Hollow and sulfonated microporous organic polymers: versatile platforms for non-covalent fixation of molecular photocatalysts

Abstract

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.

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For environmental reasons, the removal of organic pollutants from aqueous solution has attracted great attention from scientists.¹ Among the various removal strategies, photochemical transformation based on visible light absorbing dyes is an attractive method that addresses energy concerns.² Various organic dyes and metal complexes have been studied for this purpose. As the reports accumulated, the dyes used in the homogeneous systems were reviewed recently by Miranda et al.³ As mentioned in the conclusion of the review paper, one of the future issues requiring further efforts is the heterogenization of systems. Recently, our research group has sought new heterogenization strategy for the photocatalysts.⁴

Through the structural analysis of known molecular photocatalysts used for organic pollutant removal, we figured out that although dyes have structural diversity, a significant number of them have ionic characteristics due to water-solubility. Thus, we thought that the heterogenization strategy should be applicable to various dyes and ionic interaction based fixation is considerable. It has been reported that non-covalent fixation based on ionic interaction is a very efficient strategy for various heterogeneous 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 materials have been applied to various purposes such as adsorbents⁷ and catalysts.⁸ In addition, MOP materials with photocatalytic activities have been studied.⁹ However, the studies including those from our research group were based on limited dye species. Usually, photoactive species were introduced to MOP materials by coupling of pre-designed dye building blocks. As far as we are aware, a MOP based heterogenization strategy applicable to various dye systems was not reported. Moreover, MOP materials usually have hydrophobic properties due to their organic characteristics, which can limit the application to aqueous systems. However, it has been verified that the post synthetic approach for MOP materials can change their chemical properties.¹⁰

Our research group has studied functional MOP materials based on Sonogashira coupling.¹¹ Recently, we have shown that the outer shape of MOP materials can be engineered using various templates.¹² In this work, we report the synthesis of water dispersible hollow MOP materials via post-modification, the non-covalent fixation of various dyes, and their photocatalytic performance in the visible light-driven decomposition of 4-chlorophenol. Fig. 1 shows the heterogenization strategy for ionic dyes.

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

Hollow MOP (HMOP) was prepared using tetrakis(4-ethynylphenyl)methane and 1,4-diiodobenzene as building blocks for Sonogashira coupling.¹² Silica spheres (83 ± 4 nm diameter) were used as templates. For the introduction of the anchoring group, the HMOP was post-modified by sulfonation with chlorosulfonic acid.¹³ The sulfonated HMOP (S-HMOP) materials were characterized by various methods. First, according to the transmission electron microscopy (TEM), the materials showed a hollow structure with very thin shell thickness of 8.9 ± 0.6 nm. There was no difference in the outer shape of HMOP and S-HMOP (Fig. 2a and S1 in the ESI†). According to 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-ray diffraction studies, the hollow materials in this work were all amorphous (Fig. S2 in the ESI†), as reported in the literature.^6–12

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), IR spectra (j), and (k) solid phase ¹³C NMR spectra of HMOP, S-HMOP and ZnTPP loaded S-HMOP.

The chemical properties of the S-HMOP were characterized by infrared (IR) absorption and solid phase ¹³C nuclear magnetic resonance (NMR) spectroscopy. As shown in Fig. 2j and S3 in the ESI,† the vibration peaks at 3500, 1182, 1040, and 652 cm⁻¹ were attributed to sulfonic acid groups.¹⁴ The solid phase ¹³C NMR spectrum of S-HMOP showed change of aromatic ¹³C peaks,¹⁵ compared with that of HMOP, supporting that 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 was calculated as 1.76 mmol g⁻¹.

The fixation ability of the S-HMOP for the dyes was investigated. We discovered that the S-HMOP can trap cationic dyes from the aqueous solution with vivid color changes through dye fixation. The loaded cationic dyes could not be removed by sonication or washing with water and organic solvents. Anionic dyes such as eosin Y (sodium salt) were not loaded on the S-HMOP. When the HMOP was used instead of the S-HMOP, the dyes were removed after washing. These observations indicate that the dye loading on the S-HMOP is based on ionic interactions between sulfonate and cationic dyes.¹⁶

Fig. 3a shows the cationic dyes used in this study. The analysis of dye-loaded S-HMOP by TEM showed the complete maintenance of hollow morphology (Fig. 2b–h). The surface area was changed from 529 m² g⁻¹ to 233–472 m² g⁻¹ by dye loading (Table 1 and Fig. S4 in the ESI†). Fig. 3b shows the photographs and absorption spectra of dye-loaded S-HMOP, showing visible light absorption properties. The dye loading was in a range of 93–423 μmol g⁻¹, which was calculated by elemental analysis for the N contents in the materials. Compared with the loading amount (0.48–67 μmol g⁻¹) of dyes in non-porous polymer or silica in the literature,¹⁷ the dye-loaded S-HMOP showed a very efficient fixation due to the porosity, hollow structure and thin shell of supports. The loading amount of dyes in S-HMOP increased with a decrease in dye size.

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

Table 1 Properties of materials in this work

Material	Surface area	Pore volume^a	Dye loading^b
	(m^2 g^−1)	(cm^3 g^−1)	(μmol g^−1)
a Values at P/Po = 0.97.b The amount of dye loading is based on the elemental analysis for N contents in materials.
HMOP	733	0.72	—
S-HMOP	529	0.54	—
RhB-S-HMOP	443	0.42	178
MB-S-HMOP	233	0.39	423
AYG-S-HMOP	326	0.43	282
RuBPy-S-HMOP	363	0.41	181
TPP-S-HMOP	411	0.54	147
CuTPP-S-HMOP	456	0.50	110
ZnTPP-S-HMOP	472	0.52	93

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As described in the introduction section, the dyes in this work have been used as photocatalysts in homogeneous systems.³ The introduction of sulfonic acid and dyes to HMOP resulted in the hydrophilic nature of materials (Fig. S5 in the ESI†). Thus, we tested the dye-loaded S-HMOP materials as a heterogeneous system for 4-chlorophenol decomposition in water under visible light irradiation. Fig. 4 summarizes the results.

Fig. 4 (a) Photocatalytic and (b) recycle performance (4 h for each run) for the decomposition of 4-chlorophenol by dye-loaded S-HMOP. (c) Photocatalytic process. [4-Chlorophenol]_o = 1 mM (25 mL), dye: 5 mol% of 4-chlorophenol, pH_initial = 5.0, 200 W Xe lamp (4.6 mW cm⁻² intensity) with a cutoff filter (>420 nm), room temperature. For the recycle tests, 5 mol% TPP, 5 mol% ZnTPP and 10 mol% RhB on S-HMOP were used.

The 5 mol% of dyes in the S-HMOP was used for the photocatalytic decomposition of 4-chlorophenol. Without the dye materials, 4-chlorophenol was not decomposed under visible light irradiation (Fig. 4a). The dye-loaded S-HMOP materials showed a dye-dependent performance. As shown in Fig. 4a, the order of activities was methylene blue (MB) < Cu-tetrapyridium porphyrin (CuTPP) < acridine yellow G (AYG, k_obs = 0.16 h⁻¹) < rhodamine B (RhB, k_obs = 0.18 h⁻¹) < Ru(bpy)₃²⁺ (RuBPy) < Zn-tetrapyridium porphyrin (ZnTPP, k_obs = 0.44 h⁻¹) < tetrapyridium porphyrin (TPP, k_obs = 0.71 h⁻¹)-loaded S-HMOP.¹⁸ This order can be rationalized by the ¹O₂ generation ability of the dyes in the systems. For the organic dyes and porphyrins, a similar trend of photocatalytic activities was observed.¹⁹ In the literature, porphyrin dyes have been fixed on silica for the development of heterogeneous photocatalytic systems.^17a The system showed ∼85% conversion of 4-chlorophenol after 4 h using 50 mol% porphyrin dyes and 450 W Xe lamp (>420 nm).^17a In comparison, the ZnTPP-loaded S-HMOP showed superior performance, with 86% decomposition of 4-chlorophenol after 4 h using 5 mol% dyes and 200 W Xe lamp (>420 nm). The enhanced activity of ZnTPP-loaded S-HMOP can be attributed to the porosity and very thin (∼9 nm shell thickness) hollow structure of the materials, which can maximize the performance of the active dye species.²⁰

It has been well recognized that most organic dyes decompose gradually in the photocatalytic process by self-oxidation.³ As shown in Fig. 4b, the organic dyes on the S-HMOP such as RhB and TPP decomposed gradually in successive runs. Even in these cases, according to the UV/visible absorption spectroscopy of solution, the colored materials were not etched to solution. The ZnTPP on the S-HMOP showed significant maintenance of activities during three runs, which is attributable to the enhanced stability of porphyrin against self-oxidation through metallation.²¹

The decomposition process of 4-chlorophenol by homogeneous photocatalysts has been suggested to be based on ¹O₂ mediated oxidative dechlorination (Fig. 4c).^17c,22 In the photocatalytic performance of ZnTPP on the S-HMOP, the generation of ¹O₂ species and benzoquinone were directly detected by emission (emission of ¹O₂ at 1268 nm, Fig. S5 in the ESI†) and UV/visible absorption spectroscopy (absorption of benzoquinone at 245 nm), respectively, suggesting that the photocatalytic decomposition in this work follows the suggested process in the literature.^17c

In conclusion, this work shows that MOP chemistry can be applied for the heterogenization of photocatalysts. The MOP materials were engineered as hollow spheres using a silica template. Sulfonated HMOP obtained by post synthetic modification showed excellent performance as a platform for the heterogenization of photocatalysts. Among the various dyes tested, Zn-porphyrin based heterogeneous system showed promising activity and stability. We believe that more various heterogeneous 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) through the National Research Foundation of Korea and by grants no. 10047756 (Technology Innovation Program) funded by the Ministry of Trade, Industry & Energy of Korea.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Experimental details, low magnification TEM images, PXRD patterns, N₂ sorption isotherms of materials, singlet oxygen detection in the photocatalytic reaction by ZnTPP loaded S-HMOP, photocatalytic activity of ZnTPP loaded nonhollow S-MOP, and analysis of the recovered ZnTPP/S-HMOP. See DOI: 10.1039/c5ra06633f

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