Introduction of Molecular Building Blocks to Improve the Stability of Metal-Organic Frameworks for Efficient Mercury Removal

Article abstract of DOI:10.1021/acs.inorgchem.8b00704

With expanding human needs, many heavy metals were mined, smelted, processed, and manufactured for commercialization, which caused serious environmental pollutions. Currently, many adsorption materials are applied in the field of adsorption of heavy metals. Among them, the principle of many mercury adsorbents is based on the interaction between mercury and sulfur. Here, a S-containing metal-organic framework NENU-400 was synthesized for effective mercury extraction. Unfortunately, the skeleton of NENU-400 collapsed easily when exposed to the mercury liquid solution. To improve the stability, a synthetic strategy installing molecular building blocks (MBBs) into the channels was used. Modified by the MBBs, a more stable nanoporous framework was synthesized, which not only exhibits a high capacity of saturation mercury uptake but also shows high selectivity and efficient recyclability.

Full text of DOI:10.1021/acs.inorgchem.8b00704

Article
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Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
Introduction of Molecular Building Blocks to Improve the Stability of
Metal−Organic Frameworks for Efficient Mercury Removal
Shu-Yi Jiang,†,⊥ Wen-Wen He,§,⊥ Shun-Li Li,‡ Zhong-Min Su,*,† and Ya-Qian Lan*,‡
†Institute of Functional Material Chemistry, Faculty of Chemistry, Northeast Normal University, Changchun, 130024 Jilin, China
‡China Jiangsu Key Laboratory of Biofunctional Materials, School of Chemistry and Materials Science, Nanjing Normal University,
Nanjing, 210023 Jiangsu, China
§School of Chemistry and Life Science, Advanced Institute of Materials Science, Changchun University of Technology, Changchun
130012, China
S
* Supporting Information
ABSTRACT: With expanding human needs, many heavy metals
were mined, smelted, processed, and manufactured for commerci-
alization, which caused serious environmental pollutions. Currently,
many adsorption materials are applied in the field of adsorption of
heavy metals. Among them, the principle of many mercury
adsorbents is based on the interaction between mercury and sulfur.
Here, a S-containing metal−organic framework NENU-400 was
synthesized for effective mercury extraction. Unfortunately, the
skeleton of NENU-400 collapsed easily when exposed to the mercury
liquid solution. To improve the stability, a synthetic strategy
installing molecular building blocks (MBBs) into the channels was
used. Modified by the MBBs, a more stable nanoporous framework
was synthesized, which not only exhibits a high capacity of saturation
mercury uptake but also shows high selectivity and efficient
recyclability.
metal ions for network formation, the sulfur groups can remain
as free-standing secondary donor groups. Xu et al. reported the
Zr-DMBD with the hard carboxyl and soft thiol functioning as
effective mercury sorption, and Dong et al. successfully
synthesized thiol-derived Cr-MIL-101-AS by means of the
postsynthetic modification method which shows excellent
adsorbent ability in removing mercury ions from water.23,24
To efficiently remove Hg(II), we designed and synthesized a
suitable type of MOF NENU-400, which has free-standing
thioether groups. The structure of NENU-400 is arranged as 6-
connected acs (MIL-88B)25 topology with the Co3(μ3-
OH)(RCOO)6(H2O)3 clusters; the specific configurations of
the open metal sites (OMSs) can fix various monodentate guest
molecules to form a structurally decorated MOF. Unfortu-
nately, the structure of NENU-400 easily collapsed during the
process of mercury removal. To address this issue and also
maintain its surface areas and functional sites, we incorporated
predesigned molecular building blocks (MBBs) as the
upholders into the pore channels of NENU-400 to get
NENU-401 (see Scheme 1). With the MBBs, NENU-401
shows an exceptional saturated adsorption of mercury as ∼600
mg g−1 and effectively decreased Hg(II) concentration from 10
ppm to an extremely low level as 0.02 ppb. It also can efficiently
INTRODUCTION
■
Mercury has been proven to be a toxic heavy metal which is
harmful to our brains, livers, and central nervous systems.1
Many methods such as amalgamation, chemical precipitation,
magnetic separation, and ion exchange have been applied in the
remediation of mercury heavy metal pollution, which are
discharged by removing the mercury from industry products,
byproducts, and production processes. But all the methods have
the limitations of being nonreusable or having low uptake or
slow sorption kinetics, which badly affect the application in
daily practice.2−6 Therefore, constructing reusable materials to
satisfy the demands of capacity, efficiency, reusability, and
selectivity is still a challenge to chemistry science.
With characteristics such as high porosities, large surface
areas, and tunable pore surface properties, metal−organic
frameworks (MOFs) have drawn considerable attention in
significant prospects for promoting the energy and environ-
mental sustainable development, including gas adsorption
separation, catalysis, and proton conduction.7−17 With the
application as priority, functional oriented synthesis strategy
presents a progressive evolution for the construction of MOF
materials.18−20 Because of the strong Hg−S interactions, MOFs
with the introduction of free-standing sulfur groups have
proved to be excellent solid-phase sorbents to effective mercury
uptake.21,22 This represents a potentially powerful strategy for
the molecular design of MOFs; while the carboxyl groups bind
Received: March 15, 2018
© XXXX American Chemical Society
A
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Scheme 1. Schematic Representation of the Interrelated Structures for NENU-400 and Reformed NENU-401 via the
Introduction of Generated Size-Matching MBBs into the Open Channels
INT2− ligands have replaced the single coordinated water molecules
successfully (see Figure 1b). MBBs [Co2(INT)3(H2O)2] are installed
into the channels as three-connected supports which improved the
stability of the frameworks. With the tridentate MBBs, NENU-401 is
apparently more stable than NENU-400, which was shown by PXRD
results (see Figures S7 and S8). NENU-401 not only can exist stable in
air for one week at least, but also can be stable in some organic
solvents. The thermal stabilities show that NENU-400 was stable up to
∼140 °C, and then the structure decomposed. It also revealed that
NENU-401 has thermal stability up to ∼263 °C, and there is a ∼123
°C improvement (see Figure S9). Meanwhile the trigonal bipyramid-
type cage in NENU-400 has changed into a hexagonal dipyramidal
form, but the size of the cage is maintained (see Figure 1c and 1d).
With the insertion of three-connected supports into the acs net, a new
framework type, pacs (partitioned acs) has generated.27 Deeper
analysis of the channels which have been divided into six small
triangular ones shows that the diameter of the new channel is ∼3.75 Å
(see Figure 1f).
Material Characterization. The thermal stabilities of NENU-400
and NENU-401 (see Figure S9) were tested by thermogravimetric
analysis (TGA) under a dry N2 atmosphere. There was a continuous
mass loss of 27.3% from 20 to 142 °C (calcd 27.6%), corresponding to
the reduce of H2O and DMF molecules in NENU-400. The
framework was stable up to ∼140 °C, and then the structure
decomposed. NENU-401 showed a continuous weight loss of 30.22%
from 48 to 263.1 °C (calcd 29.8%), it revealed that NENU-401 has
thermal stability up to ∼263 °C (see Figure S2).
remove 99.7% mercury within a few minutes. Furthermore,
NENU-401 is not only readily regenerated and recycled
without significant loss of mercury adsorption capacity but
also has high selectivity of mercury in solutions containing
other background metal ions.
EXPERIMENTAL SECTION
■
Synthesis of NENU-400 and NENU-401. Solvothermal reactions
of Co(NO3)2·6H2O with H2DMTDC yield pink crystals of NENU-
400 {[Co3(μ3-OH)(H2O)3(DMTDC)3](NO3)10-(H2O)6(DMF)6}n
and purple crystals of NENU-401 {[Co3(μ3-OH)(DMTDC)3(INT)3]-
[Co2(OH)(H2O)2](NO3)19-(H2O)7(DMA)11}n, without or with the
introduction of INT, respectively (DMTDC = 3,4-dimethylthieno[2,3-
b]thiophene-2,5-dicarboxylic acid, INT = isonicotinate, DMF= N,N-
dimethylformamide, and DMA = N,N-dimethylacetamide). Single
crystal X-ray diffraction analysis reveals that NENU-400 crystallizes in
the trigonal space group P31c. The structure consists of Co (μ -OH)
̅
3
3
cores and DMTDC2− ligands (see Figure 1a), which is similar to the
structure of MIL-88B. Every Co(II) ion is six-coordinated with one μ3-
OH−, one coordinated water molecule, and four oxygen atoms from
four different DMTDC2− ligands.26 Every five Co3(μ3-OH) cores and
six DMTDC2− ligands compose a trigonal bipyramid-type cage (see
Figure 1c). From a topological viewpoint, the whole structure of
NENU-400 can be classified as a 6-connected acs topological network.
Also, this type of connection constructs a three-dimentional porous
framework which has hexagonal honeycomb channels running along
the c axis, with an aperture size of approximately 7.79 Å (see Figure
1e).
Powder X-ray Diffraction. To investigate stability of these two
compounds, the samples were exposed to air for one week. PXRD
patterns before and after exposure demonstrate that NENU-400
decomposed, while NENU-401 could preserve its structural integrity.
To exchange guest molecules in the frameworks and further evaluate
their chemical stabilities, NENU-400 and NENU-401 were immerged
in acetone and acetonitrile for 3 days. NENU-400 collapsed after being
immerged, on the contrary, the measured PXRD pattern of NENU-
401 is identical with its simulated PXRD pattern showing the
framework is maintained (see Figures S7 and S8).
In the structure of NENU-400, each Co3(μ3-OH) core has three
coordinated water molecules pointing to the center of the hexagonal
channel and taking a C3 symmetry. To improve the stability of NENU-
400, we try to put three-connected supports to replace these
monodentate water molecules. Considering the geometric feature of
the structure, we selected a tridentate MBB [Co2(INT)3(H2O)2]
which not only fits in its planar tritopic with C3 symmetry but also
matches the size and charge requirements of the channels. We added
isonicotinc acid into NENU-400’s reaction system and got a new
crystal NENU-401. Single crystal X-ray diffraction analysis reveals that
NENU-401 crystallizes in the same space group, which has cell
parameters nearly identical to those of NENU-400. As we expected,
RESULTS AND DISCUSSION
Adsorption Performances of NENU-400 and NENU-
401 for Mercury. To assess the absorption capacity of Hg(II),
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Figure 1. (a and b) The coordination environments of inorganic SBUs in NENU-400 and NENU-401, respectively. (c and d) Two types of cages in
NENU-400 and NENU-401, respectively. (e and f) View of the 3D structures of NENU-400 and NENU-401 along the c axis, respectively. (Purple
balls: Co; red balls: O; yellow balls: S; blue balls: N; gray balls: C).
NENU-400, and NENU-401 were exposed to Hg(II) solution
with different concentration gradient ranging from 10 to 900
ppm. After 12 hours, the crystals were extracted from the
solutions and the adsorption quantities of Hg(II) were analyzed
by ICP-MS (Inductively Coupled Plasma Mass Spectrometry).
As shown in Figure S10, there is almost no adsorption taking
place in NENU-400. This phenomenon can be ascribed to the
collapse of NENU-400 frameworks when exposed to the
Hg(II) solution. By contrast, NENU-401 with trident bearings
exhibits excellent adsorption behavior of Hg(II) (see Figure
2a). The equilibrium adsorption isotherm data fit the Langmuir
model very well, yielding a correlation coefficient as high as
0.998. The maximum capacity of NENU-401 for mercury is
596.57 mg g−1, which is higher than most thiol-functionalized
MOFs and other kinds of reported materials (see Table S4.).
The outstanding saturated Hg(II) removal capacity can be
ascribed to high accessibility of thioether groups, which are
dispersed uniformly on the channel surface of the highly porous
NENU-401.
established a system with Hg(NO3)2 (50 mL, 10 ppm) and
NENU-401 (30 mg) at 25 °C to investigate the reaction
kinetics (Supporting Information). Surprisingly, it exhibited a
considerable removal efficiency; over 99.7% of Hg(II) was
adsorbed within 10 min at equilibrium (see Figure 2c). When
the contact time was extended to 3 h, the Hg(II) concentration
was decreased to 2 ppb. The adsorption rate constant (K2,
Supporting Information) fitting with the pseudo-second-order
model was evaluated to be 0.99 g mg−1 min−1 (see Figure 2d
and e). Such high rate performance observed for NENU-401
can be attributed to the large amount of sulfur groups on the
inner surface and well-defined pore channel size, which
facilitate the diffusion of Hg(II) very well. Distribution
coefficient (Kd, Supporting Information) is used to evaluate
the affinity of sorbent for metal ions generally. The Kd value of
NENU-401 at 25 °C was calculated to be 8.3 × 106 mL g−1,
which exhibits distinct superiority compared with other
materials such as commercial resins (5.1 × 105 mL g−1),
sulfide-modified mesoporous carbons (6.82 × 105 mL g−1), and
LHMS-1 (6.4 × 106 mL g−1).28−30
In addition to the high adsorbing capacity of Hg(II), NENU-
401 is also prominent in the removal effectiveness. We
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Figure 2. (a) Hg(II) adsorption isotherm of NENU-401 after 12 h. (b) The linear regression of the adsorption isotherm by fitting the equilibrium
adsorption data with Langmuir adsorption model. (c) Hg(II) sorption kinetics of NENU-401 under the initial Hg(II) concentration of 10 ppm. (d)
Adsorption curve of NENU-401 Hg(II) versus contact time in solution. (e) Pseudo-second-order kinetic plot of NENU-401 for the adsorption at
Hg(II) concentration of 10 ppm.
Figure 3. (a) Cycle performance for Hg(II) removal. (b) Capture efficiency in removing metal ions.
Regeneration and recycling property are two important
standards to evaluate an adsorption material for practical
application. Considering the strong interaction between the
framework and Hg(II) ions, thioglycol was chosen as an eluent.
Hg(II)-captured NENU-401 was demetaled by rinsing with
thioglycol solution, and the regenerated NENU-401 was
employed for adsorption again. After 4 cycles, ICP-MS reveals
that the crystal of NENU-401 can still retain 90% of the original
capacity (see Figure 3a). In addition, powder XRD analyses
demonstrate that NENU-401 can retain high crystallinity after
every cycle (see Figure S11). Because of the thioether donors in
NENU-401, there is a considerable advantage in cyclicity over
other materials with thiol groups such as SH@SiO2, JUC-62,
Fe3O4-R6G, and so on.31−33 Selectivity is another crucial factor
for practical use of materials. NENU-401 was steeped into the
mixture solution of K(I), Na(II), Hg(II), Pb(II), Zn(II),
Ca(II), Mg(II), and Fe(III) ions at each concentration of 10
ppm. As depicted in Figure 3b, NENU-401 could remove
Hg(II) and Pb(II) effectively, but scarcely capture the rest of
the metal ions such as K(I), Na(II), Zn(II), Ca(II), Mg(II), and
Fe(III).
To study the efficient mercury removal performance in
NENU-401, we utilized XPS and FT-IR to monitor this
adsorption progress. XPS signals indicated that the appearance
of Hg(II) at 102 and 106 eV assigned to the Hg 4f7/2 and Hg
4f5/2 orbital, respectively, in the crystal of NENU-401-Hg. The
S 2p XPS spectra showed a 1 eV shift in NENU-401-Hg
comparing to that of NENU-401, which demonstrated the
strong binding interactions between the Hg(II) and S species
(see Figure S12). Furthermore, the lack of characteristic S−H
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stretching mode at 2515 cm−1 in the IR spectrum of NENU-
401-Hg could meanwhile verify the interactions between
mercury and sulfur species (see Figure S13).34
Foundation of Jiangsu Collaborative Innovation Center of
Biomedical Functional Materials.
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synthetic strategy presents a progressive evolution for the
construction of S-containing frameworks, and the development
of MOF materials for adsorption of mercury is currently
underway.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.8b00704.
■
S
Ligand synthesis, crystal data, general procedures for
Hg(II) adsorption, PXRD patterns, TGA curves, XPS
spectra, and IR spectra (PDF)
Accession Codes
CCDC 1584139−1584140 contain the supplementary crystal-
lographic data for this paper. These data can be obtained free of
charge via www.ccdc.cam.ac.uk/data_request/cif, by emailing
data_request@ccdc.cam.ac.uk, or by contacting The Cambridge
Crystallographic Data Centre, 12 Union Road, Cambridge CB2
1EZ, UK; fax: +44 1223 336033.
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■
Suarez, A. I.; Sepulveda-Escribano, A.; Vimont, A.; Clet, G.; Bazin, P.;
́
Corresponding Authors
*E-mail: yqlan@njnu.edu.cn.
*E-mail: zmsu@nenu.edu.cn.
́
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ORCID
Zhong-Min Su: 0000-0002-3342-1966
Ya-Qian Lan: 0000-0002-2140-7980
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Author Contributions
⊥S.-Y.J. and W.-W.H. contributed equally.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was financially supported by the National Natural
Science Foundation of China (Grants 21622104, 21471080,
and 21701016), the Education Department of Jilin Province
(Grant JJKH20181020KJ), the Science and Technology
Development Planning of Jilin Province (Grant
20160520124JH), the Priority Academic Program Develop-
ment of Jiangsu Higher Education Institutions, and the
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DOI: 10.1021/acs.inorgchem.8b00704
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