thermal stabilities for PPN-81 and PPN-80 (Fig. S3 and S4†),
suggesting that the presence of mesopores does not reduce
the stability. On the basis of the aforementioned results, we
conclude that PPN-80 and PPN-81 have similar network structures, while the proportion of primary –NH2 to secondary –NH–
groups as well as the porosity in two polymers are different.
The adsorption behavior of CO2, N2, and CH4 on the PPNs
were systematically studied. Despite the different adsorption
temperatures, the isotherms of CO2 on PPN-80 and PPN-81
present a similar shape (Fig. 2, S5 and S6†). High uptakes were
obtained at relatively low pressures, and hysteresis could be
clearly observed. This provides evidence for the existence of
plentiful amine groups in the PPNs, which promote the interaction of CO2 with adsorbents. The adsorbate–adsorbent
interaction was also revealed by the isosteric heat of adsorption
(Fig. S7†). At zero loading, the heat of adsorption of PPN-81
reached 72 kJ mol 1, which is higher than that of PPN-80 (at
54 kJ mol 1). The higher heat of adsorption for PPN-81 is
ascribed to the higher degree of polymerization, which is also
demonstrated by the results of IR and elemental analysis. In
other words, the total concentration of amine groups in PPN-81
is higher than that in PPN-80. With the increase of CO2 uptake,
the heat of adsorption declined progressively (Fig. S7†), which
3254 | J. Mater. Chem. A, 2015, 3, 3252–3256
Communication
Fig. 2 (A) CO2, CH4, and N2 adsorption–desorption isotherms at
295 K. (B) IAST-predicted adsorption selectivity of CO2 over CH4
and N2.
may be caused by the continuous occupation of active sites.
Regardless of the temperatures, CO2 uptake on PPN-81 is
obviously higher than that on PPN-80. For instance, the uptake
of CO2 was measured to be 84.9 mg g 1 on PPN-81 at 295 K and
1 bar while only 71.2 mg g 1 on PPN-80. The adsorption capacity
of PPN-81 is comparable to that of some reported adsorbents
under similar conditions such as porphyrin porous polymer
CuPor-BPDC (31.4 mg g 1),27 microporous metal–organic
framework {[Ni(L)2]$4H2O}n (33.9 mg g 1),28 porous polymer
network PPN-6 (53.7 mg g 1),29 and porous electron-rich
covalent organonitridic framework PECONF-4 (86.2 mg g 1).30
Unlike CO2, CH4 and N2 are barely adsorbed on the PPNs. At
295 K and 1 bar, the uptake of CH4 on PPN-81 was measured to
be only 2.9 mg g 1, and that of N2 was negligible (1.0 mg g 1). It
should be stated that the uptakes of CH4 and N2 measured on
the present materials are lower than those of normal porous
materials reported in the literature. These results suggest high
selectivities of CO2 over CH4 and N2. The ideal adsorption
solution theory (IAST) was further employed to estimate the
selectivities. In the calculation, a CO2/N2 ratio of 15/85 and a
CO2/CH4 ratio of 50/50 were used, which are typical compositions of ue gas emitted from coal-red power plants and
general feed compositions of landll gas, respectively. Both
PPNs exhibited very high selectivities of CO2/CH4 and CO2/N2,
but the selectivities on PPN-81 were measured to be generally
higher than those on PPN-80. The magnitudes of the selectivities are quite marked: the selectivity of CO2/CH4 on PPN-81
reached 1428 at 295 K and 1 bar, and the selectivity of CO2/N2
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reached as high as 4716. To our knowledge, PPN-81 exhibits the
greatest selectivities of CO2 over both CH4 and N2 among the
reported materials. The CO2/CH4 selectivity (1428) is apparently
higher than that of some well-known materials such as metal–
organic framework SIFSIX-3-Zn (231),3 Mg-MOF-74 (137),31
zeolite 13X (103),32 and porous aromatic framework PAF-30
(63).33 Similarly, the CO2/N2 selectivity (4716) is also higher than
that of typical materials, including SIFSIX-3-Zn (1818),3
Mg-MOF-74 (352),31 PPN-6-CH2DETA (442),34 and 13X (220).32
Dynamic breakthrough curves are pretty useful to evaluate an
adsorbent.35–37 Gas mixtures, specically CO2/N2 (15/85) and
CO2/CH4 (50/50), were used for column breakthrough curve
experiments. In the case of both mixtures, the breakthrough of
CO2 is obviously later than those for N2 and CH4 (Fig. S8†). These
results thus conrm the static adsorption results, pointing out
the selective adsorption of CO2 on the present materials.
The recyclability of adsorbents was investigated due to its
importance in practical applications. No loss of activity was
observed even aer six cycles, which indicates the excellent
recyclability of our materials (Fig. 3). It is worthwhile noting that
the saturated adsorbents can be regenerated at only 60 C for 100
min. The mild regeneration conditions are due to the use of
secondary amines as building blocks, which provides a proper
CO2–adsorbent interaction. The regeneration of our materials is
more energy efficient than are most reported adsorbents such as
PPN-6-CH2DETA (100 C) tethered with primary amines34 and
microporous carbon doped with nitrogen (150 C).38
By use of a nucleophilic substitution reaction of chloromethyl
benzene and ethylene diamine, two PPNs were successfully
constructed. The reaction of chloromethyl benzene with amine
groups resulted in the formation of new C–N bonds. As a result, a
great number of secondary amines were generated and functioned as bridges to connect benzene rings (Scheme 1A). A
continuous spatial network was thus fabricated. The network is
made up of not only rigid groups (benzene rings) but also exible
linkages (C–C and C–N single bonds). In the synthetic system
containing the template, the interaction between PPN precursors
and template molecules may be built via hydrogen bonding
(Scheme 1B).39,40 The template thus plays a directing role, making
the assembly and growth of PPN proceed in the continuous
solvent phase between template molecules. Hence, the mesoporous PPN-81 was constructed. In comparison with the amines
of PPN-80, those of PPN-81 are more accessible owing to the high
porosity. The better accessibility of active sites, together with the
Fig. 3
Cycling adsorption of CO2 over PPN-81 at 295 K.
This journal is © The Royal Society of Chemistry 2015
Journal of Materials Chemistry A
higher polymerization degree, is believed to be responsible for
the better adsorption performance of PPN-81 for CO2 capture.
Despite many dedicated efforts, achieving a cost-efficient
synthesis of PPNs has been a challenge with regards to monomer, catalyst, and reaction conditions. In this study, we
designed a new strategy to construct PPNs via a facile nucleophilic substitution reaction for which both monomers are
inexpensive and readily available. More importantly, the polymerization reaction can occur under mild conditions without
the addition of any catalyst. By use of a templating method, the
porosity of the material can be obviously improved, which
increases the access to active sites in frameworks. It is interesting to note that the frameworks of PPNs are comprised of
abundant secondary amines, which offer appropriate adsorbate–adsorbent interactions that are benecial to selective
adsorption and energy-saving regeneration of the adsorbents.
As a result, the present PPNs are highly active for selective
adsorption of CO2, and unprecedented high CO2/CH4 and CO2/
N2 selectivities were achieved. Furthermore, the materials can
be completely regenerated under quite mild conditions. The
cost-efficient synthesis, outstanding adsorption performance,
and energy-saving regeneration make our materials highly
promising in adsorptive separation of CO2 from mixtures such
as ue gas and natural gas.
Conclusions
Two porous polymer networks, namely PPN-80 and PPN-81,
were fabricated via a facile nucleophilic substitution reaction of
chloromethyl benzene and ethylene diamine. The presence of a
template in the synthetic system can promote the formation of
polymer with enhanced porosity and subsequently superior
adsorption performance. The plentiful secondary amines in the
frameworks endow the obtained PPNs with excellent capacity in
selective adsorption and energy-saving regeneration of the
adsorbents. By judicious choice of monomers, the present
strategy should enable secondary amines to be introduced to
frameworks with various pore structures, resulting in the
construction of new porous polymer networks that have high
potential for applications in adsorption and catalysis.
Experimental section
The PPNs were synthesized by a nucleophilic substitution
reaction of 2,4,6-tris(chloromethyl)mesitylene (namely M1) with
ethylene diamine (namely M2). In a typical process, M1 (0.561 g,
2 mmol) was dissolved in THF (50 mL) followed by the addition
of M2 (0.180 g, 3 mmol). The obtained solution was then heated
in a nitrogen atmosphere at 63 C for 24 h. Aer cooling to room
temperature, the reaction mixture was centrifuged to remove
solvent, and the precipitate was treated with an ethanol–water
(20 mL/20 mL) solution of KOH (0.504 g) at 50 C for 12 h. The
material was then washed with an ethanol–water solution three
times and dried at room temperature. The obtained white
powder was denoted as PPN-80. In a similar process, PPN-81
was synthesized by the addition of triblock copolymer P123
(0.5 g) to the initial solution containing monomers. Static
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adsorption experiments of CO2, CH4, and N2 were conducted
using an ASAP 2020 system. Adsorption–desorption isotherms
of CO2, CH4, and N2 at 273 K were measured in an ice-water
bath, while isotherms at 283 and 295 K were measured in a
water bath.
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Acknowledgements
This work was supported by the National Basic Research
Program of China (973 Program, 2013CB733504), the National
High Technology Research and Development Program of China
(863 Program, 2013AA032003), Distinguished Youth Foundation of Jiangsu Province (BK20130045), the Fok Ying-Tong
Education Foundation (141069), and the Project of Priority
Academic Program Development of Jiangsu Higher Education
Institutions.
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