Three Mechanisms in One Material: Uranium Capture by a Polyoxometalate–Organic Framework through Combined Complexation, Chemical Reduction, and Photocatalytic Reduction

Article abstract of DOI:10.1002/anie.201909718

The design and synthesis of uranium sorbent materials with high uptake efficiency, capacity and selectivity, as well as excellent hydrolytic stability and radiation resistance remains a challenge. Herein, a polyoxometalate (POM)–organic framework material (SCU-19) with a rare inclined polycatenation structure was designed, synthesized through a solvothermal method, and tested for uranium separation. Under dark conditions, SCU-19 can efficiently capture uranium through ligand complexation using its exposed oxo atoms and partial chemical reduction from U^VI to U^IV by the low-valent Mo atoms in the POM. An additional U^VI photocatalytic reduction mechanism can occur under visible light irradiation, leading to a higher uranium removal without saturation and faster sorption kinetics. SCU-19 is the only uranium sorbent material with three distinct sorption mechanisms, as further demonstrated by X-ray photoelectron spectroscopy (XPS) and X-ray absorption near edge structure (XANES) analysis.

Full text of DOI:10.1002/anie.201909718

Angewandte
Communications
Chemie
International Edition: DOI: 10.1002/anie.201909718
German Edition: DOI: 10.1002/ange.201909718
Uranium Capture Very Important Paper
Three Mechanisms in One Material: Uranium Capture by a Polyoxo-
metalate–Organic Framework through Combined Complexation,
Chemical Reduction, and Photocatalytic Reduction
Hailong Zhang, Wei Liu, Ao Li, Duo Zhang, Xiaoyan Li, Fuwan Zhai, Lanhua Chen,
Long Chen, Yanlong Wang, and Shuao Wang*
[
3]
Abstract: The design and synthesis of uranium sorbent
materials with high uptake efficiency, capacity and selectivity,
as well as excellent hydrolytic stability and radiation resistance
remains a challenge. Herein, a polyoxometalate (POM)–
organic framework material (SCU-19) with a rare inclined
polycatenation structure was designed, synthesized through
a solvothermal method, and tested for uranium separation.
Under dark conditions, SCU-19 can efficiently capture ura-
nium through ligand complexation using its exposed oxo atoms
biologically, chemically, and photocatalytic processes. Iron-
II
III
based materials, such as Fe /Fe hydroxide, FeS, zerovalent
iron, have been intensively studied for the removal of
uranium from contaminated water. The removal of U by
iron-based materials is attributed to combined adsorption and
reduction procedure. However, Fe or Fe , the active
component in the material, is easily oxidized and deactivated
in the air. On the other hand, poor reusability becomes the
critical defect of such materials.
Polyoxometalates (POMs) are a class of inorganic metal–
oxygen clusters built from the connection of {MO } polyhedra
(M = V, Nb, Mo, or W, etc., x = 5, 6). Besides multiple
applications in catalysis, medicine, and materials science,
POMs have been studied for separation and immobilization
of radioactive wastes since early 1990s. Pope et al. developed
a strategy of binding uranyl to vacant POMs, and then
thermally decomposed to inert tungsten bronze materials,
which may serve as a possible waste form of actinides. In
addition, POMs are good electron reservoirs that exhibit rich
redox properties without structural degradation. Moisy et al.
[4]
VI
II
0
VI
IV
and partial chemical reduction from U to U by the low-
VI
valent Mo atoms in the POM. An additional U photocatalytic
x
[
5]
reduction mechanism can occur under visible light irradiation,
leading to a higher uranium removal without saturation and
faster sorption kinetics. SCU-19 is the only uranium sorbent
material with three distinct sorption mechanisms, as further
demonstrated by X-ray photoelectron spectroscopy (XPS) and
X-ray absorption near edge structure (XANES) analysis.
[6]
[
7]
I
t is desirable and important to develop enrichment tech-
nologies of uranium for both sustainable development of
nuclear power and environmental protection, as uranium is
a key resource in the nuclear fuel cycle and also a global
environmental contaminant with combined radio- and chemo
VI
17ꢀn
V
(10+n)ꢀ
found heteropolyblue [P W
W O ]
could reduce
2
n
61
VI
IV
IV
16ꢀ
U
to U and stabilize U in the complex [U(P W O ) ]
2 17 61 2
[
8]
in aqueous solution. However, the use of bare POMs as
adsorbent is limited because of their high solubility, easy
aggregation nature, and small surface area. Therefore,
introducing POMs into a suitable solid matrix should be
viewed as an alternative strategy to overcome these draw-
backs.
Recently, POMs have been intensively employed as
building units with diverse coordination modes for the
designing of POM-based metal–organic frameworks
(MOFs), finding applications in many fields including but
not limited to catalysts, photochromics, and magnetic materi-
[1]
toxicity. A series of solid adsorbent materials has been
developed for uranium capture, including inorganic minerals,
mesoporous silica, carbon-based materials, and advanced
porous materials. However, these materials are still severely
limited by lacking a combined performance of stability,
removal kinetics, sorption capacity, selectivity, and reusability.
[
2]
VI
Reducing the soluble and environmentally mobile U to the
less soluble and relatively immobile U species for uranium
IV
immobilization is considered as a promising strategy for
VI
IV
achieving these goals. In fact, U can be reduced to U by
[
9,10]
als.
Although POM-based MOFs exhibit foreseeable
performance in sequestration of uranium by a combined
advantage of both POMs and MOFs, the elimination of U
[
*] H. Zhang, W. Liu, A. Li, D. Zhang, X. Li, F. Zhai, L. Chen, L. Chen,
Y. Wang, Prof. S. Wang
VI
State Key Laboratory of Radiation Medicine and Protection, School
for Radiological and interdisciplinary Sciences (RAD-X) and Collab-
orative Innovation Center of Radiation Medicine of Jiangsu Higher
Education Institutions, Soochow University
Suzhou 215123 (China)
E-mail: shuaowang@suda.edu.cn
from aqueous solutions by such materials remains unex-
plored. Among the numerous reported POM-based MOFs,
the materials derived from mixed valent e-Keggin polyoxo-
V
VI
molybdate {e-PMo Mo O Zn } (Zn-e-Keggin) units are
8
4
40
4
taken into consideration because of their inherent reducing
[10]
capability, multiple connected mode and excellent stability.
W. Liu
In this family, most of the materials were constructed from
Zn-e-Keggin nodes with bi-/tri-dentate carboxylate linkers.
Hence, there are always large unexchangeable charge-bal-
ancing cations such as tetrabutylammonium cation jammed in
the framework, generating a major barrier for achieving high
School of Environment and Material Engineering, Yantai University
Yantai 264005, Shandong (China)
Supporting information and the ORCID identification number(s) for
the author(s) of this article can be found under:
https://doi.org/10.1002/anie.201909718.
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adsorption efficiency. To address this problem, in this work,
unit can be viewed as a 3-connected node by omitting the
unexpanded TPB2 ligands. Therefore, the 2D structure can be
reduced to a rare (4·6 ) (4·6·4·6) topological network (Fig-
ure S2c).
V
VI
a neutral POM-based MOF, [e-PMo Mo O (OH) Zn ]-
8
4
37
3
4
2
2
(
TPB) ·6H O (SCU-19, TPB = 1,2,4,5-tetra (4-pyridyl) ben-
3
/2
2
zene), was successfully designed and synthesized.
Single-crystal X-ray diffraction analysis reveals that SCU-
9 crystallizes in the orthorhombic space group Pcca. The
The void space in the four-membered and six-membered
rings of the 2D layer is large enough for these 2D nets to be
mutually entangled into a layer-packing structure. Each
individual layer is catenated with an infinite number of
other inclined layers leading to a rare inclined polycatenation
2D + 2D!3D framework (Figure 1c, Figure S3 and S4).
There are two types of catenations in the polycatenation
structure of SCU-19: four-membered rings interlock with
four-membered rings and six-membered rings interlock with
six-membered rings (Figure 1d,e). In most inclined polycate-
nation systems, two or three identical sets of 2D parallel
motifs interlock in different stacking directions, resulting in
an increase of dimensionality (2D!3D) which is beneficial
1
asymmetric unit consists of one e-Keggin POM [e-
V
VI
4
PMo Mo O (OH) Zn ], one and a half coordinated TPB
8
37
3
4
ligands, and other guest molecules (Figure S1 in the Support-
ing Information; the CCDC number is given in the Support-
ing Information). The bond valence sum calculation and X-
ray photoelectron spectroscopy (XPS) analysis demonstrate
V
VI
the existence of eight Mo and four Mo ions in the e-Keggin
POM. The e-Keggin polyoxoanion is linked to four TPB
II
II
ligands via coordination with four Zn ions, while each Zn
ion is 4-coordinated by three O atoms from the POM and one
N donor from the TPB ligands (Figure 1a). The TPB ligands
can be categorized into two different types including TPB1
and TPB2. As expected, TPB1 linkers acting as four-
connecting nodes, are connected by four adjacent Zn-e-
Keggin subunits through the four N atoms, forming a linear
chain along the c axis (Figure S2a). Every TPB2 ligand
employs two meta-oriented pyridine N atoms to connect
[11]
on improving the stability of the overall structure. Interest-
ingly, four sets of 2D motifs are mutually catenated through
two different patterns in the polycatenation system of SCU-
19. To our knowledge, POM-based MOFs exhibiting inclined
polycatenation of 2D layer have never been reported. Based
on such a polycatenation, the 3D architecture of SCU-19
exhibits a series of 1D channels with the window of
approximately 7 ꢀ 7 ꢁ along the c axis. The large void
volume (21.9%, calculated by PLATON) is occupied by
solvent molecules.
II
these adjacent 1D chains by coordinating to other two Zn
ions, resulting in a 2D network (Figure 1b, Figure S2b). To
simplify the structure of the 2D layer, the TPB1 can be
regarded as a planar 4-connected node, and the Zn-e-Keggin
The phase purity was confirmed by the powder X-ray
diffraction (PXRD) patterns (Figure S5). Based on the N2
adsorption isotherm at 77 K (Figure S10), the Brunauer-
Emmett-Teller (BET) and Langmuir surface area of SCU-19
2
ꢀ1
is 32.6 and 100.8 m g , respectively. As shown in Figure S6a,
SCU-19 shows thermal stability up to 3808C. Based on the X-
ray diffraction experiments under elevated temperatures
(Figure S6b), the crystalline structure of SCU-19 is main-
tained up to 3008C. Importantly, SCU-19 is stable in water
solutions in the pH range from 2 to 12 (Figure S7). Besides,
SCU-19 also exhibits excellent radiation resistance, where no
structural and crystal degradation was observed after large
dose b irradiation or g irradiation (Figure S8).
To evaluate the removal performance of SCU-19 towards
U , batch experiments were systematically performed. The
VI
pH value of the solutions greatly affects the surface charge of
the material, which directly leads to the variation of binding
sites. Based on the initial pH dependent sorption study shown
in Figure S13, detailed sorption experiments were carried out
under pH 4.0. As shown in Figure 2a, the adsorption kinetics
VI
of SCU-19 toward U indicates a fast adsorption process. The
VI
sorption of uranium could remove 80% of U at about
9
0 min. As shown in Table S3, the adsorption of uranyl ions
onto SCU-19 can be fitted by pseudo-second-order model,
VI
suggesting that the interaction between U and SCU-19 is
controlled by chemical sorption.
The sorption isotherm was measured at 300 K (pH 4.0).
As depicted in Figure 2b, the isotherm curve can be fitted by
Figure 1. Crystal structure of SCU-19: a) connection mode between Zn-
e-Keggin and TPB ligands, b) view of the 2D layer from [210], Keggin
anions highlighted in polyhedral representation. C dark gray; N blue;
O red; Zn turquoise; H atoms are omitted for clarity. c) 3D polycatena-
tion framework. (d) and (e) arrays by inclined polycatenation of 2D
simplified layers viewed from different directions.
2
[12]
Langmuir model (R > 0.99).
The maximum sorption
VI
capacity (qmax) value of U on SCU-19 was determined to
ꢀ1
be 557.56 mgg (Table S4), higher than those of the majority
[
2]
of previously reported MOF materials (Table S5). In
2
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rated value from the view of a catalytic uptake
process, which can be further elevated when the
initial uranium concentration is higher or increasing
the reaction time. Additionally, uranium-loaded
SCU-19 samples could be regenerated by elution
with 0.01m HNO aqueous solution with a uranium
3
desorption rate of 91.57% (Figures S16,S17).
The notably high removal efficiency and capacity
of SCU-19 toward uranium could be achieved by
three possible mechanisms. The first is the adsorption
process, which greatly benefits from the unique
hydrophilic channels, and the ligand complexation
using the exposed terminal oxygen atoms of POMs in
the structure. The second is the chemical reduction
V
process, in which the Mo of the POM units in SCU-
VI
IV
1
9 may reduce parts of the adsorbed U to U . The
third is the photocatalytic reduction process, in which
photo-reduction of uranium could occur due to the
suitable photoelectric characteristics of SCU-19. To
better understand the interaction mechanism
VI
Figure 2. a) Adsorption kinetics of U by SCU-19. Experimental conditions:
ꢀ1
VI
ꢀ1
material dosage=0.5 gL , pH 4.0, initial U ꢁ10 mgL . b) Adsorption iso-
VI
VI
therms of U by SCU-19. c) Competitive sorption of coexistent ions by SCU-19
between SCU-19 and U , elemental mapping, XPS
VI
with molar ratio of metal ions to U of about 10 times. Experimental
and X-ray absorption near edge structure (XANES)
analysis were further performed. Elemental distribu-
ꢀ1
VI
ꢀ1
conditions: material dosage=0.5 gL , pH 4.0, initial U ꢁ10 mgL , and the
ꢀ1
concentration of the competing ions is around 100 mgL . d) Uptake of
VI
tion mapping of the U -loaded samples shows the
uranium by SCU-19 under dark and visible light with initial uranium concen-
trations of approximately 400 ppm, material dosage=0.5 gL , pH 4.0.
ꢀ1
presence and uniform distribution of uranium on the
surface of SCU-19 (Figure S21). In the XPS curve
fitting results, the peaks for U 4f7/2 (380.99 and
382.20 eV) in the high-resolution U 4f spectrum suggest the
addition, the presented material is of high selectivity. When
ten times excess of competing cations are present including
monovalent (Na , K ), divalent (Ca , Mg and Pb ),
approximately 10 ppm of U can be still completely removed
Figure 2c). Even in the presence of high concentration Eu ,
VI
IV
[15]
coexistence of U and U after the sorption (Figure 3a).
+
+
2+
2+
2+
IV
The relative ratio of U was calculated to be 11.7% in
samples under dark while the relative ratio of U increases to
VI
IV
3
+
(
37.5% for the light-irradiated sample (Figure 3a). The
2
+
the removal percentage toward UO2 is still as high
as 90.02%.
Photocatalytic reduction was recently demon-
VI
strated to be an efficient and green route for U
[
13]
uptake. The estimated band gap of SCU-19 based
on Kubelka-Munk Function is 2.71 eV (Figure S19),
indicating that SCU-19 can be photo-excited by
[
14]
visible light. According to the Mott–Schottky plot
Figure S20), the positive slope suggests that SCU-19
is a n-type semiconductor with a flat-band potential of
0.58 V. The conduction band (CB) and valence band
VB) energy levels were calculated to be ꢀ0.68 Vand
.03 V (vs. saturated calomel electrode (SCE)),
(
ꢀ
(
2
VI
IV
indicating that photo-reduction of U to U is
thermodynamically feasible.
[
13]
To evaluate the
photo assisted uranium removal performance of
SCU-19, the batch experiments were studied under
visible light irradiation with methanol as the sacrifi-
cial agent. Obvious differences on the sorption
capacity and kinetics were observed between the
experiments performed under dark and light irradi-
ation conditions, respectively (Figure 2d, and Fig-
ure S14, S15). Specifically, the mass of removed
uranium under light irradiation is significantly larger
than that under dark condition, resulting in a removal
Figure 3. a) U 4f XPS spectra of SCU-19 samples loaded with uranium under
dark and light conditions. b) Mo 3d XPS spectra of SCU-19 and uranium-loaded
IV
samples under dark and light conditions. c) U L -edge XANES spectra of the U
3
VI
and U references and the samples loaded with uranium under dark and light
conditions. d) Mo K-edge XANES spectra for H PMo (Mo(VI)), SCU-19, and of
3
12
ꢀ
1
capacity of 728.34 mgg and an extraction percent-
age of 91.04%. Note the first number is not a satu-
the uranium-loaded sample under dark and light conditions. The initial uranium
ꢀ
1
concentration is around 400 ppm, material dosage=0.5 gL , pH 4.0.
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V
relative ratio of Mo decreases from 66.7% to 43.2% under
reduction, and photocatalytic reduction. We believe that
more POM-based MOF materials with advanced removal
performance toward uranium and other radionuclides (e.g.
V
dark, suggesting that Mo is involved in the chemical
reduction of U to U . Besides, the relative ratio of Mo
decreases from 66.7% to 51.9% under light, indicating that
VI
IV
V
9
9
ꢀ
[18]
TcO ) could rapidly emerge in the near future.
4
VI
IV
the chemical reduction of U to U also occurs under light in
addition to the photo-reduction. As shown in Figure 3c, the
VI
absorption edge of the U -loaded samples is located between Acknowledgements
VI
IV
those of U and U standards in the U L -edge XANES
3
VI
spectrum, further illustrating that parts of U are reduced
into U under both dark and light irradiation conditions.
The Mo K-edge XANES spectra (Figure 3d) were also
recorded to determine the valence change of molybdenum,
in agreement with the result of XPS analysis.
We are grateful for funding support from the Science
Challenge Project (TZ2016004), the National Natural Science
Foundation of China (21825601, 21790374, U1532259), the
General Financial Grant from the China Postdoctoral Science
Foundation (2019M651802), and Open Project of State Key
Laboratory of Environment-friendly Energy Materials
(17kffk16) for financial support of this work. We also thank
BeamLine 14W1 of Shanghai Synchrotron Radiation Facility
(SSRF, China) and Soochow University-Western University
Center for Synchrotron Radiation Research for the technical
support.
IV
[16]
[17]
The PXRD patterns of uranium-loaded SCU-19 samples
suggest that the material could maintain integrity during the
VI
U
removal process (Figure S9). Negligible amounts of Mo
are released into the solution detected by inductively coupled
plasma-atomic emission spectrometry (ICP-AES) during the
sorption process (Table S6), further confirming the stability of
VI
SCU-19. As illustrated in Figure 4, U ions are first adsorbed
Conflict of interest
The authors declare no conflict of interest.
Keywords: metal–organic frameworks · photocatalysis ·
polyoxometalates · reduction · uranium
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VI
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V
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VI
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Manuscript received: August 1, 2019
Accepted manuscript online: September 13, 2019
Version of record online: && &&, &&&&
2011, 133, 13363 – 13374; c) J. S. Qin, D. Y. Du, W. Guan, X. J.
Angew. Chem. Int. Ed. 2019, 58, 1 – 6
ꢀ 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
5
These are not the final page numbers!

Angewandte
Communications
Chemie
Communications
Uranium Capture
Stuck on U: Uranium capture by a poly-
oxometalate–organic framework is pos-
sible through three different mechanism,
these are complexation, chemical reduc-
tion, and photocatalytic reduction. The
material features a unique 2D+2D!3D
polycatenation structure, resulting in
excellent stability toward hydrolysis and
ionization irradiation.
H. Zhang, W. Liu, A. Li, D. Zhang, X. Li,
F. Zhai, L. Chen, L. Chen, Y. Wang,
S. Wang*
&&&& — &&&&
Three Mechanisms in One Material:
Uranium Capture by a Polyoxometalate–
Organic Framework through Combined
Complexation, Chemical Reduction, and
Photocatalytic Reduction
6
www.angewandte.org
ꢀ 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2019, 58, 1 – 6
These are not the final page numbers!