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Construction of hypercrosslinked polymers with dual nitrogen-enriched building blocks for efficient iodine capture
Journal Pre-proofs Construction of hypercrosslinked polymers with dual nitrogen-enriched building blocks for efficient iodine capture Xuemei Li, Yu Peng, Qiong Jia PII: DOI: Reference:
S1383-5866(19)32662-0 https://doi.org/10.1016/j.seppur.2019.116260 SEPPUR 116260
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Separation and Purification Technology
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22 June 2019 7 September 2019 25 October 2019
Please cite this article as: X. Li, Y. Peng, Q. Jia, Construction of hypercrosslinked polymers with dual nitrogenenriched building blocks for efficient iodine capture, Separation and Purification Technology (2019), doi: https:// doi.org/10.1016/j.seppur.2019.116260
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Construction of hypercrosslinked polymers with dual nitrogen-enriched building blocks for efficient iodine capture
Xuemei Li, Yu Peng, Qiong Jia * College of Chemistry, Jilin University, Changchun 130012, China
Abstract
Nitrogen-enriched porous organic polymers are promising adsorbents for iodine capture and environmental sequestration. Herein, we constructed hypercrosslinked polymers (HCPs) with dual nitrogen-rich building blocks, 2,4,6-triphenyl-1,3,5-triazine and pyrrole. The resulting polymer presented high nitrogen content up to 8.04%, surface area of 222.8 m2 g−1, and high thermal stability. 257 wt% iodine uptake capacity was obtained as a result of several elements such as high nitrogen contents, high porosity, and π-electron conjugated structure of the polymer. Spectral studies demonstrated that iodine uptake could occur at triazine and pyrrole moieties combined with aromatic rings in the polymer chain and the iodine nature was polyiodide anions. In addition, the material could be efficiently recycled without significant loss of iodine uptake capacity. Our work presented promising results for HCPs as economical adsorbents for the removal of iodine and other contaminates for environmental remediation applications.
Keywords: triazine; pyrrole; hypercrosslinked polymers; nitrogen-enriched; iodine capture
*
Corresponding author. E-mail address: [email protected] (Q. Jia) 1
1. Introduction
Porous organic polymers (POPs), a fascinating group of porous materials, are composed of covalently bonded light atoms, which have attractive merits such as excellent chemical and physical stability, large surface area, abundant porosity, and easy tailorability [1-3]. Owing to these advantages, POPs such as covalent organic frameworks / polymers (COFs / COPs) [4-8], conjugated microporous polymers (CMPs) [9-18], covalent triazine frameworks (CTFs) [19], and porous aromatic frameworks (PAFs) [20, 21] have found their applications in the field of gas separation, e.g., iodine capture from nuclear power plant exhaust fumes. A detailed investigation reveals that iodine uptake capacity will be enhanced when nitrogen scaffolds are introduced into POPs due to the interactions between iodine and the lone pair electrons on nitrogen [22, 23]. Recently, a nitrogen-enriched molecule, triazine has been used as a building block to construct a wide variety of POPs in the field of iodine capture and the notable examples include CTFs [19] and triazine-based CMPs [12, 15]. However, high reaction temperatures, targeted functionalized monomers, or expensive metal catalysts are commonly required for the synthesis of the above-mentioned triazine-based POPs [24-26], which greatly hinders their scale-up implementation. As a subset of POPs, hypercrosslinked polymers (HCPs) possess not only the merits of POPs, but also remarkable advantages of low cost, facile preparation, and diverse synthetic methods [27-30]. So far, there have been several papers related to the synthesis of triazine-based HCPs and most of the resultant polymers have been employed in the field of gas adsorption (e.g. CO2) due to its high nitrogen contents and microporous nature. However, the application of such appealing materials to the field of iodine capture is still limited.
2
To construct HCPs with dual blocks containing adsorbate affinity units may have positive effects to the adsorption capacity. For example, the introduction of nitrogen and sulfur atoms of triazine and thiophene is responsible for CO2 adsorption as a result of the dipole-dipole interactions between the polymer and CO2 molecules [31]. In Liu et al.’s work [26], two kinds of nitrogen-containing building blocks, cyanuric chloride and pyrrole, were used to synthesize HCPs for CO2 capture. Surprisingly, triazine-based dual affinity blockscontaining HCPs have been seldom reported except the above studies focusing on CO2 adsorption. It will be a hot topic to develop HCPs with two functional groups based on triazine, the attractive nitrogen-enriched block. With these considerations in mind, herein, we targeted to design a HCP containing two adsorbate affinity building blocks, triazine and pyrrole, and used for the effective adsorption of iodine. Pyrrole, a nitrogen-containing heterocyclic molecule, has demonstrated its positive impact on iodine uptake [32]. In our previous work, we prepared a pyrrole-based HCP and an iodine uptake capacity up to 460 wt% was obtained. To further enhance the iodine uptake capacity, we synthesized a phenyl-bearing triazine-based monomer (2,4,6-triphenyl-1,3,5triazine) considering the interactions between benzene groups and iodine [23, 33]. The developed HCP rich in nitrogen affinity sites (N-HCP) exhibited high uptake efficiency for iodine due to the synergistic interactions between iodine and the lone pair electrons on nitrogen as well as extended π-conjugations.
2. Experimental section
2.1. Synthesis of triazine-based monomer
3
2,4,6-triphenyl-1,3,5-triazine (TP3) was synthesized according to a previous report with minor revisions [34]. In a typical synthesis process, 1 mL benzonitrile was charged into a round bottom flask at 0 °C. Then, 3 mL trifluoromethanesulfonic acid (98%) was added into the solution in the nitrogen atmosphere and the resultant mixture was stirred for 30 min. The reaction was left to proceed at room temperature overnight. 50 mL water was added and the mixture was neutralized with 2 mol L–1 sodium hydroxide. The resulting residue was extracted with a mixture of chloroform and acetone (50/50 v:v, 3×50 mL). The solvent was evaporated using rotary evaporators to obtain a white solid.
2.2. Synthesis of triazine and pyrrole bifunctionalized HCPs
Under a nitrogen atmosphere, 0.5 mmol TP3 and 0.5 mmol pyrrole were dispersed in 30 mL chloroform for 30 min, and then anhydrous 8 mmol FeCl3 was added into the solution. The reaction mixture was then stirred for 24 h at 25 °C. Afterwards, the mixture was transferred to a 100 mL Teflon-lined autoclave and the autoclave were heated at 150 °C for 48 h. After cooled to room temperature, the crude product was filtrated and washed by Soxhlet extraction with MeOH for 24 h.
2.3. Iodine capture
Iodine vapor uptake experiments were based on gravimetric measurements [35]. 10 mg HCP powders placing in an open cap and an excess amount of iodine were located at another sealed glass vial. The whole system was heated at 75 °C and 1 bar, which was typical fuel reprocessing conditions [36]. After specific time intervals, the N-HCP powders were cooled
4
down to room temperature and then weighed. The iodine uptake capacity was calculated using the following equation:
(m2 m1 ) 100wt % m1
(1)
where α is the iodine uptake capacity, m2 and m1 represent the mass weight of HCPs samples after and before capture of iodine.
2.4. Iodine adsorption
A stock solution of 2000 mg L–1 was prepared in cyclohexane by dissolving 0.2 g nonradioactive crystalline iodine in 100 mL cyclohexane and stored in the dark. The stock solution of iodine was diluted to obtain a range of experimental iodine concentrations. The equilibrium adsorption capacity was calculated by the following equation: Qt
( C 0 C t )V m
(2)
where C0 and Ct are initial and final concentration of iodine in cyclohexane, respectively, V is the volume of the solution (mL), and m is the mass of the adsorbent (mg). For the dynamic study, 5 mg polymer was added into 5 mL 40 mg L–1 iodine solution. After different time intervals, the iodine concentration was examined using a UV-1601PC spectrophotometer. The adsorption isotherm study was carried out with initial iodine concentrations ranging from 40 to 400 mg L−1. The concentration of iodine was measured spectrophotometrically.
3. Results and discussion
3.1. Characterization
5
N-HCP was synthesized in two steps and the synthetic scheme was presented in Fig. 1. Firstly, the monomer TP3 was synthesized by benzonitrile at room temperature in the presence of trifluoromethanesulfonic acid. Secondly, the polymerization of the monomer and pyrrole was carried out in the presence of cheap Lewis acid anhydrous FeCl3 under solvothermal conditions (in chloroform) at 150 °C. The comparisons of FTIR spectra of benzonitrile and TP3 were shown in Fig. S1. Two characteristic absorption bands associated with the triazine ring were observed at 1524 cm–1 (C–N stretching mode) and 1368 cm–1 (in-plane ring stretching vibrations), while the stretching modes of terminal C≡N group in benzonitrile at 2233 cm−1 disappeared [37, 38], suggesting the successful preparation of the triazine-based monomer TP3. Furthermore, 1H NMR and
13C
NMR spectra (Figs. S2 and S3) also
demonstrated the successful preparation of TP3. After polymerization (Fig. 2a), characteristic peaks attributed to the triazine ring at 1520 and 1357 cm−1 and the band in the range of 1649−1558 cm−1 belonging to C=C stretching vibration in the (hetero)-aromatic ring existed [26]. A new band at 1435 cm–1 corresponded to the stretching vibration of C-N-C [39], indicating the successful copolymerization of TP3 and pyrrole. The chemical environments of different carbon atoms present in N-HCP were investigated by solid-state 13C CP/MAS NMR spectroscopy (Fig. S4). The major signals in the range of 110−150 ppm were assigned to the carbon atoms of the (hetero) aromatic ring [39, 40]. The peak at 169.8 ppm was corresponding to the carbon atoms in the triazine ring [12]. Besides, the peak observed at 53.3 ppm was attributed to the alkyl carbon, suggesting that CHCl3 took part in the polymerization process [31]. To clarify the element species and further confirm the form of N element, the polymer was characterized by XPS determination (Fig. 2b). As expected, C 1s and N 1s peaks were observed in the spectrum. The high resolution N 1s spectrum of the polymer showed two peaks at 400.2 and 399.2 eV (Fig. 2b inset), which could be attributed to N atoms within pyrrolic N and the triazine units [41], respectively. Their chemical compositions were also 6
determined by CHN analysis and the contents of primary elements were confirmed as N (8.04%), C (56.07%), and H (2.96%) respectively. All the above results indicated the successful copolymerization of TP3 and pyrrole.
Fig. 1. Synthesis of N-HCP material.
Fig. 2. (a) FTIR and (b) XPS spectra of N-HCP (inset: N 1s spectra).
According to the nitrogen adsorption-desorption isotherms, Brunauer-Emmett-Teller (BET) surfaces area was calculated to be 222.8 m2 g−1. As shown in Fig. 3a, the isotherm exhibited a significant hysteresis at low pressure, indicating the presence of micropores which could be accessed with some restrictions [17]. Based on the NLDFT method, the polymer 7
exhibited supermicropores with a pore dimension centered at about 1.6 nm. The thermal stability (Fig. 3b) was estimated by the thermogravimetric analysis and the result showed that the polymer yielded more than 50% chars when heated to 800 °C. The excellent thermal stability was beneficial for the practical application of harsh conditions. Furthermore, the morphology of the polymer was evaluated by SEM and TEM (Fig. S5). SEM image showed that the polymer was composed of irregular spherical solids, which aggregated plate-shaped particles. TEM image revealed that the polymer was porous, which was in agreement with the above-mentioned BET results.
Fig. 3. (a) Nitrogen adsorption-desorption isotherms (inset: pore size distribution calculated from the NLDFT method) and (b) TG curve of N-HCP.
3.2. Iodine capture
The performance of the polymer for the capture of iodine vapor was evaluated by directly gravimetric measurements. The mass of the sample was measured at different time intervals until no obvious mass change could be observed (typically for 650 min). The maximum iodine uptake capacity was 257 wt% (Fig. 4a) and the capacity was 9-fold higher 8
than that of the typical porous silver-doped zeolite mordenite [42]. The high iodine uptake capacity might be due to three main factors, e.g., high porosity, high nitrogen contents, and πelectron conjugated structure of the polymer. As shown in Fig. 4b, compared with other triazine-based POPs, N-HCP exhibited good iodine adsorption per unit SBET. The different values of iodine adsorption per unit SBET provided a fundamental basis for the selective uptake of iodine from exhaust fumes. It was worthy to note that the iodine uptake capacity reached about 138 wt% (half of the maximum capacity) within 30 min, which was faster than other porous adsorbents. The fast capture rate might be attributed to high content of nitrogen atoms because an effective binding site is crucial at low coverage, leading to the high interactions with iodine molecules [21, 43]. From a correlation coefficient point of view, the experimental data of N-HCP were definitely better fitted by the pseudo-second-order kinetic model.
Fig. 4. (a) Gravimetric iodine uptake (inset: the pseudo-second order kinetic model) of NHCP as a function of time at 75 °C and (b) Iodine uptakes (1 bar) and per unit SBET for iodine capture.
3.3 Iodine release and adsorbent recycle
9
Reversible capture of iodine by adsorbents is of great importance for their practical applications, therefore, the thermal stability of iodine-loaded N-HCP sample was also evaluated by TGA under nitrogen atmosphere. As shown in Fig. S6, there was a sharp weight loss in the range of 80 to 330 °C, which could be attributed to the loss of loaded iodine [8]. From a practical perspective, the trapped iodine could be released and the adsorbent could be recycled. Furthermore, iodine release in organic solvents such as ethanol under ambient atmosphere was monitored by a UV-1601PC spectrophotometer. UV-Vis spectra showed two absorbance maxima at 360 and 291 nm (Fig. 5a), which could be assigned to the presence of polyiodide anions [15]. The color of the solution changed from light to dark yellow (Fig. 5b inset). The amount of iodine released from N-HCP powders increased linearly during the first 35 min and fitted well with the pseudo-zero-order kinetic model, demonstrating that the process was governed by host-guest interactions [16, 44].
Fig. 5. (a) Temporal evolution of UV-Vis adsorption spectra of the iodine released from loaded N-HCP in 3.5 mL ethanol and (b) Controlled release rate of iodine in N-HCP in the first 35 min determined by UV-Vis spectrophotometer.
10
To exploit the recyclability of the adsorbent, recycling tests were carried out by heating iodine-loaded N-HCP powders at 125 °C for 300 min. After that, the samples were reused for iodine uptake. In four consecutive runs, N-HCP could be efficiently recycled and reused without significant loss of iodine uptake (Fig. S7).
3.4. Capture mechanism
In the FTIR spectra (Fig. 6a), the peak centered at 1615 cm–1 assigned to C=C stretching vibration in the (hetero) aromatic ring shifted to 1602 cm–1. Also, the peaks at 1520 and 1357 cm–1 originated from the triazine ring shifted to 1512 and 1353 cm–1, respectively. Besides, the band at 1435 cm–1 associated with the stretching vibration of C−N−C in pyrrole ring shifted to 1432 cm–1. The results indicated that iodine uptake could occur at triazine and pyrrole moieties combined with aromatic ring. This is most likely because that a charge transfer complex is formed between N-HCP and polyiodide anions during the process of capture [33]. The resulting charge transfer interactions are enough to result from the bandshift effect. Compared with the spectrum of pristine polymer, the presence of I 3d peaks in the iodine-loaded polymer indicated the successful capture of iodine. To further demonstrate the form of captured iodine, the high resolution of I 3d spectra were shown in Fig. 6. The peaks appeared at 618.4 (3d 5/2), 619.4 (3d 5/2), 630.1 (3d 3/2), and 631.3 eV (3d 3/2), attributing to the formation of polyiodine anions (I3− and I5−) [7, 45].
11
Fig. 6. (a) FT-IR and (b) XPS spectra of N-HCP (black line) and iodine-loaded N-HCP (red line) (inset: the high resolution of I 3d spectra after iodine uptake on N-HCP).
3.5. Iodine adsorption
In addition, the iodine uptake capacity in cyclohexane solution was evaluated since it is of great importance for nuclear waste management. To get further insights into the adsorption process, adsorption kinetics and isotherms of iodine on N-HCP were investigated (Fig. 7). Temporal evolution of UV-Vis adsorption spectra in the iodine-cyclohexane solution were shown in Fig. S8. It could be observed that the adsorption capacity increased with the increase of contact time and achieved equilibrium within 7 h (Fig. 7a). The experimental data were fitted by the pseudo-first-order and pseudo-second-order kinetics models and they were expressed as follows:
ln (Qe - Qt ) = ln Qe - k1t
(3)
t 1 t = + 2 Qt k2Qe Qe
(4)
where Qe and Qt (mg g–1) are the uptake capacities of iodine on N-HCP at equilibrium and at time t (min) respectively. k1 (min–1) and k2 (g mg–1 min–1) are the pseudo-first-order and 12
pseudo-second-order rate constants, respectively. The results showed that the data of iodine uptake on N-HCP fitted well with the pseudo-second-order (R2= 0.9961) kinetic model compared with the pseudo-first-order (R2= 0.9661) kinetic model. The detailed parameters were listed in Table S1. In this study, we tested the applicability by soaking the polymers in a range of concentrations of iodine-cyclohexane solutions. As time prolonged, the pink color of the iodine-cyclohexane solution became lighter (Fig. S9). To further study the iodine adsorption isotherms, we fitted the experimental data using the non-linear Langmuir and Freundlich models as follows:
Qe
QmbCe 1 bCe
(5)
Qe KCe1 n
(6)
where Qm is the maximum adsorption capacity, b is the Langmuir constant related to binding energy, and K and n are empirical Freundlich constants associated with adsorption capacity and intensity, respectively. As is known, Langmuir model assumes monolayer adsorption with identical and energetically equivalent adsorption sites. On the contrary, Freundlich model assumes multilayer adsorption with heterogeneous surface. The parameters of isotherm models were listed in Table S2. The results showed that the correlation coefficients (R2) of Freundlich isotherm model (0.9889) was higher than that of Langmuir isotherm model (0.9606). Besides, it was worthy to note that the adsorption isotherm could be well fitted by Freundlich model since the polymer had three potential iodine trapping sites: phenyl rings, triazine moieties, and pyrrole [22].
13
Fig. 7. (a) Iodine adsorption curve of N-HCP (5 mg) at various time intervals in iodinecyclohexane solution (40 mg L–1, 5 mL, 25 °C) and (b) Adsorption isotherms with Langmuir (red line) and Freundlich (blue line) models fitting.
4. Conclusions
In summary, a triazine and pyrrole bifunctionalized task-specific hypercrosslinked polymer was successfully synthesized and applied to the capture of iodine. The high nitrogen content and surface area contributed to the satisfactory iodine uptake capacity up to 257 wt%. The process of iodine capture fitted the pseudo-second-order model and the captured iodine could be released by either immersing in ethanol solution or heating. Furthermore, the iodine uptake capacity in solution phase was evaluated, demonstrating that the adsorption was well depicted by the pseudo-second-order model and Freundlich model. Our work made great progress in HCPs synthetic methods and revealed the great potential of HCPs as an attractive adsorbent for environmental remediation.
14
Acknowledgement This project was financially supported by State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, College of Chemistry, Jilin University, China (2019-4).
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Highlights ● A nitrogen-enriched monomer with extended π-conjugations was designed. ● A cost-effective nitrogen-rich HCP with high surface area was successfully prepared. ● The resultant polymer exhibited high iodine uptake capacity up to 257 wt%. ● Nitrogen content, porosity, and π-electron structure played key roles in iodine uptake.
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S1383-5866(19)32662-0 https://doi.org/10.1016/j.seppur.2019.116260 SEPPUR 116260
To appear in:
Separation and Purification Technology
Received Date: Revised Date: Accepted Date:
22 June 2019 7 September 2019 25 October 2019
Please cite this article as: X. Li, Y. Peng, Q. Jia, Construction of hypercrosslinked polymers with dual nitrogenenriched building blocks for efficient iodine capture, Separation and Purification Technology (2019), doi: https:// doi.org/10.1016/j.seppur.2019.116260
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© 2019 Published by Elsevier B.V.
Construction of hypercrosslinked polymers with dual nitrogen-enriched building blocks for efficient iodine capture
Xuemei Li, Yu Peng, Qiong Jia * College of Chemistry, Jilin University, Changchun 130012, China
Abstract
Nitrogen-enriched porous organic polymers are promising adsorbents for iodine capture and environmental sequestration. Herein, we constructed hypercrosslinked polymers (HCPs) with dual nitrogen-rich building blocks, 2,4,6-triphenyl-1,3,5-triazine and pyrrole. The resulting polymer presented high nitrogen content up to 8.04%, surface area of 222.8 m2 g−1, and high thermal stability. 257 wt% iodine uptake capacity was obtained as a result of several elements such as high nitrogen contents, high porosity, and π-electron conjugated structure of the polymer. Spectral studies demonstrated that iodine uptake could occur at triazine and pyrrole moieties combined with aromatic rings in the polymer chain and the iodine nature was polyiodide anions. In addition, the material could be efficiently recycled without significant loss of iodine uptake capacity. Our work presented promising results for HCPs as economical adsorbents for the removal of iodine and other contaminates for environmental remediation applications.
Keywords: triazine; pyrrole; hypercrosslinked polymers; nitrogen-enriched; iodine capture
*
Corresponding author. E-mail address: [email protected] (Q. Jia) 1
1. Introduction
Porous organic polymers (POPs), a fascinating group of porous materials, are composed of covalently bonded light atoms, which have attractive merits such as excellent chemical and physical stability, large surface area, abundant porosity, and easy tailorability [1-3]. Owing to these advantages, POPs such as covalent organic frameworks / polymers (COFs / COPs) [4-8], conjugated microporous polymers (CMPs) [9-18], covalent triazine frameworks (CTFs) [19], and porous aromatic frameworks (PAFs) [20, 21] have found their applications in the field of gas separation, e.g., iodine capture from nuclear power plant exhaust fumes. A detailed investigation reveals that iodine uptake capacity will be enhanced when nitrogen scaffolds are introduced into POPs due to the interactions between iodine and the lone pair electrons on nitrogen [22, 23]. Recently, a nitrogen-enriched molecule, triazine has been used as a building block to construct a wide variety of POPs in the field of iodine capture and the notable examples include CTFs [19] and triazine-based CMPs [12, 15]. However, high reaction temperatures, targeted functionalized monomers, or expensive metal catalysts are commonly required for the synthesis of the above-mentioned triazine-based POPs [24-26], which greatly hinders their scale-up implementation. As a subset of POPs, hypercrosslinked polymers (HCPs) possess not only the merits of POPs, but also remarkable advantages of low cost, facile preparation, and diverse synthetic methods [27-30]. So far, there have been several papers related to the synthesis of triazine-based HCPs and most of the resultant polymers have been employed in the field of gas adsorption (e.g. CO2) due to its high nitrogen contents and microporous nature. However, the application of such appealing materials to the field of iodine capture is still limited.
2
To construct HCPs with dual blocks containing adsorbate affinity units may have positive effects to the adsorption capacity. For example, the introduction of nitrogen and sulfur atoms of triazine and thiophene is responsible for CO2 adsorption as a result of the dipole-dipole interactions between the polymer and CO2 molecules [31]. In Liu et al.’s work [26], two kinds of nitrogen-containing building blocks, cyanuric chloride and pyrrole, were used to synthesize HCPs for CO2 capture. Surprisingly, triazine-based dual affinity blockscontaining HCPs have been seldom reported except the above studies focusing on CO2 adsorption. It will be a hot topic to develop HCPs with two functional groups based on triazine, the attractive nitrogen-enriched block. With these considerations in mind, herein, we targeted to design a HCP containing two adsorbate affinity building blocks, triazine and pyrrole, and used for the effective adsorption of iodine. Pyrrole, a nitrogen-containing heterocyclic molecule, has demonstrated its positive impact on iodine uptake [32]. In our previous work, we prepared a pyrrole-based HCP and an iodine uptake capacity up to 460 wt% was obtained. To further enhance the iodine uptake capacity, we synthesized a phenyl-bearing triazine-based monomer (2,4,6-triphenyl-1,3,5triazine) considering the interactions between benzene groups and iodine [23, 33]. The developed HCP rich in nitrogen affinity sites (N-HCP) exhibited high uptake efficiency for iodine due to the synergistic interactions between iodine and the lone pair electrons on nitrogen as well as extended π-conjugations.
2. Experimental section
2.1. Synthesis of triazine-based monomer
3
2,4,6-triphenyl-1,3,5-triazine (TP3) was synthesized according to a previous report with minor revisions [34]. In a typical synthesis process, 1 mL benzonitrile was charged into a round bottom flask at 0 °C. Then, 3 mL trifluoromethanesulfonic acid (98%) was added into the solution in the nitrogen atmosphere and the resultant mixture was stirred for 30 min. The reaction was left to proceed at room temperature overnight. 50 mL water was added and the mixture was neutralized with 2 mol L–1 sodium hydroxide. The resulting residue was extracted with a mixture of chloroform and acetone (50/50 v:v, 3×50 mL). The solvent was evaporated using rotary evaporators to obtain a white solid.
2.2. Synthesis of triazine and pyrrole bifunctionalized HCPs
Under a nitrogen atmosphere, 0.5 mmol TP3 and 0.5 mmol pyrrole were dispersed in 30 mL chloroform for 30 min, and then anhydrous 8 mmol FeCl3 was added into the solution. The reaction mixture was then stirred for 24 h at 25 °C. Afterwards, the mixture was transferred to a 100 mL Teflon-lined autoclave and the autoclave were heated at 150 °C for 48 h. After cooled to room temperature, the crude product was filtrated and washed by Soxhlet extraction with MeOH for 24 h.
2.3. Iodine capture
Iodine vapor uptake experiments were based on gravimetric measurements [35]. 10 mg HCP powders placing in an open cap and an excess amount of iodine were located at another sealed glass vial. The whole system was heated at 75 °C and 1 bar, which was typical fuel reprocessing conditions [36]. After specific time intervals, the N-HCP powders were cooled
4
down to room temperature and then weighed. The iodine uptake capacity was calculated using the following equation:
(m2 m1 ) 100wt % m1
(1)
where α is the iodine uptake capacity, m2 and m1 represent the mass weight of HCPs samples after and before capture of iodine.
2.4. Iodine adsorption
A stock solution of 2000 mg L–1 was prepared in cyclohexane by dissolving 0.2 g nonradioactive crystalline iodine in 100 mL cyclohexane and stored in the dark. The stock solution of iodine was diluted to obtain a range of experimental iodine concentrations. The equilibrium adsorption capacity was calculated by the following equation: Qt
( C 0 C t )V m
(2)
where C0 and Ct are initial and final concentration of iodine in cyclohexane, respectively, V is the volume of the solution (mL), and m is the mass of the adsorbent (mg). For the dynamic study, 5 mg polymer was added into 5 mL 40 mg L–1 iodine solution. After different time intervals, the iodine concentration was examined using a UV-1601PC spectrophotometer. The adsorption isotherm study was carried out with initial iodine concentrations ranging from 40 to 400 mg L−1. The concentration of iodine was measured spectrophotometrically.
3. Results and discussion
3.1. Characterization
5
N-HCP was synthesized in two steps and the synthetic scheme was presented in Fig. 1. Firstly, the monomer TP3 was synthesized by benzonitrile at room temperature in the presence of trifluoromethanesulfonic acid. Secondly, the polymerization of the monomer and pyrrole was carried out in the presence of cheap Lewis acid anhydrous FeCl3 under solvothermal conditions (in chloroform) at 150 °C. The comparisons of FTIR spectra of benzonitrile and TP3 were shown in Fig. S1. Two characteristic absorption bands associated with the triazine ring were observed at 1524 cm–1 (C–N stretching mode) and 1368 cm–1 (in-plane ring stretching vibrations), while the stretching modes of terminal C≡N group in benzonitrile at 2233 cm−1 disappeared [37, 38], suggesting the successful preparation of the triazine-based monomer TP3. Furthermore, 1H NMR and
13C
NMR spectra (Figs. S2 and S3) also
demonstrated the successful preparation of TP3. After polymerization (Fig. 2a), characteristic peaks attributed to the triazine ring at 1520 and 1357 cm−1 and the band in the range of 1649−1558 cm−1 belonging to C=C stretching vibration in the (hetero)-aromatic ring existed [26]. A new band at 1435 cm–1 corresponded to the stretching vibration of C-N-C [39], indicating the successful copolymerization of TP3 and pyrrole. The chemical environments of different carbon atoms present in N-HCP were investigated by solid-state 13C CP/MAS NMR spectroscopy (Fig. S4). The major signals in the range of 110−150 ppm were assigned to the carbon atoms of the (hetero) aromatic ring [39, 40]. The peak at 169.8 ppm was corresponding to the carbon atoms in the triazine ring [12]. Besides, the peak observed at 53.3 ppm was attributed to the alkyl carbon, suggesting that CHCl3 took part in the polymerization process [31]. To clarify the element species and further confirm the form of N element, the polymer was characterized by XPS determination (Fig. 2b). As expected, C 1s and N 1s peaks were observed in the spectrum. The high resolution N 1s spectrum of the polymer showed two peaks at 400.2 and 399.2 eV (Fig. 2b inset), which could be attributed to N atoms within pyrrolic N and the triazine units [41], respectively. Their chemical compositions were also 6
determined by CHN analysis and the contents of primary elements were confirmed as N (8.04%), C (56.07%), and H (2.96%) respectively. All the above results indicated the successful copolymerization of TP3 and pyrrole.
Fig. 1. Synthesis of N-HCP material.
Fig. 2. (a) FTIR and (b) XPS spectra of N-HCP (inset: N 1s spectra).
According to the nitrogen adsorption-desorption isotherms, Brunauer-Emmett-Teller (BET) surfaces area was calculated to be 222.8 m2 g−1. As shown in Fig. 3a, the isotherm exhibited a significant hysteresis at low pressure, indicating the presence of micropores which could be accessed with some restrictions [17]. Based on the NLDFT method, the polymer 7
exhibited supermicropores with a pore dimension centered at about 1.6 nm. The thermal stability (Fig. 3b) was estimated by the thermogravimetric analysis and the result showed that the polymer yielded more than 50% chars when heated to 800 °C. The excellent thermal stability was beneficial for the practical application of harsh conditions. Furthermore, the morphology of the polymer was evaluated by SEM and TEM (Fig. S5). SEM image showed that the polymer was composed of irregular spherical solids, which aggregated plate-shaped particles. TEM image revealed that the polymer was porous, which was in agreement with the above-mentioned BET results.
Fig. 3. (a) Nitrogen adsorption-desorption isotherms (inset: pore size distribution calculated from the NLDFT method) and (b) TG curve of N-HCP.
3.2. Iodine capture
The performance of the polymer for the capture of iodine vapor was evaluated by directly gravimetric measurements. The mass of the sample was measured at different time intervals until no obvious mass change could be observed (typically for 650 min). The maximum iodine uptake capacity was 257 wt% (Fig. 4a) and the capacity was 9-fold higher 8
than that of the typical porous silver-doped zeolite mordenite [42]. The high iodine uptake capacity might be due to three main factors, e.g., high porosity, high nitrogen contents, and πelectron conjugated structure of the polymer. As shown in Fig. 4b, compared with other triazine-based POPs, N-HCP exhibited good iodine adsorption per unit SBET. The different values of iodine adsorption per unit SBET provided a fundamental basis for the selective uptake of iodine from exhaust fumes. It was worthy to note that the iodine uptake capacity reached about 138 wt% (half of the maximum capacity) within 30 min, which was faster than other porous adsorbents. The fast capture rate might be attributed to high content of nitrogen atoms because an effective binding site is crucial at low coverage, leading to the high interactions with iodine molecules [21, 43]. From a correlation coefficient point of view, the experimental data of N-HCP were definitely better fitted by the pseudo-second-order kinetic model.
Fig. 4. (a) Gravimetric iodine uptake (inset: the pseudo-second order kinetic model) of NHCP as a function of time at 75 °C and (b) Iodine uptakes (1 bar) and per unit SBET for iodine capture.
3.3 Iodine release and adsorbent recycle
9
Reversible capture of iodine by adsorbents is of great importance for their practical applications, therefore, the thermal stability of iodine-loaded N-HCP sample was also evaluated by TGA under nitrogen atmosphere. As shown in Fig. S6, there was a sharp weight loss in the range of 80 to 330 °C, which could be attributed to the loss of loaded iodine [8]. From a practical perspective, the trapped iodine could be released and the adsorbent could be recycled. Furthermore, iodine release in organic solvents such as ethanol under ambient atmosphere was monitored by a UV-1601PC spectrophotometer. UV-Vis spectra showed two absorbance maxima at 360 and 291 nm (Fig. 5a), which could be assigned to the presence of polyiodide anions [15]. The color of the solution changed from light to dark yellow (Fig. 5b inset). The amount of iodine released from N-HCP powders increased linearly during the first 35 min and fitted well with the pseudo-zero-order kinetic model, demonstrating that the process was governed by host-guest interactions [16, 44].
Fig. 5. (a) Temporal evolution of UV-Vis adsorption spectra of the iodine released from loaded N-HCP in 3.5 mL ethanol and (b) Controlled release rate of iodine in N-HCP in the first 35 min determined by UV-Vis spectrophotometer.
10
To exploit the recyclability of the adsorbent, recycling tests were carried out by heating iodine-loaded N-HCP powders at 125 °C for 300 min. After that, the samples were reused for iodine uptake. In four consecutive runs, N-HCP could be efficiently recycled and reused without significant loss of iodine uptake (Fig. S7).
3.4. Capture mechanism
In the FTIR spectra (Fig. 6a), the peak centered at 1615 cm–1 assigned to C=C stretching vibration in the (hetero) aromatic ring shifted to 1602 cm–1. Also, the peaks at 1520 and 1357 cm–1 originated from the triazine ring shifted to 1512 and 1353 cm–1, respectively. Besides, the band at 1435 cm–1 associated with the stretching vibration of C−N−C in pyrrole ring shifted to 1432 cm–1. The results indicated that iodine uptake could occur at triazine and pyrrole moieties combined with aromatic ring. This is most likely because that a charge transfer complex is formed between N-HCP and polyiodide anions during the process of capture [33]. The resulting charge transfer interactions are enough to result from the bandshift effect. Compared with the spectrum of pristine polymer, the presence of I 3d peaks in the iodine-loaded polymer indicated the successful capture of iodine. To further demonstrate the form of captured iodine, the high resolution of I 3d spectra were shown in Fig. 6. The peaks appeared at 618.4 (3d 5/2), 619.4 (3d 5/2), 630.1 (3d 3/2), and 631.3 eV (3d 3/2), attributing to the formation of polyiodine anions (I3− and I5−) [7, 45].
11
Fig. 6. (a) FT-IR and (b) XPS spectra of N-HCP (black line) and iodine-loaded N-HCP (red line) (inset: the high resolution of I 3d spectra after iodine uptake on N-HCP).
3.5. Iodine adsorption
In addition, the iodine uptake capacity in cyclohexane solution was evaluated since it is of great importance for nuclear waste management. To get further insights into the adsorption process, adsorption kinetics and isotherms of iodine on N-HCP were investigated (Fig. 7). Temporal evolution of UV-Vis adsorption spectra in the iodine-cyclohexane solution were shown in Fig. S8. It could be observed that the adsorption capacity increased with the increase of contact time and achieved equilibrium within 7 h (Fig. 7a). The experimental data were fitted by the pseudo-first-order and pseudo-second-order kinetics models and they were expressed as follows:
ln (Qe - Qt ) = ln Qe - k1t
(3)
t 1 t = + 2 Qt k2Qe Qe
(4)
where Qe and Qt (mg g–1) are the uptake capacities of iodine on N-HCP at equilibrium and at time t (min) respectively. k1 (min–1) and k2 (g mg–1 min–1) are the pseudo-first-order and 12
pseudo-second-order rate constants, respectively. The results showed that the data of iodine uptake on N-HCP fitted well with the pseudo-second-order (R2= 0.9961) kinetic model compared with the pseudo-first-order (R2= 0.9661) kinetic model. The detailed parameters were listed in Table S1. In this study, we tested the applicability by soaking the polymers in a range of concentrations of iodine-cyclohexane solutions. As time prolonged, the pink color of the iodine-cyclohexane solution became lighter (Fig. S9). To further study the iodine adsorption isotherms, we fitted the experimental data using the non-linear Langmuir and Freundlich models as follows:
Qe
QmbCe 1 bCe
(5)
Qe KCe1 n
(6)
where Qm is the maximum adsorption capacity, b is the Langmuir constant related to binding energy, and K and n are empirical Freundlich constants associated with adsorption capacity and intensity, respectively. As is known, Langmuir model assumes monolayer adsorption with identical and energetically equivalent adsorption sites. On the contrary, Freundlich model assumes multilayer adsorption with heterogeneous surface. The parameters of isotherm models were listed in Table S2. The results showed that the correlation coefficients (R2) of Freundlich isotherm model (0.9889) was higher than that of Langmuir isotherm model (0.9606). Besides, it was worthy to note that the adsorption isotherm could be well fitted by Freundlich model since the polymer had three potential iodine trapping sites: phenyl rings, triazine moieties, and pyrrole [22].
13
Fig. 7. (a) Iodine adsorption curve of N-HCP (5 mg) at various time intervals in iodinecyclohexane solution (40 mg L–1, 5 mL, 25 °C) and (b) Adsorption isotherms with Langmuir (red line) and Freundlich (blue line) models fitting.
4. Conclusions
In summary, a triazine and pyrrole bifunctionalized task-specific hypercrosslinked polymer was successfully synthesized and applied to the capture of iodine. The high nitrogen content and surface area contributed to the satisfactory iodine uptake capacity up to 257 wt%. The process of iodine capture fitted the pseudo-second-order model and the captured iodine could be released by either immersing in ethanol solution or heating. Furthermore, the iodine uptake capacity in solution phase was evaluated, demonstrating that the adsorption was well depicted by the pseudo-second-order model and Freundlich model. Our work made great progress in HCPs synthetic methods and revealed the great potential of HCPs as an attractive adsorbent for environmental remediation.
14
Acknowledgement This project was financially supported by State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, College of Chemistry, Jilin University, China (2019-4).
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Highlights ● A nitrogen-enriched monomer with extended π-conjugations was designed. ● A cost-effective nitrogen-rich HCP with high surface area was successfully prepared. ● The resultant polymer exhibited high iodine uptake capacity up to 257 wt%. ● Nitrogen content, porosity, and π-electron structure played key roles in iodine uptake.
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