Facile fabrication of bimetallic Fe-Mg MOF for the synthesis of xanthenes and removal of heavy metal ions

Article abstract of DOI:10.1039/c9ra10300g

This work reported the preparation of Mg-MOF, Fe-MOF and Fe-Mg MOF by a solvothermal technique and their characterization with FT-IR, XRD, SEM, EDS, TEM and S_BET analyses. The nanoparticle diameter ranged from 3.1 to 10.9 nm. The acidity of the

Full text of DOI:10.1039/c9ra10300g

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Facile fabrication of bimetallic Fe–Mg MOF for the
synthesis of xanthenes and removal of heavy metal
ions†
Cite this: RSC Adv., 2020, 10, 9693
W. S. Abo El-Yazeed, *ab Y. G. Abou El-Reash, L. A. Elatwy and Awad I. Ahmed *a
a
a
This work reported the preparation of Mg-MOF, Fe-MOF and Fe–Mg MOF by a solvothermal technique and
their characterization with FT-IR, XRD, SEM, EDS, TEM and SBET analyses. The nanoparticle diameter ranged
from 3.1 to 10.9 nm. The acidity of the MOFs was measured by nonaqueous potentiometric titration of n-
butylamine. It was observed that the formation of a bimetallic MOF sharply increases the surface acidity and
the catalytic activity. The catalytic results of the Fe–Mg MOF catalyzing the synthesis of 14-aryl-14-H-
dibenzo[a,j]xanthenes in comparison with those of parent MOFs showed a higher yield of the desired
product in a lower time and among various Fe : Mg, the (0.6 : 1) Fe–Mg MOF showed the highest
th
catalytic activity and acidity. Even after the 4 run, the Fe–Mg MOF catalyst still maintained nearly the
initial catalytic activity. The adsorption performance of Mg-MOF, Fe-MOF and Fe–Mg MOF was
evaluated by batch experiments. The effect of contact time, the solution pH, the adsorbent dose and the
initial concentration of the heavy metal ions was discussed. It was found that the capacity of the
bimetallic Fe–Mg MOF for Pb(II), Cu(II) and Cd(II) adsorption was higher than that of the Mg-MOF and Fe-
MOF, the kinetic data followed the pseudo-second-order kinetic model and the isothermal data obeyed
the Langmuir isotherm model. The mechanism of the removal of the heavy metal ions was discussed.
Received 8th December 2019
Accepted 28th February 2020
DOI: 10.1039/c9ra10300g
rsc.li/rsc-advances
6
ligands. MOFs have been used in different potential applica-
1
. Introduction
7
8
9
tions as drug delivery, catalytic reactions, sensing and gas
10
Metal–organic frameworks (MOFs) are a class of compounds adsorption/separation.
constructed by the coordination of clusters or metal ions with
Xanthenes compounds are considered as one of the most
organic ligands to compose one, two, or three-dimensional pore important organic compounds due to their importance in the
1
structures. Recently, MOFs have received great attention owing formation of biologically active compounds such as blood palette
to their excellent properties, such as versatile porous structures aggregation inhibitor drugs, calcium channel blocker, antibac-
2
and numerous potential applications. MOFs are unique and terial, antiviral, anti-inammatory, antihypertensive and anti-
different to other traditional porous solids owing to their tumor. They are also very useful in laser technologies, uorescent
interesting properties. These include a simple composition that materials for visualization of biomolecules, dyes and photody-
3
11
is easy to prepare compared with zeolites, the presence of namic therapy. Moreover, several polycyclic compounds con-
coordinative unsaturated active canters, tunable pore structures taining xanthene skeleton have been isolated from natural
4
and ultrahigh surface area. Moreover, MOFs have an important sources as plant species for their prospective bio-activity. Various
advantage that various frameworks can be formed by the methods for the formation of xanthenes derivatives have been
5
12,13
participation of several metal cations. MOFs can be synthe- reported in the literature.
Numerous catalysts have been used
14
sized with specic properties to improve their performances in in the preparation of xanthene, for example, Fe -HAD-SO
reaching the desired targets by deliberately and systematically sulfated starch, ZrO , WO /ZrO (ref. 17) and sulfamic acid/Cr-
2 x 2
setting their functionalities and structures; for example, the MIL-101. Most of the preparation methods suffer from some
surface area, pore size and/or shape can be controlled by varying drawbacks such as constraints of catalyst reusability, separation
the connectivity of the cations and the type of the organic of pure products, difficult workup, use of hazardous solvents and
prolonged reaction times.
O
H,
3
3
4
15
16
8
Recently; MOFs have received great attention in adsorption
a
Chemistry Department, Faculty of Science, Mansoura University, Mansoura, Egypt.
of heavy metal ions owing to unique structural features that are
E-mail: dr_ws2008@mans.edu.eg; dr.ws20087@gmail.com; awahmed@mans.edu.eg
b
8
not found in other porous materials. In general, the elastic and
Department of Chemistry, College of Sciences and Humanities in Al-Kharj, Prince
extremely porous structures of MOFs facilitate the diffusion of
the guest ions or molecules easily into the bulk structure. Also;
both the size and shape of the formed pores help in increasing
Sattam Bin Abdulaziz University, Al-Kharj 11942, Saudi Arabia
†
Electronic supplementary information (ESI) available. See DOI:
0.1039/c9ra10300g
1
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the selectivity of adsorbing denite ions or molecules. These performed using PW150 (Philips) using Cu Ka radiation source
distinct advantages make MOFs ideal sorbents in solid phase and Ni lter. The instrument was operated at a voltage of 40 kV
18
extraction of heavy metals. It was reported by the World Health and a current of 45 mA. TEM (transmission electron micros-
Organization (WHO) in 2008 that the maximum contaminant copy) images were obtained using a JEOL-JEM-2100 transition-
ꢀ
1
level (MCL) of lead in drinking water was 10 mg L . Conse- electron-microscope. SEM (scanning electron microscopy) and
quently, numerous techniques were applied for the removal of EDS (energy dispersive X-ray spectroscopy) images for surface
heavy metal ions from environmental samples, including morphological evaluation and composition were obtained
19
20
21
membrane ltration, ion exchange and/or adsorption.
using JEOL-JSM-6510LV scanning-electron-microscope, Japan.
Adsorption technology is preferred in comparison to the other The porosity of the MOF materials and BET surface area are
ꢁ
traditional techniques due to its superior efficiency and easy examined by N adsorption/desorption analysis at ꢀ196 C.
2
22
application. Various adsorbents have been used such as acti-
23
24
25
vated carbon, zeolite, ion imprinted polymer and carbon 2.4. Surface acidity measurements
26
nanotubes. However, the applications of these adsorbents
have shown some weakness. Therefore some studies have been
carried out with the intention of developing new, cheap and
sustainable adsorbents for the removal of heavy metal ions from
wastewaters. These adsorbents should possess high affinity,
selectivity and capacity towards these metals.
Here, bimetallic Fe–Mg MOFs with different Fe : Mg molar
ratios have been successfully synthesized via the solvothermal
method. Fe–Mg MOFs was utilized for the removal of Pb(II),
Cd(II) and Cu(II) from aqueous solution and as a solid Lewis acid
catalyst for the synthesis of 14-aryl-14-H-dibenzo[a,j]xanthenes.
The structure of the bimetallic Fe–Mg MOF was elucidated by
The surface acidity of MOFs have achieved by nonaqueous
titration of 0.03 g of MOF suspended in 10 mL acetonitrile with
ꢀ1
(
0.025 N) n-butylamine at 0.005 mL min , which is a basic
molecule suitable for the titrating strong acid and medium sites
28
present on the surface of the catalysts. The various electrode
potential values were measured using a double-junction elec-
trode of an Orion 420 digital A model.
2.5. Catalytic activity
A mixture of (1 mmol) benzaldehyde, (2 mmol) 2-naphthol, and
(
0.05 g) MOF was placed in a round bottom ask in an oil bath
ꢁ
and tted to a condenser at 120 C for 2 h for the synthesis of 14-
aryl-14-H-dibenzo[a,j]xanthenes. The progress in the reaction
was adjusted using TLC. Aer cooling, the reaction mixture was
placed onto crushed ice and mixed for a few minutes. The
product was ltrated, washed with cold water and aer that
recrystallized by using ethanol to yield the pure product which
was calculated as follows:
(FT-IR), (XRD), (SEM) and (TEM) analyses.
2. Experimental
2.1. Materials
Mg(NO $6H O, Fe(NO ) $9H O, terephthalic acid and dime-
3
)
2
2
3 3 2
thylformamide (DMF) from Sigma Aldrich. Lead nitrate, copper
nitrate and cadmium nitrate which provided by BDH. N-Butyl-
amine, acetonitrile, ethyl acetoacetate, benzaldehyde, urea and
Obtained weight of the product
Yield ðwt%Þ ¼
ꢂ 100 (1)
Theoretical weight of the product
2-naphthol were used as received.
The obtained product (14-aryl-14-H-dibenzo[a,j]xanthenes,
pale yellow solid) was characterized by m.p, 184–185 C, and FT-
2.2. Preparation of MOFs
ꢁ
Fe–Mg bimetallic organic frameworks with different molar IR (n ¼ 742, 805, 960, 1076, 1250, 1401, 1456, 1512, 1594, 1628,
3
+
2+
ꢀ1
ratios of Fe : Mg , (0.6 : 1 and 1 : 1), were synthesized as re- 2854, 2924, 3061 cm
.
27
ported previously. 1 mmol of Mg(NO ) $6H O, Fe(NO ) $9H O
3
2
2
3 3
2
(
0.6 mmol or 1 mmol) and 1 mmol of terephthalic acid (H BDC)
2
2.6. Batch adsorption experiment
were dissolved in 10 mL of DMF under constant stirring. The
mixture was blended completely and heated at 120 C in
a Teon-lined autoclave for 8 h. Then, the suspension was
A series of 150 mL Erlenmeyer asks were lled with 100 mL of
heavy metal ions. 0.03 g MOF was added to each conical ask
and shaken in a thermostated shaker at 150 rpm. Then, 10 mL
of the solution was collected through centrifugation at
ꢁ
cooled gradually to room temperature, collected by centrifuga-
ꢁ
tion, washed with DMF several times and dried at 120 C under
4000 rpm for 10 min. The ltrate was collected in a polyethylene
vacuum. The resulted brown powder were denoted as (0.6 : 1)
Fe–Mg MOF and (1 : 1) Fe–Mg MOF.
tube and the concentrations of Pb(II), Cu(II) and Cd(II) were
determined by dilution. 1 mL from the ltered solution with
In the same manner, monometallic Fe-MOF and Mg-MOF
2
1
mL of 4,2-pyridylazo resorcinol (PAR) and 10 mL buffer (pH ¼
were synthesized using only Fe(NO
6H O as the metal precursor.
3 3 2 3 2
) $9H O or Mg(NO ) -
0) were prepared in measuring ask, then the volume was
$
2
completed by distilled water to 25 mL, and Unicom 5625 UV/Vis
Spectrophotometer, PerkinElmer USA was used for the deter-
mination of heavy metal ions concentration. The effect of the
2.3. Instrumentation
The FTIR spectra were measured using Thermo SCIENTIFIC initial solution pH on metal ion removal was determined for pH
NICOLET iS10) FTIR spectrometer from Thermo Electron values of 3–7 by the batch removal process described above. The
(
ꢀ
1
Corporation in the range 400–4000 cm for a sample of 2 mg pH was adjusted with 0.1 M HNO
3
and 0.1 M NaOH to prepare
diluted with 200 mg KBr. Powder X-ray diffraction was acidic and neutral solutions, respectively. An adsorption
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kinetics study of the removal of Pb(II), Cu(II) and Cd(II) by MOF
was carried out at the optimum conditions.
3. Results and discussion
3.1. Characterization
3
.1.1. FT-IR analysis. The FTIR spectra of Fig. 1 display
ꢀ
1
a main characteristic peak at ꢃ3450 cm assigned for the
H–O–H stretching vibrations. The peaks which are observed at
598 cm for Mg-MOF and Fe-MOF, 1646 cm of (0.6 : 1) Fe–
Mg MOF and 1665 cm of (1 : 1) Fe–Mg MOF have been spec-
ied for the strong C]O (–COOH) stretching vibration that may
be ascribed to the existence of terephthalic acid in the MOF.
ꢀ1
ꢀ1
1
ꢀ1
29
ꢀ1
The appeared peak ꢃ1390 cm is due to the C–O stretching
ꢀ1
vibration. The three peaks at 1015, 1100 and 1145 cm were
attributed to the bending vibration of the hydroxyl group bound
29
to metal ions (M ꢀ OH) and the substitution of the aromatic
ꢀ1
ring is appeared at 825 cm . These results suggest that the
structure of Fe-MOF, Mg-MOF and Fe–Mg-MOF contains the
basic terephthalic acid skeleton and the carboxyl groups of
terephthalic acid are deprotonated. In addition, the bands at
Fig. 2 XRD of the prepared MOFs.
ꢀ1
7
58, 760 and 752 cm
vibrations.
were ascribed to metal–oxygen
30
diffraction peaks of Fe–Mg MOF (0.6 : 1) and each of Mg-MOF
and Fe-MOF, bimetallic MOF records slight shi toward lower
3
.1.2. X-ray diffraction analysis. The crystallization behav-
iors of Fe-MOF, Mg-MOF and ((0.6 : 1) and (1 : 1)) Fe–Mg MOF
are investigated using XRD analysis and were shown in Fig. 2.
XRD patterns characteristic of Fe-MOF are recognized at 2q ¼
2q and the disappearance of the diffraction peaks characteristic
for Mg-MOF and Fe-MOF conrming the incorporation of Fe in
the as-synthesized architectures. By increasing the Fe : Mg
molar ratio to 1 : 1, the intensity of these XRD peaks increases
indicated that the as-prepared Fe–Mg MOF showed the typical
diffraction pattern of Fe–Mg MOF, which simultaneously
possessed characteristic peaks belonging to Mg-MOF and Fe-
MOF. This can be attributed to the high dispersion of Fe crys-
tals on the outer cavities of Mg-MOF surface and inside the
9
4
.59, 14.31, 18.18, 19.23, 21.85, 24.83, 28.76, 29.70, 33.78, 39.43,
ꢁ
0.34, 41.84, 43.74, 45.25, 46.88, 48.72 and 50.56 (ref. 31) and
that of Mg-MOF are recorded at 2q ¼ 9.33, 12.52, 18.64, 23.68
ꢁ
and 28.24 (ref. 32) conrming the formation of metal–organic
33,34
framework
with well-developed crystallinity and without any
oxide impurities. From the comparison of the low angle
35,36
pores.
In addition, the observed simultaneous shis of the
XRD patterns to lower and higher angles may be ascribed to the
37–39

exibility and/or the breathing effect of Fe-MOF structure.
By increasing the ratio of Fe : Mg to 1 : 1, the Fe species
aggregated on the surface of Mg-MOF which in role decreases
the synergistic effect between the two metals and so the inten-
sity of the diffraction peaks characteristic of both Fe and Mg is
increased. By applying Scherer eqn (2) for the rst peak (111),
the crystal size can be easily determined. As seen from the
results that tabled in Table 1, the crystal size is decreased by
increasing the ratio of Fe : Mg to 0.6 : 1 then begins to slightly
increase by increasing the ratio to 1 : 1 conrming the effect of
Fe species on Mg-MOF.
0
:9l
D ¼
(2)
b cos q
ꢀ
where l is radiation wavelength (l ¼ 1.54 A), D is crystal size, b is
the line breadth (radians) and q ¼ angle of reection.
3.1.3. SEM, TEM and EDS studies. The surface morphol-
ogies of MOF samples were examined by photographing the
surface using scanning and transmission electron microscopes.
The images of Mg-MOF and Fe-MOF are shown in Fig. 3A(a, b)
and B(a*, b*). As shown from the gures the crystals of Fe-MOF
Fig. 1 FTIR spectra of the prepared MOFs.
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Table 1 Crystallite size, surface area acidity and activity of the prepared MOFs
Crystallite size (nm)
No. of acid
sites/g ꢂ 10
Yield (%) of
14-aryl-14-H-dibenzo[a,j]xanthenes
2
ꢀ1
ꢀ19
Sample name
From XRD peak (111)
From TEM
S
BET (m g
)
E
i
(mV)
Mg-MOF
Fe-MOF
10.6
12.1
4.5
11.0–22.3
8.0–9.6
3.1–3.9
774
983
1320
1162
116.2
137.3
273.1
180.5
16.9
28.9
63.7
34.9
38.4
41.4
93.5
72.8
(0.6 : 1) Fe–Mg MOF
1 : 1) Fe–Mg MOF
(
6.1
6.5–10.9
are shaped as a rigid zeotype octahedral structure while that of consequently new active sites by the presence of both two
40,41
1
Mg-MOF appeared as column-like with irregular sizes.
metals Fe and Mg together in the bimetallic MOF. The surface
Fig. 3A(c, d) and B(c*, d*) showed the SEM and TEM images of area of (0.6 : 1) Fe–Mg MOF is high compared with that of (1 : 1)
bimetallic Fe–Mg MOF. In the SEM images; the aggregation of Fe–Mg MOF. This can be explained, as discussed above, due to
Fe–Mg MOF crystals became more visible which shows a clear the synergistic effect between Fe and Mg metals and the high
differentiation between both single and bimetallic MOF crys- dispersion of Fe crystals inside the pores and on the outer
35,36
tals. Fe-MOF with a diameter of 8.0–9.6 nm (from TEM cavities of Mg-MOF surface.
This synergistic effect decreases
measurements, Table 1) is rstly dispersed in the pores of Mg- in (1 : 1) Fe–Mg MOF material due to the aggregation of Fe
MOF with diameter 11.0–22.3 nm then on the external surface. species on the surface of Mg-MOF. The BET results conrm the
However, in TEM images of (1 : 1) Fe–Mg MOF; the observed results of XRD, TEM and SEM analysis.
larger dark spots on the Mg-MOF surface have resulted from the
formation of aggregates of Fe-MOF nanocrystals. Similar
agglomerations about bimetallic MOFs were observed in other
3
.2. Surface acidity
The surface acidity of the MOFs was determined using
a potentiometric titration method using a basic molecule as n-
butylamine. The initial electrode potential (E ) obtained from
i
this technique displays the maximum acid strength of the
catalyst surface while the overall issue of acid sites per gram
solid catalyst (meq/g) has been obtained from the domain where
a plateau is extended. The acidity measurements resulted from
the potentiometric titration curves, Fig. 6 of MOFs catalysts
were recorded in Table 1. The following equation successfully
used to calculate the overall issue of acid sites/gram catalyst:
42,43
literature.
The SEM and TEM images are in good agreement
with the XRD results in which for (0.6 : 1) Fe–Mg MOF the peak
corresponding to Fe-MOF was found to disappear and on
increasing Fe, large aggregates were formed on Mg surface. The
EDS spectrum (Fig. 4) of (0.6 : 1) Fe–Mg MOF conrms the
presence of both Fe and Mg in the bimetallic MOF.
3.1.4.
SBET measurements. The porosity of the MOF mate-
rials and BET surface area are examined by N
2
adsorption/
ꢁ
desorption analysis at ꢀ196 C. The isotherms, Fig. 5, reveal
that all the prepared MOFs show type I isotherms typical of
microporous structure. The SBET of Fe-MOF, Mg-MOF, (0.6 : 1)
Fe–Mg MOF and (1 : 1) Fe–Mg MOF are 774, 983, 1320 and 1162
Total number of acid sites
g
m
equiv:
N
¼
ꢂ
(3)
g
1000
2
ꢀ1
m g , respectively. As illustrated the bimetallic MOFs exhibit
higher surface area than the both of monometallic MOFs. This
may be ascribed to the creation of new micropores and
where N is Avogadro's number.
It was proposed that the sites with E
i
> 100 mV are very strong
sites, the sites with 0 < E < 100 mV are strong sites, the sites with
i
Fig. 3 (A) SEM images of Mg-MOF (a), Fe-MOF (b), (0.6 : 1) Fe–Mg MOF (c) and (1 : 1) Fe–Mg MOF (d). (B) TEM images of Mg-MOF (a*), Fe-MOF
b*), (0.6 : 1) Fe–Mg MOF (c*) and (1 : 1) Fe–Mg MOF (d*).
(
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Fig. 4 EDS image of (0.6 : 1) Fe–Mg MOF.
ꢀ
100 < E
very weak sites.
i
< 0 are weak sites and the sites with E
i
< ꢀ100 mV are
28,44,45
From Table 1 and Fig. 6 and according to Fig. 6 Nonaqueous potentiometric titration for the prepared MOFs.
the above classication, both pure Mg-MOF and pure Fe-MOF
samples exhibit strong acid sites, E ¼ 116.2 and 137.3 mV,
i
respectively. For bimetallic MOF, the incorporation of Fe-MOF benzaldehyde and 2-naphthol in presence of the prepared
into Mg-MOF increases the surface acidity and the acid MOFs. The yield of xanthenes in the presence of pure Mg-MOF
strength (E
i
¼ 273.1 mV) for the sample (0.6 : 1) Fe–Mg MOF as crystals was found to be 38.4% while in presence of Fe-MOF was
a result of the high surface area and high synergistic effect found to be 41.4%, the yield was signicantly increased over
between Fe and Mg metals due to the dispersion of Fe-MOF bimetallic MOF as shown in Fig. 7. The maximum yield (93.5%)
particles on the surface and within the pores of Mg-MOF as was obtained for (0.6 : 1) Fe–Mg MOF catalyst which has the
evident from XRD, TEM and SBET analysis. The decreases in the maximum number of acid sites. The yield is improved
surface acidity and the acid strength of (1 : 1) Fe–Mg MOF, (E ¼ remarkably using bimetallic system than monometallic ones in
i
180.5 mV) may be related to the decrease in the surface area as a good agreement with the surface acidity and the total number
a result of the aggregation Fe-MOF species and formation of of acid sites. This can be ascribed to a synergetic interaction
multilayer on the Mg surface which in role decreases the between Fe and Mg in Fe–Mg MOF which plays an important
35,46–48
synergistic effect between the two metals as mentioned above in role in catalytic reactions.
The decrease in the catalytic
XRD, SEM, TEM and SBET results.
activity of the sample (1 : 1) Fe–Mg MOF was due to the
agglomeration of Fe nanoparticles on the surface of Mg which
3
.3. Catalytic activity
.3.1. Synthesis of 14-aryl-14-H-dibenzo[a,j]xanthenes. 14-
aryl-14-H-dibenzo[a,j]xanthenes can be synthesized using
3
Fig. 7 Effect of the total number of acid sites of the prepared MOF on
synthesis of 14-aryl-14-H-dibenzo[a,j]xanthenes. The error bars
represent the standard error.
Fig. 5
2
N adsorption–desorption isotherms of the prepared MOFs.
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decrease acidity and activity, Fig. 7. The reaction can occur via
Fe and Mg metals which considered as a Lewis acid catalyst
which activates the carbonyl group of the benzaldehyde. Then,
a fast Knoevenagel addition was occurred by condensation of
one molecule of b-naphthol to provide an intermediate followed
by Michael-type addition of the active methylene of the second
molecule of b-naphthol to the intermediate followed by cycli-
zation and hydrolysis afforded the nal product 14-aryl-14-H-
ðC ꢀ C ÞV
0
t
q
t
¼
(4)
m
where C
(
0
and C
t
are the initial and equilibrium concentration
mg L ) of the metal ions in the solutions under investigation
at the time (t), m is the mass of the MOF and V is the volume of
the solution (L).
As shown in Fig. S1,† the removal of Pb(II), Cd(II) and Cu(II)
ions by (0.6 : 1) Fe–Mg MOFs began fast then increases notice-
ably with the prolongation of time until equilibrium is reached
at 75 min. There is no obvious variation in the adsorbed cations
was observed aer 75 min of contact. According to these results,
ꢀ1
49–51
dibenzo[a,j]xanthenes.
.3.2. Reusability of the MOF. From the viewpoint of green
3
chemistry, we tried hard to design a green and reused catalyst.
We performed the reusing test of the MOF in the model reaction
between benzaldehyde (1 mmol) and 2-naphthol (2 mmol) in
7
5 min has been taken as the equilibrium time for Pb(II), Cd(II)
and Cu(II) ions in batch adsorption experiments. At the begin-
ning of adsorption, binding (active) sites of adsorbent were
freely available to bind adsorbate, which saturated as time
proceeds. The exhausted binding sites aer saturation repels
the coming adsorbate, also the concentration gradient between
the solution and solid surface has been changed. Previously
studies also have been reported that with the time proceeds the
adsorption may decrease due to partial coverage of the adsor-
ꢁ
the presence of 0.03 g of (0.6 : 1) Fe–Mg MOF at 120 C. Aer
each run, the MOF was separated, washed with ethanol and
ꢁ
dried at 120 C. From Fig. 8A, (0.6 : 1) Fe–Mg MOF can be
recovered and reused in the xanthenes formation reaction four
times with remarkably similar activity proving that Fe has been
effectively incorporated in the framework of Mg MOF. The FTIR
and XRD analyses were determined for the fresh and reused
57
(0.6 : 1) Fe–Mg MOF as shown in Fig. 8B and C. To detect any
5
8
bent active sites.
.4.2. Effect of pH. The solution pH is an important vari-
able inuencing the acid–base property of the adsorbent
leached amount of Fe or Mg ions during the reaction, the
leaching of the catalysts was tested by taking a small amount of
the ltrate aer centrifugation during recycling and analyzing it
by the atomic absorption spectroscopy. No traces are found
from both metals in the ltrate which indicate no leaching
process is occurred during the reaction.
3
surface. Fig. 9 showed that the adsorption of Pb(II) is increased
ꢀ1
ꢀ1
ꢀ1
from (108.4 mg g ) at pH ¼ 3 to reach maximum (140.6 mg g
)
)
at pH 5, but the removal of Cd(II) is increased from (44.3 mg g
ꢀ1
at pH 4 to reach maximum around pH 7 (134.4 mg g ), while,
Previously reported synthesis of pharmaceutically 14-aryl-14-H-
dibenzo[a,j]xanthenes compared with our catalysts showed that the
efficiency of Fe–Mg MOF was so higher than that of several re-
ꢀ1
the removal of Cu(II) is increased from 82.5 mg g at pH ¼ 3 to
ꢀ1
reach 95.6 mg g at pH ¼ 5. Thus, the optimum pH is achieved
5
9
at pH ¼ 5 for Pb(II), Cu(II) and 7 for the removal of Cd(II). At low
pH (pH less than 3), MOF offered a positive surface charge,
52
2 4
ported catalysts e.g. nano-ZnAl O , acid activated clay in Kerman
5
3
54
Province, graphene oxide-copper ferrite nanocomposite, palla-
dium nanoparticles supported on amino-functionalized MOFs and
6
0
since functional groups of the surface were protonated, which
gives rise to electrostatic repulsion among MOF and Pb(II), Cu(II)
and Cd(II) resulting in low cationic metal ions removal. When
the pH of the solution is increased, a negative surface charge
appeared on the MOF surface because of the deprotonating of
function groups which was favorable for the surface electro-
static attraction of positively charged Pb(II), Cu(II) and Cd(II)
55
zinc oxide nanoparticles. The comparison study not only depen-
ded on the percentage yield of the desired product but also the
condition required optimizing the reaction.
found in solution which make the ion exchange process
3
.4. Adsorption of Pb(II), Cd(II) and Cu(II) ions by Mg, Fe and
+
occurred. In acidic solution, hydrated hydrogen ions (H O )
Fe–Mg MOFs
3
competed with Pb(II), Cd(II) and Cu(II) on the available adsorp-
Pb(II), Cd(II) and Cu(II) ions present in positive forms; therefore,
can easily undergo favorable electrostatic interactions with the
MOF adsorbents having negatively charged framework
+
tion sites. As the pH increases, the concentration of H O
3
decreases resulting in increasing the adsorption rate of Pb(II),
Cd(II) and Cu(II). Moreover, increasing the adsorption capacity
with increasing the pH is due to the greater stability of MOF
structure in addition to the increase of the negatively charged
active centers. Furthermore, considering precipitation of both
(surface). Mg, Fe and Fe–Mg MOF were successfully used to
adsorb Pb(II), Cd(II) and Cu(II) ions which are different in ionic
radius. The individual inuences of solution pH, adsorption
time, initial cations concentration and adsorbent dose on the
uptake efficiency of Pb(II), Cd(II) and Cu(II) ions by MOF have
been studied to detect the optimum range of every single
operating factor. In order to achieve this, we noticed the inu-
ence resulted by changes in a single parameter while the other
operating parameters were maintained constant.
+
+
(
Pb(OH) and Pb(OH) ), (Cu(OH) and Cu(OH) ) that appear at
2
2
+
pH > 5 and precipitation of both (Cd(OH) and Cd(OH) ) that
2
seem at pH > 7.
3.4.3. Adsorbent dose. The effect of adsorbents weight is
one of the most signicant factors that affect the adsorption. In
this test different amounts of MOFs (10–70 mg) were added to
3.4.1. Effect of contact time. Fig. S1† shows the effect of
ꢀ1
5
0 mL solution with concentration of (100 mg L ) and appro-
changing the time of adsorption from 10 to 240 min on the
adsorption of Pb(II), Cd(II) and Cu(II) ions. The adsorbed amount
of metal ions at any time q (mg g ) is calculated by eqn (4):
t
ꢁ
priate pH, then shacked for 2 h at 25 C and by UV/Vis spec-
trophotometer, the adsorption of Pb(II), Cd(II) and Cu(II) were
ꢀ
1
56
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Fig. 8 (A) The reusability of (0.61) Fe–Mg MOF in the synthesis of xanthenes, (B) XRD and (C) FTIR of the fresh and reused (0.61) Fe–Mg MOF. The
error bars represent the standard error.
evaluated. Fig. S2,† shows the adsorption amount as a function of MOFs catalyst from (10–50 mg) and then almost constant.
of the mass of MOFs. The experimental results demonstrated This is may be due to the increase of the number of the
that the adsorption is increased with the increase in the weight deprotonating function groups on the solid MOF by increasing
its weight up to 50 mg which was favorable for the surface
61–63
electrostatic attraction of positively charged cations.
The
almost constant adsorption aer 50 mg of MOF may be attrib-
uted to the saturation of the surface active sites available for the
adsorbed cations.
3.4.4. Effect of the initial concentration of the heavy metal
ions. The adsorption isotherms of the Pb(II), Cd(II) and Cu(II) by
MOFs were studied in a series of batches using various initial
ꢀ1
ꢁ
concentrations ranging from (50–200 mg L ) at 25 C. From
Fig. S3,† It is clear that the adsorption increases with the
increase in the initial concentrations of the heavy metal ions up
ꢀ
1
to 150 mg L then almost steady constant. This can be ascribed
to the saturation of most available active sites of MOFs with
59
raising the initial concentration of Pb(II), Cd(II) and Cu(II).
Cations adsorption efficiency were signicantly higher for
ꢀ
1
initial concentration up to 150 mg L due to plenty of freely
active sites on the adsorbent surface which has been saturated
at extremely higher cation concentrations and as a result, the
Fig. 9 Effect of pH on the removal of Pb(II), Cd(II) and Cu(II) ions by (0.6 : 1)
Fe–Mg MOF sample. The error bars represent the standard error.
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2
adsorption slowed increases due to the competition between R factor ranging from 0.9980 to 9982 for Pb(II), 0.981 to 0.998
8,57
ions for the available binding sites.
for Cd(II) and 0.985 to 0.999 for Cu(II), respectively, represents
3.4.5. Adsorption isotherms. For a solid–liquid system, one the accurate t of the experimental data and more acceptable
of the most important physico-chemical studies in the charac- than that of Freundlich isotherm. It gives an indication of the
terization of sorption behavior is the equilibrium isotherm of monolayer adsorption of Pb(II), Cd(II) and Cu(II) ions on the
sorption which used to characterize the experimental adsorp- homogeneous surface of Fe–Mg MOF adsorbent. The maximum
tion data. The Langmuir isotherm is the most applied adsorp- monolayer capacity obtained from Langmuir model for the
tion model and aspects monolayer sorption behavior on the uptake of Pb(II), Cd(II) and Cu(II) is found to be 196, 191 and
ꢀ1
adsorbent surface which contains a homogeneous and denite 175 mg g , respectively. Moreover, the dimensionless separa-
64
number of active sites. The model generally expressed in the tion constant R values between 0.033–0.213, 0.117–0.293 and
L
well-known form represented in the following equation as:
0.020–0.227 for of Pb(II), Cd(II) and Cu(II) respectively indicate
the favorability of the sorption process for the three cations,
also the value of 1/n lower than unity is indicative of Langmuir
isotherm as shown in Table 2. Moreover, it is obviously from the
adsorption gures and Table 2 that the (0.6 : 1) Fe–Mg MOF
material show the better adsorption efficiency among the other
prepared MOFs. This can be explained on the basis of
increasing surface area due to the synergistic effect between the
two metals and creation of new active sites that facilitate and
improve the adsorption efficiency in this sample.
L e
K C
q
e
¼ 1
(5)
m e
þ q C
A linear form of this model expression is:
C
e
1
C
e
¼ K
þ q
(6)
q
e
L
q
max
max
ꢀ
1
where C
adsorbed molecule; q
adsorbent at equilibrium per unit mass of adsorbent; qmax (mg
e
(mg L ) is the equilibrium concentration of the
ꢀ
1
e
(mg g ) is the adsorption capacity of the
3.4.6. Adsorption kinetics. In order to design a plan for
adsorption treatment, the rate at which adsorbate rejected from
an aqueous solution is important to be predicted. The two well-
ꢀ1
ꢀ1
g
L
) indicates the theoretical sorption capacity and K in L mg
is the Langmuir sorption isotherm constant. One of the most
important descriptions of the Langmuir isotherm equation is
st
nd
known models pseudo 1 order and pseudo 2 order were
applied to the experimental data of the adsorption processes to
the dimensionless separation factor (R ), represented in the
2,64
L
evaluate the adsorption kinetics.
The integrated and linear
65
equation as:
forms of these kinetic models are seen respectively in the
following equations:
1
R
L
¼ 1
(7)
þ K
L 0
C
ln(q
e
ꢀ q
t
) ¼ ln q
e
ꢀ k
1
t
(10)
(11)
ꢀ
1
where K in L mg is the Langmuir sorption constant and C0
L
ꢀ
1
t
1
1
in mg L is the initial concentration of the adsorbed mole-
cules. The R values describe the isotherm type to be either
irreversible (R ¼ 1), unfavorable (R > 1) or
¼ 0), linear (R
favorable (0 < R < 1). On the other hand, Freundlich isotherm
model is represented by the well-known logarithmic form which
¼ k
þ
t
2
q
t
2
q
e
q
e
L
ꢀ1
ꢀ1
L
L
L
1 2
where k (min ) and k in g (mg min) are respectively the
st
nd
L
adsorption rate constants of the 1 and 2 pseudo-order; q
e
ꢀ1
and q
t
in mg g are respectively the quantities of adsorbate at
66,67
expressed in the following equation:
equilibrium and time (t). The adsorption kinetic parameters are
2
collected in Table 3. The determined R factor for the pseudo
1
/n
q ¼ K C
(8)
nd
st
e
F e
2
order model is closer to unity than that of the pseudo 1
model, Table 3, and the values of qe(Exp) of the adsorbed cations
A linear form of this expression can be represented as
follows:
are in good agreement with that of q , therefore these sorption
e2
nd
systems follow the pseudo
predominantly.
2
order kinetic model
ln q ¼ (1/n)ln C + ln K
(9)
e
e
F
3.4.7. Mechanism of adsorption. Adsorption mechanism is
where n and K
describes the adsorption or distribution coefficient giving an
F
are the Freundlich isotherm exponents. K
F
generally described through three stages are as follows:
(i) Movement of adsorbate molecules towards the external
indication of the denite quantity of the adsorbed molecule on surface of the adsorbent from the bulk of the solution (lm
an adsorbent surface at equilibrium while n indicates the diffusion).
favorability of the adsorption isotherm and measure the surface
heterogeneity or adsorption intensity. The process is more the intra particular active sites from the surface.
heterogeneous when the value of 1/n gets nearer to zero. A value
(iii) Finally adsorption of ions by the contact with active sites
of 1/n lower than unity is indicative of Langmuir isotherm while in the adsorbent.
(ii) Particle diffusion, which involves the transfer of ions to
68
1
/n above unity indicating of co-operative sorption. Langmuir
The third step does not consider the rate which controls
and Freundlich sorption isotherms for Pb(II), Cd(II) and Cu(II) steps because it is a very rapid, consequently the rate which
using (0.6 : 1) Fe–Mg MOF as adsorbent are seen in Fig. S4.† The controls steps mainly either particle diffusion or lm diffusion.
2
correlation factor coefficients, R and isotherm parameters are
Weber and Morris who rst suggested the model of intra-
calculated and recorded at Table 2. The Langmuir model with particle diffusion, which summarized that the adsorption was
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Table 2 Langmuir and Freundlich parameter for adsorption of heavy metals on the prepared MOFs
Langmuir isotherm
Heavy metal
Freundlich isotherm
ꢀ
1
ꢀ1
2
ꢀ1
2
Sample
ions
q
max (mg g
)
K
L
(L mg
)
R
R
L
K
F
(mg g
)
n
R
Mg-MOF
Fe-MOF
Pb(II)
158
179
196
186
130
163
191
185
119
161
175
172
0.237
0.074
0.582
0.282
0.048
0.103
0.151
0.145
0.078
0.290
0.079
0.068
0.97243
0.95238
0.99980
0.99741
0.98339
0.96483
0.97059
0.94089
0.96236
0.97734
0.97512
0.95824
0.078
0.213
0.033
0.066
0.293
0.163
0.117
0.121
0.204
0.065
0.020
0.227
1.61
2.88
4.28
3.98
1.36
3.78
3.85
3.73
3.34
3.43
4.47
3.89
2.32
2.27
5.37
3.70
2.09
2.17
4.32
2.15
1.02
1.03
2.57
2.04
0.96677
0.90465
0.98474
0.97055
0.96358
0.96402
0.91991
0.93427
0.92998
0.97205
0.96631
0.94908
0
1
.6 : 1 Fe–Mg MOF
: 1 Fe–Mg MOF
Mg-MOF
Fe-MOF
Cd(II)
Cu(II)
0
1
.6 : 1 Fe–Mg MOF
: 1 Fe–Mg MOF
Mg-MOF
Fe-MOF
0
1
.6 : 1 Fe–Mg MOF
: 1 Fe–Mg MOF
Table 3 Kinetic parameters for adsorption heavy metals on the prepared MOFs
Pseudo-rst-order-kinetic-model
Pseudo-second-order-kinetic-model
ꢀ1
ꢀ1
ꢀ1
2
ꢀ1
ꢀ1
ꢀ1
2
Sample
Heavy metal ions
Pb(II)
q
e(Exp) (mg g
)
q
e1 (mg g
)
k
1
(min
)
R
qe2 (mg g
)
k
2
(g mg min
)
R
Mg-MOF
Fe-MOF
130
179
196
186
130
163
191
185
119
161
172
175
124.66
167.28
191.48
190.70
124.66
130.89
192.22
173.93
111.93
128.88
164.23
165.25
0.00974
0.00647
0.00567
0.00554
0.00973
0.00757
0.00457
0.00550
0.00333
0.00753
0.01212
0.00977
0.99133 130.55
0.97538 178.57
0.96155 191.48
0.99424 187.97
0.98251 130.55
0.97035 163.67
0.97504 190.84
0.90459 188.68
0.96885 119.19
0.99945 163.67
0.95995 172.12
0.96885 176.68
0.000105
0.000052
0.000056
0.000057
0.000104
0.000267
0.000045
0.000040
0.000151
0.000267
0.000135
0.000125
0.98251
0.97784
0.95771
0.96896
0.99133
0.99945
0.97883
0.99620
0.98877
0.99988
0.99157
0.96477
0
1
.6 : 1 Fe–Mg MOF
: 1 Fe–Mg MOF
Mg-MOF
Fe-MOF
Cd(II)
Cu(II)
0
1
.6 : 1 Fe–Mg MOF
: 1 Fe–Mg MOF
Mg-MOF
Fe-MOF
0
1
.6 : 1 Fe–Mg MOF
: 1 Fe–Mg MOF
69
70
proportional to the square root of the contact time as the process of adsorption kinetic equation Boyd was utilized,
demonstrated from the following equation:
which is represented as:
ꢀ
ꢁ
1
2
6
q
t
¼ K
d
t + C
(12)
FðtÞ ¼ 1 ꢀ
expðꢀBtÞ
(13)
2
pð22Þ
3
.4.8. Boyd model. To differentiate between particle diffu-
Both intraparticle diffusion and Boyd plots are applied in
order to predict the rate limiting step. The lines don't pass
sion and lm diffusion in order to identify the slowest step in
Table 4 Comparison of the adsorption efficiency of our catalyst with the previously reported articles
Heavy metal
ion
ꢀ1
Catalyst
Isotherm
qmax (mg g
)
Ref.
Iron oxide
Pb(II)
Pb(II)
Cu(II)
Cu(II)
Cd(II)
Cd(II)
Cd(II)
Pb(II)
Cd(II)
Cu(II)
Freundlich
Langmuir
Langmuir
Langmuir
Freundlich
Langmuir
Langmuir
Langmuir
36.00
71.10
52.32
55.20
13.24
57.60
70.92
196
191
175
71
72
73
74
75
76
77
G-C
3
N
4
Cellulose
Iron nanoparticles
Biochars
Graphene
Plantain peels
Fe–Mg MOF
This study
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through the origin in intraparticle diffusion plot Fig. S5A,†
subsequently; the rate limiting step is not only intraparticle
diffusion and indicates the effect of lm diffusion (boundary
layer diffusion). It is a controlled adsorption process for all
samples by lm diffusion, due to the plots of Boyd are linear but
do not pass through the origin the parameters of intraparticle
diffusion and Boyd plot for adsorption heavy metals Pb(II), Cd(II)
and Cu(II) ions onto investigated catalysts are displayed in
Fig. S5B.†
6 S. H. Jhung, N. A. Khan and Z. Hasan, CrystEngComm, 2012,
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adsorbents applied for removal of Pb(II), Cd(II) and Cu(II). A 10 J. A. Villajos, G. Orcajo, C. Martos, J. A. Botas, J. Villacanas
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this work for the removal of Pb(II), Cd(II) and Cu(II) through 11 P. Iniyavan, S. Sarveswari and V. Vijayakumar, Tetrahedron
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reported previously in the literature and listed in Table 4. The 12 B. Xu, D. Bi, Y. Hua, P. Liu, M. Cheng and M. Gr ¨a tzel, Energy
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(
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. Conclusion
This paper offers an Fe–Mg MOF with various advantages such 16 P. Bansal, N. Kaur, C. Prakash and G. R. Chaudhary, Vacuum,
as being easily prepared in larger amounts, high catalytic 2018, 157, 9–16.
activity, straight forward synthetic route under mild conditions, 17 V. S. R. Ganga, M. K. Choudhary, R. Tak, P. Kumari,
simple handling and an easy recovery without losing its activity
which makes it an eco-friendly, green and efficient catalyst. Mg-
S. H. R. Abdi, R. I. Kureshy and N. H. Khan, Catal.
Commun., 2017, 945, 5–8.
MOF, Fe-MOF and Fe–Mg-MOF were prepared successfully, 18 S. H. Huo and X. P. Yan, Analyst, 2012, 137(15), 3445–3451.
characterized using sets of different techniques e.g. XRD, FT-IR, 19 Y. G. Abou El-Reash, A. M. Abdelghany and A. Abd Elrazak,
SEM, EDS analysis and TEM. The formation of a bimetallic MOF
sharply increased the surface acidity and the catalytic activity. 20 Y. Ibrahim, E. Abdulkarem, V. Naddeo, F. Banat and
The catalytic activity of the formation of 14-aryl-14-H-dibenzo S. W. Hasan, Sci. Total Environ., 2019, 69010, 167–180.
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Int. J. Biol. Macromol., 2016, 86, 789–798.
[
prepared MOFs were used for removal of Pb(II), Cd(II) and Cu(II)
Desalin. Water Treat., 2015, 54, 1694–1707.
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time and MOF weight on the adsorption of Pb(II), Cd(II) and 23 C. R. Aguiar, E. Fontana, J. A. Valle, A. A. Souza,
´
Cu(II) were studied. Both Langmuir and Freundlich isotherm
models were used to analyze the equilibrium.
A. F. Morgado and S. Souza, Can. J. Chem. Eng., 2016, 94,
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2
2
2
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4 A. M. Anielak and R. Schmidt, Pol. J. Environ. Stud., 2011, 20,
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Conflicts of interest
5 M. Monier, D. A. Abdel-Latif and Y. G. Abou El-Reash, J.
Colloid Interface Sci., 2016, 469, 344–354.
6 N. V. P. Aguilar, E. M. Sandoval and J. R. R. Mendez, J.
Nanopart. Res., 2010, 12, 467–480.
7 Y. Gu, D. Xie, Y. Wang, W. Qin, H. Zhang, G. Wang, Y. Zhang
and H. Zhao, Chem. Eng. J., 2019, 357, 579–588.
There are no conicts to declare.
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