Photocatalytic Degradation of Dyes Using TpPa-COF-Cl₂ Membrane ^†

Abstract

Covalent organic frameworks (COFs) are photocatalysts composed of covalent bonds of light elements and free of toxic metals. COFs are highly active against dyes. Furthermore, we aimed to improve the utility of COFs by making them into membranes. In this study, by utilizing the cross-linked structure of calcium alginate, we succeeded in forming the photocatalyst TpPa-COF-Cl₂ into a membrane without destroying its structure. This was confirmed by characterization such as FT-IR. In addition, methyl orange was decolorized at 450 nm, confirming the photocatalytic activity of the membrane.

1. Introduction

Due to the growth of the manufacturing industry, the consumption of organic dyes is continuously increasing. The discharge of waste dyes generated during these processes poses significant risks to both the environment and human health [1,2]. Degradation of organic pollutants using solar energy has emerged as a more environmentally friendly approach.

Covalent organic frameworks (COFs), composed of light elements linked by covalent bonds, are free of toxic metals and exhibit excellent stability in aqueous environments. Among various COFs, the β-ketoenamine-linked TpPa-COF, synthesized from hydroxy aldehydes and amines, has attracted attention due to its high chemical stability, tunable porosity, and good crystallinity [3]. However, its application in the removal of organic pollutants has not been extensively studied. Photocatalyst films can be used under a variety of conditions because they are easier to collect and do not require stirring, compared to powdered forms. However, forming photocatalyst films usually requires advanced technology. There have been no attempts to use COFs as photocatalyst films made with a simple and inexpensive manufacturing process.

In this study, the photocatalyst TpPa-COF-Cl₂ was fabricated into a membrane to evaluate its photocatalytic activity toward dye degradation. We prepared a film from the powdered COF to improve its reusability and enhance its potential for practical applications. The resulting membrane exhibited photocatalytic activity by decolorizing methyl orange [4].

2. Materials and Methods

1,3,5-Triformylphloroglucinol, 2,5-Dichloro-p-phenylenediamine, and 1,3,5-Trimethylbenzene were purchased from the Tokyo Chemical Industry Co. (Tokyo, Japan). 1,4-dioxane and sodium alginate were obtained from Fujifilm Wako Pure Chemical Co. (Osaka, Japan). Acetic acid, methyl orange (MO) and CaCl₂ were purchased from Nacalai Tesque, Inc. (Kyoto, Japan).

Mesitylene (4.5 mL), 1,4-dioxane (4.5 mL), acetic acid (3 M 1.5 mL), 1,3,5-trifluoro glycinol (Tp) and 2,5-dichloro-p-phenylenediamine (Pa) (molar ratio = 1:1.5) in a Schlenk tube were sonicated for 30 min. The mixture was heated and stirred (24 h, 120 °C) in a hot stirrer while cooling in a Jim Roth tube, washed with acetone, and dried to give TpPa-COF-Cl₂ powder [3].

Sodium alginate solution was prepared by placing sodium alginate (0.05, 0.1, 0.11 g) in 1.7 mL of water. TpPa-COF-Cl₂ (20, 40 mg) was added to 1.7 mL of water to prepare a TpPa-COF-Cl₂ dispersion solution. The mixture was stirred at 50 °C for 30 min. After 5 min of sonication, the mixture was allowed to come to room temperature. It was spread thinly on glass and soaked overnight in CaCl₂ solution (3 wt%). The membrane was removed from the glass and washed with water [5,6].

The membrane was submerged in a 5 ppm concentration of methyl orange (MO) solution. It was left in the dark for 30 min to obtain adsorption equilibrium. The reaction system was irradiated from the bottom with a 450 nm LED lamp for 240 min, and 2 mL of supernatant was separated from the reaction system every 60 min. The absorbance of it was then measured.

3. Results and Discussion

3.1. Characterization of TpPa-COF-Cl₂ Membrane

3.1.1. X-Ray Photoelectron Spectroscopy (XPS)

X-ray photoelectron spectroscopy (XPS) allows analysis of the composition and chemical bonding states of solid surfaces. The binding energy of C 1s was determined to be 284.8 eV (Figure 1).

The XPS spectra of the COF membrane exhibited four elemental signals: O1s, C1s, Ca2p and N1s (Figure 1a). The spectrum of C1s, peaks corresponding to C-C/C=C (284.8 eV), C-O (287.3 eV), C-N (289.0 eV), C=O (286.4 eV) and C=Cl (286.1 eV) were observed(Figure 1b). The observed peaks were assigned to specific chemical bonds as follows: C–N and C=Cl correspond to the COF structure, C=O is attributed to calcium alginate, and C–C/C=C and C–O are associated with both materials [7]. The spectrum of O1s, peaks corresponding to C-O (534.1 eV), C=O (530.8 eV) were observed. The peak at 532.6 eV corresponds to adsorbed oxygen species (Figure 1c). Ca peaks were observed, indicating the catalytic incorporation of calcium into the cross-linked structure of calcium alginate (Figure 1e).

3.1.2. Fourier Transform Infrared Spectroscopy

Intramolecular bonding was analyzed by Fourier transform infrared spectroscopy (FTIR) (Figure 2). The peaks observed at 1250 cm⁻¹, 1500–1590 cm⁻¹, and 1620 cm⁻¹ are attributed to the COF structure. The disappearance of the aldehyde peak from Tp and the amino peak from Pa suggests the formation of β-ketoenamine bond [8].

The peaks at 1419 cm⁻¹ and 1591 cm⁻¹ correspond to the asymmetric and symmetric stretching vibrations of the carboxylate groups (-COO-) in calcium alginate [9].

3.1.3. Scanning Electron Microscopy

SEM analyses was conducted to investigate the surface morphology of each catalyst [6]. The powder-form COF showed a layered structure of nanorods (Figure 3a), whereas the film-form COF showed a layered structure (Figure 3b,c). This is thought to be because calcium alginate completely covered the powder COF.

3.2. Decolorization Experiment

First, the effect of the amount of sodium alginate on the dye decolorizing effect was investigated (Figure 4). When 0.11 g of sodium alginate was used, the viscosity became too high and the mixture stuck to the container and solidified during heating and stirring. Comparing 0.05 g and 0.1 g additions, decolorizing performance was comparable. However, when 0.05 g was added, the strength of the film was low, making it difficult to handle. Therefore, the optimal amount of sodium alginate added was 0.1 g.

Next, the effect of the amount of COF used during membrane synthesis on the decolorizing effect was investigated(Figure 5). The COF film decolorized about 15% of the methyl orange in 4 h. Increasing the amount of COF did not improve the decolorization performance. The film that did not contain COF showed no decolorization at all, indicating that the COF film has photocatalytic activity.

The membrane prepared using 20 mg of COF and 0.1 g of calcium alginate was used to decolorize MO. Dye adsorption on the catalyst was examined and found to be almost non-existent (Figure 6a). MO solution without catalyst was subjected to light irradiation, but it was confirmed that light irradiation alone did not decompose MO (Figure 6b).

After 30 min of dark agitation, the MO solution with the COF membrane was irradiated with light. The results showed that most of the MO was degraded in 3 days (Figure 7).

4. Conclusions

TpPa-COF-Cl₂ membrane was successfully synthesized using the cross-linked network structure of calcium alginate. Structural analysis confirmed that the framework of TpPa-COF-Cl₂ was retained after membrane fabrication. Photocatalytic activity of the membrane was confirmed by its ability to decolorize methyl orange. The amount of sodium alginate had little impact on membrane performance and mainly affected the ease of membrane formation. Similarly, the amount of COF added did not significantly influence the photocatalytic activity. These parameters were not found to be critical for the membrane’s function. The photocatalytic activity of the membrane was demonstrated by its ability to decolorize methyl orange, with nearly complete decolorization observed within three days.