Tr?ger's base functionalized recyclable porous covalent organic polymer (COP) for dye adsorption from water

Article abstract of DOI:10.1039/d0nj01735c

Protection of the environment from the increasing threats of pollution from several chemical industries in a sustainable way has become a major global challenge. The colored effluents from the textile and dyestuff industries are major sources of pollution, contaminating water bodies. Herein, we have synthesized a covalent organic polymer TBTPACOP having a V-shaped Tr?ger's base unit in the framework with the notion that the unique cleft like architecture of TB will infuse porosity and robustness in the amorphous polymeric material. The covalent organic polymer (COP) was used for the adsorption of anionic acid dyes from aqueous effluent. The equilibrium adsorption capacity (qe) was found to be 188.96 mg g-1. The excellent dye adsorption capacity of the COP, adsorbing 95% of an acid dye (Acid Orange II) at room temperature within 70 min, along with its recyclability up to 4 consecutive cycles, makes TBTPACOP an extremely promising material for selective and efficient removal of acid dyes from aqueous effluent.

Full text of DOI:10.1039/d0nj01735c

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Tröger’s Base Functionalized Recyclable Porous Covalent Organic
Polymer (COP) for the Dye Adsorption from Water
Cite this:DOI: XXXXXXXXXXXXXXXX
Valmik P. Jejurkar,[a] Gauravi Yashwantrao,[a] and Satyajit Saha*[a]
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Abstract: Protection of the environment from the uprising threats of pollution from several chemical industries in a
sustainable way has become the major global challenge. The colored effluents from the textile and dyestuff industries
are the major sources of pollution, contaminating the water bodies. Herein, we have synthesized a covalent organic
polymer TBTPACOP having a V-shaped Tröger’s base unit into the framework with the notion that the unique cleft like
architecture of TB will infuse porosity and robustness in the amorphous polymeric material. the covalent organic
polymer (COP) was used for the adsorption of anionic acid dyes from the aqueous effluent. The equilibrium adsorption
capacity (qe) was found to be 188.96 mg/g. The excellent dye adsorption capacity of the COP by adsorbing 95% of the
acid dye (Acid Orange II) at room temperature within 70 mins, along with its recyclability up to 4 consecutive cycles,
makes TBTPACOP an extremely promising material for selective and efficient removal of acid dyes from the aqueous
effluent.
1. Introduction
separation techniques, chemical sensors, gas storage, catalysis,
optoelectronics, energy storage, as well as biomedical products, etc.20-70
There is a severe rise in the global concern for protecting the
environment from the uprising threats of pollutions created from several
chemical industries.1-3 Textile and dyestuff industries are considered as
the prime polluters due to their discharge of colored effluents into the
hydrosphere. The discharge of colored effluents into the environment
gives an undesirable color to the water even at a very low concentration,
thereby reducing sunlight penetrability, causing serious harm to marine
life. These colored effluents are normally water-soluble organic dyes
which are non-oxidizable as well as non-degradable under light or heat.
Dye contaminated water even raises the threat for several mutagenic and
teratogenic diseases by incorporating the toxic dye molecules into the
food chain.4-7 Hence, the efficient removal of toxic dyes before the
effluent is discharged into the environment throws a major challenge
towards sustainable development.
Several methods have been developed for the removal of dye from
the dye contaminated effluents. However, the development of rapid
cost-effective removal of a low level of dye contaminants is always
challenging.8-12 Decontamination techniques by coagulation, advanced
oxidation process, membrane separation techniques, electrochemical
oxidative methods along with the adsorption by activated charcoal,
multi-walled carbon nanotubes, cedar sawdust rushed brick, graphene-
based materials, chitin, fly ash, silica gels are existing but there are
persistent efforts towards the understanding the mechanism of dye
removal and development of newer methods as well as designing novel
materials for the dye removal from the effluent.13-36
Owing to their large surface area and impressive porosity, COP might
adsorb organic molecules thereby making them an ideal contender for
the removal of dyes from the effluent. However, the exploration of
porous organic materials (POMs) for water treatment is still at its infancy
and it is, therefore, necessary to rationally design COP to extend their
applications towards water treatment.
The properties of a COP are highly dependent on the morphology of
the COP as well as the constituting monomeric linker units, which can be
functionally tuned to improve the corresponding chemical property.
Several organic linkers with diverse functionalities were explored to
synthesize COPs intended for numerous applications. Depending on the
requirement of the tailor-made geometries of the COP with high
structural rigidity, the availability of the building blocks is still limited.
Herein, we have tried to introduce novel building block bearing V-shaped
Tröger’s base (TB) unit into the framework with the intention that this
unique V-shaped molecule will provide structural rigidity and incorporate
a porous hydrophobic cavity within the polymer.
Tröger’s base, although discovered in 1887, its potential was only
realized when it was first used as a chiral host material. Since then, this
unique scaffold has gained tremendous popularity and incorporation of
Troger’s base, possessing two nitrogen centers within any framework, is
known to induce the material towards molecular recognition through
non-covalent supramolecular interaction, as well as impart catalytic
properties, among several other features.71-94 Polymers decorated with
Tröger’s base have been extensively employed in generating membrane
material for gas separations. However, to the best of our knowledge,
there are no reports of Tröger’s-based porous polymer networks studied
in the context of adsorbent materials for the removal of organic dyes.
So, in order to obtain the porous material with the desirable
property, our idea was to exploit the V-shape core of the TB to construct
a covalent organic polymer by chemically linking the C2 symmetric rigid
TB with a C3-symmetric core via forming a covalent bond between them
(Fig. 1). The architecture of COP depends upon the geometry of linkers
present in an altered fashion and the proposed geometry of the COP is
presented in Fig. 1. This strategy will feature the unique cleft of Tröger’s
base in the covalent organic polymer to provide chemical and thermal
stability with a potential hydrophobic surface and opportunity to tweak
the functionality by tuning the surface morphology. The nitrogen atoms
make the pore of the framework polar, suitable for adsorbing the
substrate dye molecules through some non-covalent interaction.
Recently, covalent-organic polymers (COPs), analogous to the
covalent organic framework (COF), have stimulated extensive interest
due to their high adsorption capability, large surface area, high porosity,
chemical tunability, enhanced stability as well as easy synthetic
accessibility.20-25 Covalent organic polymers (COPs) belongs to a group of
porous organic materials constructed by linking several organic building
blocks via covalent bond formation, having applications in drug delivery,
[a] Valmik P. Jejurkar, Gauravi Yashwantrao and Dr. Satyajit Saha
Department of Dyestuff Technology,
Institute of Chemical Technology,
Mumbai-400019, India
E-mail: ss.saha@ictmumbai.edu.in
ORCID Satyajit Saha: 0000-0003-0407-7251
Supporting information for this article is available on the www. under
www.
As part of our contribution towards sustainable development
program,95-97 the present work describes the design and synthesis of an
imine linked COP, TBTPACOP consisting of Tröger’s base linker and its
application towards acid dye removal from the aqueous effluent. The
solvothermal reaction between the two reacting components, one
The synthetic procedures and the 1H and 13C NMR of all the
intermediates.

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bearing a C2-symmetric dialdehyde and the other a C3-symmetric tris-
6
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amine, yielded the mesoporous TBTPACOP in the form of black powder.
2.1. Chemicals and Reagents
The required chemicals were procured from commercial sources like
Sigma-Aldrich, Spectrochem, and Avra and were used directly. The
Fourier transform infrared (FT-IR) spectra were recorded on a Jasco FT-IR-
4100 spectrophotometer. UV/vis spectra were recorded on a Jasco V-750
spectrophotometer. The 13C solid-state CP MAS nuclear magnetic
resonance spectroscopy was carried out at ECZR Series 600 MHz NMR
spectrometer at 18 kHz. Bruker 400 and 500 MHz spectrometers were
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1
used for recording the H and 13C nuclear magnetic resonance spectra in
either CDCl3 or dimethyl sulfoxide-d6 (DMSO-d6) solvent. Perkin-Elmer
Diamond TG/DTA instrument was used to perform the thermogravimetric
analysis (TGA), performed at a heating rate of 10 °C/min under a nitrogen
atmosphere having a flow rate of 50 mL/min. Powder X-ray diffraction
patterns (PXRD) were analyzed with a Bruker D8 Advance X-ray
diffractometer. Scanning electron microscopy (SEM) images were taken
with a Hitachi S-4800 apparatus. The surface area was calculated by the
Brunauer−Emmett−Teller (BET) method. The contact angle was measured
on the ACAM D2A Contact angle meter. The 3D images recorded on the
OLS5000 3D measuring laser microscope.
Fig. 1. The proposed design strategy for the construction of Tröger’s
base incorporated COP.
The amorphous nature of TBTPACOP was confirmed by powder X-ray
diffraction (PXRD) pattern which was then structurally and
morphologically characterized by several analytical techniques like FT-
infrared spectroscopy, solid-state 13C NMR, SEM, and 3D laser confocal
microscopy and nitrogen adsorption isotherms. Dye adsorption kinetics
have also been studied. The thermogravimetric analysis revealed the high
thermal stability of the COP having a surface area of 126 m2g-1 obtained
from the BET surface area analysis. TBTPACOP exhibited superior
adsorption of Acid Orange II dye achieving excellent adsorption capability
by adsorbing 94% of the acid dye within 70 mins at room temperature
with the advantage of recyclability up to 4 cycles. The adsorption
propensity of porous TBTPACOP was selectively observed for acid dyes
over other classes of dyes suggesting selective dye adsorption based on
H-bonding interaction as well as size-dependent separation.
2.2. Synthesis and Characterization
The synthesis of TBTPACOP was performed by reacting 4,4'-(6,12-
dihydro-5,11-methanodibenzo[b,f][1,5]diazocine-2,8-diyl)dibenzaldehyde
3 with tris(4-aminophenyl)amine 6, (Scheme 1). Both the starting
materials 3 and 6 were easily accessed from para-bromo aniline and
para-nitro aniline respectively (Scheme 1). The details of the synthetic
protocols of both the starting materials are referred to in the
experimental section. Synthesis of TBTPACOP was achieved by heating
the mixture of 3 and 6 in the pressure tube for 3 days at 120 oC in
presence of mesitylene, 1,4-dioxane, and acetic acid. TBTPACOP was
obtained as a black powder by simple filtration and washing with ethyl
acetate. The TBTPACOP which was insoluble in almost all common
organic solvents was structurally characterized by infrared and solid-state
13C cross-polarization magic angle spin NMR techniques.
2. Results and Discussion
NH2
Br
N
(HCHO)n
N
= 4,4'-(6,12-dihydro-5,11-methanodibenzo[b,f]
[1,5]diazocine-2,8-diyl)dibenzaldehyde 3
TFA, 24 h,
58 %
Step-I
Br
2
Br
[Pd(PPh3)4], K2CO3,
THF: Water(3:1),
reflux, 12 h. 78%
Step-II
HO OH
B
= tris(4-aminophenyl)amine 6
1
CHO
CHO
N
N
N
F
3
AcOH
OHC
mesitylene/dioxane,
120 oC, 3 days.
NH2
N
5
NO2
91%
NH2
N
N
Step-III
K2CO3, DMSO, 120 oC,
N
N
N
N
48 h, N2, 72 %
N
H2N-NH2 /Pd/C (10%)
1,4-Dioxane/Ethanol,
79%
TBTPACOP
NO2
H2N
NH2
6
4
Scheme 1. Synthetic scheme for the preparation of TBTPACOP.
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2.3. FT-IR and 13C NMR Analysis
thermogram, TBTPACOP exhibited an exothermic glass transition
temperature at 480 oC, indicating its high thermal stability.
The infrared spectra were recorded on
a Jasco (FT-IR-4100)
spectrophotometer. The infrared analysis confirmed the bonding and
structural features of TBTPACOP. The comparison of the infrared spectra
(Fig. 2) of TBTPACOP (black line) with TPA (blue line), and TBBA (red line)
confirms the disappearance of the -NH2 and -CHO peak and appearance
of a -C=N peak. The absorption bands at 3350-3450 cm-1 and 1610 cm-1
attributes to the corresponding stretching frequencies of –N-H of NH2 in
TPA 6 whereas the peak appearing at 1710 cm-1 represents the -C=O
stretching of the –CHO of TBBA 3. Very weak peaks around 2700-2800
cm-1 appeared due to the aldehydic -C-H stretching. Thus, when
compared to the IR spectra of 3 and 6, the intensities and the positions
of the peaks appearing for TBTPACOP have shifted indicating a change in
the chemical environment of those bonds. In peak appearing at 1645 cm-1
in the FT-IR spectra of TBTPACOP (Fig. 2), confirms the presence of –C=N-
along with the absence of -CHO and -NH2 peak.
2.5. Powder XRD
The powder X-ray diffraction pattern of the synthesized TBTPACOP was
analyzed on the Bruker D8 Advance X-ray diffractometer with CuKα
radiation (1.5406 Å). The analysis of the powder XRD pattern of the
TBTPACOP at room temperature revealed a broad peak having 2θ value
21o suggesting amorphous packing of the polymeric network (Fig. 3b,
black line). The corrosponding d-spacing (100 planes) calculated from
Braggs equation (nλ = 2dsinθ) is 0.42 nm.98 This amorphous nature of the
COP could be attributed due to the incorporation of strained linkers
within the framework. The combination of V-shaped Tröger’s base
derived dialdehyde 3 and the tris amine 6 will geometrically not allow the
formation of hexagonal network formation thereby resulting in the
formation of amorphous polymer with broad diffraction peaks quite
contrary to the crystalline COFs reported in the literature.
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2.6. Effect of Organic Solvents/Alkali/Acid/Temperature on The Stability
of the TBTPACOP
The chemical stability of TBTPACOP was also probed by immersing
TBTPACOP for 1h in organic solvents like ethyl acetate and THF as well as
in 2N HCl and 2N NaOH solution and analyzing their powder XRD profile
after filtration and drying (Fig. 3b). The complete retention of the peak
positions in the PXRD patterns along with the conservation of the
amorphous nature clarifies the good resistance of TBTPACOP under
different conditions. The TBTPACOP when subjected to heat treatment at
100 oC and 150 oC for 1 h, retained the PXRD profile without any
deviation (Fig. 3b). The unchanged powder XRD profile features the
inertness of the TBTPACOP framework under different pH, temperature,
in aqueous as well as in organic solvents signifying the robustness of the
polymeric framework.
Fig. 2. Characterization of TBTPACOP by (a) FT-IR and (b) solid-state 13C
NMR spectroscopy.
The 13C (CP-MAS) NMR spectral data confirmed the incorporation of TB in
the chemical structure of the TBTPACOP. The signals appearing at the
aliphatic region of the NMR spectrum at 59.07, 65.89, and 75.80 ppm
correspond to the three -CH2-protons of the Tröger’s base linker 3. The
peak appearing at 160.91 ppm belongs to the newly formed imine carbon
in the polymer TBTPACOP and the peaks appearing in the region 122 to
140 ppm, belongs to the aromatic carbons of the polymer.
Fig. 3. (a) TGA-DSC thermogram of TBTPACOP (b) The powder X-ray
diffraction pattern of TBTPACOP treated under different conditions.
2.7. Scanning Electron Microscopy (SEM) Images
2.4. Thermogravimetric Analyses (TGA)
The field emission scanning electron microscopy (FE-SEM) provided the
deep insight into the morphological information of TBTPACOP (Fig. 4).
The synthesized TBTPACOP was ultrasonicated in distilled acetone for 10
mins and dried under a high vacuum for 48 h to remove any volatile
solvent impurities. The amorphous powder of TBTPACOP was then
mounted on the stubs using double-sided vacuum compatible carbon
tape and analyzed. The SEM images revealed the near-spherical
morphology of TBTPACOP. These observations indicate that the
polymerization through the covalent imine bond formation across the
TGA analysis was performed on a TGA4000 Perkin-Elmer instrument
under N2 atmosphere at a heating rate of 10 ◦C/min. TBTPACOP exhibited
high thermal stability up to 460 oC, as revealed from the TGA profile, and
it retained about 90% of its weight up to 460 ◦C and above 600 oC
retained 70% of its weight (Fig. 3a). The marginal weight loss observed
o
below 200 C was probably due to the release of the moisture trapped
inside the polymeric material. The simultaneous thermal stability of
TBTPACOP was also analyzed by differential scanning calorimetry (DSC)
under a nitrogen atmosphere (Fig. 3a, inset). As represented in the
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structure has led to a uniform morphology with a certain degree of
structural regularity. As represented in Fig. 4, TBTPACOP exhibited
porous nature, which might facilitate the adsorption process for the
removal of dyes from the effluent.
the IUPAC classification and displays a steep nitrogen uptake at relatively
low pressure confirming the mesoporous nature of the material (Fig. 6a).
From the BET isotherms of TBTPACOP, the surface area was found to be
126 m2g-1. Pore size distribution plot of TBTPACOP (Figure 6b) displayed a
mean pore width of 6.309 nm and pore volume of 0.199 cm3g-1 further
validating the mesoporous nature of the covalent polymer.100
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Fig. 4. Field emission scanning electron microscopy (FE-SEM) of
TBTPACOP.
Fig. 6. (a) N2 Adsorption and desorption isotherm of TBTPACOP and (b)
pore size distribution plot of TBTPACOP by Barret-Joyner-Halenda (BJH)
method.
2.8. 3D Laser Confocal Scanning Microscopy
2.10. Contact Angle Measurement and Surface Roughness Assessment
The surface analysis of the covalent organic polymer TBTPACOP was
done using a ZEISS LSM 900 confocal laser scanning microscope.99 LSM
900 enables accurate, 3D-imaging, and information about the surface
properties of nanomaterials, polymers, metals, and semiconductors. The
3D image of the TBTPACOP coated on the surface of a clean glass plate is
represented in Fig. 5. The synthesized TBTPACOP was ultrasonicated in
acetone for 10 mins and dried then under a high vacuum for 48 h to
remove volatile solvent impurities. The amorphous powder of TBTPACOP
was then mounted on the glass plate using double-sided vacuum
compatible carbon tape and analyzed by LEXT OLS5000 3D measuring
laser microscope.99 The spherical morphology is evident through the
fluorescence imaging of the polymer.
Wettability is
a much desirable property and can have several
applications that demand hydrophobic surfaces. The surface property of
the polymeric materials is generally an attribute of the nature of the
individual organic linkers or can be modified by external treatment.
However, controlling the surface wettability of COP has always been a
challenging task. So, to ascertain the surface wettability of the
TBTPACOP, the contact angle between the water droplet and the
polymer surface was measured using ACAM D2A Contact angle meter.
The measurement was done by placing a water droplet on the surface of
the clean glass surface coated with TBTPACOP and recording the
corresponding angle. The contact angle was observed to be 133.99o
indicating the hydrophobic nature of TBTPACOP, Fig. 7. The hydrophobic
nature of TBTPACOP can be reasoned due to the presence of a TB
scaffold at the periphery of the polymeric material.
Fig. 5. (a) 3D laser confocal scanning microscopic image of TBTPACOP
coated on the surface of a glass plate (b) average pore diameter
calculation.
Figure 7. Contact angle measurement displaying the contact point of
water over the surface of TBTPACOP.
2.9. BET Surface Area and Porosity Measurement
In order to evaluate the porosity and surface area parameters of
TBTPACOP, nitrogen adsorption-desorption analysis were performed on
Quantachrome Autosorb equipment in 9 mm w/o rod cell at 77 K. Prior
to the sorption experiment, the TBTPACOP was degassed at 100 ◦C for 24
h. Ads shown in the Fig. 6, the adsorption isotherm of TBTPACOP
exhibited a pattern closely related to the Type IV isotherm according to
Several industrial applications of organic polymers demand hydrophobic
surface along with the information about the surface irregularities. Since
the surface structure also influences the function of many polymeric
materials, hence the quantification of the surface roughness of the
covalent organic polymeric material TBTPACOP will help us to assess the
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material by height, depth, and interval. The covalent organic polymer
TBTPACOP was coated on a clean glass surface and the 3D topographical
images were recorded by ZEISS LSM 900 confocal laser scanning
microscope to identify the porosity of the polymeric material. Fig. 8
represents the 3D profiles of the surface of TBTPACOP and average
surface roughness providing a visual representation of the surface
topology in the form of patches of crest and troughs. The topological
analysis reveals the porous nature of the TBTPACOP. A representative
crest measuring 16.126 μm in length was considered where the height
profile was analyzed. It was observed that the surface roughness ranges
from -5 μm to 5 μm across the calculated region.
According to the Wenzel theory, surface roughness has a close
relationship with the hydrophobicity of the material.92 A rough surface
with a lot of porosity will have an abundance of air pockets located in the
grooves. With several air pockets throughout the surface, the trapped air
within those pockets will provide air cushioning to the water droplet over
it and would prevent the water from further penetrating the grooves.
Therefore, the less solid area will be wetted by water, resulting in better
hydrophobicity for the rough surfaces of the polymeric material.
TBTPACOP was then filtered out and the filtrate was analyzed
spectrophotometrically, (Fig. 9). The process was continued until a
visually clear effluent was observed. The spectrophotometric analysis of
the filtered effluents revealed that there was a gradual reduction of the
absorption maxima over time and 95% removal of dye was achieved
when stored over TBTPACOP for 70 mins at room temperature (Fig. 9d).
Beyond 70 mins, no notable change was observed. The dye adsorption
kinetics of the TBTPACOP was also visualized by observing the color
changes of the aqueous effluent over time (Fig. 9c).
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Fig. 9. (a) UV-vis spectral change due to the adsorption of Acid orange II
(D1) by TBTPACOP under different conditions (b) UV-vis spectral change
due to the adsorption of Acid orange II by TBTPACOP at different time
intervals (c) Visual change of the effluent color due to the Acid orange II
adsorption by TBTPACOP from the aqueous effluent at different time
interval (d) Graphical plot of the dye adsorption % over time by the
polymer TBTPACOP.
To further investigate the dye adsorption kinetics, the adsorption of Acid
orange II (D1) by TBTPACOP was investigated at different time intervals.
The equilibrium adsorption capacity (qt) towards Acid orange II dye was
calculated by UV-Vis spectroscopy at different time intervals (t) taking
100 ppm initial aqueous solution of D1 with 5 mg TBTPACOP. The effect
of contact time (t = 0, 5, 10, 20, 30, 40, 50, 60, and 70 min) and the
adsorption of Acid orange II (D1) by TBTPACOP is presented in Fig. 10. As
expected, the amount of D1 adsorbed by TBTPACOP enhanced with
increasing contact time. As observed, the adsorption capability of
TBTPACOP inreased rapidly initially, then gradually became slow and
reached an adsorption equilibrium when no furthur improvement in the
adsorption process was noticed.
Fig. 8. Color-coded height map of TBTPACOP by 3D confocal scanning
microscopic technique.
2.11. Dye Adsorption Study
Efficacy of dye adsorption by TBTPACOP from the aqueous effluent was
tested using Acid orange II (D1). Acid orange II (D1) is an anionic azo dye
having a reddish-yellow color. According to the NFPA and HMIS rating,
Acid orange II (D1) is considered as a grade 2 hazardous chemical that
can cause respiratory tract irritation as well as causes eyes and skin
irritation and can also penetrate through the skin. Before studying the
adsorption kinetics, to understand the effect of ultrasonication on dye
degradation, we sonicated the dye (D1) contaminated effluent for 10
mins. There was hardly any observable change in the absorption maxima
(483 nm) intensity implying the zero impact of sonication on the
degradation of the dye (Fig. 9). Thereafter 1 mg of the TBTPACOP was
added to the 10 ml of the aqueous dye effluent (1x10-3 M) and sonicated
for 5 mins. The subsequent spectrophotometric analysis revealed a 56%
reduction in the absorption intensity pointing out the involvement of
TBTPACOP in the dye adsorption process (Fig. 9). Further, the adsorption
kinetics of the dye was investigated by adding 5 mg of TBTPACOP to the
10 mL of the effluent containing 0.001 gm of Acid orange II (D1). After
ultrasonication for 5 mins, the effluent was kept without disturbing.
The equilibrium adsorption capacity of the Acid orange II (D1) by the
TBTPACOP was evaluated according to the equation provided below:
qe = (Ci - Ce )V/W
Where Ci and Ce are initial and equilibrium concentrations (mg/L) of Acid
Orange II (D1), respectively, V is the volume of Acid Orange II (D1)
solution (10 × 10−3 L), W is the amount of TBTPACOP (5×10−3 g) and the
equilibrium concentration = 5.52 mg/L. The equilibrium adsorption
capacity (qe) calculated was found to be 188.96 mg/g. The kinetics studies
were performed to evaluate the adsorption kinetics of TBTPACOP
towards dye Acid Orange II (D1).
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Fig 11 (B). The rate constant k2 for TBTPACOP was found to be 3.866 x 10-
4 mg g-1min-1. Moreover, the pseudo-second order kinetic model assumes
that the mode of adsorption is mainly via chemisorption which might
include electron transfer or electron sharing between the adsorbent and
adsorbate.
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2.12. Recyclability Study
Next, we studied the recyclability of TBTPACOP by filtering and reusing
the same polymeric material for the dye adsorption (see Fig. S1,
supporting information). It was observed that TBTPACOP indeed could be
recycled efficiently for the dye adsorption process however, the dye
adsorption capability of TBTPACOP gradually reduced from 94% to 70%
over the four cycles (Fig. 12). The reduction in the dye adsorption
efficiency beyond the 4th cycle could be because of the physical loss of
the material.
Fig. 10. Graphical plot of adsorption capacity (qt) over time.
Pseudo-first order and pseudo-second order kinetic models were used to
investigate the adsorption kinetic properties of TBTPACOP over Acid
Orange II (D1). The two different adsorption models can be expressed by
equations (1) and (2) after definite integration, by applying the initial
conditions qt = 0 at t = 0 and qt =qt at t = t.
log (qe-qt) = log qe –(k1/2.303) t
t/qt = (1/(k2 x (qe)2) + (t/qe)
.……………eq.1
……………..eq.2
where, qe and qt (mg g-1) are the amounts of organic dyes absorbed per
gram of adsorbents at adsorption equilibrium and each time t (min),
respectively.
k1 (min-1) and k2 (mg g-1 min-1) represent pseudo-first order and pseudo-
second-order rate constant. The curve fitting plots of Acid Orange II
(D1) adsorbed onto TBTPACOP for both pseudo first order and pseudo
second order kinetics are exhibited in Fig. 11 A and B respectively. All
the kinetic parameters are listed in Table 1.
Fig. 12. (a) Recyclability study of TBTPACOP for the removal of Acid
orange II from aqueous effluent (b) Graphical plot of the dye adsorption
% over different cycles.
2.13. Selective Dye Adsorption Study
Next, we decided to evaluate the ability of TBTPACOP to adsorb other
classes of dyes like direct, basic, and reactive dyes to understand the dye
adsorption mechanism. For that purpose, we have selected the following
five dyes: Acid Orange II (D1), Acid Red 54 (D2), Basic Rhodamine B (D3),
Procion green (D4), Direct Turquoise Blue (D5), (see Fig. S2, supporting
information). Dyes D1, D2, D4 are colorants containing azo chromophore
whereas D3 and D5 are basic triarylmethine dye and Cu-phthalocyanine
based direct dye, respectively. The presence of one or multiple sodium
sulfonate groups in D1, D2, D4, and D5 are responsible for the water
solubility of these dyes. However, D3 is devoid of any sulfonic acid group
but the positively charged quaternary nitrogen in D3 accounts for its
water solubility. D1 contains one sodium sulfonate group whereas D2, D5
consisting of two sodium sulfonate groups and, D4 has six sodium
sulfonate groups. D4 is a reactive dye with a relatively bigger structure
and D5 is a Cu-phthalocyanine based direct dye-containing two sodium
sulfonate group. The dye D3 is a cationic basic dye whereas all other dyes
D1, D2, D4, and D5 are anionic dyes. All these dyes being water-soluble
although with different charges and sizes will be a good contender to
understand the mechanism of dye adsorption. The dye adsorption
kinetics were studied to understand the mode of action as well as the
efficacy of TBTPACOP towards different classes of dyes (Fig. 12). The
absorption spectra of all the dyes D1-D5 before and after the treatment
with TBTPACOP for 70 mins is represented in Fig. S3 (see supporting
information). Analysis of the absorption spectra reveals that D1 and D2
were effectively adsorbed within 70 mins whereas dyes D3-D5 were
absorbed only in the range of 22-45% by the TBTPACOP, (Fig. 13).
Fig. 11. (A) Pseudo-first order kinetics plot (B) Pseudo-second-order
kinetics plot.
Table 1: The pseudo-first order and pseudo-second-order rate constants
with correlation constants (R2) of Acid Orange II over TBTPACO at 100
ppm.
C0
Pseudo-first order model
Pseudo-second order model
k1 (min-1)
R2
k2 (mg g-1
min-1)
R2
100
0.2474
0.9116
3.86605 x 10-4
0.997
ppm
The results revealed that the dye adsorption process fits well with the
pseudo second order model over the pseudo first order kinetic model
because of the superior linear correlation coefficient (R2 = 0.997)
observed as compared to the linear correlation coefficient value (R2 =
0.9116) obtained for pseudo first order model. The pseudo-second-order
rate constant k2 (g mg-1 min-1) and the adsorbed amount of dye at the
equilibrium of qe (mg g-1) can be obtained from the intercept and slope of
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adsorption kinetics have been proposed for the dye adsorption process.
It was observed that TBTPACOP has a propensity to adsorb acid dyes and
can adsorb 95% of Acid Orange II from the aqueous effluent within 70
mins. The equilibrium adsorption capacity (qe) calculated for TBTPACOP
was found to be 188.96 mg/g. Moreover, the recyclability of the COP up
to four cycles, and its ability to selectively adsorb all kinds of acid dyes
from the aqueous effluent in a very short time along with its high
porosity, good thermal stability, high hydrophobicity makes it a promising
material for the wastewater treatment and driving our mission towards
the development of more sustainable materials for the future.
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Experimental Section
Fig. 13. Study of different dye adsorption capability of TBTPACOP.
All chemicals were procured from Sigma-Aldrich, Spectrochem, and Avra
and used directly. Silica gel-coated aluminum sheets (ACME, 254F) were
used for the Thin Layer Chromatography (TLC) analysis using EtOAc and
petroleum ether as the eluents. Melting points reported were
uncorrected and were recorded using a capillary melting point apparatus
(Thomas Hoover). Fourier-transform infrared (FT-IR) spectra were
The tendency of the acid dyes D1 and D2 to get adsorbed easily
compared to the dyes D3-D5 could be due to the presence of OH and NH2
groups in the structure that can interact with the polar nitrogen atoms of
the polymer TBTPACOP via hydrogen bonding. Although dyes D3 and D4
have polar -COOH and -NH- groups but size sieving due to their relatively
bigger structure could be the probable reason for their poor absorptivity
compared to D1 and D2. Moreover, the electrostatic interaction between
the TBTPACOP and the dye molecules as the possible mode of adsorption
could be ruled out due to the absence of any ionic centers or surface
charge in the TBTPACOP, which accounts for the poor absorptivity of D5,
apart from the relatively bigger size of D5, being the other reason.
However, both D1 and D2 having a comparable size, the higher %
adsorption of D1 compared to D2 could be due to the presence of two
sodium sulfonate groups in D2 which makes it more soluble in water than
D1 and making it slightly more difficult to get adsorbed compared to D1.
Therefore, size sieving and hydrogen bond interaction could be the
possible modes of adsorption of the acid dyes from the aqueous effluent.
Moreover, slightly better adsorption rate of D4 compared to D3 and D5
may be probably due to the more number of hydrogen bond donor
groups in D4. Further, we studied the influence of other dyes on the
adsorption capability of the TBTPACOP. An equimolar mixture of D1 and
D3 were taken and their adsorption kinetics were studied upon the
addition of TBTPACOP. Analysis of the absorption spectra indicated that
the hump at around 486 nm (corresponding to the absorption maxima of
D1) has disappeared along with a minute decline in the absorption
intensity of the dye D3 (see Fig. S4, supporting information). This
observation highlights the selectivity offered by TBTPACOP towards the
acid dye adsorption over other dyes.
1
obtained with a Jasco spectrophotometer (FT-IR-4100). H and 13C NMR
spectra were recorded on Bruker 400 and 500 MHz spectrometer using
CDCl3 and dimethyl sulfoxide-d6 (DMSO-d6) as a solvent. Chemical shifts
are reported in parts per million (ppm) downfield from TMS, and the spin
multiplicities are described as s (singlet), d (doublet), t (triplet), dd
(doublet of doublets), brs (broad singlet), and bd (broad doublet).
Coupling constant (J) values are reported in Hertz (Hz).
The synthesis of the following compounds 2,8-dibromo-6,12-dihydro-
5,11-methanodibenzo[b,f][1,5]diazocine (2), 2,8-dibromo-6,12-dihydro-
5,11-methanodibenzo[b,f][1,5]diazocine
(3),
and
tris(4-
aminophenyl)amine (6) were performed following the reported
procedure.93-94, 101-102
Synthesis
of
2,8-dibromo-6,12-dihydro-5,11-
(5.0 g,
methanodibenzo[b,f][1,5]diazocine (2):93,101 4-bromoaniline
58.13 mmol) and paraformaldehyde (4.35 g, 145.3 mmol) were added to
a 250 mL round-bottom flask and cooled to -15 oC. Then dropwise 50 mL
of trifluoroacetic acid was added with vigorous stirring. The addition of
TFA was maintained at a rate of one drop per second. After the complete
addition of TFA, the flask was removed from the ice-bath and stirred for
the additional 24 hours at room temperature. The red solution was then
quenched slowly by pouring over 100 g of crushed ice containing 200 mL
of concentrated NH4OH solution. The mixture was then extracted with
3x25 mL dichloromethane. The combined organic layers were dried over
anhydrous MgSO4, filtered and concentrated to get yellow oil which was
then purified by column chromatography using ethyl acetate: petroleum
ether (1:9) to get hold of 2,8-dibromo-6,12-dihydro-5,11-
methanodibenzo[b,f][1,5]diazocine (2) as pure white solid (58%); White
3. Conclusion
In conclusion, we have introduced a V-shaped Tröger’s base unit to
design the covalent organic polymer TBTPACOP by linking a C3-
symmetric tris-amine, N’, N’-bis(4-aminophenyl)benzene-1,4-diamine
(TPA) with a C2-symmetric Trögers base scaffold featuring dialdehyde,
4,4'-(6,12-dihydro-5,11-methanodibenzo[b,f][1,5]diazocine-2,8-
diyl)dibenzaldehyde (TBBA), solvothermally. The synthesized polymeric
material was characterised by FT-IR, solid state CP-MAS NMR and also by
powder XRD, FE-SEM. The porosity of the material was furthur calculated
by BET nitrogen adsorption-desorption analysis and the hydrophobicity of
the covalent organic polymer by the contact angle measurement. The
unique cleft like geometry of the Tröger’s base featuring in the covalent
organic polymer has resulted in structurally robust, porous polymeric
material possessing hydrophobic surface and high thermal stability. The
porous nature of the polymeric material has been exploited further for
the dye removal from the aqueous effluent. A pseudo second order
o
1
solid, m.p -161-165 C; FT-IR (KBr, cm-1): 3250, 1660, 860 cm-1; H NMR
(400 MHz, CDCl3) δ 4.07 (d, J = 16.8 Hz, 2H), 4.22 (s, 2H), 4.61 (d, J = 16.8
Hz, 2H), 6.98 (d, J = 7.2 Hz, 2H), 7.02 (s, 2H), 7.25 (d, J = 7.5 Hz, 2H); 13C
NMR (100 MHz, CDCl3): δ 58.6, 67.0, 117.1, 127.1, 130.0, 130.0, 130.9,
147.1; HRMS (EI) Calcd for C15H12N2Br2, 377.9367; found, 377.9357.
Synthesis
of
4,4'-(6,12-dihydro-5,11-
methanodibenzo[b,f][1,5]diazocine-2,8-diyl)dibenzaldehyde (3):93 2,8-
dibromo-6,12-dihydro-5,11-methanodibenzo[b,f][1,5]diazocine (2) (1.0
g, 2.63 mmol), 4-formylphenylboronic acid (1.18 g, 7.89 mmol) and the
[Pd(PPh3)4](0.304 g, 0.26 mmol) were taken in a rb flask under nitrogen
atmosphere and 40 mL of THF: Water (4:1) was added followed by K2CO3
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(1.81 g, 13.2 mmol). The reaction mixture was refluxed for 24 h in dark.
After cooling, THF was removed and the mixture was extracted with ethyl
acetate, dried over Na2SO4, concentrated to get hold of the crude oil
which was further purification by column chromatography (silica gel,
ethyl acetate/pet. ether, 2:3) to yield pure compound 3 as yellow solid
(78%); FT-IR (KBr, cm-1); 1710, 1600, 1120 cm-1; 1H NMR (400 MHz, CDCl3)
δ 4.30-4.37 (m, 4H), 4.80 (d, J = 16.4 Hz, 2H), 7.16-7.31 (m, 4H), 7.45 (s,
2H), 7.63 (s, 4H), 7.87 (s, 4H), 9.99 (d, J = 6.8 Hz, 2H); 13C NMR (101 MHz,
CDCl3) δ 58.8, 66.8, 125.6, 125.9, 126.5, 127.2, 128.3, 130.2, 134.9, 135.5,
146.5, 148.5, 191.8; HRMS (EI) Calcd. for C29H23N2O2, 431.1759; found,
431.1765.
Start-Up grant (FRP (F.4-5(128)/2014(BSR), ICT Golden Jubilee research
grant (2018) and CoE-PI-TEQIP-III for the financial assistance.
Keywords: Covalent organic polymer, Trogers base, Dye adsorption, Dye
removal, Recyclable COP
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removal–a review, J. Environ. Manage. 2009, 90, 2313−2342.
2) M. Rafatullah, O. Sulaiman, R. Hashim, A. Ahmad, Adsorption of
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Synthesis of tris(4-aminophenyl)amine (6):94,102 In a 250 ml two neck
round bottom flask containing 40 mL of dry DMSO, 4-nitroaniline (a) (2.5
g, 18.1 mmol), 4-fluoro-nitrobenzene (5) (5.32 g, 4 mL, 37.7 mmol) was
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solvent was removed under vacuum. The formed solid was suspended in
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successively with H2O and methanol and dried overnight to get hold of
the product tris(4-nitrophenyl)amine as yellow solid (10.3 g, 27.1 mmol)
in 72% yield which was used as such for the next step.
In
a 250 ml round bottom flask, suspension of tris(4-
nitrophenyl)amine (5.0 g, 13.1 mmol) and 10% palladium on activated
carbon (0.329 g, 3.077 mmol) in a mixture of dry 1,4-dioxane (46 mL) and
EtOH (23 mL) was heated at 80 °C for 1 h under N2 atmosphere. Then
hydrazine monohydrate (13 mL, 414.2 mmol) was added to the reaction
mixture and the mixture was heated again at 80 °C for 48 h. On
disappearance of the starting material, palladium on activated carbon
was removed via vacuum filtration, the filtrate evaporated, and the crude
product was crystallized at low temperature. The crystallized product was
collected via vacuum filtration and dried overnight to yield 79% (3.01 g,
10.4 mmol) of tris(4-aminophenyl)amine (6); Gray solid, m.p: 240-242 oC;
FT-IR (KBr, cm-1): 3412, 3347, 3011, 1606, 1327, 1264; 1H NMR (400 MHz,
DMSO-d6) δ 6.60-6.58 (d, 6H), 6.44-6.42 (d, 6H), 4.69 (s, 6H); 13C NMR
(100 MHz, DMSO-d6) δ 127.2, 114.9, 105.6; HRMS (EI) Calcd. for C18H19N4
(M++H) 291.16097; found, 291.1611.
Synthesis of TBTPACOF: To a mixture of 1:1 mesitylene/dioxane (4 mL),
tris(4-aminophenyl)amine (6) (0.100 g, 0.344 mmol), 4,4'-(6,12-dihydro-
5,11-methanodibenzo[b,f][1,5]diazocine-2,8-diyl)dibenzaldehyde
(3)
(0.222 g, 0.5166 mmol) were added followed by aqueous acetic acid
solution (3 M, 0.714 mL). The reaction mixture was then degassed for 10
mins in a pyrex sealed tube (50 mL) and the tube was sealed tightly by
screwing and heated at 120 °C for 3 days. The precipitate formed was
collected by centrifugation, washed with THF, and dried at 120 °C under
vacuum overnight to obtain TBTPACOF in 91% yield. FT-IR; 3250, 1645.
1600 cm-1.
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Conflicts of Interest
The authors declare no conflicts of interest.
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Base
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Microporous
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V.P.J is thankful to CSIR or the Research Fellowship. G.Y is thankful to the
Swarti fellowship, Govt. of Maharashtra, India for her fellowship. The
authors also acknowledges the Department of Textile Technology, ICT
Mumbai for the ZEISS LSM 900 confocal laser scanning microscope
facility. S.S acknowledges DST-SERB, CSIR (02(0253)/16/EMR-II)), UGC-
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nλ = 2dsinθ
Where, 2θ = 21o, X-Ray wavelength = 0.15406 nm, Order of
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sample and combine them in a three-dimensional image stack. Its
aperture (usually called a pinhole) is arranged in such a way that out-
of-focus information will be blocked and only in-focus information
can be detected. Laser module GB (pigtailed; 488, 561 nm) Diode
laser (488 nm, 10 mW); laser class 3B.
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Barret-Joyner-Halenda (BJH) calculation method was followed
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Tröger’s base incorporated recyclable COP for the acid dye removal
from effluent.
Abstract: Protection of the environment from the uprising threats of
pollution from several chemical industries in a sustainable way has
become the major global challenge. The colored effluents from the textile
and dyestuff industries are the major sources of pollution, contaminating
the water bodies. Herein, we have synthesized a covalent organic
polymer TBTPACOP having a V-shaped Tröger’s base unit into the
framework with the notion that the unique cleft like architecture of TB
will infuse porosity and robustness in the amorphous polymeric materia.
The covalent organic polymer (COP) was used for the adsorption of
anionic acid dyes from the aqueous effluent. The equilibrium adsorption
capacity (qe) was found to be 188.96 mg/g for Acid orange II. The
excellent dye adsorption capacity of the COP by adsorbing 95% of Acid
Orange II at room temperature within 70 mins, along with its recyclability
up to 4 consecutive cycles, makes TBTPACOP an extremely promising
material for selective and efficient removal of acid dyes from the
aqueous effluent.
New. J. Chem.