Functional group effect of isoreticular metal-organic frameworks on heavy metal ion adsorption

Article abstract of DOI:10.1039/c8nj01150h

Evaluating the relationship between structure and function of metal-organic frameworks (MOFs) is an interesting issue that is discussed in this work. Here, four isoreticular 3D porous zinc(ii) MOFs with pcu topology, including [Zn(oba)(4-bpmb)_0.5

Full text of DOI:10.1039/c8nj01150h

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PAPER
Jason B. Benedict et al.
The role of atropisomers on the photo-reactivity and fatigue of
diarylethene-based metal–organic frameworks
rsc.li/njc

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Functional group effect of Isoreticular metal-organic frameworks on
heavy metal ion adsorption
a
b
c
d
*a
Leili Esrafili, Vahid Safarifard, Elham Tahmasebi, MD Esrafili, Ali Morsali,
a
Department of Chemistry, Faculty of Sciences, Tarbiat Modares University, P.O. Box 14115-
1
75, Tehran, Iran. E-mail: Morsali_a@modares.ac.ir;
b
Department of Chemistry, Iran University of Science and Technology, Tehran 16846-13114,
Iran.
c
Department of Chemistry, Institute for Advanced Studies in Basic Sciences (IASBS), P.O. Box
4
5195-1159, Zanjan, Iran
d
Laboratory of Theoretical Chemistry, Department of Chemistry, University of Maragheh, P.O.
Box 5513864596, Maragheh, Iran
ABSTRACT
Evaluating the relationship between structure and function of metalꢀorganic frameworks (MOFs)
is an interesting issue that has been discussed in this work. Here, four isoreticular 3D porous
zinc(II) MOFs with pcu topology, including [Zn(oba)(4ꢀbpmb) ]•(DMF)
(TMUꢀ6),
(TMUꢀ23) and
Zn (oba) (bpfn)]•(DMF) (TMUꢀ24), H oba = 4,4′ꢀoxybisbenzoic acid, bpmb = N,N′ꢀbisꢀ(4ꢀ
0
.5
1.5
[
[
Zn(oba)(bpmn) ]•(DMF)
(TMUꢀ21),
[Zn (oba) (bpfb)]•(DMF)
2 2 5
0
.5
1.5
2
2
2
2
pyridylmethylene)ꢀ1,4ꢀbenzenediamine,
bpmn
=
N,N′ꢀbisꢀ(4ꢀpyridylmethylene)ꢀ1,5ꢀ
naphthalenediamine, bpfb = N,N′ꢀbisꢀ(4ꢀpyridylformamide)ꢀ1,4ꢀbenzenediamine and bpfn =

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N,N′ꢀbis(4ꢀpyridylformamide)ꢀ1,5ꢀnaphthalenediamine, containing imineꢀ (TMUꢀ6 and TMUꢀ
1) and amideꢀ (TMUꢀ23 and TMUꢀ24) decorated pores, have been successfully synthesized by
2
mechanochemical method. Then, adsorption efficiency of these four MOFs for some heavy metal
ions was studied to evaluate the effect of type of the functional groups of the pillars in different
MOFs on adsorption process. The results indicated that the amideꢀdecorated MOFs show a better
adsorption efficiency toward metal ions than the imineꢀdecorated MOFs. In the following, TMUꢀ
+
2
2
3 was used as an efficient sorbent for extraction and removal of some heavy metal ions (Co ,
+
2
+2
+3
+2
+2
Cd , Cu , Cr , Fe , and Pb ) and its analytical performance was evaluated and determined. In
addition, DFT calculations were performed on possible coordination modes between cations and
simplified functional groups of the related pillars in each MOF and probable interaction
mechanism of the MOFs and the metal ions was evaluated.
1
. INTRODUCTION
Since the beginning of the industrial growth, human activities and natural processes have been
entering numerous potential hazardous trace heavy metal ions into the environment. Many of
these metals, such as chromium (Cr), copper (Cu), iron (Fe), and cobalt (Co) are necessary for
plants, living organisms, and the human body, [1, 2] whereas in large quantities, the same
elements are toxic. Cadmium (Cd) and lead (Pb) show extreme toxicity even at low
concentrations. Therefore, there has been an increasing necessity for analytical chemists to
evaluate the environmental and healthy quality based on an accurate determination of heavy
metal ions in environmental samples. [3ꢀ 6]
While the determination of trace heavy metal ions in environmental samples is highly desired, it
is also restricted by two main difficulties: first, the interfering effects of the matrix, second, the

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usual low concentrations of heavy metal ions which may be lower than the detection limit of
many analytical techniques. There is a crucial need for extraction and preconcentration of trace
elements from applied matrix before analyzing them through analytical techniques. [7ꢀ 9]
Solid phase extraction (SPE) is currently being used as a preconcentration or separation method
for analyzing trace analyte ions existing in various materials and complexes. [10, 11] SPE has
several advantages such as large preconcentration factors, low cost because of lower
consumption of reagent, simplicity in phase separation and ease of automation. [12, 13] In SPE
procedure, selection of suitable sorbent to achieve higher analytical performance (i.e., analytical
sensitivity, selectivity, and precision) is the most crucial step. So studying the relationship of
structure and function of a sorbent may be a fascinating subject in the analytical chemistry.
Metalꢀorganic frameworks (MOFs) are crystalline materials combined of organic linkers and
inorganic metal ions which are selfꢀassembled through coordination bonds. [14] The diversity of
metal ions and organic linkers makes it possible to generate architectures with tunable porosity.
[
15, 16] High thermal resistivity, large surface area, uniform nanosized structural cavity and
porosity makes MOFs as good candidates for diverse applications such as gas storage, [17, 18]
catalyst, [19, 20] separation, [21] drug delivery, [22, 23] sensing, [24, 25] biomedical imaging
[
26] and analytical applications. [27, 28]
Analytical chemists enjoy the potential of novel MOF materials for their application in analytical
methods. As sorbents for sampling, solid phase extraction (SPE) [29, 30] and solid phase
microextraction (SPME) [31, 32] as well as stationary phases for liquid and gas chromatography
[
33] providing them with a solution to the challenges such as extraction, sample collection,
improving sensitivity, selectivity and limit of detection. Considering all of these applications, it
is difficult to find an appropriate MOF for sample preparation. [34]

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MOFs show great promise in the analytical applications such as the capture of heavy metal ions
by the chelating effect between metal ions and functional group in the linker. [35, 36] The shape
and size of the pores lead to selectivity over the guests that may be adsorbed. Immobilization of
functional groups modifies the surface for the engineering of interface properties of solid
substrates, while the fundamental of their geometry and mechanical strength remains unchanged.
[
2, 37]This article is developed in the following order: first, synthesis of MOFs with various
functional groups. Second, discuss the effects of different functional groups in MOFs on
extraction efficiency. There are a lot of reports about the extraction and removal of heavy metal
ions with MOFs but in this work we try to explain the results that accrue with small change in
the linker and such investigation may open up new possibilities for the rational design and
synthesis of nanoporous materials for particular applications. [38]
Recently, we successfully introduced two acylamideꢀcontaining threefold interpenetration pcuꢀ
MOFs; TMUꢀ23 ([Zn (oba) (bpfb)]·(DMF) ), and TMUꢀ24 ([Zn (oba) (bpfn)]·(DMF) ) (H oba
2
2
5
2
2
2
2
=
=
4,4′ꢀoxybis(benzoic acid, bpfb = N,N′ꢀbisꢀ(4ꢀpyridylformamide)ꢀ1,4ꢀbenzenediamine and bpfn
N,N′ꢀbis(4ꢀpyridylformamide)ꢀ1,5ꢀnaphthalenediamine, where DMF is N,Nꢀ
dimethylformamide) [39], and studied their CO sorption properties. The study revealed that not
2
only the insertion of amide functional groups on the MOFs pore surface but also the accessibility
of the functional groups could affect their CO sorption and CO /N separation properties.
2
2
2
Inspired by our previous works, [40, 41] herein we report the mechanochemical synthesis of
TMUꢀ23 and TMUꢀ24 for the first time and investigate the capability of these amideꢀdecorated
MOFs as sorbents for adsorption of heavy metal ions. For further investigation of the role of the
functional groups of the MOFs in capturing of metal ions, two other isoreticular

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mechanosynthesized Zn(II)ꢀbased MOFs, [Zn(oba)(4ꢀbpmb) ]·(DMF)
1.5
(TMUꢀ6) and
0
.5
[
Zn(oba)(bpmn) ]·(DMF)
1.5
(TMUꢀ21), (bpmb
=
N,N′ꢀbisꢀ(4ꢀpyridylmethylene)ꢀ1,4ꢀ
0
.5
benzenediamine and bpmn = N,N′ꢀbisꢀ(4ꢀpyridylmethylene)ꢀ1,5ꢀnaphthalenediamine), were
selected in which the pores surface are decorated by imine functional groups [39] (Scheme 1).
This investigation helped us to examine the role of the basicity of the different Nꢀdonor ligands
(containing amide or imine group) and the oxygen containing groups in metal ion capturing and
to compare the adsorption efficiencies in four MOFs.
Also, maximum adsorption capacity of all the MOFs were examined and compared.
Subsequently, according to results of the preliminary experiments, TMUꢀ23 was selected as an
efficient sorbent for extraction and preconcentration of trace amounts of some heavyꢀmetal ions
including Cd(II), Co(II), Cr(III), Cu(II), Fe(II) and Pb(II) from aqueous samples, followed by
their determination by flowꢀinjection inductively coupled plasmaꢀoptical emission spectrometry
(
ICPꢀOES), and potential parameters affecting the performance of the target metal ions
extraction were evaluated in detail. Eventually, the proposed method was applied for the
preconcentration and determination of the heavy metal ions in real environmental water samples.
Also, DFT calculations were performed to study type of the interaction of the metal ions with
functional groups (amide or imine) of four MOFs and the obtained results were compared with
1
the experimental studies results.

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Figure1: advantage of mechanochemical methods for preparation of MOF compared to other methods.
42, 43]
[
2
. EXPERIMENTAL SECTION
.1. Materials and Physical Techniques
2
All starting materials, including Zinc(II) nitrate hexahydrate, zinc(II) acetate dihydrate, and 4,4′ꢀ
oxybis(benzoic acid) (H oba) were purchased from Aldrich and Merck. The ligands N,N′ꢀbisꢀ(4ꢀ
2
pyridylformamide)ꢀ1,4ꢀbenzenediamine
(bpfb),
N,N′ꢀbis(4ꢀpyridylformamide)ꢀ1,5ꢀ
naphthalenediamine (bpfn), N,N′ꢀbisꢀ(4ꢀpyridylmethylene)ꢀ1,4ꢀbenzenediamine (bpmb) and
N,N′ꢀbisꢀ(4ꢀpyridylmethylene)ꢀ1,5ꢀnaphthalenediamine (bpmn) were synthesized according to
ꢀ1
the previous reported methods. [39] Stock standard solutions (1000 mg L ) of Cd(II), Co(II),
Fe(II), Cr(III), Cu(II), and Pb(II) were prepared by dissolving appropriate amounts of analyticalꢀ
grade Cd(NO ) ·4H O, Co(NO ) ·6H O, Cr (NO ) ·9H O, Fe(NO ) .9H O, Cu(NO ) ·3H O,
3
2
2
3 2
2
3 3
2
3 2
2
3 2
2
and Pb(NO ) salts, respectively, from Merck in ultrapure water. Fourier transform infraꢀred (FTꢀ
3
2
IR) spectra were recorded on a Bruker Tensor 27FTIR spectrometer equipped with a Golden
Gate diamond attenuated total reflection (ATR) cell, in absorption mode at room temperature.
Powder Xꢀray diffraction (PXRD) measurements were collected on a Philips X’pert
diffractometer with monochromatic CuꢀKα radiation (λCu = 1.5406 Å). For synchronous
determination of the target elements we used a simultaneous inductively coupled plasmaꢀoptical

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emission spectrometry (ICPꢀOES) on a Varian VistaꢀPRO instrument (Springvale, Australia)
having a radial torch coupled to a concentric nebulizer and a Scott spray chamber carrying a
chargeꢀcoupled detector (CCD). To introduce the final solution into the ICPꢀOES nebulizer a
sixꢀport twoꢀposition injection valve from Tarbiat Modares University equipped with a 250 ꢁL
injection loop constructed from silicon tubing was used. Table S1 in the Supporting Information
(SI) lists the optimal conditions of instruments and the emission lines used for determining the
metal ions by ICPꢀOES.
Scheme 1. Chemical Structures of H oba, bpfb, bpfn, bpmb and bpmn.
2
Computational details
All DFT calculations were performed using the GAMESS suite of programs. The geometry of
the ligands was optimized at the B3LYP/6ꢀ31+G* level of theory. The LANL2DZ basis set with
the corresponding effective core potential was used for the metal cations. Molecular electrostatic
potentials (MEPs) of the isolated ligand were obtained on the 0.001 atomic unit surface by means
of the SASꢀWFA program using the wavefunctions generated at the aforementioned level of
theory. The Mulliken charge density analysis of the free ligands was performed at the B3LYP/6ꢀ
3
1+G* level using the GAMESS.
.2. Extraction Procedure
2

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The procedure for the solidꢀphase extraction of heavy metal ions was conducted by the following
steps: Initially a 100 mL aliquot of aqueous sample including the target metal ions was added in
a 200 mL glass beaker containing 10 mg of the MOF as sorbent. The pH value of the solution
was adjusted to 8. After stirring the solution for 10 min to speed up the adsorption of the analytes
onto the MOF, the solution was centrifuged at 6000 rpm for 2 min. Then the sorbent was
separated from the solution, and the adsorbed analytes were desorbed from the sorbent with 200
ꢁ
L of the eluent containing 0.4 M ethylenediaminetetraacetic acid (EDTA) by powerful vortex
for 2 min. After centrifuging, the eluate of volume 200 ꢁL was subjected into the nebulizer of the
ICPꢀOES instrument using the sixꢀway twoꢀposition injection valve for further determination.
To investigate the adsorption capacity of all the MOFs, 3 mg of each MOF was added to 15 mL
ꢀ1
of the metal ions solutions at different concentrations in the range of 20ꢀ200 mg L and after pH
adjusting to 6, stirred for 15 min at room temperature. As the equilibrium point was reached, the
sorbent was separated centrifugally, and the concentrations of the residual ions in solution after
equilibrium adsorption were determined by ICPꢀOES.
3
. RESULTS AND DISCUSSION
3
.1. Characterization of the MOFs
Knowing the isoreticularity concept first disclosed by Yaghi and coꢀworkers, [44] four pillared
MOF structures (TMUꢀ6, TMUꢀ21, TMUꢀ23 and TMUꢀ24) with the same network topology,
bearing different Nꢀdonor ligands were prepared. The original preparation method and the
crystallographic data of TMUꢀ6, TMUꢀ21, TMUꢀ23 and TMUꢀ24, which respectively contain
imine/amide pillars of bpmb, bpmn, bpfb and bpfn have been recently addressed by our research
group. [39]

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In order to achieve more facile preparation route of these MOFs, all MOFs were prepared via
mechanochemical method. In a typical synthesis, TMUꢀ6, TMUꢀ21, TMUꢀ23 and TMUꢀ24 were
prepared by the mechanochemical reaction (grinding by hand) of a mixture of Zn(OAc) ·2H O,
2
2
H oba, and Nꢀdonor ligand; bpmb, bpmn, bpfb and bpfn (Figure S1), respectively for 15 min
2
(SI). Comparison between the simulated and the experimental (resulting from the
mechanosynthesized powder) PXRD patterns revealed that the mechanosynthesized TMUꢀ6,
TMUꢀ21, TMUꢀ23 and TMUꢀ24 were structurally identical to their original structures prepared
by conventional heating (Figure 2).
TMUꢀ6, TMUꢀ21, TMUꢀ23 and TMUꢀ24 are neutral threefold interpenetrated MOFs formed by
{
Zn(oba)} layers connected through the N,Nꢀpillar ligands. In these structures, each nonlinear
dicarboxylate oba ligand is coordinated to three Zn(II) centers forming the {Zn(oba)} layers
along the abꢀplane. Because of the dinuclear nature of the metal cluster, these layers are formed
by two sheets of parallel oba ligands that form πꢀπ stacking interactions between them. These
{
Zn(oba)} layers are then pillared by the linear imine/amideꢀfunctionalized ligands, extending
the structure in three dimensions with 6ꢀconnected net and pcu topology. Although all
compounds are threefold interpenetrated, they still possess apparent 1D pore channels (∼8.1 ×
2
2
2
2
7
.4 Å , ∼8.2 × 7.0 Å , ∼6.8 × 6.4 Å and ∼6.7 × 5.5 Å ꢀincluding van der Waals (vdW) radiiꢀfor
TMUꢀ6, TMUꢀ21, TMUꢀ23 and TMUꢀ24, respectively) running along the ⟨101⟩ direction and
noticeable free space (33%, 30%, 32% and 30% for TMUꢀ6, TMUꢀ21, TMUꢀ23 and TMUꢀ24,
respectively) (Figure ). As we previously reported, all the MOFs are nonporous to N at 77 K but
2
are porous to CO at various temperatures. CO uptakes at 203 K were calculated to be 9.0, 4.5,
2
2
ꢀ1
3
ꢀ1
7
.2, and 6.3 mmol.g and the observed pore volumes were 0.32, 0.16, 0.27 and 0.22 cm .g for
TMUꢀ6, TMUꢀ21, TMUꢀ23, and TMUꢀ24, respectively. [39]

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Thermogravimetric analyses (TGA) of the mechanosynthesized MOFs showed a plateau in the
range of 30ꢀ~370 °C revealing that their pore channels were devoid of any guest molecules
(Figure S4, SI). Above these temperatures, the MOFs started to decompose. The prepared MOFs
were also soaked in water and some organic solvents such as ethanol, acetonitrile and
dichloromethane for at least 24 h at room temperature to determine whether they are stable in
these solvents. The PXRD patterns before and after immersion were consistent, confirming that
the structures have high stabilities in these solvents (Figure S5, S6FI).
Figure 2. Comparison of PXRD patterns of TMUꢀ6, TMUꢀ21, TMUꢀ23 and TMUꢀ24: (a,d,g,j) simulated,
(
b,e,h,k) asꢀsynthesized and (c,f,i,l) mechanosynthesized, respectively.

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Figure 3. Representation of the pores highlighting the imine and amide groups in (a) TMUꢀ6, (b) TMUꢀ
1, (c) TMUꢀ23 and (d) TMUꢀ24 (presented in spaceꢀfilling) pointing toward the pores of the
frameworks. All hydrogen atoms and the disordered guest molecules are omitted for clarity.
2
3
.2.
Optimization of Extraction Parameters
The primary experiments demonstrated that the MOF containing amide functional group has
higher metal ions adsorption efficiency than the MOF with imine group. Accordingly, TMUꢀ23
was selected as the SPE sorbent for extraction and preconcentration of heavy metal ions,
including Cd(II), Co(II), Cr(III), Cu(II), Fe(II) and Pb(II) to evaluate its analytical performance.
In order to achieve the best efficiency various factors influencing the extraction efficiency such
as concentration of the eluent, sample pH, times of adsorption and desorption, and sample and
eluent volumes were investigated and optimized. The optimization was carried out on an aqueous
ꢀ1
solution containing mixture of 100 µg L of each metal ion and the above mentioned variables
were performed by modifying one at a time and keeping the remaining constants.

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The pH of aqueous solution plays an important role in the chelation of the metal ion species with
free coordination sites of the MOF as well as in the following extraction process. The effect of
the sample pH was tested in the range of 5ꢀ11. As demonstrated in Figure S7, the maximum peak
areas were obtained at pH 8, so it was chosen as the optimum value at further studies. Decrease
in extraction efficiency by further increasing the sample pH from 8 to 11, can be attributed to
precipitation of the metal ions in hydroxide forms.
To obtain the maximum extraction efficiencies for the target analytes, amount of the sorbent was
−
1
optimized by varying its amount in the range of 5ꢀ15 mg for extraction of 100 µg L of the
analytes from 30 mL of the sample solution. In order to gain satisfactory recoveries of the target
metal ions, a certain amount of sorbent is required. The results shown in Figure S8 indicated,
indicated that the best extraction efficiencies were achieved by using 10 mg of the MOF, hence
this amount of the MOF was employed in the subsequent studies.
It is necessary to determine optimum volume of the eluent to achieve the highest enrichment
and recovery of the adsorbed analytes. Eluent volume should be enough to can soak the sorbent
and on the other hand it should not be excessive to dilute the eluate. Considering these, different
volumes (200ꢀ600 µL) of 0.4 M EDTA as eluent was examined and the results revealed that
analytes quantitative recoveries could be achieved by using 200 µL of the eluent. Accordingly,
desorption of the target metal ions was performed using 200 µL of 0.4 M EDTA in subsequent
experiments Figure S9.
To examine effect of the sample volume on the analytical performance, experiments were
performed by changing the sample solution volume, spiked with 20 ꢁg of the target analytes, in
the range of 30ꢀ250 mL using 10 mg of the MOF. The results revealed that the extraction

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efficiency was nearly constant in the volume range of 30ꢀ100 mL, and after that, a decrease was
observed in the signal intensity. Therefore, 100 mL was chosen as the optimal sample volume.
The influence of the concentration of EDTA on desorption of target ion was studied. For
ꢀ1
desorbing 100µg.L of target ion, already adsorbed on 10 mg of sorbent, 200µl of different
concentration of eluent (EDTA) have been used. As a result of experiments, 200 µl of
−
1
0
.4 mol L EDTA solutions that give maximum recovery was found as optimum eluents. A
−
1
concentration of 0.4 mol l of EDTA was selected for further studies (Figure S10). To obtain the
highest sensitivity and precision, the equilibrium time of the adsorption process should be
determined. For this purpose, further experiments were performed for study of the extraction
time by changing the time in the range of 2ꢀ20 min. As can be seen in Figure S11, it was
observed that there is an increase of the extraction efficiency along with the increasing extraction
time from 2ꢀ10 min while further increasing in time had no significant on recovery. Thus, time of
1
0 min was applied for further experiments. This rapid adsorption process can be attributed to
the short diffusion route of the metal ions to the MOF and the MOF high porosity and surface
area that cause to reaching equilibrium in a shorter contact time. The effect of the desorption
time was also studied, and it was revealed that after 2 min, no substantial change occurs in the
desorption efficiency. (Figure S12)
3
.3. Method validation
The practical applicability of the proposed extraction method was evaluated by establishing a
number of performance parameters as the figures of merit such as linearity, limit of detection
(
LOD), preconcentration factor (PF), repeatability, and extraction recovery for extraction of the
target analytes from the aqueous solutions under the optimum conditions. As summarized in
Table 1, calibration curves plotted using ten spiking levels of the metal ions with three replicate

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−
1
measurements for each point, were found to be linear in the range of 0.2−200 ꢁg L with
2
coefficients of determination (R ) ranging from 0.995 to 0.998. The limits of detection (LODs)
based on definition of 3S /m (where, S is the standard deviation of three replicate blank
b
b
−
1
measurements, and m is the slope of the calibration curve) were between 0.05ꢀ0.2 ꢁg L . The
preconcentration factors (PFs), calculated as the ratio of the slopes of the preconcentration and
direct calibration equations were found to be in the range of 40−350. The precision of the
proposed method was evaluated in terms of repeatability and RSDs% < 5.3 (n = 5) were obtained
−
1
at a concentration level of 20 ꢁg L . The ER% was calculated according to the following
equation:
C
f
×V
f
ER =
×100
0
C
0
×V
Where, C and C are the concentration of analyte in the eluent phase (after preconcentration) and
f
0
the initial concentration of analyte in the sample solution, respectively, and V and V are eluent
f
0
and sample solution volumes, respectively.
Table 1. Figures of merit of the proposed method for extraction of the
target metal ions
a
2
b
c
d
Analyte
LDR
R
LOD
PF ER (%)
RSD (%)
(n = 5)
−
1
−1
(
ꢁg L )
(ꢁg L )
+
+
2
2
Cd
Co
0.5ꢀ200
0.998
0.2
84
17
3.3
0.5ꢀ200
0.2ꢀ200
0.2ꢀ200
0.5ꢀ200
0.3ꢀ200
0.995
0.998
0.996
0.997
0.2
0.05
0.05
0.2
40
230
270
350
90
8
3.8
2.3
4.8
5.3
+
3
Cr
46
54
70
18
+
2
2
2
Cu
+
Pb
+
Fe
0.996
0.1
3.1
a
b
c
d
Linear dynamic range. Limit of detection. Preconcentration factor. Data
−
1
were calculated based on the extraction of a 20 ꢁg L sample for each
analyte.

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3
.4. Determination of the metal ions in real samples
In order to investigate the reliability and analytical performance of the proposed method for
determination of metal ions in real samples, the proposed method was verified by analyzing 100
ml of some real water samples such as tap water, and mineral water before and after spiking with
known amounts of the target metal ions. The obtained results are summarized in Table 2. As can
be seen, the relative recoveries of the target metal ions ranged from 92% to 110% and acceptable
repeatability are indicating good performance of the proposed method to apply to real samples.
Table 2. Analytical results for extraction and determination of target metal ions in real samples
+2
+2
+3
+2
+2
+2
Cd
Co
Cr
Cu
Fe
Pb
Found ± SD Found ± SD
Found ± SD
Found ± SD
Found ± SD
Found ± SD
Tap water
Non spiked
Added (5 ꢁg L )
Recovery%
<LOD
5.3±0.11
106
<LOD
5.1±0.13
102
0.3 ±0.02
0.4 ±0.01
1.1 ±0.04
<LOD
0
5
0
ꢀ
1
5.4 ±0.11
5.4 ±0.08
6.3 ±0.13
4.8 ±0.07
7
3
1
9
103
99
104
98
ꢀ
1
Added (10 ꢁg L )
9.4 ±0.3
10.1 ±0.24
10.2 ±0.09
10.4 ±0.38
10.7 ±0.25
9.3 ±0.35
1
9
6
0
0
5
6
Recovery%
94
102
99
Mineral water
<LOQ
99
96
93
Non spiked
Added (5 ꢁg L )
Recovery%
Added (10 ꢁg L )
Recovery%
<LOD
<LOD
4.6±0.10
92
0.5 ±0.02
1.0 ±0.05
<LOD
4
1
ꢀ
1
4.6 ±0.2
5.1 ±0.20
6.0 ±0.18
6.4 ±0.09
5.1 ±0.27
4
1
2
5
4
4
92
10.1±0.34
101
102
110
109
102
ꢀ
1
10.2 ±0.17
9.6 ±0.11
11.0 ±0.25
11.8 ±0.10
9.4 ±0.36
4
3
5
7
3
102
96
Mineral water
<LOD
105
108
94
Non spiked
Added (5 ꢁg L )
Recovery%
Added (10 ꢁg L )
Recovery%
<LOD
<LOD
4.8±0.21
96
9.6±0.12
96
0.6 ±0.06
0.8±0.04
<LOD
4.8±0.22
96
1
ꢀ
1
5.1 ±0.0
5.3 ±0.17
6.3 ±0.33
5.8 ±0.10
2
7
3
1
3
7
102
106
10.2±0.28
102
110
10.9±0.13
103
101
11.5±0.08
107
ꢀ
1
9.8 ±0.4
9.7 ±0.35
4
6
98
97
3
.5. Evaluation of adsorption capacity of the MOFs
Forasmuch as the MOFs have porous structure, it was expected that they have high adsorption
capacity of metal ions. For investigation of their maximum adsorption capacities, the equilibrium
adsorption experiments mentioned in the experimental section were performed at different
concentrations of the metal ions and the obtained results were evaluated by Langmuir adsorption
isotherm. With regard to the maximum adsorption capacities (Q ) of the MOFs listed in Table 3,
m

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it can be seen that all the MOFs demonstrate high and nearly same maximum adsorption
capacities for each of the metal ions. This observation is in compliance with their high porosity
and approximately same free spaces (33%, 30%, 32% and 30% for TMUꢀ6, TMUꢀ21, TMUꢀ23
and TMUꢀ24, respectively).
3
.6. Comparison of the adsorption behavior of the MOFs
In this work, our purpose was evaluation of the role of functional groups being on the pillar of
the MOFs as well as their pore size on adsorption of metal ions. Accordingly, four isoreticular
zinc(II)ꢀbased MOFs were selected while two of them (TMUꢀ6 and TMUꢀ21) had amide group
on their pillars and the other pair (TMUꢀ23 and TMUꢀ24) contained imine group. On the other
hand, TMUꢀ6 and TMUꢀ23 contain one phenyl ring and TMUꢀ21 and TMUꢀ24 had two phenyl
rings in their pillar structures (Scheme 1). This selection allowed to evaluate the effect of both
type of the functional groups and number of phenyl rings in pillars of the MOFs on metal ions
adsorption.
For comparison of adsorption behavior of four isoreticular MOFs with different pillars, their
extraction recoveries (ER%) and maximum adsorption capacities (Q ) were calculated and listed
m
in Table 3. Comparing the ER% of four MOFs studied for metal ions adsorption, it can be seen
that MOFs with amide group (TMUꢀ23 and TMUꢀ24) show higher efficiency (greater ER%) for
metal ion adsorption rather than MOFs with imine group (TMUꢀ6 and TMUꢀ21). On the other
hand, the MOFs which contain phenyl group in their pillar structure to some extent have more
ER% than MOFs with naphthyl group in their pillars (TMUꢀ23 toward TMUꢀ24 and TMUꢀ6
toward TMUꢀ21).

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Furthermore, as comparing the values of Q obtained for four MOFs (see Table 3), it can be
m
found that all the MOFs provides nearly similar adsorption capacity for each of the metal ions
while this result is not similar to the result of the comparison of the ERs%.
Since the free spaces in all the MOFs are approximately equal (33%, 30%, 32% and 30% for
TMUꢀ6, TMUꢀ21, TMUꢀ23 and TMUꢀ24, respectively), it can be concluded that at high
concentrations of metal ions free space of the MOFs is determinative of their adsorption rate.
However, at low concentrations of metal ions, the interaction of the free coordination sites
(amide or imine groups) of the MOFs with metal ions has the major role in the adsorption
efficiency. The amide group having more basicity with two coordination sites (O and N)
demonstrates greater extraction efficiency than the imine group with lower basicity and one
coordination site (N). On the other hand, it seems having naphthyl group in pillar structure could
decline the charge density of the coordination sites (in amide or imine groups) by increasing the
resonance path length in phenyl rings that results to decrease in the adsorption efficiency (ER%,
that is a measure of adsorption efficiency).
Table 3. Comparison of extraction recoveries (ER) and maximum
adsorption capacities (Q ) of four MOFs for the target metal ions.
m
MOF
+2
+2
+3
+2
+2
+2
Cd
13
Co
7
Cr
12
Cu
15
Fe
12
Pb
21
ER
Qm
ER
Qm
ER
Qm
ER
TMUꢀ6
47
11
49
17
55
14
54
4
118
9
60
14
60
54
65
43
61
10
67
18
64
15
230
18
TMUꢀ21
55
8
121
46
221
67
TMUꢀ23
TMUꢀ24
47
8
108
38
267
56

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Qm
51
49
115
63
65
256
By comparing ERs% of TMUꢀ23 and TMUꢀ5 (the azineꢀdecorated MOF studied in details in
+
2
our previous work [41] for extraction of Pb ions, it can be found that TMUꢀ23 shows much
more ER% than TMUꢀ5, nearly twice the ER% of TMUꢀ5, while ERs% of these two MOFs for
+
2
the other ions studied were almost same. From this observation it can be concluded that Pb ions
have more tendency to the amide groups rather than azine groups while the other metal ions
+
2
+2
+2
+3
studied here (Co , Cd , Cu , Cr ) have approximately similar tendency to both functional
groups (amide and azine). Due to the high attraction force of Pb ions towards the aromatic ring
as well as the the amide functional groups, the ER is enhanced. [45]
3
.7. Theoretical calculations
To analyze the trend obtained for the cations interaction with different ligands, we first analyze
the MEP plots of the isolated ligands. Figure S14 shows the MEP isosurfaces on the 0.001
atomic unit electron density isosurface of these molecules, where positive and negative regions
are indicative of a nucleophilic and electrophilic site, respectively. Table 4 shows the calculated
most negative MEP values with these ligands. As evident from Figure S14, the negative MEP
regions are mainly associated with the nitrogen or oxygen atoms, which indicate the potential of
these sites to an electrophilic attack. More especially, the most negative MEP in TMUꢀ23 and
TMUꢀ24 is mainly associated with the oxygen atom of amide group, with the values of ꢀ29.5 and
ꢀ28.5 kcal/mol, respectively. All these ligands are also characterized by a distinct negative area,

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associated with the nitrogen atom of pyridine nitrogen. The absolute MEP value at the
nitrogen/oxygen atoms of these ligands decraeses as TMUꢀ23 > TMUꢀ24 > TMUꢀ6 > TMUꢀ21.
Note that this is almost consistent with the results obtained for the adsorption of the target
cations. In particular, the MEP analysis also properly predicts that TMUꢀ21 should have the
weakest interaction with the cation species. This finding is also verfied by the calculated
Mulliken atomic charges on the oxygen or nitrogen atoms (Table 4). This indicates that the
electrostatic effects should be mainly responsible for the formation of theses cationꢀligand
complexes.
Figure 4 presents the highest occupied molecular orbital (HOMO) and lowest unoccupied
molecular orbital (LUMO) of the free ligands. It is seen that in all cases, the HOMO is mainly
localized over the phenyl or naphtyl group and has a π character. This is in accordance with the
prediction of the MEP analysis as noted above, and reveals the potential of these regions to
interact with the cation species. As expected, the energy of the HOMO of TMUꢀ21 and TMUꢀ6
is more negative than those of TMUꢀ23 and TMUꢀ24 (Figure 4), which is in agreement with the
lower tendency of the former ligands to interact with the cations considered here. Note that in the
HOMO of TMUꢀ21 and TMUꢀ6, there are some important contributions from the pyridine
groups, which is due to resonance effects within these molecules. Such resonance effects should
be responsible for the stabilization of the HOMO state of these ligands and hence their lower
potential to interact with the cation species. Moreover, by comparing the HOMO energies of the
ligands of TMUꢀ6 and TMUꢀ21 as well TMUꢀ23 and TMUꢀ24 (Figure 4), it can be found that
the HOMO of TMUꢀ21 and TMUꢀ24 ligands have more negative energies than those of TMUꢀ6
and TMUꢀ23, respectively, indicating more resonance effect of naphthyl group respect to phenyl
group; that this is consistent with the obtained experimental results.

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On the other hand, the LUMO state of these free ligands is delocalized over the entire molecular
framework and has a π* character. As evident from Figure 4, the energies of these LUMOs
increase (in absolute value) as TMUꢀ6 > TMUꢀ21 > TMUꢀ23 > TMUꢀ24. As seen, the LUMO of
TMUꢀ6 and TMUꢀ21 exhibits a more pronounced delocalization on the central phenyl or
naphthyl group than TMUꢀ23 and TMUꢀ24 ligands. These results suggest that the different
electronic structure of these ligands should be responsible for the obsereved adsorption
efficiency of the MOFs towards the considered heavy metal ions.
Table 4. The calculated most negative MEP value (VS,min) and Mulliken atomic charge (Q) over nitrogen
or oxygen atom of the isolated ligands
Ligands
TMUꢀ6
VS,min (kcal/mol)
ꢀ16.7
Q (e)
ꢀ0.157
ꢀ0.109
ꢀ0.561
ꢀ0.550
TMUꢀ21
TMUꢀ23
TMUꢀ24
ꢀ13.8
ꢀ29.5
ꢀ28.5

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Figure 4. The frontier molecular orbitals (isovalue=0.02 atomic unit) and their energies (in atomic
unit) of the free ligands
4
. Conclusions

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Here, we examined adsorption behavior of some MOFs having different functional groups in
their pillar structures for adsorption of some heavy metal ions. For this purpose, four isoreticular
zinc(II)ꢀbased MOFs were selected and synthesized by mechanochemical method such that their
differences were in type of the functional group (imine or amide) and being phenyl or naphthyl
group in their pillar structures. It was found that being of amide and phenyl groups in the pillars
causes to its more tendency to adsorption of metal ions compared to the MOF with imine and
naphthyl groups. This can be attributed to more basicity of the amide group respect to the imine
group that results to the more strength interaction of metal ions with the MOFs coordination
sites. On the other hand, the more resonance of electron density of the functional groups by the
naphthyl group compared to the phenyl causes to decline of the adsorption ability of the MOFs.
The evaluation of the adsorption behavior of the metal ions by four MOFs at high concentrations,
found that all four MOFs having almost same free space show similar Q for each of the metal
m
ions. Moreover, some theoretical studies were desighed and performed to verify the experimental
results. According to the DFT calculations, the absolute MEP values at the nitrogen/oxygen
atoms of the ligands corresponding to the MOFs decreased as TMUꢀ23 > TMUꢀ24 > TMUꢀ6 >
TMUꢀ21, indicating decrease in the charge density of the related functional groups. This also
reveals that the electrostatic interactions between the functional groups of the MOFs and the
metal ions is mainly responsible for the metal ion adsorption on MOFs. Furthermore, from
frontier molecular orbital analysis, it was found that the lower tendency of TMUꢀ6 and TMUꢀ21
ligands to adsorb the metal cations can be attributed to the resonance effects within these
molecules. This means that the electronic structure of the ligands is responsible for the different
adsorption pattern of the MOFs towards the considered heavy metal ions. Eventually, it can be
said that TMUꢀ23 with proper functional groups, high porosity and good water stability,

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providing high adsorption efficiency can be the promising material for application in extraction
+
2
and removal of metal ions from water samples especially for Pb ions.
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New Journal of Chemistry
Page 26 of 26
View Article Online
DOI: 10.1039/C8NJ01150H
Functional group effect of Isoreticular metal-organic frameworks on
heavy metal ion adsorption
a
a
b
c
*a
Leili Esrafili Vahid safarifard, Elham Tahmasebi, MD Esrafili, Ali Morsali,
We examined adsorption behavior of some MOFs having different functional groups in their
pillar structures for adsorption of some heavy metal ions
.
1