Carbazole-tagged pyridinic microporous network polymer for CO2 storage and organic dye removal from aqueous solution

Journal Pre-proof Carbazole-tagged pyridinic microporous network polymer for CO2 storage and organic dye removal from aqueous solution Narendran Rajendran, Jacob Samuel, Mohamed O. Amin, Entesar Al-Hetlani, Saad Makhseed PII:

S0013-9351(19)30798-4

DOI:

https://doi.org/10.1016/j.envres.2019.109001

Reference:

YENRS 109001

To appear in:

Environmental Research

Received Date: 8 August 2019 Revised Date:

1 December 2019

Accepted Date: 3 December 2019

Please cite this article as: Rajendran, N., Samuel, J., Amin, M.O., Al-Hetlani, E., Makhseed, S., Carbazole-tagged pyridinic microporous network polymer for CO2 storage and organic dye removal from aqueous solution, Environmental Research (2020), doi: https://doi.org/10.1016/j.envres.2019.109001. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Inc.

Carbazole-tagged pyridinic microporous network polymer for CO2 storage and organic dye removal from aqueous solution Narendran Rajendran1, Jacob Samuel2, Mohamed O. Amin1, Entesar Al-Hetlani1*, and Saad Makhseed1* 1

Chemistry Department, Faculty of Science, Kuwait University, P.O. Box 5969, 13060 Safat, Kuwait 2

Petroleum Research Center, Kuwait Institute for Scientific Research, Ahmadi, Kuwait

Email: [email protected] and [email protected]

ABSTRACT A microporous organic polymer (Cz-pyr-P) was prepared from a monomer of pyridine-imides, flanked by four carbazoles, and its application as an adsorbent for both CO2 and methylene blue dye in wastewater was investigated. The polymer was synthesised by oxidative polymerisation facilitated by FeCl3 and comprehensively characterised using routine spectroscopic, thermal, textural, and morphological analyses. With a high surface area of 1065 m2/g and a median pore width of 8.06 Å, the nitrogen-enriched Cz-pyr-P reversibly adsorbed 19.41 wt.% (273 K) and 12.78 wt.% (295 K) CO2 at 1 bar, with a good isosteric heat value of CO2 adsorption (28.5 kJ/mol). For the removal of methylene blue dye from wastewater, Cz-pyr-P exhibited excellent partition coefficient of 837.52 mg/g µM with an equilibrium time of 6 min which is shorter than reported values for other materials. The results indicate that Cz-pyr-P with desirable functionality could be utilised for reaching CO2 emission reduction targets as well as for wastewater treatment.

Keywords: Microporous organic polymer; Pyridine-based polymer; CO2 adsorption; Methylene blue; Water treatment

1

1. Introduction Owing to their non-degradable nature, bioaccumulation, and long-distance transmission, environmental pollutants are jeopardising human survival and the ecological balance in the 21st century [1]. For example, the accumulation of greenhouse gas CO2 produced by the combustion of fossil fuels and/or other anthropogenic activities is the main cause of drastic climate change. This is attributed to the increase in CO2 concentration in the atmosphere, which results in an increase in the Earth’s temperature [2]. Thus, much attention has been focused on stabilising CO2 concentrations in the atmosphere and developing efficient carbon capture platforms. Accordingly, several feasible approaches have been reported for CO2 capture and sequestration, including membrane separation, absorption, adsorption, and cryogenic distillation [3]. However, most CO2 technologies currently employed are expensive, generate by-products, consume large amounts of water, and require large amounts of energy for regeneration [4]. Another important environmental issue is the discharge of organic dyes into water sources. This phenomenon is rapidly growing and threatening many living organisms, households, fishing, and other life activities. The presence of synthetic dyes in water can retard the photosynthetic process of the aquatic ecosystem; additionally, they are carcinogenic and mutagenic [5]. For example, direct consumption of methylene blue (MB) dye causes health issues, including diarrhoea, vomiting, and inflammation. The adverse effect level of MB in rats was reported to be 50 mg/kg and greater, which makes it a serious threat [6]. Thus, the constant discharge of these pollutants from textile, paper, and paint factories into bodies of water is considered one of the primary sources of water pollution. As a result, there is a great demand for feasible and efficient approaches dedicated to the removal of pollutants from wastewater. Over the past few decades, biological, physical, and chemical methods have been explored for this purpose [7]. Of these, physical adsorption has received a great deal of attention owing to its efficiency, simplicity, reusability, and lack of harmful by-products [8]. The adsorption process is influenced by several factors, including the pH, temperature, concentration, and nature of both the adsorbate and adsorbent. Thus, careful tuning of these parameters is essential to establish the optimum conditions for water treatment and determine the adsorption rate, capacity, and mechanism.

2

Therefore, there is a constant demand to develop a novel and efficient generation of adsorbents for environmental applications including CO2 adsorption and wastewater treatment. Recently, porous organic polymers (POPs) have emerged as an attractive class of porous nanomaterials that have been applied for gas capture and separation, as well as for catalysis [9, 10]. POPs contain light-weight elements and are constructed with strong covalent bond linkages, giving them good structural stability. POPs have facile synthetic routes, versatile functionalities, a large surface area, and low density, which make them favourable candidates for gas adsorption and separation [11]. POPs can be classified as hyper-cross-linked polymers (HCPs), polymers of intrinsic microporosity (PIMs), covalent organic frameworks (COFs), conjugated microporous polymers (CMPs), covalent triazine frameworks (CTFs), porous aromatic frameworks (PAFs), extrinsic porous molecules, or porous organic cages [12]. In recent years, POPs with a πconjugated skeleton have been adopted for various applications such as adsorption of both gases [13] and organic dyes [8] and heterogeneous catalysis [14]. An extensive architectural flexibility of the chemical composition and structural rigidity are fascinating features of CMPs [15-17]. Many organic spacers, e.g. tetrazole, imidazole, carbazole, triazine, imide, and amines, have been widely used for tuning the chemical composition while maintaining the permanent porosity of these polymers, resulting in a large accessible surface area and thus a high gas storage capacity [18]. In particular, POPs with carbazole units are established materials for the adsorption and separation of gases and organic pollutants [19, 20]. Yuan et al. [21] synthesised three CMPs using the Yamamoto (CMP-YA), Suzuki (CMP-SU), and Sonogashira (CMP-SO1B2 and B3) polymerisation reactions. The CMP prepared using the Yamamoto route was utilised for CO2 adsorption and removal of methylene blue (MB), rhodamine B (RB), methyl orange (MO), and Congo red (CR). CMP-YA exhibited superior behaviour compared to the other CMPs as a result of its large surface area and homogenous microporopsity. Bera et al. [22] reported the synthesis of a 1,2,3-triazole-bonded triptycene-based microporous polymer (TTMP) through the Cu(I)-catalysed azide-alkyne Click reaction. TTMP exhibited a large surface area (822 m2/g) with high H2 (14.2 wt.%) and CO2 adsorption (16.3 wt.%) abilities. The prepared TTMP was utilised for the storage of H2 and CO2 and the removal of organic dyes from wastewater. To date, some studies have investigated the use of POPs, especially materials decorated with carbazole groups, for the removal of organic dyes from wastewater. In this study, a 3

pyridine-based CMP network polymer decorated with two imide-linked carbazole units (Cz-pyrP) was synthesised by a FeCl3-facilitated chemical oxidation protocol. Preliminary ultraviolet photoelectron spectroscopy (UPS) studies revealed the conductive nature of the synthesised polymer due to the presence of a highly conjugated monomeric backbone. Furthermore, the polymer exhibited a large surface area, ultra-porosity, and remarkable CO2 adsorption with an appreciable isosteric heat of adsorption. In addition, the efficient removal of MB was investigated for water treatment, and the effects of the dosage, dye concentration, and pH were analysed. The adsorption kinetics and isotherms of Cz-pyr-P in the context of MB removal were also discussed. Finally, a possible mechanism for the adsorption of MB dye onto the synthesised polymer was proposed. 2. Materials and methods 2.1 Chemicals All the reactions were carried out using clean, oven-dried glassware under a nitrogen atmosphere. All the chemicals used were procured from commercial vendors and used without further purification. Carbazole, potassium hydroxide, acetic acid, 2,6-diamino pyridine, N, NDimethyl formamide (anhydrous), methanol, dichloromethane, sodium hydroxide, hydrochloric acid, and MB were purchased from Sigma Aldrich; 4,5-dichloro phthalonitrile, caesium fluoride, and acetic anhydride were obtained from Alfa Aesar; ethanol and chloroform were obtained from Merck. DI water was obtained from an Elix Milli-Q water deioniser. 2.2 Synthesis of Cz-pyr-P The synthesis of Cz-pyr-P was carried out through a FeCl3-mediated Friedel-Crafts alkylation reaction. The synthesised monomer, diczpyridine (0.5 g, 1 mmol), was dissolved in 30 mL of CHCl3 in a 100-mL round-bottom flask, and 5 equivalents of anhydrous FeCl3 were added to the reaction mixture. The reaction mixture was vigorously stirred for 2 d at room temperature. The polymer was filtered and purified by washing repeatedly with chloroform, methanol, methanol/water, and tetrahydrofuran (THF). The obtained yellow powder was dried under vacuum at 120 °C for 24 h. 2.3 Instrumentation

4

Characterisation of the monomer and the polymer was carried out using the following analytical techniques: 1H and 13C nuclear magnetic resonance (NMR) spectra were recorded with a Bruker AVANCE II 600 MHz instrument, and the chemical shifts are indicated in parts per million (ppm). Ultraviolet–visible light (UV-vis) absorption spectra were obtained with an AGILENT CARY 5000 UV−Vis spectrophotometer. Fourier transform infrared (FTIR) spectra were measured with a JASCO FTIR 6300 with IRT 3000 FTIR spectrometer. High-resolution mass spectrometry (HR-MS) spectra were recorded using a Thermo GC-MS DFS instrument. Thermogravimetric analysis (TGA) was performed on a SHIMAZDU DTG-60 & thermal analyser system at a heating rate of 10 °C/min under a nitrogen atmosphere. Differential scanning calorimetry (DSC) analyses were recorded with a NETZSCH 204 F1 Phoenix at a heating rate of 10 °C/min. Accelerated surface area and porosimetry studies were performed using a Micromeritics ASAP 2020 with mmHg transducer micro-pore system for the accurate measurement of the Brunauer–Emmett–Teller (BET) and Langmuir surface areas. Scanning electron micrographs were obtained with a JEOL model JSM IT100 scanning electron microscope (SEM) from the InTouchScopeTM series with an accelerating voltage of up to 30 kV. High-resolution images of the polymers were attained with a JEOL JEM3010 high resolution transmission electron microscope (TEM). Powder X-ray diffraction (PXRD) patterns were obtained with a BRUKER D8 Advance diffractometer using a Cu-Kα1 parallel beam for the high-resolution diffraction (HRXRD) of epitaxial thin films and low symmetry powder samples. X-ray photoelectron spectroscopy (XPS) and UPS spectra were recorded with an ESCALAB 250Xi XPS System using an AlKα source with spot sizes ranging from 200–850 µm. Atomic force microscopy (AFM) images were obtained with a commercial AFM unit (SPA400-SPI4000, Seiko Instruments Inc., Japan). Zeta potential (ζ) measurements for isoelectric point determinations were performed using a Zetasizer Nano ZS, Malvern Instruments Ltd, Malvern, UK. 2.4 MB adsorption experiments The removal of MB from aqueous solution using Cz-pyr-P was evaluated through a series of experimental procedures. Initially, certain amount of Cz-pyr-P (0.2–1.0 mg) was added to 10 mL MB with the range of (1.0–20.0 mg/L) concentration. The concentration of MB remaining in the solution was determined by monitoring the change in the maximum absorbance at a 5

wavelength of 663 nm using 2 mL of sample solution in a 1 cm quartz cuvette [23, 24]. The supernatant solution was filtered through a membrane filter, and the filtrate was subjected to UVvis analysis at different time intervals (time, t) [24] The amount of MB adsorbed per unit mass of Cz-pyr-P was calculated using Equation 1 [25]:

q =

(  )

,



Equation 1

where Co and Ct (mg/L) represent the initial MB concentration and the MB concentration at time t, respectively; V is the volume of solution (L); and m is the mass of Cz-pyr-P added to the solution (g). At equilibrium, the equation can be expressed as follows:

q =

(  )



,

Equation 2

where qeq (mg/g) and Ceq (mg/L) are the amount of MB adsorbed per gram of Cz-pyr-P and the concentration of MB at equilibrium, respectively [26]. The adsorption efficiency (AE) for the investigated dye on Cz-pyr-P can be determined using Equation 3 [25]:

%AE =

(  ) 

× 100.

Equation 3

Optimisation of the Cz-pyr-P dosage was achieved by adding a certain amount of Cz-pyrP (0.2–1.0 mg) to 10 mL of a known initial concentration of MB dye solution. The solution was allowed to mix, and the absorbance of the dye in the solution was measured at different time intervals. Additionally, the effect of the contact time and initial dye concentration were investigated through a series of experiments with initial concentrations varying from 1.0– 20.0 mg/L. A stock solution of MB dye was prepared, and from this a series of dilutions were tested at a fixed adsorbent dosage. Furthermore, to scrutinise the influence of the solution pH on the removal of MB dye, 0.1 M HCl and/or 0.1 M NaOH with insignificant volumes were utilised to adjust the initial pH value for the adsorption experiments. Experiments were performed at pH values of 3.5, 5.7, and 7.3, and the adsorption profiles were evaluated at a fixed adsorbent dosage and initial concentration of the dye. All adsorption isotherm and kinetic measurements were carried out using the optimum conditions, including the adsorbent dosage, initial dye concentration, and pH of the solution. In 6

brief, certain amount of Cz-pyr-P was added into 10 mL of known initial concentration of MB dye solution at a pH of 7.3. The mixture was magnetically stirred, and the concentration of the dye in the solution was evaluated at definite time intervals and analysed spectrophotometrically. 2.5 Regeneration of Cz-pyr-P The removal of the adsorbed MB from the Cz-pyr-P was achieved by ultrasonication with DI water for 30 min, and then was filtered using vacuum filtration. The washing step was repeated several times to ensure there is no MB adsorbed on Cz-pyr-P then the adsorbent were dried under vacuum at 80 ºC. The regeneration was performed by mixing the dried adsorbent with a fresh MB solution using the procedure described in section 2.4. Using this regeneration protocol, the adsorbent was recycled three times and reused in the adsorption experiments due to the utilization of small quantity of the adsorbent. 3. Results and discussion 3.1 Synthesis and characterisation of Cz-pyr-P The monomer (Cz-pyr-M) was synthesised according to the procedure described in the Supplementary Information (SI, Section A). The synthesis of the precursor, Cz-pyr-M imide, comprised four steps (Scheme 1). Initially, 4,5-di (9H-carbazol-9-yl) phthalonitrile was synthesised through the nucleophilic substitution of carbazole with 4,5-dichlorophthalonitrile facilitated by caesium fluoride (CsF) [27]. The hydrolysis of 4,5-di (9H-carbazol-9-yl) phthalonitrile (1) with KOH produced 4,5-di(9H-carbazol-9-yl) phthalic acid (2). Then, dehydration of the acid with acetic anhydride produced the anhydride (5,6-di (9H-carbazol-9-yl) isobenzofuran-1,3-dione) (3) [28]. Finally, the monomer (4) was synthesised by imidisation of the corresponding 2,6-diaminopyridine with 5,6-di (9H-carbazol-9-yl) isobenzofuran-1,3-dione. The network polymer (Cz-pyr-P) (5) was prepared by oxidative polymerisation facilitated by anhydrous FeCl3 dissolved in CHCl3 as a solvent [29]. The crude polymeric material was purified by refluxing with chloroform, methanol, THF, and deionised water to produce a yellow powder with a high yield (>80%).

7

O N

CN

N

CN

N

(i)

O OH

(ii)

N O

OH

N

N O

O

2

1 O

O N

(iii)

3

N

N O

N N

O

N

N

4

Cz-pyr-M (Monomer)

O

O N

(iv)

N

N O

N N

O

N

N

5

Cz-pyr-P (Polymer)

Scheme 1. Synthesis of Cz-pyr-P. Reagents and conditions: (i) KOH, ethanol/water, 120 ºC; (ii) Ac2O, 110 ºC; (iii) 4,6-diaminopyridine, AcOH, 125 ºC; (iv) FeCl3, CHCl3, RT. The structural identity and purity of the monomer were evaluated using routine spectroscopic techniques. The UV–visible spectrum of Cz-pyr-M was recorded and showed peaks at 290, 331, and 392 nm (Fig. S1). The characteristic peaks of carbazole appeared at 290 nm and at 331 nm. The peak obtained at 392 nm is attributed to the intramolecular charge transfer from carbazole to the imide moiety [30]. The FTIR spectrum (Fig. S2) of the Cz-pyr-P showed peaks at 1730 cm-1 and 1786 cm-1, corresponding to C=O symmetric and asymmetric stretching of the imide functionality, respectively, further confirming the structural stability of 8

the imide linkages. Aromatic C=C stretching frequencies at 1448–1449 cm-1 were observed for both the monomer and the corresponding polymer. To confirm the Cz-pyr-M structure, 1H and 13

C NMR were performed (Figs. S3 and S4, respectively). Cz-pyr-M exhibited all the respective

carbazole proton fingerprint signals. In addition, the aromatic protons present in 3rd and 5th position of the pyridine ring were observed at 7.86 ppm (d, J = 7.8 Hz) and 8.44 ppm (t, J = 8.4 Hz) (Fig. S3). This was corroborated by the

13

C NMR, in which all the signals obtained were

ascribed to the monomer structure (Fig. S4). The carbonyl carbon signal was observed at 164 ppm, and the signals in the range from 100–150 ppm were attributed to the phenyl carbons of the monomeric structure. Cross-polarization magic angle spinning (CP-MAS)

13

C NMR

spectra were recorded for both the monomer and the polymer (Figs. S5 and S6, respectively), The Cz-pyr-P spectra included a signal at 169.2 ppm, which was assigned to the carbonyl carbon present in the imide ring; the additional chemical shifts in the range from 100–150 ppm were assigned to phenyl carbon atoms (Fig. S5) [31]. The solid-state NMR spectrum of the polymer exhibited identical carbon signals to the monomer spectrum (Fig. S6). In addition, the mass and purity of the monomer were confirmed by HRMS and high-performance liquid chromatography (HPLC) analyses, respectively (Figs. S7 and S8). To investigate the thermal stability of porous polyimides, TGA and DTG curves were recorded as a function of temperature under a nitrogen atmosphere. The DTG curves for Cz-pyrP indicate that the major decomposition occurred at 619 ºC, and the onset of decomposition occurred at 382 ºC (Fig. S9). The first degradation point at 382 ºC with a weight loss of up to 7% could be due to the release of trapped volatile materials; the maximum degradation occurred at 607 ºC (26% weight loss). DSC analysis of the Cz-pyr-M shows that the melting point is 412 ºC, which reflects its extremely rigid nature [32] (Fig. S10). PXRD analysis of the polymer showed broad peaks, which indicates the amorphous aggregation structure (Fig. S11). The chemical composition of the Cz-pyr-P was analysed using XPS (Fig. S12). The percentages of the individual atoms and their binding energies are listed in Table S1. The findings suggest that there was no carbon fraction difference in the polymer (ca.: 80.4%; obtained: 80.6%). Notably, the oxygen and nitrogen fractions varied owing to the presence of the chloride guest molecule. Additionally, the XPS spectrum confirmed the absence of peaks related to iron (708 and 710 eV), indicating the purity of the material. In addition, these polymers are

9

designed with a highly conjugated system, and thus they may have a conductive nature. A UPS study of the polymer was performed to determine the molecular orbital energies in the valence band region from 2–4 eV (Fig. S13). The region from the Fermi level (the state above this does not emit electrons) to 10 eV is generally referred to as the valence band region. The prepared material exhibited a likely secondary edge or high binding energy cut-off at 16.07 eV. Furthermore, the maximum peak intensity, λmax, for Cz-pyr-P was recorded at 8.07 eV (Fig. S14). This maximum intensity corresponds to the electrostatic interaction between the donor– acceptor features of the Cz-pyr-P. Regions between 2 and 4 eV were considered to be in the highly occupied molecular orbital state (HOMO), and the synthesised Cz-pyr-P exhibited a valence band at 2.67 eV. This is due to the presence of strong π-π conjugation in the polyimide backbone. The work function (Φp) can be calculated from the difference in binding energies between the UV photons (Helium II, 21.21 eV) and the secondary cut-off region. This Φp is related to the band gap between the conductance and valence regions of the polymer. Here, the synthesised Cz-pyr-P has a working function of 5.14 eV [33]. These preliminary results reveal that the conductive feature of the Cz-pyr-P is due to the presence of a continuous π-π conjugation system. Morphological investigations of the polymer were performed using SEM and HR-TEM (Fig. S15a-b). The Cz-pyr-P surface is identical to that of the film; it is lined with rectangular sheets and exhibits a random size distribution up to 1 µm [34]. HR-TEM images of Cz-pyr-P clearly indicate a uniform distribution of pores on the polymer surface [35]. The surface topography of the polymer was examined by AFM (Fig. S15c). AFM topographic studies were used to obtain the mean roughness (Ry) and root mean square (Rq) values and hence determine the surface roughness of the polymer. Cz-pyr-P exhibits an average roughness (Image Ra) of 80.7 nm and a root mean square (Image Rq) of 104 nm. The AFM results indicate that the polyimide has a noble surface roughness due to the high rigidity of the building block [36]. 3.2 Surface area and CO2 adsorption performance Nitrogen sorption isotherms were measured at 77 K to evaluate the textural properties of the synthesised polymer [18]. The adsorption isotherm of Cz-pyr-P exhibited very high uptake at a relatively low pressure, with moderate hysteresis upon desorption. The desorption curve of Czpyr-P (Fig. 1) is well above the adsorption curve, which corresponds to the large hysteresis extending to the region of low relative pressure. This can be attributed to the tortuosity of the

10

microporous structures of Cz-pyr-P. The apparent BET surface area determined for the polymer is 1065 m2/g. The pore size distribution was derived using non-local density functional theory (NLDFT), and the polymer contained the highest population of ultra-micropores at 0.8 nm. Czpyr-P exhibited a pyridine centre along with a stretchy imide moiety. The plane of the substituted carbazole moieties is twisted approximately 60º relative to the imide plane, and the lack of rotation makes the polymer extremely rigid. This can be clearly explained by the single-crystal analysis of phthalocyanines bearing terminal carbazoles reported in our previous study [22]. The prime factors such as ultra-microporosity and flexible amine linkers may help to maintain the intrinsic microporosity of the polymer.

Fig. 1. Nitrogen sorption isotherm and NLDFT micropore size distribution of Cz-pyr-P at 77 K

Recently, there has been extensive focus on microporous organic polymers containing nitrogen-enriched species that can selectively attract and capture CO2 molecules. For example, the presence of nitrogen species such as nitriles, basic pyridinium, carbazole, and imide groups in the framework of the porous architecture is beneficial for increasing the affinity of the polymers for CO2 gas [17]. The CO2 adsorption isotherms of Cz-pyr-P were measured up to 1 bar at 273 and 298 K (Fig. 2). The highest uptakes attained for Cz-pyr-P at 1 bar are 19.41 wt.% (4.41 mmol/g) at 273 K and 12.78 wt.% (2.91 mmol/g) at 295 K (Table 1). These results are very promising compared to other reported nitrogen-containing organic microporous polymers (Table 2). The nitrogen-rich functionalised pore walls and ultra-micropores of Cz-pyr-P facilitate the appreciable CO2 adsorption and storage. The heat of adsorption of CO2 on Cz-pyr-P 11

was calculated as 28.5 kJ/mol. This moderate heat of adsorption value is attributed to the interaction between CO2 and the nitrogen species in the Cz-pyr-P. Table 2 summarises the surface area and CO2 adsorption for some previously reported polyimides. The polyimide synthesised in this study (Cz-pyr-P) exhibits comparatively greater surface area and CO2 adsorption.

Fig. 2. Isosteric heat of adsorption and CO2 adsorption isotherms for poly-diczpyridine at 273 and 295 K

Sample

SABET

PV (cm3/g)

(m2/g)a Cz-pyr-P

1065

0.59

NLDFT pore width (Å)

CO2 uptake at 273 K and 1 bar wt.%

CO2 uptake at Qst 295 K and 1 bar (CO2)( kJ/mol) mmol/g wt.% mmol/g

8.06

19.41

4.41

12.78

2.91

28.5

Table 1. Surface area and CO2 adsorption of poly-diczpyridine a

BET surface area calculated from the nitrogen adsorption isotherm

Table 2. Comparison of the surface area and CO2 adsorption of Cz-pyr-P with those of other reported polyimides Polymer

Surface area

CO2 uptake in

m2/g

wt.% at 273 K

12

Reference

and 1 bar PEPHQDA-HBPI-CL

593

10.1

[35]

MTPA

481

12.6

[37]

MOPI-IV

660

16.7

[38]

MPI-6FA

781

13.5

[39]

TAPA-HBPI-CR

497

1.65

[40]

TTAPI

1050

14.9

[41]

Cz-pyr-P

1065

19.4

This study

The sorbent performance of the developed polymer was evaluated through the partition coefficient (PC) calculations [42]. The PC value can be calculated by using the following equation. PC =

   



…………………….. 1

Equation 4

Where AC is the adsorption capacity in mg/g, P is the partial pressure of CO2 and M is the molecular weight of CO2. Table 3, screened the PC values of the prepared polymer compared with the published values. The polymer exhibits PC value of 1.1 x 10-2 mol/ kg Pa (273K) and 7.2 x 10-3 mol/ kg Pa (273K), which is comparatively better than the reported sorbents mentioned in the table 3. This can be attributed to the high surface area and the uniform pore size distribution demonstrated by the prepared Cz-pyr-P. Table 3. Comparison of Partition coefficient (PC) of the sorbent used for the current study with other reported values in the literature Inlet No

Sorbent

Pressure

Adsorption Temperature

in bar 1 2

CAC KOH activated carbon

Capacity (mg/g)

Partition Coefficient , PC (mol/

Ref

kg Pa)

1

298K

239

1.0 x 10-3

[43]

1

273K

277

6.2 x 10-6

[44]

13

3

Zeolite 13X

1

298K

187

3.1 x 10-1

[45]

1

303K

304

8.5 x 10-5

[46]

1

298K

119

2.0 x 10-4

47

1

273K

194.1

1.1 x 10-2

This

1

298K

127.8

7.2 x 10-3

Tetraethylenepenta 4

mine activated carbon

5

6 a

Activated carbon Cz-pyr-Pa

wor k

Sorbent used in the present work

3.3 Adsorption of MB dye The prepared polymer containing pyridine functionality with high porosity is likely to interact with environmentally hazardousorganic molecules. Cz-pyr-P was further utilised as an adsorbent for removing organic MB dye from wastewater. 3.3.1 Effect of the Cz-pyr-P dosage The adsorbent dosage is a pivotal factor in the adsorption process, and it is important to identify the minimum amount of adsorbent required to achieve maximum adsorption efficiency (AE) [47]. Thus, the effect of the amount of Cz-pyr-P used for the removal of MB at different time intervals was investigated, as shown in Fig. 3 (UV-vis spectra: Fig. S16). Conspicuously, the AE of Cz-pyr-P increased from 69.5% with 0.2 mg of Cz-pyr-P to 100% for all other Cz-pyrP dosages. Additionally, with doses of Cz-pyr-P greater than 0.2 mg, a complete adsorption of the dye was observed within 8 min. Furthermore, the AE of Cz-pyr-P was evaluated using different Cz-pyr-P dosages, as shown in the inset to Fig. 3. The efficiency of MB adsorption increased with increasing adsorbent dosage, and then a state of equilibrium was achieved with a dose of 0.7 mg. Thus, this dosage was used for further experiments.

14

0.2 mg Cz-pyr-P 0.5 mg Cz-pyr-P 0.7 mg Cz-pyr-P 1.0 mg Cz-pyr-P

100 80 60 Adsorption efficiency (%)

Adsorption efficiency (%)

120

40 20

100 90 80 70 60 50 40

0

0.2

0.4

0.6

0.8

1.0

Cz-pyr-P (mg)

0

5

10

15

20

25

30

Time (min) Fig. 3. Effect of Cz-pyr-P dosage on the adsorption efficiency of MB dye at different time intervals; the inset shows the adsorption efficiency versus the Cz-pyr-P dosage.

3.3.2 Effect of contact time and initial dye concentration Experiments were conducted to investigate the effect of the contact time on the AE of Cz-pyr-P using different initial MB concentrations, as shown in Fig. 4. Initially, the AE rapidly increased , and a state of equilibrium was achieved after almost 6 min. In the initial stage of the adsorption process, a large number of vacant surface sites on Cz-pyr-P are available, which allow rapid adsorption of MB dye.. It is worth mentioning that as the initial MB concentration increases, the AE of Cz-pyr-P decreases and reached a maximum value of 48.5% with 20 mg/L of MB. This can be explained by the fact that the number of vacant sites available is fixed for a particular adsorbent and therefore, at a lower initial concentration of MB, a larger number of active adsorption sites on Cz-pyr-P are accessible, while at a higher concentration of MB, the

15

number of adsorption sites is less [48]. Consequently, the highest AE was achieved at 10 mg/L of MB dye which was used for further experiments.

Adsorption efficiency (%)

100

80

60

40 1 ppm 5 ppm 10 ppm 15 ppm 20 ppm

20

0 0

5

10

15

20

25

30

Time (min) Fig. 4. Effect of the initial dye concentration on the adsorption efficiency of Cz-pyr-P at different time intervals.

3.3.3. Effect of solution pH on the adsorption performance Given the significant influence of the solution pH on the surface chemistry (surface functional groups) of both the adsorbent and adsorbate, experiments were performed to investigate the effect of pH on the AE of Cz-pyr-P. Initially, zeta potential measurements were performed for Cz-pyr-P over a pH range of 2-12 to scrutinise the possible surface charge on the investigated material; the isoelectric point (pHPZC) was identified as 5.72, as shown in Fig. 5A. Then, the adsorption performance was assessed at pH values of 3.5, 5.7, and 7.3, as illustrated in Fig. 5B (UV-vis spectra: Fig. S17). At pH 3.5, the maximum AE of Cz-pyr-P was 96.8% after 16

30 min; upon increasing the pH to 5.7, insignificant changes in the adsorption efficiency were observed. Nevertheless, further increasing the pH of the solution to 7.3 promoted a higher AE for MB (100%) after only 6 min. This phenomenon can be attributed to the surface charge of Czpyr-P, which is positive at pH < pHPZC, thus provoking competitive effects of the H+ ions in the solution and an electrostatic repulsion between the MB molecules( cationic dye) and the positively charged active sites on Cz-pyr-P. On the other hand, at pH > pHPZC, the presence of negatively charged active sites on Cz-pyr-P stimulated the electrostatic attraction between the adsorbent and the MB molecules, thus enhancing the adsorption efficiency [48].

30

A

Zeta potential (mV)

20 IEP = 5.72

10

0

-10

-20 2

4

6

8 pH

17

10

12

Adsorption efficiency (% )

100

B

80

60

40

pH = 3.5 pH = 5.7 pH = 7.3

20

0 0

5

10

15

20

25

30

Time (min) Fig. 5. A) Isoelectric measurement of Cz-pyr-P, and B) effect of the solution pH on the adsorption efficiency of Cz-pyr-P (adsorbent dosage = 0.7 mg and MB concentration = 10 mg/L).

3.4 Adsorption equilibrium time and kinetics The adsorption of MB dye on Cz-pyr-P was investigated as a function of the contact time under optimized conditions, as shown in Fig. 6A. A rapid initial uptake of MB dye was observed in the first 4 min due to the availability of active adsorption sites. Then, a state of equilibrium was achieved after 6 min, indicating a high adsorption rate of MB on Cz-pyr-P. Thus, 6 min was used as the equilibrium time for further experiments. To obtain further insight into the adsorption process, the adsorption kinetics are used to describe the rate and adsorption mechanism of MB by Cz-pyr-P. Therefore, to determine the mechanism governing the adsorption of MB dye onto Cz-pyr-P, pseudo-first-order and pseudo18

second-order kinetic models were constructed for the experimental data using Equations 4 and 5, respectively.

ln( −  ) = ln  −  , 



=





+

 "# #

Equation 5

,

Equation 6

where qe and qt (mg/g) are the adsorbed amount of MB dye at equilibrium and time t, respectively; and k1 (min-1) and k2 (g/mg·min) are the rate constants for the pseudo-first-order and pseudo-second-order reactions, respectively. The kinetic parameters and correlation coefficients (R2) for the two models were obtained from straight-line plots, as summarised in Table 4. The pseudo-second-order plot exhibited excellent linearity with a high correlation coefficient compared to the pseudo-first-order plot, indicating the kinetics data followed a pseudo-second-order model (Fig. 6B).

140

A

qt (mg/g)

120 100 80 60 40 0

5

10

15

20

Time (min) 19

25

30

0.25

Pseudo second order

B

t/qt (min mg/g)

0.20

0.15

2

R = 0.99968 SD =0.00182

0.10

0.05

0.00 0

5

10

15

20

25

30

Time (min) Fig. 6. A) Effect of the contact time on the adsorption rate of MB on Cz-pyr-P, and B) pseudosecond-order plot for the adsorption of MB on Cz-pyr-P, where SD is standard deviation,(adsorbent dosage = 0.7 mg, initial dye concentration = 10 mg/L, solution pH = 7.3, and temperature = 25 °C).

Table 4. Fitting parameters of the pseudo-first-order and pseudo-second-order kinetic models for the adsorption of MB on Cz-pyr-P Pseudo-first-order parameters

Pseudo-second-order parameters

R2

qe (mg/g)

k1 (min-1)

R2

qe (mg/g)

k2 (g/mg·min)

0.9827

121

0.8257

0.9997

144.92

0.034

20

3.5 Adsorption isotherm To understand the nature of the interaction between MB dye and the active sites on Czpyr-P and consequently assess the adsorption capacity at equilibrium, relevant adsorption isotherms were investigated. Langmuir and Freundlich isotherm models were employed to analyse the obtained adsorption data. The Langmuir isotherm model is premised on monolayer sorption occurring on a homogenous surface; the interaction between the adsorbate and the adsorbent is negligible (i.e. there is no interaction between adsorbate and adsorbent). The Langmuir isotherm model is expressed as: 



=

 $%

+

 &' $%

,

Equation 7

where Ce (mg/L) is the concentration of MB at equilibrium, qe (mg/g) is the amount adsorbed at equilibrium, Qm (mg/g) is the maximum adsorption capacity, and KL (L/mg) is the adsorption equilibrium constant. In contrast, the Freundlich isotherm model assumes a non-ideal adsorption occurring over a heterogeneous surface and can be expressed as:

ln q ( = ln K* +

+,  ,

,

Equation 8

where Kf is the Freundlich constant, which is related to the adsorption capacity, and n is the adsorption intensity or heterogeneity. Accordingly, a plot of Ce/qe versus Ce was constructed to determine the Langmuir parameters, while a plot of ln qe versus ln Ce was employed to obtain the Freundlich parameters. Both models were used to fit the obtained equilibrium data for MB adsorption (Table 5). It is clear that the correlation coefficient obtained with the Langmuir model is much higher than the coefficient obtained with the Freundlich isotherm model. Therefore, the Langmuir isotherm model (Fig. 7) is more appropriate to describe the equilibrium data, implying that the adsorption process was relatively homogenous and a monolayer of MB dye was formed on Cz-pyr-P. Furthermore, based on the Langmuir model, the maximum adsorption capacity of MB dye was estimated to be 175.44 mg/g.

21

Langmuir isotherm

0.14 0.12

Ce/qe (g/L)

0.10 2

R = 0.9998 SD = 0.00161

0.08 0.06 0.04 0.02 0.00 0

5

10

15

20

25

Ce (mg/L) Fig. 7. Langmuir isotherm model for the adsorption of MB on Cz-pyr-P, where SD is standard deviation.

Table 5. Fitting parameters for the Langmuir and Freundlich isotherm models for the adsorption MB on Cz-pyr-P Langmuir isotherm parameters

Freundlich isotherm parameters

R2

Qm (mg/g)

KL (mL/g)

R2

n

Kf (mL/g)

0.9998

175.44

4.75

0.9921

40

148.4

3.6 Partition coefficient and comparative study The performance of the adsorbent is usually assessed by its adsorption capacity towards a particular dye. Nevertheless, the adsorption capacity is a sensitive value and can be affected by the initial concentration of the dye. Commonly, a high initial concentration of the dye drastically 22

increases the adsorption capacity value, while a low initial concentration of dye resulting in lower adsorption capacity value. Therefore, partition coefficient (PC) was employed to assess the adsorption performance of the adsorbent and it is expressed as the ratio of the concentration of the target analyte on the adsorbent solid phase to its concentration in the liquid phase [49]. Therefore, PC provides more practical assessment of the performance of the adsorbent, regardless of differences in the operating conditions such as the initial dye concentration. PC (mg/gµM) can be calculated using the following equations: PC = Adsorption capacity(Qe)/ Final concentration (Ce)

Equation 9

PC =Adsorption capacity(Qe)/(Initial concentration (Co)× Removal rate (Qt)

Equation 10

The prepared polymer showed superior PC value of 380.10 mg/gµM) which is significantly great compared to previously reported adsorbents. The enhanced performance of the Cz-pyr-P towards MB dye can be mainly ascribed to : (i) The high surface area (1065 m2/g) exhibited by Cz-pyr-P enables high loading capacity of MB dye., (ii) the presence of imide and pyridine moieties in the prepared network polymer Cz-pyr-P and (iii) Cz-pyr-P has a highly conjugated system which facilitates multiple π-π interactions between the polymer and MB dye as described in next the section. Table 6. Summary of experimental data obtained for different adsorbents on the removal of MB dye from water, maximum adsorption capacities and partition coefficients. Adsorbent

MB concentration (mg/L)

Adsorption dose, m (g/L)

Co–Ce/m (mg/g)

Partition Coefficient, Kd (mg/gµM)

Ref

4.82

Maximum adsorption (Langmuir) capacity (mg/g) 22.93

Haloxylon recurvum plant stems Fe2O3– ZrO2/BC Potato leaves powder Potato stem powder Fe2O3– SnO2/BC Cz-pyr-P

20

4

6.70

[50]

10

2

4.83

38.1

14.12

[51]

10

2

4.25

52.6

2.02

[52]

3.95

41.6

2.47

10

2

4.90

58.2

23.90

[53]

10

0.07

137.68

175.44

380.10

This work

3.7 Regeneration of Cz-pyr-P 23

Regeneration studies on the feasibility of desorbing MB molecules from Cz-pyr-P were evaluated by 3 cycles of repeated measurements as shown in Fig. 8. The Cz-pyr-P retained its dye adsorption capability of more than 85% even after 3 cycles of repeatability.

Adsorption Efficiency (%)

100

75

50

25

0

Cycle 1

Cycle 2

Cycle 3

Fig. 8. Reusability of Cz-pyr-P for removing MB from wastewater after 3 cycles. Conditions: initial concentration of MB = 10 mg L-1; and dosage of Cz-pyr-P =0.7 mg.

3.8 Proposed adsorption mechanism Several interactions may be involved in the adsorption process, including hydrogen bonding, π-π dispersion, dipole-dipoleand hydrophobic interactions [10]. FTIR spectroscopy was conducted on pristine MB, Cz-pyr-P, and Cz-pyr-P after MB adsorption to propose the mechanism that governs the adsorption of MB on Cz-pyr-P, , as shown in Fig. 9A. Upon the adsorption of MB dye, the individual peaks for MB and Cz-pyr-P could be clearly observed with no distinguishable shift, which may indicate that the adsorption of MB on Cz-pyr-P is ascribed to the physical nature.

The peaks of corresponding to stretching vibrations of C=O, C-O, and 24

C=C at 1730–1786, 1130 and 1060, and 1448–1449 cm-1, respectively, exhibited a decrease in intensity, which can be attributed to electrostatic and π-π dispersion interactions between Cz-pyrP and MB., This loss in intensity of C=C vibration frequencies can be ascribed to the geometry of MB molecule (a planar molecule) potentially enable π-π interactions between the aromatic rings present on both the adsorbent and thereby facilitatingits adsorption process [48].A schematic representation of the potential interaction between Cz-pyr-P and MB is illustrated in Fig. 9B.

Transmittance (%)

A

Poly-Diczpyridine MB dye MB dye adsorbed on Poly-Diczpyridine 4000

3500

3000

2500

2000

1500 -1

Wavenumber (cm )

25

1000

500

Fig. 9. A) FTIR spectra of Cz-pyr-P, MB dye, and MB adsorbed on Cz-pyr-P; and B) schematic illustration of the potential π-π interaction between Cz-pyr-P and MB Finally, the synthesized Cz-pyr-P microporous network polymer with a high surface area of 1065 m2/g, a narrow pore size distribution of 0.8 nm (ultramicropores) and the pyridine moiety within the resultant conjugated structure make the polymer as efficient adsorbent towards the sorption of gas molecules (CO2) and organic dye (MB). The highest CO2 uptakes attained for Czpyr-P were 19.41 and 12.78 wt.% at 273 and 295 K, respectively. These achievements are promising compared to the recently published reports. For example, Liang et al, reported the bipyridine and phenanthroline-based porous polycarbazole networks (CPOP-30 and CPOP-31) with a surface area of 880 and 1100 m2/g and pore size distribution of 0.8 and 0.9 nm respectively. Their CO2 uptakes were 12 and 8.4 wt% for CPOP-30 and CPOP-31, respectively [54]. Furthermore, a study was conducted by Song et al using hyperbranched polyimides for 26

efficient CO2 capture with surface area in the range of 43 to 294 m2/g. and maximum CO2 uptake of 8.56 wt.% at 273 K [55]. Furthermore, we rigorously investigated the potential application of Cz-pyr-P polymer for water treatment by evaluating the adsorption performance of typical water-soluble organic dyes. The polymer showed higher uptake of cationic MB with a superior adsorption capacity of 175.44 mg/g and an equilibrium time of 6 min compared to recently published papers (Table 6). The advanced adsorption of MB dye may be related to the high surface area and homogeneous microporous distribution of Cz-pyr-P. It has also been found that the interaction between the active functional groups (ie, pyridine and/or imide moieties) present in the polymer structure and the dye is strong and which could be the reason for the high adsorption efficiency of the polymer.

Conclusion A microporous organic network polymer (Cz-pyr-P) derived from a monomer of pyridine-imides flanked by four carbazoles was successfully synthesised and comprehensively characterised using routine spectroscopic techniques. The synthesised polymer exhibited a large surface area of 1065 m2/g and a median pore width of 8.06 Å, which facilitates the reversible adsorption of 19.41 wt.% (273 K) and 12.78 wt.% (295 K) CO2 at 1 bar, with a good isosteric heat value of CO2 adsorption (28.5 kJ/mol). Additionally, Cz-pyr-P was utilised for the removal of MB from waste water and exhibited an excellent adsorption capacity of 175.44 mg/g with an equilibrium time of 6 min, which is shorter than most of the reported values for other materials. The results suggest that the developed Cz-pyr-P polymer can be used for industrial applications such as water purification and gas adsorption processes. The promising results obtained by using Cz-pyr-P for dual applications: gas uptake and dye removal open up avenues for material and environmental research. Based on this study, porous organic polymers with enhanced porous properties and large surface areas will be further developed and examined for gas uptake studies and environmental applications. Additionally, different types of pollutants and their behaviour will be investigated for water treatment purposes.

Conflict of interest statement 27

The authors declare they have no conflicts of interest.

Acknowledgments The authors gratefully acknowledge the Kuwait Foundation for the Advancement of Science (KFAS), Kuwait University, RSPU Facilities No. (GS 01/01, GS 01/03, GS 02/01, GS 03/01, GS 01/05, GE 01/07, and GE 03/08), and Department of Chemistry (Faculty of Science). The Nanoscopy Science Centre is also gratefully acknowledged.

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Highlights •

Carbazole-tagged pyridinic microporous polymer (Cz-pyr-P) was successfully prepared

•

Cz-pyr-P exhibits high surface area (1065 m2/g) and pore size distribution (8.06 Å)

•

Cz-pyr-P reversibly adsorbed 19.41 wt.% (273 K) and 12.78 wt.% (295 K) CO2 at 1 bar

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Cz-pyr-P revealed MB adsorption capacity of 175.44 mg/g with 6 min equilibrium time

Novelty Statement The novelty of the present work premises on the synthesis of carbazole-tagged pyridinic microporous network polymer with decent surface area of 1065 m2/g and good CO2 adsorption efficiency of 19.41 wt.% (273 K) and 12.78 wt.% (295 K) CO2 at 1 bar. The prepared polymer was subsequently utilized for methylene blue adsorption. The adsorption rate is comparatively high (6 min) with respect to the recently reported literature.

Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:

Regards

Saad Makhseed