A new nanohybrid material constructed from Keggin-type polyoxometalate and Cd(II) semicarbazone Schiff base complex with excellent adsorption properties for the removal of cationic dye pollutants

Article abstract of DOI:10.1016/j.molstruc.2016.10.081

A novel nanohybrid material containing a Cd(II) semicarbazone Schiff base complex and phosphomolybdic acid, [Cd(H₂L⁺)₆][H₂L]⁺₄[PMo₁₂O₄₀]₄·18CH₃OH

Full text of DOI:10.1016/j.molstruc.2016.10.081

Accepted Manuscript
A new nanohybrid material constructed from Keggin-type polyoxometalate and Cd(II)
semicarbazone Schiff base complex with excellent absorption properties for the
removal of cationic dye pollutants
Saeed Farhadi, Mostafa M. Amini, Michal Dusek, Monika Kucerakova, Farzaneh
Mahmoudi
PII:
S0022-2860(16)31139-5
10.1016/j.molstruc.2016.10.081
MOLSTR 23082
DOI:
Reference:
To appear in: Journal of Molecular Structure
Received Date: 28 June 2016
Revised Date: 25 October 2016
Accepted Date: 26 October 2016
Please cite this article as: S. Farhadi, M.M. Amini, M. Dusek, M. Kucerakova, F. Mahmoudi, A new
nanohybrid material constructed from Keggin-type polyoxometalate and Cd(II) semicarbazone Schiff
base complex with excellent absorption properties for the removal of cationic dye pollutants, Journal of
Molecular Structure (2016), doi: 10.1016/j.molstruc.2016.10.081.
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Graphical Abstract

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A new nanohybrid material constructed from Keggin-type
polyoxometalate and Cd(II) semicarbazone Schiff base complex
with excellent absorption properties for the removal of cationic
dye pollutants
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Saeed Farhadi *, Mostafa M. Amini , Michal Dusek , Monika Kucerakova , Farzaneh Mahmoudi
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Department of Chemistry, Lorestan University, Khoramabad 68151-44316, Iran
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Department of Chemistry, Shahid Beheshti University , G.C, Tehran 1983963113, Iran
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Institute of Physics of the Czech Academy of Sciences, Na Slovance 2, 18221 Prague 8, Czech Republic
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*Corresponding author. Email address: sfarhadi1348@yahoo.com (S. Farhadi)
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ABSTRACT
A novel nanohybrid material containing a Cd(II) semicarbazone Schiff base complex and
+
+
.
phosphomolybdic acid, [Cd(H
2 6 2 4 40 4 3 2
L ) ][H L ] [PMo12O ] .18CH OH 4H O (1), [HL= pyridine-2-
carbaldehyde semicarbazone] was prepared by a simple sonochemical route and characterized by
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HNMR, CNMR, FT-IR, UV-vis, PXRD, FESEM, TG-DTA and BET-BJH surface area analysis.
Also the single crystal 1, was characterized by single-crystal X-ray diffraction analysis. It crystallizes
in the triclinic system with space group P-1 and is assembled into 3D supramolecular structure via
hydrogen intermolecular interactions. The nanohybrid 1 was tested for the adsorption and removal of
organic dyes such as methylene blue (MB), Rhodamine B (RhB) and methyl orange (MO) from
aqueous solutions. The effects of parameters such as the dosage of adsorbent, the initial concentration
and pH of dye solution were investigated on the removal efficiency of methylene blue. The
nanohybrid 1 exhibited excellent adsorption ability towards cationic dyes. Moreover, it could be
easily separated from the reaction solution and recycled up to three times without significant loss of
adsorption activity.
Keywords: Nanohybrid material, Polyoxometalate, Single crystal, Adsorption activity, Cationic dye
pollutants.
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1. Introduction
Water pollution with organic compounds has been one of the challenging environmental
issues in many countries [1-3]. In particular, synthetic dyes have become major water
pollutants generated mainly from textile dyeing, paper printing, color photography and
petroleum industries because about 10-15% of the total world production of synthetic dyes is
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lost during textile dyeing, more precisely, released into the textile effluents [4,5]. A wide
variety of synthetic dyes, namely, azo, polymeric, anthraquinone, triphenylmethane and
heterocyclic dyes, are used in textile dyeing processes [6,7]. Therefore, continuous discharge
of dye-bearing effluents from textile industries into natural stream and rivers poses severe
environmental problems because synthetic dyes are considered to be one of the poisonous
and harmful compounds for water, soil and human life [8]. Moreover, synthetic dyes can also
hinder sunlight’s penetration into the water, leading to the eutrophication and the die-off of
plants and animals [9,10]. Due to the complex nature and difficulty of removal of synthetic
dyes by conventional methods, textile dyeing industry is presently facing problems to safe
discharge of wastewater. So far, there are many techniques which have been reported on the
effective elimination of hazardous substances from aqueous solutions such as biological or
physical methods, photocatalytic degradation, sonocatalytic degradation, oxidation,
adsorption and so on [11–20]. Among the proposed techniques, adsorption is the procedure of
choice because it can reach good results and high efficiency using, inexpensive, eco-friendly
and simple materials [21–23]. The goal is to find a desirable adsorption material, which not
only is able to reduce the pollutant organic dyes with high efficiency and low loss, but also
allows for selective separation and recovery of raw materials [24].
In recent years, polyoxometalate (POM)-based inorganic-organic hybrid nanomaterials
have attracted considerable scientific attention in the field of environmental remediation and
removing organic dyes pollutants [25-28]. POMs as an outstanding family of metal oxide
clusters with controllable shape and size, highly electronegative, rich redox, photoactive,
oxo-enriched surfaces, and abundant topologies, have been employed in many research fields,
such as nonlinear optics, magnetism, catalysis, biological medicine and materials science [29-
32]. Up to now, a variety of strategies have been developed and used to assemble POM-based
hybrid materials by controlling reaction factors, such as metal ions, organic ligands, POM
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species, pH value and temperature, molar ratio of raw materials and reaction environment
[33-36]. Keggin-type POMs can coordinate to transition metals using their terminal or
bridging oxygens [37,38]. A prominent subclass of Keggin-type POMs is polyoxomolybdates
with range of intriguing topologies and structures based on coordination bonds or hydrogen
bonds probably due to their spherical surfaces. So far, there are many techniques which have
been reported for synthesis of inorganic-organic hybrid materials. Many POM-based hybrids
have been constructed under the commonly used solvothermal methods or some structures
have been synthesized via slow-diffusion processes, which both methods take days or weeks
to complete a hybrid material synthesis reaction, making the industrial preparation of these
materials difficult [39-41]. Recently, sonochemical synthesis as an alternative means to the
general synthetic methods has been used in the preparation of many materials. Ultrasound
induces chemical changes due to cavitation phenomena involving the formation, growth, and
instantaneously implosive collapse of bubbles in liquid, which can generate local hot spots
having a temperature of roughly 5000 °C, pressures of about 500 atmosphere, and a lifetime
of a few microseconds. This method for preparation of coordination compounds may be had
some advantages such as: it takes place in shorter reaction times, produces better yields and
also it may produce the coordination compound at nano-size scale [42-44]. Introduction of
Schiff base ligands into the chemistry of POMs opens a new direction for the synthesis of
hybrid materials with desirable abilities in different applications, which only a few results are
reported up to now [45-47].
Herein the progress in synthesis of hybrid material based on Schiff base organic ligands
and polyoxomolybdates as inorganic building blocks is referred. We report a new nanohybrid
+
+
2 6 2 4 40 4 3 2
material, [Cd(H L ) ][H L ] [PMo12O ] .18CH OH.4H O (1), containing Cd(II) pyridine-2-
carbaldehyde semicarbazone Schiff base complex and Keggin-type phosphomolybdic acid.
The nanohybrid 1 was synthesized via a sonochemical synthesis route with a good yield
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within a short time. Furthermore, the suitable single crystal 1, were synthesized via the
branched tube method for X-ray crystallography analysis. The compound 1 was fully
characterized and its adsorption ability towards some of organic dye pollutants in aqueous
solution was investigated.
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2. Experimental
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2.1. General
All chemicals and solvents were of reagent grade and used as received from Merck or
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Fluka. HNMR and CNMR spectra were recorded on a Bruker Avance 400-MHz and
Bruker AVANCE 300-MHz instrument, at room temperature with the signal of free
deuterated dimethyl sulfoxide, (CD ) SO solvent by using TMS as internal reference.
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Fourier-transform infrared spectra were recorded on Shimadzu FT-IR 8400S (Japan) with
-1
temperature controlled high sensitivity detector (DLATGS detector) and resolution of 4 cm
-1
in the scan range of 500-4000 cm using KBr pellet. The nanohybrid and single crystal 1,
were characterized using X-Ray diffract meter Philips X-Pert (Philips 8440) with radiation
Cr-Kα radiation, λ = 2.289 Å in the range of 2θ = 5°–70° at room temperature. The SEM
micrographs were obtained with Field emission scanning electron microscope (MIRA3
TESCAN) equipped with a link energy-dispersive X-ray (EDX) analyzer. The Brunauer-
Emmett-Teller surface area (BET) of the powders was analyzed by nitrogen adsorption on a
Micromeritics ASAP 2020, USA. Thermal analysis (TG–DTA) was carried on a Bahr STA-
503 instrument in air atmosphere with heating rate of 10 ˚C/min in the range of 40-820˚C.
Vibrating sample magnetometer (VSM, Iran Meghnatis Daghigh Kavir Company) was
employed to measure the magnetic parameter at room temperature. A multiwave
ultrasonic generator (Bandeline GM 2200) equipped with a converter/transducer and titanium
oscillator operating at 20 kHz with a maximum power output of 200 W was used for the
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ultrasonic irradiation. UV-vis spectra were recorded on a Carry 100 Varian
spectrophotometer in a wavelength range of 200–800 nm at room temperature.
2.2. Synthesis of ligand HL
Schiff base HL [HL: pyridine-2-carbaldehyde semicarbazone ligand] (Scheme S1), was
prepared vis the condensation of pyridine-2-carbaldehyde with semicarbazide hydrochroride
under refluxing conditions according to the reported procedure [48-51]. Yield: 80%. Anal.
Calc. for C
7 8 4
H N O: C, 45.7; H, 4.5; N, 30.1%. Found: C, 45.9; H, 4.3; N, 30.3%.
-1
Characteristic FT-IR data (KBr, cm ): 3321 m, ν(NH ); 3160 m, ν(NH); 1612 s, ν(C=N);
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1701 s, ν (C=O); 1168 m, ν(N
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N); ν(N pyridine ring) 622 w. CNMR: δ (ppm) 146.5 C(1);
1
126.1 C(2); 129.5 C(3); 125.8 C(4); 147.0 C(5); 142.6 C(6); 156.4, C(7). HNMR: δ (ppm) =
8.75 (d, 1H, C(1)H); 7.85 (t, 1H, C(2)H); 8.55 (t, 1H, C(3) H); 8.29 (d, 1H, C(4)H); 8.03 (s,
1H, C(6)H); 11.28 (s, 1H, NH).
2.3. Synthesis of nanohybrid (1)
To preparation of nanohybrid 1, 10 mL methanolic solution of cadmium(II) acetate
dihydrate (0.02 M) was sonictaed in a high-density ultrasonic probe, operating at 20 kHz with
a maximum power output of 200 W [52,53]. To this solution, 10 mL methanolic solution of
HL (0.01 M) and 20 mL methanolic solution of phosphomolybdic acid (0.1 M) were added
dropwise, respectively and the mixture was sonicated for 30 min. Then, the dark yellow
precipitate was collected, filtered and washed with methanol and dried in air. Yield 65%. FT-
-1
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IR data (KBr, cm ): 3460 m, ν (NH ); 3106 m, ν(NH); 1623 s, ν (C=N); 1686 s, ν (C=O);
-1
1164 m, ν(NN), 1080 s, ν (P–O
a
t a
); 981 s, ν (Mo–O ); 894 s, ν (Mo–O ) and 810 cm s, ν
(Mo–Ob).
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2.4. Synthesis of single crystal (1)
For the synthesis of compound 1 as suitable single crystal for structure analysis, the H
shaped branched tube is used [48, 49, 54-56]. HL ligand (0.04 g, 0.2 mmol) was placed in
one arm of a branched tube and mixtures of cadmium (II) acetate dihydrate (0.18 g, 0.6
mmol) and phosphomolybdic acid (0.36 g, 0.2 mmol) in the other arm of tube. Then, about
30 mL of methanol was carefully added, so that the bridge of the tube was filled. Then, the
tube was sealed and the arms were immersed in a bath at 60 °C, while the bridge was at
ambient temperature. After three days, polygon shape dark yellow crystals were deposited in
the cooler bridge, which were filtered off, washed with methanol and air dried. Yield: 40%.
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Characteristic FT-IR data (KBr, cm ): 3457 m, ν(NH ); 3106 m, ν(NH); 1623 s, ν(C=N);
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1686 s, ν(C=O); 1164 m, ν(NN), 1080 s, ν(P–O ); 981 s, ν(Mo–O ); 894 s, ν(Mo–O ) and
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812 cm s, ν(Mo–O
b
). CNMR: δ (ppm) 145.5 C (1); 125.8 C (2); 130.5 C (3); 124.9 C (4);
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148.0 C (5); 143.4 C (6); 156.4, C (7). H NMR: δ (ppm) = 8.77 (d, 1H, C(1)H); 7.84 (t, 1H,
C(2)H); 8.45 (t, 1H, C(3) H); 8.33 (d, 1H, C(4)H); 7.94 (s, 1H, C(6)H); 11.23 (s, 1H, NH);
7.00 (s, N pyridine ring NH), and for the methanol and water molecules 3.15 (s, H (C)H); 4.10 (s,
H (O)H).
2.5. X-ray structure analysis
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The selected single crystal 0.49 x 0.32 x 0.20 mm was mounted on a fiber glass and
placed on a four-cycle diffractometer Gemini (Rigaku Oxford Diffraction). Due to high
α
absorption we used Mo K radiation (λ = 0.71073) from a sealed X-ray tube, monochromated
with a graphite monochromator, collimated with a Mo-Enhance collimator, and detected with
a CCD detector Atlas S1. Superflip [57] was used for structure solution by charge flipping,
Jana2006 [58] for structure refinement and interpretation, Diamond4 (Diamond - Crystal and
Molecular Structure Visualization. Crystal Impact - Dr. H. Putz & Dr. K. Brandenburg GbR,
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Kreuzherrenstr. 102, 53227 Bonn, Germany, http://www.crystalimpact.com/diamond) for
plotting. Structure analysis was quite standard, the PMo12O40 did not exhibit disorder
sometimes found for this kind of compounds, and the organic part could be determined
without problems due to good quality of data. Oxygen atoms of some CH OH molecules
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were disordered. We found also some isolated atoms, probably lattice water, whose hydrogen
atoms could not be determined. Hydrogen atoms attached to carbon were kept in
geometrically expected positions with Uiso = 1.2|*Ueq of their parent atoms.
Hydrogen atoms attached to nitrogen atoms were mostly invisible in difference Fourier
maps due to the presence of heavy atoms in the structure. However, NH groups of N , N , N ,
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N10 and N11 as well as NH groups of N , N , N , N and N have been confirmed by
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spectroscopy. While atoms of NH could be attached geometrically, it was impossible for NH
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because this group is usually deviated of the triangular symmetry. Refinement of NH groups
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with distance restraint but refined positions lead to unreasonable geometry. Because for both
groups hydrogen atoms were equally invisible in the difference Fourier map, we decided to
omit all hydrogen atoms attached to nitrogen. With the neutral ligand HL, the charge of the
compound would be negative. In order to reach the charge balance, we tested possibility that
the nitrogen atoms of the aromatic rings, could be protonated. A summary of the key
crystallographic information, selected bond lengths and bond angles and also selected
hydrogen bonding parameters of 1, are listed in Tables 1 – 3.
2.6. Adsorption tests
Freshly prepared compound 1 (30 mg) were transferred into 50 mL of aqueous solution of
different dyes as methylene blue (MB), rhodamine B (RhB) and methyl organge (MO) with
−
1
initial concentration of 25 mg L . The mixture was stirred gently with the speed of 300 rpm
at 25 °C. To prevent photodegradation of dyes by nanohybrid 1, the dye solution was kept
from light by being wrapped with dark paper during the adsorption tests. After soaking for
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short time ranging from 30 seconds to 10 minutes, and in several intervals, the samples were
separated through centrifuge to remove the suspended particles. Then the upper clear
solutions were analyzed by a UV-vis spectrophotometer. The dye concentration was
determined by using Carry 100 Varian spectrophotometer. The concentrations of MB and
MO in the solution, were determined as a function of adsorption time from the change in
absorbance at 664 and 553 nm for MB and RhB solution respectively at pH= 6 and
absorbance at 463 nm for MO solution with pH= 5.
Other experiments with different dosage of compound 1, such as 15, 30 and 45 mg
towards adsorption of MB, were carry out. Furthermore, different concentration of MB dye
−
1
solutions 25, 35 and 45 mg L were prepared for investigate the ability of synthesized
nanohybrid 1, towards removal of MB molecules. Also for study the effects of initial solution
pH on the adsorption process, several experiments were carry out. The initial pH of MB
solution was adjusted to values in the range of 2–10 by dropwise adding 0.1 mol/L of NaOH
or 0.1 mol/L of HCl solutions.
The amount of dye adsorbed per unit mass of nanohybrid material (q) and the dye
removal efficiency (R) could be calculated by applying the Eqs. (1) and (2), respectively:
q
t
(mg/g) = V(C
0
-C
t
)/m
(1)
(2)
R= 100 [(C -C )/C ]
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t
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Where C and C (mg/L) are the dye concentrations at initial and t time, respectively, m (g)
0
t
represents the mass of the adsorbent and V (L) is the volume of dye solution.
After each adsorption experiments, the hybrid material 1, loaded with MB was separated
from the dye solutions by centrifugation and washed several times with water and ethanol at
room temperature. Then the regenerated compound 1, was dried overnight at 70 °C and
reused for the next adsorption experiments. The structural details and nature of some organic
pollutants used here, are listed in Table S1.
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3. Results and discussion
3.1. Synthesis
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The nanohybrid material 1, was prepared by the sonochemical route and also suitable
single crystal 1, was synthesized for structure determination. The FT-IR and PXRD results
confirmed the same composition for both synthesized samples.
3.2. Crystal structure description of compound 1
The structure of 1, determined by X-ray single crystal structure analysis and the crystal
data and structure refinement of 1, are summarized in Table 1. The compound crystallizes in
the triclinic space group P-1. As shown in Fig. 1 (a), the symmetric unit of 1, is consists of
+
+
one Cd (II) center coordinated to six protonated ligands H L , two isolated H L , two
2
2
3-
saturated Keggin-type polyoxoanions PMo12O40 , nine isolated methanol molecules which
two of them disordered, and two lattice water molecules. The coordination geometry of the
central atom Cd (II) with coordinated atoms, and the unit cell of 1, are depicted in Fig. 1(b)
and Fig. 1 (c). Both coordinated and free HL ligands, keep their hydrogen atoms, and also the
ligands are positively charged through protonation of the nitrogen of the aromatic ring which
1
are in agreement with FT-IR and HNMR data. Interestingly, the six HL ligands are
coordinated to Cd(II) center with their oxygen atoms and causes a distorted octahedral
3
-
geometry around the Cd(II) center. The polyoxoanion PMo12
O40 is a well-known α-Keggin
structure composed of twelve corner sharing MoO octahedral with the central phosphorus
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atom coordinated to four oxygen atoms in a distorted tetrahedral fashion. The P–O distances
are in the range of 1.520 to 1.543 Å, and O–P–O angles are in the range of 108.9˚-110.2˚. The
t
Mo–O distances can be classified as follows: Mo–O (terminal oxygen) of 1.654–1.714, Mo–
Ob–Mo (bridging oxygen) of 1.860–1.963, Mo–Oc–Mo (central oxygen) of 2.406–2.440
(Table 2), which are consistent with those described in the literature [38, 40]. The Cd (II)
center is bounded to six oxygen atoms of the HL, in a distorted octahedral geometry. The Cd–
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O distance as 2.252 - 2.324 Å, is shorter than the corresponding bond length of 2.490(4) Å in
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-
+
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[Cd (HL) (ꢀ1,1-N3)
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n
[48]. The PMo12
O
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polyoxoanions, [Cd(H L ) ] cations and isolated
+
H
2
L molecules are connected with several hydrogen intermolecular interactions into three
dimentional (3D) supramolecular structure (Fig. 1(d)). More details about the hydrogen
intermolecular interactions for compound 1, are listed in Table 3. The exact role of the lattice
water and methanol molecules could not be elucidated because hydrogen atoms of water and
OH groups could not be determined from X-ray analysis.
3. 3. FT-IR, NMR and UV-Vis spectroscopy
The FT-IR spectra of single crystal and nanohybrid 1, were compared with that of the free
-1
HL in the region 4000–500 cm as shown in Fig. 2 (A) The weak band ν (NH ) is observed
2
-1
-1
at 3321 cm for HL, and respectively at 3457 and 3460 cm for single crystal and
-1
-1
nanohybrid 1. The related band ν (N–H) is observed at 3160 cm for HL, and at 3106 cm
for both single crystal and nanohybrid. Observation of ν(N–H) in the spectra of compound 1
provides strong evidence for ligand coordination to the Cd(II) center in the keto form
-1
[59].The strong ν(C=N) band of HL around 1613 cm is found to be shifted to lower
-1
frequencies in 1 and appeared at 1623 cm in the spectrum of compound 1 [48-51]. An out-
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of-plane bending vibrations of the pyridine ring in the free ligand is assigned at 622 cm
which is disappeared in the single crystal and nanohybrid 1, is confirmative that the pyridine
1
nitrogen is not coordinated and according to HNMR results pyridine nitrogen is protonated
+
and the ligand HL is in the protonated form H L in the structure of nanohybrid 1. Also the
2
difference Fourier of single crystal X-ray diffraction, indicates strongly this possibility.
Furthermore, the spectrum of 1 showed four major characteristic vibrations bands, resulting
from phosphomolybdic acid with Keggin structure: ν (P–Oa), ν (Mo–Ot), ν (Mo–Oa) and ν(Mo–Ob) at
-1
1080, 980, 890 and 810 cm , respectively. By comparing the IR spectrum of the title
compound with that of H PMo O .nH O (ν = 1064 (P–O ), 963 (Mo–O ), 867 (Mo–O ) and
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782 (Mo–O
Mo–O bond, while the vibration bands of the Mo–O
that the P–O , Mo–O , Mo–O bonds become weaker while the Mo–O
somewhat stronger in 1. Furthermore, the FT-IR spectra confirms that PMo cluster in 1, still
b
) cm [39,40], red shift appears for the vibration bands of the P–O
bond is blue shifted. This demonstrates
bond become
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maintains the basic Keggin structure, in agreement with the single crystal X-ray diffraction
analysis. The FT-IR spectra of single crystal and nanohybrid 1, are in a good accordance
which confirms the same composition for them. For more details, the HL and single crystals
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1, were characterized with CNMR in (CD
3 2
) SO solution (Fig. S1). The CNMR of HL,
demonstrate all of the carbon atoms with different environments which are summarized in the
1
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synthesis section. The CNMR spectra of single crystals 1, demonstrated the same bands
related to carbon atoms of free HL with a partial chemical shift and also there is a waek band
1
at δ = 58 ppm related to carbon atoms of methanol molecules. The HNMR spectrum of HL
1
and single crystals 1, are shown in Fig. S2. HNMR studies permitted the assignment and
1
quantification of the different H chemical environments for the HL and compound 1. As
summarized in the synthesis section, all of the characterized bands of hydrogen atoms of HL
1
are observed exactly in the HNMR spectra of compound 1, with only a minor chemical shift.
In the spectra of compound 1, Presence of a band at δ = 11.23 ppm for the NH bond of HL
concluded that the HL kept its hydrogen atom. Also a broad band at δ = 7.00 ppm
demonstrate the nitrogen atoms of the aromatic rings, are protonated. Also comparing the
1
HNMR spectra of HL with single crystals 1, some new bands is observed in the spectra of
compound 1. A sharp band at δ = 3.15 ppm related to hydrogen atoms of C—H bonds of
methanol molecules and a broad band at δ = 4.1 ppm related to hydrogen atoms of O—H
bonds of methanol and water molecules.
The UV-vis spectra of HL, H PMo O (PMo ) and nanohybrid 1, were measured in the
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region of 200 to 500 nm in ethanol solvent (Fig. S3). The HL, shows three absorption bands
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at 210, 280 and 300 nm which are associated with π→ π pyridine, π→ π imine and n→ π pyridine
,
respectively. The presence of a sharp absorption band at 220-240 nm in the spectra of PMo12
corresponds to Ob/c→Mo ligand to metal charge transfer. The UV-vis spectrum of nanohybrid
1, has a sharp and obvious band at 220-240 nm corresponding to PMo12 in structure of 1.
While observation of a shoulder band at 270 nm and a broad band at 290 nm related to HL,
confirms the HL the successful synthesis of compound 1. Interestingly, the related bands of
HL in the spectra of compound 1, are shifted to lower wavelength which suggests the
coordination of HL to the metal centers. The results are in good agreement with similar works
reported in the literature [48-51].
3. 4. X-ray Powder Diffraction
To confirm the phase purity of single crystal and nanohybrid 1, X-ray powder diffraction
(XRPD) experiments were carried out and the patterns are depicted in Fig. 2 (B). In the XRD
patterns of compound 1, three peaks are observed at 2θ = 10.24°, 12.08°, 14.14° for
nanohybrid 1 and at 2θ = 10.28°, 11.57°, 14.05° for single crystal 1 (Fig. 2B (a) and (b)).
Also the experimental XRD patterns, are compared with the corresponding simulated pattern
based on the X-ray structure model (Fig. 2B (c). As can see in Fig. 2B, the nanohybrid 1 and
the pulverized single crystal 1, have the same XRD profiles. The results suggest good
agreement with obtained data and confirm the same composition without any impurity for
both nanohybrid and single crystal 1. The average size of the particles was estimated to be 50
nm using the Scherrer formula D= 0.891λ / β cosθ, where D is the average grain size, λ the
X-ray wavelength (1.5418 Å), β the full-width at half maximum of an observed peak and θ its
diffraction angle.
3. 5. EDX, SEM and BET analysis
The successful synthesis of nanohybrid 1, was further verified by the chemical
composition analyzed by EDX, which confirmed the presence of C, N, O, P, Mo and Cd
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elements. According to EDX mapping images of nanohybrid 1 (Fig. S4), these elements are
uniformly distributed in 1. The SEM images of nanohybrid material 1 (Fig. 3) reveals plate-
like morphologies [43] with diameters ranging 200 nm- 1 ꢀm, and thicknesses about 50-100
nm. The nitrogen adsorption method was used to determine BET surface area of nanohybrid
material 1. They show type III isotherm (Fig. S5) with the type H3 loop hysteresis according
to the IUPAC classification [60-62], indicating the presence of mesopores 2–50ꢁnm. The
-3
surface area and total pore volume of nanohybrid 1, are 5.9 nm and 6.16×10 cc/g,
respectively and the average pore diameter are 4.2 m²/g.
3. 6. Thermal analysis
Thermogravimetric and differential thermal analysis (TG-DTA) were performed in the
range of 40-820˚C in static air atmosphere in order to study the behavior of the nanohybrid
and single crystal 1 during the heat treatment (see Fig. 4 (a) and (b)). The TG curve of single
crystal 1, exhibits three weight loss steps in the range of 40–820 °C. The first weight loss of
6.8% (calc. 6.7%) below 200 °C corresponds to the loss of noncoordinated water and
methanol molecules. The second and third weight loss of 18.5 % (calc. 19.5%) occurred in
the range of 220–560 °C in TG curve which are attributed to the elimination of molecules of
organic ligand. Obviously these weight losses, accompanied with three exothermic peaks in
3
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the DTA curve. Collapse of PMo12O40 clusters occurs in the range of 560–820 °C without
weight loss in TG curve but an endothermic peak is observed in the DTA curve. These results
are in good agreement with the structural formula of compound 1, which obtained from
single crystal X-ray diffraction. Nanohybrid materials 1, exhibit the weight loss similar to
single crystal 1. The DTA curves, have the same general appearance for both nanohybrid and
single crystals 1, however, the exo and endothermic peaks have different intensities.
Nonetheless, the nanohybrid materials are less stable and their decomposition steps are
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shifted about 5-10 °C towards lower temperatures which is in agreement with previous
reports [44,63].
3. 7. Adsorption of dyes
In this study, the adsorption performances of nanohybrid 1, towards dye pollutants were
investigated. Firstly, the nanohybrid 1, was used for removal of different organic dyes such as
methylene blue (MB) and rhodamine B (RhB) as cationic dyes and methyl orange (MO) as
anionic dye. The UV-vis time dependent absorption spectrum of MB solution under
adsorption condition on nanohybrid 1, is depicted in Fig. 4(c) and the related C/C plots
0
versus reaction time for MB and RhB solutions, are shown in Fig. 4(d). The results show that
nanohybrid 1, can remove the MB and RhB dyes in short contact time of 5-6 min, with
adsorption capacity of 37% and 33%, and removal efficiency of 98% and 86%, respectively.
The digital images of MB dye adsorption, clearly shows the color change from blue to
colorless solution (see Fig.4(c)). The nanohybrid 1, is not efficient for removal of MO dyes
(Fig. S6). It suggests that, in this study, the dye removal was mainly achieved by electrostatic
interactions between anionic charged nanohybrid 1, and cationic charged MB and RhB dye
molecules. In the other words, the nanohybrid 1, and MO dye molecules, both have anionic
charge and repel each other, as a result, there is neither electrostatic interaction nor adsorption
property between the nanohybrid 1 and MO molecules. Furthermore the observed difference
in removal efficiency of nanohybrid 1, towards MB and RhB, is originated from this fact that,
the RhB is a dye with large molecules in contrast with MB and this causing a steric hindrance
between the RhB molecules and the active adsorption sites on nanohybrid 1, and lead to
slower removal of RhB in contrast with MB [64,65].
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For investigate the best condition for removal of MB molecules, with nanohybrid 1,
several experiments were carry out by altering other variables, including the different dosage
of adsorbent 1, different concentration of MB and different pH values.
The Fig. 5(a) and (b), show the effects of adsorbent dosage on the removal of MB.
Furthermore for more details, the related UV-vis time dependent absorption spectrum of MB
on nanohybrid 1, are depicted in Fig. S7. Along with the increase of adsorbent dosage from
15 mg/L to 30 and 45 mg/L, the removal of MB shows a significant increasing trend and the
time of adsorption, decreases from 7 to 5 and 4 min with removal efficiency of 98%. As can
see in Fig. 5 (a), the adsorption capacities decreases from 73 to 37 and 20.8 for 15, 30 and 45
mg/L dosage of adsorbent 1. The reason can be described that by increasing the dosage of
adsorbent in the same volume of MB solution (50 mL), there are not enough MB molecules
for generating driving force to overcome the mass transfer resistance of the dye and the
adsorbent are dispersed in dilute MB concentrations and the adsorption process has reached
equilibrium in short time [66]
Effects of initial MB concentrations (25, 35 and 45 mg/L) on adsorption performance was
evaluated and the results are shown in Fig. 5(c) and (d). The UV-vis time dependent
absorption spectrum are depicted in Fig. S8. Along with increasing the initial MB
concentrations, the adsorption performance completed in long time. Interestingly as can see
in Fig. 5. (d), by increasing the initial MB concentrations from 25 to 35 and 45 mg/L, the
adsorption time increased from 5 min to 9 and 45 min while the removal efficiency is same as
98%, 97% and 97% respectively. The equilibrium adsorption capacity (37.5 mg/g) at initial
MB concentration of 25 mg/L increase to adsorption capacities of 49.3 and 62 mg/g at the
concentration of 35 and 45 mg/L of MB solutions. As can see in Fig. 5 (c), with increasing
the initial concentration of MB, the adsorption capacities increase and then keep constant at a
slow speed. The results can be ascribed to the fact that, most vacant surface sites are available
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for the adsorption at the initial stage. However, with the adsorption progressing, the available
surface sites become less, and harder to be utilized due to repulsive forces between the MB
molecules adsorbed onto adsorbent 1 and bulk phase [66-68]. In fact by increasing the
concentration of MB, the driving force is increase and lead to higher adsorption capacities.
The initial solution pH, has been identified as a rather significant variable enhancing or
depressing the adsorption capacity of adsorbate molecules, which is attributed to influencing
surface binding sites or charges of the adsorbent [66,67]. The effect of initial solution pH on
MB adsorption onto nanohybrid 1, was investigated in the pH range from 2 to 10. Fig. S9
shows the UV-vis time dependent absorption spectrum of MB solution in different pH values.
The related qt/t and R/t plots are depicted in Fig. 5(e) and (f) respectively. From the results
shown in Fig. 5(f), it observed that by increasing initial pH of MB from 2 to 10, the removal
efficiency decrease from 99% to 65% while the adsorption capacities increased from 33.5 at
pH =2 to 35.2 at pH= 4 and to 37.5 at pH =6 and then the adsorption capacities decrease to 30
and 28.5 at pH=8 and 10, respectively. Also, according to results depicted in Fig. S9, the
adsorption time is increased from 5 min at pH=6 to 8 and 12 min at pHs 8 and 10 respectively
while decrease to 2 and 5 min for acidic solutions with pH 4 and 2, respectively. Moreover,
it is obvious that the adsorption capacity and removal rate can reach 33.5 mg /g and 99% at
low pH, further suggesting that acidic solutions benefited MB adsorption. This phenomenon
can be explained that POMs with highly electronegative possessed a good adsorption towards
the cationic dyes [66]. With increasing the initial pH from 6, the adsorption capacity and
removal efficiency were decrease. It can be related to this observation that, the chloride anion
in MB was exchanged with NaOH to have a displacement reaction, forming NaCl (aq) and
+
MBS OH (aq). However, the salt of NaCl might result in the deactivation of nanohybrid 1,
+
and decrease the adsorption of MBS OH (aq) on it [66, 68].
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The stability and reusability of adsorbent material, is an important factor for its practical
applications. When the adsorption process was over, the adsorbent sample saturated with dye
molecules, were separated by centrifugation due to its insolubility in water, washed with
water and ethanol several times and dried overnight at 70 °C and reused for adsorption again.
The adsorption processes were repeated for three times. Each run is carried out under the
same condition. As can see in Fig. 5. (g), there is a partial decrease in the removal efficiency
of the adsorbent 1, at the third cycle, with removal efficiency of 96% towards MB and 83%
towards RhB. Moreover, the FT-IR spectrum of nanohybrid 1, after three cycles of use, is
shown in Fig. 2A (d). It confirms that, the structure of nanohybrid 1, remains intact without
losing its adsorption efficiency over three cycles of adsorption experiments. The results
suggest the nanohybrid 1, as an efficient and reusable adsorbent to removal of cationic dyes.
4. Conclusions
In summary, a new polyoxometalate based nanohybrid material with Cd (II) semicarbazone
+
+
complex, [Cd(H L ) ][H L ] [PMo O ] .18CH OH.4H O (1), was prepared by a
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12 40 4
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sonochemical route and characterized fully. Also the single crystal 1, were synthesized with
branched tube method and used for single-crystal X-ray diffraction analysis. The FT-IR and
XRD results are in good agreement and confirm the same composition for it. The nanohybrid
material 1, was used for removal of different organic dyes from aqueous solutions. The
results reveal that, 1, can remove the cationic dyes with very good efficiency but it is not
effective for removal of anionic dyes. These results are correlated with the presence of
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PMo12
O
polyanions in the structure of 1. The nanohybrid 1, can adsorb MB and RhB
molecules in short time of 5 and 6 min with removal efficiency of 98% and 86% respectively.
Furthermore, the nanohybrid 1, can be readily recycled several times without significant loss
of its adsorption activity.
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Acknowledgements
We are grateful for financial support from Lorestan University. The crystallographic part was
supported by the project 15-12653S of the Czech Science Foundation using instruments of
the ASTRA lab established within the Operation program Prague Competitiveness - project
CZ.2.16/3.1.00/24510.
Appendix A. Supplementary data
Crystallographic data (excluding structure factors) for the structure reported in this paper
have been deposited with the Cambridge Crystallographic Data Centre as supplementary
publication no. CCDC as 1448105. Copies of these data can be obtained free of charge on
application to CCDC, 12 Union Road, Cambridge CB2 2EZ, UK; FAX: (+44) 1223 336033,
or
online
via
www.ccdc.cam.ac.uk/data_request/cif
or
by
emailing
data_request@ccdc.cam.ac.uk.
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Table 1. Crystal data and structure refinement parameters for compound 1
Chemical Formula
Formula weight
Radiation type
Crystal size (mm)
Temperature (K)
Crystal system
Space group
a (Å)
88 40 192 4
C H106CdMo48N O P
9637.3
Mo Kα
0.49 × 0.32 × 0.20
120
Triclinic
P -1
13.2377(2)
19.5954(3)
25.4879(4)
98.7446(13)
94.6958(14)
102.4346(14)
6336.23(18)
1
b (Å)
c (Å)
α (°)
β(°)
γ(°)
3
Volume (Å )
Z
-
3
ρ calculated (g cm )
2.532
-
1
µ (mm )
T min, T max
R(reflections)
wR2 (reflections)
2.51
0.772, 1
0.0587( 24832)
0.0811( 28898)
4.75, −5.11
−3
ꢂρmax, ꢂρmin (e Å )
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Table 2. Selected bond lengths (Å) and angles (deg) for compound 1
Cd1—O32
2.324 (5)
2.324 (5)
2.276 (7)
2.276 (7)
2.252 (6)
2.252 (6)
1.528 (6)
1.547 (6)
1.535 (5)
1.535 (5)
1.543 (5)
1.535 (6)
1.537 (6)
1.520 (5)
1.268 (11)
1.243 (11)
1.237 (11)
1.280 (14)
1.267 (11)
O32—Cd1—O32i
O32—Cd1—O33
O32—Cd1—O33i
O32—Cd1— O57
O32—Cd1—O57i
O33—Cd1—O33i
O33—Cd1—O57
O33—Cd1—O57i
O33i—Cd1—O57
O33i—Cd1—O57i
O57—Cd1—O57i
O69—P1—O85
O69—P1—O89
O69—P1—O91
O89—P1—O91
O76—P2—O77
O76—P2—O92
O77—P2—O87
O87—P2—O92
180.0 (5)
86.4 (2)
93.6 (2)
94.7 (2)
85.3 (2)
180.0 (5)
86.8 (2)
93.2 (2)
93.2 (2)
86.8 (2)
180.0 (5)
110.2 (3)
109.4 (3)
110.2 (3)
109.3 (3)
109.3 (3)
109.6 (3)
109.5 (3)
108.9 (3)
i
Cd1—O32
Cd1—O33
i
Cd1─ O33
Cd1─ O57
Cd1─ O57
P1—O69
P1—O85
P1—O89
P1—O91
P2—O76
P2—O77
P2—O87
P2—O92
i
O32—C23
O33—C25
O57—C42
O19—C21
O12—C16
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Table 3. Selected hydrogen bonding parameters (Å, º) for compound 1
D—H···A
D—H
0.96
0.96
0.96
0.96
0.96
0.96
0.96
H···A
D···A
D—H···A
161.18
101.44
168.31
144.21
138.34
126.21
171.31
C1—H1···O27
1.88
2.809 (8)
2.706 (10)
3.255 (12)
3.215 (14)
3.217 (13)
3.105 (13)
3.392 (9)
C1—H1···N1
2.35
2.31
2.39
2.44
2.44
2.44
C22—H16···O13ii
C22—H17···O45iii
C26—H19···O25
C32—H25···O9iii
C36—H31···O17iv
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Symmetry codes: (ii) −x, −y+1, −z; (iii) x, y+1, z; (iv) −x+1, −y, −z.
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+
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2 6 2 4 40 4 3 2
Fig. 1. (a) The symmetric unit of [Cd (H L ) ][H L ] [PMo12O ] .18CH OH.4H O (1), (b) coordination
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geometry around central atom Cd (II), (c) View of the unit cell of 1 in the direction of the crystallographic b axis
and (d) packing representation of compound 1.
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Fig. 2 (A) The FT IR spectra of : (a) HL ligand, (b) nanohybrid material 1, (c) single crystal 1, (d) nanohybrid
material 1 after adsorption experiments; and (B): PXRD patterns of (a) nanohybrid material 1, (b) single crystal
1 and (c) simulated pattern from single crystal X-ray data of compound 1.
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