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A 1D porphyrin-based rigid conjugated polymer as efficient and recyclable visible-light driven photocatalyst
Reactive and Functional Polymers 143 (2019) 104340
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A 1D porphyrin-based rigid conjugated polymer as efficient and recyclable visible-light driven photocatalyst Yang Yua, Yanwei Lia, , Yanhui Lia, Hengguo Wanga, Qinghui Zuoa, Qian Duana,b, ⁎
a b
T
⁎
School of Materials Science and Engineering, Changchun University of Science and Technology, Changchun 130022, China Engineering Research Center of Optoelectronic Functional Materials, Ministry of Education, Changchun 130022, China
ARTICLE INFO
ABSTRACT
Keywords: Porphyrin Benzobisoxazole Linear polymer Photocatalyst Rhodamine B
In this study, a novel 1D porphyrin-based polymer, polyphenylporphyrin benzobisoxazole, namely P(PPor-BBO), was synthesized via polycondensation between 5, 15-bis(4-carboxyphenyl)-10, 20-diphenylporphyrin and 4,6diaminoresor-cinol dihydrochloride. The P(PPor-BBO) is a 1D fully-conjugated heterocyclic aromatic polymer with co-linear and co-planar rigid backbone. Extended fully π-conjugated systems not only enhance chemical stability and lead to insolubility, but also improve photocatalytic active units and light-harvesting capability in the visible-light region. As a result, P(PPor-BBO) shows excellent photocatalytic performance for rhodamine B degradation (150 min, 98%) under visible-light irradiation. It also exhibits easy recyclability and sustainability, indicating it could be as environmentally-friendly catalyst for organic dye wastewater treatment.
1. Introduction In recent years, with the high-speed development of dye industry, a large number of organic dyes without nuisanceless treatment have been directly discharging into rivers, ponds and oceans. As a result of their toxicity and even carcinogenicity, it have bring about a stupendous threat to the water environment, aquatic life and human health [1–3]. Therefore, in order to prevent organic dyes from being discharged diametrically into the environment, it is exigent to take an effective measure to deal with organic dyes in wastewater. So far, among a variety of feasible environmental rectification techniques reported for toxic organic dyes, photocatalysis is a green and sustainable technology to degrade organic dyes with highly efficient and low consumption [4,5]. TiO2 is one of the most popular photocatalyst for organic dyes degradation. However, because of its wide band gap, the photosensitivity of TiO2 is mainly concentrated in the ultraviolet region, and only 4–6% of solar radiation is in the ultraviolet region reaching to the ground for TiO2 [6,7]. Yet visible light of solar radiation is far more abundant (46%) than ultraviolet [8]. Therefore, it is crucial to develop visible-light driven photocatalyst for solar energy conversion. Porphyrins are 18 π aromatic macrocyclic porphine derivatives, with the highly delocalized π electrons system. Porphyrin with the typical conjugated organic molecule is regarded as a class of attractive photocatalyst under visible light irradiation [9]. Meanwhile, porphyrins have excellent ability to generate reactive oxygen species including
⁎
1
O2, O2·− and HO· [10]. Yet porphyrins as small molecules are easily agglomerated and difficult to recycle in solution [11]. It is not conducive to the application of porphyrins as photocatalyst. In order to solve these problems, much effort has been devoted to doping inorganic nanoparticle [7,10], incorporate porphyrins into polymers as pendant groups [12], as a part of main backbone of polymer [13], and as key components of in networks including the emerging 2D covalent organic frameworks (COFs) [14] and 3D metal-organic frameworks (MOFs) [15], especially these porous 2D porphyrin-based conjugated porous polymers contribute to photodegradation of organic dyes, such as rhodamine B (RhB), congo red and methylene orange [16,17]. However, to the best of our knowledge, study of one-dimension (1D) porphyrin-based linear polymer as photocatalyst is seldom used in the application of dye degradation so far. In the present work, we designed and synthesized a 1D porphyrinbased polymer, namely P(PPor-BBO), constructed by condensation reaction between 5,15-bis(4-carboxyphenyl)-10,20-diphenylporphyrin (DiCPPor) and 4,6- diaminoresorcinol dihydrochloride (DAR·2HCl), as shown in Fig. 1. Benzobisoxazole as bridging unit induced 1D rigid fully-conjugated structure of P(PPor-BBO), which as heterogeneous photocatalyst to photodegrade RhB with recyclability and sustainability.
Corresponding authors at: School of Materials Science and Engineering, Changchun University of Science and Technology, Changchun 130022, China. E-mail addresses: [email protected] (Y. Li), [email protected] (Q. Duan).
https://doi.org/10.1016/j.reactfunctpolym.2019.104340 Received 20 June 2019; Received in revised form 22 July 2019; Accepted 13 August 2019 Available online 14 August 2019 1381-5148/ © 2019 Elsevier B.V. All rights reserved.
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Fig. 1. (a) Synthesis of P(PPor-BBO). (b) Schematic description of photocatalytic degradation process of RhB by using P(PPor-BBO) as heterogeneous photocatalyst, in the CPK of P(PPor-BBO), red: O; blue: N; gray: C. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
2. Materials and methods
o-C6H4COOCH3), 8.43 (4H, m- C6H4COOCH3), 8.79–8.87 (8H, β-Pyr) (Fig. S1). The reaction mixture of DiCPPor-OMe (132.1 mg, 0.17 mmol) in THF (10 mL) and aqueous KOH (1 mol·L−1, 20 mL) was heated to reflux for about 72 h under stirring. After the reaction mixture being cooled to room temperature, pH of the mixture solution was adjusted to 3 with HCl (aq). The reaction precipitate was filtered and washed with deionized water, and then dried under vacuum at 60 °C to give the final product DiCPPor, yield: 29%. FT-IR (KBr tablet, νmax/cm−1): 3431, 2927, 1690, 1606, 966, 802. 1H NMR (500 MHz, DMSO-d6): δ -2.94 (2H, s, N-H), 7.86 (6H, m, m, p-C6H5), 8,23 (4H, d, o-C6H5), 8.38 (8H, dd, o, m-C6H4COOH), 8.85 (8H, s, H-β), 13.33 (2H, s, –CO2H) ppm (Fig. S2). UV–vis (DMF, λmax/nm,): 417 (Soret band), 515, 549, 591, 647 (Q bands).
2.1. Materials Pyrrole, benzaldehyde and methyl 4-formylbenzoate were purchased from Aladdin. DAR·2HCl was provided by our laboratory [18]. Isopropylalcohol (IPA), 1,3-diphenylisobenzofuran (DPBF), hydrogen peroxide (H2O2) and RhB were obtained from Macklin. Propionic acid, polyphosphoric acid (PPA), tetrahydrofuran (THF), methanesulfonic acid (MSA) and N, N-dimethylformamide (DMF) were purchased from Sigma-Aldrich and used without further purification. 2.2. Synthesis of DiCPPor DiCPPor was synthesized according to a literature procedure [19]. As shown in Fig. 1, benzaldehyde (1.26 mL, 12.5 mmol) was mixed with methyl 4-formylbenzoate (2.05 g, 12.5 mmol) in propionic acid (150 mL) and heated at 140 °C, then freshly distilled pyrrole (1.76 mL, 25 mmol) in propionic acid (10 mL) was added dropwise to the mixture solution in 30 min. The reaction mixture was stirred constantly at 140 °C for 2 h. After the reaction mixture being cooled to room temperature overnight, the dark purple precipitate was collected by filtration and rinsed with deionized water and EtOH, and then dried in an oven at 60 °C. After the solid being dissolved in dichloromethane, the product was separated from mixture by silica gel column chromatography using the mixture of dichloromethane and petroleum ether (4:1) as eluent, to obtain the purple solid intermediate product 5,15-di(4carbomethoxy- phenyl)-10,20-diphenylporphyrin (DiCPPor-OMe), yield: 2.4%. 1H NMR (CDCl3, 400 MHz), δ, ppm: -2.79 (2H, NH), 4.11 (6H, COOCH3), 7.77 (6H, p,m-C6H5), 8.20 (4H, o-C6H5), 8.31 (4H,
2.3. Synthesis of P(PPor-BBO) P(PPor-BBO) was prepared via polycondensation for the first time. DAR·2HCl (24.1 mg, 0.11 mmol) was added to a stirred polyphosphoric acid (50 mL) at 70 °C under the nitrogen protection condition. A certain amount of DiCPPor (40 mg, 0.06 mmol) was added to the reaction mixture in the middle of the removal of HCl, in order to prevent DAR·2HCl from being oxidized. Until all the hydrochloric acid was removed out, the remaining DiCPPor (39.3 mg, 0.05 mmol) were added to the reaction mixture. Subsequent the reaction mixture was stirred and heated at 100 °C, 120 °C, 160 °C, 180 °C and kept warm for 6 h, respectively. The reaction solution was cooled to room temperature, and deionized water (200 mL) was added. The polymer was filtered, and then repeatedly washed with water to give the black solid of P(PPorBBO). Yield: 82%. FT-IR (KBr tablet, νmax /cm−1): 3637, 1606, 1553, 2
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1230, 1058, 801, 702. Solid-state CP/MAS 13C NMR (400 MHz): δ 162, 153, 139, 127, 111, 95 ppm. UV − vis (MSA, λmax/nm): 451, 673.
absorption peaks of DiCPPor and DAR·2HCl have almost disappeared. At the same time, the new absorption peak at 1606 cm−1 attributed to C=N stretching vibration of oxazole ring were observed. Additionally, the spectrum presented vibrational bands at 1230 and 1078 cm−1, which attributed to the C-N and = C-O-C the absorption of oxazole ring, respectively [20]. The above results prove that P(PPor-BBO) has been obtained successfully. Solid state 13C NMR has been used to further prove the structure formation of P(PPor-BBO) (Fig. 3). The carbon resonances of C9-C14 at 127 ppm were corresponding to the phenyl rings of porphyrin. The identified resonances at 95 of C3, 111 of C7, 139 of C6,8, 153 of C2,4 and 162 of C1,5 ppm confirmed the formation of oxazole rings [21,22]. The 13 C NMR result indicates the covalent bonding between DiCPPor and DAR via condensation reaction has been achieved. P(PPor-BBO) are black green solids at room temperature (Fig. 4a). SEM investigation shows that the particles of P(PPor-BBO) are about 90 nm (Fig. 4b, c, d). It is speculated that the formation of the morphology may be due to the fully conjugated rigid structure, making the molecules self-assembly through π-π stacking interactions. The intrinsic viscosity [η] of P(PPor-BBO) was measured in methanesulfonic acid solution at 30 °C by Ubbelohde viscometer. The viscosity average molecular weights (Mη) were estimated from the one-spot method by the Mark–Houwink equation: [η] = 2.77 × Mη1.8. The measured [η] and calculated (Mη) for P(PPor-BBO) are 5.28 dL g−1 and 11,080 g mol−1, respectively. The crystal structures of the DiCPPor, DAR·2HCl and P (PPor-BBO) were studied by PXRD (Fig. 5). The monomer DAR·2HCl has some independent and narrow diffraction peaks, indicating that it is a crystal structure. For the DiCPPor, there is only a very broad peak at about 2θ = 22° without any other strong diffraction peaks. P(PPor-BBO) also has more broad “steamed bread peak” at around 2θ =23° without any other strong diffraction peaks, indicating that it is amorphous polymer in nature [23]. The thermal stability of DiCPPor, DAR·2HCl and P(PPor-BBO) were reflect from the TGA curve in nitrogen atmosphere (Fig. 6). In curve of DiCPPor, three main weight losses were observed. An apparent weight loss was observed from 90 °C to 204 °C for the first stage. This may be due to physisorbed or chemisorbed water. Then a little weight loss observed at the second stage from 204 to 346 °C suggests that the thermal decomposition of carboxyl groups. The third stage of the weight loss above 346 °C corresponds to the decomposition of which may be attributed to the decomposition process of phenylporphyrin. TGA curve of DAR·2HCl showed that the weight loss in the temperature from 240°C, which root in the removal of –OH and –NH2 [18]. In general, the thermal stability of the polymer is strongly relevant to chain rigidity and chain packing. In the curve of P(PPor-BBO), the same as porphyrin, the ca. 10% weight loss from 100 °C to 423 °C may be the evaporation of the combined water. The polymer molecular chain contained many N and O atoms to form hydrogen bonds with waters. The maximum thermal decomposition temperature of P(PPor-BBO) occured at 423 °C. At the temperature, the macromolecular chain began to clot upon heating or certain groups were decomposed from the macromolecule. Additionally, even at temperatures as high as 800 °C, the weight loss of this polymer was only 26.61%. Evidently, an increase in the thermal stability of P(PPor-BBO) compared to monomer DiCPPor was attributed to the rigid structure due to the inclusion of the benzobisoxazole ring in P(PPor-BBO) [24]. The analysis of the P(PPor-BBO) thermogravimetric curve implies that P(PPor-BBO) have an eminent thermal stability. Moreover, the chemical stability of P(PPor-BBO) was examined and compared with the monomers DiCPPor and DAR·2HCl in water and various organic solvents (Table S1). The results show that P (PPor-BBO) is insoluble in water and organic solvents, only soluble in strong acid. It should be attributed to extended rigid π-conjugated backbone of porphyrin units. Additionally, after P(PPor-BBO) was dissolved in methanesulfonic acid for 24 h, the structure of the polymer almost has no change (Fig. S3). Therefore, excellent chemical stability makes it suitable as a heterogeneous photocatalyst for easy recycling.
2.4. Characterizations Fourier transform infrared spectra (FT-IR) were conducted by using a PerkinElmer Frontier spectrophotometer in the range from 4000 to 500 cm−1 using a KBr pellet. The Solid-state 13C CP/MAS NMR spectra were recorded with a Bruker Avance 400 MHz spectrometer. The morphology of polymer was observed by in a JEOL JSM-6701F fieldemission scanning electron microscopy (SEM) at 5 kV. The X-ray diffraction (XRD) measurement was performed using Rigaku Ultima IV diffractmeter over the 2θ range of 10°–80°. The thermal stability of the materials was carried out under N2 atmosphere from room temperature to 800 °C using NETZSCH STA409PC thermogravimetric analysis (TGA) meter. Ultraviolet–visible (UV–vis) spectra were recorded on a Shimadzu (UV-2600) spectrophotometer in the wavelength range of 300–800 nm. 2.5. Measurement of photocatalytic activity The photocatalytic activity experiments of the samples were surveyed by the degradation of RhB solution (1 × 10−5 mol/L, 40 mL, solvent: water) under visible-light irradiation. The light source was provided by Xenon lamp (PLS-SXE300, Perfectlight Co.) with a 400 nm cutoff filter, an output optical power density of 318.47 mw·cm−2 and an illuminated area of 40.69 cm2. The reacting suspension consisting of RhB solution and 10 mg photocatalyst, then it was stirred in dark with a magnetic stir bar for 30 min to achieve adsorption-desorption equilibrium. A 5 mL of solutions was taken out and filtered from the suspension each 30 min to test its photocatalytic activity. Finally, the photocatalytic activity of degradation RhB was detected by quantitative measuring its absorption at 554 nm through a UV–vis spectrophotometer. 3. Results and discussion 3.1. Characterization of photocatalyst The chemical structure of P(PPor-BBO) was verified by FT-IR spectrum. As shown in Fig. 2a, the absorption peak at 3431 and 1690 cm−1 were assigned to O–H and C=O stretching vibration of –COOH on DiCPPor, respectively. The absorption peak at 802 cm−1 was attributed to the bending vibration of the N–H bond on the pyrrole skeleton of DiCPPor [19]. It is suggested that DiCPPor was prepared successfully. As shown in Fig. 2b, the broad peak at 3210–2800 cm−1 were assigned the superposition of ammonia hydrochloride and hydroxyl of DAR·2HCl. The characteristic peaks indicate that the DAR·2HCl was not oxidized. Fig. 2c shows that the characteristic
Fig. 2. FTIR spectra of (a) DiCPPor, (b) DAR·2HCl and (c) P(PPor-BBO). 3
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Fig. 3. Solid state
13
C NMR spectrum of P(PPor-BBO).
Fig. 4. (a) Photographic, (b-d) SEM images of P(PPor-BBO).
3.2. Photocatalytic performance
was evaluated by the change of absorption peak at 554 nm of the original RhB solution (1 × 10−5 mol/L) with exposure time under visiblelight irradiation. In order to better study the photocatalytic activity of P (PPor-BBO), blank experiment of RhB degradation without any photocatalyst was observed in Fig. S5. The result shows that the self-degradation of RhB without any photocatalyst under visible-light irradiation was negligible. The time-dependent absorption peaks of RhB degradation under visible-light irradiation in the presence of P(PPorBBO) (Fig. S6) compared with DiCPPor (Fig. S7) was studied, respectively. The results clearly show that photodegradation rate of RhB under P(PPor-BBO) as photocatalyst was obviously higher (63%, 150 min) than that of DiCPPor (40%, 150 min). The excellent photocatalytic performance may be attributed to 1D large π-conjugated system and completely rigid coplanar structure of P(PPor-BBO), which
P(PPor-BBO) as visible-light driven photocatalyst, the light absorption capability was important. It was dissolved in methanesulfonic acid and evaluated by UV–vis spectroscopic measurement (Fig. S4). There were two obvious absorption peaks at 451 and 672 nm in visible region. It shows remarkably red-shifted electronic absorption spectra compared with DiCPPor due to extended π-conjugate degree of porphyrin unit (Fig. S4a). Generally, the UV–vis characteristic absorption spectrum of porphyrin compound has a B band and four Q bands. But solvent effect of strong acid have resulted in the P(PPor-BBO) and DiCPPor only showed one Q bands (Fig. S4b). The photocatalytic activity of P(PPor-BBO) towards degradation of RhB was investigated under visible-light irradiation. Wherein, the degradation activity of RhB 4
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(Fig. S9). The result shows the very weak change in absorption peaks of RhB before and after 150 min. It confirms that the fading of dye mainly comes from photocatalytic degradation, not adsorption. 3.3. Photocatalytic mechanism In order to study the primary reactive species generated from P (PPor-BBO) in the photocatalytic degradation process, DPBF as single oxygen for 1O2 was performed in the scavenger experiments [25]. The photocatalytic properties of P(PPor-BBO) to generate 1O2 was investigated in DMF and monitored by time-dependent absorption plot of the DPBF at 415 nm. The results show that P(PPor-BBO) can rapidly generate 1O2 and consume up to approximative 90% of DPBF in 70 s under visible light conditions (Fig. 8). This phenomenon proves that P (PPor-BBO) as photocatalyst can efficiently generate 1O2 to degrade the RhB. A proposed mechanism of visible-light driven photodegradation of RhB by P(PPor-BBO) photocatalyst is proposed as follows:
Fig. 5. XRD pattern of (a) DiCPPor, (b) DAR·2HCl and (c) P(PPor-BBO).
P(PPor BBO) + hv 1P(PPor
BBO) + ISC
3P(PPor
BBO) + 3O2
H2 O2 + hv 1O , 2
1P(PPor
BBO)
3P(PPor
BBO)
P(PPor BBO) + 1O2
2OH
OH·+ RhB
Oxidation products
(1) (2) (3) (4) (5)
As shown in the above equations, when the P(PPor-BBO) is irradiated by visible- light, the excited singlet state 1P(PPor-BBO)⁎ is formed (eq.1). Then, 1P(PPor-BBO)⁎ is converted to the triplet state 3P (PPor-BBO)⁎ through intersystem crossing (ISC, eq. 2). The 3P(PPorBBO)⁎ interacts with the groud state triplet oxygen (3O2) to generate the active 1O2 (eq. 3). Moreover, H2O2 could also produce OH· free radicals under visible light irradiation (eq. 4). The formation of active species including 1O2 and OH· resulted in significantly enhanced photocatalytic activity in the P(PPor-BBO) catalytic system, they can react with RhB, and RhB is degraded to nontoxic oxidation products (eq. 5).
Fig. 6. TGA of (a) DiCPPor, (b) DAR·2HCl and (c) P(PPor-BBO).
improved catalytic active site and light-harvesting capability of the porphyrin units in the visible-light region. H2O2 as oxidant could help the photocatalyst to drastically degrade RhB in photocatalytic reaction (Fig. S8). Fig. S8 shows the time-dependent UV–vis spectra of RhB solution in the presence of H2O2 under visible-light irradiation (58%, 150 min). The RhB degradation in the presence of P(PPor-BBO) including H2O2 under visible-light irradiation was illustrated in Fig. 7. The photocatalytic degradation efficiency of RhB using the P(PPorBBO)/H2O2 could reach 98% during 150 min under visible-light irradiation. It is inferred that the enhanced activities originate from the synergistic effect between P(PPor-BBO) and H2O2. In addition, In order to observe that whether the fading phenomenon of RhB comes from the adsorption by catalyst, RhB solution in the presence of photocatalyst P (PPor-BBO) was magnetic stirred in dark for 150 min and then detected
3.4. Reusability and stability Moreover, the reusability of the photocatalyst also plays an important role in organic dye wastewater treatment, implying industrial low cost. P (PPor-BBO) is insoluble in water, the used photocatalyst could be separated from RhB solution by simple filtration or centrifugation, and then dried under vacuum at 60 °C. As shown in Fig. 9, the photocatalytic activity of P(PPor-BBO) for degradation of RhB under visible-light irradiation only reduced a little after four cycles, which could be due to residual organic contaminants RhB on the surface of photocatalyst during
Fig. 7. UV–vis absorbance spectra of RhB with (a) P(PPor-BBO)/H2O2 under visible-light irradiation, (b) Comparison of the decay rate of RhB (black) alone, and in the presence of DiCPPor (red), H2O2 (blue), P(PPor-BBO) (green) and P(PPor-BBO)/H2O2 (pink) under visible light irradiation. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.) 5
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Fig. 8. Photocatalytic efficiency of RhB on (a) Time-dependent absorption spectra of DPBF (in DMF) in the presence of P(PPor-BBO) (b) The decay rate of DPBF in the presence of P(PPor-BBO).
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Fig. 9. Photocatalytic degradation of RhB over P(PPor-BBO)/H2O2 under visible-light irradiation during four cycles.
the test period. According to the results, P(PPor-BBO) have excellent cycle stability under visible-light irradiation. 4. Conclusions In this work, an efficient 1D photocatalyst, P(PPor-BBO) was successfully fabricated through polycondensation between DiCPPor and DAR·2HCl. 1D rigid conjugated structural features endows P(PPor-BBO) with excellent chemical stability and photocatalytic performance in heterogeneous catalysis. As the heterogeneous photocatalyst, P(PPorBBO) exhibited superior photocatalytic capacity that 98% RhB was degraded after 150 min under visible-light irradiation. Meanwhile, P (PPor-BBO) possessed good thermal stability and reusability. This work may provide a new insight into the design of other novel 1D porphyrin materials as photocatalysts for removal of organic pollutants under sunlight irradiation. Acknowledgements This work was supported by the Science and Technology Innovation Fund of Changchun University of Science and Technology (grant numbers XJJLG-2016-05, XJJLG-2017-07); Research Foundation of Education Department of Jilin Province (grant numbers JJKH20190582KJ); and Research Foundation of Science & Technology Department of Jilin Province (20190303069SF). Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.reactfunctpolym.2019.104340. 6
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[17]
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Reactive and Functional Polymers journal homepage: www.elsevier.com/locate/react
A 1D porphyrin-based rigid conjugated polymer as efficient and recyclable visible-light driven photocatalyst Yang Yua, Yanwei Lia, , Yanhui Lia, Hengguo Wanga, Qinghui Zuoa, Qian Duana,b, ⁎
a b
T
⁎
School of Materials Science and Engineering, Changchun University of Science and Technology, Changchun 130022, China Engineering Research Center of Optoelectronic Functional Materials, Ministry of Education, Changchun 130022, China
ARTICLE INFO
ABSTRACT
Keywords: Porphyrin Benzobisoxazole Linear polymer Photocatalyst Rhodamine B
In this study, a novel 1D porphyrin-based polymer, polyphenylporphyrin benzobisoxazole, namely P(PPor-BBO), was synthesized via polycondensation between 5, 15-bis(4-carboxyphenyl)-10, 20-diphenylporphyrin and 4,6diaminoresor-cinol dihydrochloride. The P(PPor-BBO) is a 1D fully-conjugated heterocyclic aromatic polymer with co-linear and co-planar rigid backbone. Extended fully π-conjugated systems not only enhance chemical stability and lead to insolubility, but also improve photocatalytic active units and light-harvesting capability in the visible-light region. As a result, P(PPor-BBO) shows excellent photocatalytic performance for rhodamine B degradation (150 min, 98%) under visible-light irradiation. It also exhibits easy recyclability and sustainability, indicating it could be as environmentally-friendly catalyst for organic dye wastewater treatment.
1. Introduction In recent years, with the high-speed development of dye industry, a large number of organic dyes without nuisanceless treatment have been directly discharging into rivers, ponds and oceans. As a result of their toxicity and even carcinogenicity, it have bring about a stupendous threat to the water environment, aquatic life and human health [1–3]. Therefore, in order to prevent organic dyes from being discharged diametrically into the environment, it is exigent to take an effective measure to deal with organic dyes in wastewater. So far, among a variety of feasible environmental rectification techniques reported for toxic organic dyes, photocatalysis is a green and sustainable technology to degrade organic dyes with highly efficient and low consumption [4,5]. TiO2 is one of the most popular photocatalyst for organic dyes degradation. However, because of its wide band gap, the photosensitivity of TiO2 is mainly concentrated in the ultraviolet region, and only 4–6% of solar radiation is in the ultraviolet region reaching to the ground for TiO2 [6,7]. Yet visible light of solar radiation is far more abundant (46%) than ultraviolet [8]. Therefore, it is crucial to develop visible-light driven photocatalyst for solar energy conversion. Porphyrins are 18 π aromatic macrocyclic porphine derivatives, with the highly delocalized π electrons system. Porphyrin with the typical conjugated organic molecule is regarded as a class of attractive photocatalyst under visible light irradiation [9]. Meanwhile, porphyrins have excellent ability to generate reactive oxygen species including
⁎
1
O2, O2·− and HO· [10]. Yet porphyrins as small molecules are easily agglomerated and difficult to recycle in solution [11]. It is not conducive to the application of porphyrins as photocatalyst. In order to solve these problems, much effort has been devoted to doping inorganic nanoparticle [7,10], incorporate porphyrins into polymers as pendant groups [12], as a part of main backbone of polymer [13], and as key components of in networks including the emerging 2D covalent organic frameworks (COFs) [14] and 3D metal-organic frameworks (MOFs) [15], especially these porous 2D porphyrin-based conjugated porous polymers contribute to photodegradation of organic dyes, such as rhodamine B (RhB), congo red and methylene orange [16,17]. However, to the best of our knowledge, study of one-dimension (1D) porphyrin-based linear polymer as photocatalyst is seldom used in the application of dye degradation so far. In the present work, we designed and synthesized a 1D porphyrinbased polymer, namely P(PPor-BBO), constructed by condensation reaction between 5,15-bis(4-carboxyphenyl)-10,20-diphenylporphyrin (DiCPPor) and 4,6- diaminoresorcinol dihydrochloride (DAR·2HCl), as shown in Fig. 1. Benzobisoxazole as bridging unit induced 1D rigid fully-conjugated structure of P(PPor-BBO), which as heterogeneous photocatalyst to photodegrade RhB with recyclability and sustainability.
Corresponding authors at: School of Materials Science and Engineering, Changchun University of Science and Technology, Changchun 130022, China. E-mail addresses: [email protected] (Y. Li), [email protected] (Q. Duan).
https://doi.org/10.1016/j.reactfunctpolym.2019.104340 Received 20 June 2019; Received in revised form 22 July 2019; Accepted 13 August 2019 Available online 14 August 2019 1381-5148/ © 2019 Elsevier B.V. All rights reserved.
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Fig. 1. (a) Synthesis of P(PPor-BBO). (b) Schematic description of photocatalytic degradation process of RhB by using P(PPor-BBO) as heterogeneous photocatalyst, in the CPK of P(PPor-BBO), red: O; blue: N; gray: C. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
2. Materials and methods
o-C6H4COOCH3), 8.43 (4H, m- C6H4COOCH3), 8.79–8.87 (8H, β-Pyr) (Fig. S1). The reaction mixture of DiCPPor-OMe (132.1 mg, 0.17 mmol) in THF (10 mL) and aqueous KOH (1 mol·L−1, 20 mL) was heated to reflux for about 72 h under stirring. After the reaction mixture being cooled to room temperature, pH of the mixture solution was adjusted to 3 with HCl (aq). The reaction precipitate was filtered and washed with deionized water, and then dried under vacuum at 60 °C to give the final product DiCPPor, yield: 29%. FT-IR (KBr tablet, νmax/cm−1): 3431, 2927, 1690, 1606, 966, 802. 1H NMR (500 MHz, DMSO-d6): δ -2.94 (2H, s, N-H), 7.86 (6H, m, m, p-C6H5), 8,23 (4H, d, o-C6H5), 8.38 (8H, dd, o, m-C6H4COOH), 8.85 (8H, s, H-β), 13.33 (2H, s, –CO2H) ppm (Fig. S2). UV–vis (DMF, λmax/nm,): 417 (Soret band), 515, 549, 591, 647 (Q bands).
2.1. Materials Pyrrole, benzaldehyde and methyl 4-formylbenzoate were purchased from Aladdin. DAR·2HCl was provided by our laboratory [18]. Isopropylalcohol (IPA), 1,3-diphenylisobenzofuran (DPBF), hydrogen peroxide (H2O2) and RhB were obtained from Macklin. Propionic acid, polyphosphoric acid (PPA), tetrahydrofuran (THF), methanesulfonic acid (MSA) and N, N-dimethylformamide (DMF) were purchased from Sigma-Aldrich and used without further purification. 2.2. Synthesis of DiCPPor DiCPPor was synthesized according to a literature procedure [19]. As shown in Fig. 1, benzaldehyde (1.26 mL, 12.5 mmol) was mixed with methyl 4-formylbenzoate (2.05 g, 12.5 mmol) in propionic acid (150 mL) and heated at 140 °C, then freshly distilled pyrrole (1.76 mL, 25 mmol) in propionic acid (10 mL) was added dropwise to the mixture solution in 30 min. The reaction mixture was stirred constantly at 140 °C for 2 h. After the reaction mixture being cooled to room temperature overnight, the dark purple precipitate was collected by filtration and rinsed with deionized water and EtOH, and then dried in an oven at 60 °C. After the solid being dissolved in dichloromethane, the product was separated from mixture by silica gel column chromatography using the mixture of dichloromethane and petroleum ether (4:1) as eluent, to obtain the purple solid intermediate product 5,15-di(4carbomethoxy- phenyl)-10,20-diphenylporphyrin (DiCPPor-OMe), yield: 2.4%. 1H NMR (CDCl3, 400 MHz), δ, ppm: -2.79 (2H, NH), 4.11 (6H, COOCH3), 7.77 (6H, p,m-C6H5), 8.20 (4H, o-C6H5), 8.31 (4H,
2.3. Synthesis of P(PPor-BBO) P(PPor-BBO) was prepared via polycondensation for the first time. DAR·2HCl (24.1 mg, 0.11 mmol) was added to a stirred polyphosphoric acid (50 mL) at 70 °C under the nitrogen protection condition. A certain amount of DiCPPor (40 mg, 0.06 mmol) was added to the reaction mixture in the middle of the removal of HCl, in order to prevent DAR·2HCl from being oxidized. Until all the hydrochloric acid was removed out, the remaining DiCPPor (39.3 mg, 0.05 mmol) were added to the reaction mixture. Subsequent the reaction mixture was stirred and heated at 100 °C, 120 °C, 160 °C, 180 °C and kept warm for 6 h, respectively. The reaction solution was cooled to room temperature, and deionized water (200 mL) was added. The polymer was filtered, and then repeatedly washed with water to give the black solid of P(PPorBBO). Yield: 82%. FT-IR (KBr tablet, νmax /cm−1): 3637, 1606, 1553, 2
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1230, 1058, 801, 702. Solid-state CP/MAS 13C NMR (400 MHz): δ 162, 153, 139, 127, 111, 95 ppm. UV − vis (MSA, λmax/nm): 451, 673.
absorption peaks of DiCPPor and DAR·2HCl have almost disappeared. At the same time, the new absorption peak at 1606 cm−1 attributed to C=N stretching vibration of oxazole ring were observed. Additionally, the spectrum presented vibrational bands at 1230 and 1078 cm−1, which attributed to the C-N and = C-O-C the absorption of oxazole ring, respectively [20]. The above results prove that P(PPor-BBO) has been obtained successfully. Solid state 13C NMR has been used to further prove the structure formation of P(PPor-BBO) (Fig. 3). The carbon resonances of C9-C14 at 127 ppm were corresponding to the phenyl rings of porphyrin. The identified resonances at 95 of C3, 111 of C7, 139 of C6,8, 153 of C2,4 and 162 of C1,5 ppm confirmed the formation of oxazole rings [21,22]. The 13 C NMR result indicates the covalent bonding between DiCPPor and DAR via condensation reaction has been achieved. P(PPor-BBO) are black green solids at room temperature (Fig. 4a). SEM investigation shows that the particles of P(PPor-BBO) are about 90 nm (Fig. 4b, c, d). It is speculated that the formation of the morphology may be due to the fully conjugated rigid structure, making the molecules self-assembly through π-π stacking interactions. The intrinsic viscosity [η] of P(PPor-BBO) was measured in methanesulfonic acid solution at 30 °C by Ubbelohde viscometer. The viscosity average molecular weights (Mη) were estimated from the one-spot method by the Mark–Houwink equation: [η] = 2.77 × Mη1.8. The measured [η] and calculated (Mη) for P(PPor-BBO) are 5.28 dL g−1 and 11,080 g mol−1, respectively. The crystal structures of the DiCPPor, DAR·2HCl and P (PPor-BBO) were studied by PXRD (Fig. 5). The monomer DAR·2HCl has some independent and narrow diffraction peaks, indicating that it is a crystal structure. For the DiCPPor, there is only a very broad peak at about 2θ = 22° without any other strong diffraction peaks. P(PPor-BBO) also has more broad “steamed bread peak” at around 2θ =23° without any other strong diffraction peaks, indicating that it is amorphous polymer in nature [23]. The thermal stability of DiCPPor, DAR·2HCl and P(PPor-BBO) were reflect from the TGA curve in nitrogen atmosphere (Fig. 6). In curve of DiCPPor, three main weight losses were observed. An apparent weight loss was observed from 90 °C to 204 °C for the first stage. This may be due to physisorbed or chemisorbed water. Then a little weight loss observed at the second stage from 204 to 346 °C suggests that the thermal decomposition of carboxyl groups. The third stage of the weight loss above 346 °C corresponds to the decomposition of which may be attributed to the decomposition process of phenylporphyrin. TGA curve of DAR·2HCl showed that the weight loss in the temperature from 240°C, which root in the removal of –OH and –NH2 [18]. In general, the thermal stability of the polymer is strongly relevant to chain rigidity and chain packing. In the curve of P(PPor-BBO), the same as porphyrin, the ca. 10% weight loss from 100 °C to 423 °C may be the evaporation of the combined water. The polymer molecular chain contained many N and O atoms to form hydrogen bonds with waters. The maximum thermal decomposition temperature of P(PPor-BBO) occured at 423 °C. At the temperature, the macromolecular chain began to clot upon heating or certain groups were decomposed from the macromolecule. Additionally, even at temperatures as high as 800 °C, the weight loss of this polymer was only 26.61%. Evidently, an increase in the thermal stability of P(PPor-BBO) compared to monomer DiCPPor was attributed to the rigid structure due to the inclusion of the benzobisoxazole ring in P(PPor-BBO) [24]. The analysis of the P(PPor-BBO) thermogravimetric curve implies that P(PPor-BBO) have an eminent thermal stability. Moreover, the chemical stability of P(PPor-BBO) was examined and compared with the monomers DiCPPor and DAR·2HCl in water and various organic solvents (Table S1). The results show that P (PPor-BBO) is insoluble in water and organic solvents, only soluble in strong acid. It should be attributed to extended rigid π-conjugated backbone of porphyrin units. Additionally, after P(PPor-BBO) was dissolved in methanesulfonic acid for 24 h, the structure of the polymer almost has no change (Fig. S3). Therefore, excellent chemical stability makes it suitable as a heterogeneous photocatalyst for easy recycling.
2.4. Characterizations Fourier transform infrared spectra (FT-IR) were conducted by using a PerkinElmer Frontier spectrophotometer in the range from 4000 to 500 cm−1 using a KBr pellet. The Solid-state 13C CP/MAS NMR spectra were recorded with a Bruker Avance 400 MHz spectrometer. The morphology of polymer was observed by in a JEOL JSM-6701F fieldemission scanning electron microscopy (SEM) at 5 kV. The X-ray diffraction (XRD) measurement was performed using Rigaku Ultima IV diffractmeter over the 2θ range of 10°–80°. The thermal stability of the materials was carried out under N2 atmosphere from room temperature to 800 °C using NETZSCH STA409PC thermogravimetric analysis (TGA) meter. Ultraviolet–visible (UV–vis) spectra were recorded on a Shimadzu (UV-2600) spectrophotometer in the wavelength range of 300–800 nm. 2.5. Measurement of photocatalytic activity The photocatalytic activity experiments of the samples were surveyed by the degradation of RhB solution (1 × 10−5 mol/L, 40 mL, solvent: water) under visible-light irradiation. The light source was provided by Xenon lamp (PLS-SXE300, Perfectlight Co.) with a 400 nm cutoff filter, an output optical power density of 318.47 mw·cm−2 and an illuminated area of 40.69 cm2. The reacting suspension consisting of RhB solution and 10 mg photocatalyst, then it was stirred in dark with a magnetic stir bar for 30 min to achieve adsorption-desorption equilibrium. A 5 mL of solutions was taken out and filtered from the suspension each 30 min to test its photocatalytic activity. Finally, the photocatalytic activity of degradation RhB was detected by quantitative measuring its absorption at 554 nm through a UV–vis spectrophotometer. 3. Results and discussion 3.1. Characterization of photocatalyst The chemical structure of P(PPor-BBO) was verified by FT-IR spectrum. As shown in Fig. 2a, the absorption peak at 3431 and 1690 cm−1 were assigned to O–H and C=O stretching vibration of –COOH on DiCPPor, respectively. The absorption peak at 802 cm−1 was attributed to the bending vibration of the N–H bond on the pyrrole skeleton of DiCPPor [19]. It is suggested that DiCPPor was prepared successfully. As shown in Fig. 2b, the broad peak at 3210–2800 cm−1 were assigned the superposition of ammonia hydrochloride and hydroxyl of DAR·2HCl. The characteristic peaks indicate that the DAR·2HCl was not oxidized. Fig. 2c shows that the characteristic
Fig. 2. FTIR spectra of (a) DiCPPor, (b) DAR·2HCl and (c) P(PPor-BBO). 3
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Fig. 3. Solid state
13
C NMR spectrum of P(PPor-BBO).
Fig. 4. (a) Photographic, (b-d) SEM images of P(PPor-BBO).
3.2. Photocatalytic performance
was evaluated by the change of absorption peak at 554 nm of the original RhB solution (1 × 10−5 mol/L) with exposure time under visiblelight irradiation. In order to better study the photocatalytic activity of P (PPor-BBO), blank experiment of RhB degradation without any photocatalyst was observed in Fig. S5. The result shows that the self-degradation of RhB without any photocatalyst under visible-light irradiation was negligible. The time-dependent absorption peaks of RhB degradation under visible-light irradiation in the presence of P(PPorBBO) (Fig. S6) compared with DiCPPor (Fig. S7) was studied, respectively. The results clearly show that photodegradation rate of RhB under P(PPor-BBO) as photocatalyst was obviously higher (63%, 150 min) than that of DiCPPor (40%, 150 min). The excellent photocatalytic performance may be attributed to 1D large π-conjugated system and completely rigid coplanar structure of P(PPor-BBO), which
P(PPor-BBO) as visible-light driven photocatalyst, the light absorption capability was important. It was dissolved in methanesulfonic acid and evaluated by UV–vis spectroscopic measurement (Fig. S4). There were two obvious absorption peaks at 451 and 672 nm in visible region. It shows remarkably red-shifted electronic absorption spectra compared with DiCPPor due to extended π-conjugate degree of porphyrin unit (Fig. S4a). Generally, the UV–vis characteristic absorption spectrum of porphyrin compound has a B band and four Q bands. But solvent effect of strong acid have resulted in the P(PPor-BBO) and DiCPPor only showed one Q bands (Fig. S4b). The photocatalytic activity of P(PPor-BBO) towards degradation of RhB was investigated under visible-light irradiation. Wherein, the degradation activity of RhB 4
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(Fig. S9). The result shows the very weak change in absorption peaks of RhB before and after 150 min. It confirms that the fading of dye mainly comes from photocatalytic degradation, not adsorption. 3.3. Photocatalytic mechanism In order to study the primary reactive species generated from P (PPor-BBO) in the photocatalytic degradation process, DPBF as single oxygen for 1O2 was performed in the scavenger experiments [25]. The photocatalytic properties of P(PPor-BBO) to generate 1O2 was investigated in DMF and monitored by time-dependent absorption plot of the DPBF at 415 nm. The results show that P(PPor-BBO) can rapidly generate 1O2 and consume up to approximative 90% of DPBF in 70 s under visible light conditions (Fig. 8). This phenomenon proves that P (PPor-BBO) as photocatalyst can efficiently generate 1O2 to degrade the RhB. A proposed mechanism of visible-light driven photodegradation of RhB by P(PPor-BBO) photocatalyst is proposed as follows:
Fig. 5. XRD pattern of (a) DiCPPor, (b) DAR·2HCl and (c) P(PPor-BBO).
P(PPor BBO) + hv 1P(PPor
BBO) + ISC
3P(PPor
BBO) + 3O2
H2 O2 + hv 1O , 2
1P(PPor
BBO)
3P(PPor
BBO)
P(PPor BBO) + 1O2
2OH
OH·+ RhB
Oxidation products
(1) (2) (3) (4) (5)
As shown in the above equations, when the P(PPor-BBO) is irradiated by visible- light, the excited singlet state 1P(PPor-BBO)⁎ is formed (eq.1). Then, 1P(PPor-BBO)⁎ is converted to the triplet state 3P (PPor-BBO)⁎ through intersystem crossing (ISC, eq. 2). The 3P(PPorBBO)⁎ interacts with the groud state triplet oxygen (3O2) to generate the active 1O2 (eq. 3). Moreover, H2O2 could also produce OH· free radicals under visible light irradiation (eq. 4). The formation of active species including 1O2 and OH· resulted in significantly enhanced photocatalytic activity in the P(PPor-BBO) catalytic system, they can react with RhB, and RhB is degraded to nontoxic oxidation products (eq. 5).
Fig. 6. TGA of (a) DiCPPor, (b) DAR·2HCl and (c) P(PPor-BBO).
improved catalytic active site and light-harvesting capability of the porphyrin units in the visible-light region. H2O2 as oxidant could help the photocatalyst to drastically degrade RhB in photocatalytic reaction (Fig. S8). Fig. S8 shows the time-dependent UV–vis spectra of RhB solution in the presence of H2O2 under visible-light irradiation (58%, 150 min). The RhB degradation in the presence of P(PPor-BBO) including H2O2 under visible-light irradiation was illustrated in Fig. 7. The photocatalytic degradation efficiency of RhB using the P(PPorBBO)/H2O2 could reach 98% during 150 min under visible-light irradiation. It is inferred that the enhanced activities originate from the synergistic effect between P(PPor-BBO) and H2O2. In addition, In order to observe that whether the fading phenomenon of RhB comes from the adsorption by catalyst, RhB solution in the presence of photocatalyst P (PPor-BBO) was magnetic stirred in dark for 150 min and then detected
3.4. Reusability and stability Moreover, the reusability of the photocatalyst also plays an important role in organic dye wastewater treatment, implying industrial low cost. P (PPor-BBO) is insoluble in water, the used photocatalyst could be separated from RhB solution by simple filtration or centrifugation, and then dried under vacuum at 60 °C. As shown in Fig. 9, the photocatalytic activity of P(PPor-BBO) for degradation of RhB under visible-light irradiation only reduced a little after four cycles, which could be due to residual organic contaminants RhB on the surface of photocatalyst during
Fig. 7. UV–vis absorbance spectra of RhB with (a) P(PPor-BBO)/H2O2 under visible-light irradiation, (b) Comparison of the decay rate of RhB (black) alone, and in the presence of DiCPPor (red), H2O2 (blue), P(PPor-BBO) (green) and P(PPor-BBO)/H2O2 (pink) under visible light irradiation. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.) 5
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Fig. 8. Photocatalytic efficiency of RhB on (a) Time-dependent absorption spectra of DPBF (in DMF) in the presence of P(PPor-BBO) (b) The decay rate of DPBF in the presence of P(PPor-BBO).
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Fig. 9. Photocatalytic degradation of RhB over P(PPor-BBO)/H2O2 under visible-light irradiation during four cycles.
the test period. According to the results, P(PPor-BBO) have excellent cycle stability under visible-light irradiation. 4. Conclusions In this work, an efficient 1D photocatalyst, P(PPor-BBO) was successfully fabricated through polycondensation between DiCPPor and DAR·2HCl. 1D rigid conjugated structural features endows P(PPor-BBO) with excellent chemical stability and photocatalytic performance in heterogeneous catalysis. As the heterogeneous photocatalyst, P(PPorBBO) exhibited superior photocatalytic capacity that 98% RhB was degraded after 150 min under visible-light irradiation. Meanwhile, P (PPor-BBO) possessed good thermal stability and reusability. This work may provide a new insight into the design of other novel 1D porphyrin materials as photocatalysts for removal of organic pollutants under sunlight irradiation. Acknowledgements This work was supported by the Science and Technology Innovation Fund of Changchun University of Science and Technology (grant numbers XJJLG-2016-05, XJJLG-2017-07); Research Foundation of Education Department of Jilin Province (grant numbers JJKH20190582KJ); and Research Foundation of Science & Technology Department of Jilin Province (20190303069SF). Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.reactfunctpolym.2019.104340. 6
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