Iodine sequestration using cyclophosphazene based inorganic-organic hybrid nanoporous materials: Role of surface functionality and pore size distribution

Article abstract of DOI:10.1016/j.molliq.2019.03.053

Cyclophosphazene based inorganic-organic hybrid nanoporous materials having triazine and pyrrolic functionality named as HNM-1 and CHNM-1, respectively, with varying nitrogen content, hierarchical pore size and high specific surface area have been explore

Full text of DOI:10.1016/j.molliq.2019.03.053

Accepted Manuscript
Iodine sequestration using cyclophosphazene based inorganic-
organic hybrid nanoporous materials: Role of surface
functionality and pore size distribution
Raeesh Muhammad, Paritosh Mohanty
PII:
S0167-7322(19)30477-5
DOI:
Reference:
https://doi.org/10.1016/j.molliq.2019.03.053
MOLLIQ 10594
To appear in:
Journal of Molecular Liquids
Received date:
Revised date:
Accepted date:
25 January 2019
7 March 2019
9 March 2019
Please cite this article as: R. Muhammad and P. Mohanty, Iodine sequestration using
cyclophosphazene based inorganic-organic hybrid nanoporous materials: Role of surface
functionality and pore size distribution, Journal of Molecular Liquids, https://doi.org/
10.1016/j.molliq.2019.03.053
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ACCEPTED MANUSCRIPT
Iodine Sequestration Using Cyclophosphazene Based Inorganic-
Organic Hybrid Nanoporous Materials: Role of Surface
Functionality and Pore Size Distribution
Raeesh Muhammad and Paritosh Mohanty*
Functional Materials Laboratory, Department of Chemistry, Indian Institute of Technology
Roorkee, Roorkee, Uttarakhand-247667, India
*E-mail: pmfcy@iitr.ac.in, paritosh75@gmail.com
Abstract: Cyclophosphazene based inorganic-organic hybrid nanoporous materials having
triazine and pyrrolic functionality named as HNM-1 and CHNM-1, respectively, with varying
nitrogen content, hierarchical pore size and high specific surface area have been explored for
iodine sequestration application. The presence of electron rich functionality in the HNM-1 and
CHNM-1 provides a large number of active sites for the adsorption of iodine. Maximum
gravimetric iodine capture capacity of 223 and 120 wt.% were estimated in HNM-1 and CHNM-
1 specimens, respectively, under fuel processing condition i.e. 75 °C and ambient pressure.
Moreover, both the specimens have shown very good iodine release behavior in organic solvents.
The recyclability of HNM-1 and CHNM-1 for iodine sorption has shown the retention of
adsorption capacity by 80 and 86%, respectively, even after five cycles. The iodine sorption both
in vapor as well as solution phases is greatly influenced by the nitrogen content and pore size
distribution of the framework materials.
Keywords: Cyclophosphazene based hybrid materials; iodine sequestration; nanoporous
materials; physicochemical properties
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1. Introduction
Climate crisis owing to uncontrolled anthropogenic release of CO2 in atmosphere has emerged as
one of the greatest threats for society [1],[2],[3],[4]. Hence, the quest for reliable alternate energy
source having minimum impact on the environment is very high [5],[6],[7],[8]. Towards this,
nuclear energy has been envisioned as the clean and carbon free energy source but the large-
scale energy production concerns the release of huge amount of harmful nuclear waste
[6],[7],[8],[9]. 129I is one of the radioactive components of nuclear waste which emerges during
the fission of nuclear fuel in nuclear power plant [10],[11]. 129I has long half-life (1.57 x 107
years) and easily incorporate itself into human metabolism and creates health issues
[11],[12],[13],[14]. Thus, to develop the nuclear energy at mass scale, there is an urgent need to
find the cost-effective materials for efficient capture and storage of iodine which has been a
challenging task [11].
Adsorptive removal of iodine has emerged as a more effective solution as compared to
caustic and acidic scrubbing methods, as these suffers from recyclability problem [6],[11],[15].
Recently, natural and synthetic silver-exchanged zeolite have been used for the remediation of
radioactive iodine in nuclear plant [11],[12]. Although, these materials are hydrothermally stable
but their limited iodine uptake has been the major bottleneck towards their utilization under mass
scale condition [13]. In addition to above, many more adsorbents like carbonaceous materials
[16], porous graphene [17], composites materials [18], metal organic frameworks (MOFs) [6],
covalent organic frameworks (COFs) [19],[20], and porous organic polymers [8],[9],[21] have
been studied towards adsorptive removal of iodine. The recent studies on iodine capture shed
some light on the facts that, the iodine removal capacity is not greatly influenced by only specific
surface area, and in fact, the pore surface functionality, pore volume and pore size distribution do
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play pivotal role [22],[23],[24]. The enrichment of the frameworks by the heteroatoms have been
found to improve the interaction with the iodine and ultimately improvement in the iodine uptake
capacity. Liao et al. reported that the amine functionalized conjugated microporous polymers
demonstrated high iodine uptake (336 wt.%) owing to improved affinity of framework with
iodine [13]. Han and coworkers, reported azo bridged porphyrins and phthalocyanine bridged
framework having iodine uptake capacity of 220 wt.% [25]. Similarly, Juneta et al. have found
that presence of imine functionality in the silsesquioxane cores resulted in high iodine uptake
capacity (485 wt.%) [26].
As discussed above, the framework enrichment by heteroatoms facilitate iodine uptake
owing to better interaction between host and guest, and towards this, use of cyclophosphazene
moiety as a source of heteroatoms has been exclusively explored by Mohanty and co-workers
[27], [28],[29],[30],[31],[32],[33]. Here cyclophosphazene moiety not only helps in introduction
of heteroatoms but also serve as a three-dimensional building block and with its paddle-wheel
structure, which creates the void naturally in the framework [27],[28],[29],[30],[31],[32],[33].
Herein, we report the iodine sequestration application of cyclophosphazene based inorganic-
organic hybrid nanoporous materials having pyrrolic and triazines moieties with varying nitrogen
content and controlled textural properties.
2. Materials and Method
2.1. Synthesis of compound-I
A common precursor ‘compound-I’ was used for the synthesis of both HNM-1 and CHNM-1
following previous reports [29],[30]. In a typical synthesis, 10 mmol of phosphonitrilic chloride
trimer (PNC, 99%, Sigma Aldrich India) dissolved in 50 ml of THF (AR, SRL India) was
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condensed with 61 mmol of 4-hydroxybenzaldehyde (99%, Sigma Aldrich India) dissolved in
100 ml THF in presence of 121 mmol of K2CO3 (AR, Fisher Scientific India) under argon
atmosphere. The reaction was found to be completed after stirring of reaction mixture for 48 h at
RT. The detailed synthesis condition and processing/purification of compound-I has been
1
13
31
reported elsewhere [29]. The formation of compound-I was confirmed by H, C and P NMR
spectra [29]. 1H NMR (CDCl3): δ 9.95 (s), 7.75 (d), 7.17 (d). 13C NMR (CDCl3): δ 190, 155, 134,
132, 122 and 31P NMR (CDCl3): δ 7.04.
2.2. Synthesis of HNM-1: In a typical synthesis of HNM-1, condensation of 1 mmol of
compound-I with 3 mmol of melamine (99%, Sigma Aldrich India) in 30 ml DMSO (AR, Fisher
Scientific India) under reflux condition in presence of N2 atmosphere after 48 h resulted in an
off-white powder [29]. The product was washed with methanol, tetrahydrofuran followed by
soxhlet extraction with diethyl ether, and dried at 100 °C in hot air oven.
2.3. Synthesis of CHNM-1: In a typical synthesis of CHNM-1, 1 mmol of compound-I was
condensed with 6 mmol of freshly distilled pyrrole (AR, SRL India) in 5 ml 1,4-Dioxane (AR
Fisher Scientific, India) at 70 °C for 1 hour under N2 atmosphere followed by solvothermal
treatment at 180 °C for 48 h, which resulted in a brown monolith [30]. The obtained monolith
was washed with methanol and tetrahydrofuran several times followed by drying at 80 °C in
vacuum oven.
2.4. Characterization: The chemical environment and structural elucidation of the framework
materials were carried out using FT-IR (Spectrum Two, PerkinElmer), powder X-ray diffraction
(XRD; Ultima IV, Rigaku), elemental analysis (varioMICRO) and XPS (HI-5000 VersaProbe
III ULVAC-PHI Inc) analysis. The microstructural characterizations were conducted using field
emission scanning electron microscopy (FESEM; MIRA3, TESCAN), and transmission electron
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microscopy (TEM; TECNAIG2S-TWIN) instruments. The thermal stability of these framework
materials was analyzed using EXSTAR TG/DTA6300 TGA analyzer under argon atmosphere.
The textural parameters were estimated by N2 sorption analysis using Autosorb iQ2
(Quantachrome Instruments, USA) at -196 °C and 1 bar after degassing of the samples at 120 °C
for 12 h. The specific surface area (SABET) was estimated using BET model and the pore size
distribution (PSD) was estimated using density functional theory (DFT) model.
2.5. Iodine sorption: The iodine vapor adsorption experiment was conducted using gravimetric
analysis under the reaction condition similar to nuclear fuel reprocessing. Before the
experiments, both the specimens were activated at 100 °C for 12 h under vacuum. For every
experiment, 20 mg of the sample was place in an open mouth glass vessel which was placed
inside the tightly capped glass vessel containing excess of solid Iodine at 75 °C under ambient
pressure. The iodine uptake capacity was estimated by the equation (1):
푚 −푚
푡
푖 × 100 푤푡%
( )
1
Iodine uptake capacity =
푚
푖
where, mi and mt are initial mass of the sample and sample mass after time t of the iodine
adsorption experiment, respectively.
The iodine adsorption experiment in solution phase was investigated using hexane as a
liquid media. In general, 300 ppm iodine solution in hexane was used for all the investigations.
About 10 mg of the adsorbents were soaked in the solution for various time intervals at 25 °C.
The extent of iodine adsorption from the hexane solution was evaluated by standard UV-visible
spectroscopic method using Shimadzu UV-1800 UV-visible spectrophotometer. Irrespective of
the adsorption media, the release experiments were carried out in organic media using absolute
ethanol as solvent at 25 °C. The recyclability test was carried out by soaking the iodine loaded
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specimens in absolute ethanol for 12 h and separated the solid adsorbents using filtration
followed by activating the specimens at 100 °C overnight in vacuum.
O
O
O
O
O
Cl
Cl
O
P
N
P
N
K2CO3 , THF
N
N
N
P
N
O
O
O
+
P
P
N2 Atmosphere,
48 Hour
Cl
Cl
P
O
O
Cl
Cl
OH
NH2
N
N
H
N
O
O
H2N
N
NH2
Compound-I
D
,
M
N
2
e
n
S
a
h
a
x
O
,
8
4
t
o
i
m
,
r
HN
D
.
e
C
o
4
4
f
,
l
8
N
1
u
HN
0
NH
8
h
x
N
1
NH
N
NH
N
N
N
N
N
HN
NH
N
H
NH
N
N
HN
H
N
NH
HN
N
N
NH
H
N
N
H
N
N
N
H
N
H
O
N
O
HN
NH
NH
O
O
P
N
HN
NH
N
N
P
N
NH
NH
N
N
P
O
N
N
N
P
N
O
HN
HN
P
HN
N
O
O
O
P
N
N
O
H
O
O
NH
N
H
H
N
N
N
H
N
N
N
N
HN
NH
N
NH
H
N
N
HN
N
H
N
NH
NH
NH
N
N
HN
N
N
HN
HN
N
N
NH
N
HN
NH
CHNM-1
HNM-1
Scheme-1. Reaction scheme for the synthesis of CHNM-1 and HNM-1.
3. Results and Discussion
The successful condensation of the compound-I with melamine and pyrrole to form HNM-1 and
CHNM-1, respectively, (Scheme-1) was confirmed from the FT-IR and XPS investigations. The
observation of bands at 3440 cm-1 due to -N-H stretching (for secondary amine instead of
multiplet for primary amine) and 2925 cm-1 due to -C-H stretching of methine linkage, and the
absence of aldehyde band at ~ 1700 cm-1 confirm the condensation in both the cases as shown in
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Figure S1 [29],[30],[34]. The multiplet of bands in the region of 1220 to 1130 cm-1 confirms the
incorporation of cyclophosphazene ring in the specimens [27],[28],[29],[30]. It is important to
note that the band at 1695 cm-1 in CHNM-1 is observed due to -C=N stretching of the pyrrole
ring and should not be confused with the aldehyde band [30]. Further, the XPS investigation
supports the formation of HNM-1 and CHNM-1 as per the Scheme-1. The XPS survey scans
shown in the Figure S2, confirm the presence of P, C, N, and O in framework materials. The in-
depth study on the chemical environment of the individual elements i.e. P, C, N and O, was
gained by the their high-resolution (P 2p, C 1s, O 1s and N 1s) XPS spectra as shown in Figure
1. The C 1s high resolution XPS spectrum for HNM-1 shown in Figure 1(a), has three peaks at
284.8, 285.3 and 288.2 eV owing to the presence of methine, aromatic and triazine carbons,
respectively [35],[36],[37],[38]. Similarly, the C 1s XPS spectrum for CHNM-1 also have shown
three peaks at 284.8, 285.2 and 286.3 eV, and these could be associated to methine, aromatic and
pyrrolic carbons, respectively [37],[39]. The N 1s high resolution XPS spectrum for HNM-1
shown in Figure 1(b) has two peaks at 399.8 and 398.6 eV for the ‘N’ of the cyclophosphazene
ring and triazine ring, respectively, while the N 1s spectrum of CHNM-1 has shown the peaks for
the ‘N’ of cyclophosphazene and pyrrolic ring at 399.0 and 396.7 eV, respectively
[36],[37],[38],[39].
The O 1s high resolution XPS spectrum of CHNM-1 has two peaks associated with the
‘O’ of framework (Scheme-1) and ‘O’ from the tape used for the sample mounting in XPS
analysis (can be seen in the XPS survey scan of silicon tape, Figure S2) at 533.4 and 531.4 eV,
respectively [36], but the O 1s high resolution XPS spectrum of HNM-1 in addition to the peaks
for ‘O’ of the framework at 532.9 eV and ‘O’ of the tape at 531.6 eV, have an extra peak at
530.0 eV originated from the ‘O’ of DMSO trapped in the pores of the HNM-1, Figure 1(c). The
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P 2p high resolution XPS spectra shown in Figure 1(d), have shown a single peak for both HNM-
1 and CHNM-1 at 133.4 and 133.1 eV, respectively, indicating the only one type of phosphorous
present in the frameworks and incorporation of the cyclophosphazene ring in the framework
materials [36].
CHNM-1
CHNM-1
(b)
(a)
HNM-1
HNM-1
290
288
286
284
282
402
400
398
396
Binding Energy (eV)
Binding Energy (eV)
CHNM-1
(c)
CHNM-1
(d)
HNM-1
HNM-1
136 134 132 130 128
Binding Energy (eV)
536
534
532
530
528
Binding Energy (eV)
Figure 1. XPS high resolution scans; (a) C 1s, (b) N 1s, (c) O 1s and (d) P 2p for HNM-1 and
CHNM-1.
The HNM-1 and CHNM-1 were further subjected to elemental analysis. As per the
proposed structures in Scheme-1, theoretically, HNM-1 would consist of 41.8 % C, 46.8 % N
and 2.9 % H, while CHNM-1 with 70.4 % C, 10.3 % N and 3.9 % H. The experimental
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elemental composition was found to be 40.2 % C, 42.2 % N and 4.5 % H for HNM-1, and 62.8
% C, 8.4 % N and 3.5 % H for CHNM-1. The experimentally estimated elemental composition is
matching well with the theoretically calculated elemental composition. The thermal gravimetric
analysis performed under argon atmosphere indicates that HNM-1 and CHNM-1 are stable up to
a temperature of 370 °C, Figure S3. Both of these materials are amorphous in nature as no sharp
crystalline peak was observed in the XRD patterns (Figure S4). The microstructural analysis of
HNM-1 and CHNM-1 were carried out using FESEM and TEM instruments. Both the samples
consist of agglomerated small particles of irregular shapes in the size range of 50 to 200 nm. The
TEM images of HNM-1 and CHNM-1 shown in Figure 2(c and d), respectively show the
presence of small pores in the nanopore regime. The SAED patterns shown in the inset of figure
2(c & d) indicate that both the materials are amorphous and further corroborate the observation
of XRD.
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Figure 2. FESEM images of (a) HNM-1 and (b) CHNM-1, and TEM images of (c) HNM-1 and
(d) CHNM-1. The SAED pattern of samples has been shown in the inset of their respective TEM
images.
The textural properties of the HNM-1 and CHNM-1 has been analyzed by N2 sorption
analysis. In Figure 3, a type-I isotherm is observed for HNM-1 while a combination of type-I and
type-IV isotherm can be seen for CHNM-1. Both the samples show the remarkable N2 uptake at
low pressure region (P/P0 > 0.1) indicating the presence of micropores. This was further
supported by the pore size distribution analysis carried out using DFT method. The majority of
pores in HNM-1 are centered at 1.2 nm with significant proportion in ultra-micropore region at
0.65 nm and a very small proportion in mesopore region also [inset of Figure of 3(a)]. The
CHNM-1 has two types of pores and these were found to be centered at 1.65 and 3.46 nm [inset
of Figure of 3(b)]. The specific surface area estimated by the BET method was found to be 934
and 1283 m2 g-1 for HNM-1 and CHNM-1, respectively (Figure S5). The total pore volume
estimated at P/P0 of 0.90 was calculated to be 0.87 and 1.09 cm3 g-1 for HNM-1 and CHNM-1,
respectively.
(a)
HNM-1
0.25
(b)
800
2000
0.20
0.15
0.10
600
1500
0.05
0.00
CHNM-1
0.4
0
10
20
30
40
400
200
0
Pore width (nm)
1000
500
0
0.3
0.2
0.1
0.0
Ads
Des
Ads
Des
0
10
20
30
Pore width (nm)
0.0
0.2
0.4
0.6
0.8
1.0
0.0
0.2
0.4
0.6
0.8
1.0
Relative Pressure (P/P0)
Relative Pressure (P/P0)
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Figure 3. Nitrogen sorption isotherms of (a) HNM-1 and (b) CHNM-1. The pore size
distribution of HNM-1 and CHNM-1 has been shown in the inset of figure 2(a) and 2(b),
respectively.
The presence of heteroatoms such as N, O and P in the framework along with hierarchical
pore geometry and controlled textural properties has encouraged us to investigate the iodine
adsorption behavior. As discussed above, both HNM-1 and CHNM-1 were employed for the
iodine sequestration, both in vapor and in liquid phase. In the vapor phase adsorption, where
gravimetric method was used for the estimation of the extent of the iodine adsorption, it has been
observed that there is a continuous increase in the mass of the sample over a period of time up to
5 h and 6 h for CHNM-1 and HNM-1, respectively. Beyond this time, there is almost no increase
in the mass observed and hence, considered as the saturation is achieved as shown in Figure 4(a).
The quicker saturation in case of CHNM-1(5 h) as compared to HNM-1 (6 h) can be attributed to
larger pore size which in turn facilitates a better mass transfer. However, it is interesting to see a
higher iodine uptake in the HNM-1 (223 wt.%) as compared CHNM-1 (120 wt.%), although, the
CHNM-1 has higher SABET as well as pore volume. This unusual iodine adsorption behavior
could be attributed to the significantly larger nitrogen content in HNM-1 (42.2 wt.%) as
compared to CHNM-1 (8.4 wt.%), which makes it more electron rich for facilitating the
adsorbate-adsorbent interaction. Thus, it could be concluded that the primary factors affecting
the iodine adsorption in the present case are the electron rich heteroatom enrichment in the
framework as well as the pore size. The iodine capture capacity of HNM-1 is better than many
reported adsorbents like SCMP-1 (222 wt.%) [23], NTP (180 wt.%) [21], NIP-CMP (202 wt.%)
[9], NBDP-CPP (150 wt.%) [40], S4-LDH (155 wt.%) [41], NH2-HMNOS (202 wt.%) [42], and
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comparable to BDP-CPP-2 (223 wt.%) [40], HCMP-3 (336 wt.%) [13], PG-700 (346 wt.%) [17]
and AzoPPN (290 wt.%) [25].
The change in color of the samples after iodine adsorption from off white to brown in
HNM-1 and brown to dark brown in CHNM-1, as shown in the Figure 4(b), provide compelling
evidence of iodine adsorption. It was further confirmed by the comparison of the TGA data of
iodine loaded HNM-1 and CHNM-1 named as I2@HNM-1 and I2@CHNM-1, respectively, with
the pristine specimens. A significantly higher mass loss was observed for I2@HNM-1 and
I2@CHNM-1 in the temperature range of 80 to 300 °C as compared to the HNM-1 and CHNM-
1, owing to the release of iodine upon heating [Figure 4(c & d)].
The energy dispersive X-ray (EDX) spectroscopy and elemental mapping were used to
confirm the presence of iodine in I2@HNM-1 and I2@CHNM-1 (Figure S6 and S7). The EDX
spectra of I2@HNM-1 and I2@CHNM-1 show the peaks for C, N, P, O and I. The peaks of P, C,
N and O originated from the framework materials while the peak for I is appeared from the
iodine adsorbed on I2@HNM-1 and I2@CHNM-1.
250
HNM-1
(a)
(b)
200
150
100
50
CHNM-1
0
0
2
4
6
8
10
Time (h)
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(c)
100
80
60
40
20
0
(d)
100
80
60
40
HNM-1
20
CHNM-1
I2@CHNM-1
I2@HNM-1
0
200
400
600
800
1000
200
400
600
800
1000
Temperature (°C)
Temperature (°C)
Figure 4. (a) Gravimetric iodine uptake of HNM-1 and CHNM-1 estimated at 75 °C. (b) Change
in the color of HNM-1 and CHNM-1 after iodine adsorption. TGA of (c) HNM-1 and I2@HNM-
1, (d) CHNM-1 and I2@CHNM-1 under argon atmosphere.
The mechanism of iodine adsorption and its interaction with framework materials, HNM-
1 and CHNM-1, were investigated by XPS and Raman spectroscopy. The XPS survey scan of
I2@HNM-1 and I2@CHNM-1 shown in Figure S8, show the peaks for P, C, N & O present in
the framework materials in addition to peaks for iodine (I 3d and I 4d). These peaks could be
ascribed to the iodine adsorbed on the materials. Further, the high resolution XPS spectra of I
3d5/2 show the peaks at 618.3 and 620.0 eV for I2@HNM-1, and 619.4 and 621.4 eV for
̶
̶
I2@CHNM-1. These peaks could be ascribed to polyiodide ion I3 and I5 , Figure 5(a & b) [11].
Further, Raman spectra measured by a Renishaw inVia Raman spectrometer at λext of 514 nm
show the bands at 106 and 168 cm-1 for I2@HNM-1 and I2@CHNM-1. These peaks further
confirm the presence of polyiodide ions [Figure 4 (c & d)] [11],[16],[43],[44].
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(a)
(b)
I 3d5/2
I 3d5/2
I2@CHNM-1
I2@HNM-1
I 3d3/2
I 3d3/2
635
630
625
620
632
628
624
620
616
Binding Energy (eV)
Binding Energy (eV)
(c)
(d)
I2@HNM-1
I2@CHNM-1
100
150
200
250
300
150
200
250
300
Wavenumber (cm-1)
Wavenumber (cm-1)
Figure 5. I 3d high resolution XPS spectra of (a) I2@HNM-1 and (b) I2@CHNM-1. Raman
spectra of (c) I2@HNM-1, and (d) I2@CHNM-1.
The HNM-1 and CHNM-1 have also been investigated for the iodine adsorption from
organic solvents. 10 mg of the adsorbent was soaked in the 300 ppm iodine solution in hexane.
The iodine uptake in HNM-1 and CHNM-1 was rapid during the first 30 min of the adsorption
experiment and later on the increase in iodine uptake was gradual [Figure 6(a)]. A complete
saturation was observed after 720 min. The higher adsorption capacity of HNM-1 as compared to
the CHNM-1 led to the complete removal of iodine from the solution leaving behind a
completely colorless solution after 12 h, whereas, in CHNM-1, a faint iodine color still exists
even after 18 h of the adsorption experiment which is also visible in Figure 6(b & c). The
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adsorptive removal efficiency of 98.5 and 87.8 % were estimated for HNM-1 and CHNM-1,
respectively.
0.8
0.6
0.4
0.2
0.0
0.8
0.6
0.4
0.2
0.0
(b)
(c)
100 (a)
HNM-1
CHNM-1
10 min
30 min
60 min
120 min
180 min
240 min
360 min
720 min
1080 min
80
60
40
20
0
HNM-1
CHNM-1
10 min
30 min
60 min
120 min
180 min
240 min
360 min
720 min
1080 min
HNM-1
CHNM-1
before
after
0
200
400
600
800 1000
Contact time (min)
400
500
600
700
400
500
600
700
Wavelength (nm)
Wavelength (nm)
Figure 6. (a) Iodine adsorption by HNM-1 and CHNM-1 in hexane solution. (b and c) the
absorption of iodine with time. The change in the color of the solution before and after the
adsorption is shown in the inset of figure 6(a).
The time dependent release of iodine in ethanol from I2@HNM-1 and I2@CHNM-1 is
linear with respect to time. The rate of iodine release in CHNM-1 was faster than HNM-1 which
further confirms the strong host guest interaction in HNM-1 owing to presence of triazine unit
than CHNM-1, Figure S11. The UV-vis spectra shown in Figure S12(a & b) have two maxima at
291 and 360 nm ascribed to polyiodide ion [13],[41]. Further, the recyclability of the HNM-1
and CHNM-1 was carried out. The iodine loaded samples were soaked in absolute ethanol
overnight and then dried. After five cycles, the HNM-1 and CHNM-1 have shown the iodine
removal efficiency of 80 % and 86%, respectively as shown in Figure S13. The significant loss
in iodine capture for HNM-1 after first cycle could be ascribed to stronger affinity of triazine
moiety in HNM-1 towards the iodine.
4. Conclusion
In summary, two types of cyclophosphazene based inorganic-organic hybrid nanoporous
materials i.e. HNM-1 and CHNM-1 have been explored for iodine adsorption both in vapor and
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liquid phase. Both the specimens, due to presence of electron-rich heteroatoms, have shown good
affinity towards the iodine which resulted in maximum iodine capture capacity of 223 and 120
wt.% for HNM-1 and CHNM-1, respectively, in vapor phase. The specimens have also shown
excellent release of iodine in organic media owing to the hierarchical pore structure of the
framework. These specimens have shown the very good adsorption retention capacity (80% for
HNM-1, 86% for CHNM-1) even after five cycle. These results demonstrate that the tuning of
physicochemical properties such as heteroatom enrichment, pore size and pore volume could
pave the way for improved iodine uptake capacity.
AUTHOR INFORMATION
Corresponding Author
*Email: pmfcy@iitr.ac.in, paritosh75@gmail.com
ORCID ID
Raeesh Muhammad: 0000-0003-3448-5846
Paritosh Mohanty: 0000-0003-2765-0129
Notes
The authors declare no competing financial interest.
Acknowledgements
This research was funded by Department of Science and Technology, Govt. of India [Grant No.
DST/TDT/TDP-03/2017(G)]
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Graphic Image
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Vapor Phase
Liquid Phase
Before
After
Before
After
Sample
CHNM-1
CHNM-1
CHNM-1 HNM-1
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Highlights
 Cyclophosphazene based hybrid materials have been explored for iodine sequestration
 Iodine sequestration both in liquid and vapor phase have been studied
 Maximum iodine adsorption capacity of 223 wt.% have been estimated
 The physicochemical properties played the pivotal role in iodine sequestration
24