Phosphazene functionalized silsesquioxane-based porous polymers for absorbing I2, CO2 and dyes

Article abstract of DOI:10.1016/j.polymer.2021.123491

Porous polymers have been widely used as adsorbents to cope with environmental issues. Two parallel series of hybrid silsesquioxane-based phosphazene functionalized porous polymers (PCS-OP-1, 2, 3 and PCS-CP-1, 2, 3) have been prepared by varying the molar ratio of hexaphenoxycyclotriphosphazene (OP) or hexaphenylcyclotriphosphazene (CP) with octavinylsilsesquioxane (OVS) in the Friedel-Crafts reaction, respectively. PCS-OP-3 and PCS-CP-3 with hierarchical micropore/mesopore coexisting structures and high surface areas are chosen to absorb I₂ vapor, dyes and CO₂. The adsorption capacity of PCS-OP-3 is higher than PCS-CP-3, which is 1.51 g g^?1 for iodine vapor, 731 mg g^?1 for Congo red (CR), 151 mg g^?1 for Methylene Blue (MB) and 1.74 mmol g^?1 for CO₂. This study provides a feasible method to prepare and tune phosphazene functionalized silsesquioxane-based porous polymers.

Full text of DOI:10.1016/j.polymer.2021.123491

Polymer 218 (2021) 123491
Contents lists available at ScienceDirect
Polymer
journal homepage: http://www.elsevier.com/locate/polymer
Phosphazene functionalized silsesquioxane-based porous polymers for
absorbing I2, CO2 and dyes
,b,*
Yiqi Wanga, Mikhail Soldatova, Qingzheng Wanga, Hongzhi Liua
a Key Laboratory of Special Functional Aggregated Materials, Ministry of Education, School of Chemistry and Chemical Engineering Shandong University, Jinan, PR
China
b Key Laboratory of Organosilicon and Materials Technology of the Ministry of Education, Hangzhou Normal University, Hangzhou, 31112, PR China
A R T I C L E I N F O
A B S T R A C T
Keywords:
Porous polymers have been widely used as adsorbents to cope with environmental issues. Two parallel series of
hybrid silsesquioxane-based phosphazene functionalized porous polymers (PCS-OP-1, 2, 3 and PCS-CP-1, 2, 3)
have been prepared by varying the molar ratio of hexaphenoxycyclotriphosphazene (OP) or hexaphenylcyclo-
triphosphazene (CP) with octavinylsilsesquioxane (OVS) in the Friedel-Crafts reaction, respectively. PCS-OP-3
and PCS-CP-3 with hierarchical micropore/mesopore coexisting structures and high surface areas are chosen
to absorb I2 vapor, dyes and CO2. The adsorption capacity of PCS-OP-3 is higher than PCS-CP-3, which is 1.51 g
gꢀ 1 for iodine vapor, 731 mg gꢀ 1 for Congo red (CR), 151 mg gꢀ 1 for Methylene Blue (MB) and 1.74 mmol gꢀ 1 for
CO2. This study provides a feasible method to prepare and tune phosphazene functionalized silsesquioxane-based
porous polymers.
Phosphazene
Silsesquioxane
Adsorption
1. Introduction
stable because of inorganic moieties and easily modified because of
organic moieties [14–18]. These above-mentioned properties inspired
Recently, environmental pollution induced by dyes, CO2 and radio-
active nuclear waste (e.g., 129I, 85Kr, 127Xe) has attracted the attention of
governments and organizations worldwide because of its long-term
harmful influence on both human health and nature [1–3]. The main
sources of pollutants include municipal, industrial and agricultural ac-
tivities, and it is urgent to find new effective materials and methods for
their removal from wastes.
researchers to study hybrid materials intensely during the recent de-
cades. Cage-like silsesquioxanes (SQs), which have three-dimensional
structure, are inorganic–organic hybrid molecules with general chemi-
cal formula of (RSiO1.5)n (n = 6, 8, 10–12, 14), where cubic T8 cages
with n = 8 are the most extensively studied [19–25]. SQs possess an
excellent chemical and thermal stability, and can react with various
substances to prepare micro-mesoporous polymers with high surface
area [26–28]. Octavinylsilsesquioxane (OVS) is one of the mostly used
cage monomers for preparation of silsesquioxane-based porous mate-
rials through hydrosilylation reaction [16,29–31], Heck coupling reac-
tion [28,32], Friedel-Crafts reaction [33–36], cationic polymerization
[26] etc. It’s worth mentioning that the Friedel-Crafts reaction is a
simple method with mild reaction condition, high efficiency and
low-cost catalyst to prepare porous polymers in a large scale compared
to Heck reaction and hydrosilylation reaction, which need noble cata-
lysts and strict condition [29–33]. Recently, Zhang et al., synthesized a
new porous polymer by Friedel-Crafts reaction, which showed an
excellent thermal stability and iodine loading capacities (1.37 g gꢀ 1 at
90 ◦C) [37].
Currently, adsorption is considered as an efficient way to remove
pollutants from human activities wastes, due to variability of good ad-
sorbents and its technological simplicity [3–6]. Activated carbon, zeo-
lites and carbon nanotubes were widely used in adsorption with low cost
and good reversibility [7–9]. However, the low adsorption capacity and
removal efficiency become obstacles to widespread use. Metal-organic
frameworks (MOFs) [10–13] is a kind of porous three-dimensional
crystal for adsorption due to large specific surface area, while it is
water-instable and highly cost. Thus, it is of growing need for finding an
ideal adsorbent exhibiting high adsorption capacity, thermal and
chemical stability as well as low cost.
Inorganic-organic hybrid porous polymers contain both organic and
inorganic units, which make them highly thermally and chemically
The performance of the adsorbents is not only affected by stability,
* Corresponding author. Shandong University, Jinan, PR China.
E-mail address: liuhongzhi@sdu.edu.cn (H. Liu).
https://doi.org/10.1016/j.polymer.2021.123491
Received 13 November 2020; Received in revised form 27 January 2021; Accepted 29 January 2021
Available online 4 February 2021
0032-3861/© 2021 Elsevier Ltd. All rights reserved.

Y. Wang et al.
Polymer 218 (2021) 123491
but also determined by the typical adsorption parameters, such as the
specific surface area and pore volume [33,38]. Moreover, the adsorption
performance also depends on functional sites in pores providing strong
interaction with the molecules, which ensures the high adsorption ca-
pacities [39–43]. Therefore, in order to improve the adsorption capacity
of absorbent for I2, CO2 and dyes, we should focus on new strategy to
introduce special active sites, or increase porosity as well as stability. For
example, we can introduce some rigid or/and heteroatom-containing
structures into the adsorbent [37,44,45]. The phosphazene compounds
can be easily modified by various functional groups to impart them such
unique properties as good biocompatibility, thermal stability and sol-
vent resistance [46], which makes them to be widely applied in bio-
materials [47], catalysis [48], flame retardant materials [49],
adsorbents [50,51], membranes [52], and so on. For example, Zhang
et al. synthesized metal-containing cyclomatrix polyphosphazene ma-
terials showing potential applications in charge-selective adsorption
[53]. Liu et al. reported organic–inorganic hybrid cyclomatrix poly-
phosphazene submicron-spheres which can selectively adsorb and
separate dyes from an aqueous solution [54]. Soldatov et al. prepared a
hybrid phosphazene-containing silsesquioxane-based porous polymer
by Heck coupling OVS with hexa (4-bromophenoxy)cyclo-
triphosphazene, which showed the highest adsorption capacities for ions
such as Pb2+, Hg2+, Cu2+ [32]. Generally, specific binding sites for
iodine molecules involve heteroatoms (nitrogen, phosphorus, sulfur and
dichloromethane (CH2Cl2) and 1,2-dichloroethane (CH2ClCH2Cl) were
dried over CaH2 for 12 h and used as freshly distilled.
2.2. Synthesis of monomers (OP and CP)
The monomers OP and CP were prepared according to a similar
procedure described in Ref. [60].
Hexaphenoxycyclotriphosphazene (OP): Sodium phenolate was
prepared by adding dropwise phenol (2.0 g, 21.6 mmol) dissolved in 20
mL anhydrous THF to sodium hydride (0.52 g, 21.6 mmol) in the re-
action flask. The solution of Hexachlorocyclotriphosphazene (0.94 g,
2.7 mmol) in 20 mL of dry THF was added slowly via syringe and the
mixture was stirred under reflux for 48 h. After reaction, the solution
was filtered, washed with brine, dried and the solvent was removed by
rotary evaporation. The product was recrystallized from ethyl acetate
and dried under vacuum. Yield: (1.6 g, 85%). 1H NMR (Figure S1(a),
300.53 MHz, CDCl3, δ): 7.10–7.00 (m, 18H), 6.84 (d, 12 H). 13C NMR
(Figure S1(b), 75.57 MHz, CDCl3, δ): 121.09, 124.89, 129.44,
150.64.31P NMR (Figure S1(c), CDCl3, δ): 8.68 (s).
Hexaphenylcyclotriphosphazene (CP): Diphenylphosphinamide
(2.0 g, 9.2 mmol) and triphenylphosphine (2.9 g, 11.1 mmol) were
dissolved in dry dichloromethane (30 mL), carbon tetrachloride (1.3 mL,
9.2 mmol) and triethylamine (1.3 mL, 9.2 mmol), and then the reaction
mixture was refluxed at 45 ◦C for 5 h. After reaction, the mixture was
filtered. The solution was washed with brine, dried. After the solvent
was removed, the product was recrystallized and dried under vacuum.
Yield: (1.4 g, 77%). 1H NMR (Figure S2(a), 300.53 MHz, CDCl3, δ): 7.70
(m, 18H), 7.23 (d, 12 H). 13C NMR (Figure S2(b), 75.57 MHz, CDCl3, δ):
127.93, 130.33, 130.76, 139.04.31P NMR (Figure S2(c), CD2Cl2, δ):
15.02 (s).
boron) and
π bonded fragments (aromatic rings, double and triple
bonds) [44]. For example, triphenylamine-functionalized silsesquiox-
ane-based porous polymer prepared by Scholl coupling was employed to
capture I2 vapor and offered a high adsorption capacity of 4 g gꢀ 1 within
1 h since it had a high nitrogen atom content and an extended conju-
gation structure [55]. Thus poly- and cyclophosphazenes, containing
alternating phosphorus and nitrogen atoms in the backbone, might serve
as efficient iodine adsorbents [56]. Mohanty et al. reported cyclo-
phosphazene based inorganic-organic hybrid nanoporous materials and
explored their adsorption of iodine vapor, which resulted in maximum
iodine capture capacity of 223 wt% [57]. Geng et al. used biimidazole to
2.3. Synthesis of hybrid porous polymers (PCS-OPs and PCS-CPs)
Typical procedure for PCS-OP-3: The molar ratio of OP to OVS for
PCS-OP-3 is 1:3.75. OVS (1.2 g, 1.875 mmol), OP (0.35 g, 0.5 mmol),
anhydrous aluminum chloride (1.5 g, 11.3 mmol) and 1,2-dichloro-
ethane (45 mL) were charged in an oven-dried flask. The mixture was
first stirred at room temperature for 0.5 h , and then refluxed for 24 h.
After cooling to room temperature, the mixture was filtered and solid
was washed with THF, methanol, dichloromethane, sequentially, in
order to remove unreacted monomers or residual catalyst. The product
was further purified under the Soxhlet extractor with methanol for 48 h
and dichloromethane for 48 h, respectively, and then dried in vacuum at
80 ◦C for 24 h to obtain brown solid. Yields: PCS-OP-3 (1.6 g, 103%),
PCS-CP-3 (1.8 g, 120%).
react
apart
with
2,4,6-trichloro-1,3,5-triazine
and
hexa-
chlorocyclotriphosphazene by nucleophilic substitution to form POPs
(TBIM and HBIM), which showed the astonishing adsorption capacities
for iodine molecules with the adsorption capacities of 9.43 and 8.11 g
gꢀ 1, respectively [58]. However, it has not been reported that combining
OVS with cyclophosphazene by Friedel-Crafts reaction to prepare
porous materials and using them as adsorbents.
In this work, we selected two typical cyclophophazene monomers,
hexaphenoxycyclotriphosphazene
(OP)
and
hexaphenylcyclo-
triphosphazene (CP), as functional units, and then crosslinked them with
octavinylsilsesquioxane (OVS) by Friedel–Crafts reaction catalyzed with
AlCl3. We synthesized two parallel series of phosphazene/
silsesquioxane-based functional porous polymers (PCS-OP-1, 2, 3 and
PCS-CP-1, 2, 3), which exhibited high specific surface areas and high
thermal stabilities. Their porosities can be tuned easily by changing the
molar ratio of OP (or CP ) with OVS. In this study, we selected PCS-OP-3
and PCS-CP-3 with high surface areas in two parallel series to investigate
their applications in adsorbing iodine vapor, dyes and CO2. The
adsorption capacity of PCS-OP-3 is 1.51 g gꢀ 1 for iodine vapor, 731 mg
gꢀ 1 for CR, 151 mg gꢀ 1 for MB and 1.74 mmol gꢀ 1 for CO2, which is
higher than PCS-CP-3 considering its higher surface area.
Moreover, PCS-OP-1, PCS-OP-2, PCS-CP-1 and PCS-CP-2 were syn-
thesized by OP (or CP) and OVS in different proportions. When the
molar ratio of OP (or CP) to OVS was 1: 0.75, PCS-OP-1 and PCS-CP-1
were obtained; when the molar ratio was 1:1.5, PCS-OP-2 and PCS-CP-
2 were obtained. Yields: PCS-OP-1 (0.4 g, 68%), PCS-OP-2 (0.8 g,
97%), PCS-CP-1 (0.2 g, 37%), PCS-CP-2 (0.7 g, 90%).
2.4. Procedure for the uptake of I2 vapor by PCS-OP-3 and PCS-CP-3
A sample of PCS-OP-3 or PCS-CP-3 powder (50.0 mg) and an excess
of crystalline iodine were placed in a sealed polypropylene container
and heated at 70 ◦C under ambient pressure. After specific period of
time, the container was cooled down to room temperature and the PCS-
OP-3 or PCS-CP-3 sample was quickly weighed. The I2 uptake of PCS-OP-
2. Experimental
3 or PCS-CP-3 was calculated by weight gain:
α = (m2 ꢀ m1)/m1, where
2.1. Materials
α
is the I2 uptake, m1 and m2 are the masses of the PCS-OP-3 or PCS-CP-3
samples before and after being exposed to I2 vapor, respectively.
2.5. Procedure for the adsorption of dyes by PCS-OP-3 and PCS-CP-3
The solution mode adsorption studies were carried out for aqueous
Unless otherwise noted, all reagents were purchased from commer-
cial suppliers and used without further purification. Octavi-
nylsilsesquioxane (OVS) was synthesized by previous reports [59]. THF
was dried over Na/benzophenone and used as freshly distilled before
synthesis. Carbon tetrachloride (CCl4), triethylamine (Et3N),
2

Y. Wang et al.
Polymer 218 (2021) 123491
solution of dyes, including Congo red (CR), and methylene blue (MB). In
the process of dyes adsorption, 5 mg of PCS-OP-3 or PCS-CP-3 were
dispersed in 20 mL of an aqueous solution of dyes with different con-
centration. The mixture was stirred at room temperature for 24 h until
the equilibrium was reached. The dyes concentration in the solution
before and after adsorption was determined by an Ultraviolet–visible
(UV–vis) spectrophotometer (497 nm for CR and 664 nm for MB).
2.6. Material characterization
Fourier-transformed infrared (FT-IR) spectra were characterized
using a Bruker TENSOR-27 infrared spectrophotometer from 4000 cmꢀ 1
to 400 cmꢀ 1 at a resolution of 4 cmꢀ 1. The sample was prepared using a
conventional KBr disk method. Solid-state 13C CP/MAS NMR, 29Si MAS
NMR and 31P MAS NMR spectra were recorded on a Bruker AVANCE-
500 NMR spectrometer operating under a magnetic field strength of
9.4 T. The resonance frequencies at this field strength were 125 99 and
202.6 MHz for 13C NMR ,29Si NMR and 31P NMR, respectively. A
Chemagnetics 5 mm triple-resonance MAS probe was used to acquire
13C, 29Si and 31P NMR spectra. 29Si NMR spectra with high power proton
Scheme 1. Preparation of hybrid porous polymers by cross-linking OVS with
OP and CP via a Friedel-Crafts reaction, respectively. Some possible fragments
formed in the networks are shown as examples.
decoupling were recorded with a π/2 pulse length of 5 μs, a recycle delay
PCS-CP-3 were used for the further studies.
of 120 s, and a spinning rate of 5 kHz. The 13C chemical shifts were
referenced externally via the resonance of tetramethylsilane (TMS) of 0
ppm. The 31P chemical shifts were referenced externally via the reso-
nance of NH4H2PO4 at an isotropic chemical shift of 1.058 ppm.
Elemental analysis was conducted using an Elementarvario EL III
elemental analyzer. Field-emission scanning electron microscopy (FE-
SEM) experiments were characterized using a HITACHI S4800 spec-
trometer. High-resolution transmission electron microscopy (HR-TEM)
experiments were performed using a JEM 2100 electron microscope
(JEOL, Japan) with an acceleration voltage of 200 kV. Thermal gravi-
metric analysis (TGA) was performed on a Mettler-Toledo model SDTA
-854 TG A system under a N2 atmosphere at a heating rate of 10 ◦C
minꢀ 1 from room temperature to 800 ◦C. Powder X-ray diffraction
(PXRD) images were collected on a Riguku D/MAX 2550 diffractometer
under Cu-Kα radiation, at 40 kV and 200 mA, with a 2θ range of 5–80◦
(scanning rate of 10◦ minꢀ 1) at room temperature. The UV–vis adsorp-
tion spectra of dye solutions were obtained by a TU-1901 spectropho-
tometer at room temperature.
Infrared spectra of the polymers along with the precursors are shown
in Figure S3. The band located at around 1100 cmꢀ 1
(
ν Si–O–Si) and 1200
cmꢀ 1
(
ν
)
N
segments arises from silsesquioxane and cyclo-
–
–
P
phosphazene units in polymers, respectively. The C–H stretching vi-
bration peak of vinyl groups at 3053 cmꢀ 1 decreases dramatically after
the Friedel-Crafts reaction, and a new peak at 2967 cmꢀ 1 appears
demonstrating the formation of CH2 linkage, which indicates that the
polymers are successfully synthesized. Furthermore, a peak at 3440
cmꢀ 1 is attributed to Si–OH group, which is formed after Si–O–Si or Si–C
bond cleavage caused by AlCl3 during the reaction.
The solid-state NMR measurements were performed to investigate
the structures of PCS-OP-3 and PCS-CP-3. In 29Si spectra signals, the
peaks that appeared at approximately ꢀ 66.5, ꢀ 69.5 and ꢀ 79 ppm
(Fig. 1) suggested the formation of T3 units (Tn = CSi(OSi)n (OH)3-n), and
generation of regioisomers; signals at ꢀ 100 ppm suggest the formation
of Q3 units (Qn = Si(OSi)n (OH)4-n). Compared with the 29Si MAS NMR
spectrum of OVS with one peak at ꢀ 79 ppm, the presence of Qn signals is
explained by partial collapse of cages in the framework caused by AlCl3
during Friedel-Crafts reaction. The signal at ꢀ 80.7 ppm should be
N2 sorption isotherm measurements were characterized with a
Micromeritics surface area and pore-size analyzer. Before measurement,
the samples were degassed for 12 h at 150 ◦C. A sample of ca. 100 mg
and UHP-grade N2 (99.999%) gas source were adopted in the N2 sorp-
tion measurements at 77 K and collected with a Quantachrome Quad-
rasorb apparatus. BET surface areas were confirmed over a P/P0 range
from 0.01 to 0.20. Nonlocal density functional theory (NL-DFT) pore-
size distributions (PSDs) were determined by using the carbon/slit-
cylindrical pore mode of the Quadrawin software. The CO2 adsorption
capacity was measured at 273 K and 1.0 bar. Before measurement, the
samples were degassed at 150 ◦C under vacuum for about 15 h.
–
assigned to the “Si” atom from unreacted “Si–CH CH ” group, which
–
2
agreed with the FTIR results, and further revealed there were some
unreacted vinyl groups. The relative intensity ratio T3/(T + Q) is up to
0.9 for PCS-OP-3 and 0.8 for PCS-CP-3, which means that less than 20%
cages in the network are cleaved.
The solid-state 13C CP/MAS NMR spectra of the PCS-OP-3 and PCS-
CP-3 are shown in Figure S4. The broader peaks at about 130 ppm
should be assigned to aromatic carbons of OP and CP, as well as the
carbon atoms of unreacted vinyl groups [61]. Compared with OVS, the
new peaks from 0 to 50 ppm are mainly ascribed to the alkyl carbons of
Si–CH2–CH2-OP (CP) and Si–CH(CH3)OP(CP) units, again confirming
that the formation of single C–C bonds derived from vinyl units after
Friedel-Crafts reaction. The solid-state 31P MAS NMR spectra of the
PCS-OP-3 and PCS-CP-3 are shown in Fig. 2. The spectrum of PCS-OP-3
exhibits a distinct signal at 8.7 ppm (Fig. 2a) and confirms that no re-
actions happen on phosphazene rings, such as ring opening. The spec-
trum of PCS-CP-3 exhibits divided signals at 16 and 23 ppm (Fig. 2b)
indicating there are reactions on phosphazene rings. The data of
solid-state 31P MAS NMR spectra explains the element analysis results
(as listed in Table S1) that the content of nitrogen in PCS-CP-3 is lower
than PCS-OP-3 and theoretically calculated data.
3. Results and discussion
3.1. Synthesis and characterization
The preparation routes for two series of phosphazene/silsesquioxane
hybrid porous polymers were depicted in Scheme 1. Hexaphenox-
ycyclotriphosphazene (OP) and hexaphenylcyclotriphosphazene (CP)
were used as phosphazene units and they are different by the presence of
oxygen atoms in organic substituents or not. The crosslinking between
cyclotriphosphazenes and OVS was performed by Friedel-Crafts reaction
with AlCl3 as catalyst and 1,2-dichloroethane as solvent. Two parallel
series of hybrid polymers assigned as PCS-OPs (PCS-OP-1, 2 and 3) and
PCS-CPs (PCS-CP-1, 2 and 3) were prepared by varying the molar ratio
between OP or CP and OVS, respectively. As two representative samples,
PCS-OP-3 with the highest surface area as well as its analogous product
Therefore, the FT-IR, 29Si NMR, 13C NMR and 31P spectroscopy and
element analysis results clearly demonstrate that hexaphenoxycyclo-
triphosphazene or hexaphenylcyclotriphosphazene can react with OVS
by Friedel-Crafts reaction to produce PCS-OP and PCS-CP, respectively.
3

Y. Wang et al.
Polymer 218 (2021) 123491
Fig. 1. Solid-state 29Si MAS NMR spectra of OVS, PCS-OP-3 (a) and PCS-CP-3 (b).
Fig. 2. Solid-state 31P MAS NMR spectra of PCS-OP-3 (a) and PCS-CP-3 (b).
3.2. Thermal property and morphology
isotherms show sharp uptake at low relative pressures, while at higher
relative pressure show a gradually increasing tendency in uptake with
hysteresis, indicating that the polymers contain both micro- and meso-
pores. The polymers exhibit the type I and type IV nitrogen sorption
isotherms according to the IUPAC classification. Among them, PCS-OP-3
(Fig. 3 (a)) and PCS-CP-3 (Fig. 3 (b)) have H3 hysteresis loop isotherm
and there are no obvious saturated adsorption platforms, which in-
dicates that the pore structures are very irregular [64,65].
It is worth noting that OVS has a good thermal stability (Td5%
>
280 ◦C) in air, as shown in Figure S5. The resulted porous polymers
showed higher thermal stability than OVS due to the higher crosslinking
networks [62]. The 5% weight loss temperature (Td5%) of PCS-CP-3 was
320 ◦C, which was ca. 30 ◦C higher than PCS-OP-3. It can be explained
by the presence of P–O bonds in PCS-OP-3, which can decrease thermal
stability. Compared with some other OVS-based materials [27,28,38,
63], the 5% weight loss temperature (Td5%) of PCSs are lower. This result
should be ascribed to the cleavage of partial silsesquioxane cage or ring
The Brunauer-Emmett-Teller surface areas (SBET) of PCS-OP-3 and
PCS-CP-3 were measured as 686 m2 gꢀ 1 and 462 m2 gꢀ 1 and the
micropore surface areas (Smicro) were calculated to be 152 m2 gꢀ 1 and
202 m2 gꢀ 1 by the t-plot method, respectively. The presence of oxygen
atoms in OP makes the linker between cage and phosphazene ring in
PCS-OP-3 more flexible and their relative positions are easily adjusted
and optimized. However, some phosphazene rings were cleaved in PCS-
CP-3 since the stiff linkage between cage and ring, which can be
demonstrated by solid 31P MAS NMR spectra (Fig. 2). Therefore, PCS-
OP-3 offers a larger pore volume and higher surface area than PCS-CP-
3 [66]. Nonlocal density functional theory (NLDFT) is used to evaluate
the pore size distribution (PSD). The PSD curve of PCS-OP-3 showed a
unique bimodal structure, which indicated the presence of micropores at
~1.4 nm and mesopores at ~4.2 nm, respectively; while the PCS-CP-3
exhibited a hierarchical structure of micropores centered at around
1.7 nm and a relatively broad distribution of mesopores with diameters
of 2.9–10.3 nm due to ring-opening or rearranged phosphazene rings of
PCS-CP-3. The total pore volumes (Vtotal) estimated at P/P0 of 0.99 were
separately calculated to be 0.64 cm3 g ꢀ 1 and 0.45 cm3 g ꢀ 1 for PCS-OP-3
and PCS-CP-3, and the ratios of the micropore volume (Vmicro) to Vtotal
for PCS-OP-3 and PCS-CP-3 were 0.10 and 0.19, respectively. A brief
summary of porosity data is listed in Table S2.
of phosphazene, which were evidenced from solid state 29Si NMR or 31
P
NMR spectra. Powder X-ray diffraction (PXRD) analysis was performed
to determine the morphology of monomer and resulted materials. As
shown in Figure S6, the broad diffraction peaks at about 22◦ (2θ)
demonstrated that although OVS and phosphazene monomers were
crystalline ,PCS-OP-3 and PCS-CP-3 were amorphous. This may be
ascribed to random cross-linking between cages restricting crystalline.
In order to analyze the particle size and morphology of PCS-OP-3 and
PCS-CP-3, the field emission scanning electron microscopy (FE-SEM)
was performed. The FE-SEM images (Figure S7(a), Figure S8(a)) illus-
trate that samples have irregular shapes and a wide range of diameters
from 100 nm to several micrometers. The high-resolution transmission
electron microscopy (HR-TEM) images (Figure S7(b), Figure S8(b))
show no evidence to confirm the existence of long-range ordered
structures in polymers.
3.3. Porosity
The porosities of the polymers were characterized by N2 adsorption-
desorption measurements at 77 K. As shown in Fig. 3, the fully reversible
PCS-OP-3 and PCS-CP-3 with high surface areas and same molar
4

Y. Wang et al.
Polymer 218 (2021) 123491
Fig. 4. Iodine uptake curve for PCS-OP-3 and PCS-CP-3 at 70 ◦C and ambient
pressure. Inset: photograph showing the color change of PCS-OP-3 and PCS-CP-
3 powder before and after iodine vapor capture. (For interpretation of the
references to color in this figure legend, the reader is referred to the Web
version of this article.)
increase iodine vapor adsorption capacity [42,44,67,68]. The iodine
capture performance may be attributed to three factors: (1) high surface
area, considerable pore volume and hierarchical micropore/mesopore
provide enough space for iodine capture and good diffusion of iodine
molecules into the adsorbent bulk [69,70]; (2) the N-containing
frameworks offers binding sites to form N⋅I halogen bonds and enhance
the interaction between iodine molecules and the frameworks [69,71,
72]; (3) phosphazene rings with
efficient iodine uptakes [40,44].
π-conjugations could contribute to
The higher I2 adsorption capacity in PCS-OP-3 compared to PCS-CP-3
may be ascribed to its higher specific surface area. The diameters of
micropores of the polymers are less than 1 nm, close to the dynamic
diameters of iodine molecules (0.56 nm), which favor adsorbing iodine
vapor molecules [73]. And the mesopore structures of the polymer are
helpful for the mass-transfer of iodine molecules, which can improve the
adsorption rates of iodine molecules [74]. The adsorption capacities of
PCS-OP-3 and PCS-CP-3 for iodine vapor are moderate, perhaps due to
the low content of phosphazene rings.
Fig. 3. N2 adsorption/desorption isotherm and pore size distribution of PCS-
OP-3(a) and PCS-CP-3(b).
ratios of reactants in these two parallel series were selected to further
investigate their applications in adsorbing iodine vapor, dyes and CO2,
respectively.
3.4. Adsorption of iodine vapor
The mechanism of iodine adsorption and the interaction of I2 with
PCS-OP-3 and PCS-CP-3 were studied by FT-IR, TGA, Raman spectros-
copy and the energy dispersive X-ray (EDX) spectroscopy. TGA was used
to determine the binding strength of adsorbed iodine in the framework.
As shown in Figure S9, compared with PCS-OP-3 and PCS-CP-3, the mass
loss of PCS-OP-3@I2 and PCS-CP-3@I2 was significantly higher. When
the temperature reached 281 ◦C, the weight of PCS-CP-3@I2 was
reduced by 51%. The results are close to the weight increment induced
by I2 capture.
Since the hybrid porous polymers possess high thermal and chemical
stability, high porosity and heteroatoms such as N, P and O in the
framework, they were investigated as adsorbents for iodine vapor. Small
amounts of the polymers were placed into pre-weighed vials and
exposed to I2 vapor in sealed polypropylene containers. The iodine
loading capacities were measured by the weight of samples at different
time intervals until no further changes in mass were observed (see the
Supporting Information for details). Under ambient pressure and 70 ◦C,
the amount of loaded iodine of PCS-OP-3 and PCS-CP-3 gradually
increased at initial 1–2 h, and then reached saturation within 10 h.
During the adsorption process, the color of the samples gradually
changed from brown to black, which indicated that I2 had diffused into
frameworks of porous polymers (Fig. 4), and the calculated iodine
adsorption capacities of PCS-OP-3 and PCS-CP-3 were 1.51 and 1.27 g
gꢀ 1, respectively.
Since N atoms present in the structure, charge transfer in I2 mole-
cules occurs easily and polarizes into polyiodide (I3ꢀ , I5ꢀ ). The structure
of the trapped iodine inside the pores was revealed by Raman spec-
troscopy (Fig. 5) [70]. The characteristic peaks of PCS-OP-3@I2 and
PCS-CP-3@I2 were dominated by the peaks around at 107, 142, and 168
cmꢀ 1, which were mainly attributed to I3ꢀ and I5ꢀ anions [70]. The
strong peak, located at 168 cmꢀ 1, attributed to the I5ꢀ stretching vi-
bration [58]. The characteristic peaks of PCS-OP-3@I2 at around 107
cmꢀ 1, 138 cmꢀ 1 and 168 cmꢀ 1 belonged to both perturbed di-iodine
molecules and asymmetric I3ꢀ ions of polyiodides [I3ꢀ ⋅I2], whereas
the peaks of PCS-CP-3@I2 at about 142 cmꢀ 1 and 168 cmꢀ 1 were the
results of perturbed di-iodine units for [Iꢀ ⋅(I2)2] [68]. The change of
Previous studies on capture of volatile iodine by porous polymeric
absorbents showed that the adsorption included chemical capture and
physical deposition. Among them, the strong affinity of absorbents to
iodine molecules and favorable pore parameters, such as high specific
surface area, pore volume and pore size distribution, would lead to
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Polymer 218 (2021) 123491
Fig. 5. Raman spectra of PCS-OP-3, PCS-OP-3@I2(a), PCS-CP-3 and PCS-CP-3@I2(b).
iodine valence status indicates that chemisorptions occur, which com-
bines with physisorption to obtain a large amount of iodine adsorption
[71]. According to the EDX images (Figure S10 and S11), the peaks of P,
C and N originated from the framework materials, while the peaks of I
arose from the iodine adsorbed on PCS-OP-3@I2 and PCS-CP-3@I2,
which further revealed that porous polymers could adsorb iodine. The
PCS-OP-3@I2 and PCS-CP-3@I2 in the air were also compared with those
in the iodine atmosphere. It was observed that the mass of polymers in
the iodine atmosphere remained same, while the mass of polymers in the
air decreased (Figure S12). This means some iodine deposited on the
surface of polymers.
Further, the experimental adsorption data of PCS-OP-3 and PCS-CP-3
were simulated with pseudo-first-order and pseudo-second-order kinetic
models (Figure S13). As listed in Table S3, the adsorption process
matched well with the pseudo-second-order kinetic model which had
good linear correlation coefficients R2 value of more than 0.999. On the
basis of the data presented above, the pseudo-second-order kinetic
model was more appropriate to describe the adsorption process.
Notably, the trapped iodine can be readily and easily removed from
the framework of iodine-loaded materials in organic solvents such as
ethanol [55]. For the further study of such releasing process, about 80
mg of PCS-OP-3 or PCS-CP-3 samples and an excess of crystalline iodine
were placed in a sealed polypropylene container and heated at 70 ◦C
under ambient pressure for 24 h. After that, the containers were cooled
down to room temperature; the PCS-OP-3@I2 and PCS-CP-3@I2 sample
were quickly weighed. Then, the samples were washed by fresh ethanol,
the color of the ethanol solution gradually changed from colorless to
orange, which clearly indicated that the iodine molecules were desorbed
from the polymeric frameworks. Then the samples were dried under
vacuum at 60 ◦C for 24 h, the quantities of PCS-OP-3 and PCS-CP-3 had
not obviously changed by weighing. The regenerated PCS-OP-3 and
PCS-CP-3 samples were reused for next cycle by exposing to iodine vapor
again. As shown in Fig. 6, after five adsorption cycles, the adsorption
abilities of PCS-OP-3 and PCS-CP-3 showed a decrease, because the
polymers had N binding sites and the charge transfer in the I2 molecule
can easily occur in the I2 molecules and polarize into polyiodide, which
is not easy to wash off, but the removal capacities were stable above 60%
[72]. The two polymers of PCS-OP-3 and PCS-CP-3 after five cycles were
denoted as PCS-OP-3-I2 and PCS-CP-3-I2, respectively. The FTIR and BET
analyses were used to characterize the stabilities of PCS-OP-3-I2 and
PCS-CP-3-I2. The FTIR spectra shown in Figure S14 indicate that there
are no significant differences in the structures of samples before and
after 5 cycles of iodine vapor adsorption. Compared with PCS-OP-3 and
PCS-CP-3, SBET of PCS-OP-3-I2 and PCS-CP-3-I2 decreased to 396.7 and
95.2 m2 gꢀ 1, respectively (Figure S15 and Table S4). It is explained that
some pores are blocked by residual iodine [75]. The cyclic experiments
Fig. 6. Recyclability of PCS-OP-3 and PCS-CP-3 for capturing iodine
from vapor.
reveal that the polymers could be attractive recyclable adsorbents for
iodine vapor capture in practical application.
3.5. Dyes adsorption
It is noteworthy that porous materials were capable for removing
dyes from aqueous solution, so the dyes adsorption of PCS-OP-3 and
PCS-CP-3 was further studied. In order to estimate the adsorption
behavior of PCS-OP-3 and PCS-CP-3 in dye-water solution, an UV–vis
spectrophotometer was employed to measure the changes in intensities
of the maximum absorption wavelengths of model dyes, Congo red (CR)
and Methylene Blue (MB).
As shown in Fig. 7, the initial adsorption amount was a function of
the initial concentration of dye solutions, while the adsorption capac-
ities reached the maximum value having nothing to do with the
increasing dyes concentrations. The experimental results indicated that
the maximum equilibrium adsorption capacity (Qe) of the two adsor-
bents in water were 731 and 151 mg gꢀ 1 for CR and MB onto PCS-OP-3,
respectively, and 522 and 73 mg gꢀ 1 for CR and MB onto PCS-CP-3. The
Qe of PCS-OP-3 was larger than that of PCS-CP-3, regardless of dyes,
which was due its higher SBET and Vtotal. In addition, the PCS-OP-3
possesses more electron-rich oxygen atoms than PCS-CP-3, which
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Polymer 218 (2021) 123491
Fig. 7. Equilibrium adsorption isotherms of dyes on PCS-OP-3 and PCS-CP-3.
Fig. 8. Recyclability of PCS-OP-3 and PCS-CP-3 for capturing CR.
3.6. CO2 capture
easily bind cationic dyes through the coulombic force [33,38]. As listed
in Table S5, MB is a small-sized cationic dye, while CR is opposite. For
PCS-OP-3 and PCS-CP-3, the adsorption capacity of CR was higher than
that of MB, indicating that larger dyes could enter the mesopores and be
blocked by micropores, whereas small-sized dyes could easily enter and
leave the pores [27,45].
The mitigation of carbon dioxide emission into the atmosphere has
attracted tremendous attention due to the fact that the excessive emis-
sion of CO2 into the atmosphere may cause “greenhouse effect” [76].
Among the different strategies for CO2 abatement, capture and storage
are considered to be advantageous owing to the low energy consumption
and ease of operation [77,78]. Therefore, it is vital to develop practical
and effective materials with high CO2 adsorption capacity. Porous ma-
terials based on cage-like organosiloxanes (PCSs) have been attempted
to store CO2 by virtue of their abundant porosities [79,80]. The CO2
adsorption curves of PCS-OP-3 and PCS-CP-3 are shown in Fig. 9. The
adsorption capacities of PCS-OP-3 and PCS-CP-3 are calculated as 1.74
mmol gꢀ 1 (7.65 wt%) and 1.31 mmol gꢀ 1 (5.76 wt%) at 273.0 K/101
kPa, respectively, which are higher or comparable in comparison with
some other PCSs [27,38,63,81]. The higher surface area of PCS-OP-3
accounted for its larger CO2 absorption capacity compared to PCS-CP-3.
The high CO2 adsorption capacities of PCS-OP-3 and PCS-CP-3 can be
associated with basic nitrogen atoms in the network, which increase the
affinity of the polymers to carbon dioxide molecules via dipole-
quadrupole interaction between CO2 and nitrogen, thereby enhancing
the CO2 adsorption performance [55]. In addition, the porous polymers
In order to further study the adsorption behavior from a theoretical
perspective, the isotherm curves were analyzed according to Langmuir
and Freundlich models. The related parameters are listed in Table S6
and the details are shown in Figure S16. The correlation coefficients of
Langmuir isotherm model showed a better linear fitting than the cor-
relation coefficients of Freundlich isotherm model, indicating that
monolayer adsorption occurred in a finite number of identical sites for
PCS-OP-3 and PCS-CP-3.
In order to obtain the equilibrium time and fully understand the
adsorption mechanism, the adsorption kinetics of PCS-OP-3 and PCS-CP-
3 in CR aqueous solution were studied. As shown in Figure S17, we can
see that the kinetic adsorption curves can be divided into two stages: the
dye adsorption efficiency increased rapidly during the first 40 min, and
then a slow increase was observed until it reached equilibrium. The
removal efficiencies of 94% and 95% were individually calculated by
adsorption data for CR solutions of PCS-OP-3 and PCS-CP-3 at 3 h,
respectively.
Pseudo-first-order and pseudo-second-order kinetic models were
used to study the adsorption mechanism and the correlative kinetic
parameter and curves are listed in Table S7 and Figure S18. In com-
parison with the pseudo-first-order model, the pseudo-second-order
model shows a better linear fitting with the correlation coefficient
(R2) values of 0.9989 and 0.9995 for PCS-OP-3 and PCS-CP-3,
respectively.
The encapsulated dyes can be easily removed from the framework of
dye-loaded materials in organic solvents. To further study of this
releasing process, PCS-OP-3 and PCS-CP-3 were dispersed in the solution
of CR with concentration of 40 mg Lꢀ 1 for 24 h, and the residual CR
concentration was measured by UV–Vis spectroscopy. After that, the
dye-loaded samples were soaked sequentially into fresh methanol and 2
M hydrochloric acid, the color of the methanol solution gradually
changed from colorless to orange, which clearly indicated that CR
molecules were desorbed from the polymer frameworks. As shown in
Fig. 8, after five cycles, the adsorption capacities of PCS-OP-3 and PCS-
CP-3 still remain same, revealing that the porous polymers could be
promising and recyclable adsorbents for removal of dyes from
wastewater.
Fig. 9. CO2 adsorption isotherms of PCS-OP-3 and PCS-CP-3 at 273 K.
7

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Polymer 218 (2021) 123491
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In summary, we synthesized two parallel series of silsesquioxane-
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PCS-CPs via Friedel-Crafts reaction. The high surface area, considerable
pore volume, hierarchical micropore/mesopore and N, P-containing
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1.51, 0.73 g gꢀ 1 and 1.74 mmol gꢀ 1, respectively. Moreover, PCS-OP-3
and PCS-CP-3 showed a good recyclability and can be reused at least for
five cycles. All these results show the perspectives of phosphazene
containing PCSs for adsorbing iodine vapor, dyes and CO2.
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The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influence
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Acknowledgements
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This research was supported by the National Natural Science Foun-
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Program of Shandong province (No. 2019GHZ034), Ministry of Science
and Technology of the People’s Republic of China and Seed Fund Pro-
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