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Hollow and sulfo

aDepartment of Chemistry, Department of E

Suwon 440-746, Korea. E-mail: sson@skk

031-290-4572bDepartment of Organic and Nanoengineer

Korea

† Electronic supplementary information (Emagnication TEM images, PXRD patternsinglet oxygen detection in the photoS-HMOP, photocatalytic activity of Znanalysis of the recovered ZnTPP/S-HMOP.

Cite this: RSC Adv., 2015, 5, 47270

Received 13th April 2015Accepted 20th May 2015

DOI: 10.1039/c5ra06633f

www.rsc.org/advances

47270 | RSC Adv., 2015, 5, 47270–4727

nated microporous organicpolymers: versatile platforms for non-covalentfixation of molecular photocatalysts†

Nojin Park,a Daye Kang,a Min Cheol Ahn,a Sungah Kang,a Sang Moon Lee,a

Tae Kyu Ahn,a Jae Yun Jaung,b Hee-Won Shin*a and Seung Uk Son*a

Sulfonated hollow microporous organic polymers (S-HMOP) were

prepared by template synthesis and post synthetic approach. The S-

HMOP showed excellent non-covalent fixation ability towards

various cationic dyes through ionic interaction. Among various dye

systems, Zn-porphyrin loaded S-HMOP showed promising activity

and stability in the decomposition of 4-chlorophenol under visible

light irradiation.

For environmental reasons, the removal of organic pollutantsfrom aqueous solution has attracted great attention fromscientists.1 Among the various removal strategies, photochem-ical transformation based on visible light absorbing dyes is anattractive method that addresses energy concerns.2 Variousorganic dyes and metal complexes have been studied for thispurpose. As the reports accumulated, the dyes used in thehomogeneous systems were reviewed recently by Miranda et al.3

As mentioned in the conclusion of the review paper, one of thefuture issues requiring further efforts is the heterogenization ofsystems. Recently, our research group has sought new hetero-genization strategy for the photocatalysts.4

Through the structural analysis of known molecular photo-catalysts used for organic pollutant removal, we gured out thatalthough dyes have structural diversity, a signicant number ofthem have ionic characteristics due to water-solubility. Thus, wethought that the heterogenization strategy should be applicableto various dyes and ionic interaction based xation is consid-erable. It has been reported that non-covalent xation based on

nergy Science, Sungkyunkwan University,

u.edu; hwshin91@gamil.com; Fax: +82-

ing, Hanyang University, Seoul 133-791,

SI) available: Experimental details, lows, N2 sorption isotherms of materials,catalytic reaction by ZnTPP loadedTPP loaded nonhollow S-MOP, andSee DOI: 10.1039/c5ra06633f

4

ionic interaction is a very efficient strategy for various hetero-geneous catalytic systems.5

Recently, various microporous organic polymers (MOPs)have been prepared via coupling reactions of building blocks.6

Due to their high surface area and robustness, these materialshave been applied to various purposes such as adsorbents7 andcatalysts.8 In addition, MOP materials with photocatalyticactivities have been studied.9 However, the studies includingthose from our research group were based on limited dyespecies. Usually, photoactive species were introduced to MOPmaterials by coupling of pre-designed dye building blocks. Asfar as we are aware, a MOP based heterogenization strategyapplicable to various dye systems was not reported. Moreover,MOPmaterials usually have hydrophobic properties due to theirorganic characteristics, which can limit the application toaqueous systems. However, it has been veried that the postsynthetic approach for MOP materials can change their chem-ical properties.10

Our research group has studied functional MOP materialsbased on Sonogashira coupling.11 Recently, we have shown thatthe outer shape of MOP materials can be engineered usingvarious templates.12 In this work, we report the synthesis ofwater dispersible hollow MOP materials via post-modication,the non-covalent xation of various dyes, and their photo-catalytic performance in the visible light-driven decompositionof 4-chlorophenol. Fig. 1 shows the heterogenization strategyfor ionic dyes.

Hollow MOP (HMOP) was prepared using tetrakis(4-ethynylphenyl)methane and 1,4-diiodobenzene as buildingblocks for Sonogashira coupling.12 Silica spheres (83 � 4 nmdiameter) were used as templates. For the introduction of theanchoring group, the HMOP was post-modied by sulfonationwith chlorosulfonic acid.13 The sulfonated HMOP (S-HMOP)materials were characterized by various methods. First,according to the transmission electron microscopy (TEM), thematerials showed a hollow structure with very thin shell thick-ness of 8.9� 0.6 nm. There was no difference in the outer shapeofHMOP and S-HMOP (Fig. 2a and S1 in the ESI†). According to

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Fig. 1 Synthesis of the sulfonated hollow microporous organic poly-mer (S-HMOP) and the heterogenization of ionic dyes.

Fig. 2 TEM images of S-MOP (a) and dye loaded S-MOP (b–h). N2

isotherm curves (i) at 77 K, pore size distribution diagrams (inset), IRspectra (j), and (k) solid phase 13C NMR spectra ofHMOP, S-HMOP andZnTPP loaded S-HMOP.

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N2 isotherm analysis, S-HMOP materials showed a 529 m2 g�1

surface area and microporosity, compared with the 733 m2 g�1

surface area of HMOP (Fig. 2i). According to powder X-raydiffraction studies, the hollow materials in this work were allamorphous (Fig. S2 in the ESI†), as reported in the literature.6–12

The chemical properties of the S-HMOP were characterizedby infrared (IR) absorption and solid phase 13C nuclearmagnetic resonance (NMR) spectroscopy. As shown in Fig. 2jand S3 in the ESI,† the vibration peaks at 3500, 1182, 1040, and652 cm�1 were attributed to sulfonic acid groups.14 The solidphase 13C NMR spectrum of S-HMOP showed change ofaromatic 13C peaks,15 compared with that of HMOP, supportingthat sulfonic acid was introduced at phenyl rings (Fig. 2k).According to the elemental analysis for sulfur contents(5.66 w%), the amount of sulfonic acid groups in S-HMOP wascalculated as 1.76 mmol g�1.

The xation ability of the S-HMOP for the dyes was investi-gated. We discovered that the S-HMOP can trap cationic dyesfrom the aqueous solution with vivid color changes through dyexation. The loaded cationic dyes could not be removed bysonication or washing with water and organic solvents. Anionicdyes such as eosin Y (sodium salt) were not loaded on theS-HMOP. When theHMOPwas used instead of the S-HMOP, thedyes were removed aer washing. These observations indicatethat the dye loading on the S-HMOP is based on ionic interac-tions between sulfonate and cationic dyes.16

Fig. 3a shows the cationic dyes used in this study. Theanalysis of dye-loaded S-HMOP by TEM showed the completemaintenance of hollowmorphology (Fig. 2b–h). The surface areawas changed from 529 m2 g�1 to 233–472 m2 g�1 by dye loading

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(Table 1 and Fig. S4 in the ESI†). Fig. 3b shows the photographsand absorption spectra of dye-loaded S-HMOP, showing visiblelight absorption properties. The dye loading was in a range of93–423 mmol g�1, which was calculated by elemental analysis forthe N contents in the materials. Compared with the loadingamount (0.48–67 mmol g�1) of dyes in non-porous polymer orsilica in the literature,17 the dye-loaded S-HMOP showed a veryefficient xation due to the porosity, hollow structure and thinshell of supports. The loading amount of dyes in S-HMOPincreased with a decrease in dye size.

As described in the introduction section, the dyes in thiswork have been used as photocatalysts in homogeneoussystems.3 The introduction of sulfonic acid and dyes to HMOPresulted in the hydrophilic nature of materials (Fig. S5 in theESI†). Thus, we tested the dye-loaded S-HMOP materials as aheterogeneous system for 4-chlorophenol decomposition inwater under visible light irradiation. Fig. 4 summarizes theresults.

The 5 mol% of dyes in the S-HMOP was used for the pho-tocatalytic decomposition of 4-chlorophenol. Without the dye

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Fig. 3 (a) Dyes used in this study. (b) Photographs and absorptionspectra of S-HMOP and dyes-loaded S-HMOP.

Table 1 Properties of materials in this work

Material

Surface area Pore volumea Dye loadingb

(m2 g�1) (cm3 g�1) (mmol g�1)

HMOP 733 0.72 —S-HMOP 529 0.54 —RhB-S-HMOP 443 0.42 178MB-S-HMOP 233 0.39 423AYG-S-HMOP 326 0.43 282RuBPy-S-HMOP 363 0.41 181TPP-S-HMOP 411 0.54 147CuTPP-S-HMOP 456 0.50 110ZnTPP-S-HMOP 472 0.52 93

a Values at P/Po ¼ 0.97. b The amount of dye loading is based on theelemental analysis for N contents in materials.

Fig. 4 (a) Photocatalytic and (b) recycle performance (4 h for eachrun) for the decomposition of 4-chlorophenol by dye-loadedS-HMOP. (c) Photocatalytic process. [4-Chlorophenol]o ¼ 1 mM(25 mL), dye: 5 mol% of 4-chlorophenol, pHinitial ¼ 5.0, 200 W Xe lamp(4.6 mW cm�2 intensity) with a cutoff filter (>420 nm), roomtemperature. For the recycle tests, 5 mol% TPP, 5 mol% ZnTPP and10 mol% RhB on S-HMOP were used.

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materials, 4-chlorophenol was not decomposed under visiblelight irradiation (Fig. 4a). The dye-loaded S-HMOP materialsshowed a dye-dependent performance. As shown in Fig. 4a, the

47272 | RSC Adv., 2015, 5, 47270–47274

order of activities was methylene blue (MB) < Cu-tetrapyridiumporphyrin (CuTPP) < acridine yellow G (AYG, kobs ¼ 0.16 h�1) <rhodamine B (RhB, kobs ¼ 0.18 h�1) < Ru(bpy)3

2+ (RuBPy) <Zn-tetrapyridium porphyrin (ZnTPP, kobs ¼ 0.44 h�1) < tetra-pyridium porphyrin (TPP, kobs ¼ 0.71 h�1)-loaded S-HMOP.18

This order can be rationalized by the 1O2 generation ability ofthe dyes in the systems. For the organic dyes and porphyrins, asimilar trend of photocatalytic activities was observed.19 In theliterature, porphyrin dyes have been xed on silica for thedevelopment of heterogeneous photocatalytic systems.17a Thesystem showed �85% conversion of 4-chlorophenol aer 4 husing 50 mol% porphyrin dyes and 450W Xe lamp (>420 nm).17a

In comparison, the ZnTPP-loaded S-HMOP showed superiorperformance, with 86% decomposition of 4-chlorophenol aer4 h using 5 mol% dyes and 200 W Xe lamp (>420 nm). Theenhanced activity of ZnTPP-loaded S-HMOP can be attributed tothe porosity and very thin (�9 nm shell thickness) hollowstructure of thematerials, which canmaximize the performanceof the active dye species.20

It has been well recognized that most organic dyes decom-pose gradually in the photocatalytic process by self-oxidation.3

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As shown in Fig. 4b, the organic dyes on the S-HMOP such asRhB and TPP decomposed gradually in successive runs. Even inthese cases, according to the UV/visible absorption spectros-copy of solution, the colored materials were not etched tosolution. The ZnTPP on the S-HMOP showed signicant main-tenance of activities during three runs, which is attributable tothe enhanced stability of porphyrin against self-oxidationthrough metallation.21

The decomposition process of 4-chlorophenol by homoge-neous photocatalysts has been suggested to be based on 1O2

mediated oxidative dechlorination (Fig. 4c).17c,22 In the photo-catalytic performance of ZnTPP on the S-HMOP, the generationof 1O2 species and benzoquinone were directly detected byemission (emission of 1O2 at 1268 nm, Fig. S5 in the ESI†) andUV/visible absorption spectroscopy (absorption of benzoqui-none at 245 nm), respectively, suggesting that the photocatalyticdecomposition in this work follows the suggested process in theliterature.17c

In conclusion, this work shows that MOP chemistry can beapplied for the heterogenization of photocatalysts. The MOPmaterials were engineered as hollow spheres using a silicatemplate. Sulfonated HMOP obtained by post synthetic modi-cation showed excellent performance as a platform for theheterogenization of photocatalysts. Among the various dyestested, Zn-porphyrin based heterogeneous system showedpromising activity and stability. We believe that more variousheterogeneous photocatalytic systems can be developed by non-covalent heterogenization strategy of this work.

Acknowledgements

This work was supported by grants NRF-2012-R1A2A2A01045064 (Midcareer Researcher Program) throughthe National Research Foundation of Korea and by grants no.10047756 (Technology Innovation Program) funded by theMinistry of Trade, Industry & Energy of Korea.

Notes and references

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3 M. L. Marin, L. Santos-Juanes, A. Arques, A. M. Amat andM. A. Miranda, Chem. Rev., 2012, 112, 1710.

4 (a) J. Lee, J. Kwak, K. C. Ko, J. H. Park, J. H. Ko, N. Park,E. Kim, D. H. Ryu, T. K. Ahn, J. Y. Lee and S. U. Son, Chem.Commun., 2012, 48, 11431; (b) N. Kang, J. H. Park, K. C. Ko,J. Chun, E. Kim, H. W. Shin, S. M. Lee, H. J. Kim,T. K. Ahn, J. Y. Lee and S. U. Son, Angew. Chem., Int. Ed.,2013, 52, 6228; (c) J. H. Park, K. C. Ko, N. Park, H.-W. Shin,E. Kim, N. Kang, J. H. Ko, S. M. Lee, H. J. Kim, T. K. Ahn,J. Y. Lee and S. U. Son, J. Mater. Chem. A, 2014, 2, 7656.

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5 J. M. Fraile, J. I. Garcıa and J. A. Mayoral, Chem. Rev., 2009,109, 360.

6 Reviews on microprous organic materials: (a) R. Dawson,A. I. Cooper and D. J. Adams, Prog. Polym. Sci., 2012, 37,530; (b) F. Vilela, K. Zhang and M. Antonietti, EnergyEnviron. Sci., 2012, 5, 7819; (c) A. Thomas, Angew. Chem.,Int. Ed., 2010, 49, 8328; (d) A. I. Cooper, Adv. Mater., 2009,21, 1291; (e) M. Mastalerz, Angew. Chem., Int. Ed., 2008, 47,445; (f) C. Weder, Angew. Chem., Int. Ed., 2008, 47, 448.

7 Selected examples: (a) X. Yang, B. Li, I. Majeed, L. Liang,X. Long and B. Tan, Polym. Chem., 2013, 4, 1425; (b)C. D. Wood, B. Tan, A. Trewin, F. Su, M. J. Rosseinsky,D. Bradshaw, Y. Sun, L. Zhou and A. I. Cooper, Adv. Mater.,2008, 20, 1916.

8 Selected examples: (a) K. Thiel, R. Zehbe, J. Roeser,P. Strauch, S. Enthaler and A. Thomas, Polym. Chem., 2013,4, 1848; (b) C. Bleschke, J. Schmidt, D. S. Kundu,S. Blechert and A. Thomas, Adv. Synth. Catal., 2011, 353,3101; (c) X. Du, Y. Sun, B. Tan, Q. Teng, X. Yao, C. Su andW. Wang, Chem. Commun., 2010, 46, 970.

9 Selected examples: (a) K. Kailasam, J. Schmidt, H. Bildirir,G. Zhang, S. Blechert, X. Wang and A. Thomas, Macromol.Rapid Commun., 2013, 34, 1008; (b) K. Zhang, D. Kopetzki,P. H. Seeberger, M. Antonietti and F. Vilela, Angew. Chem.,Int. Ed., 2013, 52, 1432; (c) Z. Xie, C. Wang, K. E. DeKraand W. Lin, J. Am. Chem. Soc., 2011, 133, 2056.

10 (a) B. Kiskan and J. Weber, ACS Macro Lett., 2012, 1, 37; (b)T. Ratvijitvech, R. Dawson, A. Laybourn, Y. Z. Khimyak,D. J. Adams and A. I. Cooper, Polymer, 2014, 55, 321; (c)H. Urakami, K. Zhang and F. Vilela, Chem. Commun., 2013,49, 2353.

11 (a) H. C. Cho, H. S. Lee, J. Chun, S. M. Lee, H. J. Kim andS. U. Son, Chem. Commun., 2011, 47, 917; (b) J. Chun,J. H. Park, J. Kim, S. M. Lee, H. J. Kim and S. U. Son,Chem. Mater., 2012, 24, 3458; (c) H. S. Lee, J. Choi, J. Jin,J. Chun, S. M. Lee, H. J. Kim and S. U. Son, Chem.Commun., 2012, 48, 94; (d) N. Kang, J. H. Park, J. Choi,J. Jin, J. Chun, I. G. Jung, J. Jeong, J.-G. Park, S. M. Lee,H. J. Kim and S. U. Son, Angew. Chem., Int. Ed., 2012, 51,6626; (e) J. Chun, S. Kang, N. Kang, S. M. Lee, H. J. Kimand S. U. Son, J. Mater. Chem. A, 2013, 1, 5517.

12 (a) N. Kang, J. H. Park, M. Jin, N. Park, S. M. Lee, H. J. Kim,J. M. Kim and S. U. Son, J. Am. Chem. Soc., 2013, 135, 19115;(b) J. Chun, S. Kang, N. Park, E. J. Park, X. Jin, K.-D. Kim,H. O. Seo, S. M. Lee, H. J. Kim, W. H. Kwon, Y.-K. Park,J. M. Kim, Y. D. Kim and S. U. Son, J. Am. Chem. Soc.,2014, 136, 6786; (c) B. H. Lim, J. Jin, S. Y. Han, K. Kim,S. Kang, N. Park, S. M. Lee, H. J. Kim and S. U. Son, Chem.Commun., 2014, 50, 7723; (d) J. Jin, B. Kim, N. Park,S. Kang, J. H. Park, S. M. Lee, H. J. Kim and S. U. Son,Chem. Commun., 2014, 50, 14885; (e) J. Yoo, N. Park,J. H. Park, J. H. Park, S. Kang, S. M. Lee, H. J. Kim, H. Jo,J.-G. Park and S. U. Son, ACS Catal., 2015, 5, 350.

13 W. Lu, D. Yuan, J. Sculley, D. Zhao, R. Krishna andH.-C. Zhou, J. Am. Chem. Soc., 2011, 133, 18126.

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15 Similar change of aromatic 13C peaks by sulfonation ofphenyl rings using chlorosulfonic acid was reported inFig. S13† in (ref. 13).

16 K. A. Cavicchi, ACS Appl. Mater. Interfaces, 2012, 4, 518.17 (a) W. Kim, J. Park, H. J. Jo, H.-J. Kim and W. Choi, J. Phys.

Chem. C, 2008, 112, 491; (b) Y. Shiraishi, T. Suzuki andT. Hirai, New J. Chem., 2010, 34, 714; (c) R. Zugle,E. Antunes, S. Khene and T. Nyokong, Polyhedron, 2012,22, 74; (d) M. Hu, Y. Xu and J. Zhao, Langmuir, 2004, 20, 6302.

18 The photodecomposition of 4-chlorophenol by AYG, RhB,TPP, and ZnTPP-loaded S-HMOP materials showed rst-order kinetics, as reported in the literaure (ref. 16, 18 and19). The kobs values were obtained by plotting ln C/Co

versus t based on the equation, ln C/Co ¼ �kobst.

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19 D. Gryglik, J. S. Miller and S. Ledakowicz, Sol. Energy, 2001,77, 615.

20 As a control material, ZnTPP/S-nonhollow-MOP wasprepared (See the Experimental Sections in the ESI†). TheZnTPP/S-nonhollow-MOP showed less loading (42 mmmol/gZnTPP) of ZnTPP and lower photocatalytic activiy thanZnTPP/S-HMOP (Fig. S7 in the ESI†).

21 According to TEM analysis, the hollow shape of ZnTPP/S-HMOP retained aer three cycles. However, surface area ofthe materials decreased from 472 m2 g�1 to 310 m2 g�1,which is attributable to the entrapped chemicals viadecomposition of 4-chlorophenol (Fig. S8 in the ESI†).

22 E. Silva, M. M. Pereira, H. D. Burrows, M. E. Azenha,M. Sarakha and M. Bolte, Photochem. Photobiol. Sci., 2004,3, 200.

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