Highly efficient iodine capture by layered double hydroxides intercalated with polysulfides

Article abstract of DOI:10.1021/cm5036997

We demonstrate strong iodine (I₂) vapor adsorption using Mg/Al layered double hydroxide (MgAl-LDH) nanocomposites intercalated with polysulfide (S_x^2-) groups (S_x-LDH, x = 2, 4, 6). The as-prepared LDH/polysulfide hybrid materials display highly efficient iodine capture resulting from the reducing property of the intercalated polysulfides. During adsorption, the I₂ molecules are reduced to I₃^- anions by the intercalated [S_x]^2- groups that simultaneously are oxidized to form S₈. In addition to the chemical adsorption, additional molecular I₂ is physically captured by the LDH composites. As a result of these parallel processes, and despite their very low BET surface areas, the iodine capture capacities of S₂-LDH, S₄-LDH, and S₆-LDH are 1.32, 1.52, and 1.43 g/g, respectively, with a maximum adsorption of 152% (wt %). Thermogravimetric and differential thermal analysis (TG-DTA), energy dispersive X-ray spectroscopy (EDS), and temperature-variable powder X-ray diffraction (XRD) measurements show the resulting I₃^- ions that intercalated into the LDH gallery have high thermal stability (￥350 °C). The excellent iodine adsorption performance combined with the facile preparation points to the S_x-LDH systems as potential superior materials for adsorption of radioactive iodine, a waste product of the nuclear power industry.

Full text of DOI:10.1021/cm5036997

Article
pubs.acs.org/cm
Highly Efficient Iodine Capture by Layered Double Hydroxides
Intercalated with Polysulfides
†
,‡
‡
‡
†
‡
§
†
Shulan Ma, Saiful M. Islam, Yurina Shim, Qingyang Gu, Pengli Wang, Hao Li, Genban Sun,
†
,‡,§
Xiaojing Yang, and Mercouri G. Kanatzidis*
†Beijing Key Laboratory of Energy Conversion and Storage Materials, College of Chemistry, Beijing Normal University, Beijing
1
00875, China
‡
Department of Chemistry, Northwestern University, 2145 Sheridan Road, Evanston, Illinois 60208, United States
Material Science Division, Argonne National Laboratory, 9700 S Cass Ave, Argonne, Illinois 60439, United States
§
*
S Supporting Information
ABSTRACT: We demonstrate strong iodine (I ) vapor adsorption
2
using Mg/Al layered double hydroxide (MgAl-LDH) nanocomposites
2−
intercalated with polysulfide (Sx ) groups (S -LDH, x = 2, 4, 6). The as-
x
prepared LDH/polysulfide hybrid materials display highly efficient
iodine capture resulting from the reducing property of the intercalated
−
polysulfides. During adsorption, the I molecules are reduced to I
2
3
2−
anions by the intercalated [Sx] groups that simultaneously are oxidized
to form S . In addition to the chemical adsorption, additional molecular
8
I2 is physically captured by the LDH composites. As a result of these
parallel processes, and despite their very low BET surface areas, the
iodine capture capacities of S -LDH, S -LDH, and S -LDH are ∼1.32,
2
4
6
1.52, and 1.43 g/g, respectively, with a maximum adsorption of 152% (wt %). Thermogravimetric and differential thermal
analysis (TG-DTA), energy dispersive X-ray spectroscopy (EDS), and temperature-variable powder X-ray diffraction (XRD)
−
measurements show the resulting I3 ions that intercalated into the LDH gallery have high thermal stability (≥350 °C). The
excellent iodine adsorption performance combined with the facile preparation points to the S -LDH systems as potential superior
x
materials for adsorption of radioactive iodine, a waste product of the nuclear power industry.
INTRODUCTION
which from the cost perspective can be a significant drawback
for their extensive use. Porous metal−organic frameworks
■
Nuclear power can be a clean, reliable energy source to meet
global population demands if the challenge of nuclear waste
pollution can be tackled. Consequently, appropriate manage-
ment of nuclear waste is a main safety concern associated with
14−21
(
MOFs) are another kind of adsorbent for iodine capture,
22
1
,2
but their relatively low thermal stability or steric hindrance of
2
3,24
the organic ligands
limits their application. Aerogels are a
25−27
1
29
new family for iodine capture.
We have developed
the production of nuclear energy. The radionuclide I is one
of the byproducts and is a hazardous species with its long half-
nonoxide aerogels made with metal sulfides for iodine
7
immobilization and found that these materials are promising
life (∼10 years) that must be captured and reliably stored as
2
8,29
1
31
as a potential replacement for AgZ.
However, the complex
nuclear waste. Although I is a short-lived (half-life of 8.02
days) radionuclide, it also requires immediate trapping because
of its high volatility and involvement in human metabolic
preparation processes of aerogels can restrict their widespread
30−32
use.
Thus, it is necessary to develop improved alternative
3
−5
iodine sorbents with higher loading capacity, lower cost, and
easier preparation steps than existing materials.
processes.
Due to the harmful effects of radioactive iodine
6
,7
on human health, their safe and long-term capture using
7
,8
Layered double hydroxides (LDHs) are well studied anion-
exchanging materials with low cost and excellent adsorptive
appropriate adsorbents and subsequent storage is extremely
important.
There are many existing adsorbent materials for iodine
3
3
34−36
properties. They have versatile applications in catalysts,
3
7
9
10
two-dimensional nanoreactors, adsorbents for certain
guests, and other functional materials.
capture, including zerovalent iron, Illite minerals, Al−O−F
38
36,39−41
1
1
12
13
Considerable
materials, zeolite-based materials, and functionalized clays.
12
efforts have been devoted to the adsorption of environmental
Ag-based porous zeolitic (AgZ) materials have shown high
iodine loading capacity and removal efficiency. In the United
States, the most prevalent iodine sorbent studied is AgZ, while
Japanese and European researchers have studied AgNO3-
impregnated alumina and silica. However, both AgZ and
AgNO -impregnated sorbents require silver to bind I (g),
42−44
contaminants using LDH materials.
Functionalized LDHs
Received: October 9, 2014
Revised: November 20, 2014
3
2
©
XXXX American Chemical Society
A
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Chemistry of Materials
Article
vacuum (P < 10− mbar), whereas survey scans and high-resolution
scans were collected using a pass energy of 25 eV. Binding energies
were referred to the C 1s binding energy at 284.6 eV. A low-energy
electron flood gun was employed for charge neutralization. Prior to
XPS measurements, powders were pressed on cupper foil and
mounted on stubs and successively put into the entry-load chamber
to pump. Fitting of the peaks was made by using the Avantage
software.
8
with anionic cyclodextrin cavities were reported to adsorb
molecular iodine. Recently, we demonstrated the intercalation
of polysulfide anions into the LDH gallery to give rise to
4
5
materials (S -LDH) that exhibit outstanding heavy metal ion
x
4
6
47
removal as well as Hg(0) vapor capture properties. Here,
we show for the first time that the S -LDH materials are
x
excellent in achieving rapid iodine capture as well via reduction
of the polysulfides.
The metal contents of the solid samples were determined using
inductively coupled plasma atomic emission spectroscopy (ICP-AES,
EXPERIMENTAL SECTION
Jarrel-ASH, ICAP-9000). Solutions of 0.1 M HNO were used to
dissolve the solids. CHN analyses of the samples were conducted using
an Elementar Vario EL elemental analyzer.
3
■
Materials. MgAl−CO −LDH was prepared by a hexamethylene-
3
4
8,49
−
2−
tetramine hydrolysis method,
and through a NO /CO anion-
3 3
exchange of MgAl−CO −LDH, the MgAl−NO −LDH was ob-
3
3
4
8,50
RESULTS AND DISCUSSION
tained.
Potassium polysulfides of K S (x = 2, 4, 6) were
2 x
■
synthesized by the reactions of K and S in liquid ammonia as
Characterization of S -LDH and LDH−NO −CoS . The
3
2
x
3
4
reported. The S -LDH composites were prepared via an exchange of
x
synthesis of polysulfide/LDH materials was accomplished via
−
2−
46
NO3 with [Sx] as we previously reported. Using the S -LDH and
4
Co(NO ) ·6H O as reacting materials, LDH-NO −CoS was
3
2
2
3
4
4
7
obtained.
I Vapor Capture. Naturally occurring, nonradioactive I was used
2
2
in all experiments. Loading was conducted under conditions relevant
to fuel reprocessing (350 K/77 °C and ambient pressure). The Sx-
LDH composites were tested for I vapor capture in a closed vial setup
2
0
32
similar to that previously reported for Hg adsorption. Other
materials such as S , K S , LDH-NO −CoS , and MgAl-NO -LDH
8
2
4
3
4
3
were used for control experiments. An amount of 0.20 g of I was
2
placed on the bottom of a glass vial, and 0.05 g of solid sorbent sample
was placed above, supported by conical shaped filter paper at the top
of the vial. The vial was capped, wrapped with Teflon tape, and
transferred inside a bigger glass vial in order to prevent leakage of I2
vapor and to ensure homogeneous transfer of heat to the inner vials.
The bigger vial containing several smaller vials was placed in the sand
bath at a temperature of ∼75 °C. After a period of time (1−2 days),
the sand bath was cooled to room temperature, and the sample mass
difference (before and after adsorption) was measured to estimate the
captured amount of iodine. Elemental distribution maps using energy
dispersive X-ray spectroscopy (EDS) and X-ray photoelectron
spectroscopy (XPS) analyses were done to confirm the iodine
presence in the samples after adsorption.
Physical Characterization and Chemical Analyses. Powder X-
ray diffraction (XRD) patterns were collected with a Phillips X’pert
Pro MPD diffractometer using Cu Kα radiation, at room temperature,
with a step size of 0.0167°, a scan time of 15 s per step, and 2θ ranging
from 4.5 to 70°. The generator setting was 40 kV and 40 mA. Variable
temperature XRD patterns were collected using a step size of 0.0167°,
a scan step time of 20 s, and a 2θ range of 4.5 to 70°. The temperature
ascending rate of 20 °C/min was used, and the holding time at each
selected temperature was 3 min. The temperature was raised in 50 °C
steps from 50 to 600 °C. The change of temperature was controlled by
the TCU2000 Temperature Control Unit (Anton Parr Co.). Fourier
transformed infrared (FT−IR) spectra of the samples were recorded
on a Nicolet-380 Fourier-Transform infrared spectrometer using the
KBr pellet method. Raman spectra were taken on a microscopic
confocal Raman spectrometer, using a 633 nm He−Ne laser.
Thermogravimetric and differential thermal analysis (TG-DTA)
measurements were performed with a ZRY-2P thermal analyzer in
an air atmosphere. Scanning electron microscopy (SEM), elemental
distribution mapping, and energy dispersive spectroscopy (EDS)
measurements were carried out using a Hitachi S-4800 microscope.
UV−vis absorption spectra were recorded on a TU-1901 spectropho-
tometer using suspensions of I-loaded samples in acetonitrile.
Figure 1. XRD patterns of the precursors (a) CO −LDH and (b)
3
NO −LDH; the exchanged products (c) S −LDH, (d) S −LDH, (e)
3
2
4
S -LDH, and (f) LDH-NO −CoS , I-adsorbed samples of (g) S -
6
3
4
2
LDH-I, (h) S -LDH-I, (i) S -LDH-I, (j) K S −I, (k) KI, (l) LDH-
4
6
2 4
CoS −I, and (m) NO -LDH-I. The d values are given in nanometers.
4
3
an ion-exchange reaction of NO with [Sx]2− according to the
−
3
46
scheme:
LDH(NO
Based on ICP, CHN analyses, and charge balance
) + K
S
→ LDH(S
) + KNO
3
2
x
x
3
considerations, the compositions of S -LDH, S -LDH, and S -
2
4
6
L D H
w e r e
e s t i m a t e d
t o
b e
Mg0.67Al0.33(OH) (S )0.14(NO3)0.01(CO3)0.02·0.8H O,
2
2
2
Mg0.67Al0.33(OH) (S )0.13(NO3)0.01(CO3)0.03·0.8H O, and
2
4
2
Mg0.67Al0.33(OH) (S ) (NO ) (CO3)0.05·0.3H O, respec-
2
6
0.11
3 0.01
2
The surface area was measured by nitrogen adsorption/desorption
2−
tively. The CO3 anions in the formula are adventitious and
at 77 K at relative pressures (P/P ) in the range of 0.05−0.30 with a
o
are introduced from air or water because of the high affinity of
Micromeritics Tristar II system, using the Brunauer−Emmett−Teller
48
CO22 for LDH layers. After ion-exchange of [Sx] with
−
2−
(
BET) model. The samples were degassed at 298 K under a vacuum
−
NO , the basal spacing (d ) of 0.89 nm in NO −LDHs
for 12 h before analysis to remove any adsorbed impurities. XPS
studies were performed using a Thermo Scientific ESCALAB 250 Xi
spectrometer equipped with a monochromatic Al Kα X-ray source
3
basal
3
increased to 0.80/0.81/0.79 nm (Figure 1c−e). In view of the
51
thickness of the LDH layer (0.48 nm) and dbasal values of S -
x
(
1486.6 eV) and operated at 300 W. Samples were analyzed under a
LDHs (∼0.80 nm), the observed gallery height of ∼0.32 nm (=
B
dx.doi.org/10.1021/cm5036997 | Chem. Mater. XXXX, XXX, XXX−XXX

Chemistry of Materials
Article
longer S 2 ions in the gallery. For LDH−NO −CoS (Figure
−
6
3
4
1
f), the 0.88 nm dbasal is close to that of NO -LDH and suggests
3
that the NO − resides in the gallery while the “CoS ” exists as
3
4
47
an amorphous phase.
The IR spectra (Figure 2) provide clear evidence for the ion-
−1
exchange reaction. The absorption bands at 1354 cm (Figure
−
1
2
a) and 1384 cm (Figure 2b) respectively correspond to
2−
−
−1
CO3 and NO . In the S -LDH samples, the 1384 cm band
3
x
(
Figure 2c) is very weak, implying a nearly complete
−
2−
2−
substitution of NO3 by [S ] . The double charged [S ]
x
x
provides a stronger driving force for the ion-exchange than the
−
48,52−54
singly charged NO .
4
Moreover, the bands at 663 and
3
47 cm− (Figure 2c) assigned to ν(M−O) and δ(O−M−O)
vibrations arise from the LDH layer in S -LDH. Raman
spectra (Figure 3) further demonstrate the presence of [Sx]
1
46
4
2
−
anions as a result of the successful ion exchange. As shown in
Figure 3a, the peaks at 228, 267, 434, and 484 cm− correspond
1
5
5
to the symmetric and asymmetric S−S vibrations of K S , and
2
4
after intercalation, these main S−S vibration bands (Figure 3b)
remain, with only a little shift and some change of the relative
intensity.
XPS spectra were employed (Figure 4) to probe the
polysulfides in LDH. The assignment of the binding energies
for S -LDH is summarized in Table 1. Figure 4a,d,g reveal
x
Figure 2. FT-IR spectra of (a) CO −LDH, (b) NO −LDH, (c) S −
3
3
4
broad bands in the range of 159−166 eV, indicating the
LDH, I-laden samples of (d) S -LDH-I, (e) S -LDH-I, (f) S -LDH-I,
2
4
6
presence of multiple oxidation states of sulfur as expected for a
and (g) SO -LDH.
56,57
4
polysulfide.
In S -LDH, the weak peak at 167.6 eV (peak 4
6
2−
in Figure 4g) suggests a small degree of oxidation of [S ] to
x
SO32 /SO4
−
2− 58
.
The XPS spectra of the materials after I2
adsorption reveal the occurrence of I based peaks and will be
discussed below.
Iodine Capture. The iodine vapor capture by S -LDH is
x
rapid as judged from the color change of the solid samples from
yellow to brown within a period of 30 min. A control
experiment carried out with MgAl-NO -LDH showed no color
3
change even after 3 days of exposure to iodine vapor,
highlighting the key role of the intercalated polysulfides in
the iodine adsorption process. The capture capacity for iodine
was directly obtained by weighing the samples before and after
adsorption. The adsorption capacities for S -LDH, S -LDH,
2
4
and S -LDH were calculated to be 1.32, 1.55, and 1.43 g/g,
6
respectively (see Table 2). The theoretical chemisorption
capacities listed in Table 2 are based on the balanced reaction
−
2−
of S -LDH with I , in which the molar ratio of [I ] /[S ] is 2
x
2
y
x
based on their charge ratio of 2.
For reactions between I and S -LDH, there are two possible
2
x
schemes:
Figure 3. Raman spectra of (a) K S , (b) S -LDH, (c) S -LDH-I, (d)
2
4
4
2
S -LDH-I, (e) S -LDH-I, (f) K S -I, and (g) S .
4
6
2
4
8
2
−
LDH − [(S ) ] (s) + (m·y)I (g)
x
m
2
−
0
.80−0.48) suggests a similar flat lying arrangement of zigzag
→
LDH − [I ] (s) + (m·x)/8S (s)
y
2m
8
(1)
(2)
2−
47
[
S ] anions in the interlayer. For S -LDH, a clear Bragg
x
6
peak at 0.38 nm is possibly associated with the (006) reflection
2
−
LDH − [(S ) ] (s) + y/2I (g) + 2mO (g)
x
m
2
2
of CO -LDH. Changing the starting K S /LDH ratios did not
3
2 6
−
2−
remove the 0.38 nm peak. Thus, steric hindrance from the
→
LDH − [I ](s) + LDH − [(SO ] ] (s)
y
4
m
2
−
2−
2−
[
S ] chains may prevent the coexistence of CO with [S6]
6
3
2−
in the gallery, ultimately forcing CO3 to form a separate phase
CO -LDH), whose (003) reflection at 0.76 nm may overlap
According to IR spectra (Figure 2d−f), there are no clear
2− −1
(
observable [SO ] bands around 1108 cm as in Figure 2g.
4
3
with the 0.79 nm peak (the (003) reflection of S -LDH phase).
XRD results also do not show the d
of 0.87 nm for SO4-
6
basal
4
8,59,60
It is noteworthy that in S -LDH and S -LDH the intensity of
LDH.
Since there is no SO -LDH phase observed, eq 2 is
2
4
4
(
003) and (006) reflections is similar, which is due to the
ruled out, and therefore S is the most likely oxidation product
8
2
−
presence of the high electron density Sx species in the gallery.
In S -LDH, the 006 reflection is stronger than the (003)
reflection, because of the even higher electron density of the
as shown in eq 1.
−
Additionally, in eq 1, the polyiodide anions of [I ] and/or
6
3
−
45
[I ] can form, while molecular I is adsorbed on the surface
5
2
C
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Chemistry of Materials
Article
Figure 4. XPS spectra for S 2p and I 3d with deconvolution of corresponding XPS peaks. Parts a, d, and g correspond to those before absorption
S −LDH), and the remaining are those after iodine absorption (S -LDH-I).
(
x
x
Table 1. XPS Data and Assignment of S and I States for Sx-
LDH before/after Adsorption
Table 2. I Vapor Capture Results for S -Containing
2 x
a
Samples
binding energy
theoretical
a
b
c
(
eV)
163.6
62.3/161.0
164.8/163.7
assignment
adsorption capacity
chemisorption capacity
S2-LDH before I adsorption
after I adsorption
2p of S0
samples
S2-LDH
g/g
wt %
g/g
wt %
2p of [S ]2
−
1
n
1.32
1.55
1.43
0.84
no
132%
155%
143%
84%
1.26
1.05
0.93
126
105
93
2p of S0
S4-LDH
S6-LDH
K2S4
2p of [S ]2
−
162.6
168.8
630.5
619.0
n
2p of S6+ (SO42−)
d
1.25
125
3d3/2 of I
S8
−
3d5/2 of In
LDH−NO −CoS4
no
3
0
S4-LDH
before I adsorption
after I adsorption
163.3
2p of S
NO −LDH
no
3
1
2
0
−
−
1
1
62.2
60.9
2p of S
2p of S
2p of S
a
b
Three samples were used to evaluate these results. The values were
obtained from the mass difference of samples by weighing them before
and after iodine adsorption. Sample: 0.05 g. I : 0.20 g. T: 60 °C. Time:
24 h. These values were based on the reacting formula: LDH-
[(S ) ] + 3m·I → LDH[I ] + (m·x)/8·S , where moles of I are
double those of [S ] . S -LDH: 1/85 × 0.14 × 2 × 380.7 = 1.26 g/g.
-LDH: 1/90 × 0.11 ×
2 × 380.7 = 0.93 g/g. The value was deduced by the reaction formula
of 2K S + 2I → 4KI + S .
164.8/163.7
2
2−
c
163.2
168.8
630.3
618.8
2p of [S ]
n
2p of S6+ (SO42−)
3d3/2 of In−
2−
−
−
x
m
2
3
2m
8
3
2−
x
2
3d5/2 of I −
S
-LDH: 1/93 × 0.13 × 2 × 380.7 = 1.05 g/g. S
n
4
6
d
before I adsorption
164.6/163.3
62.2
162.8/165.8
2p of S0
2p of [Sn]2
−
1
2
4
2
8
S6-LDH after I adsorption
2p of [Sn]2−/S0
2p of SO32−/SO42−
3d3/2 of In−
−
−1
1
6
6
67.4/169.4
30.2
for [I ] , a band at 167/168 cm
5
corresponding to
Since our present I-
6
2,63
coordinated iodine should appear.
laden samples show an intense band at 108 cm (∼110 cm )
−1
−1
18.7
3d5/2 of In−
−1
a
but not at 167/168 cm band (Figures 3c−e), it suggests the
− −
62
The binding energies were obtained from the deconvoluted peak
presence of [I3] rather than [I ] . Nevertheless, based on
positions.
5
−
−1
calculated vibrational frequencies of I , the 150 cm band
5
−
may be due also to some I . In K S −I, there are no bands
5
2 4
−
−
of LDH. Raman spectroscopy was employed to detect the [I ]
corresponding to any [I ] , because the anion of the product in
y
x
4
5
−
−1
−
species. For [I ] , an intense band at ∼110 cm (ν1,
the reaction (K S + 2I → 4KI + S ) is the only I . Although
3
2
4
2
8
4
5,61,62
−
symmetric stretch) is expected,
and sometimes a 150
[IO ] may also be a potential product because of possible air
exposure at the beginning of the adsorption tests, its signature
3
−1
62
58
cm band assigned to Fermi resonance is also present, but
D
dx.doi.org/10.1021/cm5036997 | Chem. Mater. XXXX, XXX, XXX−XXX

Chemistry of Materials
Article
a
Table 3. Iodine Adsorption Capacity of Various Absorbents in This Work and References
surface area
2
iodine state
T (°C)
(m /g)
capacity (wt %) iodine form
reference
b
c
Sx-LDH
I2 (g)
I2 (g)
I2 (g)
I2(g)
70
9−13
132−155
25
I /I -LDH
this work
2
n
e
d
Ag@Mon-POF
70
690
AgI
27
69
28
28
28
25
25
75
22
f
74
g
ZIF-8
77
1630
125
I2
h
i
2
SnS /SnS
125
125
125
140
140
-
270
13
23
360
287
-
67−68
33
SnI (S )
g
p
4
8
j
SnS33
I2 (g)
I2 (g)
I2 (g)
I2 (g)
I2 (g)
SnI4
j
SnS50
53
SnI (S )
4
8
2
k
l
Cg-5C
239 (20 days)
I2
I2
k
l
Cg-5P
BYA) [PbBr ]
87
m
n
(
75
BYA-I2
2
4
o
p
[
(CH ) NH ]1.66[Cd1.84Na0.66(BDC) ]·DMF·0.5EtOH
cyclohexane solution of I2
cyclohexane solution of I2
cyclohexane solution of I2
R. T.
R. T.
R. T.
R. T.
-
17
I2
3
2
q
2
3
r
MIL-53
Cu I -(DABCO) ]I
-
20−60
118
1
I2
s
−
t
2
[
-
I /I
64
70
4
3
2
u
In−
v
MgAl-NO -LDH
I2 water solution
10
3
a
b
c
The ‘-’ refers to no related data reported in the reference. LDH intercalated with polysulfides. Intercalation and electrostatic interaction of iodine
d
+
e
anion and LDH layer. Ag@Mon-POF: polymeric organic framework (Mon-POF) adsorbs Ag to deposit Ag nanoparticles into the pores. Reaction
of I and Ag to form AgI. Zeolitic imidazolate framework-8. Interactions between polarizable I and MeIM (2-methylimidazole) ligand, i.e. I···H
and I···C contacts. Sn S chalcogel granue (SnS ) or powder (SnS ). Iodine binding with S . PAN(polyacrylonitrile)-chalcogel hybrid. Pt−Ge−S
chalcogen aerogels. Pearson’s HSAB principle based on the affinity of a soft Lewis base such as chalcogen (i.e., S, Se) for a soft Lewis acid such as
I2(g). Hybrid perovskite (BYA) [PbBr ] in which the BYA is an alkyne-ammonium group. Formation of BYA-I molecules via covalent C−I
bonds. Eight-connected self-interpenetrating porous MOF. Charge-transfer (CT) between I and π-electron walls. Al-based MOFs Charge
f
g
2
2
h
i
j
k
2
3
g
p
n
l
m
n
2
4
2
o
p
q
r
2
s
transfer complexes between electron donors as amino and iodine. Copper(I) halide-based MOF, DABCO = N,N′-dimethyl-1,4-
t
u
diazabicyclo[2.2.2]octane. Adsorption and intermolecular interactions. Treatment of wastewater containing high concentrations of iodine
v
Electrostatic attraction of iodine and LDH layers.
is not observed in the XPS data (Figure 4). Based on the above
analyses, the product of iodine absorption by S -LDH should
x
−
be I .
3
The UV/vis absorption bands appearing at 295 and 360 nm
−
further verify the presence of [I ] , which correspond to the
3
6
4−67
spin and symmetry-allowed σ−σ* and π−σ* transitions.
Thus, the iodine adsorption process of S -LDH involves the
x
redox reaction and subsequent intercalation according to eq 1.
That is, the I molecules are reduced by polysulfides to produce
2
−
I3 anions, which ultimately intercalate into the LDH interlayer;
meanwhile, the [Sx] are oxidized by I to form S . On the
2−
2
8
2−
Figure 5. Photographs showing the state of samples before and after
iodine adsorption.
basis of the affinity order of anions to the LDH layer (CO
>
3
−
2−
−
−
−
−
−
−
48
−
SO4 > OH > F > Cl > Br > NO > I ), I or I ions
3
n
have very low affinity for the LDH layer; therefore, generally, it
Figure 6. Variable temperature XRD patterns of S -LDH-I heated in air and standard calculated patterns of related products.
4
E
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Chemistry of Materials
Article
Figure 7. SEM images of I-laden samples of (a,a′) S -LDH-I, (b,b′) S -
2
4
LDH-I, (c,c′) S -LDH-I, and (d,d′) K S −I.
6
2 4
−
is very difficult to synthesize I intercalated LDH. Bastianini et
n
68
Figure 8. (a) SEM image, (a′) EDS and average chemical composition
al. studied the intercalation of molecular iodine in non-
aqueous solutions into LDH-Cl but could not obtain a pure
LDH-I hydrotalcite phase, because the LDH had a poor affinity
of S -LDH-I, and corresponding SEM image and EDS data for the
4
heated samples in the air for 1 h under temperatures of (b, b′) 150 °C,
(c, c′) 350 °C, (d, d′) 450 °C.
−
−
−
for I ions which prevented complete I /Cl exchange. Sasaki
6
7
−
et al. also reported the intercalation of I into CoFe-LDH,
3
−
27
for which the I3 ions are the redox active species used for
partial oxidation of Co to Co . All these cases suggest that it
is difficult to obtain In intercalated LDH through normal anion
Ag@Mon-POF, the zeolitic imidazolate framework-8 (ZIF-
15 25
2
+
3+
8), and Cg-5P (Pt−Ge−S chalcogen aerogel).
−
Nearly all of the previously reported iodine adsorbing
2
exchange methods. Therefore, the I absorption processes we
materials have very large surface areas (287−1630 m /g), and it
2
describe here also amount to a facile method for preparing such
iodide intercalated LDH materials.
is this physical surface characteristic that mainly contributes to
the final capture result. The very high adsorption capacity of
our materials occurs despite the low surface areas of only ∼10
The theoretical chemisorption capacities for iodine are
2−
2
determined based on the amount of [Sx] in S -LDH and
m /g (Figure S2, Supporting Information) and suggests that
x
the reaction
the chemically functional polysulfides groups play a critical role
in iodine adsorption (rather than the physical surface properties
alone). Both the reductive property of polysulfides and the
intercalative function of LDH materials contribute to the total
2
−
LDH‐[(S ) ] (s) + 3m·I (g)
x
m
2
−
→
LDH‐[I ] (s) + (m·x)/8·S (s)
(3)
3
2m
8
high iodine capture. For pure MgAl-NO -LDH (lacking the
3
in which there are twice as many equivalents of I3− as [S ] .
2−
polysulfide guests), the adsorption capacity is only ∼1%.
70
x
Taking S -LDH as an example with its molecular weight of 85,
Detailed information and comparisons between adsorbents and
iodine at different states as well as the iodine form after
adsorption are summarized in Table 3. This unique property of
our materials provides important insights for designing
additional superior iodine adsorbents in the future.
2
the amount of [S2] is 0.14 mol, and the weight of I3− is 380.7
g. Therefore, the chemisorption capacity is calculated to be 1.26
g/g (= 1/85 × 0.14 × 2 × 380.7). The details with some
examples and the results are listed in Table 2. As shown in the
table, the amounts of iodine captured by both chemisorption
and physisorption are all greater than the calculated theoretical
chemisorption amounts. In comparison to the reported MOF
2−
To further validate our results with S -LDH, several control
x
experiments were carried out using various key materials such
as crystalline S , crystalline K S , LDH-NO −CoS , and MgAl-
8
2
4
3
4
69
materials where the highest I uptake loading is 125% (wt %),
NO -LDH as absorbers. These materials lack one or more key
2
3
the S -LDH materials give higher adsorption capacities of 132−
properties the S −LDH systems have: S has S−S bonding, but
x
x
8
1
55%. Table 3 compares the I adsorption capacities of some
its oxidation state is zero; K S exhibits S−S bonds similar to
2
2 4
reported adsorbents and S -LDH materials in this work. The
those in S -LDH and possesses the reductive capacity to
x
x
adsorption capacities of S -LDH are much higher than those of
capture I , but it lacks the LDH layer; LDH-NO −CoS has
x
2
3
4
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Chemistry of Materials
Article
Figure 9. SEM and elemental distribution maps of S -LDH-I and the heated samples in the air for 1 h: (a) SEM of S -LDH-I and a-1−a-4 maps for
4
4
Mg, Al, S, I. (b) SEM of 150 °C-heated sample, and b-1−b-4 maps for Mg, Al, S, I. (c) SEM of 350 °C-heated sample, and c-1−c-4 maps for Mg, Al,
S, I. (d) SEM of 450 °C-heated sample, and d-1−d-4 maps for Mg, Al, S, I.
NO -LDH shows no iodine adsorption. All of these control
3
experiments emphasize the importance of the intercalated
polysulfide sites (in the galley space of LDH) in achieving the
high iodine adsorption efficiency.
Structures and Morphologies of I-Laden Samples. As
shown in the XRD patterns of Figure 1g−i, after iodine
adsorption, the samples (labeled as S -LDH-I) reveal a strong
x
Bragg reflection at 0.41 nm and low intensity reflections at 0.31,
0
.20, and 0.17 nm. The 0.41 nm reflection is consistent with the
(
006) reflection of the iodide intercalated MgAl-LDH with a
48
dbasal of 0.82 nm, while the (003) reflection at 0.82 nm is
unobserved. This results from the very heavy nature of the
iodine atoms in [I3]− ions, which places massive electron
density on the (006) plane of the crystal structure. Bastianini et
al. also found that in In− intercalated ZnAl-LDH (dbasal = 0.81
nm), the relative intensity of the (006) reflection is stronger
than other (00l) reflections, and this was interpreted as the
increase in electron density of the interlayer region, due to the
presence of heavy iodine atoms halfway between adjacent
brucite sheets.
68
Figure 10. (a) SEM image of K S −I. (b) EDS and average chemical
2
4
composition for the area a. (c−e) Elemental distribution maps for S, K,
I in a.
polysulfide groups, but they are coordinated to Co2+ and thus
When organic solvents such as acetone are used to wash the
have a weak reductive capacity; MgAl-NO -LDH is the starting
3
S -LDH-I sample, leaching of I is observed forming a red
4
2
material and has only LDH layers with no reductive capacity.
As seen in Table 2, crystalline K S does display some iodine
solution (Figure 5c) along with an insoluble yellow solid
Figure 5d). The solids, separated by filtration, showed XRD
2
4
(
capturing capability but it is lower compared to its theoretical
patterns with an obvious Bragg peak at 0.82 nm (Figure 1h).
chemisorption capacity and much lower than that of S -LDH.
x
acetone
−
LDH-NO −CoS and S show even lower iodine capture. The
The dissolution reaction in acetone could be LDH-[I
3
]⎯⎯⎯⎯⎯⎯→
3
4
8
−
−
limited iodine capture by LDH-NO −CoS is possibly from the
LDH-I + I , and the remaining solid is LDH-I . The electron
density of the interlayer I ions is lower than [I ] , and this
2
3
4
−
−
coordinated S species that inhibit the reaction with iodine. In
3
the absence of polysulfide species, the parent materials MgAl-
results in the occurrence of the (003) reflection at 0.82 nm.
G
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Chemistry of Materials
Article
iodine capture using K S as a sorbent produces the KI cubic
2
4
crystals, which was verified by the XRD pattern in Figure 1j.
Clear evidence for the formation of KI was obtained from EDS
and elemental distribution mapping (Figure 10).
Thermal Behavior of I-Laden Samples. The thermal
decomposition behavior of the I-loaded samples was studied
with TG-DTA followed by EDS analysis of the residues. Figure
1
1 shows the TG-DTA curves of the I-laden samples along with
the S -LDH precursor for comparison. For S -LDH-I, there
4
4
were four obvious mass loss steps in the temperature ranges of
5
0−150, 150−250, 250−350, and 350−450 °C. The first step
with the large mass loss of ∼40% (50−150 °C) is attributed to
the removal of physisorbed iodine and I molecules from
2
decomposition of I3− and perhaps some hydration water. In
addition, some sulfur loss could be occurring, which is
consistent with the decrease of the S fraction as shown by
EDS analyses, discussed below (Figure 8a′,b′). The second and
third mass loss steps (total ∼20%, 150−250 and 250−350 °C)
68
appear to come from further removal of I and S and
2
6
8,72
dehydroxylation of brucite-like layers.
The fourth step
(
350−500 °C) is attributed to the release of I− due to
subsequent sublimation of any metal iodide formed.
When the S -LDH-I sample was heated in the air to 150, 350,
4
and 450 °C for 1 h, the EDS analyses (see Figure 8) along with
the elemental distribution maps showed a decrease of the S and
I contents. The significant amount of iodine remaining in the
−
3
50 °C-heated sample corresponds to an I intercalated phase
after the I molecules were removed. The iodine content of the
2
Figure 11. TGA-DTA curves of (a) S −LDH−I, (b) S −LDH−I, (c)
2
4
4
50 °C-heated sample (Figure 8d′) is drastically reduced (≤1%
S −LDH−I, and (d) S −LDH (in air).
6
4
in atomic ratio), as the structure collapses, hydroxyl
condensation to water occurs and the as-formed metal iodides
evaporate. The elemental distribution maps of the residue
(Figure 9d-4) confirmed the extremely low iodine content at
this stage.
This is consistent with simulated PXRD patterns of model
71
structures of LDH by Kamath et al. The yellow color of the
solids comes from S . The variable temperature XRD patterns
8
(
Figure 6) of S -LDH-I showed an obvious peak at 0.82 nm
The variable temperature XRD (Figure 6) of I-laden samples
4
when heated up to 400 °C, similar to that observed in the
(i.e., S -LDH-I) was used to investigate the structural changes
4
acetone-washed sample (Figure 1h). This further verifies the
with increasing temperature. At 100 °C, the (003) reflection
(0.81−0.82 nm) was absent, and this stems from the fact that
formation of I− intercalated LDH, resulting from the
−
decomposition of [I ] and the removal of molecular I by
3
2
the (006) reflection gains intensity from the very heavy
−
heating or acetone washing.
intercalated [I ] ions, which place large electron density on the
3
The Raman spectra show a series of peaks corresponding to
(
006) basal planes, as shown in Figure 1h. Between 100 and
2
−
the S phase (Figure 3g) verifying the oxidation of [S ] to S8.
8
x
400 °C, a series of basal (00l) Bragg reflections being
characteristic of a layered LDH phase were clearly observed
XPS spectra (Figure 4) also showed the formation of S and the
8
presence of iodine after adsorption. There were very strong and
as considerable I loss lowered the electron density on the
2
clear I 3d peaks (∼630 eV for I 3d and ∼619 eV for I 3d ),
3
/2
5/2
(
006) basal planes, ultimately allowing the (003) plane
which are consistent with the 3d binding energies of iodine
associated with the brucite layer to be shown. In addition,
the peak with d-spacing of 0.15 nm corresponding to the (110)
plane of the 2D LDH sheets was clearly discernible. The
consistent basal spacing of 0.81 nm (100−400 °C) suggests
5
8
ions. Similar to the starting materials, the I-laden samples
showed S 2p peaks corresponding to various oxidation states of
S. The strongest peak appeared at 163.7 [peak 2] in S -LDH-I,
2
1
63.7 [peak 2] in S -LDH-I, and 162.8 [peak 1] in S -LDH-I
4 6
58
−
−
o
that the iodine remains intercalated in LDH as I and [I ] ions
3
corresponding to the binding energies of S 2p for Sn
confirms the presence of S . The peaks around 166−170 eV in
the samples indicate the presence of SO
define the same gallery height. When the sample was heated to
450 °C, however, the XRD patterns changed dramatically,
signaling the collapse of the layered structure. The halo at 0.21
nm can be assigned to MgO (Figure 6d). The spinel phase
8
2−
58
coming from the
4
partial oxidation upon exposure to air during handling.
SEM images (Figure 7a−c′) indicate that the resulting iodide
intercalated LDH samples maintain the hexagonal prismatic
shape of the LDH precursor, implying that the iodine
adsorption process is followed by intercalation of the resulting
73
which should occur during LDH calcinations was not
discernible here. Any metal iodide species is expected to be
volatile and sublimes out of the samples in these temperatures.
This suggests a large transformation of the structure takes place
in this temperature range. The results agree well with the TGA
results discussed above, which show a significant mass loss in
the range of 400−450 °C.
−
I3 anions. EDS (Figure 8a′ and Figure S3 and S4, Supporting
Information) and elemental distribution maps (Figure 9 and
Figure S3 and S4, Supporting Information) show the presence
of a significant amount of S and I in the samples. Additionally,
H
dx.doi.org/10.1021/cm5036997 | Chem. Mater. XXXX, XXX, XXX−XXX

Chemistry of Materials
Article
(
9) Lee, J.; Cha, D.; Oh, Y.; Ko, K.; Song, J. J. Hazard. Mater. 2009,
CONCLUDING REMARKS
■
1
(
64, 67.
10) Kaplan, D. I.; Serne, R. J.; Parker, K. E.; Kutnyakov, I. V.
Environ. Sci. Technol. 2000, 34, 399.
11) Miller, A.; Wang, Y. J. Environ. Radioact. 2014, 133, 35.
The polysulfide-containing S -LDH intercalates are strong
x
iodine vapor capture agents despite their low surface areas.
The LDH layers act as a supporting substrate and provide these
polysulfide compounds with a protective yet accessible space
for iodine vapor. The reducing property of the polysulfide
species is the driving force for the chemisorption of iodine. The
(
(12) Chapman, K. W.; Chupas, P. J.; Nenoff, T. M. J. Am. Chem. Soc.
2010, 132, 8897.
(13) Riebe, B.; Dultz, S.; Bunnenberg, C. Appl. Clay Sci. 2005, 28, 9.
−
(
14) Zeng, M.-H.; Wang, Q.-X.; Tan, Y.-X.; Hu, S.; Zhao, H.-X.;
Long, L.-S.; Kurmoo, M. J. Am. Chem. Soc. 2010, 132, 2561.
15) Sava, D. F.; Rodriguez, M. A.; Chapman, K. W.; Chupas, P. J.;
Greathouse, J. A.; Crozier, P. S.; Nenoff, T. M. J. Am. Chem. Soc. 2011,
33, 12398.
16) Liu, Q.-K.; Ma, J.-P.; Dong, Y.-B. Chem. Commun. 2011, 47,
185.
17) Shi, X.; Yang, J.; Salvador, J. R.; Chi, M. F.; Cho, J. Y.; Wang, H.;
I2 molecules are reduced to I3 ions while the polysulfide ions
oxidize to form S . Moreover, there is a physisorption
8
(
component to the process contributing to the already high
efficiency for I (g) capture. The S -LDH, S -LDH, and S -LDH
2
2
4
6
1
(
7
(
materials showed iodine capture capacities of 1.32, 1.52, and
.43 g/g, reaching a high adsorption rate of 152% by weight.
These are comparable to or higher than previously reported
materials such as the best MOF materials with their highest I2
uptake of 125%. The results reported here demonstrate that
1
Bai, S. Q.; Yang, J. H.; Zhang, W. Q.; Chen, L. D. J. Am. Chem. Soc.
2011, 133, 7837.
high surface area materials are not always necessary for the
(18) Chapman, K. W.; Sava, D. F.; Halder, G. J.; Chupas, P. J.;
Nenoff, T. M. J. Am. Chem. Soc. 2011, 133, 18583.
effective capture of massive amounts of iodine. Compared with
+
2+
(
19) Yin, Z.; Wang, Q.-X.; Zeng, M.-H. J. Am. Chem. Soc. 2012, 134,
857.
20) Falaise, C.; Volkringer, C.; Facqueur, J.; Bousquet, T.; Gasnot,
L.; Loiseau, T. Chem. Commun. 2013, 49, 10320.
21) Bennett, T. D.; Saines, P. J.; Keen, D. A.; Tan, J.-C.; Cheetham,
A. K. Chem.Eur. J. 2013, 19, 7049.
22) Luo, Y. H.; Yu, X. Y.; Yang, J. J.; Zhang, H. CrystEngComm
014, 16, 47.
the Ag -loaded zeolitic materials or the Pt -containing
aerogels, the lower cost of environmentally safe and abundant
4
(
elements of S -LDH combined with their significant iodine
x
adsorption capacity make them promising for highly efficient
I2(g) capture. The latter is of importance to manage radioactive
waste iodide relevant to the nuclear energy industry.
(
(
2
ASSOCIATED CONTENT
(23) Wang, X.-L.; Qin, C.; Lan, Y.-Q.; Shao, K.-Z.; Su, Z.-M.; Wang,
E.-B. Chem. Commun. 2009, 410.
■
*
S
Supporting Information
(24) Long, D. L.; Hill, R. J.; Blake, A. J.; Champness, N. R.;
UV/vis absorption spectra for I-laden sample (S -LDH-I)
4
Hubberstey, P.; Proserpio, D. M.; Wilson, C.; Schroder, M. Angew.
Chem., Int. Ed. 2004, 43, 1851.
dispersed in acetonitrile; BET surface areas of S -LDH and S -
2
4
LDH; SEM images; EDS and elemental distribution maps for
Mg, Al, S, and I of S -LDH-I and S -LDH-I. This materials are
(
́ ̌
25) Riley, B. J.; Chun, J.; Ryan, J. V.; Matyas, J.; Li, X. S.; Matson, D.
2
6
W.; Sundaram, S. K.; Strachan, D. M.; Vienna, J. D. RSC Adv. 2011, 1,
available free of charge via the Internet at http://pubs.acs.org.
1704.
(
26) San
Colloid Interface Sci. 2006, 300, 437.
27) Katsoulidis, A. P.; He, J. Q.; Kanatzidis, M. G. Chem. Mater.
012, 24, 1937.
28) Riley, B. J.; P, D. A.; Chun, J.; Matyas
G.; Law, J. D.; Kanatzidis, M. G. Environ. Sci. Technol. 2014, 48, 5832.
29) Riley, B. J.; Chun, J.; Um, W.; Lepry, W. C.; Matyas, J.; Olszta,
́
chez-Polo, M.; Rivera-Utrilla, J.; Salhi, E.; Von Gunten, U. J.
AUTHOR INFORMATION
■
*
(
2
(
Corresponding Author
Tel. +1-847-467-1541. Fax: +1-847-491-5937. E-mail: m-
kanatzidis@northwestern.edu.
́ ̌
, J.; Lepry, W. C.; Garn, T.
(
Notes
M. J.; Li, X. H.; Polychronopoulou, K.; Kanatzidis, M. G. Environ. Sci.
Technol. 2013, 47, 7540.
The authors declare no competing financial interest.
(
30) Mohanan, J. L.; Arachchige, I. U.; Brock, S. L. Science 2005, 307,
ACKNOWLEDGMENTS
■
397.
(
31) (a) Bag, S.; Trikalitis, P. N.; Chupas, P. J.; Armatas, G. S.;
This work is supported by the National Science Foundations of
China (NSFC; Nos. 21271028, 51272030,and 21271001) and
the Department of Energy USA (NEUP program, Project 12-
Kanatzidis, M. G. Science 2007, 317, 490. Shafaei-Fallah, M.; He, J. Q.;
Rothenberger, A.; Kanatzidis, J. J. Am. Chem. Soc. 2011, 133, 1200−
1
202.
3438).
(32) Oh, Y.; Morris, C. D.; Kanatzidis, M. G. J. Am. Chem. Soc. 2012,
1
34, 14604.
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