Porphyrin-containing materials for photodegradation of organic pollutants in wastewaters: a review

Abstract

Industrialization and town urbanization have led to an exponential need for clean water and new wastewater treatment strategies. Currently, photocatalysis is the most appealing method to destroy organic pollutants in water, and the use of sunlight, with the assistance of a solid photocatalyst, is considered one of the most sustainable approaches. The use of heterogeneous photocatalysts has unique advantages, namely their recovery and reuse for several cycles without loss of activity. Due to their remarkable optical and photophysical properties, porphyrin (Por) dyes can be used in homogenous and heterogeneous photocatalysis. Different supports can be used to immobilize Pors, allowing the final material to extend its absorption into the white light region since most of the supports only absorb UV light. This review focuses on the photocatalytic performance of non-immobilized and immobilized porphyrins in the photodegradation of organic pollutants for wastewater treatment application.

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1. Introduction

Water is the most important natural resource on earth and must be preserved. However, it has been contaminated because of anthropogenic activities. Industrialization and town urbanization have led to enormous problems regarding effluent gathering and treatment. In many countries, most industries discard their wastewater directly into aquatic environments without previous suitable treatment. In India, for instance, most of the industrial effluents are coloured due to the untreated wastewaters from the painting and textile industries. This leads to loss of water quality, which can be hazardous to aquatic life and human health. Because most synthetic dyes are slowly degraded in the environment, it is necessary to remove or degrade those contaminants as much as possible before the effluents are discharged into the rivers to solve or mitigate this problem.^1–4

Researchers have developed sustainable wastewater treatment strategies in the past few decades.^5–10 When developing a new approach for water treatment, its safety and sustainability must be considered. In this context, photocatalysis arises as a green alternative to currently used methods (rapid sand filtration, upflow roughing filters, and slow sand filtration, among others) that show several disadvantages such as high maintenance and expensive costs.¹¹

In the case of photocatalysis, it only requires a photocatalyst and light to produce reactive oxygen species (ROS) that will degrade the pollutant. The use of sunlight, with the assistance of a solid photocatalyst (which may be recycled and reused),^12,13 is a valuable and green approach to destroy organic contaminants. During the photocatalytic process with semiconductors, the adsorption process plays an important role since it is necessary for the dye first to be adsorbed into the photocatalyst's surface. This process is based on a surface phenomenon using a solution which has an adsorbable solute (like dyes) that comes in contact with the adsorbent with a highly porous surface.¹⁴ Adsorption was already used as an effective separation technique due to its high efficiency in removing dyes. In fact, activated carbon was already used to remove phenols, pesticides, and dyes.¹⁵

The efficiency of a photocatalytic process depends on the nature of the photocatalyst as well as the source of light. When irradiated, the catalyst is photo-activated by photons with the same or higher energy than its band gap energy. Typically, higher irradiation intensity leads to higher photocatalytic efficiency.¹⁶ Knowing that sunlight is a safe and renewable energy, its use in photodegradation studies offers many advantages, such as cleanness and abundance. However, it requires the development of catalysts that can be activated by sunlight.¹⁷

Titanium dioxide has become the most widely used photocatalyst in the past few years due to its high stability, cheapness, non-toxicity, and high efficiency. Furthermore, TiO₂ can absorb near-UV light (315–400 nm) and form electron (e⁻)–hole (h⁺) pairs which can directly oxidize or reduce substrates. Nonetheless, there are several drawbacks in using bare TiO₂ in photocatalysis which include a relatively large band-gap and its wavelength light absorption being restricted to UV light.^18,19 Therefore, to improve the use of sunlight, it is necessary to modify TiO₂ to extend its wavelength absorption to white light. This modification includes non-/transition-metal ion doping^20–22 and dye sensitization.²³ Besides TiO₂, it is also common to find zinc oxide,²⁴ silicon dioxide,²³ polymers²⁵ and g-C₃N₄ (ref. 26) that can also be modified to increase their absorption spectra to white light.^27,28 Among many dye sensitizers, porphyrins (Pors)^29,30 have received particular attention due to their valuable features in water treatment. In fact, due to their strong absorption in the visible region and photoelectrical properties, Pors have been extensively used as catalysts for the photooxidation of organic pollutants.²³

Pors are aromatic tetrapyrrolic macrocycles with interesting absorption spectra,³¹ typically displaying a strong absorption band around 400 nm (the Soret band) and other bands with lower intensity in the 500–650 nm region (the Q bands).^32,33 Por derivatives can undergo several reaction types, including inner core complexation with metal ions such as Zn(II), Cu(II), Co(II), Sn(III), and Fe(II), among others.^34–36 The presence or absence of a metal ion in the macrocycle core can significantly modify its reactivity and photocatalytic performance.^37,38 Due to their biological, photophysical, and photochemical properties, tetrapyrrolic macrocycles have also been applied in areas such as sensors,^39–41 artificial oxygen transporters,⁴² biomimetic models,⁴³ semiconductors⁴⁴ and medicine.^45–48 However, considering the water pollution problems, (photo)catalysis is one of the most relevant areas where these dyes can be used.^49,50 Moreover, Pors combined with (or supported on) inorganic materials, such as TiO₂, ZnO, SiO₂, mixtures of two inorganic supports (e.g., TiO₂–SiO₂, etc.) or nanoparticles, among others, act as photocatalytic materials for wastewater treatment that give rise to higher degradation rates, allow their facile recovery and their reusability without significant loss of activity.⁵¹

This review presents several successful examples regarding the photocatalytic degradation of water pollutants using non-immobilized and immobilized porphyrins.

2. Mechanism of the photocatalytic degradation of organic pollutants

The degradation of pollutants may occur by indirect photolysis in the presence of immobilized or non-immobilized porphyrins that can absorb irradiation and subsequently generate ROS such as singlet oxygen (¹O₂), hydroxyl (HO·), peroxyl (ROO·), and superoxide anion (O₂˙⁻) radicals.^23,52 This methodology requires light, a photosensitizer (PS), and molecular oxygen (³O₂). As depicted in Fig. 1, a PS is excited from the ground state (S₀) to an excited singlet state (S₁ or S₃) after irradiation. From this point, the excited molecule can return to the ground state by different processes, including emission (fluorescence) and vibration (heat) or by reacting with substrates. However, an excited triplet state (T₁) with a much longer lifetime can be formed if the decay can go through an intersystem crossing. From this point, the PS can perform photoreactions by two competitive mechanisms: type I mechanism, an electron transfer from the PS to the substrate occurs, leading to the formation of reactive radicals, while in the type II mechanism there is an energy transfer from the PS's triplet state to molecular oxygen, generating ¹O₂.⁵³

Fig. 1 Modified Jablonski's diagram to elucidate the formation of ROS.

Some Pors have high quantum yields for triplet state formation, determining their increased ability to generate ROS.⁵⁴ This quantum yield can be increased by the insertion of some metal ions in their inner core.⁵⁵

3. Photocatalytic degradation of organic pollutants in water with non-immobilized porphyrins

Non-immobilized porphyrins are frequently evaluated as photocatalysts, namely in preliminary assays aiming to study the adequate experimental conditions for the photodegradation of pollutants before their immobilization in organic or inorganic matrixes. In cases where the applied porphyrins are water-insoluble, their recovery and reuse do not necessarily require immobilization in any support. A brief discussion concerning the photodegradation of common organic water pollutants (e.g., dyes, nitroaromatics, phenols, medicines) by non-immobilized porphyrins is presented in this section.

Natural nitroaromatic compounds are rare, but they are frequently used as dyes, pesticides, or explosives, for instance. Their use has become so intensive that it led to the contamination of soil and groundwater.⁵⁶ To find efficient ways to degrade 2,4,6-trinitrotoluene (2,4,6-TNT), Hikal and Harmon⁵⁷ developed a crystalline structure (Fig. 2) by self-assembly of porphyrins Zn1 and H₂2, which bear sulfonato and pyridinium groups, respectively. Crystals of Zn1–H₂2 were characterized by optical microscopy to determine that, depending on the molar ratio of the porphyrins in the starting solution, they may reach a length between 30–55 μm and width of 2–50 μm. Moreover, it was observed that the UV–vis spectrum of the crystalline structure and the spectra of the two monomers are quite different (broadened and blue and/or red-shifted Soret and Q bands). It is essential to highlight that the formation of the crystals led to a dramatic reduction of the intensity of the emission bands accompanied by a red-shift and split of the bands. These materials were able to reduce 80% of 2,4,6-TNT under a tungsten lamp for 3 h, with the photocatalyst being highly photostable under the experimental conditions (Table 1). The authors studied the degradation products and identified trinitrobenzene.

Fig. 2 Representation of the interactions between Zn1 and H₂2 in the solid-state; and structures free-base and metal M3.

Table 1 Non-immobilized porphyrins used to degrade TNT and MO pollutants

Por	Pollutant	Irradiation time (min)	Light intensity (mW.Cm^-2)	Irradiation source	Degradation rate (%)	Ref.
a pH = 6. b pH = 7. c pH = 10.
Zn1–H22	TNT	180	5.1	UV–vis	80	57
H23	MO	240	—	Sunlight	70^a	58
					16^b
					10^c
Cu3					28^a
					10^c
Ag3					25^a
					10^c
Sn3Cl2					100^a
					60^b
					90^c

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Methyl orange (MO) is a water-soluble dye used as a pH indicator in textiles, foodstuffs, farming chemical products, paper, leather industries, etc. However, its release into the environment, even in low quantities, causes serious pollution problems. In this sense, Salker and co-worker⁵⁸ used meso-tetraphenylporphyrin (H₂3) and the corresponding metalloporphyrins M3 (M = Cu(II), Ag(II), or Sn(IV)Cl₂, Fig. 2 and Table 1) for the photocatalytic degradation of MO. The fluorescent H₂3 and Sn3Cl₂ were revealed to be more efficient for MO photodegradation than the non-fluorescent Cu3 and Ag3.⁵⁸ Although only small amounts of porphyrin are needed for these photocatalytic studies, the porphyrins used in this work could be recovered, which is a very relevant point for this type of application.

4. Wastewater treatment using polymeric porphyrins or porphyrins immobilized on polymers

In the past few years, due to their low-cost and facile synthesis along with appropriate mechanical properties and excellent electrical and electrochemical activities, organic polymers have attracted increased attention as materials for solar cells, fuel cells, catalysts, adsorption, light emitting diodes, separation processes, magnetism, and rechargeable lithium batteries.²⁵

Both polymeric porphyrins and porphyrins immobilized on polymers have been used as photocatalysts to degrade organic pollutants in water. A few examples are discussed in the following sections.

4.1. Polymeric porphyrins

Rahman and co-workers⁵⁹ described that Congo red (CR), a stable water-soluble anionic azo dye with a benzidine core (a cancer-causing substance), is hard to degrade. This dye is dangerous for human health and has a high ecotoxicity in the environment. There are several methods for removing this dye from water, however adsorbents seem to be the best due to their high efficiency, simplicity, and cheapness.^59,60 Thus, the combination of the adsorption of this pollutant from wastewaters with its photodegradation is a trustworthy way to eliminate it. For that reason, Sun and co-workers⁶¹ prepared the polymeric dendrite-like nanostructure Fe4 (Fig. 3) and used it as a photocatalyst to degrade Congo red. According to scanning electron microscopy (SEM), each branch of the dendrite-like nanostructure possesses a diameter size of about 30–80 nm. The chiasma and tangle of the nanostructure formed the final microstructure with a size of about 30 μm. This microporous material showed high selectivity (when compared with MO, methyl violet (MV), methyl red (MR) and rhodamine B (RhB)) and efficiency: 88.3% of the CR dye was decomposed in just 120 s under white light irradiation (Table 2). Despite using N,N-dimethylformamide (DMF) in the photocatalytic studies, this material proved to be a good candidate for azo dye photodegradation.

Fig. 3 Structure of H₂6 and polymers Fe4, H₂5, H₂7, and M8 (M = Cu, Zn, Co, Mn, or Fe).

Table 2 Polymeric porphyrins used for the degradation of several pollutants

Por	Pollutant	Irradiation time (min)	Irradiation source	Degradation rate (%)	Ref.
Fe4	CR	2	Visible	88	61
	MO	4		30
	MV			10
	MR			10
	RhB			0
H25	RhB	80	Visible (λ > 420 nm)	100	64
H26		120		42
H27				53
Co8	AO	240	Visible (λ > 420 nm)	100	65
		180	Sunlight
		480	Visible (λ > 420 nm)
Fe8
Cu8
Mn8
Zn8

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RhB has been used for dyeing cotton, silk, paper, bamboo, and other products and consequently has been found in wastewaters produced by these industries. Cationic RhB dyes are water-soluble and can interact with the negatively charged surface of cell membranes and penetrate cells. For that reason, they are considered more toxic than the anionic ones. There are several methods to eliminate this pollutant,^62,63 but Zhao and co-workers⁶⁴ reported a new one where the porphyrin-based porous organic polymer H₂5 (Fig. 3) is used as a photocatalyst for the degradation of RhB and also methylene blue (MB). After verifying that MB was unstable under white light irradiation, the authors proceeded with the photocatalytic studies only for RhB. The commercially available photocatalyst TiO₂ (PC-500), meso-tetrakis(p-aminophenyl)porphyrin (H₂6), and polymeric porphyrin H₂7 (prepared from the reaction of H₂6 and terephthalaldehyde) were used as controls. The experimental results showed that polymer H₂5 possesses high photocatalytic activity for RhB: 100% photodegradation after white light irradiation for 80 min (Table 2). The authors proposed that linking porphyrin building blocks in an amorphous way contributes positively to a higher degradation effect, prevents aggregation of the porphyrin molecules (π–π stacking), and ensures the porosity and accessibility of the active sites of the porphyrin molecule. One interesting conclusion of this work was the selectivity of H₂5 for MB degradation in the presence of a mixture of both dyes (MB/RhB 1 : 1).

The presence of dyes like acridine orange (AO) in watercourses negatively affects the quality of aquatic and human lives. In this context, Chen and co-workers⁶⁵ reported the photodegradation of AO in an aqueous solution using polymeric metalloporphyrins M8 (M = Cu, Zn, Co, Mn, or Fe) (Fig. 3 and Table 2). It was observed that Co8 was the best photocatalyst under a high-pressure mercury lamp, and the authors assessed its photocatalytic behaviour under an iodine tungsten lamp, or natural sunlight. Polymer Co8 (Fig. 3) revealed a complete photodegradation of AO under natural sunlight irradiation after 3 h. (Table 2). This work showed that polymeric metalloporphyrins could be successfully employed in wastewater treatment using solar light irradiation.

4.2. Porphyrins immobilized on polymers

Linking porphyrins to organic polymers can increase their photocatalytic activity and reusability.^66–68 Indeed, several organic polymers (e.g., poly(N-isopropylacrylamide), polyacrylonitrile, polystyrene, etc.) incorporating porphyrins, and even self-assembled porphyrin polymers, have been developed for wastewater treatment, namely for the photodegradation of dyes.

Bhagat and co-worker⁶⁹ prepared a polymer-supported Brønsted acid-functionalized Zn9 complex tangled with a benzimidazolium moiety (Fig. 4) and assessed its ability as a photocatalyst for the degradation of MO and CR (Table 3) using atmospheric air or H₂O₂. After irradiation for 30 min under atmospheric air, a complete degradation of the two dyes was achieved. The catalyst could be reused up to five times without loss of activity.

Fig. 4 Structure of polymeric porphyrin derivatives 9 (M = 2H or Zn).

Table 3 Porphyrins immobilized on polymers used for the degradation of several pollutants

Por	Pollutant	Irradiation time (min)	Light source	Light source	Recycle	Ref.
a H2O2. b mg g^−1 of Por/material.
Zn9	MO	30	Visible (λ > 400 nm)	100	5 cycles (5% loss)	69
		25^a
	CR	20		85
		20^a
	MR	30		95
		30^a		90
Zn10	Reactive orange	250		10	—	70
Cu10				20
Ni10				20
Co10				25
Pd10				15
Fe10				40
Fe11				40	4 cycles (36% loss)
	Reactive blue	180	UV	100	—
	Reactive black
	Reactive red
	Reactive light yellow			90
H212	PCP	360	Visible (λ > 400 nm)	150.31^b		71
	TCP			30.51^b
	DCP			46.55^b
H212	PCP			35		72
	TCP			15
	DCP			0
H213	PCP	120		100
	TCP	360
	DCP			35
H214	Benzene	—	UV	95		73
			Visible (λ > 400 nm)
	Toluene		UV	90
			Visible (λ > 400 nm)	80
	Ethyl benzene		UV	95
			Visible (λ > 400 nm)	50
	Xylene		UV	95
			Visible (λ > 400 nm)
H215	TCP	360		100		74
H216
Zn17	MB	180		72	7 cycles (0% loss)	75
Zn18		150		75	7 cycles (30% loss)	76
Zn18	RhB	240		86	5 cycles (5% loss)	77
Zn19				82
Zn20				73
Zn21				69

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4.2.1. Porphyrins supported in polyacrylonitrile fibres. Qi and co-workers⁷⁰ reported the construction of uniform polyacrylonitrile polymer and polyacrylonitrile copolymer nanofibre mat supported metalloporphyrins M10 (M = Zn(II), Cu(II), Ni(II), Co(II), Fe(III), and Pd(II)) and Fe11, respectively by electrospinning (Fig. 5 and Table 3). These polymers showed high photocatalytic activity to degrade several azo dyes in an aqueous solution under white light irradiation. The polymer nanofibre mat supported Cu10 and Fe10 were the most effective ones. Using the polymer Fe10 and the copolymer Fe10/nanofibre mat, over 60–80% of the azo dyes were degraded in less than 2 h of irradiation. It is important to mention that the copolymer immobilized Fe11 shows a higher photocatalytic activity. The immobilization of the metalloporphyrins into the nanofibre mats allowed the recovery of the photocatalysts without significant reduction of their photocatalytic activities (e.g., Fe11/nanofibre mat was reused up to four cycles, and photodegradation of 64.5% was still observed in the last cycle).

Fig. 5 M10 (M = Zn(II), Cu(II), Ni(II), Co(II), Fe(III), and Pd(II)) and Fe11 supported in a polyacrylonitrile or polyacrylonitrile copolymer nanofibre mat.

Lee, Baek, and co-workers^71,72 reported the porphyrin star copolymer H₂12 (Fig. 6) that was able to degrade a series of chlorophenols: 2,4-dichlorophenol (2,4-DCP), 2,4,6-trichlorophenol (2,4,6-TCP), and pentachlorophenol (PCP). In 2013, Lee, Baek, and co-workers⁷³ synthesized the free-base porphyrin star copolymer H₂14, and in 2018 the same authors⁷⁴ developed two palladium(II) porphyrins coupled to two different copolymers, H₂15 and H₂16, in order to evaluate their photocatalytic activity in the degradation of 2,4,6-TCP in water under white light irradiation. Using low energy and extending the absorption wavelength, H₂12 could efficiently degrade the chlorophenols in an aqueous solution under white light, reaching removal efficiencies ranging from 30.51 to 150.31 mg g⁻¹ of Por/material (Table 3).⁷¹ The same authors⁷² compared two different types of porphyrin core star copolymers: the same poly(dimethylaminoethyl acrylate)porphyrin H₂12 and H₂13, respectively, to efficiently degenerate chlorophenols. Even though they used another block copolymer H₂13, the degradation was still more efficient when using H₂12, accomplishing a photodegradation rate of 35% and 100% (Table 3), respectively. This difference can be associated with the higher hydrophobicity of H₂12 due to the presence of polystyrene polymer in its structure. In this case, the interaction of the hydrophobic contaminants (chlorophenols) with the hydrophobic block (polystyrene) of the material induced a hydrophobic–hydrophobic interaction which could enhance the access to the porphyrin core (photocatalyst), increasing the removal efficiency. The degradation products were identified as hydroquinone and posterior CO₂ and H₂O.

Fig. 6 Free-base and palladium porphyrin star copolymers.

Baek and co-workers⁷³ firstly reported that the water-soluble amphiphilic star block copolymers H₂14 could efficiently capture toxic organic compounds such as benzene, toluene, ethylbenzene, and xylene (Table 3) from water due to the hydrophobic–hydrophobic interaction with the inner polystyrene block segments and degrade them by the UV-induced photo-oxidation reaction. They also reported the recovery of the star block copolymers by increasing the temperature above their lower critical solution temperature (LCST) where the thermosensitive H₂14 forms water insoluble agglomerates.

According to the second study,⁷⁴ the amphiphilic porphyrin star block copolymers were also homogenously soluble in water. It was also concluded that the water-soluble star block copolymers with hydrophobic core–shell structures, such as H₂15 and Pd16, were the most effective for the degradation of 2,4,6-TCP in water under white light irradiation when compared with the corresponding homopolymer. The explanation for those results was the effectiveness of the hydrophobic core near the metalloporphyrin in capturing and binding the hydrophobic chlorophenols, contributing to their easier degradation. The authors considered that selecting an inner hydrophobic block to design amphiphilic star block copolymers is vital for the degradation of 2,4,6-TCP in water.

4.2.2. Porphyrins supported on poly(N-isopropylacrylamide). Duan and co-workers^75,76 developed polymers 17 and 18 (Fig. 7 and Table 3) containing a Zn(II) porphyrin and evaluated their ability as photocatalysts for the oxidative degradation of MB under white light irradiation. One polymer has a well-defined linear poly(N-isopropylacrylamide) (PAM) (Zn17–PAM), and the other is a well-defined star-shaped PAM (Zn18–PAM). Both polymers were synthesized via atom transfer radical polymerization (ATRP). Polymer Zn17–PAM, in the presence of H₂O₂, could degrade 72% of MB when irradiated with white light for 180 min at its lower critical solution temperature (LCST). This photocatalyst could be reused up to 7 times without losing its catalytic activity. The photocatalytic studies using Zn18–PAM were made at pH 2 and at the LCST (29 °C) and in the presence of H₂O₂ for 150 min under white light irradiation, where 75% of MB was degraded. This photocatalyst was reusable for up to 7 cycles keeping the photodegradation rate at 70% under the same conditions.

Fig. 7 Structures of polymers Zn17–Zn21.

Gao and co-workers⁷⁷ reported the synthesis of the Zn(II) porphyrin-based multi-arm star polymers Zn18, Zn19, Zn20, and Zn21 (Fig. 7 and Table 3), where n = 1–4. The polymer used was the same as that reported by Duan and co-workers.^75,76 All porphyrins were prepared via ATRP and their ability as photocatalysts to degrade RhB under white light irradiation was assessed. The authors were interested in establishing a relationship between the number of arms at the porphyrin and the respective photocatalytic activity and stability. Interestingly, as the number of arms increased, the stability and photocatalytic activity also increased. Thus, the best photocatalyst was Zn18, which can be recycled up to 5 times without activity loss.

4.3. Porphyrins in metal–organic frameworks

Metal–organic frameworks (MOFs) are emerging classes of materials used in photocatalysis, especially those based on metalloporphyrins.⁷⁸ Nagaraja and co-workers⁷⁹ used the Ni22 nickel MOF (Fig. 8 and Table 4) in the photodegradation of nitroaromatic compounds under white light for 90 min. When compared with Ni22 alone, the Ni22 MOF revealed an enhanced photocatalytic activity (>90% degradation) which may be attributed to a combined catalytic effect. Also, this MOF was recycled and reused for 3 cycles without loss of catalytic activity and structural rigidity.⁷⁹ The reusability of the catalysts without loss of stability indicates that they can be applied to treat dye-polluted waters. The degradation products were identified as the respective amino derivatives.

Fig. 8 Representation of the Ni22 porphyrin and the Ni22 nickel MOF structure.⁷⁹

Table 4 Porphyrins in MOFs used for the degradation of several pollutants

MOF	Por	Pollutant	Irradiation time (min)	Light source	Degradation rate (%)	Recycle	Ref.
a 6 μM. b 500 nm. c 150 nm. d 300 nm. e pH = 3. f pH = 9. g pH = 7. h pH = 5. i In the presence of H2O2.
NiMOF	Ni22	Nitrobenzene	90	Visible (λ = 420 nm)	100	3 cycles (5% loss)	79
		Nitrotoluene	60			—
		Chloronitrobenzene	105
		Bromonitrobenzene	210
		Di-nitrobenzene	300
		Nitronapthalene	1440		34
ZrMOF		Naphthalene-1,5-diol	0	Visible (λ > 420 nm)	100		80
PCN-224	H222	Tetracycline	90		10^a	—	81
					50^b
					98^c
			2		100^d	5 cycles (5% loss)
		Ciprofloxacin	10		90^d
			90		90^c	—
					25^b
					5^a
PCN-222	H222	RhB	120 in the dark + 120 min irradiation	UV	100	3 cycles (30% loss)	83
	Ni22					3 cycles (10% loss)
	H222	Basic violet 14			99	—
	Ni22				100
	H222	Crystal violet			56
	Ni22				70
	H222	AB			47
	Ni22				83
Ln-MOF	H222	Naphthalene-1,5-diol	20	Visible λ > 420 nm	60	—	84
Cu-MOF	Cu22	RhB	360		82^e		85
		MB			40^f
		CR			89^g
		Tetracycline			71^h
		Norfloxacin			44^f
		Oxytocin			46^f
La-MOF	H222	MO	125		16		82
					26^i
		MB			24
					61^i
	Fe22	MO			5
					55^i
		MB			16
					93^i

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Wang and co-workers⁸⁰ synthesized zirconium ultrathin 2D zirconium MOF nanosheets using Ni22 (Fig. 9 and Table 4) and evaluated their photocatalytic activity against naphthalene-1,5-diol. The authors reported good photocatalytic activity and high photostability of these nanosheets when compared with PCN-222, a well-known zirconium metalloporphyrin MOF. These nanosheets' most increased photocatalytic activity might be due to their nanometre thickness, allowing rapid mass transport and a high percentage of exposed catalytically active surfaces.

Fig. 9 Zirconium ultrathin 2D metal–organic framework nanosheets using Ni22. Adapted from ref. 80.

More recently, Xue, Wang and co-workers⁸¹ developed a porphyrin MOF (PCN-224), composed of a Zr₆O₄(OH)₄ cluster and the porphyrin ligand H₂22 (Table 4), to be used in the photocatalytic degradation, under white light irradiation, of antibiotics such as tetracycline and ciprofloxacin. Three different particle sizes were prepared: 6 μM, 150 nm, 300 nm, and 500 nm. The best photocatalyst was found to be 300 nm-PCN-224 followed by 150 nm-PCN-224, 500 nm-PCN-224 and 6 μM-PCN-224. The best photocatalyst could achieve total degradation of tetracycline after 2 min of irradiation and 90% of ciprofloxacin after 10 min of irradiation. No photocatalytic degradation was observed in the absence of light. Also, no significant changes were observed after 5 cycles of photocatalytic activity.

Kumar and co-workers⁸² developed a porphyrin H₂22 and Fe22 La-MOF to be used in the photocatalytic degradation of MO and MB under visible light irradiation. After 125 min of irradiation the H₂22–La-MOF could degrade 16% of MO and 24% of MB. On the other hand, the Fe22–MOF could achieve a photocatalytic rate of 5 and 16% for MO and MB, respectively. These photocatalytic rates could be enhanced after the addition of H₂O₂ (Table 4).

B. Zhang, X. Zhang and co-workers⁸³ prepared a bimetallic porphyrin MOF of PCN-222(Ni/Hf) by inserting Ni(II) in the inner core of H₂22 and coordinating Hf(IV) with the carboxyl groups of the porphyrin. The authors assessed the photocatalytic activity of the MOF for the degradation of four pollutants: RhB, basic violet 14, crystal violet and acid black 210 (AB) – Table 4. The degradation process was not strictly dependent on light since it could reach total degradation of basic violet 14 and RhB after 120 min in the dark. Regarding the degradation of crystal violet and acid black, even under irradiation, it could only reach ∼47% and 60% degradation, respectively. These results were much better than those obtained using PCN-222(Hf) (without Ni(II)). The stability of PCN-222(Ni/Hf) is also enhanced when compared with PCN-222(Hf): after three cycles, the photocatalytic activity decreased by only 10% while a reduction of 30% was observed for PCN-222(Hf).

Regarding lanthanides, Huang and co-workers,⁸⁴ using the same porphyrin, prepared a series of porphyrin-based 2D MOFs (Ln–H₂22, Ln = Ce, Sm, Eu, Tb, and Yb) (Fig. 10 and Table 4) with different thicknesses and assessed their ability as photocatalysts for the degradation of naphthalene-1,5-diol in order to establish a metal/thickness–activity relationship. The results showed that the thinner the thickness of the nanosheets, the higher the light harvesting ability and the better the photocatalytic activity. Interestingly, the most effective MOF incorporated Yb, which showed outstanding performance due to the effective energy and electron transfer between H₂22 and Yb³⁺.

Fig. 10 Representation of Ln–H₂22 MOF nanosheets.

Zhang and co-workers⁸⁵ prepared a Cu22-based MOF through a rapid microwave hydrothermal route. The resulting nanosheets displayed excellent crystallinity and were used as photocatalysts for the degradation of Congo red, MB and RhB as well as some antibiotics like tetracycline, norfloxacin, and the hormone oxytocin (Table 4). After adsorption of the pollutants and irradiation with white light (λ > 420 nm), the Cu22-MOF showed good photocatalytic activity reaching the highest photodegradation rates for Congo red (89%) and RhB (81.2%) after 6 h.

5. Titanium oxide-supported porphyrins

TiO₂-based materials are excellent photocatalysts for the oxidation of pollutants in water. They present several advantages: high photocatalytic efficiency, physical and chemical stability, low cost, and low toxicity. However, bare TiO₂ can only be excited by UV light and for that reason, it shows no response to white light.⁸⁶ Considering that UV radiation is less than 5% of natural sunlight and that 43% of the solar spectral energy is in the white light spectrum, it is highly desirable to develop TiO₂-based photocatalysts that can absorb visible light in order to improve the use of solar light.⁸⁶ There are several strategies to increase the light absorption in the visible region, such as doping with noble metals, dye sensitization and even combining TiO₂ with other semiconductors. Porphyrin's sensitization is a cheap and efficient method to extend the absorption into the visible region. Despite being a good photocatalyst by itself, the sensitization of TiO₂ with porphyrins significantly increases its photocatalytic activity under sunlight and achieves high degradation rates of organic pollutants.^18,87 A brief discussion about the photodegradation of organic contaminants by porphyrins immobilized on TiO₂ will be considered in the following examples.

Chang and co-workers⁸⁸ and Ji and co-workers⁸⁹ prepared several TiO₂ materials sensitized with M3 (Fig. 2): Ni3–TiO₂, Fe3Cl–TiO₂, Co3–TiO₂, Mn3–TiO₂, and Cu3–TiO₂ and studied their photoactivity against 2,4-DCP and MO. Chang and co-workers⁸⁸ proved that under white light (λ = 419 nm) irradiation for 4 h, the Ni3–TiO₂ nanocomposite could degrade 81% of 10 ppm 2,4-DCP. The degradation was time and 2,4-DCP concentration-dependent. This study showed that the synthesized nanohybrid could efficiently eliminate pollutants present in wastewaters using sunlight. On the other hand, Ji and co-workers⁸⁹ found that bare TiO₂ could only degrade less than 10% of MO even after 180 min of light irradiation. However, the photocatalytic activity of Cu3 and M3 (M = Ni(II), Fe(III), Co(II), or Mn(II)) impregnated TiO₂ could efficiently be enhanced. Moreover, the Cu3–TiO₂ composite was the best to degrade MO followed by, in decreasing order, Ni3–TiO₂, Mn3–TiO₂, Fe3Cl–TiO₂, and Co3–TiO₂. UV light could increase the photocatalytic activity of all photocatalysts, with Cu3–TiO₂ reaching 98% degradation after 1.5 h.

Microemulsions can be used as “reactors” due to their unique properties like ultralow interfacial tension, sizeable interfacial area, thermodynamic stability, and, more importantly, their ability to solubilize otherwise immiscible liquids.⁹⁰ Having that in mind, Carrero, Madriz, and co-workers^91,92 reported the use of TiO₂, Ti(IV) citrate, and Zn3 (Fig. 2) as photocatalysts in oil-in-water (O/W) and water-in-oil (W/O) microemulsions to degrade 4-chlorophenol (4-CP). According to the authors, TiO₂ and titanium(IV) citrate were at the water phase, while Zn3 was in the oil phase due to its hydrophobic character. The surfactant cetyltrimethylammonium bromide (CTAB) has a positively charged head positioned at the water interface, forming an interphase or pseudophase, attracting the negatively charged Ti(IV) citrate complex when present. The photodegradation results are presented in Table 5.

Table 5 Photodegradation of 4-chlorophenol^91,92

Type of microemulsion	Por	Metal complex	Support	Surfactant	Light source	Photodegradation (%)
a Best results obtained by Madriz and co-workers.^92
W/O	—	—	TiO2	CTAB	UV	34
O/W	—	—	TiO2	CTAB	UV	70
W/O	—	—	TiO2	CTAB	Visible	0
O/W	—	—	TiO2	CTAB	Visible	0
W/O	Zn3	—	—	CTAB	UV	39
O/W	Zn3	—	—	CTAB	UV	52
W/O	Zn3	—	—	CTAB	Visible	62
O/W	Zn3	—	—	CTAB	Visible	26
W/O	—	Ti(IV)-Cit	—	CTAB	UV	48
O/W	—	Ti(IV)-Cit	—	CTAB	UV	20
W/O	—	Ti(IV)-Cit	—	CTAB	Visible	20
O/W	—	Ti(IV)-Cit	—	CTAB	Visible	20
W/O	Zn3	—	TiO2	CTAB	UV	29
O/W	Zn3	—	TiO2	CTAB	UV	27
W/O	Zn3	—	TiO2	CTAB	Visible	11
O/W	Zn3	—	TiO2	CTAB	Visible	25
O/W	Zn3	Ti(IV)	—	CTAB	UV	51
W/O	Zn3	Ti(IV)	—	—	UV	75^a
W/O	Zn3	Ti(IV)	—	—	Visible	86^a

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In the experiments, after irradiation of the microemulsions with UV light, it was possible to observe that the 4-CP degradation in O/W (Table 5) was much more effective (70%) than in W/O microemulsions (34%). However, there was no 4-CP degradation in the experiments with the TiO₂ suspension under white light irradiation. The higher photodegradation rates in O/W can be explained by the fact that the TiO₂ particles act individually and do not form aggregates that can decrease the exposed surface and the photoactivation of the particles. As expected, adding Zn3 did not affect the photodegradation rate given the lack of interaction between TiO₂ and Zn3 from their different solubilities. Moreover, the Zn3 microemulsion without TiO₂ was irradiated with visible light, showing a higher degradation rate in W/O (62%) than in O/W (26%).⁹¹ On the other hand, when irradiated with UV light, the O/W showed a higher degradation rate (52%) than W/O (39%). The addition of Ti(IV) to Zn3 microemulsion increased the photodegradation rate of 4-CP. A few years later, the same authors⁹² used the same porphyrin Zn3, without CTAB, and found that the best conversion percentage of 4-CP (86%) was obtained using W/O microemulsions of titanium citrate mixed with Zn3 and irradiation with white light for 30 min.

Palmisano and co-workers, SŁota and co-workers and Mele and co-workers^93,94 reported the synthesis of a polycrystalline TiO₂ impregnated with the free-base porphyrin H₂23 or selected metalloporphyrins M23 (M = Cu(II), Fe(III)Cl, or Mn(III)OAc) (Fig. 11), which were used as catalysts for the photodegradation of 4-nitrophenol (4-NP).

Fig. 11 Free-base porphyrin H₂23, Cu23, Fe23Cl, Mn23OAc and Cu24–Cu27 used to sensitize TiO₂.

According to the first study, the prepared materials H₂23 and Cu23 were found to be photostable even after 5 h of light irradiation. Regarding the photocatalytic activity, it is possible to affirm that the TiO₂ samples that contain Cu23 are better photocatalysts than the corresponding free-base H₂23. The photocatalytic activity under UV light for 2 h of bare TiO₂ was compared with that of the synthesized hybrids to establish the role of the central metal in M23 (M = 2H, Cu(II), Fe(III), or Mn(III)).⁹⁴ The obtained results showed that different metals can modulate photocatalyst efficiency. The authors also accessed the stability of the prepared materials where it was possible to observe that all the materials were stable. For example, Cu23–TiO₂ and Fe23Cl–TiO₂ were more photoactive than bare TiO₂ (anatase), whereas Mn23OAc–TiO₂ showed a lower photoreactivity. The metal-free porphyrin H₂23 also showed slightly higher photoactivity than the corresponding bare TiO₂, indicating that the porphyrin macrocycle is photoactive even in the absence of metal.⁹⁴

Li and co-workers⁹⁵ developed four Cu(II) porphyrins (Cu24–Cu27) with one to four peripheral ester groups (Fig. 11) to be used to sensitize TiO₂ and to use the composites as photocatalysts for the degradation of 4-NP. The photocatalytic properties of mesoporous TiO₂ and Cu24– to Cu27–TiO₂ hybrids were evaluated to degrade 4-NP in aqueous media and under white light irradiation. The results showed that the photocatalytic activity order is: Cu27–TiO₂ > Cu26–TiO₂ > Cu25–TiO₂ > Cu24–TiO₂ (Table 6). The different number of peripheral groups directly modulates the photocatalytic activity, influencing the electron–hole separation process, and even after 6 cycles, the photocatalytic activity only decreased by 20% for the best photocatalyst. The loss of photodegradation efficiency could be assigned to the in situ generated hydroxyl radicals attacking the porphyrin molecules.

Table 6 Porphyrins–TiO₂ used for the photodegradation of 4-NP developed by several authors^93–99

Por	Irradiation time (min)	Light source	Degradation rate (%)	Recycle	Ref.
H223	300	Visible (λ = 366 nm)	100	—	93
Cu23	240
Cu23	120				94
Fe23	180
Mn23
Cu24	60	White light		6 cycles (50% loss)	95
Cu25	55			6 cycles (45% loss)
Cu26	50			6 cycles (40% loss)
Cu27	45			6 cycles (30% loss)
Cu28	400	Visible (λ = 420 nm)	98	—	96
H228			90
Cu29			99	6 cycles (15% loss)
H229			98	—
Pt28	360	UV–vis	99	6 cycles (10% loss)	97
Pt29			97	—
Pt30			96
Cu30	400		90		98
Cu31			98
Cu32			95
Cu33			100
Cu34		Visible (λ > 420 nm)	90	6 cycles (5% loss)	99
Cu35			89
Cu36			87

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A series of studies using TiO₂ impregnated with meso-tetraphenylporphyrin (TPP–H₂3) derivatives bearing 3-phenoxypropyloxy groups at ortho-, meta- or para-positions (Fig. 12 and Table 6) were performed in order to evaluate the photocatalytic activity of the composites in 4-NP degradation.^96,97 The ortho, meta and para tetra-substituted free-base porphyrins H₂28–H₂30 (Fig. 12) and the corresponding complexes Cu28–Cu30 and Pt28–Pt30 were used in those studies. Later, Li and co-workers^98,99 intended to assess the influence of the number of meta- and para-substituents by synthesizing the mono, di, tri, and tetrasubstituted free-base porphyrins H₂30–H₂33 as well as their Cu(II) derivatives Cu30–Cu33.

Fig. 12 Free-base, Cu(II), Pt(II), and Sn(IV) porphyrins 28–36.

The first study⁹⁶ was carried out in aqueous media under visible light (λ = 420 nm) irradiation. Among the synthesized photocatalysts, Cu29–TiO₂ was revealed to be the most efficient in 4-NP degradation, followed by Cu28–TiO₂, H₂29–TiO₂, and H₂28–TiO₂. TiO₂ impregnated with ortho-substituted porphyrins (H₂29–TiO₂ and Cu29–TiO₂) exhibited higher photoactivities than TiO₂ synthesized with para-substituted porphyrins (Cu28–TiO₂ and Pt28–TiO₂). This can be due to the lower symmetry of the ortho-substituted structures, which can lead to a minor extent of aggregation and less important intermolecular π–π interactions. All photocatalysts were stable after irradiation for 5–7 h. In this study, TiO₂ impregnated with porphyrins could extend the light absorption efficiency and this depends on the amount of porphyrin loaded into the TiO₂ surface. When using the best photocatalyst, almost all 4-NP (99%) was degraded after 400 min of irradiation.

According to Li and co-workers,^97,98 all Pt(II) and Cu(II) porphyrins (Pt28–Pt30 and Cu30–Cu33, Fig. 12 and Table 6) combined with TiO₂ enhanced the photocatalytic efficiency when compared with bare TiO₂. Overall, Pt(II) complexes/TiO₂ showed better photocatalytic activity in the degradation of 4-NP compared with the Cu(II) porphyrins/TiO₂ after 200 min of irradiation. However, when the irradiation time was extended to 300 min, all derivatives induced a complete degradation of the pollutant. Among the ortho-, meta- and para-substituted porphyrins, the ortho derivative Pt29–TiO₂ was the best photocatalyst. Regarding the effect of the number of peripheral meta-substitutions on the photocatalysts' efficiency, the best results were obtained with the tetrasubstituted Cu30 followed by the trisubstituted Cu33, disubstituted Cu32 and monosubstituted Cu31.⁹⁸ Similarly, Li and co-workers⁹⁹ reached the same results for the para-substituted porphyrins, with the trisubstituted Cu36–TiO₂ being the best catalyst followed by disubstituted Cu35–TiO₂ and monosubstituted Cu34–TiO₂ (Fig. 12 and Table 6). To justify those results, the authors^98,99 assumed that the number of substituents does not influence the redox potential of the sensitizers but, instead, it influences the interaction between porphyrins and the porphyrin–TiO₂ surface as well as the electron transfer process from the sensitizer to TiO₂. Porphyrins with more substituents can adsorb more dissolved oxygen and, consequently, an increased photocatalytic activity is observed. Catalyst recycling studies showed that there is no significant loss of activity after six cycles.

Li and co-workers¹⁰⁰ and Mu and co-workers¹⁰¹ used the three isomeric Sn(IV) porphyrin derivatives Sn3(OH)₂ (Fig. 2), Sn28(OH)₂–Sn30(OH)₂ (Fig. 12) to prepare the corresponding TiO₂–Sn(IV) porphyrin hybrids via axial functionalization. The ability of the resulting hybrids to photodegrade 4-NP and MO was then evaluated under visible and UV light. Under white light, the three composites showed higher photocatalytic activity than bare TiO₂, with the Sn28(OH)₂–TiO₂ hybrid being the best photocatalyst with a photocatalytic rate of 98.1% after 500 min of light irradiation (Table 7). However, under UV light irradiation, the photocatalytic efficiency of all hybrids was also enhanced when compared with bare TiO₂. Regarding the photodegradation efficiencies of 4-NP, an efficient degradation is obtained with the peripheral substituents at the para-positions as well as the presence of axial ligands, which can prevent π–π interactions and decrease the aggregation of the porphyrin molecules, especially for the most planar Sn28(OH)₂.

Table 7 Porphyrins–TiO₂ used for the photodegradation of 4-NP, MO and RhB developed by several authors

Por	Pollutant	Irradiation time (min)	Light source	Degradation rate (%)	Recycle	Ref.
Sn28	4-NP	500	Visible (λ > 400 nm)	98.1	—	100
		450	UV–vis	100
Sn29		500	Visible (λ > 400)	90
		450	UV–vis	100
Sn30		500	Visible (λ > 400)
		450	UV–vis
Sn3		180		90	7 cycles (5% loss)	101
	MO			95
Cu37	RhB	30		100	—	103
	NP	60		90	6 cycles (5% loss)
Cu38	RhB	30		98	—
	NP	60		90	6 cycles (40% loss)
Cu39	RhB	30		100	—
	NP	60		98	6 cycles (5% loss)
Cu38		160		90	—	104
Cu39		65	Visible (λ > 350 nm)	90		106
Zn39				85
Ni39				80
Zn40		180	UV	94		107
Cu40				98
Co40				94
Fe40Cl				93
Zn41				95
Cu41				98	6 cycles (10% loss)
Co41				95	—
Cu42		160	Visible (λ > 350 nm)	100		108
		120	UV (λ = 100–400)
Zn43	MO	150	UV–vis (λ > 100)	89	6 cycles (10% loss)	109
Zn44				91	—
Zn45				84
Zn46				73

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Moreover, it is important to say that the structure of Sn29(OH)₂ (Fig. 12) can have more stressed and stereochemically complicated conformations than others. On the other hand, the substituents of porphyrin Sn28(OH)₂ have just a small deviation from planarity, which causes a perfect parallel plane between the porphyrin and the TiO₂ surface. This means that Sn28(OH)₂ can easily approach the TiO₂ surface enabling the initial electron injection process from the porphyrin's excited state into the TiO₂ CB leading to an effective photocatalytic effect. As expected, the structure of Sn29(OH)₂ makes it inaccessible to the surface of TiO₂ and blocks the participation of the porphyrin in the photocatalytic effect. Despite the photocatalytic activity being higher than Co(II) and Zn(II) porphyrins, it is still lower than that of Cu(II) porphyrins, which proves that the presence of different metal ions plays an important role in the photocatalytic process.¹⁰² In the presence of Sn3(OH)₂–TiO₂ more than 85% degradation was accomplished with 180 min of irradiation. Also, the characterization studies showed an interaction between the hydroxyl groups of Sn3(OH)₂ and TiO₂. The authors also established the relevance of the O₂ concentration. Recycling studies with the photocatalyst Sn3(OH)₂–TiO₂ showed that it can be reused seven times without loss of activity.¹⁰¹

Li and co-workers^103,104 evaluated the effect of the different peripheral substituents in porphyrin derivatives in the photodegradation of the pollutants RhB and 4-NP. They found that among Cu37–TiO₂, Cu38–TiO₂, and Cu39–TiO₂ (Fig. 13), the best photocatalyst for RhB degradation was Cu39–TiO₂ (Table 7), but for the degradation of 4-NP it was the second best photocatalyst. Their results confirmed that the impregnation of TiO₂ with Cu39 enhanced its photocatalytic activity concerning the degradation of both 4-NP and RhB. The photocatalytic activity of Cu38–TiO₂ was assessed regarding the photodegradation of 4-NP in an aqueous solution under white light irradiation with an appropriate amount of H₂O₂. Under those conditions, a remarkable photocatalytic activity was observed, with almost complete degradation of 4-NP after 160 min of irradiation. Cu38–TiO₂ showed a catalytic efficiency of 86.7% even after 6 cycles. The superior photocatalytic activity of Cu38–TiO₂ is probably due to the presence of the heterojunction along with the higher surface, caused by its mesoporous structure. Cu38–TiO₂ was also used as a photocatalyst in 4-NP degradation in another work.¹⁰⁵

Fig. 13 Free-base and Cu(II) porphyrin derivatives 37–39, cardanol-substituted porphyrins 40 and 41 and porphyrin derivatives 50–52. Used to prepare the Por–TiO₂ hybrid materials.

Li and co-workers¹⁰⁶ evaluated the photocatalytic activity of the TiO₂-based materials Cu39–TiO₂, Zn39–TiO₂ and Ni39–TiO₂ (Fig. 13) in aqueous solution under halogen lamp irradiation and compared it with bare TiO₂. All porphyrinic materials revealed enhanced photocatalytic efficiency in the photodegradation of 4-NP, with Cu39–TiO₂ being the best photocatalyst followed by Zn39–TiO₂ and Ni39–TiO₂ (Table 7); bare TiO₂ was the least active. This result can be justified by the better combination of the excited potential of the CB of both Cu39 and TiO₂. Compared with Cu2–TiO₂, the higher catalytic activity of Cu39–TiO₂ is probably due to the peripheral OH group that can act as an anchoring group allowing a better interaction between porphyrin and TiO₂. This is beneficial to the initial electron injection from the porphyrin excited state to the TiO₂ CB, leading to an effective photoactivity.

Vasapollo and co-workers¹⁰⁷ used cardanol, a by-product of the food industry, for the synthesis of the “cardanol-based” free-base porphyrins H₂40 and H₂41 as well as their Zn(II), Cu(II), Co(II), and Fe(III) complexes (Fig. 13). After their immobilization on TiO₂, the materials were tested as sensitizers in 4-NP degradation under UV irradiation for 180 min (Table 7). All metallated complexes (Zn40, Cu40, Co40, Fe40Cl, Zn41, Cu41, Co41, and Fe41Cl) showed a slightly better photocatalytic activity (≈98%) than the free-base derivatives H₂40 and H₂41 (≈94%). Recycling and stability experiments revealed that all photocatalysts are stable, with the most effective ones being the Cu(II) complexes that could be used for six cycles without loss of activity. The better photoactivity of Cu(II) complexes can be assigned to the Cu(II)–Cu(I) photocatalytic redox cycle. Finally, it was concluded that the oxidizing species responsible for the degradation of 4-NP do not interact with the photocatalyst since the C–C double bond of the cardanol unit appears to be present at the end of the process.

Li and co-workers¹⁰⁸ also reported the synthesis of the free-base H₂42 and its Cu(II) complex Cu42 (Fig. 14). These compounds were used to sensitize TiO₂ and the catalytic ability of the resulting hybrids to photodegrade 4-NP was evaluated. The photocatalytic assays were performed in aqueous media under white light and UV irradiation in the presence of H₂O₂. The addition of H₂O₂ to the photocatalytic system prompted a high photocatalytic activity of Cu42–TiO₂ after white light irradiation for 160 min (Table 7). An enhanced photocatalytic activity was also observed when irradiated with UV light but only a small increase when H₂O₂ was added.

Fig. 14 Free-base porphyrin 42, its Cu(II) complex, Zn(II) porphyrins Zn43–Zn46, M3 and mono(substituted-phenyl)porphyrins 47–49 immobilized on TiO₂.

Chen and Shen¹⁰⁹ prepared TiO₂ modified with four Zn(II) porphyrins 43–46 (Fig. 14) in order to study the effects of the spacer length of a peripheral substituent on the photodegradation of MO in an aqueous solution under white light. The results showed that all porphyrins enhanced the photocatalytic activity of bare TiO₂ with the following order: Zn44–TiO₂ > Zn43–TiO₂ > Zn45–TiO₂ > Zn46–TiO₂ > bare TiO₂. The photocatalytic activity of Zn44–TiO₂ was much higher than the others (Table 7) because it presented the best match with the CB potential of TiO₂. Furthermore, the spacer length of the peripheral substituent can influence the interaction between the Zn(II) porphyrin and TiO₂, consequently influencing the initial electron injection process from the porphyrin excited singlet state to the CB of TiO₂, essential for an effective photocatalytic effect. The authors performed reusability studies and all the photocatalysts can be reused 6 times without significant loss of activity (10%).

meso-(Hydroxyphenyl)porphyrins and their derivatives are frequently used to sensitize TiO₂ and the resulting materials typically show high photocatalytic activity.^110–117 For instance, Mu and co-workers¹¹⁰ used porphyrins H₂47, Zn47 and Sn47Cl₂ (Fig. 14) to sensitize TiO₂ nanoparticles in order to improve the photocatalytic activity of bare TiO₂ nanoparticles. Among the four catalysts (H₂47, Zn47, Sn47Cl₂, bare TiO₂), the sensitizer Sn47Cl₂–TiO₂ showed the highest photocatalytic activity, being able to degrade 86% of MO after 180 min irradiation with white light (Table 8). The results also confirmed that the central metal ion in the porphyrin significantly influenced the photocatalytic activity of the hybrid materials.

Table 8 meso-(Hydroxyphenyl) porphyrins–TiO₂ used for the photodegradation of 4-NP and MO

Por	Pollutant	Irradiation time (min)	Light source	Degradation rate (%)	Recycle	Ref.
N.E. – not evaluated.
Sn47Cl	MO	180	Visible (λ > 320 nm)	86	—	110
Zn47				50
H247				60
Cu47	4-NP	300	UV–vis (λ = 380–780 nm)	100		111
		60	UV–vis (λ = 340–780) nm	90		112
Cu48		300	UV–vis (λ = 380–780 nm)			111
		60	UV–vis (λ = 340–780 nm)	95		112
Cu49		300	UV–vis (λ = 380–780 nm)	100		111
		60	UV–vis (λ = 340–780 nm)			112
H250			Visible λ >350 nm	N.E.		113
Cu50				96
Co50				81
Zn50				86
Cu51		300		100		114
Cu52		45			6 cycles (15% loss)	115
Co52		75			6 cycles (22% loss)
Zn52					6 cycles (19% loss)
Cu53		60		97	—	116
		40		100	6 cycles (10% loss)
Cu 54		60			—	117
		30			6 cycles (10% loss)

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Li and co-workers¹¹¹ immobilized the mono(substituted-phenyl)porphyrins Cu47, Cu48 and Cu49 (bearing hydroxy, ester or carboxy groups, respectively, Fig. 14) on TiO₂ and used the resulting hybrids for the photodegradation of 4-NP in aqueous solution under simulated solar irradiation. It was found that porphyrins Cu47–Cu49 enhanced the catalytic activity of bare TiO₂ for 4-NP photodegradation after 300 min of irradiation with a xenon lamp (35% with bare TiO₂ and almost complete degradation using the hybrids) – Table 8. Cu49–TiO₂ was the best photocatalyst followed by Cu47–TiO₂, Cu48–TiO₂, and bare TiO₂. Also, the authors concluded that a larger polarity of the substituent groups and a stronger interaction between Cu47–Cu49 and TiO₂ accelerate the electron injection process and improve the electron–hole separation. Cu49 has the largest polarity and its impregnation on TiO₂ afforded the highest photocatalytic activity. Therefore, the polarity of the porphyrin substituents is also an important factor to be considered in the photodegradation of organic pollutants. The photodegradation results confirmed that the impregnation of TiO₂ with Cu47, Cu48, and Cu49 increased the photodegradation of 4-NP and RhB. It is important to highlight that the different peripheral substituents in the porphyrin macrocycle led to different catalytic activities; porphyrins containing –OH or –COOH groups (Cu47 and Cu49) were the best ones for the degradation of 4-NP and RhB. These results can be justified by the strong interaction of these groups with the surface of TiO₂.

Later, the same authors compared the photodegradation efficiency of porphyrins Cu47–Cu49 (Fig. 14) immobilized on “home-prepared” and commercial TiO₂ (p-TiO₂ and c-TiO₂, respectively).¹¹² The results showed that the commercial c-TiO₂ caused 84.9% degradation of 4-NP in aqueous solution under metal halide lamp irradiation for 60 min. However, under the same experimental conditions, the “home-prepared” p-TiO₂ just exhibited a photodegradation rate of 43.6%. All Cu47–TiO₂ hybrids showed higher photocatalytic activity than p-TiO₂ in the following order: Cu49–c-TiO₂ ≈ Cu49–p-TiO₂ > Cu47–c-TiO₂ ≈ Cu48–p-TiO₂ > Cu47–p-TiO₂ > c-TiO₂ > Cu48–c-TiO₂ > p-TiO₂. It is well known that the surface area along with the hydroxylation level of TiO₂ could affect the interaction of the sensitizer and TiO₂, which, in turn, improves the photocatalytic activity. Moreover, the anchoring group also plays an important role which could explain the higher activity of Cu49; the carboxy group could accelerate the electron transfer from the porphyrin to TiO₂. The formation of porphyrin dimers Cu47–Cu47, Cu48–Cu48, and Cu49–Cu49 on the lower and less hydroxylated surface area of c-TiO₂ was also detected. It is important to highlight that regardless of the porphyrin chosen to sensitize TiO₂, the monosubstituted porphyrin structure is more beneficial for increasing photodegradation than the dimeric structures.

In order to understand the influence of the metal ion in 5,15-bis(4-hydroxyphenyl)porphyrins in the photodegradation of 4-NP, Li and co-workers¹¹³ immobilized the free-base H₂50 along with the metallated porphyrins Cu50, Zn50, and Co50 (Fig. 15) on the surface of TiO₂. The four porphyrins and the corresponding TiO₂ composites were then evaluated as photocatalysts to degrade 4-NP. The impregnation of porphyrins M50 (M = 2H, Cu(II), Zn(II), or Co(II)) did not influence the particle size of TiO₂ but increased its photocatalytic activity. The four photocatalysts enhanced the photoactivity in the following order: Cu50–TiO₂ (95.8%) > Zn50–TiO₂ (85.6%) > Co50–TiO₂ (80.8%) > Cu3–TiO₂ (66.8%) > bare TiO₂ (60%) after irradiation for 60 min (Table 8). These results showed that the metal ion plays, effectively, an important role in the photodegradation efficiency. Also, the OH anchoring group of the metalloporphyrin is a factor that improves its efficiency as a photocatalyst. Moreover, the results of photoluminescence showed that the three photocatalysts M50 (M = Cu(II), Zn(II), or Co(II)) could increase the separation efficiency of the photoinduced electron and hole. The highest photocatalytic activity was observed with the Cu50–TiO₂ hybrid.

Hu and co-workers¹¹⁴ synthesized the dicarboxylic acid Cu51 (Fig. 15 and Table 8), impregnated it on TiO₂ and evaluated the photocatalytic activity of the resulting composite on the degradation of 4-NP in aqueous media under xenon lamp irradiation. The results showed that the Cu51–TiO₂ composite's photoactivity was enhanced when compared with bare TiO₂ due to the interaction between the carboxy group of the porphyrin and the hydroxyls anchored in the TiO₂ surface.

Fig. 15 Porphyrin derivatives 50–52.

As already mentioned, the metalloporphyrins with carboxy groups exhibit higher photocatalytic activity due to the strong interactions between the –COOH groups and TiO₂. Also, the role of the central metal is revealed to be crucial, and for that reason, Li and co-workers¹¹⁵ studied the influence of different metals on the photocatalytic activity. Cu52–TiO₂ is still the most efficient photocatalyst, followed by Co52–TiO₂, Zn52–TiO₂ (Fig. 15), and bare TiO₂. The Cu(II) ions can easily accept electrons to reach a steady state, causing the isolation of electron holes when compared with Co(II) and Zn(II) in M52–TiO₂. All the metalloporphyrin composites could be recycled 6 times with a maximum loss of 22% (Table 8) in the photodegradation efficiency.

The two porphyrinic mesoporous materials Cu53–TiO₂ and Cu54–TiO₂ (Fig. 16) are efficient photocatalysts for the degradation of 4-NP (Table 8), but the highest efficiency was observed for Cu54–TiO₂.^116,117 The chemical interaction between the carboxy groups of Cu54 and TiO₂ is stronger than the physical interaction of the ester groups of Cu53 with TiO₂.¹¹⁶ As the interaction strength increases, the electron transfer efficiency also increases, leading to a better photocatalytic effect. According to the authors, the electron transfer occurs through space, not through the linker group that connects the porphyrin core to the anchoring group. Moreover, when considering the steric constraint of porphyrins, Cu54 bonds to the TiO₂ surface by two carboxy groups, with a certain planar deviation, whereas Cu54 places vertical to the surface of TiO₂. As a result, the photocatalytic efficiency is higher for Cu54–TiO₂ than for Cu53–TiO₂. Moreover, the efficiency is maintained at 90% after six cycles.

Fig. 16 meso-Tetra(substituted-phenyl)porphyrin derivatives 22, 53–57.

Tasseroul and co-workers¹¹⁸ designed xerogels based on TiO₂ modified with the free-base meso-tetrakis(4-carboxyphenyl)porphyrin H₂22 and its Ni(II) complex Ni22 (Fig. 16) in order to assess their potential to degrade 4-NP (Table 9). An improvement of the photocatalytic activity was observed for the xerogels dried for 10 days, with this effect being more pronounced with Ni22. The high specific surface area of the xerogels allowed the adsorption of 4-NP to be increased and, consequently, the reaction rate to increase.

Table 9 Tetracarboxylic porphyrins M22, H₂56 and Zn64 immobilized on TiO₂ for the photodegradation of several pollutants

Por	Pollutant	Irradiation time (min)	Light source	Degradation rate (%)	Recycle	Ref.
a Dried in 3 days. b Dried in 10 days. c 80 mg of photocatalyst. d 20 mg of photocatalyst. e Presence of SA.
H222	4-NP	360	Visible (λ > 350 nm)	0^a	—	118
				30^b
Ni22				42^b
H222	MB	180	UV (λ = 254 nm)	74		119
Mn22Cl				30
H222	Acid chrome blue K	15	Visible (λ > 400 nm)	94		120
H256		60		96
H222	Famotidine	180		100	1 cycle (33% loss)	123
		30	Natural sunlight	90	—
	Tamsulosin	180	Visible (λ > 400 nm)	20
	Solifenacin			25
H222	Oxolinic acid		Sunlight simulator	60		124
Zn22				52
Cu22				68
Sn2				50
H222	Oxytetracycline			60
Zn22				35
Cu22				62
Sn2				60
H222	Atrazine	60	Visible (λ > 420 nm)	25		126
Fe22Cl				57
Cu22				82
Zn22				40
H222	MB	30		98^c		128
				59^d
Cu22				91^c
				50^d
H222		60		100^c		129
Cu22		400		90^c		130
H222		80		70	10 cycles (10% loss)	135
Zn64				84
Fe22		120	UV–visible (λ > 100 nm)	85^e	5 cycles (10% loss)	136
				68

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To degrade MB, Viana and co-workers¹¹⁹ modified TiO₂ nanoparticles with H₂22 (Fig. 16) and Mn22Cl (Fig. 16). The carboxylic group, which is chemically linked to the TiO₂ nanoparticle, enhances the electronic coupling between the porphyrin and the semiconductor. In the case of the free base H₂22, the porphyrins could be anchored to TiO₂ by one or two of the carboxylic groups that appear to be perpendicular to the TiO₂ surface but Mn22Cl was anchored in a non-perpendicular way to TiO₂, suggesting that the filling of the TiO₂ pores by these molecules occurs, leading to a reduction of the catalyst surface area, in the case of Mn22Cl, decreasing its photocatalytic activity when compared with H₂22.

Shi and co-workers¹²⁰ modified TiO₂ with H₂22 or H₂56 (Fig. 16) and used the composites to degrade acid chrome blue K in aqueous media under visible-light irradiation. The results revealed that both H₂22–TiO₂ and H₂56–TiO₂ could efficiently degrade this dye in 15 min under natural sunlight (94.0% and 96.1% degradation, respectively, Table 9).

The presence of pharmaceuticals in the environment, from wastewater of the pharmaceutical industry, excreted by patients on drug therapy, and drugs flushed down the toilet, is worrying.¹²¹ Famotidine, a histamine H2 receptor antagonist used to prevent peptic ulcers,¹²² can be considered a wastewater pollutant if not destroyed properly. Nolan and co-workers¹²³ prepared a hybrid composed of TiO₂ modified with H₂22 and assessed its capability as a catalyst in the photodegradation of this drug using both visible and solar light irradiation. After reusing the photocatalyst, it could maintain 67% of the original photocatalytic activity. A careful analysis of the photodegradation products of famotidine using the H₂22–TiO₂ composite revealed that it did not lead to complete mineralization of the drug, but instead generated a range of degradation products, mainly the S-oxide of famotidine. The optimized conditions used to degrade famotidine were also used to degrade other pharmaceuticals (tamsulosin and solifenacin) but without success. It was concluded that the H₂22–TiO₂ composite is substrate specific.

D'Urso and co-workers¹²⁴ studied the ability of TiO₂ modified with cationic porphyrins M2 (M = 2H, Zn(II), and Sn(IV)(OH)₂) (Fig. 2) and anionic porphyrins H₂22, Zn22, and Cu22 (Fig. 16 and Table 9) to degrade oxolinic acid and oxytetracycline, two antibiotics used in aquaculture.¹²⁵ These pharmaceuticals are photodegraded in the presence of bare TiO₂ but it would be important to enhance its catalytic activity using porphyrins. Preliminary studies indicated that porphyrin H₂22 was the best ¹O₂ generator while Cu22 could not generate ¹O₂. All cationic porphyrins exhibited similar results. Despite these results, the Cu22–TiO₂ composite was considered the best photocatalyst among the hybrid materials, being slightly specific for oxolinic acid. Sn2(OH)₂ was chosen for immobilization on TiO₂ due to the presence of the hydroxyl axial ligands, but it did not show significant results in the photodegradation of both pharmaceuticals.

In 2009, Chovelon and co-workers¹²⁶ used porphyrins H₂22, Fe22Cl, Cu22, and Zn22 (Fig. 16) supported on TiO₂ to degrade atrazine¹²⁷ in aqueous solution under white light irradiation. Atrazine was only degraded after the addition of H₂O₂ and the best results were obtained with Cu22–TiO₂, which achieved 82% degradation after 1 h of irradiation. The photodegradation rates with the remaining composites were much lower: Fe22Cl (57%), Zn22 (40%) and H₂22 (25%) – Table 9. The authors could identify a series of intermediates after a first allylic oxidation.

Later, Zheng and co-workers¹²⁸ also used modified TiO₂ with H₂22 and Cu22 (Fig. 16 and Table 9) to degrade MB under white light. The best photocatalyst was also Cu22–TiO₂, with a MB photodegradation rate of 59.3% upon 40 min of irradiation (20 mg amount of photocatalyst). This efficiency could be increase to 92% with the increase on the amount of photocatalyst to 80 mg. Wang and co-workers also confirmed that H₂22–TiO₂ and Cu22–TiO₂ can efficiently degrade MB under white light irradiation.^129,130 Their studies on the photodegradation of MB revealed that the sensitization of TiO₂ with H₂22 and Cu22 enhances its photoactivity and provides good photocatalysts for wastewater treatment.

TiO₂ nanoparticles modified with H₂57 (Fig. 17) were prepared and used as photocatalysts for photodegradation of MB by Tsai and co-workers.¹³¹ The photocatalytic activity of H₂57–TiO₂ was assessed by performing MB degradation in aqueous media under white light. The absorption spectrum of the H₂57–TiO₂ nanoparticle was extended to 300–500 nm, which greatly enhanced the photocatalytic activity when compared with bare TiO₂ reaching 90% degradation of MB after 60 min of solar irradiation (Table 10).

Fig. 17 Free-base fluorinated H₂57 and Cu(II) derivatives Cu58–Cu61.

Table 10 Several porphyrins immobilized on TiO₂ for the degradation of several pollutants^{131–134,137–139}

Por	Pollutant	Irradiation time (min)	Light source	Degradation rate (%)	Recycle	Ref.
a In the presence of H2O2 and TEOA. b 0.001 molar ratio content. c 0.003 molar ratio content. d 0.006 molar ratio content.
H257	MB	60	Natural sunlight	90	—	131
Cu58	4-NP	120	Visible (λ > 420)	100		132
Cu59		45			5 cycles (30% loss)
	MB	50
	RhB	120
Cu60	4-NP			80	—
Cu61				60
Fe62Cl		180	Natural sunlight	100^a	3 cycles (10% loss)	133
H263	MB	60	UV	30	—	134
			Visible (λ > 400)	60
Fe63Cl			UV	85
			Visible (λ > 400)	0
H265	Acid black 1	120	Natural sunlight	100		137
H266	α-Terpinene	40	Visible light (λ > 410 nm)		4 cycles (6% loss)	138
H267	4-NP	480	UV–vis	52^b	4 cycles (20% loss)	139
				55^c	4 cycles (25% loss)
				61^d	4 cycles (10% loss)

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In 2016, Halder and Bhavana¹³² studied the role of conformation and the effect of the substituents in the meso-positions of porphyrins derivatives Cu58, Cu59, Cu60, and Cu61 (Fig. 17 and Table 10) adsorbed on TiO₂ in the photocatalytic degradation of 4-NP, RhB and MB. The activity of Cu59–TiO₂ is significantly higher than that of Cu58–TiO₂ and higher than both Cu60–TiO₂ and Cu61–TiO₂. For instance, the time required to degrade 4-NP with Cu59–TiO₂ is 45 min while 2 h are necessary when Cu58–TiO₂ is used. With Cu60–TiO₂ and Cu61–TiO₂ a complete degradation of 4-NP is not achieved in 2 h. Similar results were obtained for RhB and MB. The differences observed in the photocatalytic activity of these compounds were explained considering the position of the substituents and their conformations. The best photocatalyst (Cu59–TiO₂) was found to be stable under the experimental conditions and its efficacy was maintained after five cycles.

The photocatalytic activity of hemin anchored TiO₂ (Fe62Cl–TiO₂, Fig. 18 and Table 10) was investigated by ArunaKumari and Devi¹³³ through the degradation of 4-NP in aqueous solution under solar light irradiation but with the assistance of an appropriate amount of triethanolamine (used as an electron donor). The structural characterization of the catalyst confirmed that the chemisorption of the hemin molecules on the surface of TiO₂ is through O=C–O–Ti bonds which can not only enhance the extent of adsorption but also act as electron transfer channels that facilitate the electron transfer process. The photocatalyst can be reused 3 times without loss of activity. Pal and co-workers¹³⁴ investigated the potential application of hematoporphyrin-functionalized titania nanohybrids (H₂63–TiO₂ and Fe63Cl–TiO₂) (Fig. 18 and Table 10) for the photodegradation of MB in aqueous solution under visible and UV light. Under UV light, Fe63Cl–TiO₂ shows higher photocatalytic activity due to the cooperative functions of Fe63Cl and TiO₂ in generating reactive species. On the other hand, H₂63–TiO₂ shows higher photocatalytic activity under white light due to the absence of Fe(III) ions that obstruct the electron transfer from the porphyrin macrocycle to TiO₂.

Fig. 18 Structures of porphyrins 62 and 63.

In 2020, Son and co-workers¹³⁵ reported the synthesis of the porphyrin–squaraine-based sensitizer Zn64 (Fig. 19) that shows broad absorption in the visible and NIR regions, an essential feature in photocatalytic studies. This panchromatic porphyrin derivative was then immobilized on TiO₂ and the resulting composite was used as a catalyst for the photodegradation of MB. The photocatalytic studies were performed under white light irradiation and the results were compared with bare TiO₂ and H₂22–TiO₂ (a well-established good photocatalyst). After 80 min of irradiation, Zn64–TiO₂ exhibited the highest photocatalytic activity (∼80%) followed by H₂22–TiO₂ (∼70%) > TiO₂ (∼20%). As predicted, the fast and efficient activity of Zn64 is due to its broad absorption ability. The addition of H₂O₂ slightly improved the photocatalytic activity. The photocatalyst can be reused up to 10 cycles without significant loss of activity.

Fig. 19 Structure of the porphyrin–squaraine-based panchromatic sensitizer (Zn64) immobilized on TiO₂.

In 2015, Yao and co-workers¹³⁶ bonded porphyrin Fe22Cl to TiO₂ particles using 2-hydroxy-5-sulfosalicylic acid (SSA) or salicylic acid (SA) as a bridging molecule (Fig. 20A and B). These materials were then used as photocatalysts in the photodegradation of MB. The main objective was to check the influence of the bridge between the porphyrin and the TiO₂ support on the photocatalytic activity. The photocatalytic activity of both derivatives Fe22Cl–SSA–TiO₂ and Fe22Cl–SA–TiO₂ was evaluated under white light. Adsorption studies showed that Fe22Cl–SSA–TiO₂ and Fe22Cl–SA–TiO₂ present a higher capacity to adsorb MB than bare TiO₂. The FT-IR results showed that the Fe22Cl molecules, rather than being adsorbed onto the surface of TiO₂, are linked to TiO₂ through stable π-conjugated chemical bonds (Fig. 20A and B). The Fe22Cl–SSA–TiO₂ hybrid showed the best photocatalytic activity (99.3%) under visible light when compared with Fe22Cl–SA–TiO₂ (85.2%), Fe22Cl–TiO₂ (68.9%) and TiO₂ (30.7%). Even after being recycled 5 times, the best photocatalyst could degrade ca. 90% of MB. However, a slight decrease of photoactivity was observed when SA was used, suggesting that the bridge between the porphyrin and TiO₂ was weaker. Moreover, the degradation efficiency of MB with Fe22Cl–TiO₂ seems to decrease rapidly after recycling, which could be due to a bleaching process that is suppressed when SSA is used. The authors suggested that there is a break of the aromatic rings of MB, which was further degraded to CO₂ and H₂O.

Fig. 20 Representation of TiO₂ modified with porphyrin Fe22Cl using 2-hydroxy-5-sulfosalicylic acid (A) or salicylic acid (B) bridging molecules.

In 2017, Sobral and co-workers¹³⁷ prepared and used composite H₂65–TiO₂ (Fig. 21) for the photodegradation of acid black 1 under direct sunlight. The authors found that the photocatalytic activity of bare TiO₂ was efficiently improved with its sensitization with H₂65; the photocatalyst H₂65–TiO₂ could efficiently degrade 90.1% of acid black 1 after 90 min of direct sunlight irradiation, reaching a complete degradation after 120 min.

Fig. 21 Representation of H₂65–TiO₂ and H₂66–TiO₂.

In 2012, Cai and co-workers¹³⁸ developed TiO₂ microspheres modified with porphyrin H₂66 (Fig. 21). To sensitize TiO₂ microspheres with H₂66 the authors used allyl bromide and 3-mercaptopropyltrimethoxysilane (MPS) as linkers and studied the capacity of the photocatalyst in α-terpinene degradation under visible light irradiation. The structural characterization of this material revealed that H₂66–MPS was chemisorbed on the surface of TiO₂ through a Si–O–Ti bond (Fig. 21). Regarding the photodegradation assays, when compared with bare TiO₂, the presence of the H₂66–linker–TiO₂ greatly enhanced the degradation of α-terpinene under white light. The photocatalyst was reused four times with high efficiency (7% loss).

Mahy and co-workers¹³⁹ modified TiO₂ with three different ratios of a sylilated meso-tetrakis(4-aminophenyl)porphyrin derivative (H₂67–Si) and evaluated the activity of the resulting hybrids H₂67–Si–TiO₂ (Fig. 22) on the photocatalytic degradation of 4-NP under white light. The hybrid materials proved to be efficient photocatalysts and an improvement in photoactivity is observed when the amount of porphyrin improves.

Fig. 22 Representation of the porphyrin-based hybrid silica-titania photocatalyst 67.

After 24 h of testing, the photodegradation rate remains constant and no leaching was revealed. However, in the recycling tests, a decrease of photoactivity towards bare TiO₂ activity was observed due to bond breaking between H₂67–Si and TiO₂. Remarkably, H₂67–Si–TiO₂ was revealed to be nearly 6 times more photoactive than commercial Evonik P25 for 4-NP degradation under white light.

5.1. Other titanium oxide supports

5.1.1. Titanate nanotubes. In 2011, Kang and co-workers¹⁴⁰ studied the effects of anchoring modes on the properties of titanate nanotubes (TiNTs) modified with two porphyrins: Zn3 and Zn47 (Fig. 14). Both Zn3–TiNTs and Zn47–TiNTs are capable of MO degradation under white light irradiation (Table 11). In fact, the greater the amount of Zn3 or Zn47 loaded into TiO₂, the higher the photocatalytic activity. The structural characterization of the hybrids showed that Zn47 was bonded mainly on the outer surfaces of the TiNTs through hydrogen bonds while Zn3 was physically adsorbed into the pore channels of the TiNTs via a capillary process. The different anchoring modes influence the separation of photogenerated electrons and holes and the stability of the Zn47/Zn3 coupled TiNTs. The anchoring mode of Zn3 on the TiNTs allows an efficient photoinduced electron transfer which consequently confers a photocatalytic activity seven times higher than that of Zn47–TiNTs after 30 min of irradiation.

Table 11 Porphyrins immobilized on titanate tubes for the degradation of MO and MB

Por	Pollutant	Irradiation time (min)	Light source	Degradation rate (%)	Recycle	Ref.
Zn3	MO	30	Visible (λ > 370 nm)	80		140
Zn47		60		30
H222	MB	180		99		141
Zn22		50	Visible (λ > 420 nm)	100	5 cycles (10% loss)	142
Cu22		40	Visible (λ > 390 nm)		—	143
Fe22Cl		120		85	5 cycles (5% loss)	144

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In 2013, Wan and co-workers¹⁴¹ studied the photocatalytic degradation of some organic dyes with H₂22 (Fig. 16) modified TiNTs. Within one year apart, Zheng and co-workers,¹⁴² and Wan and co-workers^143,144 modified TiNTs with metallated porphyrins to obtain Zn22–TiNTs, Fe22Cl–TiNTs and Cu22–TiNTs, respectively, to be used as catalysts in the photodegradation of MB (Table 11).

According to Wan and co-workers,¹⁴¹ a degradation rate of 99.4% was achieved after white light irradiation for 3 h using 5% of H₂22 loaded into TiO₂ nanotubes. Photocatalytic studies showed that the best composite was also efficient in the photodegradation of other organic pollutants, namely RhB.

On the other hand, the studies reported by Zheng and co-workers,¹⁴² and Wan and co-workers^143,144 revealed that the photocatalysts Fe22Cl–TiNTs, Cu22–TiNTs and Zn22–TiNTs have good activity for MB degradation (higher than 90%) under white light irradiation (Table 11). Moreover, the unique cooperative effect caused by the high surface area, along with the strong adsorption ability and low gap energy, makes Fe22Cl–TiNTs and Zn22–TiNTs good photocatalysts.

5.1.2. BaTiO₃. Devi and Nithya¹⁴⁵ modified BaTiO₃ with hemin (Fe62Cl, Fig. 18) and evaluated the photocatalytic activity of the resulting material in the degradation of 4-CP under solar light irradiation, compared with bare BaTiO₃. After irradiation for 150 min, a 91.8% degradation of 4-CP was observed using Fe62Cl–BaTiO₃ while only 13.4% degradation was achieved with BaTiO₃. A complete (100%) photodecomposition of 4-CP was observed after irradiation for 60 min using the photocatalyst Fe62Cl–BaTiO₃ and H₂O₂. Trapping experiments with TBA and EDTA allowed identifying the main active species in the oxidative degradation process as hydroxyl free radicals and electron holes (h⁺).

6. Silica-supported porphyrins

Silica has excellent optical properties, exceptional adsorption and ability of selective separation of chemicals. Moreover, its large surface area, along with its good mechanical and thermal properties, makes silica a good material for active phase-support in catalysis.¹⁴⁶ Several types of silica structures can be used for supporting dyes (namely porphyrins) and the resulting hybrids have been frequently used as photocatalysts in water pollutant degradation. The role of those types of silica (nano)structures in photocatalysis is discussed in the following examples.

6.1. SiO₂ modified with porphyrins

Regarding SiO₂ hybrids, Cai and co-workers¹⁴⁷ reported the fabrication of silica microspheres functionalized with free-base porphyrin units. The hybrids H₂68 and H₂69 (Fig. 23) were used as catalysts for the photodegradation of naphthalene-1,5-diol (Table 12) in water and illumination with an iodine tungsten lamp. Hybrid H₂69 revealed much higher activity than SiO₂ alone or the porphyrin H₂68. After irradiation for 50 min, the degradation rate was 92.7% with hybrid H₂69 but only 42.9% in the presence of the unsupported porphyrin. The enhanced catalytic performance of hybrid H₂69 can be related to the presence of sulfonic groups. The authors also evaluated the reusability of microspheres H₂69 and found that their catalytic performance has slightly decreased after being reused five times.

Fig. 23 Representation of SiO₂–porphyrin microspheres 68 and 69.

Table 12 SiO₂ modified with porphyrins for the degradation of several pollutants

Por	Pollutant	Irradiation time (min)	Light source	Degradation rate (%)	Recycle	Ref.
H268	Naphthalene-1,5-diol	50	Visible light (λ > 410 nm)	43	—	147
H269				93	5 cycles (5% loss)
Sn3(OH)2	Naproxen	120	Visible light (λ > 400 nm)	30	—	148
	Trimethoprim			40
	Atorvastatin			70
	Tolmetin			75
	Losartan			100
	Propanolol
	Cimetidine
	Ranitidine
	Caffeine		Fluorescent lamp (visible light)	0
	Acetaminophen
	Amoxicillin
	Naproxen			50
	Trimethoprim			60
	Atorvastatin			70
	Tolmetin			90
	Losartan			100
	Propanolol
	Cimetidine
	Ranitidine
Fe22Cl	Phenol	90	Visible light (λ > 400 nm)	95		150
Cu71 (X = PF6^−)	2,6-DBP		Visible light (λ = 350–800 nm)	80		151
Cu71 (X = ClO4^−)		50		64
Cu72 (X = PF6^−)				55
Cu74 (X = ClO4^−)		50		61
H257	Metoprolol	720	Simulated solar irradiation	90		152

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Lee and co-workers¹⁴⁸ reported the preparation of two photocatalysts consisting of a Sn(IV) porphyrin derivative Sn3(OH)₂ supported on silica and the amino-C₆₀ derivative 70 also supported on silica (Fig. 24) and evaluated the influence of the two sensitizers on the photodegradation of a range of phenolic pharmaceutical compounds (Table 12) frequently found as water pollutants. Both photosensitizers were activated under white light to generate ¹O₂. The study demonstrated that Sn3(OH)₂ allowed more effective oxidation of the pharmaceuticals than 70. However, a decrease in the activity of Sn3(OH)₂–SiO₂ was observed during the recycling tests due to a self-destructive reaction via¹O₂ induced oxidation of the porphyrin macrocycle. On the other hand, the photocatalytic activity of the aminoC₆₀–SiO₂70 did not decrease when it was reused for five cycles.

Fig. 24 Representation of the photocatalysts Sn3(OH)₂–SiO₂ and aminoC₆₀–SiO₂70 used for the degradation of phenolic pharmaceuticals.

In 1993, Bauer and co-workers¹⁴⁹ reported that a simple photo-Fenton reaction (Fe²⁺, H₂O₂ and light) was adequate to increase the photodegradation of the wastewater pollutant 4-CP when compared with the same reaction in the dark. More recently, Diaz-Uribe and co-workers¹⁵⁰ produced a catalyst composed of meso-tetrakis(4-carboxyphenyl)porphyrin iron(III) (Fe22Cl) (Fig. 16) adsorbed on silica and evaluated its efficiency in the oxidation of phenol assisted by white light. In this study, the phenol concentration was reduced from 100 mg L⁻¹ to ca. 5 mg L⁻¹ after 90 min of white light irradiation via oxidation through the photo-Fenton reaction. Hydroquinone, catechol, and p-benzoquinone were identified as intermediate oxidation products but the chain reaction proceeds until the formation of short chain acids that are finally transformed into carbon dioxide and water. The main ROS responsible for the degradation of phenol was OH˙⁻ and its presence during the photooxidation process was corroborated by chemical trapping with terephthalic acid. The study was carried out at four pH values (1.8, 2.4, 2.8, and 3.2), but it was found that the highest rate constant (3.6 ± 0.2 × 10⁻² min⁻¹) and the highest yield of the photooxidation process occurred at pH 2.8.

Setsune and co-workers¹⁵¹ synthesized the copper(I) complexes of an N²¹,N²²-etheno bridged octaethylporphyrin (Cu71 (X = PF₆⁻ or ClO₄⁻)) and the copper(I) complex of N²¹,N²²-etheno bridged meso-tetraphenylporphyrin (Cu72) with different counter ions (e.g., PF₆⁻ and ClO⁻) (Fig. 25 and Table 12). These complexes were also immobilized on silica gel and used as catalysts for the photooxidation of phenols (2,6-di-t-butylphenol (2,6-DBP) and 4-propylphenol) both under homogeneous and heterogeneous conditions. A plausible mechanism for the heterogeneous photooxidation of 4-propylphenol is shown in Fig. 25.

Fig. 25 Copper(I) complexes of the N²¹,N²²-etheno bridged porphyrins 71 and 72, and the proposed mechanism of the photodegradation of 4-propylphenol catalysed by these complexes immobilized on silica gel.

For the homogenous catalysis, an acetonitrile solution of 2,6-DBP and Cu71 (X = PF₆⁻) was irradiated with white light for 90 min. The only detected oxidation product was 2,6-di-tert-butyl-1,4-benzoquinone, formed in 80% yield (determined by ¹H NMR). On the other hand, Cu71 (X = ClO₄⁻) achieved a conversion rate of 64% after 50 min of irradiation; without photoirradiation, only 7% of 2,6-DBP was modified. The efficiency of Cu72 was not very different from the one of OEP derivative Cu71 (X = ClO₄⁻). The N²¹,N²²-etheno bridge porphyrin derivatives could afford higher phenol conversion than the free-base H₂73 and the tetradentate Cu73 complex. This could be due to their relative photostabilities since a Pd complex of N²¹,N²²-etheno bridge porphyrin was reported to be more photostable than the free-base porphyrin.^153,154 The di(μ-hydroxo)dicopper(II) diporphyrin Cu74 (obtained by air-oxidation of complex Cu71 (X = PF₆⁻)) was found to have a similar reactivity in photooxidizing 2,6-DBP (61%) as Cu71 (X = PF₆⁻), which indicated that derivative Cu71 (X = PF₆⁻) acts in the photodegradation reaction through the formation of complex Cu74. Regarding the heterogenous catalysis, the phenol derivative was degraded within 60 min of irradiation with white light when incubated with Cu71 (X = PF₆⁻). Also, this compound was immediately oxidized into a complex Cu74-type which proved to be essential for the photoreaction process. It is important to highlight that, contrary to the homogenous catalysis, the photooxidation of phenol derivatives using Cu71 (X = PF₆⁻) was faster than with its respective free-base in the heterogeneous one. The final composites could be easily recovered and reused up to three times without loss of activity.

Metoprolol is a well-known prescribed β-blocker that is not entirely removed from water in sewage treatment plants. For that reason, Simões and co-workers¹⁵² studied the photocatalytic activity of the free-base H₂57 as a photosensitizer, under homogeneous and heterogeneous conditions, in metoprolol degradation. The promising results led to the study of H₂57 immobilized on 3-aminopropyl silica (Fig. 26).

Fig. 26 Representation of the H₂57–silica photocatalyst (more para-F atoms may be substituted).

Under homogeneous conditions and with simulated solar radiation, H₂57 was found to be efficient in the photodegradation of metoprolol, with ca. 90% degradation after 12 h. Similarly, the use of the heterogeneous catalyst H₂57–silica allowed 60% degradation of metoprolol after irradiation for 12 h. Despite the decrease in the photodegradation efficiency, the use of the immobilized porphyrin allows its easy recovery. Studies performed in a sample from the secondary effluent of a wastewater treatment plant fortified with metoprolol revealed that the efficiency of H₂57–silica was not decreased by the complex matrix of the effluent. It was confirmed that singlet oxygen was the main component responsible for the photodegradation of metoprolol.

6.2. MCM-41 silica modified with porphyrins

Pereira and co-workers¹⁵⁵ reported the preparation of MCM-41 with encapsulated porphyrins M75 (M = 2H or Cu(II)) and M76 (M = 2H or Cu(II)) (Fig. 27) in a ship-in-a-bottle way, and the resulting materials were used to degrade 2,4,6-trimethylphenol and two pesticides: fenamiphos and diuron. These pesticides are considered potential groundwater contaminants and it is necessary to find efficient and economical ways to eliminate them. The four immobilized porphyrins M75 (M = 2H or Cu(II)) and M76 (M = 2H or Cu(II)) @MCM-41 were evaluated as photocatalysts for the degradation of 2,4,6-trimethylphenol under polychromatic light (300–460 nm). The best results were obtained with the photocatalyst H₂76@MCM-41, achieving a degradation of 80%. The studies were extended to the degradation of fenamiphos and diuron in an aqueous solution. The degradation of both pollutants was possible in the presence of O₂ under white light irradiation using H₂76@MCM-41 as a photocatalyst, which was recovered by filtration and reused with no significant decrease of its catalytic activity.

Fig. 27 Representation of MCM-41 immobilized porphyrins 75 and 76.

7. Immobilized porphyrins on a diversity of supports

7.1. Self-assembly of supramolecular materials

Bhosale and co-workers¹⁵⁶ developed a photocatalyst based on a supramolecular nanostructure induced by arginine and the free-base meso-tetrakis(4-carboxyphenyl)porphyrin (H₂22, Fig. 8) that can degrade RhB. In the presence of L-/D-arginine, H₂22 forms one-dimensional, self-assembled supramolecular nanobelts with a well-defined morphology that shows an enhanced photocatalytic performance with a maximum RhB degradation up to 90% (when compared with only 15% degradation induced by the H₂22 monomer) after 3 h under white light irradiation.

7.2. Porphyrin–ZnO composites

Zinc oxide is an efficient photocatalyst. In fact, it has been reported^157–159 that ZnO can achieve more significant photocatalytic activity than TiO₂, the most used inorganic material in catalysis. When modified with porphyrin derivatives, the photocatalytic activity of ZnO is substantially improved, as shown in the next examples concerning the degradation of RhB, MB, MO, and 4-NP.

Kang and co-workers¹⁶⁰ reported the formation of an inorganic–organic material composed of ZnO microrods and nano-heteroaggregates containing the anionic and cationic porphyrins Co1 and H₂77 (Fig. 28). The modified ZnO microrods that revealed a photocatalytic activity higher than those of the porphyrin monomers and pure ZnO prompted a total photodegradation of RhB in water, at room temperature, after white light irradiation (λ ≥ 420 nm) for 60 min (Table 13).

Fig. 28 ZnO microrods modified by a porphyrin heteroaggregate.

Table 13 Porphyrin–ZnO composites for the degradation of several pollutants

Por	Pollutant	Irradiation time (min)	Light source	Degradation rate (%)	Recycle	Ref.
a In the presence of Cu(II) ions. b In the presence of Fe(III) ions. c In the presence of triethanolamine.
Co1/H277	RhB	60	Visible (λ ≥ 420 nm)	100	—	160
Cu78		210		63		161
		36	UV–vis (λ = 380–780 nm)	95	5 cycles (12% loss)
H278		30	Sunlight	98	4 cycles (15% loss)	162
H279
H263	MB	60	Visible light (λ > 395 nm)	55	—	37
				80^a
				17^b
H263			UV	25
				90^a
				100^b
H262		50	Solar	85	4 cycles (0% loss)	163
	RhB	80
	Eosin yellow
Fe62Cl	4-NP	120	UV	99	—	164
		180	Solar	99^c
Sn22Cl2	MO	240	Visible light (λ > 400 nm)	85		165

---

Li and co-workers¹⁶¹ reported the synthesis of an organic–inorganic photocatalyst made of Cu78 (Fig. 29) impregnated onto the surface of ZnO. The photocatalytic activity of Cu78–ZnO was evaluated in the photodegradation of RhB under either UV–vis (λ = 380–780 nm) or visible (λ ≥ 420 nm) lights. When irradiated with white light, 63% of RhB was photodegraded with Cu78–ZnO after 210 min (Table 13), but only 23% of RhB was degraded using ZnO (ZnO does not absorb white light). The photocatalyst's reusability was evaluated, and a decrease in the catalytic efficiency from 95% to 83% was observed after five cycles, indicating a slow reduction of the photocatalytic activity.

Fig. 29 Porphyrin derivatives used to prepare Cu78–ZnO, H₂79–ZnO and Cu79–ZnO hybrids.

Mele and co-workers¹⁶² accomplished the synthesis of composite nanomaterials based on ZnO impregnated with a lipophilic tetrasubstituted porphyrin bearing hydrogenated cardanol units (H₂79 and Cu79) (Fig. 29). The FTIR analysis showed that non-covalent interactions occur between H₂79 and the ZnO support. Sunlight irradiation for 30 min was sufficient to achieve almost complete decomposition (98%) of RhB in the presence of both photocatalysts (H₂79–ZnO and Cu79–ZnO) – Table 13. These results indicate that these hybrids may be used to eliminate pollutants efficiently from water under ambient conditions. The photocatalysts' reusability was assessed, and it was observed that after four cycles, their photodegradation efficiency was kept above 83%.

As already mentioned, MB is extensively used in the dye industry that can produce hazardous environmental effects in aquatic and human beings.^166,167 To remove it from water, Pal and co-workers,³⁷ and more recently Thomas and co-worker,¹⁶³ reported the formation of light-harvesting nanohybrids composed of ZnO nanorods incorporating organic pigments, namely hematoporphyrin (H₂63) (ref. 37) and protoporphyrin IX (H₂62) (ref. 163) (Fig. 18 and Table 13).

Pal and co-workers³⁷ showed that the photoactivation of H₂63–ZnO nanohybrids with white light leads to the formation of reactive oxygen species and consequently to the photodegradation of MB. Overall, this work³⁷ revealed that this nanohybrid is an effective material for wastewater treatment. Moreover, in another work, they used the nanohybrids Cu62–ZnO and Fe62Cl–ZnO for the photodegradation of MB under white light in the presence of naturally abundant Cu(II) and Fe(III) ions and discovered that these ions could enhance or retard, respectively, the photodegradation of MB. In the presence of Cu(II) ions, 80% degradation of MB was achieved while only 17% degradation was observed in the presence of Fe(III) ions; these results may be correlated with the additional structural stability of the porphyrin macrocycle metallated with Cu(II).³⁷

The photocatalyst composed of protoporphyrin IX–ZnO nanorods (H₂62–ZnO, Fig. 18) was also reported by Thomas and co-workers.¹⁶³ The photocatalytic activity of the prepared nanorods was evaluated for several pollutants, such as MB, RhB, and eosin yellow, achieving >80% degradation after 50 or 80 min of solar light irradiation (Table 13). The photocatalyst was found to be stable and reusable for four cycles, and the photocatalytic activity was retained. It is noteworthy that the spectroscopic results revealed that H₂62 was chemisorbed to the ZnO surface through O=C–O–Zn bonds. It becomes important to highlight that H₂62 was not the best photocatalyst despite having a good photocatalytic activity. The best was perylene-3,4,9,10-tetracarboxylic dianhydride–ZnO followed by fluorescein–ZnO and phosphotungstic acid–ZnO.¹⁶³

Due to its high stability, 4-NP is a water-soluble highly toxic and hazardous pollutant. Once dissolved in water, its removal by conventional methods is extremely difficult.^168–170 However, Devi and co-workers¹⁶⁴ reported the preparation of a ZnO-based photocatalyst able to degrade 4-NP under UV and solar light irradiation. According to the authors, almost total degradation of 4-NP occurs after 120 min under UV light irradiation in the presence of ZnO modified with hemin (chloro(protoporphyrinato)iron(III)) (Fe62Cl, Fig. 18 and Table 13) and H₂O₂. On the other hand, the same degradation rate is achieved after 180 min with triethanolamine as an electron donor under solar light irradiation. The presence of electron donors enhances the efficiency of the degradation process by regenerating neutral hemin from its oxidized form. The neutral hemin can then absorb white light, undergo excitation, and promote a new cyclic process.¹⁶⁴

MO, due to its toxicity in water, is frequently used in photodegradation studies.¹⁷¹ Rahimi and co-workers¹⁶⁵ reported the photodegradation of 85% of MO using ZnO nanorods with tin(IV) porphyrin derivative Sn22Cl₂ after white light irradiation for 240 min. It was found that O₂˙⁻ is the principal species responsible for the photodegradation of MO.

7.3. CdS nanocomposites

Cadmium sulphide CdS nanocomposites^172,173 are an emerging class of materials used in photocatalysis. Various research groups¹⁷⁴ used the free-base H₂22 (Fig. 30) to prepare CdS nanocomposites used to degrade RhB. It is important to highlight that the photodegradation of RhB with H₂22–CdS was more efficient under solar light irradiation than with UV light. The photocatalyst was reused five times without loss of its stability.¹⁷⁴

Fig. 30 Representation of CdS nanocomposites modified with H₂22 for the photodegradation of dyes.

7.4. Porphyrin–polyoxometalate hybrid materials

Polyoxometalates (POMs) are inorganic metal oxide clusters with remarkable properties. They already have been applied in photocatalysis, electrochemistry, photochromic materials, among others.^175,176 Their possible recuperation and reusability make them appealing photocatalysts for the oxidation of organic compounds.^175–177 In 2014, Rabbani and co-workers¹⁷⁸ reported the synthesis of an organic–inorganic framework where H₂3 (Fig. 2) was immobilized on the surface of the polyoxometalate H₃PMo₁₂O₄₀. The photocatalytic activity of the hybrid material H₂3–H₃PMo₁₂O₄₀ regarding the photodegradation of MB in water under white light irradiation was evaluated and the results confirmed that its efficiency for MB degradation is higher than bare H₃PMo₁₂O₄₀. These hybrid materials are, thus, a good alternative for MB photodegradation in an aqueous medium.

7.5. Porphyrin–Bi₂MoO₆ composites

The photocatalytic performance of Bi₂MoO₆ for water splitting and degradation of organic compounds under visible-light irradiation is well known.¹⁷⁹ However, Bi₂MoO₆ only absorbs white light in the 420–500 nm region. To extend its absorption range to use the sunlight fully, Zhang and co-workers¹⁸⁰ introduced different amounts of Cu22 (Fig. 8) into Bi₂MoO₆ and studied its influence in the photodegradation of RhB. As expected, the resulting composite Cu22/Bi₂MoO₆ revealed enhanced photocatalytic activity, which can be attributed to the broadened absorption spectrum as well as the increased charge separation rate in the composite. It is important to highlight that the amount of Cu22 loaded into Bi₂MoO₆ is an important factor for the photocatalytic activity of the composite since above a certain fraction of porphyrin/Bi₂MoO₆ a decrease of the photocatalytic activity was observed. This result can be explained by the fact that more active reaction sites on the surface of Bi₂MoO₆ become occupied. The photocatalyst can be reused for five cycles without loss of activity.

7.6. Porphyrin–bismuth ferrite hybrids

Condorelli and co-workers¹⁸¹ functionalized multiferroic bismuth ferrite (BFO) with porphyrin H₂22 (Fig. 8) and the resulting hybrid showed an enhanced photocatalytic activity for the degradation of MB and RhB when compared with bare BFO. The photocatalytic studies were performed in water under simulated solar light irradiation. This hybrid was reused up to four cycles without loss of activity.

7.7. Magnetic nanoparticles

Kim and co-workers¹⁸² reported the use of water-soluble magnetic nanoparticles functionalized with a Pt(II) 5,15-bis(carboxyphenyl)porphyrin (Pt80, Fig. 31 and 32) as a catalyst for the photodegradation of 2,4,6-TCP in aqueous media under white light irradiation. After immobilization on the magnetic nanoparticles, porphyrin Pt80 maintains its optical and functional properties along with a high singlet oxygen quantum yield. When irradiated with white light, the excited nanoparticles convert ³O₂ into ¹O₂ and other ROS that are responsible for the degradation of 2,4,6-TCP. Despite their high solubility in water, the magnetic property of the microstructure allows its recovery and reuse up to seven cycles by applying a magnetic field.

Fig. 31 Representation of magnetic nanoparticles–porphyrin hybrids used in this sub-section.

Fig. 32 Representation of the silica magnetic nanoparticles functionalized with H₂57, Pt80 H₂81, H₂82 and Zn83.

Tomé and co-workers¹⁸³ developed silica core–shell magnetic nanoparticles (NP) functionalized with three different porphyrins, H₂57, H₂81, and H₂82 (Fig. 32) and assessed their ability as photocatalysts for the degradation of 17β-estradiol. All non-immobilized porphyrins (H₂57, H₂81, and H₂82) could achieve complete degradation of 17β-estradiol after 8 h of white light irradiation. However, when immobilized in magnetic nanoparticles, H₂82–NP could induce a complete degradation of the hormone, whereas H₂57–NP and H₂81–NP could only achieve 50% degradation after the same time of irradiation.

In 2021, Simões and co-workers¹⁸⁴ developed a silica coated nanomagnet porphyrin hybrid and compared the ability of the Zn83–nanomagnet hybrid (Fig. 32) as a photocatalyst for the degradation MO with that of non-immobilized porphyrins H₂83 and Zn83. The photocatalytic assays were performed under white light irradiation in the presence of hydrogen peroxide. After 270 min of irradiation, complete degradation of the pollutant was achieved by using non-immobilized Zn83 or the Zn83–nanomagnet hybrid. In the absence of H₂O₂, 60% of MO was degraded when the Zn83–nanomagnet was used, whereas only 10% degradation was observed with the non-immobilized Zn83 and H₂83.

7.8. Metal-free graphite-like carbon nitride (g-C₃N₄)

Regarding this type of support, Liu and co-workers²⁸ published a review exploring the ability of graphitic carbon nitride modified with porphyrins, phthalocyanines or other similar dyes as photocatalysts for H₂ evolution, CO₂ reduction and pollutant degradation. Also, Raizada and co-workers¹⁸⁵ reviewed the removal of contaminants from wastewater through photocatalysis by non-noble metals doped on this support. Concerning this last application, they mentioned the work of Zhu and co-workers¹⁸⁶ where the hybridization of graphite-like carbon nitride (g-C₃N₄) with Cu22 (Fig. 8) is explored in the photodegradation of phenol under white light (Table 14). The Cu22 molecules are easily assembled on the surface of g-C₃N₄ nanosheets through covalent bond and intermolecular interactions, such as electrostatic interaction and π–π stacking interactions between Cu22 and the graphene nanosheets. When compared with pure g-C₃N₄, Cu22–g-C₃N₄ showed higher photocatalytic activity under white light irradiation. The enhancement of the photocatalytic activity derives from the efficient electron transfer process from the photoexcited Cu22 molecules to g-C₃N₄.

Table 14 Porphyrin–metal-free graphite-like carbon nitride (g-C₃N₄) for the degradation of several pollutants

Por	Pollutant	Irradiation time (min)	Light source	Degradation rate (%)	Recycle	Ref.
Cu22	Phenol	480	Visible (λ > 420 nm)	25	—	186
H222	RhB	60	Visible (λ > 380 nm)	100	4 cycles (0% loss)	187
Cu3		90	Visible (λ > 420 nm)	69	—	188
Ni3				93
Co3				100
Fe22		120		98		189

---

Cai and co-workers¹⁸⁷ adsorbed the metal-free porphyrin H₂22 (Fig. 8) on g-C₃N₄ (through π–π stacking interactions) and the resulting hybrid revealed a doubled photocatalytic activity for RhB degradation (Table 14) when compared with bare g-C₃N₄. The immobilization of the porphyrin on g-C₃N₄ broadened its absorption spectrum and improved its photocatalytic activity.

Zhu and co-workers¹⁸⁸ developed a series of Cu3, Ni3, and Co3 modified g-C₃N₄ (Fig. 33) in order to assess the photocatalytic degradation of RhB. Co3–g-C₃N₄ exhibited the highest photocatalytic efficiency reaching a total degradation (99.8%) after 90 min of irradiation, followed by Ni3–g-C₃N₄, Cu3–g-C₃N₄, and pristine g-C₃N₄. The addition of metalloporphyrins, particularly Co3, is favourable for the interfacial charge transfer process, by minimizing the electron–hole recombination, enhancing the photodegradation efficiency of RhB.

Fig. 33 Representation of the hybrid structures of Cu3–, Ni3–, and Co3–g-C₃N₄.

Moreover, Li and co-workers¹⁸⁹ constructed a visible light-driven PCN-222 g-C₃N₄ heterojunction using Fe22 (Fig. 8) to decompose RhB and ofloxacin. The composite 1.0 wt% PCN-222–g-C₃N₄ showed the best photocatalytic activity for ofloxacin and RhB degradation under white light irradiation. The photocatalytic activity of this composite was ca. 3 times higher than that of pristine g-C₃N₄, reaching ∼98% after irradiation for 120 min. This enhanced photocatalytic activity was attributed to the extended light response range, fast generation, efficient separation of the photogenerated carrier and enhanced adsorption capacity of organic contaminants. The prepared PCN-222–g-C₃N₄ photocatalyst exhibited excellent recyclability and can be reused four times without activity loss.

7.9. Layered hydroxides

Recently, Liu and co-workers¹⁹⁰ constructed an organic–inorganic hybrid palladium porphyrin/NiFe-LDH (Pd22/NiFe-LDH and H₂22/NiFe-LDH) (Fig. 16) and evaluated its ability as a photocatalyst for the degradation of tetracycline under white light irradiation. The results showed that Pd22/NiFe-LDH is more efficient, achieving a degradation rate of 91% when compared to H₂22/NiFe-LDH (74%) and bare NiFe-LDH (67%). This enhanced photocatalytic activity is due to the higher production of O₂˙⁻ and HO˙.

7.10. Lignin

Calvete, Pereira and co-workers¹⁹¹ described the photocatalytic degradation of trimethoprim and sulfamethoxazole, highly prescribed antibiotics, using a white mercury lamp, air as an oxidant and a photocatalyst based on the encapsulation of stable meso-tetra(2,6-dichlorophenyl) porphyrin (H₂84) (Fig. 34) into acetylated lignin nanoparticles. The authors could achieve almost compete degradation (99 ± 1%) of trimethoprim and sulfamethoxazole after 120 min and 240 min, respectively. The combination of these antibiotics is usually known as cotrimoxazole (also an antibiotic) and for that reason the authors also accessed the ability of this nanoparticle for its photodegradation. After 120 min of irradiation, it was possible to achieve the same photocatalytic rate. It is important to mention that there was no degradation in the dark or just in the presence of the lignin nanoparticles (without porphyrin) for these antibiotics. The photocatalyst could be reused up to 10 times without significant loss of activity (∼5% loss).

Fig. 34 Porphyrin H₂84 bearing 2,6-dichlorophenyl groups.

7.11. Agarose

Hou and co-workers¹⁹² developed a Tb-porphyrin (Tb22Cl) (Fig. 16) aerogel using agarose as a building block for the photocatalytic degradation of RhB. After 75 min under visible light irradiation (λ > 420 nm), it was possible to achieve 61% of degradation.

8. Immobilized porphyrins on modified inorganic supports

8.1. TiO₂–Zr-MOF

In 2017, Feng and co-workers¹⁹³ developed an efficient catalyst composed of a zirconium-based porphyrin–MOF PCN-222 immobilized onto the surface of TiO₂ nanoparticles via 4-mercaptopyridine, which is axially bonded to the metal centre of porphyrin Zn22 (Fig. 35). The composite was effective for RhB degradation under white light irradiation due to the synergistic effect of its components, being able to degrade 75% of RhB after irradiation for 270 min. Fluorescence spectroscopy studies confirmed that the enhanced photocatalytic activity resulted from the electron transfer from PCN-222 to TiO₂ and efficient charge separation. Moreover, the composite showed excellent stability and recyclability.

Fig. 35 Representation of zirconium-based porphyrin Zn22–MOF PCN-222 immobilized onto TiO₂ nanoparticles via 4-mercaptopyridine.

8.2. Doped TiO₂

Huang and co-workers¹⁹⁴ modified La-doped TiO₂ with Cu85 (Fig. 36) and assessed its ability as a photocatalyst for MO degradation. Under white light, the photocatalytic activity of La-TiO₂ is higher than bare TiO₂ and it increases with the sensitization by Cu85.

Fig. 36 Structure of the porphyrin 85 immobilized on La-doped TiO₂ and porphyrin used to prepare triad hybrid multilayers on quartz surfaces 86.

In another study,¹⁹⁵ it was observed that the maximum degradation of 4-NP was obtained when the free-base H₂23 (Fig. 11) was used with TiO₂ doped with Fe(III). This catalyst showed an enhanced photocatalytic activity compared with TiO₂ or Fe–TiO₂, causing a complete degradation of 4-NP after irradiation for 45 min. Finally, the photocatalyst can be recycled at least three times without loss of activity.

The photocatalytic activity of micro and nanostructures of TiO₂ impregnated with complexes of metalloporphyrin M23 (M = Mn(III), Fe(III), Ni(II), and Cu(II)) (Fig. 11) under UV light irradiation was also evaluated.¹⁹⁶ The highest photocatalytic activity was achieved with impregnated micro-TiO₂ catalysts, which indicates an improvement of the electron transfer process between the sensitizer and TiO₂ compared with the nano-crystalline structure. Cu23 revealed excellent performance when used in the micro-anatase composite.¹⁹⁶

Rahimi and co-workers¹⁹⁷ used the free-base porphyrin H₂22 to sensitize vanadium-doped TiO₂. Two photocatalysts were prepared: one containing an ester-like linkage formed between a carboxyl group of H₂22 and a hydroxyl group of the TiO₂ surface (H₂22–V-TiO₂) and the other an amide bond (H₂22-CO–NHR-V-TiO₂) (Fig. 37). These materials were used as catalysts for the photodegradation of MO in an aqueous solution under visible light irradiation. The photocatalytic activity of H₂8–V-TiO₂ was much higher than that of H₂8-CO–NHR-V-TiO₂ with rates of 95% and 62%, respectively, after irradiation for 180 min.

Fig. 37 Representation of vanadium doped TiO₂ modified with H₂22 using a 3-aminopropylsilane bridging unit.

Yao and co-workers¹⁹⁸ modified nitrogen doped TiO₂ powder with metalloporphyrins Co3, Ni3, Co55, and Ni55 (Fig. 2 and 16) and evaluated the photocatalytic ability of the resulting materials to degrade MB. Nitrogen-doped anatase powder (N-TiO₂) modified by each of the four metalloporphyrins exhibited higher degradation efficiency than the unmodified N-TiO₂. The results showed that Co3–N-TiO₂ and Co55–N-TiO₂ exhibited higher degradation efficiency than the corresponding nickel complexes Ni3 and Ni55–N-TiO₂, with Co3–N-TiO₂ being the best photocatalyst with 94.1% degradation of MB after 180 min irradiation with white light. Additionally, the composites were reused up to four times without significant loss of activity.

Rahimi and co-workers,¹⁹⁹ and Abou-Gamra and co-workers²⁰⁰ also used H₂22 (Fig. 16) on Co-doped TiO₂ and regular TiO₂ and assessed the ability of the composites to degrade MB and RhB. The results using H₂22–Co-TiO₂ (ref. 199) and H₂22–TiO₂ (ref. 200) showed that these materials are more efficient than bare Co-TiO₂ and TiO₂, respectively, for the degradation of MB and RhB under white light. The sensitization of metal-doped TiO₂ enhances the photocatalytic activity due to the formation of O=C–O–Ti bonds that can function as an electron transfer channel, accelerating the electron injection to the CB of TiO₂. Moreover, the composites can directly use sunlight, allowing their use in wastewater treatment.

Rabbani and co-workers²⁰¹ synthesized an Ag-doped mesoporous TiO₂ (Ag-TiO₂) powder modified with Zn22 (Fig. 16) and the composite was used to degrade 4-NP and MB. The results revealed that Zn22–Ag-TiO₂ has a higher activity than Ag-TiO₂ under white light but a lower activity under UV light irradiation. The higher activity of Ag-TiO₂ under UV light can be justified by the high band gap, being only activated under UV light. The presence of porphyrin molecules on its surface (Zn22–Ag-TiO₂) leads to a decrease of active sites for UV light absorption. Regarding the reusability of Zn22–Ag-TiO₂, it was verified that there were no significant changes in their photocatalytic activity for four cycles.

Rahimi and co-workers²⁰² used fluorine-doped TiO₂ (F-TiO₂) nanoparticles modified with H₂22 or Zn22 (Fig. 16) to photodegrade RhB. The photocatalytic assays were done under UV and white light irradiation. The results showed that, under white light, Zn22–F-TiO₂ and H₂22–F-TiO₂ are better catalysts than bare F-TiO₂. The O=C–O–Ti bonds formed between Zn22 or H₂22 and TiO₂ act as an electron transfer channel and accelerate the electron injection to the CB of F-TiO₂.

8.3. TiO₂–organic polymers

Considering that photocatalysts resulting from the association of porphyrins with either TiO₂ or organic polymers are very effective for the photodegradation of organic pollutants, in 2012, Qi and co-workers²⁰³ prepared a fibrous polymer mat-supported Fe3Cl–TiO₂ and used it in the photodegradation of azo dyes. The photocatalyst could degrade MO and other azo dyes in up to 98.5% in 5 h. Even after four cycles, the degradation rate remains at 87.0%. These results could contribute to preparing a membrane photo-reactor for continuous wastewater treatment.

In 2016, Cantarella and co-workers²⁰⁴ embedded TiO₂ modified with H₂22 (Fig. 16) in poly(methyl methacrylate) (PMMA), aiming to obtain a photocatalyst with no need for recovery after water treatment. The photocatalyst was used in MB degradation and even after 8 cycles, its ability to degrade MB did not decrease. These PMMA materials are stable, cheap and can be prepared by simple methods and, thus, are considered a potential tool for removing organic pollutants from wastewater.

Qi and co-workers²⁰⁵ supported a porphyrin-polyacrylonitrile fibre mat (Fig. 38) in TiO₂ and the resulting material (Fe87–PANC–TiO₂ mat) was used as a photocatalyst for methyl red degradation in an aqueous solution under white light. A ca. 75% methyl red degradation after irradiation for 120 min was reported. The photocatalyst could be recovered and reused up to 3 times but with a loss of 10% activity after each cycle.

Fig. 38 Fe87(OH) supported in a polyacrylonitrile fibre mat.

8.4. TiO₂–SiO₂

Charge-transfer catalysts of TiO₂–SiO₂, particularly Pt-modified TiO₂–SiO₂ catalysts,²⁰⁶ have higher activity than bare TiO₂. Either TiO₂ or SiO₂ is widely used as an inorganic support for photocatalytic activity. However, their limitations require the immobilization of a photoactive molecule on the support. Using a TiO₂–SiO₂ composite modified with a photoactive porphyrin molecule enhances the photocatalytic activity. For that reason, Cai and co-workers²⁰⁷ prepared TiO₂–SiO₂ modified with Zn(II) carboxyphenyl porphyrins with different para-substituents to be used as photocatalysts for α-terpinene degradation. The photodegradation of α-terpinene with Zn88–, Zn89–, and Zn90–TiO₂–SiO₂ (Fig. 39) revealed that their photocatalytic activity was much higher than that of bare TiO₂–SiO₂ under white light irradiation. After irradiation for 50 min, the best photocatalyst was Zn89–TiO₂–SiO₂ (95.6% degradation), followed by Zn90–TiO₂–SiO₂ (84.1%) and Zn88–TiO₂–SiO₂ (76.9%).

Fig. 39 Zn(II) porphyrins immobilized on TiO₂–SiO₂.

Qian and co-workers,²⁰⁸ using the layer-by-layer method, constructed triad hybrid multilayers on quartz surfaces formed by zinc meso-tetra(4-pyridyl)porphyrin (Zn86) (Fig. 35) and pyridine-functionalized TiO₂ (TiO₂-Py) nanoparticles connected by Pd(II) ions. The photocatalytic activity of the nanocomposites for the degradation of MO was assessed in aqueous media under irradiation with visible light (λ > 420 nm) at room temperature. After 48 h, the photocatalyst could degrade 50% of MO, but a complete degradation was achieved only after 8 days. When reused, the photocatalytic activity decreased by about 10%.

8.5. Surface-modified TiO₂ nanoparticles

Roques-Carmes and co-workers²⁰⁹ coupled porphyrin derivatives H₂91 and chlorin H₂92 to surface-modified TiO₂ nanoparticles (Fig. 40) and the photodegradation of MB and MO was used to assess the photocatalytic efficiency of the prepared catalysts under simulated solar and visible light. Porphyrin H₂91 and chlorin H₂92 serve as visible–light antennas, thus extending the photoresponse of the hybridized TiO₂ nanoparticles towards the visible light region.

Fig. 40 Representation of the TiO₂–APTES–PS NPs.

Enhanced photocatalytic efficiency towards the degradation of MB and MO was observed for the (3-aminopropyl)triethoxysilane (APTES)-modified nanosystems, especially for TiO₂–APTES–H₂92 that lead to 91% degradation of MO under visible light irradiation. Similar nanoparticles prepared using SiO₂ instead of TiO₂ revealed low photocatalytic activity when compared with those with TiO₂.

8.6. TiO₂ nanotubes–reduced graphene oxide

Wan and co-workers²¹⁰ fabricated ternary composites of TiO₂ nanotubes (TiNT) with reduced graphene oxide (rGO) and porphyrin H₂22 and assessed their capability as photocatalysts for the degradation of MB under white light. The experimental results showed more than 92% of MB degradation, about 4.3 times higher than the pure TiNT. The authors suggest a mechanism in which rGO acts as the adsorbent, electron acceptor, and transporter to increase the separation of the electron–hole pairs, accelerating the decomposition of organic pollutants. In turn, porphyrin has a critical role in capturing photons and expanding the absorption wavelength to the visible light region.

The same authors also reported the formation of a similar porphyrin/rGO–TiNT nanocomposite but using Cu22.²¹¹ In this case, experimental results showed a photodegradation of more than 95% MB, which is about 5 times higher than that of the pure TiNT.

8.7. TiO₂–graphene

Rahimi and co-workers²¹² prepared a TiO₂–graphene nanocomposite pillared with porphyrin Sn3(OH)₂ (Fig. 41) and used this material as a photocatalyst to degrade MO in an aqueous solution under white light irradiation. This pillared graphene–TiO₂–Sn3(OH)₂ allowed the complete photocatalytic degradation of MO in 180 min. It was determined that the OH radical is the main active species responsible for the degradation of MO.

Fig. 41 Representation of Sn3 functionalized pillared graphene–TiO₂.

8.8. TiO₂–MOF

Lü, Zhu and co-workers²¹³ published a series of 2D M22 (M = Cu(II), Zn(II), Co(II), and Mn(II))–MOFs/TiO₂ (Fig. 16) binary nanocomposites and their ability as photocatalysts for the degradation of RhB under visible light irradiation. After 30 min of irradiation, the best photocatalyst was Cu22–MOF/TiO₂ (97%) followed by Zn22–MOF/TiO₂ (88%), Co22–MOF/TiO₂ (65%) and Mn22–MOF/TiO₂ (57%).

8.9. Fe₃O₄@ZrO₂

Ghafuri and co-workers²¹⁴ synthesized a mixture of inorganic supports of Fe₃O₄@ZrO₂ derivatized with H₂22, Zn22, and Co22 as well as the free-base porphyrin H₂93, and its metal complexes Zn93 and Co93 (Fig. 42). The photocatalytic properties of materials with carboxy porphyrins H₂22–, Zn22–, Co22–Fe₃O₄@ZrO₂ and 2-naphthyl esters, H₂93, Zn93, and Co93–Fe₃O₄@ZrO₂ for the degradation of MB were evaluated and compared. The results showed that carboxy and 2-naphthyl benzoate derivatives lead to different degradation rates, as shown in Fig. 42.

Fig. 42 Porphyrins used to sensitize Fe₃O₄@ZrO₂ and the MB photodegradation rates under white light irradiation for 3 h.

The results of the photodegradation of MB allowed us to conclude that: a) the presence of a porphyrin increases the activity of the inorganic support; b) carboxy porphyrin derivatives are more active than the corresponding 2-naphthyl esters; c) Zn(II) and Co(II) porphyrins are more active than the corresponding free-base porphyrins; d) Co(II) porphyrins are more active than Zn(II) porphyrins.

8.10. SiO₂–ZnO

Sobral and co-workers²¹⁵ reported the chemical modification of the inorganic support SiO₂–ZnO (prepared by mixing 70% SiO₂ and 30% ZnO) with meso-tetrakis(p-aminophenyl)porphyrin (H₂65) using 3-glycidoxypropyltrimethoxysilane as a coupling agent. This material (Fig. 43) was used as a catalyst for the photodegradation of naphthol black blue under direct sunlight and white light. A 60% degradation of naphthol black blue was achieved with both types of light but at 120 and 180 min of irradiation time, respectively. Moreover, the catalyst was found to be reusable.

Fig. 43 Representation of H₂65 immobilized on SiO₂–ZnO.

8.11. SiO₂ (quartz)–MOF

Mn22–MOF (Fig. 44) with catalytically active sites was built upon functionalized quartz glass surfaces using a layer-by-layer self-assembly method.²¹⁶ With a porous reticular structure, the Mn22–MOF films, in the presence of H₂O₂, could efficiently degrade 90% of MB under visible-light irradiation for 180 min. The catalyst is easily recovered, and after being reused five times, it can still degrade over 85% of MB.

Fig. 44 Mn22–MOF films immobilized on quartz glass.

8.12. Reduced graphene oxide–Ag nanoparticles

Sun and co-workers²¹⁷ reported the preparation of organic/inorganic ternary nanohybrids constituted by a Sn(IV) porphyrin (Sn62Cl₂), Ag nanoparticles, and reduced graphene oxide (Fig. 45). The nanohybrids revealed superior catalytic activity for the degradation of RhB under white light, reaching 42% degradation in 3 h. Moreover, the ternary nanohybrid was also an active catalyst for reducing 4-NP to 4-aminophenol by NaBH₄: 97% of 4-NP was reduced within 17 min.

Fig. 45 Representation of Sn62Cl₂, AgNPs, on rGO.

More recently, La, Nguyen and co-workers²¹⁸ developed the same hybrid (g-C₃N₄/Ag nanoparticles) using the porphyrin H₂22 (Fig. 16). The authors evaluated the ability of the hybrid in the photocatalytic degradation of RhB after exposure to sunlight. The authors could achieve a maximum of 97% degradation of RhB using H₂22-g-C₃N₄/Ag which was better than using bare g-C₃N₄/Ag (79%) and H₂22 (89%). The incorporation of H₂22 in the hybrid allowed its recovery and reusability for four times with a 12% loss of activity. The authors identified O₂˙⁻ and HO˙ as the species responsible for the photodegradation mechanism.

8.13. S or P and N co-doped graphene quantum dots

In 2016, Mahyari and co-workers²¹⁹ explored the properties of Fe(III) meso-tetra(4-sulfonatophenyl)porphyrin (Fe1Cl) (Fig. 2) supported on S and N co-doped graphene quantum dots (GQDs) as a recyclable heterogeneous photocatalyst for aerobic oxidation of benzyl alcohols under white light irradiation. The synthesized photocatalyst revealed high conversion rates and selectivity for the oxidation of benzyl alcohols to the corresponding aldehydes in water at room temperature. This study provided a promising efficient photocatalyst, recyclable up to 5 cycles without activity loss, to be used in wastewater treatment. The same authors²²⁰ extended this study to P and N co-doped graphene quantum dots modified with cobalt(II) porphyrin 1 (Co1). The composite also showed high performance and recyclability as a heterogeneous photocatalyst in the aerobic oxidation of benzyl alcohols under white light irradiation.

8.14. BiVO₄–graphene nanocomposites

Rahimi and co-workers²²¹ prepared a composite of BiVO₄–graphene impregnated with porphyrin H₂22 (Fig. 16) and evaluated its ability as a photocatalyst to degrade MO in aqueous solution under white light irradiation. The introduction of graphene in the BiVO₄ composite enhanced its photocatalytic activity up to 2 times. Incorporation of porphyrin H₂22 allowed the photocatalytic activity of the resulting nanocomposite to be improved under white light irradiation. After the photosensitization with porphyrin, the photocatalytic activity of the resulting H₂22–BiVO₄–graphene nanocomposite increased 3.5 times and allowed the degradation of 78%, 87%, and 97% of MO at 30 min, 60 min, and 150 min of irradiation, respectively.

8.15. Bi₂WO₆–reduced graphene oxide

Bismuth tungstate-based materials (Bi₂WO₆) have been widely used as photocatalysts in environmental, energy, and biological fields, including water treatment, air purification, and inactivation of bacteria.^222,223

Huang and co-workers²²⁴ constructed a ternary H₂22/rGO/Bi₂WO₆ Z-scheme heterojunction and evaluated its photocatalytic activity. The composite revealed enhanced photocatalytic activity towards the degradation of the antibiotic tetracycline under white light (removal of 83.6%) when compared with pure Bi₂WO₆, rGO/Bi₂WO₆, and H₂22/Bi₂WO₆. Recycling experiments with the composite showed that the tetracycline removal efficiency remained 79.3% after five cycles.

9. Summary and outlook

This work outlines the significant advances in the photocatalytic oxidation of water pollutants using simple porphyrin derivatives as well as porphyrins linked to a range of organic or inorganic materials, namely polymers, fibres, carbon nanostructures, ZnO, SiO₂, and TiO₂ (or mixtures of these supports), under UV, visible, and/or solar light irradiation. The presence of different metals in the porphyrin core resulted in different photocatalytic activities, with the highest degradation rates being achieved with Cu(II) porphyrin derivatives.

Moreover, by sensitizing TiO₂ or SiO₂ supports with porphyrins, the photocatalytic activity was greatly enhanced due to the reduction of the material's band gap and the absorption of visible light. This sensitization could be made through covalent or non-covalent interactions. It is important to highlight that the photocatalytic mechanism is based on the excitation of the porphyrin followed by electron transfer to the CB of the support. The photogenerated holes and the excited electrons can react with O₂ or H₂O to form reactive oxygen species, which will degrade the pollutant. In some cases, the addition of H₂O₂ is necessary to obtain high rates of pollutant degradation.

The light source (UV, visible, or solar light) is also an important parameter. When solar/white light is used, the support can also be excited along with the porphyrin, but only a small or no excitation of the porphyrin moiety occurs with UV light. The use of solar light is considered a huge advantage in photocatalysis since it reduces costs and takes advantage of all solar spectrum. It is also important to mention that almost all photocatalysts discussed here are reusable, with no significant loss of activity in most cases, and in some cases, the photocatalyst is specific for a selected pollutant. While the presented photocatalytic studies highlighted in this review are promising, there is a noticeable absence of research on identifying the degradation products.

This review shows that the scientific community is closer to finding effective methods for the degradation of pollutants that are difficult to remove from water. However, scale-up studies with the photocatalysts' production and practical applications (in the field) of this technology are (urgently) needed.

Abbreviations

2,4,6-TCP	2,4,6-Trichlorophenol
2,4,6-TNT	2,4,6-Trinitrotoluene
2,4-DCP	2,4-Dichlorophenol
2,6-DBP	2,6-Di-t-butylphenol
4-CP	4-Chlorophenol
4-NP	4-Nitrophenol
AO	Acridine orange
ATRP	Atom transfer radical polymerization
BFO	Bismuth ferrite
CdS	Cadmium sulphide
CB	Conduction band
CEES	2-Chloroethyl ethyl sulfoxide
CEESO	2-Chloroethyl ethyl sulfoxide
CTAB	Cetyltrimethylammonium bromide
DMF	Dimethylformamide
GO	Graphene oxide
LCST	Lower critical solution temperature
MB	Methylene blue
MO	Methyl orange
MOF	Metal–organic framework
NIR	Near-infrared
PAM	Poly(N-isopropylacrylamide)
PAN	Polyacrylonitrile
PCP	pentachlorophenol
PMMA	Poly(methyl methacrylate)
POM	Polyoxometalate
Por	Porphyrin
RhB	Rhodamine B
ROS	Reactive oxygen species
SA	Salicylic acid
SSA	2-Hydroxy-5-sulfosalicylic acid
TiNT	Titanate nanotube
TPP	meso-Tetraphenylporphyrin
UV	Ultra-violet
VB	Valence band

Conflicts of interest

There are no conflicts of interest to declare.

Acknowledgements

Thanks are due to the University of Aveiro and Fundação para a Ciência e a Tecnologia (FCT) for the financial support to LAQV-REQUIMTE (LA/P/0008/2020 DOI: https://doi.org/10.54499/LA/P/0008/2020, UIDP/50006/2020 DOI: https://doi.org/10.54499/UIDP/50006/2020, and UIDB/50006/2020 DOI: https://doi.org/10.54499/UIDB/50006/2020), Centro de Química Estrutural (UIDB/00100/2020 DOI: https://doi.org/10.54499/UIDB/00100/2020 and UIDP/00100/2020 DOI: https://doi.org/10.54499/UIDP/00100/2020), and IMS (LA/P/0056/2020 DOI: https://doi.org/10.54499/LA/P/0056/2020) research units, and to the FCT project P2020-PTDC/QUI-QOR/31770/2017, through national funds and where applicable co-financed by the FEDER-Operational Thematic Program for Competitiveness and Internationalization-COMPETE 2020, within the PT2020 Partnership Agreement. S. Gamelas thanks FCT for her PhD scholarship (SFRH/BD/143549/2019).

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