A new pillared Cd-organic framework as adsorbent of organic dyes and as precursor of CdO nanoparticles

Article abstract of DOI:10.1016/j.poly.2019.114265

A new neutral cadmium-organic framework with a pillared layer structure, [Cd₃(BTC)₂(4-bpdb)₂] (H₃BTC = benzene-1,3,5-tricarboxylic acid; 4-bpdb = 1,4-bis(4-pyridyl)-2,3-diaza-1,3-butadiene), has been synthesized via solvothermal reaction of cadmium nitrate with the tricarboxylic acid H₃BTC and the linear bispyridyl linker 4-bpdb. The complex has been characterized by single crystal X-ray diffraction, showing to possess a 3D porous network of jcr7 topology. The capability of the prepared MOF in adsorbing the organic dyes Congo Red (CR) and Neutral Red (NR), together with kinetics and thermodynamics of their adsorption, have been investigated in detail. The adsorption process was well described by pseudo-first order and pseudo-second order kinetics for CR and NR, respectively. In addition, conversion of the MOF 3D architecture into nano-sized cadmium oxide particles has also been studied.

Full text of DOI:10.1016/j.poly.2019.114265

Polyhedron 176 (2020) 114265
Contents lists available at ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
A new pillared Cd-organic framework as adsorbent of organic dyes and
as precursor of CdO nanoparticles
a
b
Samira Gholamali Ghomshehzadeh a, Valiollah Nobakht a, , Nahid Pourreza , Pierluigi Mercandelli ,
⇑
Lucia Carlucci b
a Department of Chemistry, Faculty of Sciences, Shahid Chamran University of Ahvaz, Ahvaz, Iran
b Dipartimento di Chimica, Università degli Studi di Milano, via Camillo Golgi 19, 20133 Milano, Italy
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 3 May 2019
Accepted 23 November 2019
Available online 16 December 2019
A new neutral cadmium-organic framework with a pillared layer structure, [Cd3(BTC)2(4-bpdb)2]
(H3BTC = benzene-1,3,5-tricarboxylic acid; 4-bpdb = 1,4-bis(4-pyridyl)-2,3-diaza-1,3-butadiene), has
been synthesized via solvothermal reaction of cadmium nitrate with the tricarboxylic acid H3BTC and
the linear bispyridyl linker 4-bpdb. The complex has been characterized by single crystal X-ray diffrac-
tion, showing to possess a 3D porous network of jcr7 topology. The capability of the prepared MOF in
adsorbing the organic dyes Congo Red (CR) and Neutral Red (NR), together with kinetics and thermody-
namics of their adsorption, have been investigated in detail. The adsorption process was well described
by pseudo-first order and pseudo-second order kinetics for CR and NR, respectively. In addition, conver-
sion of the MOF 3D architecture into nano-sized cadmium oxide particles has also been studied.
Ó 2019 Elsevier Ltd. All rights reserved.
Keywords:
MOF
Pillar ligands
Cd(II) complex
Congo Red
Neutral Red
Adsorption
1. Introduction
ligands to give rise to 3D structures with the desired network
topology [4].
Metal-Organic Frameworks (MOFs) are an interesting category
of materials containing metal or metal-cluster nodes and multi-
topic organic linkers [1]. Tunability of cluster nodes and organic
linkers offers infinite possibilities to design and synthesize a vari-
ety of MOF structures with fascinating topologies and useful prop-
erties. Moreover, MOF structures obtained by self-assembly
methods can undergo a wide range of chemical functionalization
via post-synthetic modification (PSM) reactions [2]. The ease of tai-
lorability combined with outstanding features, such as ultrahigh
Brunauer–Emmett–Teller (BET) surface area and high thermal sta-
bility and porosity, brought these materials to the attention of
many scientists in both academia and industry [3]. In order to pre-
pare pre-designed MOFs, the best choice is to employ rigid linkers
containing carboxylate or pyridyl donating groups. In particular,
the combination of carboxylates with pyridyl-based linkers as pil-
lars is an intriguing strategy to obtain designed structures. Indeed,
carboxylate ligands usually form 2D-sheet structures with a highly
predictable network topology by means of coordination to the
metal ions. The remaining coordination sites of the metal centers
are then occupied by the nitrogen atoms of the pyridyl-based
Metal-organic frameworks exhibit a wide range of properties
and potential applications, including gas sorption and separation,
catalysis, sensing, and biomedical applications [5]. Furthermore,
results have proved that MOFs offer a promising toolbox for
preparing advanced nanomaterials that are not easily obtainable
using conventional methods [6]. Among metal-oxide nanoparticles
(NPs), cadmium oxide is a well-known n-type semiconductor and
shows applications in various fields such as optoelectronics, solar
cells, transparent electrodes and gas sensors [7]. Up to now, several
approaches have been developed to produce CdO nanostructures
including solvothermal, sol-gel, sonochemical, precipitation, and
wet chemical methods [8]. Thermal decomposition of inorganic
precursors or Cd-organic coordination networks is also a valuable
route to obtain CdO nanostructures with different shapes and
sizes [6].
On the other hand, the presence of organic and inorganic pollu-
tants in waters and wastewaters is a serious problem and causes
worldwide concern, due to their harmful effects and potential car-
cinogenic hazards [9]. Significant amounts of organic dyes have
been produced by textile, leather, paper, and plastic factories
which should be treated before their discharging into the environ-
ment. Several approaches, such as adsorption, coagulation, ion
exchange, filtration, photocatalysis, and advanced oxidation have
been used to remove organic dyes from aqueous media [10].
⇑
Corresponding author.
E-mail address: v.nobakht@scu.ac.ir (V. Nobakht).
https://doi.org/10.1016/j.poly.2019.114265
0277-5387/Ó 2019 Elsevier Ltd. All rights reserved.

2
S.G. Ghomshehzadeh et al. / Polyhedron 176 (2020) 114265
Among them, adsorption of dyes is an attractive method due to its
simplicity, effectiveness, and affordability [11]. Recently, adsorp-
tion by MOFs of toxic heavy metal ions and organic pollutants from
the effluents has become of great interest for researchers [12].
Zhao et al. employed several cationic indium-organic frameworks
for the adsorption and separation of organic dyes through anion-
exchange processes [13]. Several reports concerning the selective
adsorption and separation of anionic or cationic organic dyes by
positively- or negatively-charged MOFs via ion-exchange processes
have been reported up to now [14]. Adsorption of organic dyes into
the pores or on the surface of neutral MOFs has also been reported
[15]. In this context, several mechanisms of interaction between
the dye molecules and the MOF surface have been proposed [16],
such as electrostatic interaction, acid–base reactions, hydrogen
gradually cooled to room temperature over 24 h. Yellow needle-
shaped single crystals of [Cd3(BTC)2(4-bpdb)2] suitable for single
crystal X-ray diffraction analysis were obtained, collected by filtra-
tion, and dried in air (0.18 g, yield: 48% based on Cd). Elemental
analysis (%) calc. for C51H47Cd3N11O15: C 44.03, H 3.41, N 11.07;
found: C 44.16, H 3.30, N 11.09.
2.4. Adsorption experiments for Congo Red and Neutral Red
Before dye adsorption experiments, to increase the surface area
of the adsorbent, crystals of Cd-MOF were ground in an agate mor-
tar. The adsorption experiments were carried out by adding 5 mg
of the adsorbent powder into a 100 mL beaker containing 20 mL
of the dye solution (CR or NR) at different initial concentrations.
The mixture was left under stirring for 80 min (CR) and 95 min
(NR) to allow to reach the equilibrium. The solution was then
transferred to a tube and centrifuged at 4000 rpm for 5 min. The
supernatant solution was removed and the absorbance was mea-
sured at 499 nm and 526 nm for CR and NR, respectively. The equi-
bonding,
pꢀ ꢀ ꢀp stacking, and hydrophobic interactions.
In this work, a new Cd(II) metal-organic framework with an
interesting network topology has been prepared via solvothermal
synthesis using a tricarboxylate linker in combination with a bis-
pyridyl ligand. The efficiency of the synthesized MOF in the
adsorption of some organic dyes has been studied in detail. In addi-
tion, spherical CdO nanoparticles have been obtained via calcina-
tion of the MOF at 500 °C.
librium
concentration
of
the
dyes
was
determined
spectrophotometrically using
a
calibration plot for each dye
(A = 0.0278C + 0.0520 and A = 0.0533C + 0.2533 for CR and NR,
respectively). The amount of adsorbed dye at time t was calculated
using the following equation: qt = (C0 ꢁ Ct)V/m, where qt is the
adsorption capacity, C0 and Ct are the initial and the equilibrium
concentration of the dye, V is the volume of the sample solution
and m is the mass of the adsorbent.
2. Experimental
2.1. Materials and physical measurements
The starting materials were purchased from commercial
sources and used without further purification. Infrared spectra
(4000–400 cmꢁ1) were recorded as KBr discs with a BOMEN
MB102 or Perkin Elmer Spectrum Two FT-IR spectrometers. Ele-
mental analyses for C, H, and N were performed on a Thermo Fin-
nigan Flash EA 1120 CHN analyzer. UV–Visible spectra were
recorded on a GBC Cintra 1010 spectrophotometer. Particle size
and morphology of CdO NPs were determined respectively by
dynamic light scattering using a Qudix Inc. Scatteroscope I and
by scanning electron microscopy using a KYKY-EM3200e. Powder
X-ray diffraction (PXRD) patterns were recorded on a Philips X’Pert
2.5. Synthesis of CdO nanoparticles
Cd-MOF (0.207 g) was transferred into a 25 mL crucible and
heated at 500 °C for 5 h. Upon gradual cooling, red-brown nanopar-
ticles of CdO were obtained, collected (0.05 g, yield: 72%), and char-
acterized by FT-IR spectroscopy, PXRD analysis, particle size
analysis, and scanning electron microscopy.
2.6. Single crystal X-ray diffraction analysis
A full-sphere dataset was collected at 150(2) K on a Bruker Apex
II diffractometer using graphite-monochromatized Mo K
tion (k = 0.71073 Å), operating in -scan mode. Even if the reflec-
Pro diffractometer (Cu K
a radiation, k = 1.54184 Å) in the 5–
a radia-
50°/90° 2h range. The PXRD pattern was simulated on the base of
single-crystal data using Mercury [17]. Topological analysis was
performed using ToposPro [18].
x
tions could be indexed with a monoclinic unit cell, a careful
inspection of the diffraction spots in the reciprocal space and a
subsequent analysis with CELL_NOW [20] showed that the crystal
was a non-merohedral twin composed of two triclinic domains of
comparable dimensions (the twin operator being a 180° rotation
about b*). All attempts to isolate a non-twinned specimen, chang-
ing also the crystallization conditions, were unsuccessful. The
frames has been integrated refining a single triclinic unit cell and
two orientation matrices (one for each domain). A multi-scan
absorption correction based on a multipolar spherical harmonic
expansion of equivalent intensities was applied to all data using
TWINABS [20].
The structure was solved using direct methods and refined with
a full-matrix least-squares procedure based on F2, using all data
[20]. The limited number of observed reflections, due to the partial
superposition of the diffraction patterns of the two domains and to
the overall low quality of the crystal, did not allow a free anisotro-
pic refinement of the structure. To overcome the problem, some
restraints on the atomic displacement parameters have been
employed (ISOR, RIGU, and SIMU). In addition, the three dimethyl-
formamide solvate molecules were restrained to have a similar
geometry (SAME) and were refined isotropically, with a common
atomic displacement parameter. Hydrogen atoms were placed at
geometrically calculated positions and refined riding on their
parent carbon atoms.
2.2. Preparation of 1,4-bis(4-pyridyl)-2,3-diaza-1,3-butadiene
The ligand 1,4-bis(4-pyridyl)-2,3-diaza-1,3-butadiene (4-bpdb)
was prepared according to a previously published method [19].
Typically, 0.53 mL (11 mmol) of hydrazine (80 wt% solution in
water) was added dropwise to a solution of 4-pyridinecarboxalde-
hyde (2.1 mL, 22 mmol) in ethanol (15 mL). Two drops of formic
acid were added and the mixture was stirred at room temperature
for 24 h. The yellow solid was filtered, washed several times with
ethanol and diethyl ether (2 ꢂ 5 mL), and dried in air (yield: 86%).
2.3. Synthesis of [Cd3(BTC)2(4-bpdb)2] (Cd-MOF)
Cd(NO3)2ꢀ4H2O (0.102 g, 0.33 mmol), benzene-1,3,5-tricarboxylic
acid (H3BTC, 0.046 g, 0.22 mmol), and 1,4-bis(4-pyridyl)-2,3-diaza-
1,3-butadiene (4-bpdb, 0.046 g, 0.22 mmol) were dissolved in 7 mL
DMF in three separate test tubes.
The three solutions were warmed at 80 °C for 10 min. Then the
solution of 4-bpdb was added to that of Cd(NO3)2ꢀ4H2O and, after
stirring for 10 min, also that of H3BTC was added to the mixture.
The resulting yellowish solution was placed in a teflon-lined stain-
less steel autoclave and heated at 90 °C for 72 h. The mixture was

S.G. Ghomshehzadeh et al. / Polyhedron 176 (2020) 114265
3
Table 1
Crystallographic data and structure refinement details for Cd-MOFꢀ3DMF.
Cd-MOFꢀ3DMF
Formula
Formula mass
T (K)
C
42H26Cd3N8O12ꢀ3(C3H7NO) = C51H47Cd3N11O15
1391.19
150(2)
Crystal system
Space group
a (Å)
b (Å)
c (Å)
triclinic
P–1
10.2682(9)
15.9251(14)
17.0692(15)
85.187(1)
77.217(1)
85.961(1)
2708.5(4)
2
a
(°)
b (°)
c
(°)
V (Å3)
Z
Dcalcd (g cmꢁ3
)
1.706
Fig. 1. Experimental (red) and simulated (green, from single-crystal data) powder
X-ray diffraction patterns. (For interpretation of the references to colour in this
figure legend, the reader is referred to the web version of this article.)
No. of reflns collected
No. of independent reflns
No. of observed reflns
Rint
17,661
9129
4971
0.0665
(sin h/k)max (Åꢁ1
)
0.841
gen atoms of three distinct BTC3ꢁ ligands in the equatorial posi-
tions (CdAO in the range 2.292(6)–2.660(6) Å) and two nitrogen
atoms of two symmetry-related 4-bpdb molecules in the axial
positions (CdAN in the range 2.291(8)–2.306(8) Å). The chelat-
ing-bridging coordination mode adopted by the two BTC3ꢁ ligands
about Cd1 and Cd2 gives rise to dimeric ‘‘[CdBTC]2” units charac-
terized by a CdꢀꢀꢀCd distance of 3.8163(10) Å (see Figs. 2a and
S1a). The third independent metal atom Cd3 is connected to three
‘‘[CdBTC]2” dimeric units by four distinct BTC3ꢁ ligands, leading to
Data/restraints/params
S (all data)
9129/1326/560
0.919
0.0631
R1 [I > 2
r
(I)]
wR2 (all data)
0.1641
Crystal data and details of structure refinement are listed in
Table 1, while relevant geometrical parameters describing the
coordination environment of the three independent cadmium
atoms are listed in Table S1.
a
CdO6 coordination environment with a distorted trigonal-
prismatic geometry (CdAO in the range 2.294(7)–2.382(7) Å).
Fully deprotonated BTC3ꢁ ligands act as m5-bridging groups via
two chelating-bridging carboxylate moieties and one chelating car-
boxylate moiety (2.21 and 1.11 according to the Harris notation
[22], respectively) (Fig. S1a).
3. Results and discussion
3.1. Synthesis of [Cd3(BTC)2(4-bpdb)2]
Extended connection of Cd(II) centers by the BTC3ꢁ linkers in
the ac plane forms a wavy thick layer with a thickness of about
5.7 Å (Fig. S1c). Along the b direction these thick layers are pillared
by bridging 4-bpdb ligands which, connecting Cd1 and Cd2
atoms belonging to adjacent layers, give the single 3D framework
(Fig. 2b,c). The N. . .N distance along the bridging trans-4-bpdb
ligands is 11.4 Å, while the pillaring distance, defined as the
CdANꢀꢀꢀNACd separation, is 15.9 Å. In order to have a better insight
into the complex 3D structure, a topological analysis was also per-
formed. Simplification of the structure by the program ToposPro
[18] gives a (4-c)(5-c)2(5-c)2-connected network with point sym-
bol (42.67.8)2(43.62.8)(45.64.8)2. In this network description the
4-c node coincides with Cd3 atoms, one of the two 5-c nodes coin-
cides with Cd1 and Cd2 atoms while the other lies on the BTC3ꢁ
ligands. Moreover, this 3D network can be seen as derived from a
(3-c)2(4-c)(5-c) 2D network with point symbol (42.6)2(43.62.8)
(45.64.8)2 that, by pillaring, transforms Cd1 and Cd2 atoms from
3-c to 5-c nodes and the 2D thick layers to a 3D framework
(Fig. 2b,c). The topological type for this 3D net is jcr7, as reported
in the database of ToposPro (personal.ttd). Interestingly, this is the
second example of a coordination polymer with jcr7 topology. The
other crystal structure showing this topology is strictly related to
the present species being an isostructural pillared MOF in which
the only difference can be found in the bispyridyl ligand, namely
[Cd3(BTC)2(pbptz)2] (pbptz = 3,6-bis(4-pyridyl)-1,2,4,5-tetrazine)
[23]. The presence of different pillars leads to slightly different
NꢀꢀꢀN and CdꢀꢀꢀCd distances (11.2 and 15.8 Å vs 11.4 and 15.9 Å in
[Cd3(BTC)2(pbptz)2] and in the present structure, respectively). A
more evident difference concerns the thickness of the [Cd3(BTC)2]
layers in the two structures (5.0 vs 5.7 Å, respectively). The
increased thickness of the layer in the present structure may be
attributed to a skewed orientation of the NꢀꢀꢀN vector of the 4-bpdb
Metal-organic coordination networks containing linear bis(pyr-
idyl) pillars and multi-carboxylate ligands show a variety of struc-
tures and network topologies, which are strongly influenced by the
geometry of the multi-carboxylate linkers and the coordination
geometry of the metal centers. In particular, self-assembly of the
4-bpdb pillar ligand with linear or V-shaped benzenedicarboxylate
linkers (1,4-BDC and 1,3-BDC, respectively) afforded 3D MOFs with
pcu topology [4a,b] or more varied topologies [4d,21], respectively.
In this work, we use the 1,3,5-benzentricarboxylate ligand as a
planar 3-connected linker and the 4-bpdb ligand as a long bis(pyr-
idyl) pillar in presence of large Cd(II) ions to generate a MOF con-
taining metal centers with high coordination numbers.
Solvothermal reaction of Cd(NO3)2ꢀ4H2O, 4-bpdb, and H3BTC in a
3:2:2 M ratio was carried out at 90 °C in DMF obtaining single crys-
tals of Cd-MOF of formula [Cd3(BTC)2(4-bpdb)2].
The new species has also been prepared as a bulk by applying
more quantities of starting materials in order to obtain a sufficient
amount of microcrystalline material to be employed in the subse-
quently described dye adsorption and CdO nanoparticles prepara-
tion experiments. The purity of the obtained bulk material has
been checked by comparing its experimental PXRD spectrum with
the spectrum simulated on the basis of single-crystal data (Fig. 1).
3.2. Crystal structure and characterization of [Cd3(BTC)2(4-bpdb)2]
The compound crystallizes as a twin in the triclinic space group
P–1 with Z = 2 (Table 1) and shows an open 3D framework archi-
tecture. The asymmetric unit contains three crystallographically
independent Cd(II) ions, two BTC3ꢁ anions and two 4-bpdb mole-
cules (Fig. 2a). Three solvate dimethylformamide molecules are
also present in the asymmetric unit. Cd1 and Cd2 have a distorted
pentagonal bipyramidal geometry, being coordinated to five oxy-

4
S.G. Ghomshehzadeh et al. / Polyhedron 176 (2020) 114265
Fig. 2. (a) A view of the coordination environment of the three independent cadmium atoms Cd1, Cd2, and Cd3. Superscripts i–v refer to symmetry-related positions
described in the footnote of Table S1. (b) View of the 3D pillared framework along the a axis. (c) View of the simplified 3D network of jcr7 topology along the c axis and (d)
view along the b axis showing the simplified 2D thick layer and the position of its nodes of different connectivity.
ligand with respect to the mean plane of the layer (in the pbptz
derivative this vector is exactly orthogonal to the layer).
The pillared 3D structure displays free void distributed in chan-
nels running along c, decorated by the azine groups of the 4-bpdb
ligands, corresponding to about 30% of the unit cell volume (Platon
[24]). A depiction of the channels is reported in Fig. S2.
The infra-red spectrum of Cd-MOF shows characteristic bands
of the BTC3ꢁ ligand in the range 370–1610 cmꢁ1 (Fig. S3). The
bands observed at 1373 and 1435 cmꢁ1 can be attributed to the
symmetric stretching modes of the carboxylate groups; their rela-
tive intensity 2:1 is in accordance with the presence in the crystal
structure of two chelating-bridging (2.21) carboxylate groups and
one chelating (1.11) carboxylate group [23]. The corresponding
two asymmetric vibration bands are observed at higher wave num-
ber, 1574 and 1615 cmꢁ1; their apparent intensity ratio is per-
MOF shows better adsorptions performance for CR and NR
(Fig. S4). During the adsorption of dye molecules the color of the
adsorbent changes from yellow to red for CR and NR, to blue for
MB and to orange for MO, while the color of the solution fades with
time (Fig. S5). Thus CR and NR dyes were selected for more in
depth investigations. The change in the absorption spectra of solu-
tions of CR and NR, together with the fading of their color follow-
ing their treatment with the Cd-MOF powder (Fig. 3) clearly
indicate the removal of the dyes from the solution due to their sig-
nificant adsorption on the MOF.
3.3.1. Adsorption isotherms
Adsorption isotherms present the relationship between the
equilibrium concentration of the dye in solution and the amount
of species adsorbed on the adsorbent, at a given temperature. They
provide valuable information about the adsorption mechanism
that can be employed in designing new adsorbing systems. In this
study, data has been analyzed using the most commonly employed
adsorption isotherm models (Langmuir, Freundlich, and Temkin
equations).
turbed by the overlap of the higher energy band with the m(C@N)
stretching bands of the 4-bpdb ligand, which in the free ligand
appear at 1594 and 1628 cmꢁ1
.
3.3. Adsorption of organic dyes
The adsorption behavior of the organic dyes Congo Red (CR),
Neutral Red (NR), Methylene Blue (MB), and Methyl Orange (MO)
by the synthesized MOF was investigated. Results indicate that the
The Langmuir equation describes a system constituted by a
homogeneous surface, whose adsorption sites are all energetically
identical, onto which the ability of a molecules to bind to a site is

S.G. Ghomshehzadeh et al. / Polyhedron 176 (2020) 114265
5
Fig. 3. Temporal evolution of the UV–Vis absorption spectra of (a) CR and (b) NR in water. The photographs show the color of the dye solutions, and of Cd-MOF before and
after 1 and 24 h of CR and NR adsorption, respectively.
Table 2
independent of whether or not nearby sites are occupied [25]. Its
Langmuir, Freundlich, and Temkin isotherms adsorption parameters.
expression is:
Isotherm
model
Parameters
Congo Red (CR) Neutral Red (NR)
Ce
1
Ce
¼
þ
qe KLqm qm
Langmuir
qm (mg/g)
KL (L/g)
192.3
3.71
243.9
0.17
in which Ce is the equilibrium concentration of the dye on the
adsorbent (mg Lꢁ1), qe is the amount of dye adsorbed per unit mass
of adsorbent at the equilibrium concentration (mg gꢁ1), qm is the
maximum adsorption capacity (mg gꢁ1), and KL is Langmuir con-
stant, related to the sorption energy. Values of qm and KL can be
determined from a plot of 1/qe vs 1/Ce.
The Freundlich equation describes a system constituted by a
heterogeneous surface onto which molecules can be adsorbed
forming a multilayer [26]. Its linear form is:
R2
0.991
65.11
0.990
29.63
Freundlich
KF [(mg/g)/(mg/
L)1/n
n
]
4.40
1.25
R2
0.043
30.65
32.33
0.384
0.981
6.97
31.61
0.893
Temkin
KT (L/g)
b (J/mol)
R2
1
n
tion. The pseudo-first order kinetic model is given by the Lagergren
equation:
log qe ¼ log KF þ log Ce
in which KF and n are parameters related to the adsorption
capacity and intensity, respectively. Values of KF and n can be
determined from a plot of log qe vs log Ce.
The Temkin equation assumes that, due to an indirect adsor-
bate/adsorbate interaction, heat of adsorption decrease linearly
with the increase of surface coverage (rather than logarithmically,
as implied by the Freundlich equation) [27]. Its expression is:
k1t
log ðqe ꢁ qtÞ ¼ log qe ꢁ
ln 10
in which qe and qt are the amounts of dye adsorbed onto the
adsorbent (mg gꢁ1) at the equilibrium and at time t, respectively
and k1 is the first-order rate constant (l minꢁ1) for the adsorption.
A plot of log(qe ꢁ qt) vs t can be used to obtain the rate parameters.
The pseudo-second order kinetics is described by the equation:
RT
b
RT
b
qe ¼
ln KT þ
ln Ce
t
1
t
¼
þ
qt k2q2e qe
in which KT is the equilibrium binding constant (L gꢁ1) and b is
related to the heat of adsorption (J molꢁ1). Values of KT and b can
be determined from a plot of qe vs ln Ce.
The isotherm constants and the correlation coefficients were
determined for the Langmuir, Freundlich, and Temkin isotherms
from a plot of 1/qe vs 1/Ce, plot of log qe vs log Ce and a plot of qe
vs ln Ce, respectively. The results listed in Table 2 indicate that,
for both dyes the adsorption process can be described by the Lang-
muir model (R2 equal to 0.991 and 0.990 for CR and NR, respec-
tively). In addition, the adsorption of NR can also be described,
even if with a less accuracy, by the two other models.
in which k2 is the second-order rate constant (g mgꢁ1 minꢁ1). A
plot of t/qt vs t can be used to obtain the rate parameters. The
pseudo-first order and pseudo-second order kinetic models for
the adsorption of CR and NR was investigated using the above
equations. Results indicate that the adsorption of CR can be fitted
by a first-order kinetic model with a correlation coefficient of
0.991 while the adsorption of NR can be fitted to a second-order
model with a correlation coefficient of 0.990 (Fig. S6).
Various mechanisms have been reported for dye adsorption by
MOFs, including anion exchange for ionic MOFs, encapsulation,
electrostatic interactions, and also weak interactions, such as H-
3.3.2. Adsorption kinetics
bond and p-p stacking [28,29]. In this work, as clearly indicates
Kinetic models of pseudo-first order and pseudo-second order
were applied to predict the variation with time of the concentra-
tion of the adsorbed dyes. The pseudo-first order model is most
widely used for the adsorption of an analyte from aqueous solu-
by the crystal structure of Cd-MOF, the channels’ cross section
does not allow the entrance of the dye molecules into the frame-
work. On the other hand, due to the overall neutral nature of the
structure, ion exchange and encapsulation are not the predominant

6
S.G. Ghomshehzadeh et al. / Polyhedron 176 (2020) 114265
mechanisms of adsorption. However,
p
-
p
stacking interactions
are formed. CdO NPs were characterized by FT-IR spectroscopy,
between the aromatic rings of BTC3ꢁ and 4-bpdb and the aromatic
rings of CR and NR, in combination with H-bonds, may be respon-
sible for the adsorption of the dye molecules on the external sur-
face of the Cd-MOF particles.
PXRD, particle size analysis, and scanning electron microscopy.
As it is usual for metal-oxide nanoparticles, FT-IR spectrum of
the obtained material is simple, due to the release of the organic
parts of the 3D architecture (Fig. S3). The peak observed at
618 cmꢁ1 is assigned to the CdAO bond, while the peaks with weak
to medium intensity at 1660 and 3440 cmꢁ1 are attributed to the
bending and stretching vibrations of the water molecules adsorbed
by CdO [31].
The PXRD pattern of the red-brown product confirms the for-
mation of the cubic monteponite structure of CdO (JCPDS No.
05–0640, a = 4.6953 Å). The sharp and well defined peaks in the
PXRD pattern indicate the crystalline nature of CdO NPs; no other
impurity peaks were observed in the product (Fig. 4a). SEM images
of the particles showed their spherical morphology (Fig. 4b) and a
particle size analysis gave an average particle size of 48 nm with a
standard deviation of 15 nm (Fig. S7).
Adsorption of CR and NR was also investigated by FT-IR spec-
troscopy (Fig. S3). Due to the superposition of the vibration bands
of the loaded dyes with those of the organic linkers constituting
the MOF skeleton, it is difficult to precisely assign the observed
peaks in the spectra, however the band detected at 1043 cmꢁ1
may be assigned to the m(S@O) stretching of the sulfonate groups
of CR.
3.4. Synthesis of cadmium-oxide nanoparticles
Cadmium-oxide nanoparticles (CdO NPs) show unique physical
and chemical properties and applications in electrochemical
devices, information displays, sensor devices, and smart windows.
Size, morphology, and physico-chemical properties of CdO NPs
depends on various factors such as: method of preparation, size,
morphology, and type of precursors [30]. For these reasons we
attempted the preparation of CdO NPs by thermal decomposition
of the [Cd3(BTC)2(4-bpdb)2] precursor under normal atmospheric
pressure at 500 °C. During the calcination process, organic compo-
nents of the 3D framework burn and cadmium oxide nanoparticles
4. Conclusion
A new pillared porous cadmium-organic framework with an
uncommon topology has been prepared by means of a mixed-
ligand strategy under solvothermal conditions. The topology of
the 3D net corresponds to a (4-c)(5-c)2(5-c)2-connected network
of type jcr7. It was found that this MOF could efficiently adsorb
organic dyes with an adsorption capacity of 192.3 and 243.9 mg gꢁ1
for CR and NR, respectively. The adsorption process of MOF was
well fitted by pseudo-first order and pseudo-second order reaction
kinetics for CR and NR solutions, respectively. Moreover, the MOF
can be used as a precursor in a simple preparation of CdO
nanoparticles.
Declaration of Competing Interest
The authors declare that they have no known competing finan-
cial interests or personal relationships that could have appeared
to influence the work reported in this paper.
Acknowledgements
The Authors thank Shahid Chamran University of Ahvaz (Grant
No.: SCU.SC98.206) and the Università degli Studi di Milano (Piano
di Sviluppo di Ateneo, azione B, progetti di interesse interdisci-
plinare PSR2015-1716FDEMA-07) for financial support. PM and
LC thank one of the reviewers for helpful advices about the crystal-
lographic study.
Appendix A. Supplementary data
CCDC 1902635 contains the supplementary crystallographic
data for Cd-MOF. These data can be obtained free of charge via
http://www.ccdc.cam.ac.uk/conts/retrieving.html, or from the
Cambridge Crystallographic Data Centre, 12 Union Road, Cam-
bridge CB2 1EZ, UK; fax: (+44) 1223-336-033; or e-mail:
deposit@ccdc.cam.ac.uk. Supplementary data to this article can
be found online at https://doi.org/10.1016/j.poly.2019.114265.
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