The targeted design of dual-functional metal-organic frameworks (DF-MOFs) as highly efficient adsorbents for Hg2+ ions: Synthesis for purpose

Article abstract of DOI:10.1039/c9dt03933c

Designing adsorbents with accessible chelating sites and achieving high contaminant purification efficiency are still important to overcome environmental remediation challenges. As one of the significant global concerns, the presence of heavy metal ions i

Full text of DOI:10.1039/c9dt03933c

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The targeted design of dual-functional metal–
organic frameworks (DF-MOFs) as highly efficient
adsorbents for Hg ions: synthesis for purpose†
Cite this: Dalton Trans., 2019, 48,
2
+
17831
Leili Esrafili,‡ Maniya Gharib‡ and Ali Morsali
*
Designing adsorbents with accessible chelating sites and achieving high contaminant purification
efficiency are still important to overcome environmental remediation challenges. As one of the significant
global concerns, the presence of heavy metal ions in the environment has attracted increasing attention
due to their toxicity, carcinogenicity, and bioaccumulation in the food chain. Herein, we performed a tar-
geted design of a new dual-functionalized metal–organic framework (DF-MOF) by incorporating different
percentages of the N1,N3-di(pyridine-4-yl) malonamide ligand (S) into urea-containing MOF (TMU-32);
the produced material was labeled as TMU-32S (with 33%, 65%, and 100% incorporation percentages).
Designing DF-MOF is our “design-for-purpose” approach for the decoration of MOF walls by suitable
functional groups, resulting in high removal capacity of heavy metal ions. Among the TMU-32S series
2
+
having different concentrations of the S ligand, TMU-32S-65% demonstrated exceptional Hg ion selec-
tively. To the best of our knowledge, this is the first report of mixed urea-malonamide-based MOF, which
provides a proper coordination site to strongly coordinate with Hg ions, along with 1428 mg g−1
maximum adsorption capacity. Generally, we attributed the impressive implementation of TMU-32S-65%
to the synergistic effects of both hydrophilic chelating urea and the malonamide functional group. Hence,
the results reported in this work indicate the exceptional potential of DF-MOFs for the high accomplish-
ment of environmental remediation.
2+
Received 5th October 2019,
Accepted 3rd November 2019
DOI: 10.1039/c9dt03933c
rsc.li/dalton
for water treatment owing to high adsorption capacity, a com-
paratively low cost, simple design, and easy operation.
1
. Introduction
5,6
Water pollution with heavy metals has led to a noticeable Traditional adsorbent materials, nanosized metal oxides, zeo-
worldwide concern because they can accumulate in the vital lites, activated carbon, functionalized polymers, and graphene
organs of humans and animals and also have deleterious oxides have been widely applied for the adsorption of heavy
1
–3
7,8
effects on the environment and living species.
Because of metal ions. However, these adsorbents suffer from one or
the toxic and non-biodegradable nature of mercury, it causes several disadvantages including low adsorption capacity, slow
serious problems for environmental remediation and can adsorption kinetics, limited selectivity and recovery process.
cause various toxic effects even at ultra-trace concentrations, Therefore, the development of a new adsorbent for more
such as birth defects, kidney and neurological sickness, brain effective adsorption has become an increasingly important
damage and cancer. Accordingly, the effective elimination of task to overcome the existing drawbacks and maximize the
2
+
Hg from the environment and industrial wastes is very adsorption capacity with economical preparation methods and
4
9,10
crucial. New strategies are being developed for the detection good recycling ability.
and removal of this toxic metal ion. Compared with various Metal–organic frameworks (MOFs), a new fascinating class
methods, adsorption is an effective and economical method of crystalline materials with permanent porosity, have been
widely applied in gas storage, sensing, catalysis, separation,
1
1,12
etc.
Due to their unique characteristics such as tunability,
Department of Chemistry, Faculty of Sciences, Tarbiat Modares University,
P.O. Box 14115-175, Tehran, Iran. E-mail: Morsali_a@modares.ac.ir;
Tel: +0098-21-82884416, Cell Phone: +0098-9122420568
high surface area, and diverse designable functionalities, they
have drawn much attention in the field of the adsorptive
13,14
removal of various heavy metals.
The functional group of
†
Electronic supplementary information (ESI) available: Chemicals and charac-
organic ligands plays an important role in the MOF backbone
terization methods, NMR, TGA, IR spectroscopy and synthesis details. See DOI:
1
5,16
for high adsorption.
As a result, they can show host–guest
10.1039/C9DT03933C
‡
These authors contributed equally to this work.
interactions with the substrate. Using strong adsorption sites
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that can be purposefully incorporated into the pore surfaces of framework, and the efficiency of the adsorbent was enhanced
MOFs irrespective of whether by post-synthetic or pre-design- when both urea and malonamide groups existed in the frame-
ing approaches, high densities of chelating functional groups work at the same time and approximately in the same amount.
have been obtained and applied for heavy metal ion Urea as a hydrophilic functional group with carbonyl and
1
4,15,17,18
decontamination.
amine groups together is potentially a good candidate for
The functionalization method can be categorized as a metal ion adsorption from aqueous solutions. The urea-based
direct or post-synthesis modification method. Sometimes, the metal–organic frameworks have attracted attention due to
direct synthesis of the MOF structure by using specific functio- their hydrophilic nature and also their ability to form double
nalized ligands is not possible. Consequently, we faced a chal- hydrogen bonds, making them good candidates for organoca-
lenge in synthesis. To overcome this challenge, we employed a talysis, anion recognition, and highly selective CO
2
2
4,25
new strategy to swap the MOF linker while retaining their crys- adsorption.
As is obvious, the majority of reported MOFs
1
9
tallinity. Solvent-assisted ligand exchange (SALE) is a post- cannot be used directly in many applications as they are not
synthetic method, which provides a route to incorporate the stable in water due to the hydrolysis of metal and organic
desired ligand that is unobtainable via the traditional syn- linkers, but it should be noted that water-stable MOFs with
thetic pathways. In this work, by introducing multiple adsorp- sufficient hydrophilicity can have applications in biomedical
tion sites into the MOF backbone, we attempted to improve technologies, water cleaning, and heat transformation. On the
the features of these compounds and increase their efficiency other hand, the malonamide motif can be considered as a
in the removal of heavy metal ions. As for this approach, first, “double amide”, which has been extensively studied for its
2
6–28
we synthesized hydrophilic MOF containing the urea func- high performance in gas adsorption applications.
The
tional group (TMU-32) that is appropriate for heavy metal malonamide functional group (–NH–CO–CH –CO–NH–) of
2
2
0,21
removal, especially lead, according to our previous work.
daughter MOFs includes two adjacent amide groups with loca-
With the aim to incorporate a more effective and also hydro- lized free non-bonding lone pairs, which can be considered as
philic functional group into the MOF pore wall, we immobi- an effective Lewis basic moiety compared to urea predecessors.
lized a malonamide functional group (N1,N3-di(pyridine-4-yl) In this study, we synthesized dual-functionalized MOFs,
malonamide) with different percentages. Due to their chelat- TMU-32 and TMU-32S, with different percentages of urea and
ing effect on heavy metal cations, we hypothesized that the malonamide ligands, and their applications for mercury
decoration of the MOF walls with both malonamide and urea adsorption were investigated. Among the three TMU-32S
struts can promote their Lewis basic properties and improve compounds, i.e., TMU-32S-33%, TMU-32S-65%, and
2
2,23
the removal capacity of heavy metal ions
(Fig. 1). The TMU-33S-100% with different percentages of the malonamide
resulting material was found to be isostructural to the parent ligand, TMU-32S-65% exhibited excellent selectivity towards
Fig. 1 An overview about the synthesis of TMU-32S through the linker exchange; representation of (a) white crystal of TMU-32 and (b) light brown
crystal of TMU-32S.
17832 | Dalton Trans., 2019, 48, 17831–17839
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2
+
1
Hg due to the synergistic effects of both hydrophilic urea of replacement, H NMR demonstrated that 33%, 65%, and
and malonamide functional group. To the best of our knowl- 100% of urea was replaced by the S ligand, respectively. The
edge, the maximum mercury uptake capacity of TMU-32S is intensity of the S peaks increased gradually in different
significantly higher than that of some reported porous exchange days, while those of L2 decreased (Fig. S2†). Also, the
materials.
functional groups of the S ligand in different percentages were
observed with an FT-IR spectrometer. In order to compare the
functional group difference of TMU-32S and parent TMU-32,
the CO stretching frequency was also probed by FTIR spec-
troscopy. In the infrared spectra of the S ligand, the character-
2
. Results and discussion
2
.1. Structure characterization
−1
istic sharp carbonyl (CvO) stretching appears at 1688 cm as
Herein, we reported the selective aqueous phase detection and the strongest band. Consistent with the successful incorpor-
removal of mercury ions by hydrophilic urea and malonamide ation of the desired compound, the carbonyl peak shifted by
pillared metal–organic frameworks termed as TMU-32 and about 13 cm− and appeared at 1675 cm as a new peak,
1
−1
TMU-32S. The white crystals of TMU-32, [Zn(oba)(L2)]·2DMF·H O which was separate from the carbonyl group of the urea ligand
2
1
(H
2
oba: 4,4′-oxydibenzoic acid; L2 = 1,3-di(pyridin-4-yl)urea), (1738 cm− ) (Fig. S3†).
were obtained by the reaction of the oba and L2 urea-based
The thermal stabilities of the parent and daughter struc-
ligands and zinc(II)–nitrate tetrahydrate in DMF after 72 h at a tures were investigated by TGA, as shown in Fig. S4.† The
temperature of 100 °C to give suitable X-ray quality crystals, encapsulated DMF solvent was removed from the framework
which were recently reported by our group. The synthesis of oba upon heating to about 120 and 290 °C, which caused ∼2% and
and L2 linkers was straightforward and achieved in a few steps 20% weight loss of the material, respectively. Moreover, this
starting from 4-aminopyridine (ESI†).
framework displayed thermal stability up to 320 °C, after
The asymmetric unit of TMU-32 with the monoclinic P ˉ1 which a distinctive weight loss occurred in the range of
space group consists of two crystallographically independent 370–500 °C, thereby suggesting the disintegration of the net host.
Zn atoms. Zn1 is surrounded by four coordination sites includ- Furthermore, after activation and evacuation of the cavities, the
1
ing two oxygen atoms of two different oba ligands via κ O and thermal stabilities of the daughter compounds TMU-32S-65%
two nitrogen atoms from two different L2 ligands, resulting in and 100% were evaluated by TGA. The thermal stability analysis
distorted tetrahedral geometry. The Zn2 site also possesses a of TMU-32S-65% indicated two steps at 150 °C and 300 °C,
distorted tetrahedral coordination sphere occupied by the which were attributed to the removal of the S ligands and the
second nitrogen atoms of the two pyridyl L2 ligands and the decomposition of the structure, respectively. However, the TGA
second carboxylate spacer of the same two oba ligands result for TMU-32S-100% did not show two steps, and the
(
Fig. S5†). The BET measurements revealed that this frame- decomposition of the structure started from 230 °C (Fig. S5†).
2
−1
work has adsorption capacity of 462 m g toward N at 77 K.
The investigation of the Brunauer–Emmett–Teller (BET)
2
TMU-32S was synthesized by a typical SALE method based on surface area confirmed the modification of TMU-32 through
a single crystal of TMU-32 as a precursor. Two mmol (0.512 g) the SALE process, as evidenced by the increase in the measur-
of the malonamide (S) ligand was dissolved in the solution of able surface area of the daughter compound. It can be
1
5 ml and sonicated for 5 minutes to achieve complete dis- observed from the data provided in Fig. S6† that the BET
solution. Then, approximately 1 mmol (∼0.5 g) of the TMU-32 surface area of TMU-32S compared to that of TMU-32
2
−1
2
−1
MOF crystals soaked in this mixture were dried in an oven at increased from 462 m g to 560 m g due to ligand replace-
0 °C and the ligand solution was refreshed every day. The ment. The powder X-ray diffraction (PXRD) patterns of TMU-32
8
successful incorporation of S was initially evidenced by their in comparison with those of its daughter compound
optical change. Compared to TMU-32, which consisted of (TMU-32S) confirmed that the MOFs synthesized via SALE
white cubic crystals, the TMU-32S crystals attained light-brown were structurally similar to their parents (Fig. 2). However, the
coloration after the incorporation of the S ligand (Fig. S1†); the small shift observed in the PXRD pattern of TMU-32S-100%
further proceeding of SALE was monitored by FTIR spectroscopy, (2θ = 6°) assigned to the 010 and 01–1 facets may be due to the
1
H NMR, BET, TGA, and XRD analysis. Using acetonitrile in the cavity growth and malonamide ligand exchange (Fig. 3).
activation process, one can eliminate the DMF residuals, result-
2
+
2
.2. Hg adsorption by TMU-32 and TMU-32S
ing in a higher surface area and active potential interacting sites.
These activation routes included conventional solvent exchange Earlier, Custelcean and co-workers have demonstrated that the
for 4 days, followed by vacuum drying at 60 °C. crystallization process in the formation of one-dimensional
The confirmation of the urea : S ratios in these compounds coordination polymers functionalized with urea groups can be
2
9,30
was carried out by the digestion of the mixed-ligand structures exploited as a separation tool.
Following these studies, a
1
in the D SO /DMSO-d solution and analysis by H-NMR spec- urea-containing metal–organic framework was designed and
2 4 6
2
+
troscopy. The two doublet peaks at 7.96 and 8.73 ppm were used for the investigation of Pb adsorption applications in
related to the urea-L2 ligand and a singlet peak located at our previous work, which showed significant efficiency with
.53 ppm and two doublet peaks at 7.85 and 7.66 ppm were 909 mg g− adsorption capacity. TMU-32S is an example of
1
3
attributed to the malonamide (S) linker. After 3, 6, and 10 days hydrophilic MOF containing urea and malonamide-functiona-
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Fig. 2 The coordination environment around Zn (a) TMU-32 and (b) TMU-32S; (c) surface of these pores is functionalized with urea groups (urea
groups are highlighted in blue).
Fig. 3 The PXRD patterns of the as-synthesized TMU-32 (black), TMU-32S with different exchange percentages.
17834 | Dalton Trans., 2019, 48, 17831–17839
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lized ligands. The incorporation of hydrophilic urea along with
Effect of pH. pH plays a key role in the adsorption of target
the chelating malonamide functional groups into the TMU-32 metal ions and affects the charge transfer on the adsorbent
backbone imparted a hydrophilic character to the framework interface. The value of pH can be considered as a measure for
structure, and the ligand chelating effect on the metallic the percentage of the replaced ligand. The pH of the solution
cations makes it a suitable platform for the removal of heavy of TMU-32 was 4.8, and it decreased to 4.2 for fully exchanged
metals from aqueous environments. An interesting contrast TMU-32S-100%. Also, the effect of pH in the range of 3–8 on
2
+
can be observed while comparing the miscibility of MOF in the removal of Hg was studied, and the initial concen-
the water phase with its behavior in the organic (toluene and tration used for the study was 100 mg L− . It was illustrated
dichloromethane) phase. Due to hydrophilicity, TMU-32S pre- that the removal efficiency increased on decreasing the pH
ferred the aqueous phase rather than the organic phase values until a certain pH value of 4.4 was reached, which was
1
(
Fig. S6 and S7†). The malonamide functional group (–NH– the ambient pH for TMU-32S-65%. This may be because the
CO–CH –CO–NH–) includes two adjacent Lewis basic carbonyl synergic effect of the malonamide and urea ligands results in
groups with delocalized free non-bonding lone pairs, which an increase in the sorption of Hg . Of course, other factors
2
2
+
+
2+
are in equilibrium with an isomer called enol. Compared to such as the competition between the H and Hg ions for
+
the carbonyl oxygen atoms, the delocalization of these non- the sorption sites in the presence of excessive H should also
bonding electrons makes enol oxygen atoms more negative be considered. Accordingly, the pH of 4.4 was selected as the
and accessible for chelating with positive species. Additionally, optimal value for the subsequent adsorption experiments
the aromaticity and lone pair–lone pair repulsions between the (Fig. 4).
2
+
adjacent carbonyl and amidic (–NH–) groups were also respon-
The adsorption of TMU-32 and TMU-32S towards Hg . To
2+
sible for the enriched basicity of these O atoms of the carbonyl investigate the uptake capacity of these TMUs in Hg removal,
groups. Thus, upon comparing TMU-32 with TMU-32S, we different concentrations of the Hg solutions ranging from 50
2
+
could conclude that TMU-32S had stronger Lewis basic sites, to 300 ppm were examined at an ambient temperature. As
which could act as acceptors in the adsorption process.
illustrated in Fig. S7† and Fig. 5, by increasing the initial con-
2
+
TMU-32S was designed to contain bifunctional groups; it centration of the Hg solutions, a rapid increase in the
2
+
can be a promising material for heavy metal removal due to its adsorption of the Hg ions on all TMUs was observed, indicat-
chelating ligands and hydrophobic properties. Based on this ing that these potential adsorbents have a strong affinity for
hypothesis, we investigated the removal capacity of TMU-32S mercury ions, which can be ascribed to the abundant metal
towards different metal ions; about 7 mg of the activated com- chelating groups within the MOF cavities. Moreover, the
pound was dispersed in 100 mL aliquot of aqueous samples experimental adsorption data were well fitted with the
2
+
containing Hg in various concentrations and kept at room Langmuir isotherm, signifying monolayer-type adsorption
temperature for 20 min. At the end of the adsorption time, the (Fig. 5 and S7†). By fitting with the linear Langmuir model,
solution was centrifuged and analyzed by an ICP-OES instru- the saturated adsorption capacities of TMU-32S-33%, 65%,
2
+
−1
ment. In order to study the heavy metal ion adsorption pro- and 100% for Hg were calculated to be 714 mg g , 1428 mg
perties, a series of optimization experiments including the
−
1
−1
g , and 956 mg g , respectively, which were higher than
adsorption isotherm, effect of pH, adsorption kinetics, and those of various other porous materials. Nonlinear regression
recyclability were conducted.
is a more general tool that can be used to estimate the models’
Fig. 4 The effect of sample pH on removal efficiency.
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In this study, all the model parameters were analyzed by linear
and non-linear regression models using the curve fitting tool
of the Origin program with both Hill and Hyperb models. The
linear pseudo-second-order model exhibited a high coefficient
2
of determination (R = 0.98), which was statistically significant
2
than that of the pseudo-first-order model (R = 0.92). The q
t
values for both Hill and Hyperb nonlinear regressions were
calculated to be 1525 mg g− and 1537 mg g , respectively.
Therefore, the results were compatible with the chemical reac-
tions between the positive mercury ions and the adsorption
sites, which indicated that the transfer, exchange or sharing of
electrons occurred between the target ion and the MOF back-
bone (Fig. 6 and S8†).
1
−1
Thus, incorporating the malonamide functional group into
2
+
the MOF backbone has a profound effect on the selective Hg
recognition over other heavy metal ions. It can be noted that
developing an adsorbent for the selective removal and binding
2
+
of the Hg ions in the presence of other heavy metal ions is
highly desired but still remains a challenge. Based on this
design, the negative surface charge of the carbonyl groups
2+
Fig. 5 Nonlinear curve fitting with Langmuir and Hg ion.
2
+
created an electrostatic interaction with the Hg ions, which
was also the main driving force in the adsorption process.
Owing to the moderate porosity of these MOFs, the higher
adsorption capacity of TMU-32S 65% could be attributed
to preferable host-guest interactions of multiple accessible
adsorption sites formed by the malonamide and urea groups
inside the pore wall which created electrostatic attractions
parameters, and it can be applied even if the isotherm model
cannot be linearized. The nonlinear Langmuir coefficients cal-
2
+
culated for the adsorption of Hg onto TMU-32S-33%, 65%,
and 100% were 784 mg g , 1507 mg g−1, and 1012 mg g
−
1
−1
,
respectively. The excellent performance of TMU-32S-65% can
be attributed to its bifunctionality, which can facilitate the
diffusion of metal ions to the chelating sites of both urea and
2
+
between Hg ions and framework.
2
+
PXRD, ICP, and AAS analyses were applied to investigate the
water stability of the framework. Although the PXRD pattern of
the framework indicated the retention of the structure, ICP
and AAS analyses revealed almost 0.65% degradation of the
framework during the adsorption process. It should be noted
that the degradation rate did not change significantly on
increasing the water exposure time up to 24 h. Further analyses
revealed that the partial framework degradation could be
because of the direct exposure to water and the presence or
absence of analytes did not affect the decomposition rate.
Moreover, after the sensing process, the PXRD patterns of the
TMU-32S-65% and regenerate samples showed high structural
stability and confirmed that the structures’ crystallites did not
change (Fig. S10†).
malonamide struts. Additionally, the adsorption of Hg from
aqueous media by using the bare TMU-32 sample was investi-
gated as a control experiment, and we observed 450 mg g−
1
capacity (Table 1).
Study of sorption kinetics. Adsorption kinetics provides
important information about adsorption pathways and mecha-
nisms. The kinetic experiment was accomplished using the
initial concentration of 100 ppm under the condition of pH
4
.4, which is the ambient pH for TMU-32S-65% at room temp-
erature. The adsorption capacity increased sharply during
1
1
–30 min and approached adsorption equilibrium within
7 min. Further explanation was given by investigating
different kinetic models such as pseudo-first-order and
pseudo-second-order kinetic models to simulate the adsorp-
2
+
Adsorption mechanism. In order to investigate the probable
adsorption mechanism of TMU-32S, the FTIR spectrum of the
adsorbent was compared before and after the adsorption of
the target ion. The malonamide functional group (NH–CO–
tion kinetics of Hg on TMU-32S-65%, which showed high
adsorption capacity compared to that of other daughter MOFs.
CH –CO–NH) of TMU-32S can be described as a double amide
by considering the presence of acidic CH , which favors the
2
2
2+
Table 1 The Langmuir factors for the sorption of Hg ions
enol form with inter-molecular hydrogen bonding. The electro-
negativity of the carbonyl (CvO) groups in both malonamide
and urea functional groups is larger than the electronegativity
of the amido (–HN–) organic functional group. Thus, the car-
bonyl groups adsorb positive ions easily and consequently, the
polarity of this functional group may reduce significantly upon
chelating with the target ions. There are strong interactions
Langmuir
(nonlinear)
Langmuir (linear)
Q
(mg g
m
Q
m
−1
b (L mg−1
2
(mg g−1) b (L mg−1
)
Adsorbents
)
)
R
TMU-32
450
714
1428
956
0.045
0.035
0.0065
0.0081
0.987
0.994
460
784
0.998 1507
0.985 1012
0.038
0.023
0.027
0.007
TMU-32S 33%
TMU-32S 65%
TMU-32S 100%
2
+
between Hg and the O atoms of both malonamide and urea
17836 | Dalton Trans., 2019, 48, 17831–17839
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Fig. 6 Nonlinear pseudo-second-order kinetics: (a) Hill and (b) Hyperb models for Hg(II) adsorption on TMU-32S-65%.
functional groups, which can limit the CvO vibration modes tively, showed high removal capacity in comparison to
and subsequently decrease their vibrational frequency, result- TMU-32S-65%.
−
1
−1
ing in a significant redshift from 1675 cm to 1657 cm for
Selectivity. A mixed solution of metal ions containing Cu(II),
−
1
−1
malonamide and 1740 cm to 1734 cm for the carbonyl Cd(II), Pb(II), Co(II), and Cr(III) was prepared for selectivity
−
1
group of the urea strut. Also, the peak at 1294 cm , which was experiments; the concentration of all the heavy metal ions was
ascribed to the characteristic C–N vibration mode, shifted to set to 50 mg L− . The results are shown in Fig. 8. The adsorp-
1
312 cm− after loading Hg . Hence, excellent selectivity tion ability of TMU-32S-65% toward Hg was noticeably
1
2+
2+
1
2
+
towards Hg was displayed due to the synergistic effects of higher than that towards other ions.
both hydrophilic urea and malonamide organic functional
groups (Fig. 7).
Regeneration and reusability tests. Recyclability is an impor-
tant characteristic of the materials used in environmental
2
+
The comparison of the maximum adsorption capacity of remediation as these adsorbents can be reused if Hg adsorp-
TMU-32S-65% with those of other adsorbents reported in the tion is reversible. For investigating the adsorption–
literature for mercury ion removal is listed in Table 2. As is desorption efficiency of the studied MOFs, 7 mg of the adsor-
considered, the efficiency of the proposed MOF as an bent was added to a 100 ml solution of 100 mg L− of Hg
1
2+
adsorbent was higher as compared with those of previously ions. In this study, 2 ml of 0.2 M of EDTA.2Na solution was
2
+
reported adsorbents for the removal of heavy metal ions used to desorb the Hg ions, and the total concentration of
2
+
2+
such as Hg . The usually used adsorbents with high the Hg ions was then measured by ICP-OES. As shown in
surface areas but without proper functional groups cannot Fig. S8,† although the five samples show different reusability
provide enough adsorption activated sites and thus prevent for two frameworks, the worst of them still retained more
2
+
Hg diffusion to the MOF cavity. The Fe-BTC@PDA and than 60% of its previous uptake capacity. To explore the
2
+
COF-SH composites using polydopamine (PDA) containing effect of Hg
adsorption on the BET surface area of
adsorption/desorption experiment
amine as well as catechol and SH functionalities, respec- TMU-32S-65%, the
N
2
2+
m
Table 2 Comparison of maximum adsorption capacities (q ) of some adsorbents for the removal of Hg metal ions
Adsorption capacity (mg g− ) of Hg ion
1
Method
Time to adsorption equilibrium (min)
Ref.
MOF
Fe-BTC@PDA
BioMOF
UiO-66-NHC(S)NHMe
Thiol-HKUST-1
1670
900
769
714
713
380
1350
1480
Adsorption
Adsorption
Adsorption
Adsorption
Adsorption
sensing
1<
30
240
120
120
—
27
31
32
33
34
35
36
[
Ni(3-bpd)
2
(NCS)
2
]n
LMOF-263
COF-S-SH
TMU-32S 65%
Adsorption
Adsorption
20
17
This work
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2
+
Fig. 7 The IR spectrum of TMU-32S 65% before and after Hg adsorption and the schematic representation of framework interaction with the metal ions.
3
. Conclusion
One of the most promising approaches to achieve high metal-
ion removal efficiency is employing a new strategy to design
new functionalized host materials. In this regard, metal–organic
frameworks can be effectively functionalized or tailored to
incorporate the linkers containing a series of urea and malona-
mide moieties, which involve host–guest interactions. Herein, a
hydrophilic urea-malonamide-containing MOF was designed
and studied for the detection and removal of mercury ions. The
results clearly demonstrated the power of using highly pre-orga-
2
+
nized receptors for Hg recognition. The structural pre-organiz-
ation induced by the network formation enhanced the potential
for the selective recognition of a particular ion. Accordingly, the
results demonstrated the exceptional ability of TMU-32S to
2
+
selectively recognize Hg ions. Thus, incorporating this unpre-
cedented motif into the backbone of TMU-32 gives rise to the
highest uptake capacity of 1428 mg g− for Hg(II) ions and
results in high removal efficiency (>97%) in just 17 min.
1
Fig. 8 The adsorption of various toxic metals by TMU-32–65%.
was also performed. The results showed that the BET of
TMU-32S-65% after mercury adsorption decreased, indicating the
Conflicts of interest
2+
presence of Hg inside the pore space. Furthermore, the BET of
TMU-32S-65% after 3 and 5 adsorption–desorption cycles demon-
strated that after 3 cycles, the porosity decreased by only 10
percent, while in the fifth cycle, a large amount of pore volume
was still occupied. The decrease in the adsorption capacity in the
The authors declare no competing financial interests.
Acknowledgements
fifth cycle could be attributed to the decrease in the BET surface We are gratefully acknowledged from Tarbiat Modares
area (Fig. S11–S13†).
University for the support of this research.
17838 | Dalton Trans., 2019, 48, 17831–17839
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9 M. Gharib, L. Esrafili, A. Morsali and P. Retailleau, Dalton
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References
1
2
3
4
5
6
L. Esrafili, V. Safarifard, E. Tahmasebi, M. Esrafili and 20 A. Azhdari Tehrani, L. Esrafili, S. Abedi, A. Morsali,
A. Morsali, New J. Chem., 2018, 42, 8864–8873.
P. A. Kobielska, A. J. Howarth, O. K. Farha and S. Nayak,
Coord. Chem. Rev., 2018, 358, 92–107.
L. Carlucci, D. M. Proserpio, J. Wang, P. C. Junk and T. Liu,
Inorg. Chem., 2017, 56, 1446–1454.
21 A. Hakimifar and A. Morsali, Inorg. Chem., 2018, 58, 180–
187.
F. Rouhani and A. Morsali, Chem. – Eur. J., 2018, 24, 17170–
1
7179.
22 C. Cuillerdier, C. Musikas, P. Hoel, L. Nigond and X. Vitart,
Sep. Sci. Technol., 1991, 26, 1229–1244.
23 A. Rout, K. Venkatesan, T. Srinivasan and P. V. Rao,
J. Hazard. Mater., 2012, 221, 62–67.
24 J. M. Roberts, B. M. Fini, A. A. Sarjeant, O. K. Farha,
J. T. Hupp and K. A. Scheidt, J. Am. Chem. Soc., 2012, 134,
3334–3337.
T. Wu, C. Liu, B. Kong, J. Sun, Y. Gong, K. Liu, J. Xie, A. Pei
and Y. Cui, ACS Cent. Sci., 2019, 5, 719–726.
S. Perumal, R. Atchudan, D. H. Yoon, J. Joo and
I. W. Cheong, Ind. Eng. Chem. Res., 2019, 58, 9900–9907.
Y. Zou, X. Wang, A. Khan, P. Wang, Y. Liu, A. Alsaedi,
T. Hayat and X. Wang, Environ. Sci. Technol., 2016, 50,
7
290–7304.
25 P. W. Siu, Z. J. Brown, O. K. Farha, J. T. Hupp and
K. A. Scheidt, Chem. Commun., 2013, 49, 10920–10922.
26 Z. Rao, K. Feng, B. Tang and P. Wu, ACS Appl. Mater.
Interfaces, 2017, 9, 2594–2605.
7
8
9
G. Zhao, J. Li, X. Ren, C. Chen and X. Wang, Environ. Sci.
Technol., 2011, 45, 10454–10462.
A. Sayari, S. Hamoudi and Y. Yang, Chem. Mater., 2005, 17,
2
12–216.
27 D. T. Sun, L. Peng, W. S. Reeder, S. M. Moosavi, D. Tiana,
D. K. Britt, E. Oveisi and W. L. Queen, ACS Cent. Sci., 2018,
4, 349–356.
E. Tahmasebi, M. Y. Masoomi, Y. Yamini and A. Morsali,
Inorg. Chem., 2014, 54, 425–433.
1
0 T. He, Y.-Z. Zhang, X.-J. Kong, J. Yu, X.-L. Lv, Y. Wu, 28 D. T. Sun, N. Gasilova, S. Yang, E. Oveisi and W. L. Queen,
Z.-J. Guo and J.-R. Li, ACS Appl. Mater. Interfaces, 2018, 10,
6650–16659.
1 H. Furukawa, K. E. Cordova, M. O’Keeffe and O. M. Yaghi,
J. Am. Chem. Soc., 2018, 140, 16697–16703.
29 R. Custelcean, T. J. Haverlock and B. A. Moyer, Inorg.
Chem., 2006, 45, 6446–6452.
1
1
1
1
1
1
1
Science, 2013, 341, 1230444.
2 F. ZareKarizi, M. Joharian and A. Morsali, J. Mater. Chem.
A, 2018, 6, 19288–19329.
3 F. Rouhani, F. Rafizadeh-Masuleh and A. Morsali, J. Am.
Chem. Soc., 2019, 141(28), 11173–11182.
30 R. Custelcean, B. A. Moyer, V. S. Bryantsev and B. P. Hay,
Cryst. Growth Des., 2006, 6, 555–563.
31 M. Mon, F. Lloret, J. Ferrando-Soria, C. Martí-Gastaldo,
D. Armentano and E. Pardo, Angew. Chem., Int. Ed., 2016,
55, 11167–11172.
4 M. Y. Masoomi, A. Morsali, A. Dhakshinamoorthy and 32 A. Abbasi, T. Moradpour and K. Van Hecke, Inorg. Chim.
H. García, Angew. Chem., Int. Ed., 2019, 58(43), 15188–15205. Acta, 2015, 430, 261–267.
5 O. K. Farha and J. T. Hupp, Acc. Chem. Res., 2010, 43, 1166– 33 F. Ke, L.-G. Qiu, Y.-P. Yuan, F.-M. Peng, X. Jiang, A.-J. Xie,
175. Y.-H. Shen and J.-F. Zhu, J. Hazard. Mater., 2011, 196, 36–43.
6 P. Deria, J. E. Mondloch, O. Karagiaridi, W. Bury, 34 S. Halder, J. Mondal, J. Ortega-Castro, A. Frontera and
1
J. T. Hupp and O. K. Farha, Chem. Soc. Rev., 2014, 43, 5896–
912.
7 A. U. Czaja, N. Trukhan and U. Müller, Chem. Soc. Rev.,
009, 38, 1284–1293.
P. Roy, Dalton Trans., 2017, 46, 1943–1950.
35 N. D. Rudd, H. Wang, E. M. Fuentes-Fernandez, S. J. Teat,
F. Chen, G. Hall, Y. J. Chabal and J. Li, ACS Appl. Mater.
Interfaces, 2016, 8, 30294–30303.
5
1
1
2
8 M. Mon, J. S. Ferrando-Soria, T. Grancha, F. R. Fortea- 36 Q. Sun, B. Aguila, J. Perman, L. D. Earl, C. W. Abney,
Pérez, J. Gascon, A. Leyva-Pérez, D. Armentano and
E. Pardo, J. Am. Chem. Soc., 2016, 138, 7864–7867.
Y. Cheng, H. Wei, N. Nguyen, L. Wojtas and S. Ma, J. Am.
Chem. Soc., 2017, 139, 2786–2793.
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