Effect of Functional Groups on the I2 Sorption Kinetics of Isostructural Metal–Organic Frameworks

Article abstract of DOI:10.1002/bkcs.12201

In this work, the effect of functional groups on I₂ sorption kinetics is investigated using two different types of isostructural metal-organic frameworks, UiO-66-X series (X = H, Br, NO₂, NH₂, (OH)₂, and (COOH)₂) and M₂(m-DOBDC) series, (M = Co2+, Mg2+, and Ni2+; m-DOBDC^4? = 4,6-dioxo-1,3-benzenedicarboxylate). Among the UiO-66-X series, UiO-66-(COOH)₂ exhibits the fastest sorption kinetics and the highest sorption capacity due to dipole-induced dipole interactions between carboxylic acid groups and I₂ molecules. In addition, faster I₂ chemisorption is observed in M₂(m-DOBDC) because of electrophilic aromatic substitution of m-DOBDC^4? with I₂. The I₂ sorption mechanisms are further supported by fitting the I₂ adsorption kinetics data to pseudo-first-order and pseudo-second-order kinetic models.

Full text of DOI:10.1002/bkcs.12201

Communication
DOI: 10.1002/bkcs.12201
BULLETIN OF THE
B. Lee and J. Park
KOREAN CHEMICAL SOCIETY
Effect of Functional Groups on the I Sorption Kinetics of Isostructural
2
Metal–Organic Frameworks
*
Byeongchan Lee and Jinhee Park
Department of Emerging Materials Science, Daegu Gyeongbuk Institute of Science and Technology
DGIST), Daegu 42988, South Korea. *E-mail: jinhee@dgist.ac.kr
(
Received October 16, 2020, Accepted December 16, 2020, Published online January 19, 2021
In this work, the effect of functional groups on I sorption kinetics is investigated using two different
2
types of isostructural metal-organic frameworks, UiO-66-X series (X = H, Br, NO , NH , (OH) , and
2
2
2
4−
(
COOH) ) and M (m-DOBDC) series, (M = Co2+, Mg2+, and Ni2+; m-DOBDC = 4,6-dioxo-1,3-
2 2
benzenedicarboxylate). Among the UiO-66-X series, UiO-66-(COOH) exhibits the fastest sorption kinet-
2
ics and the highest sorption capacity due to dipole-induced dipole interactions between carboxylic acid
groups and I molecules. In addition, faster I chemisorption is observed in M (m-DOBDC) because of
2
2
2
4−
electrophilic aromatic substitution of m-DOBDC with I . The I sorption mechanisms are further
2
2
supported by fitting the I adsorption kinetics data to pseudo-first-order and pseudo-second-order kinetic
2
models.
Keywords: Metal–organic frameworks, Iodine capture, Functional group effects, Sorption kinetics
1
2
13–16
17
18
Effective utilization of nuclear energy is essential in order
to meet the rapid increase in global energy demand. How-
ever, the radioactive nuclides generated from the fission of
nuclear fuels are hazardous to human health and the envi-
ronment, as witnessed in the 2011 Fukushima-Daiichi acci-
HKUST-1, ZIF-8,
UiO-66 series, MIL series,
19
20
21
MFM-300 series, Tb-MOF, and SBMOF have been
evaluated for I capture. To design an ideal MOF sorbent,
the interactions of the adsorbed I molecules with the build-
ing blocks of MOFs must be enhanced. Nevertheless, there
is a lack of in-depth research focusing on the effect of func-
2
2
1
dent. A major constituent of nuclear waste is radioactive
129
131
129
iodine (e.g.,
I and
I). The
I isotope is one of the
tional groups on the I sorption of MOFs. Previous studies
2
longest-lived radioactive isotopes with a half-life of
have reported the enhancement of the sorption capacities and
kinetics by employing MOFs functionalized with electron-
rich and basic functional groups such as amine and pyridine,
as these groups can form strong charge-transfer complexes
7
131
1.57 × 10 years. Meanwhile,
I, with a relatively short
half-life (8.02 days), is generated in large amounts. These
radioisotopes of iodine pose significant health hazards, such
as hypothyroidism and thyroid cancer because of the emis-
17,18
with molecular I2.
However, these reports might be mis-
2
sion of beta (β) and gamma (γ) radiation. Thus, the secure
understood to suggest that basic functional groups are more
sequestration of radioactive iodine is essential for safe
nuclear waste management. Conventionally, iodine was
sequestrated by filtration processes using wet scrubbers,
sand bed filters, and metallic filters, which were followed
effective in enhancing I sorption than are acidic functional
2
groups. Therefore, this study investigated the effect of func-
tional groups on I capture by examining the I sorption
2
2
kinetics of the UiO-66-X series containing various functional
groups, including acidic and basic groups (X = H, Br, NO
NH , (OH) , and (COOH) ) (Figure 1(a)). As another strat-
egy to enhance the adsorption kinetics and interactions
between the MOFs and the I molecules, the use of a ligand
consisting of a functional group that can react with I was
investigated. We previously reported the I adsorption of the
isostructural MOFs, Co (m-DOBDC) (m-DOBDC = 4,6-
dioxo-1,3-benzenedicarboxylate) and Co (p-DOBDC) (p-
= 2,5-dioxo-1,4-benzenedicarboxylate), and
found that they followed different I adsorption mechanisms,
chemisorption and physisorption, respectively (Figure 1
3
by adsorption in porous sorbents. However, several draw-
,
2
backs are associated with the use of traditional porous sor-
bents such as triethylenediamine-impregnated activated
carbon and silver-exchanged zeolites. Activated carbon has
2
2
2
2
a low ignition temperature and readily reacts with NO to
2
x
form explosive compounds, because of which its use is
2
4−
restricted to mild sorption conditions such as low tempera-
2
4
ture and NO -free environment. In addition, silver-
2
x
4−
exchanged zeolites have low sorption capacities because of
DOBDC
5
their small accessible surface areas. Therefore, there is a
2
need to investigate new kinds of I adsorbents that have
2
22
2+
high sorption capacity and strong affinity toward I2.
(b)). While I
is adsorbed on the open Co sites in Co (p-
2 2
Metal–organic frameworks (MOFs) are promising adsor-
DOBDC), I adsorption by Co (m-DOBDC) is facilitated by
the electrophilic aromatic substitution (EAS) of m-DOBDC .
2
2
4−
bents for I capture because of their large surface areas and
2
6–11
easy structural tunability.
Various MOFs such as
The adsorbed I converts the aryl C5 H bond into an aryl
2
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BULLETIN OF THE
KOREAN CHEMICAL SOCIETY
Figure 1. (a) Crystal structure of the UiO-66-X series and interactions of various organic linkers with I . (b) I adsorption mechanisms of
2
2
2
+
2+
2+
Co (p-DOBDC) and M (m-DOBDC) (Mg , Co , and Ni ). I removal efficiencies (%) of (c) UiO-66-X series and (d) M (m-DOBDC)
series and Co (p-DOBDC) as functions of exposure time. H-NMR spectra of H m-DOBDC and H 5-I-m-DOBDC derived from
e) I @Ni (m-DOBDC) and (f) I @Mg (m-DOBDC). Color code: light blue, M (Mg , Co , Ni , and Zr ); gray, C; yellow, O; blue,
2
2
2
1
2
2
4
+
4
2
2+
2+
4+
(
2 2 2 2
N; white, H; and purple, I.
−
C5 I bond, while generating an iodide ion (I ), which binds
readily prepared from substituted terephthalates (BDC-X,
X = H, Br, NO , NH , (OH) , (COOH) ) by following pre-
2+
to an open Co site. Upon increasing the I loading, a tri-
2
2
2
2
2
−
iodide ion (I ) is formed by the interaction of an I molecule
with an I ion. In this study, these findings were extended to
examine the I sorption by an isostructural M (m-DOBDC)
viously reported procedures with slight modifications
3
2
−
23–26
(Figure 1(a)).
series makes it useful for examining the effects of acidic
and basic functional groups on the I sorption. The forma-
The synthetic feasibility of this MOF
2
2
2+
2+
series (M = Mg and Ni ).
2
The UiO-66-X series, which is built up with thermally
and chemically stable 12-connected Zr6 clusters, was
tion of these MOFs was confirmed by powder X-ray
diffraction (PXRD) (Figure S2). The porosities of the
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Table 1. I
2
adsorption kinetic studies of the UiO-66-X, Co
2 2
(p-DOBDC), and M (m-DOBDC) series.
Pseudo-first-order
Pseudo-second-order
k1 (h−
1
)
R
2
k2 (gÁmg Áh−1)
−1
R
2
UiO-66-H
UiO-66-Br
UiO-66-NO2
UiO-66-NH
UiO-66-(OH)
UiO-66-(COOH)2
0.021 (Æ0.001)
0.018 (Æ0.001)
0.017 (Æ0.002)
0.016 (Æ0.001)
0.022 (Æ0.001)
0.024 (Æ0.001)
0.016 (Æ0.001)
0.15 (Æ0.02)
0.975
0.962
0.935
0.984
0.954
0.999
0.984
0.919
0.792
0.981
0.0000067 (Æ0.0000036)
0.00030 (Æ0.00017)
0.00027 (Æ0.00017)
0.000026 (Æ0.000014)
0.000032 (Æ0.000021)
0.000019 (Æ0.000002)
0.000022 (Æ0.000010)
0.0021 (Æ0.0001)
0.034
0.913
0.865
0.860
0.809
0.990
0.876
0.999
0.999
0.999
2
2
Co
Co
2
(p-DOBDC)
(m-DOBDC)
2
Mg (m-DOBDC)
0.089 (Æ0.022)
0.045 (Æ0.002)
0.0015 (Æ0.0007)
2
Ni
2
(m-DOBDC)
0.00024 (Æ0.00002)
solvent-evacuated samples were revealed by N adsorption–
To explore the effect of ligand reactivity on I capture,
2
2
desorption isotherms, which were comparable to previously
four isostructural MOFs, Co (p-DOBDC), Co (m-
2 2
DOBDC), Mg (m-DOBDC), and Ni (m-DOBDC), were
2 2
2
3–27
reported isotherms (Figure S4).
As expected, the intro-
duction of functional groups into the pores decreased the
N uptake and the surface areas (Table S1). Thus, UiO-
2
synthesized by following previously reported proce-
29,30
dures.
N adsorption–desorption isotherms of the pre-
2
6
6-(COOH) , the MOF containing two bulky COOH
pared samples revealed that the MOFs had comparable
surface areas (Figure S5 and Table S1). While the I2
2
3
groups, exhibited the lowest N uptake (151.4 cm /g at
2
2
0.99 P/P ) and surface area (492.3 m /g).
removal efficiency of Co (p-DOBDC) slowly reached
0
2
The I sorption kinetics were examined by immersing
23.2% in 30 h, Co (m-DOBDC), Mg (m-DOBDC), and
2
2
2
the desolvated MOFs into a solution of I in cyclohexane.
Ni (m-DOBDC) rapidly removed I in 30 h with 78.1%,
2 2
2
UV–vis spectroscopy was used to determine the concentra-
76.1%, and 76.9% efficiencies, respectively (Figure 1(d)).
After the decomposition of I -adsorbed M (m-DOBDC),
tion of the I solution and to examine the I uptake of the
2
2
2
2
UiO-66-X series as a function of exposure time (Figure 1
NMR spectroscopy was performed, which revealed the for-
mation of the iodo-substituted ligand via the conversion of
the C5 H bond into the C5 I bond (Figure 1(b)). The
intensity ratio of the peaks corresponding to the C5 proton,
(c) and Figure S6).
Despite having the largest surface area, UiO-66-H
showed the lowest I removal efficiency in the series, i.e.,
2
2
2.4% in 150 h. The introduction of Br and NO groups in
6.4 ppm, and the C2 proton, 8.3 ppm, of H m-DOBDC
2
4
the MOFs did not considerably affect the I uptake, imply-
was lower in I -adsorbed M (m-DOBDC) than in the pris-
2
2
2
ing that the halogen bonding and dipole-induced dipole
tine MOFs (Figures 1(e)-(f)). These results indicate that the
functionalization of organic linkers is a useful strategy to
interactions between these functional groups and I were
2
insignificant. The I2 removal efficiency of UiO-66-NH2
was 70.1% in 150 h, which was much higher than that of
UiO-66-H, in accordance with the previous reports. In par-
ticular, although UiO-66-(OH)2 contains two hydroxyl
groups, its sorption kinetics and removal efficiency were
enhance the affinity of MOFs toward the I molecules.
To obtain further information on the adsorption kinetics,
2
the I adsorption data were fitted to Lagergren’s pseudo-
2
first-order3 and pseudo-second-order kinetic models
1,32
(Table 1).
The data for the UiO-66-X series were better
comparable to those of UiO-66-NH . This result suggests
fitted to the pseudo-first-order model than to the pseudo-
2
that the amine group, which is a better electron donor than
the hydroxyl group, formed a more stable charge-transfer
second-order model, implying that the rate-determining step
was the physisorption of I (Figures S7-S18). As mentioned
2
complex with the I molecule.
Interestingly, despite having the smallest surface area, UiO-
above, the addition of functional groups on the linkers
affected the I2 sorption kinetics; among the UiO-66-X
2
6
6-(COOH) showed the fastest sorption kinetics and the highest
series, UiO-66-(COOH) exhibited the highest rate constant
(0.024 h ) owing to the high affinity provided by the
dipole induced-dipole interactions between the carboxylic
2
2
−1
I removal efficiency, which reached 82.0% in 150 h. Presum-
2
δ+ δ−
ably, the I molecule was easily polarized into the [I -I ] state
2
28
because of its large electron cloud, which enabled it to simulta-
neously interact with the electron-rich oxygen atom (carbonyl-
oxygen) and the acidic hydrogen of the carboxylic acid group
via double dipole-induced dipole interactions. However, the
hydroxyl or amine group can only form single dipole-induced
acid groups and the polarizable I molecules. In addition,
2
unlike Co (p-DOBDC), whose adsorption kinetic data were
2
fitted to the pseudo-first-order model, the I2 adsorption
kinetic data of the M (m-DOBDC) series were better fitted
2
to the pseudo-second-order model (Figures S19-S26). This
dipole interactions with polarized I . As a result, UiO-
indicates that in M (m-DOBDC), adsorption occurred via
2
2
6
6-(COOH) possessed superior I sorption properties.
the chemisorption of I , which was facilitated by the EAS
2
2
2
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of the electron-rich aryl C H bond with I . This result is
consistent with the proposed sorption mechanism.
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In summary, the effect of functional groups on I adsorption
2
was investigated using MOFs, which were functionalized with
acidic and basic functional groups, as well as the substituent
aryl C H group. Among the UiO-66-X series, UiO-
6
6-(COOH) exhibited the highest I removal efficiency owing
2 2
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2
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2
Acknowledgments. This work was supported by the
National Research Foundation (NRF) of Korea funded by the
Ministry of Science and ICT (NRF-2016M2B2A9912217).
This study was also supported by the DGIST R&D Program
of the Ministry of Science and ICT (20-HRHR-ET-09).
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Supporting Information. Additional supporting informa-
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