Photocatalytic decolourization of a new water-insoluble organic dye based on phenothiazine by ZnO and TiO₂ nanoparticles

2.1 General

Reagent grade chemicals purchased from Sigma-Aldrich and used without further purification. Aqueous solutions of the dye were prepared by using distilled water. ¹H and ¹³C NMR spectra were measured on a Bruker Avance 600 MHz spectrometer using in CDCl₃-d₆ solution. Infrared spectra and mass spectrometry were measured on a PerkinElmer spectrum 100 FTIR spectrometer and Agilent GC 7000 mass spectrometer, respectively. LC-mass spectrometry was performed using Agilent LC 6320 Ion Trap Mass Spectrometer. UV–Vis absorption spectra were determined in DMF/water (90/10, v/v) on Shimadzu UV–Vis Spectrophotometer. The melting points were measured with a Stuart Scientific melting point apparatus and uncorrected. ZnO nanoparticle (particle size < 100 nm) and TiO₂ (P25) of particle size < 25 nm were purchased from Sigma-Aldrich.

2.2 Synthesis of compound I

Into a round-bottomed flask containing a mixture of phenothiazine (2.4 g, 12 mmol), 1-bromodecane (4.48 g, 18 mmol), and KI (catalytic amount) in 50 ml DMSO, KOH (2 g, 35.71 mmol) was added portion-wise with stirring at room temperature. The reaction was completed after 5 h, as judged by the TLC, then 250 ml of water was added, and the organic product gets extracted with CHCl₃ (4 × 40 ml). The combined extracts were collected, washed with 20% (w/w) NH₄Cl (50 ml), and water. Drying the combined extracts with anhydrous Na₂SO₄, filtrated and finally dried in vacuo. Column chromatography (SiO₂) of the crude product using hexane as eluent gives I as oil product, TLC System: Petroleum ether: Ethyl acetate (PE/EtOAc, 8/2), Yield 78%, ¹H NMR (600 MHz, CDCl₃) δ 0.90 (3H, t, J = 7.2 Hz, CH₃) ,1.3 (12H, m, CH₂), 1.45 (2H, m, CH₂), 1.82 (2H, m ,CH₂), 3.86 (2H, t, J = Hz, N-CH₂), 6.90 (2H, d, J = 7.8 Hz, Ar-H), 6.93 (2H, t, Ar-H), 7.16 (2H, d, J = 7.2 Hz, Ar-H), 7.18 (2H, t, H-Ar). ¹³C NMR (125 MHz, CDCl₃) δ 14.33, 22.89, 27.12, 27.18, 29.47, 29.50, 29.70, 29.76, 29.93, 32.10, 47.73, 115.68, 122.57, 125.12, 127.38, 127.63, 145.47 FTIR, $\nu$ , cm⁻¹: Aliphatic and olefinic C–H appears at 2922, 2852 and 3064, respectively. The peaks at 1594, 1572, 1457 are due to C⚌C stretch vibrations.

2.3 Synthesis of compound II

A cooled POCl₃ (15 ml) was dropwise into an ice-cooled DMF (40 ml) at 0 °C with stirring. Afterward, the reaction temperature raised gradually to room temperature, where the reaction continued for 90 min with stirring to be completed. The flask of the reaction was cooled to 0 °C, and compound I (66 mmol) was added, followed by removing the ice-bath to allow the reaction mixture to get warm gradually to reach 80 °C, where it kept running with stirring for 2 h. The mixture was then quenched after being cooled and mixed with ice–water and basified (sat. aqueous K₂CO₃ solution). The product was extracted from the mixture with CHCl₃ (4 × 40 ml) washed with brine and water. Drying the combined extracts with anhydrous Na₂SO₄, filtrated and finally dried in vacuo. Column chromatography (SiO₂) of the crude product (eluent: PE/EtOAc, 8:2) on silica gel gives a yellow solid product. Yield 91%. ¹H NMR (600 MHz, CDCl₃) δ 3.91 (t, 3H, N-CH₂), 6.91 (d, 1H, J = 8.4 Hz, Ar-H), 6.92 (d, 1H, J = 9.0 Hz, Ar-H) , 6.99 (td, 1H, J = 7.2 Hz, Ar-H) , 7.14 (dd, H, J = 7.8, 1.2 Hz) ,7.19 (td, 1H, J = 7.5, 1.2 Hz, Ar-H) , 7.61 (d,1H, J = 1.8 Hz, Ar-H) , 7.67 (dd, 1H, J = 8.4, 1.8 Hz, Ar-H) , 9.82 (s, 1H, CHO). ¹³C NMR (125 MHz, CDCl₃) δ 14.30, 22.82, 26.96, 27.03, 29.35, 29.39, 48.23, 115.01, 116.16, 123.78, 124.04, 125.25, 127.76, 127.78, 128.64, 130.27, 131.26, 143.67, 150.99, 190.26. FTIR, $\nu$ , cm⁻¹: Aliphatic and olefinic C—H bands appear at 2923, 2852 and 3061, respectively. The peaks at 2724 and 1686 are due to C—H and C⚌O stretching vibration of aldehyde group, respectively.

2.4 Synthesis of 2-((10-decyl-10H-phenothiazin-3-yl)methylene)malononitrile III

A mixture of malononitrile (0.2 g, 3 mmol) and compound II (1.1 g, 3 mmol) and in 7 ml of basic ethanol solution (composed from mixing 1:3:96 by volume of piperidine, glacial acetic acid, and absolute ethanol, respectively) was reacted overnight with stirring at room temperature, filtered off and recrystallized from ethanol to produce dye III, Yield 52%, M.P. 65–57 °C. ¹H NMR (600 MHz, CDCl₃) δ 3.91 (t, 2H, N-CH₂), 6.88 (d, 1H, J = 9.0 Hz, Ar-H), 6.91 (d, J = 8.4 Hz, Ar-H), 7.01 (td, 1H, J = 7.8 Hz, Ar-H), 7.11 (dd, 1H, J = 7.5, 1.2 Hz, Ar-H) , 7.20 (td, 1H, J = 7.8, 1.8 Hz, Ar-H) , 7.51 (s, 1H, H-C⚌C), 7.57 (d, 1H, J = 2.4 Hz, Ar-H), 7.78 (dd, 1H, J = 9.0, 2.4 Hz, Ar-H) ¹³C NMR δ 14.33, 22.88, 26.90, 26.98, 29.36, 29.47, 29.66, 29.72, 32.08, 48.42, 113.76, 114.88, 115.12, 116.28, 123.21, 124.38, 125.20, 125.39, 127.84, 128.00, 129.75, 131.55, 142.69, 151.05, 157.54.

FTIR, $\nu$ , cm⁻¹: Aliphatic C—H bands appear at 2923, 2851. The peak at 2224 is due to CN group and the bands at 1600, 1581, 1563 are due to C⚌C. MS (ESI): M⁺; found: 415.1, C₂₆H₂₉SN₃ requires 415.6.

2.5 The photocatalytic method

The photocatalytic decolorization of the dye was made following the method reported in the literature (Hamdy et al., 2014). In this method, black UV light (λ > 367 nm) was used as the light source for the irradiation of the sample. The dye sample aqueous solution (50 ml, 8.02 × 10⁻⁵ M) was prepared by dilution of a concentrated DMF solution of the dye in DMF/water, 1/9 v/v. Two catalysts, namely TiO₂ (P25) and ZnO, were used for the photocatalytic decolorization of the dye. Typically, the catalyst (TiO₂ (P25) or ZnO, 100 mg) and 50 ml of the aqueous dye solution (8.02 × 10⁻⁵ M) were mixed in a beaker. The sample was subjected to sonication for about 10 min and then placed into the photoreactor. Dark chemisorption of the catalyst was firstly allowed by stirring the samples (700 rounds per minute (rpm)) for 15 min at room temperature. Afterward, an aliquot of the dye sample was taken, and the remaining dye solution was subjected to the irradiation. For the kinetic studies, several aliquots were taken every 10 min, filtered (0.2 mm PTFE Millipore membrane filter) to remove the catalyst, and finally analyzed optically at the maximum wavelength of absorption of the dye at 538 nm to determine the remaining concentration of the dye (Fig. 1-a). Assuming that the photocatalysis proceeds via first-order kinetics, then the reaction rate constant (K) can be obtained from the following equation:

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Fig. 1

(Top left) the UV–Vis spectra obtained as a results of 2-((10-decyl-10H-phenothiazin-3-yl)methylene)malononitrile dye degradation over TiO₂ (P25) under UV–Vis illumination. (Top right) the degradation profile of 2-((10-decyl-10H-phenothiazin-3-yl)methylene)malononitrile dye over TiO2 (P25), ZnO, and without catalyst (photolysis). Bottom left) the kinetics of the 2-((10-decyl-10H-phenothiazin-3-yl)methylene)malononitrile dye degradation. Bottom right) the first order rate constants of the 2-((10-decyl-10H-phenothiazin-3-yl)methylene)malononitrile dye degradation.

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ln(C₀/C) = −Kt, where C₀ and C are the dark chemisorption concentration and the concentration at time t, respectively. K is the reaction rate constant. Thus, plotting ln(C₀/C) versus t results in a straight line with a slope equal –K. Two or three runs of photocatalysis were repeated using a fresh catalyst, and the standard deviation obtained was about 5%.

3.1 Synthesis

The synthesis of dye III was made, as shown in Scheme 1. Thus, the N-alkylation of phenothiazine via SN2 reaction afforded the corresponding product I, which underwent Vilsmeier formylation reaction to give the corresponding aldehyde II in high yield. Condensation of compound II with malononitrile via Knoevenagel reaction afforded the corresponding dye III in good yield. The chemical structure of these synthesized compounds was confirmed by IR, NMR, and high-resolution mass spectra (see supplementary file).

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Scheme 1

The synthetic route of the dye.

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3.2 Photocatalytic studies

The photocatalytic decolourization of water-insoluble dyes is not common, and to the best of our knowledge, as yet, no report for such an approach has appeared. It is believed that the success in the decolorizing organic-based hydrophobic dye, such as the presented one, would pave the way for the decontamination of water-insoluble organic pollutants. Thus, the title dye was subjected to photodecolorization from aqueous solution (1:9, DMF/water v/v). It was observed that at zero time (dark chemisorption), the dye concentration remains almost constant, indicating the absence of chemisorption. Having been informed by that, it was necessary to study whether TiO₂ (p25) would catalyze the decolourization of the dye. Fig. 1 top (right) reveals the gradual decreases in the dye absorption upon irradiation time to reach almost complete decolourization after 30 min. Fig. 1 top (left) reveals the time curve of the decolourization of the dye in the absence of catalyst and in the presence of TiO₂ (p25) and ZnO. It is clearly observed that the dye is almost stable during all tested irradiation time in the absence of catalysts, whereas in the presence of catalysts, TiO₂ (p25) shows superior efficiency at a higher rate than ZnO. Analyzing the data, as shown in Fig. 1 bottom (left) and bottom (right), the rate using TiO2 (0.095 min⁻¹) is higher than the rate constant using ZnO (0.055 min⁻¹). The effect of solution pH on the rate of photocatalytic decolourization of the dye was performed at three different pHs of 5, 7, and 9. The colour of the dye did not change, and the obtained first-order ate constant did not change a lot (only negligible difference), indicating that the pH, although it affects the catalysts but has no effect on the organic-based hydrophobic dye and thus no effect on the photocatalytic process. The results of pH effect are presented in the supplementary file.