Microporous cyanate resins: Synthesis, porous structure, and correlations with gas and vapor adsorptions

Article abstract of DOI:10.1021/ma3008652

Three silicon and nitrogen-centered cyanate monomers tetrakis(4- cyanatophenyl)silane, tetrakis(4-cyanatobiphenyl)silane, and tris(4-cyanatobiphenyl)amine were designedand synthesized, which were then polymerized via thermal cyclotrimerization reaction to create highly porous cyanate resin networks with systematically varied nodes and linking struts. The chemical structures of monomers and polymers were confirmed by ¹H NMR, FTIR, solid-state ¹³C CP/MAS NMR spectra, and elemental analysis. The products are amorphous with 5% weight-loss temperatures over 428 °C. The results based on N₂ and CO₂ adsorption isotherms show that the pores in these polymers mainly locate in the microporous region, and the BET surface areas are up to 960 m² g^-1, which is the highest value for the porous cyanate resin reported to date. The nitrogen- and oxygen-rich characteristics of cyanate resins lead to the networks strong affinity for CO₂ and thereby high CO₂ adsorption capacity of 11.1 wt % at 273 K and 1.0 bar. The adsorption behaviors of H ₂, CO₂, benzene, n-hexane, and water vapors were investigated by correlating with the chemical composition and porosity parameters of the networks as well as the physicochemical nature of adsorbates.

Full text of DOI:10.1021/ma3008652

Article
pubs.acs.org/Macromolecules
Microporous Cyanate Resins: Synthesis, Porous Structure, and
Correlations with Gas and Vapor Adsorptions
Hao Yu, Changjiang Shen, Mengzhe Tian, Jing Qu, and Zhonggang Wang*
Department of Polymer Science and Materials, State Key Laboratory of Fine Chemicals, School of Chemical Engineering,
Dalian University of Technology, Dalian 116024, People’s Republic of China
S
* Supporting Information
ABSTRACT: Three silicon and nitrogen-centered cyanate
monomers tetrakis(4-cyanatophenyl)silane, tetrakis(4-cyanato-
biphenyl)silane, and tris(4-cyanatobiphenyl)amine were designed
and synthesized, which were then polymerized via thermal cyclo-
trimerization reaction to create highly porous cyanate resin
networks with systematically varied nodes and linking struts.
The chemical structures of monomers and polymers were con-
firmed by 1H NMR, FTIR, solid-state 13C CP/MAS NMR spectra,
and elemental analysis. The products are amorphous with 5%
weight-loss temperatures over 428 °C. The results based on N2
and CO2 adsorption isotherms show that the pores in these
polymers mainly locate in the microporous region, and the BET surface areas are up to 960 m2 g−1, which is the highest value for the
porous cyanate resin reported to date. The nitrogen- and oxygen-rich characteristics of cyanate resins lead to the networks strong
affinity for CO2 and thereby high CO2 adsorption capacity of 11.1 wt % at 273 K and 1.0 bar. The adsorption behaviors of H2, CO2,
benzene, n-hexane, and water vapors were investigated by correlating with the chemical composition and porosity parameters of the
networks as well as the physicochemical nature of adsorbates.
contorted and bulky enough to prevent them from efficiently
packing. These finds open a window for a variety of micorporous
polymers prepared under relatively mild conditions such as
conjugated microporous polymers (CMPs),6 microporous aromatic
frameworks (PAFs),7 and element−organic frameworks (EOFs).8
The involved chemical reactions include Yamamoto,7a,9 Ullmann,10
Suzuki,11 Sonogashira−Hagihara couplings,6 and “click” chemistry12
as well as various condensation polymerizations.13
In comparison with crystalline networks, the current knowl-
edge of control on pore shape, volume, size, and distribution of
amorphous microporous polymers is still far less sufficient since
their pore-formation mechanism and interconnecting behavior
are more complicated. Recent studies have revealed interesting
effects of building blocks and synthetic conditions on the net-
work topology and pore morphology. For the conjugated micro-
porous polymers (CMPs), changing the strut length of the
monomers appears to significantly affect their surface area and
pore size.6a,12a Moreover, the pore properties can even be remark-
ably altered by the reaction medium. For example, the CMP
network synthesized in N,N-dimethylformaide exhibits high
surface area up to 1260 m2 g−1, whereas that obtained in toluene
has lower surface areas of 690 m2 g−1.14 Our previous study shows
that the polyimide networks based on pyromellitic dianhydride has
large surface area of 1407 m2 g−1 and narrow pore size distribution
INTRODUCTION
■
Microporous organic polymers with large surface area and pore
size smaller than 2 nm are attracting great attention in scientific
community due to their academic significance and potential
applications in heterogeneous catalysis, gas storage and selec-
tive separation, optics and chemical sensing, etc.1 Relative to
other microporous materials such as inorganic zeolites and metal−
organic frameworks (MOFs), the outstanding advantages for
microporous organic polymers lie in that they are constructed
from pure organic building blocks via robust covalent bonds and
essentially free of metal. As a result, they are chemical stable in
acid−base and high-temperature moist environments, where the
utilizations of zeolites and MOFs are seriously restricted.
In the past several years, numerous microporous polymers
have been reported in the literature which, according to the aggre-
gation structure, can be roughly classified as two categories: long-
range ordered 2D or 3D crystalline networks and amorphous
polymer networks. Typical crystalline porous networks are
covalent organic frameworks (COFs)2 and covalent triazine-
based frameworks (CTFs)3 obtained under thermodynamic
control by means of reversible borate chemistry and cyclotri-
merization of nitrile groups, respectively. However, in reality,
the reversible thermodynamic reactions for the crystalline networks
are rather limited. Fortunately, the investigations on hyper-cross-linked
polymers (HCPs)4 and polymers of intrinsic microporosity (PIMs)5
reveal that amorphous polymer networks also possess large surface
areas in the cases that the segments of network are sufficiently
opened up by the rigid cross-linking nodes or if the segments are
Received: April 30, 2012
Revised: May 24, 2012
Published: June 5, 2012
© 2012 American Chemical Society
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Thermogravimetry measurement (TG) was performed in the
nitrogen atmosphere on a NETZSCH TG 209 thermal analyzer at a
heating rate of 10 °C min−1. The temperature range was from room
temperature to 800 °C. In order to eliminate the effect of possible
adsorption of moisture from atmosphere, all the samples were dried at
150 °C under vacuum for 10 h before measurements.
Field-emission scanning electron microscopy (FE-SEM) experi-
ments were performed on a Nova NanoSEM 450 scanning electron
microscope. The samples were sputter-coated with gold.
centered at 4−8 Å, whereas the replacement of pyromellitic
dianhydride with naphthalene-1,4,5,8-tetracarboxylic dianhydride
leads to the polymers heterogeneous topological architecture, very
broad pore size distribution, and greatly reduced surface area.15
The pore structure and physicochemical nature of pore wall
are of vital importance for the final gas adsorption property of
microporous polymers. The theoretical and experimental
studies indicate that the narrow pore structure with a diameter
of 0.6−0.7 nm can enhance the affinity for hydrogen and
thereby significantly improve its adsorption capacity,1b,16 while
the introduction of heteroatoms results in high CO2 adsorption
capability and isosteric heat of adsorption owing to the dipole−
quadrupole interactions or hydrogen-bonding effect between
the pore surface and CO2 molecule.17
Cyanate resins (CEs) are important thermosetting engineer-
ing plastics and excellent matrix resins for advanced composite
materials in aeronautics and astronautics industries due to their
high heat resistance, excellent dimensional stability, low moisture
absorption, low dielectric loss, and good mechanical property.18
During curing, every three cyanate groups undergo thermal cyclo-
trimerization to give a sym-triazine ring linked by aryl ether link-
ages, resulting in a uniformly cross-linked network. To our knowl-
edge, microporous cyanate resins are seldom investigated so far
except for a preliminary study in our group.19
All the adsorption and desorption measurements for gases, organic
vapors, and water were performed on an Autosorb iQ (Quantach-
rome) analyzer. Prior to measurements, the samples were degassed at
120 °C under high vacuum for 24 h. N2 adsorption and desorption
isotherms were measured at 77 K. The apparent surface areas (SBET
)
for N2 were calculated using the Brunauer−Emmett−Teller (BET)
model in the relative pressure (P/P0) range from 0.1 to 0.25 for CE-1
and from 0.01 to 0.125 for CE-2 and CE-3. Microporous volumes
were calculated using the t-plot method based on the Halsey thickness
equation, while the total porous volumes were obtained from the N2
isotherm at P/P0 = 0.90. Pore size distributions were derived from the
N2 and CO2 adsorption isotherms using the nonlocal density func-
tional theory (NLDFT). H2 adsorption isotherms were measured at 77
and 87 K up to 1.0 bar. CO2 adsorption isotherms were measured at
273 and 298 K up to 1.0 bar. Water vapor adsorption isotherms were
measured at 293 K, while benzene and n-hexane vapors adsorption
isotherms were measured at 298 K.
Synthesis of Tetrakis(4-methoxyphenyl)silane (1). A solution of 1-
bromo-4-methoxybenzene (7.0 mL, 54.5 mmol) in anhydrous THF
(100 mL) was stirred at −78 °C under dry N2 and treated drop-
wise with a solution of n-butyllithium (24.8 mL, 2.5 M in hexane,
61.6 mmol) over 1 h. The resulting mixture was stirred at −78 °C for
another 1 h, and then SiCl4 (0.5 mL, 4.6 mmol) was added dropwise.
The mixture was stirred overnight. Water was added slowly, and the
product was extracted with diethyl ether. The combined extracts were
washed with water and brine, dried over MgSO4, and filtered. Volatiles
were removed by evaporation under reduced pressure, and the residue
was recrystallized from CH2Cl2 and hexane to afford white solid. Yield
82%; mp 241−244 °C. IR (KBr, cm−1): 3019 (Ar−H), 2950, 2838
(CH3), 1591, 1500 (Ar), 1245, 1026 (Ar−O−C). 1H NMR (400
MHz, CDCl3): δ (ppm) 7.72−7.70 (d, 8H), 7.62−7.60 (d, 8H), 7.59−
7.57 (d, 8H), 7.00−6.98 (d, 8H), 3.85 (s, 12H).
With the above considerations in mind, herein, three cyanate
monomers with tetrahedral and triangular-pyramidal shapes
were designed and synthesized. Each arm of the monomers
contains one or two benzene rings. Thus, after cross-linking,
the nodes and linking struts in the networks are systematically
varied, which enables us to examine the relationship between
building block and pore morphology and helps to thoroughly
understand the influence of pore structure and chemical
composition on the adsorption properties of H2, CO2, N2,
organic, and water vapors of the cyanate resin networks.
EXPERIMENTAL SECTION
■
Materials. Tetrachlorosilane, cyanogen bromide, 1-bromo-4-
methoxybenzene, 1,4-dibromobenzene, tris(4-bromophenyl)amine,
boron tribromide (BBr3), triisopropyl borate, and n-butyllithium
were purchased from J&K Chemical Co., Ltd. Nonylphenol, acetone,
diethyl ether, tetrahydrofuran (THF), dichloromethane, and triethyl-
amine were provided by Shanghai Chemical Reagent Co. Diethyl ether
and THF were purified by refluxing over sodium with the indicator
benzophenone complex. Dichloromethane and triethylamine were
purified by distillation over calcium hydride. Acetone was purified by
refluxing over phosphorus pentoxide and distilled prior to use. Other
chemical reagents were of reagent grade and used as received. All
chromatographic separations were carried out on silica gel (400 mesh)
purchased from Qingdao Make Silica Gel Drier Co., Ltd.
Synthesis of Tetrakis(4-hydroxyphenyl)silane (2). A solution of 1
(3.6 g, 7.88 mmol) in anhydrous CH2Cl2 (64 mL) was added BBr3
(15 mL of 1 M CH2Cl2 solution) dropwise at −78 °C. The resulting
mixture was stirred overnight. The reaction mixture was poured into
water (300 mL) and stirred vigorously for 30 min. The white needle
crystal was obtained by vacuum filtered, washed with water, and
purified by recrystallization from methanol. Yield 40%; mp 287−
290 °C. IR (KBr, cm−1): 3307 (O−H), 3034 (Ar−H), 1595, 1520
1
(Ar), 1246 (Ar−O). H NMR (400 MHz, acetone-d6): δ (ppm) 8.50
(s, 4H), 7.36−7.34 (d, 8H), 6.88−6.86 (d, 8H).
Synthesis of Tetrakis(4-cyanatophenyl)silane (3). A solution of
cyanogen bromide (1.78 g, 16.8 mmol) in anhydrous acetone (30 mL)
was stirred at −30 °C and added simultaneously dropwise with a
solution of 2 (1.4 g, 3.5 mmol) in acetone (30 mL) and a solution of
triethylamine (1.9 mL, 14.0 mmol) in acetone (30 mL) over 1 h. After
addition was completed, the mixture was stirred rapidly at −10 °C for
1 h and at 10 °C for another 1 h. The resulting mixture was evaporated
to dryness and washed with water. The white needle crystal was
obtained by purified through silica gel column chromatography
(CH2Cl2). Yield 75%; mp 175−176 °C. IR (KBr, cm−1): 2267, 2235
Instrumention. Fourier transform infrared spectra (FTIR) were
recorded using a Nicolet 20XB FT-IR spectrophotometer in the 400−
4000 cm−1 region. Samples were prepared by dispersing the complexes
in KBr and compressing the mixtures to form disks.
1H NMR spectra were measured on a 400 MHz Varian INOVA
NMR spectrometer, using tetramethylsilane as an internal reference.
Solid-state 13C CP/MAS (cross-polarization with magic angle
spinning) spectra were recorded on a Varian Infinity-Plus 400 spectro-
meter at 100.61 MHz at an MAS rate of 10.0 kHz using zirconia rotors
4 mm in diameter using a contact time of 4.0 ms and a relaxation delay of
2.0 s. The 1H π/2 pulse was 3.75 μs, and two-pulse phase modulation
(TPPM) decoupling was used during the data acquisition.
1
(CN). H NMR (400 MHz, CDCl3): δ (ppm) 7.59−7.57 (d, 8H),
7.41−7.39 (d, 8H).
Synthesis of 4-Methoxyphenylboronic Acid (4). A solution of 1-
bromo-4-methoxybenzene (13.5 mL, 105 mmol) in anhydrous THF
(300 mL) was stirred at −78 °C under dry N2 and treated dropwise
with a solution of n-butyllithium (42.0 mL, 2.5 M in hexane, 105 mmol)
over 1 h. The resulting mixture was stirred at −78 °C for another hour,
and then triisopropyl borate (24.3 mL, 105 mmol) was added
dropwise. The mixture was stirred at −78 °C for a further hour and
Elemental analyses were determined with an Elementar Vario EL III
elemental analyzer.
A wide-angle X-ray diffraction (WAXD) study of the sample was per-
formed using Rigku D/max-2400 X-ray diffractometer (40 kV, 200 mA)
with a copper target at a scanning rate of 2°/min, scanned from 5° to 60°.
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then allowed to warm to room temperature overnight. 20 mL of water
was added to stop the reaction. The mixture was stirred vigorously for
30 min, then filtered, and successively washed with water and hexane.
The resulting white solid was dried under vacuum. Yield 90%; mp
202−204 °C. IR (KBr, cm−1): 3740−3055 (OH), 3022 (Ar−H), 2956,
Synthesis of Tris(4-hydroxybiphenyl)amine (10). A solution of 9
(4.5 g, 7.98 mmol) in anhydrous CH2Cl2 (160 mL) was added BBr3
(12 mL of 1 M CH2Cl2 solution) dropwise at −78 °C. The resulting
mixture was stirred overnight. The reaction mixture was poured into
water (300 mL) and stirred vigorously for 30 min. The white needle
crystal was obtained by vacuum filtered, washed with water, and purified
by recrystallization from methanol. Yield 58%; mp 260−262 °C. IR
(KBr, cm−1): 3500−2400 (O−H), 3030 (Ar−H), 1599, 1495 (Ar),
1
2839 (CH3), 1606 (CC), 1248 (B−OH). H NMR (400 MHz,
D2O): δ (ppm) 7.28−7.26 (d, 2H), 6.69−6.67 (d, 2H), 4.61 (s, 2H +
H2O), 3.58 (s, 3H).
1
1281 (Ar−N), 1258 (Ar−O). H NMR (400 MHz, acetone-d6): δ
Synthesis of Tetrakis(4-bromophenyl)silane (5). A solution of 1,4-
dibromobenzene (25.0 g, 106.0 mmol) in anhydrous diethyl ether
(300 mL) was stirred at −78 °C under dry N2 and treated dropwise
with a solution of n-butyllithium (42.4 mL, 2.5 M in hexane,
106.0 mmol) over 1.5 h. The resulting mixture was stirred at −78 °C
for 15 min and at 0 °C for 1 h, and then SiCl4 (3.0 mL, 26.5 mmol) in
diethyl ether (20 mL) was added dropwise. The mixture was kept for a
further hour and then allowed to warm to room temperature
overnight. The solvent of acetic acid (5 mL) in water (5 mL) was
then added, and the resulting mixture was stirred vigorously for 30
min. Volatiles were removed by evaporation under reduced pressure.
The white solid was obtained by vacuum filtered and washed with
water and then with methanol. The product was dried under vacuum
at 50 °C and used without further purification. Yield 75%; mp 241−
244 °C. IR (KBr, cm−1): 1377 (Si−Ph), 1066, 1010 (C−Br). 1H NMR
(400 MHz, CDCl3): δ (ppm) 7.54−7.52 (d, 8H), 7.39−7.37 (d, 8H).
Synthesis of Tetrakis(4-methoxybiphenyl)silane (6). 4-Methoxy-
phenylboronic acid (4) (4.2 g, 27.6 mmol), tetrakis(4-bromophenyl)-
silane (5) (3.0 g, 4.6 mmol), and Pd(PPh3)4 (1.1 g, 1.0 mmol) were
dissolved in deoxygenated dioxane (90 mL) under N2 purge. A
solution of deoxygenated 2 M K2CO3 aqueous solution (11.1 g,
40.0 mmol) was added, and then the mixture was refluxed for 48 h.
After filtration, the white needle crystal was obtained, which was washed
with water and further purified through silica gel column chromato-
graphy (CH2Cl2:hexane = 2:1). Yield 89%; mp 233−237 °C. IR (KBr,
cm−1): 3034 (Ar−H), 2959, 2833 (CH3), 1604, 1520 (Ar), 1251, 1080
(ppm) 8.40 (s, 3H), 7.53−7.47 (q, 12H), 7.13−7.11 (d, 6H), 6.90−
6.88 (d, 6H).
Synthesis of Tris(4-cyanatobiphenyl)amine (11). A solution of
cyanogen bromide (3.9 g, 37.0 mmol) in anhydrous acetone (50 mL)
was stirred at −30 °C and added simultaneously dropwise with a
solution of 10 (5.5 g, 10.5 mmol) in acetone (30 mL) and a solution of
triethylamine (4.4 mL, 31.5 mmol) in acetone (30 mL) over 1 h. After
addition was completed, the mixture was stirred rapidly at −10 °C for 1 h
and at 10 °C for another 1 h. The resulting mixture was evaporated to
dryness and washed with water. The white needle crystal was obtained by
purified through silica gel column chromatography (CH2Cl2). Yield 78%;
1
mp 248−250 °C. IR (KBr, cm−1): 2273, 2237 (CN). H NMR (400
MHz, CDCl3): δ (ppm) 7.66−7.64 (d, 6H), 7.50−7.48 (d, 6H), 7.38−
7.36 (d, 6H), 7.25−7.23 (d, 6H).
Preparation of Porous Cyanate Resins. The three cyanate
resins obtained by the thermal cyclotrimerization from tetrakis(4-
cyanatophenyl)silane, tetrakis(4-cyanatobiphenyl)silane, and tris(4-
cyanatobiphenyl)amine are named as CE1, CE2 and CE3, respectively.
They were prepared under the same conditions, so only a typical
procedure for CE-1 is described as follows: To a mixture of 3 (1.3 g)
and diphenyl sulfone (15 g) was added nonylphenol (0.013 g). The
resulting mixture was heated under nitrogen atmosphere at 170 °C for
4 h, 190 °C for 4 h, 230 °C for 8 h, 270 °C for 8 h, 290 °C for 4 h, and
330 °C for 2 h. The gel obtained was rushed and extracted with THF
in a Soxhlet apparatus for 48 h. The solid was then dried under
vacuum at 150 °C for 2 days. Yield 95%.
1
(Ar−O−C). H NMR (400 MHz, CDCl3): δ (ppm) 7.72−7.70 (d,
8H), 7.62−7.60 (d, 8H), 7.59−7.57 (d, 8H), 7.00−6.98 (d, 8H), 3.85 (s,
12H).
RESULTS AND DISCUSSION
■
Synthesis of Tetrakis(4-hydroxybiphenyl)silane (7). A solution of 6
(4.0 g, 5.26 mmol) in anhydrous CH2Cl2 (160 mL) was added BBr3
(10 mL of 1 M CH2Cl2 solution) dropwise at −78 °C. The resulting
system was stirred overnight. The mixture was poured into water (300 mL)
and stirred vigorously for 30 min. The white needle crystal was washed with
water and purified by recrystallization from methanol. Yield 80%; mp 179−
182 °C. IR (KBr, cm−1): 3307 (O−H), 3034 (Ar−H), 1595, 1520 (Ar),
Synthesis and Characterization of Cyanate Monomers
and Cyanate Resin Networks. The synthesis routes to the
three cyanate monomers with tetrahedral or triangular-pyramidal
configurations are outlined in Schemes 1−3. The starting com-
pound 1-bromo-4-methoxybenzene was first lithiated by BuLi and
then reacted with SiCl4 to give 1. After demethylation, 1 was con-
verted into 2, which was further reacted with cyanogen bromide to
produce tetrakis(4-cyanatophenyl)silane (3). For the synthesis of
tetrakis(4-cyanatobiphenyl)silane, 1-bromo-4-methoxybenzene
was boronized to give 4, while the reaction between 1,4-dibromo-
benzene, BuLi, and SiCl4 led to 5. Then 6 was synthesized by
Suzuki cross-coupling between 4 and 5. The subsequent pro-
cedures of demethylation and cyanate esterification were similar to
that of 3. In addition, the Suzuki cross-coupling between 4 and
tris(4-bromophenyl)amine afforded 9. The demethylation of 9
gave tris(4-hydroxybiphenyl)amine (10), which was reacted with
cyanogen bromide in the presence of triethylamine to produce the
target product tris(4-cyanatobiphenyl)amine (11).
1
1246 (Ar−O). H NMR (400 MHz, acetone-d6): δ (ppm) 8.52 (s, 4H),
7.69 (s, 16H), 7.60−7.58 (d, 8H), 6.97−6.95 (d, 8H).
Synthesis of Tetrakis(4-cyanatobiphenyl)silane (8). A solution of
cyanogen bromide (2.8 g, 26.5 mmol) in anhydrous acetone (40 mL)
was stirred at −30 °C and added dropwise with a solution of 7 (4.0 g,
5.7 mmol) in acetone (40 mL) and a solution of triethylamine
(3.2 mL, 22.8 mmol) in acetone (40 mL) over 1 h. After addition was
completed, the mixture was stirred rapidly at −10 °C for 1 h and at
10 °C for another 1 h. The resulting mixture was evaporated to dryness
and washed with water. The white needle crystal was obtained by purified
through silica gel column chromatography (CH2Cl2). Yield 72%; mp
266−268 °C. IR (KBr, cm−1): 2264, 2237 (CN). 1H NMR (400 MHz,
CDCl3): δ (ppm) 7.78−7.76 (d, 8H), 7.74−7.72 (d, 8H), 7.66−7.64 (d,
8H), 7.44−7.42 (d, 8H).
The chemical structures of the cyanate monomers were
confirmed by FTIR and 1H NMR. In the FTIR spectra (Figure
1S in Supporting Information), the appearance of stretching vibra-
tions of −OCN at 2273 and 2237 cm−1 confirms the formation of
cyanate group. The absorption bands at 1445, 1108, and 761 cm−1
are characteristics of the C−Si bond for tetrakis(4-cyanatophenyl)-
silane and tetrakis(4-cyanatobiphenyl)silane. The absorption at 1285
cm−1 is attributed to the Ar−N bond in tris(4-cyanatobiphenyl)-
amine. In addition, all the resonance peaks in the 1H NMR spectra
for cyanate monomers (Figures 2S−4S) can be well assigned to the
corresponding protons.
Synthesis of Tris(4-methoxybiphenyl)amine (9). Tris(4-
bromophenyl)amine (3.0 g, 6.2 mmol), 4-methoxyphenylboronic
acid (4) (4.3 g, 28.0 mmol), and Pd(PPh3)4 (1.1 g, 1.0 mmol) were
dissolved in deoxygenated dioxane (90 mL) under N2 purge. A
solution of deoxygenated 2 M K2CO3 (11.1 g, 40.0 mmol) was added,
and then the mixture was refluxed for 48 h. After filtration, the crude
product was purified through silica gel column chromatography to
obtain white solid (CH2Cl2:hexane = 1:1). Yield 91%; mp 237−
240 °C. IR (KBr, cm−1): 3032 (Ar−H), 2955, 2833 (CH3), 1603, 1495
1
(Ar), 1248, 1080 (Ar−O−C), 1281 (Ar−N). H NMR (400 MHz,
acetone-d6): δ (ppm) 7.62−7.60 (d, 6H), 7.59−7.57 (d, 6H), 7.19−
7.17 (d, 6H), 7.02−7.00 (d, 6H), 3.84 (s, 9H).
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Scheme 1. Synthesis Route to Tetrakis(4-cyanatophenyl)silane
Using diphenyl sulfone as solvent, three cross-linked cyanate
resins (CE-1, CE-2, CE-3) were prepared from tetrakis(4-
cyanatophenyl)silane, tetrakis(4-cyanatobiphenyl)silane, and
tris(4-cyanatobiphenyl)amine, respectively. The syntheses of
cyanate resin networks are illustrated in Scheme 4. All the poly-
merizations were carried out at the same concentration of
5 wt %, and 1.0 wt % of nonylphenol was added in the system
to catalyze the cyclotrimerization reaction. In order to avoid
rapid cross-linking and obtain the homogeneous network, the
polymerizations were conducted initially at around 170 °C for 4
h. Then the temperature was raised stepwise to 330 °C and
allowed to polymerize at this high temperature for 2 h to ensure
a sufficient cross-linking.
The obtained products are insoluble in any common solvents
such as dimethyl sulfoxide (DMSO), N,N-dimethylformamide
(DMF), N,N-dimethylacetamide (DMAc), N-methylpyrroli-
done (NMP), and tetrahydrofuran (THF), suggesting the
hyper-cross-linked structure. The XRD patterns (Figure 5S)
show that they are amorphous in nature. After curing, the
cyanate resins display strong characteristic peaks at 1562 and
1367 cm−1 in the FTIR spectra (Figure 6S), indicative of the
generation of large amount of triazine rings. The disappearance
of −O−CN stretching bands at 2273 and 2237 cm−1 demon-
strates that cyanate groups have been completely consumed
under the polymerization conditions. The presence of weak
adsorption at 1720 cm−1 for CE-1 and CE-2 implies the trace
amount of dimerized linkage [−OCONC(NH2)O−].20 The
solid-state 13C CP/MAS NMR spectra for the three networks
are illustrated in Figure 7S. The resonance peak at 174 ppm
belongs to the carbon atom of triazine ring, while the signal
at 155 ppm is attributed to the O-substituted phenyl carbon.
The N-substituted phenyl carbon of CE-3 locates at 147 ppm.
Other signals of phenyl carbons are observed at chemical
shifts between 137 and 116 ppm. The calculated chemical
compositions for the networks are roughly consistent with
the measured values by elemental analysis (Table 1S), except
for hydrogen. The absorbed moisture in the porous cyanate
resin networks may account for the higher hydrogen contents
than the theoretical values.
To investigate the thermal stability of the cyanate resins, the
TG and derivative TG (DTG) curves as a function of tem-
perature in a nitrogen atmosphere were recorded (Figure 8S).
The results show that they are stable over 400 °C with the 5%
weight losses for CE-1, CE-2, and CE-3 at 428, 453, and 486 °C,
and the char yields at 800 °C are 57.8, 65.3, and 72.7 wt %,
respectively. In addition, the DTG curves exhibit two thermal
steps of degradation. The first stage for all the samples occurs at
almost the same temperature (ca. 450 °C), which is associated
with the cleavage of trizaine ring in the network.18b The second
one in the range from 550 to 600 °C is due to the decomposi-
tion of the aromatic skeleton.
The products were observed by field-emission scanning
electron microscopy (Figure 1). Similar to other microporous
polymers,6 21 the surface morphologies of three samples are
c,
homogeneous, composed of loosely packed tiny particles.
Moreover, the particles have irregular shape with rather rough
surface. The particle diameters of CE-1 and CE-2 are between
20 and 40 nm, whereas the sizes of CE-3 are apparently larger,
ranging from 50 to 80 nm.
Porous Structure and Pore Size Distribution of
Cyanate Resin Networks. The porosity parameters of the
cyanate resins were determined by nitrogen sorption at 77 K.
As shown in Figure 2, all the three sorption isotherms exhibit a
steep rise of uptake at the low relative pressure region (<0.01),
indicative of the presence of substantial micropores. In the
relative pressure range from 0.1 to 0.8, the isotherms of CE-2
and CE-3 are flat which, according to the classification of IUPAC,
can be attributed to type I sorption. In contrast, the nitrogen
uptake in CE-1 continuously increases with the pressure,
displaying a combination of type I and IV characteristics. CE-1,
therefore, consists of both microporous and mesoporous
structures. On the other hand, when the relative pressure
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Scheme 2. Synthesis Route to Tetrakis(4-cyanatobiphenyl)silane
exceeds 0.8, all of them show significant increase of nitrogen
uptake, suggesting the presence of macroporous structure. This
can be interpreted as the interparticulate voids arising from the
loose packing of small particles as observed in the FE-SEM
micrographs. In addition, for each sample, the desorp-
tion isotherm lies above the adsorption one to form an unclosed
loop, displaying a typical desorption−adsorption hysteresis effect.
A similar phenomenon has also been observed for other micro/
mesoporous polymer materials and is attributed to the
deformation of pore structure because of the softness of organic
and molecular mechanics optimizing, the energy-minimized
stuctures are relaxed through dynamic calculation at 298 K for
200 ps based on the COMPASS force field to obtain the models
as presented in Figure 3. For CE-1 and CE-2, each monomer
contains four cyanate groups, and every three cyanate groups from
different monomers condense to form a triazine ring. If the cross-
linking reactions are sufficient and all the cyanate groups have
been consumed, the topological models show that part of voids in
the networks will be further divided by one rigid tetrahedral node.
As a consequence, the networks should simultaneously consist of
microropores and mesopores with the sizes of 0.65 and 2.3 nm for
CE-1 and 0.89 and 3.2 nm for CE-2, respectively. By contrast, for
CE-3, its monomer has only three cyanate groups. After curing,
the triphenylamine and triazine nodes are mutually connected to
yield cyclic structure with the size of 2.5 nm.
polymer skeleton and swelling effect in liquid nitrogen.1b,6 22
c,
In order to illustrate the topological structures and elucidate
the possible pore-forming mechanism of the cyanate resin
networks, the computer simulations were conducted using
Materials Studio software. The models were constructed by con-
necting three neighboring tetrahedral or triphenylamine units via
a symmetrical 1,3,5-triazine ring. The mutual penetration of
networks was not taken into account. After successive geometry
Using nitrogen as probe, the experimental pore size distri-
butions for the cyanate resin networks were obtained from the
adsorption isotherms by the nonlocal density functioanl therory
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Scheme 3. Synthesis Route to Tris(4-cyanatobiphenyl)amine
Scheme 4. Representative Synthesis Route to Cyanate Resin from Tetrahedral Monomers
Figure 1. FE-SEM images of the obtained cyanate resins.
(NLDFT). Figure 4 shows that the major pores lie in the
microporous region. In addition, it is interesting to observe that
all the samples have the similar microporous sizes ranging from
0.7 to 0.9 nm. For CE-2 and CE-3, only a small amount of
mesopores is detected. The mesoporous size is 2.6−3.1 nm for
CE-2 and around 2.6 nm for CE-3, which value agrees with the
simulated one in Figure 3. As a matter of fact, since the poly-
merization proceeded under kinetic control, the interpenetra-
tion of networks, especially for those having longer struts, is
inevitable.6 The larger freedom and flexibility because of the
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Figure 4. Pore size distribution obtained by the NLDFT method
(filled symbols: derived from CO2 adsorption isotherm at 273 K; open
symbols: derived from N2 adsorption isotherm at 77 K).
Figure 2. Nitrogen adsorption−desorption isotherms of CE-1 (+400),
CE-2 (+200), and CE-3.
Table 1. Pore Structure Parameters of the Cyanate Networks
Obtained by N2 Adsorption
longer arms in CE-2 and CE-3 monomers are relatively easier
for the mutual penetration of networks so that almost all the
mesopores have been divided into micropores and even ultra-
micropores (<7 Å) by the penetrated segments. In the case of
CE-1 monomer, the shorter arm restricts its motion ability.
Moreover, the steric hindrance also restricts the interpenetra-
tion of netowrks to some extent. As a consequence, in contrast
to CE-2 and CE-3, CE-1 exhibits more mesopores. On the
other hand, the pore distribution curves in Figure 4 obtained
from nitrogen adsorption isotherms did not detect ultramicro-
pore in the samples. The reason may be due to the complexity
of pore channels in the cyanate resin networks and that the
large kinetic diameter of N2 molecule (3.64 Å) hinders its
accessibility to the ultramicropore.23 Considering that CO2 has
the rodlike molecular structure and smaller kinetic diameter
(3.30 Å), it is more suitable as molecule probe to reveal the
ultralmicroporous information. As shown in Figure 4, indeed,
the analysis of CO2 adsorption isotherms at 273 K indicates
that there are large amounts of pores with size in the range
between 4 and 6 Å in the three cyanate resin networks.
a
b
c
d
e
SBET
SL
Vmicro
Vtotal
pore size
(nm)
sample
(m2 g−1
)
(m2 g−1
)
(cm3 g−1
)
(cm3 g−1
)
CE-1
CE-2
CE-3
960
588
540
1417
810
0.126
0.155
0.140
0.97
0.47
0.43
0.823
0.783
0.863
750
a
b
Brunauer−Emmett−Teller surface area. Langmuir surface area.
c
Micropore volume derived using the t-plot method based on the
d
Halsey thickness equation. Total pore volume determined from the
N2 isotherm at P/P0 = 0.90. Micropore size derived from N2 isotherm
e
using the NLDFT method.
filling of pore by the penetrated segments reduces the porosity
and surface area. Cooper and co-workers also reported that the
longer strut length led to the CMPs lower surface areas.6,13a
Although CE-1 has the larger surface area than CE-2 and CE-3,
its microporous volume derived using the t-plot method is only
0.126 cm3 g−1, lower than those of CE-2 (0.155 cm3 g−1) and
CE-3 (0.140 cm3 g−1). In addition, it should be mentioned that
the polymerization condition can remarkably influence the pore
property of the product. In our previous preliminary study, the
CE-1 was cured without adding any catalyst, and the final
reaction temperature was only 270 °C. The FTIR spectra
demonstrated that the reaction was incomplete. As a result, the
The porosity data of the three cyanate resins are listed in
Table 1. CE-1 exhibits the BET surface area of 960 m2 g−1 and
Langmuir surface area of 1417 m2 g−1, which are the largest
among the three samples. The lower surface areas in CE-2 and
CE-3 can be caused by the network interpenetration, and the
Figure 3. Energy-optimized molecule models for CE-1 (left), CE-2 (middle), and CE-3 (right).
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measured BET surface area was only 528 m2 g−1.19 To solve the
above problem, nonylphenol was employed here to catalyze the
reaction, and at the final stage, the product was further treated
at 330 °C to facilitate the sufficient curing. The results indicate
that the experimental improvement is quite effective for the
enhancement of surface area.
Adsorptions of CO2 and H2 and Their Correlations
with Pore Structure. The CO2 adsorption isotherms for the
three samples at 273 and 298 K are presented in Figure 5. The
In this study, the CO2 isosteric enthalpies of adsorption (Qst)
were calculated from the sorption isotherms measured at
different temperatures in term of Clausius−Clapeyron equa-
tion:24
Q st
ln P =
+ C
(1)
RT
where R is the gas constant and C is a constant, whereas P and
T are the pressure and temperature at the equilibrium state,
respectively.
The CO2 isosteric enthalpies (Qst) as a function of adsorbed
amount are plotted in Figure 6. The Qst values and the uptake
Figure 5. CO2 adsorption isotherms of the three cyanate resin
networks.
absorbed CO2 amount continually increases with the pressure,
implying that the CO2 adsorption by the porous network has
not reached its equilibrium or saturated state in the measure-
ment pressure range. The data in Table 2 show that, at 273 K
Figure 6. Variation of CO2 isosteric enthalpies with the adsorbed
amount.
capacities have the same ranking order of CE-1 > CE-3 > CE-2.
The data in Table 1S show that the N/C and O/C atomic
ratios of CE-1 are 0.17 and 0.19, respectively, which are the
highest among the three samples, indicating that, besides the
larger surface area, the high contents of oxygen and nitrogen
atoms in CE-1 significantly enhance the affinity for CO2, lead-
ing to its high CO2 uptake. The further comparison between
CE-2 and CE-3 reveals that they have the similar pore distribu-
tion, and the surface area of CE-3 is even smaller than that of
CE-2, but CE-3 exhibits apparently higher CO2 uptake and
isosteric enthalpy. The reason can be due to the presence of
more nitrogen atoms in CE-3 coming from the triazine ring and
triphenylamine moieties. In addition, the Qst values show a
dropping trend with the adsorbed CO2 amount, suggesting that
the CO2 molecule prefers to adsorb on the pore surface of
cyanate resin networks.
Table 2. Uptakes of Gases, Organic, and Water Vapors in the
Cyanate Networks
a
b
c
d
e
CO2
H2
benzene
(wt %)
n-hexane
(wt %)
water
sample
(wt %)
(wt %)
(wt %)
CE-1
CE-2
CE-3
11.1
7.91
8.50
1.11
0.98
1.04
58.5
35.1
34.4
35.7
29.8
24.9
22.0
6.9
6.3
a
b
CO2 uptake determined at 1 bar and 273 K. H2 uptake determined
c
at 1.0 bar and 77 K. Benzene uptake obtained from the benzene vapor
adsorption isotherm at P/P0 = 0.9 at 298 K. n-Hexane uptake
d
determined from the n-hexane vapor adsorption isotherm at P/P0 =
e
0.9 at 298 K. H2O uptake determined from the water vapor adsorp-
tion isotherm at P/P0 = 0.9 at 293 K.
and 1.0 bar, CE-1, CE-2, and CE-3 have the CO2 uptake of
11.1, 7.91, and 8.50 wt %, respectively, which are higher than
many porous polymers possessing larger surface area at the
same condition, such as COF-5 (5.9 wt %, 1670 m2 g−1),2b
COF-103 (7.6 wt %, 3530 m2 g−1),2b and BLP-1H (7.4 wt %,
1360 m2 g−1).23 The high CO2 uptakes are probably attributed
to the abundant nitrogen and oxygen atoms in the cyanate resin
networks. It has been reported that the introduction of electron-
rich atoms, e.g., nitrogen and oxygen, can yield strong dipole−
quadrupole interaction with CO2, leading to a significant increase
in the adsorption enthalpy. For example, the amine functionality in
MOF results in an increased CO2 adsorption enthalpy from 30
to 50 kJ mol−1,17d while the MOF with immobilized functional
OH groups on pore surfaces exhibits high adsorption enthalpy
of 40.6 kJ mol−1.17e
In an effort to gain more insight into the physicochemical
nature of CO2 adsorption in the networks, the CO2 isotherms
at different temperatures were analyzed using the virial equation25
ln(n/P) = A0 + A1n + A2n2 + ...
(2)
where n is the adsorbed amount at pressure P and A0, A1, etc., are
virial coefficients.
At low surface coverage, A2 and higher terms can be neglected.
A0 is related to the gas−material interaction, whereas A1 reflects
gas−gas interaction. The Henry’s law constant (KH) can be
calculated by the equation KH = exp(A0). Figure 7 shows that
the virial plots of CO2 adsorption for the three networks follow
a good linear relationship, from which the A0, A1, and KH can be
calculated. Based on the K′ at different temperature, the limiting
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Figure 9. Variation of isosteric enthalpies of H2 with the adsorbed
amount.
Figure 7. Virial plots of CO2 adsorptions for CE-1, CE-2, and CE-3.
enthalpies of adsorption (Q0), i.e., Qst at zero surface CO2
coverage, were derived from the slope of the plot of ln KH vs 1/
T. The data of A0, A1, and KH and Q0 are listed in Table 2S. It
can be seen that CE-1 has the largest A0, KH, and Q0, which is
consistent with the ranking sequence of CO2 uptake capacities and
isosteric enthalpies obtained by the Clausius−Clapeyron equation.
Moreover, it is noteworthy that the Q0 values of the three
networks are higher than those of porous materials such as ZTF-1
(25.4 kJ mol−1)17b and BILP-1 (26.5 kJ mol−1)13d and comparable
to those of MOFs with organic ammonium ions or Lewis basic
amine in pores (30−55 kJ mol−1).26
It is widely recognized that the H2 adsorption capacity in a
porous material is mainly dominated by its microporous area
and microporous volume.1b,16 Nevertheless, the data in Table 2
show that CE-1 has the apparently lower microporous volume
than CE-2 and CE-3, but it exhibits the H2 uptake of 1.11 wt %
at 77 K and 1.0 bar, which is even slightly higher than CE-2
(0.98 wt %) and CE-3 (1.04 wt %). The reason can be
attributed to CE-1 having the highest H2 adsorption enthalpy
among the three networks.
The virial plots of H2 adsorption at 77 and 87 K are shown in
Figure 10. All of them exhibit good straight lines. The limiting
Hydrogen adsorption isotherms of the three networks at 77
and 87 K are presented in Figure 8, from which the isosteric
Figure 10. Virial plots of H2 adsorption for CE-1, CE-2, and CE-3.
Figure 8. H2 adsorption isotherms of three cyanate resin networks.
enthalpies of adsorption (Q0) of hydrogen obtained through the
similar method to CO2 are 9.42 kJ mol−1 for CE-1, 7.54 kJ mol−1
for CE-2, and 7.75 kJ mol−1 for CE-3, higher than or comparable to
other porous organic polymers such as COFs (6.0−7.0 kJ mol−1),2b
polyimide networks (5.3−7.0 kJ mol−1),15a benzimidazole-linked
polymer (7.9 kJ mol−1),13d POFs (8.3−9.0 kJ mol−1),28 and
polyanilines (5.0−9.3 kJ mol−1).29
Adsorptions of Organic and Water Vapors in the
Cyanate Resin Networks. The adsorption isotherms of benzene
and n-hexane vapors measured at 298 K and water vapor
measured at 293 K are shown in Figure 11. It can be seen that,
for benzene and n-hexane, after a steep increase of uptake at the
low relative pressure (P/P0 < 0.1), the adsorptions iso-
therms display apparent convex shapes with respect to the pressure,
indicating that there is a strong interaction between cyanate
resin networks and organic molecules. As a consequence, besides
enthalpies of H2 at different adsorbed amounts were calculated
and presented in Figure 9. The results show that CE-1 has
significantly higher Qst value than the other two samples. The
reason can be due to the large amount of tetraphenylsilane
moieties in CE-1 network. The lower electronegativity of the
silicon with respect to the carbon atom leads to the ionic
characteristic of Si−C bond,27 and the electron deficiency of
silicon can be transferred to the neighboring phenyls through
the σ−π conjugation, which is advantageous for the affinity of
H2 for the polymer. CE-2 also contains silicon, but its atomic
ratio of Si/C is only 1.92, far less than that of 3.57 in CE-1.
Similar to CO2, the Qst values of H2 decrease with the adsorbed
amount, meaning that the interaction between H2 and polymer is
stronger than that between H2 molecules.
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polymers are different from that of PFD values. As a matter of fact,
the vapor uptake in a porous polymer is contributed by both its
total pore volume and PDF value. Apparently, in the present
polymer system, the former factor plays the more important role.
CONCLUSIONS
■
Three porous cyanate resins CE-1, CE-2, and CE-3 were
successfully synthesized in diphenyl sulfone solution by the
thermal cyclotrimerization of tetrakis(4-cyanatophenyl)silane,
tetrakis(4-cyanatobiphenyl)silane, and tris(4-cyanatobiphenyl)-
amine, respectively. The complete conversion of cyanate groups
into triazine rings were confirmed by FTIR, solid-state 13C CP
MAS spectra, and elemental analysis. The computer modeling
shows that the networks should consist of large amount of
mesopores, but using N2 and CO2 as molecule probes, the
NLDFT results show that the major pores of the three samples
locate in the microporous region, and the pore sizes are at 7−9
and 4−6 Å, respectively. The reason is attributed to the
subdivision of mesopore by the interpenetration of networks.
Among the three samples, the atom ratios of N/C and O/C in
CE-1 are the highest. Consequently, the electron-rich N and O
endow CE-1 with strong affinity for CO2 with limiting isosteric
heat of adsorption up to 39.9 kJ/mol and high CO2 adsorption
capacity of 11.1 wt %. On the other hand, the presence of
electron-deficient silicon atom and its σ−π conjugative effect with
neighboring phenyls lead to CE-1 enhanced affinity for H2 and
thereby higher H2 adsorption than CE-2 and CE-3 even though
the microporous volume of CE-1 is the lowest among them. The
uptakes of organic and water vapors were investigated in term of
the total pore volume, pore filling degree, oleophilic/hydrophilic
property of inner surface of pore wall, and the π−π and hydrogen-
bonding interactions between polymer and vapor molecule.
Figure 11. Organic and water vapor adsorption isotherms as a
function of the relative pressure at 298 and 293 K for three porous
cyanate resins (benzene + 30%, hexane + 10%, water).
adsorption, the absorption of organic molecules in the polymer
framework may also contribute to the total sorption capacity to
some extent. In contrast, isotherms for water adsorption are typical
type III adsorption, suggesting a weak affinity of networks toward
water molecule. The uptakes of benzene, n-hexane and water for
cyanate resins at P/P0 = 0.9 are listed in Table 2. At 298 K, the
benzene uptake for CE-1 is 58.5 wt %, much higher than those for
carbon material F42C (39.5 wt %)30 and metal azolate framework
MAF-2 (20.6 wt %),31 showing application potential in the
removal of organic pollutants. On the other hand, for each vapor,
the adsorption capacities in the three polymers are ranked as CE-1
> CE-2 > CE-3, which is consistent with the order of total pore
volumes other than micropore volumes, implying that the
mesopores and interparticulate macropores also contribute to
the vapor uptakes.
To further understand the effect of physicochemical nature of
porous polymer on the vapor adsorption, the pore filling degree
(PFD), i.e., the ratio of adsorbed vapor volume to the total pore
volume, is utilized here as a measure of the affinity of vapor
molecule on the pore wall of a polymer. The calculated PFD
values of benzene are 69%, 85%, and 91% for CE-1, CE-2, and
CE-3, respectively, which is the same as the trend of phenyl
contents in polymers (7.99, 9.95, and 10.06 mmol g−1, res-
pectively), meaning that the phenyl groups in the networks
provide the abundant affinity sites for aromatic benzene mol-
ecule by virtue of the strong π−π interaction. On the other
hand, the PFD values for organic and water vapors can reflect
the hydrophilic/oleophilic nature of the inner pore surface of a
porous polymer. For example, for water adsorption, relative to
CE-2 and CE-3, the higher content of oxygen atom and
nitrogen-rich triazine ring in CE-1 results in it stronger hydro-
philicity by hydrogen-bonding interaction, and consequently,
the water PFD value of 23% for CE-1 is apparently higher than
those of CE-2 and CE-3 (around 15%). In contrast, the PDF
values of n-hexane are 56%, 96%, and 88% for CE-1, CE-2, and
CE-3, respectively, implying that CE-2 and CE-3 are more
oleophilic than CE-1. In addition, the comparison reveals that
the ranking order of vapor adsorption capacities in the three
ASSOCIATED CONTENT
■
S
* Supporting Information
1
Eight figures and two tables, including the FTIR and H NMR
spectra of three cyanate monomers, FTIR and solid-state 13C CP/
MAS NMR spectra, X-ray diffraction patterns, and TG curves
as well as the data of elemental analysis, virial parameters and
isosteric enthapies of the cyanate resin networks. This material is
available free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*E-mail: zgwang@dlut.edu.cn; Tel: 0086-411-84986096.
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The National Science Foundation of China (Nos. 51073030
and 20874007) and Program for New Century Excellent
Talents in University of China (No. NCET-06-0280) are
gratefully acknowledged. We thank Mr. Xin Li and Mr. Zhan
Wang (Quantachrome Instruments) for the technical support
of adsorption measurements for gases, organic, and water
vapors.
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