Selective and reversible adsorption of cationic dyes by mixed ligand Zn(II) coordination polymers synthesized by reactant ratio modulation

Article abstract of DOI:10.1039/c7dt03667a

Dye capture and separation through coordination polymers (CPs) has been a promising research field in recent times due to the toxic and nondegradable nature of organic dyes released into the environment from various industries as well as the reusability of CPs for the said purpose. Here, we report the synthesis and characterization of two mixed ligand CPs {[Zn₂(HBTC)₂(L)(H₂O)₂](C₂H₅OH)₃}_n (CP1) and {[Zn₅(BTC)₂(L)₃(OH)₄(H₂O)₂](H₂O)₄(CH₃OH)₁₁}_n (CP2) (where H₃BTC = 1,3,5-benzene tricarboxylic acid and L = 1,4-bis(4-pyridyl)-2,3-diaza-1,3-butadiene) by the stoichiometric variation of the precursors. The crystal structure analysis revealed that CP1 is a 2D network composed of a [Zn₂(HBTC)₂(H₂O)₂]_n motif linked via terminal nitrogen atoms of L and CP2 is a 3D framework in which symmetrically disposed two-dimensional {[Zn₅(BTC)₂(L)₃(OH)₄(H₂O)₂]}_n sheets composed of pentanuclear [Zn₅(RCO₂)₆(μ₃-OH)₂(μ₂-OH)₂(H₂O)₂] SBUs are pillared by L ligands. Adsorption and separation of cationic dyes by CP1 and the solid-state fluorescence properties of both CPs have been investigated. Cationic dyes (RhB, MB, and MV) can be effectively adsorbed by CP1 from their aqueous solution (61%, 90%, and 97%, respectively) while the anionic dye methyl orange (MO) remains uncaptured. Dye desorption studies and CP1 as a column chromatographic filler for the separation of cationic dyes in water have also been demonstrated.

Full text of DOI:10.1039/c7dt03667a

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ISSN 1477-9226
PAPER
Joseph T. Hupp, Omar K. Farha et al.
Efficient extraction of sulfate from water using
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Selective and Reversible Adsorption of Cationic Dyes by Mixed
Ligand Zn(II) Coordination Polymers Synthesized by Reactants
Ratio Modulation
eceived 00th January 20xx,
Accepted 00th January 20xx
Yadagiri Rachuri,a,b Sreevalsa Subhagan,b Bhavesh Parmar,a,b Kamal Kumar Bishtc and Eringathodi
DOI: 10.1039/x0xx00000x
Suresha,b
*
www.rsc.org/
Dye capture and separation through coordination polymers (CPs) has been a promising research field in recent times due
to the toxic and nondegradable nature of organic dyes released to the environment from various industries as well as the
reusability of CPs for the said purpose. Here, we report the synthesis and characterization of two mixed ligand CPs
{[Zn2(HBTC)2(L)(H2O)2](C2H5OH)3}n (CP1) and {[Zn5(BTC)2(L)3(OH)4(H2O)2](H2O)4(CH3OH)11
}
(CP2), (where, H3BTC = 1,3,5–
n
benzene tricarboxylic acid and L
precursors. Crystal structure revealed that CP1 is a 2D network composed of [Zn2(HBTC)2(H2O)2]n motif linked via terminal
nitrogen atoms of and CP2 is 3D framework in which symmetrically disposed two dimensional
= 1,4–bis(4–pyridyl)–2,3–diaza–1,3–butadiene) by stoichiometry variation of the
L
a
{[Zn5(BTC)2(L)3(OH)4(H2O)2]}n sheets composed of pentanuclear [Zn5(RCO2)6(µ3–OH)2(µ2–OH)2(H2O)2] SBUs are pillared by L
ligands. Adsorption and separation of cationic dyes by CP1 and solid–state fluorescence properties of both CPs have been
investigated. Cationic dyes (RhB, MB, MV) can be effectively adsorbed by CP1 from their aqueous solution (61%, 90%, and
97%) while anionic dye methyl orange (MO) remains uncaptured. Dye desorption studies and CP1 as a column
chromatographic filler for the separation of cationic dyes in water has also been demonstrated.
waste before being discharged into nature. To perform this
task, several approaches have been exploited such as chemical
oxidation, photocatalytic degradation, biological treatment,
Introduction
Environment protection has been a consensus of governments
and scientists all over the world and billions of expenditures
were cast to treat the pollution such as heavy metals,1–2
organic waste water pollution3, nuclear radiation pollution4
etc. Organic dyes are one of the major environmental
pollutants in water because of their non–degradable nature
and wide distribution. Synthetic organic dyes have been widely
used in paper, printing, textiles, plastics and cosmetics
industries. Due to large amount of dyes being used in textile
and paper industries, dyed wastewater is discharged to natural
water sources which are harmful not only to fish and other
aquatic organisms but also for human health. Considering
environmental and safety perspective, development of
feasible technology is essential to remove dyes from liquid
adsorption etc.5–9 Among these methods, adsorption
perceived as one of the most feasible, effective, simple
approach and number of materials such as activated carbon,
zeolites, layered double hydroxides, porous organic polymers
and metal–organic frameworks (MOFs) have been developed
as dye adsorbents.10–16 In view of anionic or cationic nature
and different sizes of dye molecules, Coordination Polymers
(CPs) or MOFs offering tunable pore size and framework
charge are promising materials for adsorption and separation
of dyes. Hence, based on the nature of CPs/MOFs as host,
adsorption by different supramolecular interactions such as
hydrogen bonding, π···π stacking, electrostatic interactions etc.
can be exploited in the selective uptake of dyes.16-18
CPs or MOFs are crystalline materials which are well known
over the past decades due to their various applications in the
area of gas storage/separation, removal of hazardous
molecules, catalysis and biomedecine.19–34 The tunability of
pore size/shape by changing the connectivity of the inorganic
moiety and sensible choice of the multidentate ligands in
CPs/MOFs and thermal/chemical stability of these materials
has captured particular interest towards diverse
applications.35–37 Highly structured and ordered CPs can be
synthesized in many different mechanical or chemical
procedures such as solvo–thermal method, slow evaporation
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of a precursor solution, layering of solutions or slow diffusion were determined using BRUKER SMART APEX (CCD)
of the components and mechano–chemical method.38–40 diffracteomter. Liquid and solid–state UV–Vis spectra were
Besides choice of the metal nodes with versatile coordination recorded using Varian model CARY 500/ Shimadzu UV–3101PC
geometry and multidentate organic linkers, stoichiometry of spectrometer and photoluminescence spectra were recorded
the precursors, reaction condition such as temperature, at room temperature utilizing Fluorolog Horiba Jobin Yvon
solvents used in the reaction and pH of the reaction media etc. spectrophotometer. Field Emission–Scanning Electron
can play important role in fabrication and molding CPs/MOFs Microscopy (FE–SEM) was used for SEM images of CP1
with multidimensional framework and topology. Structural morphology studies. Zeta potential (ζ) measured using a
alterations in CPs/MOFs have been studied and reported by Zetasizer Nano ZS light scattering apparatus (Malvern
manipulating reaction conditions such as temperature, Instruments, U.K.) with a He−Ne laser (633 nm, 4 mW) at
solvent, pH and stoichiometry of the precursors in the 298.15 K.
synthetic approach.41–45
Synthesis of CP1 and CP2
We have been involved in the fabrication of functional
Preparation of Stock ligand solution for CP1 (SL1):
CPs/MOFs with different dimensionality utilizing N–donor
ligands and multi–carboxylic acids. Recently, we have reported
some mixed ligand luminescent coordination polymers for
selective sensing and detection of noxious chemicals such as
nitroaromatics, toxic heavy metals and chromium
oxyanions.40,46–47 In continuation of our ongoing research,
herein we report Zn(II) based 2D and 3D CPs,
{[Zn2(HBTC)2(L)(H2O)2](C2H5OH)3}n(CP1)and
H3BTC (42 mg, 0.2 mmol) was added to NaOH (25 mg, 0.64
mmol) in 20 mL of water with constant stirring to make clear
solution. L (42 mg, 0.2 mmol) was dissolved in 20 mL ethanol
separately. Both solutions were mixed together by constant
stirring for 15 minutes, filtered and used as stock ligand
solution.
Preparation of Stock ligand solution for CP2 (SL2):
{[Zn5(BTC)2(L)3(OH)4 (H2O)2](H2O)4(CH3OH)11}n
H3BTC = 1,3,5–benzene tricarboxylic acid and L = 1,4–bis(4–
pyridyl)–2,3–diaza–1,3–butadiene) synthesized by mixed
(CP2), (where,
H3BTC (210 mg, 1 mmol) was added to NaOH (144 mg, 3.6
mmol) in 20 mL of water with constant stirring to make clear
solution. L (210 mg, 1 mmol) was dissolved in 20 mL ethanol
separately. Both solutions were mixed together by constant
stirring for 15 minutes, filtered and used as stock ligand
solution.
ligand strategy. Subtle changes in the molar concentrations of
the reactants under the same experimental condition resulted
in different products with diverse structures and networks.
Accordingly, we were able to synthesize 2D and 3D Zn(II) CPs
adopting the same layering technique by simply changing the
stoichiometry of the precursors such as metal ion and ligands.
The present study describes reactants ratio modulated
Growth of CP1 Single Crystals:
2 mL of stock ligand solution (SL1) was carefully layered above
the 4 mL of Zn(NO3)2·6H2O (120 mg in 10 mL, 0.4 mmol)
solution with 4 mL buffer (methanol & water 1:1) between the
layers in a narrow test tube. The test tube was covered and
kept for slow diffusion of reactants at room temperature
which afforded yellowish block shaped crystals within a week
synthesis
of
two
CPs,
their
structural details,
photoluminescence properties and the application of CP1 for
selective cationic dye adsorption in aqueous media.
time (Yield
∼76%). Elemental analysis (%) Cal. for
C36H40N4O17Zn2: C, 46.42; H, 4.33; N, 6.01; found: C, 43.8; H,
4.70; N, 5.67; IR cm–1 (KBr): 3451(b), 1631(s), 1579(s), 1449(s),
1381(s), 1111 (w), 1023 (w), 759(s), 719(s), 573(m) and
482(m).
Experimental
Material and General Methods
Precursors for synthesis of 1,4–bis(4–pyridyl)–2,3–diaza–1,3–
butadiene (L) 4–pyridine carbaldehyde, hydrazine hydrate, as
well as other chemicals such as 1,3,5–benzene tricarboxylic
acid, metal salts and solvents were purchased from
commercial sources. Distilled water was used for synthetic
manipulations. The reagents and solvents were used as
received without any further purification. L was synthesized by
slight modification of the synthetic procedure described in
literature and detail procedure is mentioned in supporting
data.48 CHN analyses were done using elementar vario MICRO
Growth of CP2 single Crystals:
2 mL of stock ligand solution (SL2) was carefully layered above
4 mL of Zn(NO3)2·6H2O (297 mg in 10 mL, 1 mmol) solution
with 4 mL buffer (methanol & water 1:1) in between. Pale
Yellowish block shaped crystals were harvested by diffusion of
reactants in the test tube within two weeks (Yield
∼45%).
Elemental analysis (%) Cal. For C65H96N12O33Zn5: C, 41.08; H,
5.09; N, 8.84; found: C, 41.43; H, 4.28; N, 8.92; IR cm–1 (KBr):
3382(b), 1622(s), 1573(s), 1444(s), 1376(s), 1108(w), 1023(w),
764(m), 721(m), 570 (w), 518 (w) and 474 (w).
1
CUBE analyzer. H NMR spectra for the ligand L was recorded
Bulk Synthesis of CP1 by conventional (reflux) method:
on Bruker AX500 spectrometer (500 MHz) at temperature 25
°C and was calibrated with respect to internal reference TMS.
IR spectra were recorded using KBr pellet method on a Perkin
Elmer, G–FTIR spectrometer. TG analysis was carried out using
Mettler Toledo and Netzsch. X–ray powder diffraction (PXRD)
data were collected using a PANalytical Empyrean (PIXcel 3D
detector) system with CuKα radiation. Single crystal structures
For synthesis of CP1 in bulk quantity, 105 mg of (0.5 mmol)
H3BTC was initially mixed with 15 mL aqueous solution of 84
mg (1 mmol) of NaHCO3. To this 15 mL of methanolic solution
of 105 mg (0.5 mmol) of L was added slowly, followed by 297
mg (1 mmol) of 15 mL aqueous solution of zinc nitrate
resulting a pale yellowish turbid precipitate immediately. The
mixture was allowed to reflux with stirring for 6 h at 70 °C. The
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mixture was centrifuged, washed with water followed by monochromatized MoKα radiation (λ = 0.71073 Å) at 150 K for
methanol for several times and dried under ambient both the CPs. No crystal decay was observed during the data
conditions (Yield
H, 4.33; N, 6.01; found: C, 44.83; H, 3.53; N, 6.53; IR cm–1 (KBr): used for data collection and reduction, respectively49. In both
3448 (b), 1630 (s), 1565 (s), 1439 (s), 1380 (s), 1100 (w), 1023 cases, absorption corrections based on multi scans using the
∼89%). Elemental analysis (%) Cal.: C, 46.42; collections. The SMART and SAINT software packages were
(w), 758 (s),725 (s), 577 (m), 492 (m).
SADABS software was also applied.50 The structures were
solved by direct methods SHELXL-2017/1 and were refined on
F2 by the full–matrix least–squares technique.51 All non–
Dye adsorption experiments
Freshly prepared compound CP1 (100 mg) was transferred to hydrogen atoms were refined anisotropically till convergence
the aqueous solutions of methyl orange (MO), methylene blue is reached. The hydrogen atoms of the organic moiety for both
(MB), rhodamine B (RhB) and methyl violet (MV) (10 mL, 5×10– CPs were either located from difference Fourier map or fixed
5 M), respectively. The dye solutions containing the adsorbents stereo chemically. Hydrogen atoms for the coordinated water
were mixed well by constant stirring for 3 hours. After molecules were located from the difference Fourier map and
adsorption for a pre–determined time, solutions were allowed the O–H distances were kept fixed using DFIX command. In the
to centrifuge to isolate the solid CPs from the liquid and the case of CP1, the lattice solvent molecules were highly
concentration of the dye was determined by UV–Vis disordered and the diffused peaks appeared was modelled
spectroscopy. After completion of adsorption process the sensibly by assigning them as ethanol moiety which is used in
amount of the dye adsorbed onto the CP1 was determined the preparation of stock solution in crystallization experiment.
according to the change of concentration before and after Hydrogen atoms of coordinated hydroxyl group is located and
adsorption experiment. The amount of dye adsorbed after 3 h, included in the structure of CP2. After the complete
qe (mg/g), was obtained by the following equation.
refinement of CP2 along with the located lattice solvent
molecule, large number of diffused peaks with residual
ꢀꢁ = (ꢂꢃ − ꢂꢁ )V × MW/m
(1)
electron density ranging from
observed in the difference Fourier map which can be
∼
3.5 e/Å3 to 2 e/Å3 were
where Ci and Ce are initial and equilibrium concentrations of attributed to disordered solvent molecule present in the
dyes (moles/L). V is the volume of the solution (L), MW is the crystal lattice. Attempts to model these disordered peaks were
molecular weight of dyes and m is the mass of adsorbent used unsuccessful since residual electron density was weak and
(g).
there were no obvious major site occupations for the lattice
The removal percentage η of dyes was calculated using the solvent molecule. PLATON/SQUEEZE52 was used to mask the
following equation:
η = (ꢂꢃ − ꢂꢁ )/ꢂꢃ × 100%
diffraction data for the contribution from disordered lattice
solvent (methanol) molecule. The solvent accessible void
volume and the corresponding electron counts/unit cell
estimated was 426 Å3/141e for CP2. This electron count
corresponds to approximately eight disordered methanol
(2)
Dye release study
The dye adsorbed samples, CP1@MV
, CP1@MB and molecules present in the unit cell as solvent of crystallization
CP1@RhB (10 mg) were transferred into 0.5 mL MeOH in a for CP2. Final cycles of least–square refinements with the
cuvette separately. UV–Vis spectra were employed to monitor modified data set after masking the contribution from the
the desorption ability of CP1@dye at scheduled time intervals. disordered solvent molecules significantly improved the R–
values and Goodness of Fit of the structural data in CP2. The
Chromatographic column for dye separation
molecular formula provided in the crystallographic table
A chromatographic column (15 cm height and 0.5 cm includes the lattice solvent molecules.
diameter) was packed with aqueous slurry of CP1 (up to 5 cm),
for the individual separation experiments. In each independent
Results and discussion
experiment aqueous solutions of the respective dyes RhB, MB,
MV and MO (10–4 M) was passed through the column at the
Crystal and Molecular Structure of {[Zn2(HBTC)2(L)(H2O)2]
room temperature. To study the separation experiments of
(C2H5OH)3}n (CP1)
mixture of dyes, MO+RhB, MO+MB and MO+MV (1:1 v/v) were
The X–ray structure of CP1 reveals a 2D framework in which
the asymmetric unit is composed of one crystallographically
allowed to pass through the chromatographic column. The
time taken for the whole separation is approximately 15 min.
unique Zn(II), one monoprotonated trimesic acid (HBTC), half a
The UV–Vis spectra of effluents were studied to confirm the
molecule of N–donor ligand L (with a twofold axis bisecting the
separation capability.
azine bond), one coordinated water and 1.5 lattice ethanol
molecule. Each Zn1 within the basic dimeric unit, holds a
X–ray crystallography
distorted trigonal bipyramidal coordination are linked through
pairs of symmetrically disposed HBTC ligands with opposite
Summary of crystallographic data and selected bond lengths
orientations via μ3–ƞ2ƞ0ƞ1ƞ0 mode of coordination through
and bond angles for CP1 and CP2 are given in Table S3 and S4.
Single–crystal X–ray data collection was performed on a Bruker
SMART APEX CCD diffractometer equipped with graphite
carboxylate oxygen O5 and O1 in the formation of 1D stepped
substructure running along b–axis (Fig. 1a&1b).
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[Zn2(HBTC)2(H2O)2]n motif is further axially coordinated to N
atoms from L to complete the distorted [ZnNO4] trigonal
bipyramidal geometry around the metal center generating a
2D supramolecular architecture. Notably, the L coordinate in
bidentate mode along c–axis with [Zn2(HBTC)2(H2O)2]n motif
alternately in an outward fashion along two sides through the
terminal nitrogen atoms generating the 2D corrugated
rectangular network in CP1 (Fig. 1c). The coordination sphere
of each central Zn(II) atom is provided by four oxygen atoms
(O5, O5#, and O1 from HBTC and O7 from water) and one
nitrogen N1 from L, displaying a distorted trigonal bipyramidal
geometry. The deviation of Zn(II) from the mean coordination
strong intermolecular dimeric association involving O–H···O
interaction is observed between partially occupied hydrogen
of the carboxylic acid from the uncoordinated HBTC moiety
between the neighboring two dimensional networks. Further,
coordinated water hydrogen act as a donor and involved in
intermolecular O–H···O contacts and the disordered lattice
ethanol molecule which is anchored within the rectangular
cavity of the 2D nets. In addition to this O–H···O contact
phenyl hydrogen H14 of L is making a C–H···O interaction
across the dimeric unit with the carboxylate oxygen O6 to
stabilize the 2D framework in the crystal lattice.
trigonal plane, O5–O1–N1, is 0.250 Å, and the axial
∠O5#–
Crystal and Molecular Structure of {[Zn5(BTC)2(L)3(OH)4(H2O)2]
(H2O)4(CH3OH)11}n (CP2)
Zn1–O7 angle 175.18(6)°. Nearest Zn···Zn separation within the
dimeric unit is 3.493(4) Å and within the double layered
[Zn2(HBTC)2]n motif bridged by HBTC is 9.643(5) Å.
CP2 represents a 3D coordination framework and crystallizes
ꢄ
in the triclinic space group P1. The asymmetric unit of CP2
consists two Zn cations (Zn1 and Zn2 at general position) and
half of Zn3 at special position (0.5, 0.5, 1.0), one completely
deprotonated BTC, one and a half molecules of L, two bridging
hydroxyl ions, one coordinated water molecules along with
two water molecules and two methanol molecules as solvent
of crystallization. The framework {[Zn5(BTC)2(L)3(OH)4(H2O)2]
(H2O)4(CH3OH)11}n is based on a secondary building unit (SBU),
[Zn5(RCO2)6(µ3–OH)2(µ2–OH)2(H2O)2]
composed
of
a
pentanuclear Zn(II) cluster, in which Zn3 is occupying the
center of symmetry and surrounded by four symmetrically
disposed Zn nodes (Zn1 and Zn2) bridged via carboxylate as
well as hydroxyl oxygen atoms. The pentanuclear Zn(II) cluster
with coordination geometry around the metal center is
depicted in Fig. 2a. The central metal ions Zn3 and Zn2 are
hexacoordinated and adopt a distorted octahedral geometry
while the coordination around Zn1 is distorted trigonal
bipyramidal. Thus, in Zn3, the square base is composed of the
symmetrically oriented carboxylate and hydroxyl oxygen
atoms O3 and O8 (Zn3-O3 = 2.075(5), Zn3-O8 = 2.062(5)
)
which also acts as a bridging ligand with the neighboring Zn1 in
the cluster formation. Another symmetric hydroxyl oxygen O9
at the axial position (Zn3–O9 = 2.138(5)) completes the
octahedral geometry around Zn3 metal center. Hence,
carboxylate oxygen O3 and hydroxyl oxygen O9 is involved in
μ2–type bridging connecting Zn1, Zn3 and Zn2, Zn3
respectively while hydroxyl oxygen O8 is making μ3
–
coordination linking all the three metal centers Zn1, Zn2 and
Zn3. Octahedral coordination of Zn2 is provided by carboxylate
oxygen atoms O2 and O4 of different BTC ligands, hydroxyl
oxygen atoms O8 and O9, O7 from water and monodentate
coordination through N3 from the dangling L. Square base
around Zn2 is constituted by O2, O4, O7 and O8 and the axial
coordination provided by N3 from L and hydroxyl oxygen O9.
Zn1 possesses a trigonal bipyramidal coordination in which the
trigonal base is provided by carboxylate oxygen atoms O1, O6
and hydroxyl oxygen O8 with the metal ion up by 0.287Å from
the trigonal base towards the axially coordinated N1 with axial
Fig. 1 (a) Coordination environment around metal center depicting trigonal
bipyramidal geometry in CP1, (b) A layered [Zn2(HBTC)2(H2O)2]n 1D stepped
substructure running along b–axis and (c) Two–dimensional pillar–layer type
corrugated rectangular network in CP1
.
Exobidentate nitrogen atoms of L crosslinked between the
adjacent [Zn2(HBTC)2(H2O)2]n motif with Zn···Zn distance
15.439 Å generating the two dimensional motif in CP1. The
through–space apertures within the 2D nets measure 15.439 Å
× 9.643Å and the lattice methanol molecule. In an attempt to
understand the supramolecular interaction packing and
hydrogen bonding interactions are analyzed. Interestingly,
angle
∠N1–Zn1–O3 = 171.54°(5). The nearest Zn(II)···Zn(II)
distance of the symmetrically oriented pentanuclear cluster is
Zn1···Zn2 = 3.441Å, Zn1···Zn3 = 3.324 Å and Zn2···Zn3 = 3.113 Å
respectively. As represented, the pentanuclear SBUs (Fig. 2a)
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are extended into a 2D sheet by versatile bridging mode of the data (Fig. S1a&b). The chemical composition of CP1 and CP2
carboxylate oxygen atoms from the completely deprotonated were also estimated and ascertained by TGA, elemental and
BTC moiety and the hydroxyl oxygen atoms O8 and O9.
single crystal analyses. FTIR spectra for CP1 and CP2 were
recorded by suspending the compounds in KBr pellets (Fig. S2).
For CP1 and CP2 two pairs of νasym and νsym frequencies
appeared at 1631, 1449 cm–1 (Δ
= 198) and 1622, 1444 cm–1 (Δ
ν
= 182), 1579, 1381 cm–1 (Δ
ν
ν
= 178), 1573, 1376 cm–1 (Δν =
197) respectively, corresponding to the carbonyl functionality
of the BTC ligand. Broad bands observed for both CPs in the
region, 3693–3307 cm–1 are attributable to the O–H stretching
modes of coordinated or lattice water molecules. The weak
absorption bands in the fingerprint region, at 573, 482 cm–1 for
CP1, 570, 518, 474 cm–1 for CP2 are assigned to Zn–O and Zn–
N vibration modes. Thermogravimetric analyses (TGA) were
performed in the range of 30–700 °C in inert atmosphere in
order to gauge the thermal behavior and stability of CPs (Fig.
S3). The TG curve of CP1, exhibits weight loss in the range of
50–130 °C which is corroborated to the lattice methanol and
coordinated water molecules (cal. 17.4%, obs. 15.02%),
subsequently the framework decomposed at ca. 330 °C. CP2
expelled lattice methanol and coordinated/uncoordinated
water molecules in the range of 45–251 °C (cal. 18.02%, obs.
18%), followed by the decomposition of the 3D network at ca.
340 °C.
Fig. 2 (a) Coordination geometry around the Zn(II) metal center in the formation
of symmetrically disposed pentanuclear metal cluster in CP2, (b) 2D sheet like
architecture generated in CP2 by the coordination of completely deprotonated
BTC ligand and hydroxyl groups, (c) 3D coordination polymeric net observed in
CP2 and (d)
π···π stacking interaction between the pyridyl rings between
different L ligand moieties of CP2
.
Accordingly, from the BTC ligand carboxylate oxygen O1, O2 is
involved in μ2–ƞ1ƞ1, O3, O4 in μ3–ƞ2ƞ1 and O5, O6 is making
ƞ1ƞ0 mode of binding and hydroxyl oxygen atoms O8 and O9
are involved in μ3 and μ2 mode of coordination in extending the
pentanuclear metal clusters in the formation of 2D sheet like
network in CP2 (Fig. 2b). Two dimensional sheets thus formed
is further pillared alternatively from through N1 of the
symmetrical L ligand in bidentate mode with Zn1 with the
neighboring sheets generating a 3D coordination polymeric net
(Fig. 2c). Interestingly, pairs of the dangling L ligands with
opposite orientation occupied between the square grid
Photoluminescence properties
Photoluminescence properties of MOFs comprising of d10
metal ions and electron rich multidentate ligands have been
extensively studied due to strong emission properties and
potential applications in the field of photo luminescent
chemical sensors.53-57 Luminescence properties of CP1
, CP2, L
and H3BTC were investigated in the solid state and in the
suspension of DMF at room temperature. The emission peaks
for free ligands L and H3BTC in the solid state appeared at 410,
388 nm upon excitation 300, 320 nm respectively, which can
be assigned to the ligand centered π*→π or π*→n electronic
transitions. CP1 and CP2 exhibited emission band nearly at the
same wavelength ca. 445 nm upon excitation at 320 nm (Fig.
S4 & Fig. S5). Nevertheless, suspension of both CPs in DMF
through unidentate coordination with Zn2 and involved in π···π
stacking interaction between the pyridyl rings from either side
(Fig. 2d) (Cg···Cg distance ranging from 3.80 to 3.86Å). The
coordinated and the lattice water molecules O7 and O11 act as
donors and involved in strong O-H···O interactions. Thus, H7B
from O7 is making O-H···O contact with the O1AA of the
methanol and both hydrogen atoms H11A and H11B is making
O-H···O interaction with methanol oxygen O0AA and the
hydroxyl oxygen O9 respectively. Interestingly lattice water
hydrogen H10A is making O-H···N contact with N6 of the
dangling L. All these hydrogen bonding interactions stabilizes
the molecule in the crystal lattice and details of pertinent
hydrogen bonding interactions are given in Table S1.
(∼414 nm) showed shift in emission maximum (∼31nm) which
can be due to solvent effect. In comparison with the emission
of free ligands L and H3BTC, the change in emission
wavelengths for CP1 and CP2 can be attributed to the ligand to
metal charge transfer phenomenon (LMCT).58-59
Selective dye adsorption from aqueous solutions by CP1
Removal of organic dyes from industrial effluents is ongoing
research in the area of separation technology. Recently,
MOFs/CPs have been employed for the separation of
hazardous chemical species from air and industrial effluents.
Especially, charged or functional CPs have been reported as
potential materials for selective adsorption of cationic or
anionic dyes from aqueous solutions.60-62 Generally, there are
three main aspects for the adsorption process: size selectivity,
ion exchange and the electrostatic attraction. Dye adsorption
from aqueous solutions has been reported in the literature,
based on the adsorption process.63-65 These investigations
PXRD, IR and TGA analyses
CP1 synthesized by flexible methods and CP2 have been
characterized by various analytical methods including X–ray
crystallography. Phase purity of CP1 synthesized by different
methods and CP2 crystals obtained by diffusion has been
confirmed by comparing the experimental PXRD profile with
simulated patterns of single crystal X–ray diffraction (SXRD)
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revealed that dyes show selective adsorption depending on dye (5×10–5 M) and allowed to mix by constant stirring at room
the nature of framework. For instance, anionic frameworks temperature for 3 h in dark condition. After the mentioned
prefer cationic dyes and vice–versa 66-67
Neutral functionalized scheduled time for dye adsorption process, color intensity for
.
frameworks towards selective adsorption of cationic dyes over two cationic dyes (RhB, MB) was reduced to some extent and
anionic dyes is reported scarcely.68-69
appreciable color change was observed in the case of MV
(which almost became colorless). Contrastingly, no color
change was shown in the case of the anionic dye MO (Fig. 3a).
Above results clearly indicate the potential of neutral CP1 as
an adsorbent towards cationic dyes over the anionic one. The
concentration of dyes after adsorption experiment was
assessed by UV–Vis spectrometry. The drop–in dye
concentration observed (from 5×10–5 M initial concentration)
within 3 h time is 1.97×10-5 M, 4.67×10–6 M, 1.55×10–6 M and
∼
4.78×10-5 M for the respective dyes RhB, MB, MV and MO
(Fig. 3c–f). Calibration experiments were performed to assess
the amount of adsorbed dye after a given time using the
absorbance of standard aqueous solutions of dyes (Fig. S6).
Based on the equation (2) the calculated adsorption efficiency
(
η
whereas for MO was 20% (Fig. 3g). The adsorption amount of
) for RhB, MB and MV were 61%, 90%, and 97% respectively,
∼
cationic dyes (qe, mg/g) by CP1 was calculated based on
equation (1) and was found to be 1.45, 1.5 and 2 mg/g for RhB,
MB and MV respectively. At higher dye concentrations (10–4 M)
the efficiency of adsorption was increased which turns out to
be 2.8, 2.65 and 3.52 mg/g respectively. These experiments
clearly demonstrated the potential of neutral network CP1 for
the effective removal application of cationic dyes. As depicted
Scheme 1 Molecular structures of dyes used in the present investigation.
In the present investigation, we have explored CP1 as a
selective adsorbent for cationic dyes from aqueous solutions.
An efficient adsorbent material should enjoy outstanding
adsorption property toward the dyes, good chemical stability
and competence for selective removal and separation. To
perform adsorption and separation experiments four organic
dyes have been selected, viz., cationic dyes RhB, MB, MV and
anionic dye, methyl orange (MO) (Scheme 1).
in Fig. 3b, the color of the dye adsorbed CP1 (CP1@dye) in
presence of the respective cationic dyes changed completely
from pale yellow, whereas no substantial color change was
observed in the case CP1@MO
.
Fig. 3 (a) The digital photographs of dye solutions before and after adsorption
representing the cationic dye adsorption over anionic dye by CP1, (b) Color
Fig. 4 FE–SEM micrographs of CP1, before and after adsorption of cationic dyes.
change observed in CP1@Dye material after respective dyes adsorption by CP1
,
As a result of dye adsorption, pale yellow CP1 is converted to
(c–f) UV–Vis spectral changes based on treating CP1 with different
cationic/anionic dyes after 3h and (g) Removal efficiency plot of MO, RhB, MB
and MV after 3h by CP1 (black bars: initial concentration; Pink bars:
concentration after 3 h).
light brown, green and violet colored material in the case of
CP1@RhB, CP1@MB and CP1@MV
. Furthermore, the
CP1@dye materials were characterized by solid state UV–Vis,
FTIR spectra and TG analysis (Fig. S7, S8&S9) and the
morphology of CP1 before and after adsorption of cationic
dyes have been studied by FE–SEM analysis (Fig. 4). FE–SEM
To evaluate the adsorption capacity, 100 mg of CP1 was
suspended into 10 mL of aqueous solutions of the respective
6 | J. Name., 2012, 00, 1-3
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micrographs of CP1@dye indicated the agglomeration with the solution. Motivated by the adsorption results of cationic
spherical morphology of micro–crystalline CP1. PXRD patterns dye, compound CP1 was employed as column
a
of CP1@dye showed the preservation of framework structures chromatographic filler for the separation of dyes from water.
implying that CP1 retains the structural integrity and can be
used efficiently for the removal of MV from the group of dyes
selected in present investigation (Fig. S10). Study of dye
adsorption and separation using functionalized CPs with good
selectivity toward the targeted dyes become a promising
research topic. The selective adsorption of organic dye from
the mixture becomes important considering the recovery of
valuable chemicals from industrial waste water. The selectivity
of CP1 towards cationic dyes encouraged us to investigate
separation study of cationic and anionic dye mixture. For the
selective adsorption, anionic dye methyl orange was taken
along with other three cationic dyes in equimolar ratio (1:1, 10
mL) for the separate batch studies. CP1 (100 mg) was added in
mixture of aqueous dye solutions and allowed to mix
thoroughly by stirring for 3 h in dark condition. Selective
adsorption of the cationic dyes from organic dye mixture by
CP1 was also monitored by UV–Vis spectrophotometer.
Fig. 6 (a–d) UV–Vis spectra of aqueous solutions of dye before separation (black)
and after separation through the CP1 chromatographic column (orange) for
individual dyes (RhB, MB, MV and MO), (f–h) dye mixture and (e–i) digital
photographs illustrating selective separation of cationic dyes by the
chromatographic column of CP1 (A, indicates before separation and B, indicates
after separation).
As displayed in Fig. 6, when 10–4 M solutions of individual
cationic dye stream (RhB, MB and MV) is passed the column
and the collected solutions after passing through the column
found to be colorless, indicating the separation of dyes from
aqueous solutions by adsorption of the column material.
Similar experiments were also conducted for mixed–dye
combination (RhB+MO, MB+MO and MV+MO) stream passing
through the column. The collected solutions after passing
through the column displaying pale orange color indicate the
presence anionic dye MO in solution and subsequent
separation of cationic dyes RhB, MB and MV in the CP1
column. These results further affirm strongly CP1 to be an
excellent candidate for the efficient and selective adsorption
and separation of cationic dyes over anionic dyes.70-71 There
are various factors which can influence the selectivity of
specific molecular dye adsorption by MOFs.72-73 The observed
Fig. 5 (a–f) Selective dye adsorption with mixture of anionic and cationic dyes;
UV–Vis spectral changes and digital photographs showing selective adsorption of
efficient adsorption of CP1 towards MV from selected pool of
RhB, MB and MV over MO by CP1
.
cationic dyes may also be explained on the basis of crystal
structure analysis. In fact, the concomitant effect of
supramolecular/electrostatic interaction between framework
and cationic dye molecule as well as accessible layered gap
between 2D network to accommodate the dye molecule may
As depicted in Fig. 5, UV–Vis spectra clearly disclosed that
absorption intensity of RhB ( 553 nm), MB ( 662 nm) and MV
582 nm) decreases significantly, while the MO absorption
intensity ( 463 nm) showed only marginal decrease. It is also
∼
∼
(∼
∼
probably account for the adsorption of cationic dye by CP1
.
observed by naked eye that color of the mixture of respective
dyes in presence of CP1 after 3 hours of stirring turned to
orange, indicating that methyl orange was left in the solution.
Both spectroscopic as well as visual detection experiments
clearly validate the selective adsorption of respective cationic
dyes by CP1 from the mixture, retaining the MO anionic dye in
Due to the free/coordinated carboxyl groups which can act as
an effective acceptor/donor in H–bonding interactions with
the cationic dyes and the layered gap between 2D network
structure in CP1 can induce the adsorption capacity towards
cationic dyes by various supramolecular interactions. As
revealed in the crystal structure, carboxylate/ carboxylic acid
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of the HBTC ligand of CP1 can act as an active site in experiment. Methanol was employed as a releasing media to
supramolecular/electrostatic interaction with the cationic dyes assess the dye releasing behavior of CP1@dye. Fig. 7a–b
favoring the adsorption process on the surface of the CP1 host shows that the adsorbed cationic dye (MV) molecules are
by accommodating the dye molecule between the 2D layers. progressively released in methanol as represented in UV–Vis
Probably the uncoordinated carboxylic acid group or the spectra indicating the enhancement in the intensity at 578 nm.
coordinated water oxygen lined on the surfaces of the two– The same batch study was performed for release of MB and
dimensional network can involve in supramolecular RhB (Fig. S11&S12). The colorless solution gradually changes to
interactions via H–bonding with the cationic dye and plays an the respective dye color (violet) with time indicating the
important role in the adsorption process.
steady release of dye molecule as shown in digital
photographs (Fig. 7c).
Conclusions
In summary, two Zn(II) based mixed ligand luminescent CPs
containing N–donor ligand L and rigid aromatic tricarboxylate
anions have been synthesized by versatile routes through
reactant ratio modulation. Crystal structure revealed that CP1
is a 2D network composed of [Zn2(HBTC)2(H2O)2]n double
layered motif linked via terminal nitrogen of L. CP2 is a 3D
framework in which the 2D net, {[Zn5(BTC)2(L)3(OH)4 (H2O)2]}n
is composed of symmetrically disposed pentanuclear Zn(II)
SBUs pillared by L ligands. Both CPs were characterized by
various analytical techniques such as CHN, FTIR, TGA and PXRD
to evaluate the phase purity and structural integrity. CP1 was
found to exhibit efficient adsorption of cationic dye methyl
violet (MV) compared to other cationic dyes including anionic
dye methyl orange. In addition, selective adsorption of cationic
dye from mixture of dyes, dye release experiments of CP1 as
well as photoluminescence properties of both CPs were also
investigated. Selective adsorption of cationic dye by neutral
CPs is scantly reported in the literature; simultaneous
influence of supramolecular interaction between the
carboxylate /water oxygen on the surface of the coordination
polymer with the cationic dye and layered gap between 2D
network in CP1 to dwell in the dye moiety may be the
probable basis for the adsorption property. Moreover, the
chromatographic column of CP1 shows a preference for
separating the cationic dyes and selectivity of cationic dyes
from the mixture of cationic and anionic dyes in aqueous
solution, highlighting the efficient aqueous–phase separation
of organic dyes. Scaling up of CP1 synthesis and its application
is a significant advancement for the methods available as an
adsorbent for the aqueous phase separation of cationic dyes
and water purification.
Scheme 2 The possible mechanism of dye adsorption on CP1 by electrostatic and
H–bonding interactions.
Fig.
7 (a) Time dependent UV–Vis spectra depicting enhancement in the
absorbance (λmax = 578 nm) suggesting increase in MV release with time, (b)
absorbance–time profile showing linear plot for the release of MV (30 to 360
min) and (c) digital photographs showing progressive color change due to release
of adsorbed MV in 0.5 mL MeOH after soaking 10 mg of CP1@MV
.
Hence, the protonated amino nitrogen and free nitrogen
atoms of cationic dye MV can comprehend strong N–H···O/O–
H···N interactions with the carboxylic group of the BTC moiety
of CP1 in enhancing the adsorption capacity MV up to 97%
among other cationic dyes (Scheme 2). Observed decrease in
Conflicts of interest
There are no conflicts to declare.
zeta potential (ζ) of CP1@MV (–11.5 mV) compared to CP1 (
–
Acknowledgements
21 mV) also corroborate the strong electrostatic interactions
between adsorbent and adsorbate (Table S2).74-75 The
desorption process is important and interesting in practical
applications. After CP1 was loaded with the respective cationic
dyes, the obtained materials (CP1@dye) were soaked in
methanol for six hours to accomplish the organic dye release
Authors acknowledge the CSIR, India (Project, Grant no. MLP–
0017) for financial support, Mr. Parthrajsinh Sodha for TGA
data, Ms. Riddhi Laiya for PXRD data, Mr. Viral Vakani for
elemental analysis, Ms. Megha Yadav for FTIR data, Mr. Jayesh
Chaudhari for FE–SEM images and AESD&CIF for all round
8 | J. Name., 2012, 00, 1-3
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30 L. Wen, X. Xu, K. Lv, Y. Huang, X. Zheng, L. Zhou, R. Sun, D. Li,
ACS Appl. Mater. Interfaces, 2015, , 4449.
31 X. Zhao, K. Wang, Z. Gao, H. Gao, Z. Xie, X. Du and H. Huang,
Ind. Eng. Chem. Res., 2017, 56, 4496.
32 A. C. Kathalikkattil, D. -W. Kim, J. Tharun, H. -G. Soek, R.
Roshan and D. -W. Park, Green Chem., 2014, 16, 1607.
33 D. Briones, B. Fernández, A. J. Calahorro, D. Fairen-Jimenez,
R. Sanz, F. Martínez, G. Orcajo, E. S. Sebastián, J. M. Seco, C.
S. González, J. Llopis and A. Rodríguez-Diéguez, Cryst. Growth
Des., 2016, 16, 537.
analytical support. YR and BP acknowledge UGC and CSIR
(India) for SRF respectively. Authors are grateful to the
crystallographic reviewer for providing suggestion and
assistance in resolving the problems in the crystal structure
analysis. Publication Registration Number: PRIS 105/2017.
7
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