Synthesis of 5,10,15,20-tetrakis[4-(naphthalen-2-yloxycarbonyl)phenyl]porphyrin (TNBP) and its complexes with zinc and cobalt and an investigation of the photocatalytic activity of nanoFe₃O₄@ZrO₂–TNBP

Abstract

5,10,15,20-Tetrakis[4-(naphthalen-2-yloxycarbonyl)phenyl]porphyrin (TNBP) and its porphyrin complexes with zinc and cobalt and nanoFe₃O₄@ZrO₂–TNBP photocatalyst were synthesized with high yields and the structure and morphology were characterized by X-ray diffraction (XRD), UV-Vis, FT-IR and ¹H-NMR spectroscopy and scanning electron microscopy (SEM). The photocatalytic activity was investigated by photo degradation of methylene blue in aqueous solution under visible light irradiation. The results demonstrated that porphyrin significantly enhanced the visible photocatalytic activity of Fe₃O₄@ZrO₂ magnetic nanoparticles in the degradation of methylene blue. The findings also demonstrate that Fe₃O₄@ZrO₂–TNBP photocatalysts exhibited higher photoactivity than bare Fe₃O₄@ZrO₂ nanoparticles under light irradiation.

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Introduction

The 4n + 2 aromatic macrocycle porphyrin which includes pyrroles and methine carbons in a square plane is one of the most major pigments that can be found in nature. Because of the enormous variety of properties of porphyrin such as its coordination chemistry, emission capabilities, optical and electronic abilities, a range of porphyrins and conjugated porphyrin arrays have been constructed along the years.^1–4 Porphyrins and derivatives are commonly used components in reactions and have several properties. Catenanes and rotaxanes linked with porphyrins, resulted in new types of compounds with different properties. They incorporate the electronic properties of porphyrins with those of the linked compounds. Photosynthesis in certain bacteria and plants is mainly founded on porphyrinoid pigments. Porphyrinic macrocycles are modulated into arrays in the Polymerase Chain Reaction (PCR), where light energy is finally converted to chemical energy. Also tris and tetrakis-porphyrin arrays can play roles in PCR.⁵ Porphyrins play key roles in numerous synthetic multicomponent reactions, simulating the operation of the photosynthetic reaction centres.⁶ Also, porphyrins have enormous potential as the light resulting component of dye-sensitised nanocrystalline TiO₂ solar cells.⁷

Porphyrins have strong absorptions and can be used to reduce phenol pollutants (by photooxidation of them). Tetrakis porphyrin and its metal derivatives are also strong sensitizers for this action.⁸

In the earth’s history, natural metalloporphyrins were formed early and performed a significant function in transport goals in biological activities; also in redox reactions, metabolism and photosynthesis. Metalloporphyrins are generally appropriate to tailor sensors and catalysts with functionalization of the surface on the nanoscale.⁹

After reviewing the properties of porphyrins and metalloporphyrines¹⁰ and also the importance of the purification contaminated wastewater, the properties of metalloporphyrins have been widely considered for the purification of water.

The entry of organic contaminants into drinking water from industrial waste water containing dyes, caused by an increase in their use causes serious problems. Biological purification of these pollutants due to its limited effectiveness is not appropriate. Special attention has been paid to the removal or degradation of many contaminants, including methylene blue using metalloporphyrin absorbents.

On the other hand, magnetic nanoparticles because of their high capacity load transfer and easy separation by an external magnetic field have attracted the attention of many researchers. Nanoparticles have wide applications in data storage, sensors, catalysts, biotechnology and environmental fields. Superparamagnetic Fe₃O₄ nanoparticles due to high levels of chemical and magnetic dipole interactions are very sensitive to oxidation and aggregation. Different methods are used to protect and prevent their oxidation. For better stabilization of the porphyrin on the substrate, use of ZrO₂ and Fe₃O₄@ZrO₂ is more suitable compared with other organic and inorganic compounds.^10–12

Our research shows that porphyrin with its four pyrrolic rings set on a square surrounding substance (and metalloporphyrin) improved the photocatalytic performance of Fe₃O₄@ZrO₂ magnetic nanoparticles for the degradation of organic pollutants. These findings demonstrate that Fe₃O₄@ZrO₂–TNBP photocatalysts exhibit higher photoactivity than bare Fe₃O₄@ZrO₂ nanoparticles under light irradiation.

Due to these extensive uses of porphyrins, we also drew attention to this compound and we decided to synthesis the metalloporphyrin.^13,14 In 2014, Xiangqing Li and coworkers reported the synthesis of a silanized porphyrin cobalt monomer¹⁵ and we used their method to synthesize 5,10,15,20-tetrakis[4-(naphthalen-2-yloxycarbonyl)phenyl]porphyrin and its metal complexes with zinc and cobalt. We synthesized Fe₃O₄@ZrO₂–TNBP as photocatalyst and used it as a substrate of Zn and Co porphyrin. Its photocatalytic efficiency on the degradation of methylene blue under visible light irradiation was examined and the results indicated that this porphyrin can greatly enhance the visible photocatalytic activity of Fe₃O₄@ZrO₂ magnetic nanoparticles on the degradation of methylene blue. We found that electron transfer from porphyrin to Fe₃O₄@ZrO₂ magnetic nanoparticles plays a strategic role in the photocatalytic processes of the Fe₃O₄@ZrO₂–TNBP system and thus leads to an increase the photocatalytic activity.

Experimental

Materials and methods

All of the chemicals used in this work were obtained from Merck and used without further purification.

¹H-NMR spectra were recorded with a Bruker Avance 500 MHz, in chloroform with tetramethylsilane (Me₄Si) as an internal standard. Infrared (IR) spectra were recorded on a Shimadzu FT-IR-8400S spectrophotometer using a KBr pellet. The DRS spectra were recorded via a Shimadzu MPC-2200 spectrophotometer. The UV-Vis spectra were recorded on a Shimadzu (mini 1240 double beam) spectrophotometer in the wavelength range of 400–800 nm. The particle morphologies were observed by scanning electron microscopy (SEM) at 26 kV (KYKY-EM3200). The X-ray diffraction measurement was performed using graphite monochromatic copper radiation (Cu Kα) at 40 kV, 40 mA over the 2θ range of 5–80°. Magnetic properties of the particle were assessed with a vibrating-sample magnetometer (VSM, Lake Shore 7410). A commercial HH-15 model vibrating sample magnetometer was utilized for the collection of magnetic particles.

Synthesis of 5,10,15,20-tetrakis[4-(naphthalen-2-yloxycarbonyl)phenyl]porphyrin

In the first step, TCPP (0.1 g, 0.1 mmol) was dissolved in THF (10 mL). Then thionyl chloride (1 mL) was added drop wise to this solution, and after the addition was completed, the solution was stirred at 70 °C for 2 h. After this the excess thionyl chloride was removed by rotary evaporation, and the remaining material was dissolved in THF (10 mL) and then β-naphthol (0.06 g, 0.4 mmol) was added into this solution. The mixture was refluxed at 70 °C for 8 h. Finally, TNBP was obtained after filtration. IR (KBr): ν_max = 3278, 2925, 2860, 1716, 1272, 808, 646 cm⁻¹. UV-Vis: λ_max = 420 nm (Soret band), 514, 549, 589, 645 nm (Q bands).

Synthesis of Zn(II) 5,10,15,20-tetrakis[4-(naphthalen-2-yloxycarbonyl)phenyl]porphyrin (ZnTNBP)

Zn(CH₃COO)₂ (0.1 g, 6 mmol) and TNBP (0.15 g, 1 mmol) were dissolved in 50 mL DMF and refluxed for 3 h. After the solution was cooled to room temperature, the precipitate was collected by filtration, washed with water and dried under vacuum. IR (KBr): ν_max = 2923, 2854, 1716, 1272, 802, 713 cm⁻¹. UV-Vis: λ_max = 420 nm (Soret band), 513, 547 nm (Q bands). ¹H-NMR (500 MHz, CHCl₃): δ in ppm = 8.64–8.43 (s, 8H, CH pyrrole), 8.42–8.2 (m, 28H, CH β-naphthol), 8.02–8.1 (d, 8H, CH Ar), 7.72–7.64 (d, 8H, CH Ar).

Synthesis of Co(II) 5,10,15,20-tetrakis[4-(naphthalen-2-yloxycarbonyl)phenyl]porphyrin (CoTNBP)

For the synthesis of CoTNBP, a mixture of CoCl₂ (0.09 g, 6 mmol) and TNBP (0.09 g, 1 mmol) was dissolved in 25 mL DMSO and the solution was refluxed at 190 °C for 24 h.¹⁶ A brown precipitate was obtained, which was filtered and then purified by column chromatography and dried under vacuum. IR (KBr): ν_max = 2925, 2856, 1716, 1271, 811, 640 cm⁻¹. UV-Vis: λ_max = 428 nm (Soret band), 548 nm (Q band).

Preparation of nanoFe₃O₄@ZrO₂–TNBP photocatalyst

Fe₃O₄@ZrO₂–TNBP photocatalyst was obtained by mixing TNBP (0.01 g) and Fe₃O₄@ZrO₂ nanoparticles (0.1 g). The mixture was dissolved in 50 mL DMF and the solution stirred and refluxed for 6 h. The solid product was collected by filtration, washed with water and dried under vacuum.

Photocatalytic degradation

For 30 mL of the MB aqueous solution (C₀ = 72 mol L⁻¹), 30 mg of Fe₃O₄@ZrO₂–TNBP photocatalyst was used. Before irradiation, the suspension was stirred in dark conditions for 30 min. Then, in the presence of irradiation, samples were taken every 30 min for 4 h. The photocatalytic degradation of the samples was recorded by UV-Vis spectrum.

Result and discussion

According to Scheme 1, tetrakis[4-(naphthalen-2-yloxycarbonyl)phenyl]porphyrin (TNBP) and its complexes with zinc and cobalt were synthesized and characterized by UV-Vis and FT-IR spectra.

Scheme 1 Synthesis of (1) ZnTNBP, (2) CoTNBP.

FT-IR spectra analysis

The FT-IR spectra in KBr were recorded in the range of 400–4000 cm⁻¹. As shown in Fig. 1, the carbonyl vibration band C=O of the ester group appears as an intense band at 1716 cm⁻¹ and indicates that the respective TNBPs were successfully synthesized. The N–H bending vibration band appears at 3200 cm⁻¹ that confirms the pyrrole ring. C–H stretching of Ar group vibrations appears at 2900 cm⁻¹. Moreover the disappearance of the N–H vibration around 3200 cm⁻¹ in Fig. 2 shows that the related metalloporphyrins were also quantitatively prepared.

Fig. 1 FT-IR spectrum of TNBP.

Fig. 2 FT-IR spectra of metalloporphyrins.

UV-Vis spectra analysis

The UV-Vis absorption spectra of TNBP and its metal complexes in DMF shown in Fig. 3 indicate that the Soret bands of TNBP and ZnTNBP appear in the region of 420 nm. Four weaker absorptions which relate to Q bands appear at higher wavelengths (about 500–650 nm). The coordination of TNBP with zinc and cobalt reduces the intensity of the Soret band and the number of the Q bands from 4 bands to 2 bands, which indicates that metalloporphyrins are produced and due to their symmetry, weak bands reduced. Because of the d¹⁰ orbitals of Zn, there is very little effect on the gap energy transfer of π → π* in the porphyrin ligand. This makes the Soret and Q bands of porphyrin (TNBP and ZnTNBP) appear at around the same area.

Fig. 3 UV-Vis absorption spectra of TNBP and metalloporphyrin in DMF.

DRS analysis

As shown in the DRS spectra in Fig. 4, we find that there is no absorption above 400 nm for Fe₃O₄@ZrO₂, while the supported catalyst shows the characteristic peaks of the Soret band and Q bands which makes it apparent that the porphyrin is successfully established onto the Fe₃O₄@ZrO₂ nanoparticle surface maintaining the porphyrin structure.

Fig. 4 DRS spectra of Fe₃O₄@ZrO₂/porphyrin.

XRD study

The magnetic nanoparticles sensitized by ZnTNBP porphyrin, were analyzed by XRD (Fig. 5). Since porphyrins are amorphous and magnetic nanoparticles are crystalline, the XRD pattern does not show the exact structure of the porphyrin. As can be seen, porphyrins reduce the intensity of the nanoparticles peaks, but did not cause a change in the crystal structure of the Fe₃O₄@ZrO₂ nanoparticles. Peaks appearing at 30.9 and 44.35 degrees are from Fe₃O₄ and peaks appearing at 21.28, 5.31, 17.34, 19.50 and 1.60 degrees are related to the structure of ZrO₂. According to this data, the structure of the nanoparticle is monoclinic that is corresponding to the ZrO₂ standard card (01-083-0944) and Fe₃O₄ standard card (01-085-1436).

Fig. 5 XRD pattern of Fe₃O₄@ZrO₂–ZnTNBP.

SEM study

From SEM images, morphology, particle size and surface uniformity can be identified. SEM images of Fe₃O₄@ZrO₂ and Fe₃O₄@ZrO₂–ZnTNBP nanoparticles are shown in Fig. 6. The sizes of the nanoparticles are between 37 to 61 nm. According to the pictures, the nanoparticles are spherical and have a good uniformity. As can be seen, the porphyrin has not created a change in the monoclinic morphology of the nanoparticles.

Fig. 6 SEM images of nanoparticles of (a) Fe₃O₄@ZrO₂ and (b) Fe₃O₄@ZrO₂–ZnTNBP.

VSM spectrum of Fe₃O₄@ZrO₂–ZnTNBP

From the results of the analysis of a vibrating sample magnetometer spectrum, which are given in Fig. 7, it is estimated that the magnetic nanoparticles superparamagnetic behavior was reduced after their composition with porphyrin.

Fig. 7 VSM spectra of Fe₃O₄@ZrO₂–ZnTNBP.

Investigation of the photodynamic degradation of methylene blue (MB)

To evaluate and compare the use of the synthesized photocatalysts, degradation of methylene blue was performed at a concentration of 10 ppm in the presence of light (lamp LED 5 W). Results after three hours of exposure by UV-Vis spectra were recorded on a chart. As can be seen in Fig. 9, porphyrins with different functional groups on the substrate nanoparticles show different degradation. The degradation ratios are given in the Table 1.

Fig. 8 Schematic of the photocatalytic mechanism.

Table 1 Degradation percentages

Catalyst	Degradation percentage
No catalyst	10
Fe3O4@ZrO2	24
Fe3O4@ZrO2/TNBP	57
Fe3O4@ZrO2/TCPP	60
Fe3O4@ZrO2/ZnTNBP	65
Fe3O4@ZrO2/CoTNBP	73
Fe3O4@ZrO2/ZnTCPP	95
Fe3O4@ZrO2/CoTCPP	98

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Fig. 9 Absorption spectrum of the destruction of MB (10 mL, 10 ppm) after 3 h irradiation by 10 mg catalyst in room temperature.

The phenyl rings of TNBP were attached by π bond and van der Waals to the surface of the magnetic nanoparticles of zirconia iron oxide and by the effect of ring resonance, the electron transfer action is performed better and results in an increase of the degradation of methylene blue into nanoparticles. A greater number of phenyl rings leads to increased resonance and electron transfer and increased destruction of pollutants. But tetra(4-carboxyphenyl)porphyrin is better than TNBP at inclusion on the nanoparticle surface due to the establishment of hydrogen bonds by the carbonyl group and the hydroxyl group with the surface area of the nanoparticles. As a result, the electron transfer is performed easier and better, and the degradation of methylene blue to TNBP increased.

Metals in the center of the tetrapyrrole rings of the porphyrin increased the photocatalysis activity compared to the free porphyrin. Metalloporphyrins with electron transfer (from the metal to the ligand) enhance catalyst activity and the degradation percentage of methylene blue.

Photocatalysis process

It is generally believed that the photocatalytic activity of ZrO₂ is only in the range of UV light due to a broad E_g of about 5 eV.

So, being coated on the surface of Fe₃O₄ magnetic nanoparticles, with a low band gap of approximately 1.6 eV or lower can create a material that can effectively absorb sunlight. The presence of porphyrin improved the photocatalytic performance of Fe₃O₄@ZrO₂ magnetic nanoparticles.

As be seen in Fig. 8, under visible light irradiation, the photocatalytic process of Fe₃O₄@ZrO₂–porphyrin was sensitised and with a photon transition, an electron converted from the ground state to the excited state of porphyrin and creates positive and negative charges. The generated electron not only transferred to the porphyrin surface but also injected into the conduction band of ZrO₂, owing to the CB level of ZrO₂ being near the CB of porphyrin. On the other hand, the electrons in the valence band (VB) of ZrO₂ are preferentially excited to its conduction band (CB) under irradiation and therefore generate an equal amount of holes in its VB. The photogenerated holes transfer from the VB of ZrO₂ to the VB of porphyrin. Therefore, the probability of electron–hole recombination can be reduced.

The Fe₃O₄ superparamagnetic nanoparticles can be separated from the suspension by an external magnetic field due to the magnetic dipole interactions.

The accumulation of electrons in the CB of ZrO₂ can react with molecular oxygen to produce ˙O₂. Similarly, the holes on the porphyrin surface can attract electrons from water and hydroxyl ions from porphyrin to generate ˙OH through an oxidative process. These produced radicals can effectively destroy the methylene blue^17–19 (eqn (1)–(6)). There for the modification of Fe₃O₄@ZrO₂ surface can improve the catalytic properties.²⁰

[Fe₃O₄@ZrO₂ − TNBP] + hυ → [Fe₃O₄@ZrO₂ − TNBP]* (1)

[Fe₃O₄@ZrO₂ − TNBP]* → Fe₃O₄@ZrO₂[TNBP]⁺ + e_CB⁻ (2)

e_CB⁻ + O₂ → O₂˙⁻ (3)

h_VB⁺ + OH⁻ → OH˙ (4)

O₂˙⁻ + MB → oxidation products (5)

OH˙ + MB → oxidation products (6)

Kinetic diagram

In order to investigate degradation by the photocatalyst, synthetic graphs were drawn based on the reaction rate obtained by the following equation:

D = [C₀ − C/C₀] × 100

C₀ is the initial uptake of contaminant in the absence of photocatalyst and C is the adsorption after the exposure time. Samples were taken in the first 30 minutes in the dark without light, and the equilibrium adsorption–desorption is achieved. After the measurement is completed in the presence of light, photocatalytic degradation is finished. As seen in the Fig. 10, a small percentage of degradation of methylene blue occurs in the absence of catalyst. Nanoparticles of zirconia iron oxide catalyst show a small amount of contaminant degradation. But a strong role of porphyrin in the photocatalytic degradation is seen. CoTNBP and ZnTNBP deposition on the nanoparticles of the substrate due to increased electron transfer reaction increases the efficiency of degradation. In this chart, the highest percentage of degradation is for catalyst particles sensitized with CoTCPP, which due to its better stabilization on the surface, can almost completely destroy the methylene blue (Fig. 11).

Fig. 10 Kinetic diagram of MB (30 mL, 10 ppm) degradation under vs. irradiation time by 30 mg catalyst.

Fig. 11 Degradation percentage of MB (10 ppm) using 10 mg of (1) no catalyst (2) Fe₃O₄@ZrO₂, (3) Fe₃O₄@ZrO₂/TNBP, (4) Fe₃O₄@ZrO₂/TCPP, (5) Fe₃O₄@ZrO₂/ZnTNBP, (6) Fe₃O₄@ZrO₂/ZnTCPP, (7) Fe₃O₄@ZrO₂/CoTNBP, (8) Fe₃O₄@ZrO₂/CoTCPP.

Catalytic stability

After completing the degradation with the photocatalyst, the catalyst was separated and washed several times with acetone and no change was seen in its structure. FT-IR and DRS spectra of catalyst confirmed the presence of porphyrin on the catalyst (Fig. 12 and 13).

Fig. 12 DRS spectra of Fe₃O₄@ZrO₂/CoTNBP (a) before completion of the reaction (b) after completion of the reaction.

Fig. 13 FT-IR spectra of Fe₃O₄@ZrO₂/ZnTNBP (a) before completion of the reaction (b) after completion of the reaction.

Conclusions

The FT-IR, UV-Vis, DRS, SEM, XRD, ¹H-NMR and VSM techniques were used to characterize 5,10,15,20-tetrakis[4-(naphthalen-2-yloxycarbonyl)phenyl]porphyrin and its metal complexes with zinc and cobalt. These data confirmed the synthesis of this product and after that, to investigate the use of synthesized photocatalyst, degradation of methylene blue was performed in the presence of light.

The results showed that having a metal in the center of the tetrapyrrole rings of the porphyrin increased photocatalytic activity more compared to the free porphyrin and increased the percentage degradation of methylene blue. Also, the porphyrin and metalloporphyrins have a strong role in the photocatalytic degradation and deposition of nanoparticles on the substrate (increasing the electron transfer reaction will increase the efficiency of degradation by the photocatalyst).

Acknowledgements

The authors gratefully acknowledge the partial support from the Research Council of the Iran University of Science and Technology.

Notes and references

1. T. Tanaka and A. Osuka, Chem. Soc. Rev., 2015, 44, 943–966.
2. H. K. Hombrecher, S. Ohm and D. Koli, Tetrahedron, 1996, 52, 5441.
3. S. M. S. Ló, D. R. B. Ducatti, M. E. R. Duarte, S. M. W. Barreira, M. D. Noseda and A. G. Gonçalves, Tetrahedron Lett., 2011, 52, 1441.
4. M. A. Schiavona, L. S. Iwamoto, A. G. Ferreirab, Y. Iamamotoa, M. V. B. Zanonic and M. D. Assisa, J. Braz. Chem. Soc., 2000, 11, 458.
5. A. Faiz, V. Heitz and J. Sauvage, Chem. Soc. Rev., 2009, 38, 422–442.
6. M. J. Crossley, P. J. Sintic, J. A. Hutchison and K. P. Ghiggino, Org. Biomol. Chem., 2005, 3, 852.
7. M. J. Griffith, K. Sunahara, P. Wagner, K. Wagner, G. G. Wallace, D. L. Officer, A. Furube, R. Katoh, S. Mori and A. J. Mozer, Chem. Commun., 2012, 48, 4145.
8. M. Niskanen, M. Kuisma, O. Cramariuc, V. Golovanov, T. I. Hukka, N. Tkachenkoa and T. T. Rantalab, Phys. Chem. Chem. Phys., 2013, 15, 17408.
9. T. E. Shubina, H. Marbach, K. Flechtner, A. Kretschmann, N. Jux, F. Buchner, H. P. Steinru, T. Clark and J. M. Gottfried, J. Am. Chem. Soc., 2007, 129, 9476.
10. L. Sun, Y. Li, M. Sun, H. Wang, S. Xu, C. Zhang and Q. Yang, New J. Chem., 2011, 35, 2697.
11. H. Jiang, P. Chen, S. Luo, X. Tu, Q. Cao and M. Shu, Appl. Surf. Sci., 2013, 284, 942.
12. Y. W. Wu, J. Zhang, J. Liu, L. Chen, Z. Deng, M. Han, X. Wei, A. Yu and H. Zhang, Appl. Surf. Sci., 2012, 258, 6772.
13. X. Huang, K. Nakanishi and N. Berova, Chirality, 2000, 12, 237.
14. J. Rochford and E. Galoppini, Langmuir, 2008, 24, 5366.
15. Z. Xiangqing Li, S. Kang, L. Qin, G. Lib and J. Mu, Chem. Commun., 2014, 50, 9064.
16. T. Nakazono, A. R. Parent and K. Sakai, Chem. Commun., 2013, 49, 6325.
17. G. Yao, J. Li, Y. Luo and W. Sun, J. Mol. Catal. A: Chem., 2012, 361–362, 2935.
18. C. Wang, J. Li, G. Mele, G. M. Yang, F. X. Zhang, L. Palmisano and G. Vasapollo, Appl. Catal., B, 2007, 76, 218226.
19. L. Gomathi Devi and R. Kavitha, RSC Adv., 2014, 4, 28265.
20. H. Ghafuri, A. Rashidizadeh, B. Ghorbani and M. Talebi, New J. Chem., 2015, 39, 4821–4829.

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