Visible-Light Photocatalysts and Their Perspectives for Building Photocatalytic Membrane Reactors for Various Liquid Phase Chemical Conversions

Abstract

Photocatalytic organic synthesis/conversions and water treatment under visible light are a challenging task to use renewable energy in chemical transformations. In this review a brief overview on the mainly employed visible light photocatalysts and a discussion on the problems and advantages of Vis-light versus UV-light irradiation is reported. Visible light photocatalysts in the photocatalytic conversion of CO₂, conversion of acetophenone to phenylethanol, hydrogenation of nitro compounds, oxidation of cyclohexane, synthesis of vanillin and phenol, as well as hydrogen production and water treatment are discussed. Some applications of these photocatalysts in photocatalytic membrane reactors (PMRs) for carrying out organic synthesis, conversion and/or degradation of organic pollutants are reported. The described cases show that PMRs represent a promising green technology that could shift on applications of industrial interest using visible light (from Sun) active photocatalysts.

1. Introduction

The development of advanced green chemical technologies and processes in organic synthesis and environmental conversions has become one of the most important challenges for chemical researchers in the last decades [1,2] and, particularly, for the ones using heterogeneous photocatalysis (HPC). Too much interest is devoted on using the solar source as a non-exhaustible energy and, in this aspect, visible light photocatalysts play a very crucial role.

Many conventional catalytic processes for chemical production usually employ toxic and hazardous substances as catalysts or solvents, thus, they are not very sustainable in terms of resources, environmental impact and energy efficiency [1,2,3,4]. The main reason, from a catalytic point of view, that makes photocatalysis sustainable is the photonic activation mode of the photocatalyst instead of the thermal activation typical of traditional industrial processes [5]. Ideal materials for photocatalysis should possess qualities such as high photoactivity towards near UV and visible region of the electromagnetic spectrum, resistance to photocorrosion, inertness, low cost and low environmental toxicity [1,2,3,4]. Photocatalytic reactions generally utilize semiconductors to directly absorb photons above their band gap energy and produce carriers consisting of electrons and holes, thus promoting oxidation and reduction reactions [1,2,3,4,6]. The produced electron/hole couple can undergo two different fates: (i) the couple can quickly recombine, in the bulk or on the surface of the semiconductor, releasing the accumulated energy as heat or photons; (ii) the couple can migrate to the surface of the semiconductor where the promoted electron and the hole can, reduce and oxidize the adsorbed substrates, respectively. In order to employ the photogenerated couples, to induce photocatalytic reactions, the electron/hole recombination should be avoided.

The action of ultraviolet (UV), visible (VIS) or infrared (IR) radiation is necessary to obtain the initiation of a chemical reaction catalyzed by a photocatalyst [3,6,7]. The study of the photocatalytic reactions started in 1972 when Fujishima and Honda [8] discovered that UV light induced water cleavage using a TiO₂ photoanode in combination with a Pt counter electrode immersed in an aqueous electrolytic solution. Since then, photocatalysis has attracted significant attention for its promising applications, mainly in the environmental sector, and particularly, in wastewater purification [6,9,10,11,12,13]. In this process high-energy UV-light frequently induces unselective reactions in which organic and inorganic pollutants in liquid phases are totally degraded to innocuous substances. However, in last years there is an increase in the interest of application of photocatalysis in reactions of synthesis such as selective reduction and oxidation, generating less by-production formation, and this can be achieved by using visible light as energy source [14,15,16,17,18]. Indeed, the imminent energy crisis and growing pollution issues, increased the interest in the utilization of renewable energy sources, such as wind or solar light [19]. In particular, solar energy can be used for the photocatalytic degradation of harmful and toxic organic substances performing photoreduction and photooxidation processes. Furthermore, the photocatalytic process can generate renewable and green energy carriers (e.g., H₂ (and O₂)) from water splitting [6,20,21,22]; it can help to decrease pollution (e.g., photocatalytic degradation of organic pollutants) [1,11,12,13,23]; it can provide energy conversion and materials storage (e.g., photocatalytic reduction of CO₂) [24,25,26,27,28,29,30,31]; and it can synthesize organic substances providing a sustainable pathway for green synthesis [1,2,3,4].

Potential advantages that makes HPC a sustainable approach are [1,2,3,4,13,32,33,34]: (i) possibility to carry out reactions under mild conditions such as room temperature and pressure; (ii) use of environmentally friendly and safer catalysts (e.g., TiO₂) avoiding the use of dangerous heavy metal catalysts; (iii) reducing/avoiding the use of strong chemical oxidant/reducing agents (e.g., the use of mild oxidants such as molecular oxygen); (iv) possibility to convert contaminants to innocuous by-products; (v) limiting the use of auxiliary additives; (vi) application to a wide range of substrates in liquid, solid, and gaseous phases; (vii) possibility to use renewable solar energy; and (viii) opportunity to couple HPC with other physical and chemical technologies (e.g., membrane separation and membrane reactors, respectively). Coupling HPC and semipermeable membranes gives systems named photocatalytic membrane reactors (PMRs). These represent a very interesting technology because they minimize the environmental and economic impacts obtaining the advantages to perform chemical reactions and separation processes in a single step [1,2,3,4,35,36,37]. The main benefits of using a membrane are: (i) recovery of the catalyst (immobilizing it on/in the membrane or using the membrane to maintain it in suspension), (ii) selective separation of the molecules employed in the reaction limiting the formation of by-products thus enhancing the selectivity. PMRs can be also coupled with various materials such as adsorbents incorporated in membrane, photocatalysts combined with magnetic material or coated on optical fibers, etc. Moreover, one of the main advantages of the employment of solar light in photocatalytic membrane reactors (PMRs), avoiding UV radiation, is the lowest degradation of polymeric membranes elongating their lifetime. In addition, while most photocatalysts can be activated only by ultraviolet light energy, which is only about 5% in the solar spectrum, visible light is 45% and near-infrared (NIR) is 50% of the solar light energy. Thus, in view of efficient solar-energy utilization, efforts to extend the light response from UV to visible light are necessary.

In the present paper a focus on the most recent advances in the use of visible light photocatalysts for carrying out chemical conversions in liquid phases in batch and PMRs are reported. Some applications on the photocatalytic reduction and oxidation of organics, water treatment and degradation of organic pollutants are reported and their perspectives are discussed.

2. Problems of UV-Light Photocatalysts and Requirement for Vis-Light Photocatalysts

Solar-driven photocatalytic reactions are an attractive clean and effective way to convert solar energy to perform chemical reactions [6,38,39,40]. The entire process of the heterogeneous photocatalysis in liquid phase can be divided in five independent steps: (1) transfer of reagents from the liquid phase to the catalyst surface; (2) adsorption of reactants; (3) reaction in the adsorbed phase; (4) desorption of the products; (5) removal of the products from the interface solid/liquid. Instead, the step (3), which involves the photocatalytic reaction, can be divided in four stages [3]: (a) light absorption followed by the separation of the electron–hole couple; b) adsorption of reagents; (c) redox reaction; (d) desorption of products. The redox reaction happens in the electronic structure of the semiconductor which is characterized by a conduction band (CB) and a valence band (VB) separated by a band gap of energy (E_G). When the semiconductor is excited by photons with energy equal to or higher than the band gap energy level (hv ≥ E_G), electrons receive energy from the photons and are thus promoted from VB to CB. Electrons and holes that migrate to the surface of the semiconductor can, respectively, reduce and oxidize the reactants adsorbed on the semiconductor surface and this depends on the redox potential (or energy level) of the substrate (Figure 1).

The photocatalyst’s ability to carry out a particular reaction depends on the relative position of the energy levels of the catalyst and the substrate. The adsorbed molecule can be reduced if its reduction potential is higher than that of the photoelectrons or it can be oxidized if its potential is lower than that of the photohole. The semiconductor photocatalyst possess intrinsic band structure that determines its electronic transport properties, including its electron energy-level transition [19]. The photoactivity is promoted by the light absorption, redox potential, charge-carrier mobility and can be improved by modifying the band structure [4]. The most widely used semiconductor for photocatalysis is TiO₂ because it shows advantages as: wide availability, low cost, nontoxicity, and long-term stability [32]. Nevertheless, the TiO₂ photocatalyst is photoactive only in the UV range and its activity is strongly influenced by the recombination of photogenerated electrons and holes [1,2,3,4,32,41].

Generally, to increase the light absorption response, the main strategy, reported by different authors, is improving the number of light-active sites on the photocatalyst and restricting charge-carrier recombination [19]. The enhancement of light-active sites can be obtained by two methods: (1) design of semiconductors at nanoscale size improving their surface area, (2) increase of highly active facets of the photocatalyst. Charge-carrier separation can be improved by controlling the morphology and the heterostructure construction with suitable band alignment. Moreover, the presence of open pores in some semiconductors can permit the diffusion of the reactants also into the photocatalyst material. Indeed, by a one-step and template-free process for the synthesis of meso-crystalline TiO₂, Bian et al. [41] obtained an increased charge-separation efficiency and a large specific surface area, which enhanced the photocatalytic properties in the photocatalytic oxidation of p-chlorophenol (4-CP) and rhodamine B (RhB), as well as the photocatalytic reduction of Cr⁶⁺ in the aqueous phase. Besides, depositing fine noble-metal particles (e.g., Pt, Ag, Au, Pd, and Rh) on the photocatalyst surface is a practical route to enhance charge separation [19]. In this hybrid nanostructure, a Schottky barrier by Fermi level equilibration between the photocatalyst and metal deposits can be established.

2.1. Selectivity of Visible Light Photocatalysts

Most semiconductors photocatalysts, can be activated only by ultraviolet light [6], but UV light in the solar spectrum is only about 5%, instead visible light is 45%, and near-infrared (NIR) is 50% of the solar light energy [19]. Thus, for efficient solar-energy utilization, efforts to extend the light response from UV to visible light and to NIR light should be made. Moreover, high-energy UV-light frequently induces an increase of by-products formation due to the excessive energy input in the reacting environment for the higher production of reducing and/or oxidizing agents. This problem can be limited by using visible light as energy source [42,43,44]. The mechanism of the visible-light response for visible-light active photocatalysts is the same as that for UV-light-active photocatalysts. The only difference is the lower photon energy needed to activate the photocatalytic cycle that probably causes higher selectivity.

For example, the Palmisano group [45,46,47,48,49,50,51,52] performed various photocatalytic oxidations of aromatic alcohols to aldehydes in water making a comparison of selectivity between rutile and anatase TiO₂ nanoparticles prepared in-house under UV light irradiation. They showed results in terms of selectivity of about 45–74% over rutile TiO₂ nanoparticles for the photo-oxidation of 4-substituted aromatic alcohols to the corresponding aldehydes [48], 41% and 62% of selectivity over anatase and rutile TiO₂ nanoparticles for oxidation of 4-methoxybenzyl alcohols, respectively [49,52], and 60% over rutile TiO₂ nanoparticles for oxidation of benzyl alcohols to benzaldehyde [46]. To improve the selectivity to desired products some researchers studied the photocatalytic oxidation under visible light.

For example, Zhang et al. [53] presented a system working under visible light irradiation constituted by a dye-sensitized anatase TiO₂- TEMPO (2,2,6,6-tetramethylpiperidinyloxyl) nanoparticles which showed selectivity of over 93% for oxidation of a series of aromatic alcohols and of 98% for benzyl alcohols. Similarly, Higashimoto et al. [54] reported high selectivity (>99% for most of the studied aromatic alcohols) for the photocatalytic oxidation of aromatic alcohols to aldehydes under visible-light irradiation over anatase TiO₂ nanoparticles. Li et al. [44] also, reported over 99% selectivity of the partial photocatalytic oxidation of benzyl alcohol over rutile TiO₂ nanorods under visible-light irradiation. These results are better than that reported under UV light irradiation. The nanorods were prepared by a hydrothermal reaction using rutile TiO₂ as a substrate at 150 °C. A tentative reaction mechanism was proposed by the authors (Figure 2): benzyl alcohol was first adsorbed on the surface of rutile TiO₂ to form a complex which could absorb visible-light to generate electrons and holes, then it was oxidized into the benzaldehyde by electrons transfer and holes-attracted release of H atom. In addition, the unidirectional property of nanorods favored the electron transfer and thus the selectivity [44].

2.2. Visible-Light-Active Photocatalysts

To expanse the light-harvesting region from UV to visible, obtaining photocatalysts active under visible light, different methods have been used [19]. These ones concern nanocatalysts [55,56], noble metal loading [57,58,59,60,61,62,63], ion doping [64,65], dye sensitization [66], fabrication of nanostructured semiconductors for photocatalysis, including nanosheet, nanotube, and nanorod, various fabrication routes for TiO₂ nanotubes such as hydrothermal, anodization, and template methods [67,68,69,70,71] and new materials such as graphene based materials [20,21,66,72]. Indeed, to avoid photogenerated charge-carrier recombination on the photocatalyst, a design and manufacture of suitable heterostructures is very important to enhance the charge carrier separation. Several semiconductor materials possessing visible-light photoactivity have been prepared to utilize more efficiently solar energy, for example, CdS, CdSe, InP, GaZnON, WO₃, Ag₂O, Cu₂O, BiVO₄, Bi₂MoO₆, Bi₂WO₆, RbPb₂Nb₃O₁₀, and others [19]. Among them, CdS is one of the most known, with a long research history [42].

In the last years the development of new materials such as graphene and graphene oxide (GO) as photocatalysts is increasing, for their sustainability and flexibility, in the modification and design of the photocatalysts. In addition to these advantages, the photocatalytic activity of GO under visible light can be implemented by the presence of cationic or anionic dyes [66]. Some dyes having visible light sensitivity are used in photocatalytic systems [73]. Under visible light illumination, the excited dyes can inject electrons into the conduction band of semiconductors to initiate the photocatalytic reactions. Lavorato et al. [66], reported the use of [Ru(bipy)₃]²⁺ dye molecules located in the intergallery space of incompletely delaminated GO by a simple cationic exchange at 60 °C obtaining a photocatalyst that was able to produce hydrogen from water/methanol solutions, under laser light at 532 nm and solar simulated light with or without a cut filter to eliminate all UV light. The authors proposed the following photosensitization mechanism: upon light excitation, [Ru(bipy)₃]²⁺ reaches the triplet excited state, which has an energy of −1.06 eV with respect to the same reference electrode [74]. Consequently, from a thermodynamic point of view, electron injection from the [Ru(bipy)₃]²⁺ LUMO into the CB of GO will be favorable [66]. Another photocatalyst used to increase the visible light absorption of a photocatalyst mainly active under UV light such as TiO₂ is alizarin red S (ARS). Li et al. [42], reported the selective oxidation of alcohols with atmospheric O₂ by using a visible light LED as irradiation source and 0.67 mol% of ARS anchored onto TiO₂ surface as sensitized photocatalyst [75]. Sometimes, the electron transfer, between the photosensitizer/semiconductor and the reactant, can be limited by their oxidation/reduction potential. To limit this problem, it is possible to introduce a co-catalyst (such as an organometallic complex) [2] that acts as electro mediator between the photosensitizer/semiconductor and the reactant. In particular, Figure 3, Li et al. [42] reported the following advantages to activate a photocatalyst with this approach: (1) easy charge separation process in space; (2) decrease of the energy barrier of reactants by multistep electron-transfer; (3) increase of reactions selectivity with complexes as active sites; and (4) design to apply on specific reactions.

Noble metals, including Pt, Au, Pd, Rh, Ni, Cu and Ag have been reported to be very effective for enhancement of TiO₂ photocatalytic activity decreasing the possibility of electron–hole recombination and reducing the band gap energy of the photocatalyst, thus shifting the radiation absorption towards higher wavelengths, permitting to use visible light [57]. Recently, the reduction of various organic compounds has been investigated by using visible light TiO₂ based semiconductors loaded with Pd nanoparticles (NPs), also in membrane reactors, for acetophenone reduction [57,59]. Noble metals can improve the photocatalytic properties of TiO₂ NPs [6,60,61,76,77,78,79] because they absorb the excited electrons from TiO₂ limiting the recombination of electrons and holes. Moreover, the localized surface plasmon resonance (LSPR) effect of noble metals, which is the collective electron oscillation on the surface of noble metals, contributes to the strong absorption of visible light. In the experiments on photocatalytic water splitting of an alcohol-water solution, Seh et al. [78] reported that, by using Au/TiO₂ as photocatalyst, the amount of hydrogen produced was 138 mL compared with 7 mL obtained by using pure TiO₂ in 3 h. Mendez et al. [60] found that the absorption edge energy of TiO₂ (3.15 eV) changed after 0.5 wt% Au loading (2.98 eV), therefore, the absorption peak was red-shifted and the Au loading improved the absorption of visible light. Moreover, the duration of the plasmon-excited electrons decay of Au/TiO₂ NPs was 1.5 ns better than a few picoseconds of an individual Au NP [61], thanks to the migrations of electrons from the Au NP surface to the TiO₂ NP, extending the decay time and effectively limiting the recombination of electrons-holes in Au NP.

In recent years, the doping of TiO₂ with nitrogen has grown interest due to the photoactivation by solar light [79,80]. Azami et al. [79] reported a preparation of N-doped TiO₂ by mixing urea and TiO₂ powder under microwave irradiation. The presence of nitrogen makes N-doped TiO₂ active under visible light irradiation with reduced band gap to 2.9 eV. It was reported that the N-doped TiO₂ photocatalyst had good photocatalytic activity with complete degradation of Reactive Red 4 after 60 min. Nitrogen has been also used to improve the photocatalytic activity of graphene. Lavorato et al. [20] reported a new preparation of a nitrogen doped graphene photocatalyst obtained by pyrolysis of chitosan at different temperatures. This type of photocatalyst was more active than GO, which showed photocatalytic activity only under UV light. N-doped graphene exhibited high efficiency for the photocatalytic generation of hydrogen from water/methanol mixtures in the absence of Pt or any other metal with similar efficiency using UV or visible-light.

Other semiconductors that draw great attention on their utilization as photocatalysts are metal-organic frameworks (MOFs) that consist of metal clusters interconnected with organic linkers (e.g., MOF-5, UiO-66(Zr), ZIF-8, MIL-125(Ti), etc.) [81]. These semiconductors show limited efficiency under solar light illumination because of the lack of visible light response, but their effective use under visible light can be easily achieved by the modification of their organic linkers or metal centers. For example, NH₂-MIL-125(Ti) (denoted as NH₂-M125), as the isostructural MOF of MIL-125(Ti), constituted by cyclic Ti₈O₈(OH)₄ oxoclusters and 2-aminoterephthalate ligands, shows extended absorption spectra into the visible-light region (around 550 nm) because of its amino functionality moieties.

3. Outline on Applications of Vis-Light Photocatalysts

3.1. Water Splitting

Hydrogen generation by using visible light is very attractive because it allows to convert and store solar energy in a clean and effective way [1,6,20,42,60,66]. Photocatalysis is one of the main approaches to produce hydrogen by utilizing solar energy. As explained in the paragraph 2, different methods have been ideated to improve the visible light activity of photocatalysts. Among them noble metals such as Au, Ag and Pt, which work as co-catalysts, are very effective to enhance the photocatalytic performance of TiO₂ for water splitting. In the following some types of photocatalysts tested in water splitting and some experimental data are discussed. A summary of the photocatalysts and the main results is reported in

Table 1. Summary of some photocatalysts tested in water splitting and main results.

Photocatalyst	Main Results	Ref.
Au/TiO2 nanoparticles (NPs)	super-linear dependence on light intensity in the range 0–7 kW/m2, and then a sub-linear dependence in the range 7–9 kW/m2	[6]
[Ru(bipy)3]2+@GO	3290 µmol h−1g−1 of hydrogen production under laser irradiation at 532 nm	[66]
N-doped graphene	About 55 µmol after 3 h of visible-light (532 nm) laser irradiation	[20]
phosphorus-doped grapheme (Pt–(P)G-4)	H2-generation rate 282 µmol h−1 g−1 under UV/Vis irradiation using triethanolamine	[98]
2%NiS/20%g-C3N4/SrTiO3 (2NS/20CN/STO)	hydrogen production rate of 1722.7 μmol h−1 g−1	[100]
CaTiO3/Pr3+ Y2SiO5/RGO	Up conversion of visible light into UV light	[82]

.

The overall photocatalytic water splitting process involves two half reactions, an oxygen evolution reaction (OER) and a hydrogen evolution reaction (HER) [82] which compete with their reverse reaction forming again water. At present, many semiconductor materials have been used as photocatalysts in hydrogen evolution reaction. Some semiconductor materials reported by various authors, such as CdS [83], CdSe [84], InNbO₄ [85], BiVO₄ [86], Bi₂MoO₆ [87], Bi₂WO₆ [88], TaON [89], g-C₃N₄ [90,91], (Ga1-xZnx)(N1-xOx), ZnS [92], are capable to utilize visible light [93], but some of these photocatalysts can be degraded by the photocorrosion [82,94,95]. In particular, metal sulfides are subject to rapid photocorrosion through the oxidizing holes which convert some of the excitation energy to promote a chemical decomposition of the semiconductor generating sulfur and metal ions. For example, ZnS and CdS nanoparticles (NPs) have poor stability during the photocatalytic process due to photo-corrosion [92,96] despite: (i) the superior photocatalytic activity for hydrogen production under visible or solar light, (ii) the rapid generation of the electron-hole pairs by photoexcitation, and (iii) the highly negative reduction potentials of excited electrons. Instead, other semiconductors, active only under UV light, such as TiO₂ [32] and SrTiO₃ [97], are stable and efficient for hydrogen evolution.

To enhance the application of sunlight irradiation in water splitting reaction, there are various methods that can be adopted such as doping of metal or nonmetal elements or dye sensitization on various semiconductors [1,4,6,42,61,94]. For example, Au is a metal largely used to dope titanium dioxide improving the visible light absorption of the photocatalyst in the visible range. Au/TiO₂ nanoparticles (NPs) showed good performance in photocatalytic hydrogen generation because of the localized surface plasmon resonance (LSPR) effect of Au/TiO₂ NPs [6]. A parameter that plays an important role in the photocatalytic reaction rate of Au/TiO₂ NPs is the light intensity. Very recently, Xing et al. [6] found that the hydrogen production rate has a super-linear dependence on light intensity in the range 0–7 kW/m², and then a sub-linear dependence in the range 7–9 kW/m². Probably Au’s LSPR effect plays a key role in the super-linear relationship because at high intensity the faster recombination rate of electrons and holes limits the growth.

Other interesting new materials successfully used for water splitting under visible-solar light are graphene-based materials [20,21,66]. By coupling [Ru(bipy)₃]²+ dye molecules and delaminated GO, it can be obtained a photocatalyst that can produce hydrogen from water/methanol solutions, under laser light at 532 nm and solar simulated light with or without a cut filter to eliminate all UV light [66]. Pristine graphene is not suitable for water-splitting, but by doping heteroatoms (e.g., N, P, etc.), gapless metallic graphene can be turned into a semiconductor. Lavorato et al. [21] reported the possibility of using alginate, a natural polysaccharide from algae, as a simultaneous graphene precursor and a templating agent for ceria nanoparticles for UV-visible light water splitting (λ > 380 nm). They prepared a series of materials consisting of highly crystalline ceria nanoparticles embedded on a few-layer graphene matrix. Changing the pyrolysis temperature and the weight percentage of ceria/alginate, the best sample preparation exhibited about 3 times higher photocatalytic activity for water oxidation to oxygen than commercial ceria. In another work Lavorato et al. [20] studied the photocatalytic activity for hydrogen production of N-doped graphene obtained by pyrolysis of a natural biopolymer such as chitosan. The best photocatalyst was obtained by using a pyrolysis temperature of 900 °C. It showed a similar activity under laser light irradiation at 355 and 532 nm and it was also able to generate hydrogen upon simulated sunlight illumination. Nitrogen doping could increase the hole scavenging by electron donors, extending the lifetime of electrons, resulting in high charge extraction and creating efficient photocatalysis. Garcia et al. [98], obtained a photocatalytic activity under visible light by using phosphorus-doped graphene obtained by the pyrolysis of natural alginate that contained H₂PO₄⁻ at 900 °C. This material catalyzes the generation of H₂ from a water/methanol mixture upon irradiation with UV or visible light.

Another method to design photocatalysts active in the visible light is the formation of a heterojunction [99]. This is an interface between unequal band structures with different semiconductors with suitable band gap alignment. The speed of electron transfer is promoted lowering recombination sites of photoexcited charge carriers due to potential gradient of heterojunction between semiconductors. Luo et al., synthesized 2%NiS/20% g-C₃N₄/SrTiO₃ (2NS/20CN/STO) as photocatalyst to improve the photocatalytic activity of both g-C₃N₄ and SrTiO₃ semiconductors for H₂ evolution under UV-visible light irradiation [100]. In fact, the band energy levels of g-C₃N₄ and SrTiO₃ match, for constructing, an efficient heterojunction, which can enable the separation and transfer of photogenerated charge carriers in the organic-inorganic g-C₃N₄/SrTiO₃ composite because the valence band (VB) of SrTiO₃ is more positive than that of g-C₃N₄, while the conductance band (CB) of g-C₃N₄ is more negative than that of SrTiO₃. Moreover, both g-C₃N₄ and SrTiO₃ have efficient thermodynamic potential for hydrogen production from water splitting, because their CB levels are more negative than the hydrogen production level. NiS, which has a small band gap of 0.5 eV, was used as cocatalyst. Heterojunction construction and NiS cocatalyst enhanced the H₂ production rate for the sample 2%NiS/20%g-C₃N₄/SrTiO₃ 2 (2NS/20CN/STO) due to an effective charge separation via transportation of photogenerated electrons to NiS. The authors reported that this photocatalyst (2NS/20CN/STO) showed a photocatalytic hydrogen production rate of 32.8, 8.9 and 4.2 times the value obtained with pure g-C₃N₄, SrTiO₃ and 20%g-C₃N₄/SrTiO₃ (20CN/STO), respectively.

Recently, Gao et al. [82], studied the possibility of using photocatalytic materials active under UV light to obtain overall water splitting by starting with visible light irradiation. They synthesized the following photocatalysts: Pr³⁺-CaTiO₃ (Pr³⁺-CTO), Pr³⁺-Y₂SiO₅ (Pr³⁺-YSO) and Pr³⁺-Y₂SiO₅/Pr³⁺-CaTiO₃ (Pr³⁺-CTYS). A complex photocatalyst CaTiO₃/Pr³⁺ Y₂SiO₅/RGO (reduced graphene oxide) that was able to upconvert visible light into UV light which was then used to excite UV, formed charge pairs by taking advantage of up conversion luminescence properties of Pr³⁺-Y₂SiO₅ (Pr³⁺-YSO). The RGO greatly facilitated the forward electron transfer from photoexcited Pr³⁺-CTO to Pt co-catalyst and suppressed back electron transfer. Moreover, to avoid hydrogen-oxygen reverse recombination reaction to water formation, the oxygen formed in the reaction mixtures was removed by an artificial gill. The conjectured mechanism of RGO/Pr³⁺-CTYS/Pt for photocatalytic hydrogen evolution is described in Figure 4.

3.2. Photocatalytic Reductions

To develop a sustainable chemistry, the photocatalytic reduction reactions involving various semiconductor materials can proceed under mild conditions using various light sources, including solar energy [101]. We have reviewed the state-of-the-art in heterogeneous photocatalytic systems derived from various semiconductors, their composites, and other materials for the selective reduction of different classes of organic compounds including CO₂, ketones and nitro compounds.

3.2.1. Conversion of CO₂

CO₂ reduction under visible light can supply sustainable liquid fuels and alleviate global warming. Very recently, various authors reported the CO₂ reduction under visible light [24,27,28,29,102,103]. In particular CO₂ reduction by water to alcohols is very interesting. Microwave-synthesized carbon-dots (mCD) possess unique hole-accepting nature, prolonging the electron lifetime (t_50%) of carbon nitride (CN) by six folds, favoring a six-electron product [24]. mCD-decorated CN stably produced stoichiometric oxygen and methanol from water and CO₂ with nearly 100% selectivity to methanol and internal quantum efficiency of 2.1% in the visible region.

In the following some types of photocatalysts tested in CO₂ conversion and some experimental data are discussed. A summary of the photocatalysts and the main results is presented in

Table 2. Summary of some photocatalysts tested in CO₂ conversion and main results.

Photocatalyst	Main Results	Ref.
Microwave-synthesized carbon-dots (mCD)	Carbon dioxide reduction nearly 100% selectivity to methanol by pure water	[24]
Nickel silicate hydroxide	CO yield up to 1.71 × 104 μmol g−1 h−1 with 99.2% selectivity under visible light	[25]
g-C3N4/Pt/3DOM-TiO2@C	CH4 yield of 6.56 μmol h−1 for 0.1 g catalyst and 5.67% of quantum efficiency	[96]
Supported Cu2O on activated-dolomite (Cu/DA)	CH3OH generation of 38 μmol g−1 h−1	[97]
Cu-modified TGCN (TiO2/g-C3N4)	CH3OH yield of 2574 mmol g−1	[98]
[(Au/A-TiO2)@g-C3N4]	Formation rates of CH4 is 37.4 μmol g−1 h−1	[100]
CoAl-LDH/MoS2-x	Modulation of the syngas proportion (H2:CO) from 1:1 to 9:1	[101]

.

An interesting CO₂ conversion strategy includes the use of adsorbents to remove harmful heavy metals in polluted water and their conversion into valuable photocatalysts for CO₂ reduction without secondary treatment [25]. Calcium silicate hydrate (CSH) nanosheets, with ultrathin thickness (2.8 nm), ultrahigh surface area (637.2 m² g⁻¹), and abundant surface hydroxyls, are much favorable for both heavy metals removal and photocatalysis processes. Chen et al. [25] studied four typical heavy metals including Cu²⁺, Zn²⁺, Ni²⁺, and Pb²⁺. For Ni²⁺ they observed that it was spontaneously converted into a new semiconductor nickel silicate hydroxide that showed a suitable energy level for reducing CO₂ into CO (CO yield up to 1.71 × 10⁴ μmol g⁻¹ h⁻¹ with 99.2% selectivity under visible light).

Wang et al. [103] reported an artificial photosynthesis for visible-light-driven CO₂ reduction with H₂O. They fabricated all-solid-state Z-scheme ternary photocatalysts, consisting of two isolated photochemical systems of graphitic carbon nitride (g-C₃N₄) and three-dimensional ordered macroporous carbon-coated TiO₂ (3DOM-TiO₂@C) combined with Pt nanoparticles as electron-transfer system (g-C₃N₄/Pt/3DOM-TiO₂@C). Photonic crystal structure and carbon-coated nanolayers of 3DOM-TiO₂@C support enhanced visible light-harvesting efficiency. The vectorial photoelectron transferring from TiO₂@C → Pt → g-C₃N₄ boosts the separation and surface enrichment efficiencies of photogenerated electrons and holes. The results showed high yields of CH₄ and quantum efficiency (6.56 μmol h⁻¹ for 0.1 g catalyst and 5.67%, respectively) during conversion of CO₂ with H₂O under visible light.

Cu₂O photocatalyst represents an outstanding choice to convert the CO₂ molecule into solar fuels [104]. Nevertheless, its capacity to adsorb CO₂ is limited and for this reason Luévano-Hipólito et al. [104], supported Cu₂O on activated-dolomite (Cu/DA) for the photocatalytic CO₂ conversion to solar fuels (CH₃OH and HCOH) under LED-visible light. Pure-Cu₂O resulted in the highest HCOH production (419 μmol g⁻¹ h⁻¹) with a low amount of CH₃OH generated (11 μmol g⁻¹ h⁻¹). Instead, in the case of supported Cu₂O in DA the generation of CH₃OH (38 μmol g⁻¹ h⁻¹) was favored up to 3.4 times. Adekoy et al. [105] reported that visible light driven photoconversion of CO₂ over Cu-modified TGCN (TiO₂/g-C₃N₄) composite yielded 2574 mmol g⁻¹ of CH₃OH [105,106]. Z-scheme-type photocatalysts with two photochemical systems of graphic-C₃N₄ (g-C₃N₄) and anatase TiO₂ (A-TiO₂) nanocrystals combined with Au nanoparticles (NPs) [(Au/A-TiO₂)@g-C₃N₄] designed by Wang et al. [107] exhibited excellent photoreduction of CO₂ under visible light irradiation and 37.4 μmol g⁻¹ h⁻¹ of CH₄ was produced.

To limit the increased global energy crisis and environmental problems it is important to use renewable materials such as syngas, hydrocarbons, and alcohols. A route for their production is CO₂ photoreduction under visible light [108]. High value-added chemicals in the industry can be obtained by reduction of syngas with different H₂:CO ratios. Qui et al. [108], proposed a novel route to produce syngas for solar-to-fuel conversion in an artificial photosynthetic system. They carried out interfacial engineering by designing a series of heterostructured layered double-hydroxide/MoS₂ nanocomposites (CoAl-LDH/MoS2-x) via electrostatic self-assembly for CO₂ photoreduction. The authors reported that were able to modulate the syngas ratio (H₂:CO) from 1:1 to 9:1 by visible-light irradiation (λ > 400 nm) under the control of the interface-rich heterostructures.

3.2.2. Photocatalytic Hydrogenation of Ketones

The reduction of carbonyl compounds, such as acetophenone (AP), in the corresponding alcohols, such as phenyl ethanol (PE), plays an important role in organic synthesis [57,59,109]. In particular, phenyl ethanol (PE), is a common precursor for the preparation of analgesic and anti-inflammatory drugs as well as fragrances and perfumes [57,59,109]. The choice of solvent and electron-hydrogen donor, to achieve the photocatalytic transfer hydrogenation of organic molecules, is essential. In the following some types of photocatalysts tested in ketone hydrogenation and some experimental data are discussed. A summary of the photocatalysts and the main results is reported in

Table 3. Summary of some photocatalysts tested in ketone hydrogenation and main results.

Photocatalyst	Main Results	Ref.
Iridium based complex supported on CdS	Pyruvate TON 790, cyclohexane carboxaldehyde TON 381, Acetophenone TON 220	[42]
Aminopyridyl cobalt complex active and [Cu(bathocuproine)(Xantphos)](PF6) photoredox catalyst	High selectivity towards the hydrogenation of aryl ketones was observed in the presence of terminal olefins, aliphatic ketones, and alkynes	[110]
RhB–TiO2	Phenylethanol production initial reaction rate 0.70 mol dm−3 h−1	[111]
Pd/TiO2	Phenylethanol Productivity 22 mg gTiO2 −1 h−1	[57]
Pd/TiO2/FAU	Phenylethanol Productivity 99.6 mg gTiO2 −1 h−1	[59]

TON = Turnover Number.

.

Li et al. [42] reported a photocatalytic transfer hydrogenation of carbonyl compounds, under visible light irradiation, by coupling the semiconductor CdS with four types of iridium complexes (Figure 5). The iridium complexes are activated by the photoexcited electron from CdS, and they play dual functions, acting as the acceptor of electron as well as the catalytic sites for organic transformation. The authors reported high selectivity values on visible light photocatalytic hydrogenation of various ketones.

Call et al. [110] presented a methodology to reduce aromatic ketones and both aliphatic and aromatic aldehydes using a photocatalytic system consisting of a robust and well-defined aminopyridyl cobalt active complex and [Cu(bathocuproine)(Xantphos)](PF₆) photoredox catalyst illuminated by visible light with H₂O and amine (Et₃N or iPr₂EtN) as electron donor and the hydride source. Tests were carried out in aqueous mixtures (60–80% water). High selectivity towards the hydrogenation of aryl ketones was observed in the presence of terminal olefins, aliphatic ketones, and alkynes. The catalytic system also showed good selectivity for the reduction of acetophenone in the presence of aliphatic aldehydes.

Kotani et al. [111] reported the extended photocatalytic UV response of TiO₂ towards visible light region by combination of dyes (fluorescein (Fl) and rhodamine B (RhB)) withTiO₂ for the photocatalytic hydrogenation of aromatic ketones with triethylamine as a sacrificial electron donor.

Recently Molinari’s group [57,59] synthesized different titanium based photocatalysts for the photocatalytic hydrogenation of acetophenone. Hydrogenation tests were carried out using water as solvent and formic acid as electron hydrogen donor also in a PMR under UV and visible light irradiation (see Section 4.2.2). In another work [109] the authors tested the photocatalytic activity of TiO₂ and Pd/TiO₂ for the same photocatalytic reaction under UV light using ethanol as both solvent and hydrogen-electron donor and a water/ethanol solution. By comparing the results obtained in batch [57,109] the authors found that the best solvent for this reaction was ethanol, indeed, the yield obtained using water as solvent with 7.5% of formic acid (1.9 M) was double than that obtained using water/ethanol 50/50, but it was lower than that using pure ethanol. This means that, in perspective to apply the photocatalytic hydrogenation in a membrane reactor, ethanol could not be the best solvent for all type of polymeric membranes.

3.2.3. Photocatalytic Hydrogenation of Nitrocompounds

Nitroaromatic compounds cause severe health hazards to human, animal, and aquatic life because they are dangerous and toxic organic chemicals [112]. The intensively use of these compounds in wars and explosive industrial applications, caused a considerable environmental pollution. For this reason, some researchers are studying the reduction of nitroaromatic compounds to their corresponding aromatic amines [112]. This reaction is even more interesting if it is performed by using ecofriendly alternatives such as photocatalysis under visible light. In the following some types of photocatalysts tested in nitro compounds hydrogenation and some experimental data are discussed. A summary of the photocatalysts and the main results is reported in

Table 4. Summary of some photocatalysts tested in nitro compounds hydrogenation and main results.

Photocatalyst	Main Results	Ref.
Eosin Y on resin (REY)	4-Nitrophenol (4-NP) conversion 60% in 50 min.	[114]
g-C3N4@ Fe3O4NPs	Visible light activity higher than Fe3O4NPs	[112]
3.0 wt% CQDs/ZnIn2S4	Full conversion of nitrobenzene after irradiation for 16 h	[115]

.

Recently, the photo-reduction of nitro derivatives was performed by using dye sensitized TiO₂ and transition metal nanoparticles [113]. Various nitrobenzenes were reduced to anilines using Ru-sensitized TiO₂ photocatalysts by using high power LEDs (Luxeon, 3 W, 530 nm) or sunlight as irradiation source. Gazi et al. [114] reported the study of several dyes with the absorption band in the visible region (e.g., alizarin S red, bromophenol blue, bromopyrogallol red, rhodamine B, methylene blue, rose Bengal and eosin Y) for the reduction of 4-nitrophenol (4-NP) to 4-aminophenol (4-AP) by using NaBH₄ as electron donor. The best result in terms of conversion (60% in 50 min) was obtained by using eosin Y on resin (REY) as catalyst.

Graphite-like graphitic carbon nitride (g-C₃N₄) has grown significant interest in the last few years. Kumar et al. [112] reported a simple method for preparing g-C₃N₄@ Fe₃O₄NPs based on thermal polycondensation of melamine and coprecipitation of ferric and ferrous salts. The obtained material (g-C₃N₄@Fe₃O₄NPs) had a porous structure, abundant surface functional groups, strong magnetism, electrostatic attraction, and electron donor−acceptor interaction. This photocatalyst offered the efficient adsorption of chromium species in a wide pH range, showing high removal efficiency towards Cr(VI) and Cr(III). In addition, g-C₃N₄@Fe₃O₄NPs can catalyze the NaBH₄-mediated reduction of 2-nitroaniline (2-NA) and 4-nitroaniline (4-NA) under the irradiation of visible light with activity higher than Fe₃O₄NPs. Furthermore, the authors reported that this material, through magnetic decantation, can be reused in four successive cycles of 2-NA and 4-NA reduction.

Very recently, Wang et al. [115] reported the hydrogenation of nitrobenzene to produce aniline, azoxybenzene and azobenzene by using CQDs/ZnIn₂S₄ nanocomposite under visible light. An optimized production of aniline was obtained over 3.0 wt% CQDs/ZnIn₂S₄ nanocomposite in a reaction medium of TEOA:MeOH (1:1), while a strong alkaline reaction medium promoted N-N coupling to produce azoxybenzene and azobenzene.

3.3. Total and Partial Oxidations

The selective oxidation is one of the major transformations in organic chemistry. Photocatalytic oxidation of organic compounds by solar light is a promising strategy for environmentally benign conversion processes [116]. In the last years the possibility of using cheap and eco-friendly processes, compared to the utilize of strong oxidants which generate toxic wastes, has become imperative. For this reason, interest is growing to perform these reactions by photocatalytic mode.

3.3.1. Oxidation of Primary and Secondary Alcohols to Aldehydes and Ketones

There are many studies on this research subject but here only some very recent articles are reported on the photocatalytic oxidation of organic compounds under visible light [116,117,118,119,120,121,122]. Gogoi et al. [116] tested the photocatalytic activity of TiO₂ and Au-TiO₂ for oxidation of primary and secondary alcohols to aldehydes and ketones under visible light irradiation. As expected, Au-TiO₂ was found to be more efficient than TiO₂ alone. In the following some types of photocatalysts tested in the oxidation of primary and secondary alcohols and some experimental data are discussed. A summary of the photocatalysts and the main results is reported in

Table 5. Summary of some photocatalysts tested in oxidation of alcohols and main results.

Photocatalyst	Main Results	Ref.
Au-TiO2	Au-TiO2 higher photocatalytic activity than TiO2	[116]
Pd/(NH4)xWO3/WO3	80% selectivity for benzaldehyde production with ca. 84% conversion of benzyl alcohol.	[117]
CeO2/ZnIn2S4	The benzaldehyde and hydrogen yields obtained were 664.1 and 1496.6 μmol gcat−1 h−1, respectively.	[118]
BiFeO3-(CNTs) (PPy)	Benzyl alcohol conversion rate and selectivity to benzaldehyde of about 83.2% and 99.3%, respectively.	[119]
Au/TiO2 nanorod	55% yield of benzaldehyde with an external quantum efficiency of 3.4%	[120]
g-C3N4/TiO2	turnover frequency (TOF) of over 550 h−1	[121]
S-doped g-C3N4	H2 production rate 13.2 times higher than bulk g-C3N4, H2 and benzaldehyde production rate of 3.76 and 3.87 μmolh−1, respectively.	[122]

.

Lv et al. [117] studied a new type of photocatalyst, (Pd/(NH₄)_xWO₃/WO₃, working under visible light irradiation, that exhibited higher selectivity in comparison to Pd/WO₃ nanorods in the partial conversion of aqueous benzyl alcohol into benzaldehyde (80% selectivity for benzaldehyde production with ca. 84% conversion of benzyl alcohol).

Photocatalytic alcohol oxidation can improve green fuels synthesis applications such as hydrogen production [118]. A new Z-scheme catalyst was constructed for this purpose: ZnIn₂S₄ nanosheets were grown on the surface of CeO₂ nanorods. This 2D/1D composite can form solid Z-scheme heterostructures that combines the outstanding visible-light absorption of ZnIn₂S₄ nanosheets, and the fast charge transport of CeO₂ nanorods, with an improved photocatalytic efficiency. The benzaldehyde and hydrogen yields obtained were 664.1 and 1496.6 μmol gcat⁻¹ h⁻¹, respectively.

Guo et al. reported a good oxidation efficiency and selectivity towards the conversion of benzyl alcohol into benzaldehyde under visible-light irradiation by using a series of novel Z-scheme BiFeO₃-(carbon nanotubes (CNTs)-polypyrrole(PPy) prepared via hydrothermal reaction and oxidative polymerization [119]. This photocatalyst presented a benzyl alcohol conversion rate and selectivity to benzaldehyde of about 83.2% and 99.3%, respectively.

Verma et al. [120] tested three different catalysts based on TiO₂ doped with Au: Au/TiO₂ nanotube, Au/TiO₂ nanorod and Au/TiO₂ microporous. The results, obtained carrying out the reaction with thermal activation conditions in dark, were compared with those performed under visible light irradiation (λ > 420 nm) at 373 K. The most active photocatalyst, under the same reaction conditions, was found to be Au/TiO₂ nanorod. It showed a performance of 33% yield with the thermal activation in dark and 55% yield of benzaldehyde with an external quantum efficiency of 3.4% under visible light irradiation (λ ≥ 420 nm) at 373 K for 4 h. The enhanced catalytic performance was attributed to the Au-LSPR phenomenon.

Recently, significant progresses have been made in the photocatalytic activity of graphitic-carbon nitride (g-C₃N₄) [121]. Mohammadi et al. [121] synthesized spray-dried g-C₃N₄/TiO₂ granules, through a pyrolysis process of urea and TiCl₄ as the precursors. g-C₃N₄, g-C₃N₄/TiO₂, and spray-dried g-C₃N₄/TiO₂ granules (TCNg) were used for the photocatalytic oxidation of benzyl alcohol (BA) to benzaldehyde (BAL) in a batch reactor under visible light irradiation. The results showed that TCNg, was more active on the photocatalytic oxidation of benzyl alcohol with an average turnover frequency (TOF) of over 550 h⁻¹.

Zhang et al. [122] proposed an interesting utilization of solar energy by coupling H₂ generation and selective oxidation of benzyl alcohol. They synthesized heteroatom doped g-C₃N₄ hierarchical mesoporous spheres. S-doped g-C₃N₄ showed 13.2 times higher H₂ production rate than bulk g-C₃N₄ under visible-light. Having the photocatalyst a dual function, as H₂ production and selective oxidation of benzyl alcohol, it exhibited outstanding activity with a H₂ and benzaldehyde production rate of 3.76 and 3.87 μmol h⁻¹, respectively.

3.3.2. Partial Oxidation of Alkanes and Alkenes: Oxidation of Cyclohexane

Selective activation of C (sp³) and H bonds can be used to develop new strategies to produce high-value organic molecules for their use in the chemical, pharmaceutical and agricultural, industries [123]. An interesting photocatalytic reaction of green chemistry is the cyclohexane oxidation to cyclohexanol (A) and cyclohexanone (K) (the mixture of both is called KA oil) with the use of clean and low-cost molecular oxygen as the oxidant and solar light as the driving force [123,124,125]. The obtained products are important intermediates to fabricate nylon-6 and nylon-66. In the following some types of photocatalysts tested in the partial oxidation of alkanes and alkenes and some experimental data are discussed. A summary of the photocatalysts and the main results is reported in

Table 6. Summary of some photocatalysts tested in partial oxidation of alkanes and alkenes and main results.

Photocatalyst	Main Results	Ref.
NH2MIL-125 TiO2	Photocatalytic activity three times higher than that of NH2-MIL-125(Ti)	[81]
MoS2@Cu/Cu2O@C	Conversion of 1.31%, selectivity to cyclohexanol and to cyclohexanone of 59 and 39%, respectively	[128]
BiOI	Conversion and selectivity of cyclohexane to cyclohexanol 0.0011% and over 80%, respectively	[123]

.

Cyclohexanol and cyclohexanone can be produced from cyclohexane in the liquid phase using titanium dioxide under mild conditions [126,127]. However, the bandgap of titanium dioxide (3.2 eV) implies the absorption of photons with wavelengths below 387 nm to promote photoelectrons from the valence band to the conduction band. To extend the absorption spectra of TiO₂ to the visible light region limiting recombination of photoelectrons and photoholes, Zhao et al. [81], synthesized a new photocatalyst named NH₂-MIL-125(Ti)/TiO₂ obtained by coupling metal organic frameworks MOFs (see Section 2) and TiO₂. Despite, the bandgap obtained for this photocatalyst was 2.70 eV, which meant wavelengths lower than 460 nm, the authors reported an enhancement of the photocatalytic performance of NH₂-MIL-125(Ti)/TiO₂ three times higher than that of NH₂-MIL-125(Ti) under visible light (λ > 420 nm) for the photocatalytic cyclohexane oxidation using molecular oxygen as oxidant at room temperature. Another photocatalyst based on MOFs (MoS₂@Cu/Cu₂O@C) was reported to be active under visible light (λ > 420 nm) for cyclohexane oxidation. Peng et al. [128], reported the synthesis of this new composite photocatalyst prepared by immersing the guest (NH₄)₂MoS₄ (ammonium tetrathiomolybdate) into Cu-metal–organic framework (MOF) polyhedra host and subsequently pyrolyzing the Cu-MOF–guest polyhedra to encapsulate MoS₂ into Cu/Cu₂O@C. This photocatalyst showed higher photocatalytic cyclohexane oxidation performance compared to the pristine MoS₂ or Cu/Cu₂O@C. This behaviour is probably due to its heterojunction band reconstruction and to the presence of Cu nanoparticles that capture electrons and prevents electron−hole recombination. Other types of semiconductors that show photocatalytic activity under visible light are bismuth oxyhalides (BiOX, X=Cl, Br, I) because they have adequate chemical stability and desirable electrical and optical properties [129,130].

Among the bismuth oxyhalides, bismuth oxyiodide (BiOI), which is stable in water and has low toxicity, exhibits the narrowest bandgap (1.8–1.9 eV) and highest photocatalytic performance within the visible light region [131]. Contreras et al. [123], studied different samples of bismuth oxyiodide and they reported that all BiOI photocatalysts showed higher selectivity for cyclohexanol than cyclohexanone.

3.3.3. Photocatalytic Production of Vanillin

Vanillin (4-hydroxy-3-methoxybenzaldehyde, C₈H₈O₃) is a biophenol relatively abundant (in the form of a β-D-glucoside) in the green pods of the orchid Vanilla planifolia which was isolated for the first time by Gobley in 1858 [132]. This substance has many applications in food, perfume and pharmaceutical industries [17,132]. Only 1% is obtained by extraction from the plant Vanilla planifolia which requires a very long and expensive process to obtain the “natural” product. The remaining 99% is produced by chemical syntheses which allow one to obtain a product almost 100 times cheaper than the natural one [133,134,135]. An attractive and alternative synthesis to the biotechnological and chemical processes for the aromatic aldehydes may be the photocatalysis at room temperature and atmospheric pressure, even more if it can be conducted under visible light [136,137]. In the following some types of photocatalysts tested in the photocatalytic production of vanillin and some experimental data are discussed. A summary of the photocatalysts and the main results is reported. in

Table 7. Summary of some photocatalysts tested in vanillin production and main results.

Photocatalyst	Main Results	Ref.
ZnFe2O4	Selectivity to vanillin up to 99% at a conversion over 98% and turn-over frequency values up to 1600 h−1 in the presence of H2O2 and base	[139]
1Ce/TiO2-BS	Maximum conversion of 39.7% with selectivity values of 99% towards vanillin	[141]
5Zr/TiO2-BS	Maximum conversion of 52.4% with selectivity values of 99% towards vanillin	[141]

.

Parrino et al. studied the photocatalytic oxidation of trans-ferulic acid to vanillin (FA, C₁₀H₁₀O₄) in aqueous solution under visible light by using different commercial and home-prepared TiO₂ [138]. The main product was CO₂, whereas secondary oxidation products were organic species such as vanillin, caffeic acid, homovanillic acid, and vanillylmandelic acid. Among different alternative sources to produce vanillin, the valorization of renewable lignocellulosic biomass can be considered as interesting step towards sustainability [139]. For this purpose, lignin, an aromatic amorphous polymeric material, can be used as a starting material for the production of vanillin. The oxidative depolymerization can allow the conversion of lignin to small phenolic compounds such as vanillin, syringaldehyde and p-hydroxybenzaldehyde [134,135]. Vanillyl alcohol (VAL) is considered a lignin model substrate that can be converted selectively into vanillin.

Al-Hunaiti et al., synthesized NPs using a route phytomediated by Boswellia carterii extract [139]. The spinel ZnFe₂O₄ with a relatively narrow band gap of about 1.9 eV had visible-light response, good photochemical stability and low cost. The authors reported that ZnFe₂O₄ showed selectivity to vanillin up to 99% at a conversion over 98% and turn-over frequency values up to 1600 h⁻¹ in the presence of H₂O₂ and base. Furthermore, ZnFe₂O₄ particles are magnetic semiconductor materials [140] and can be magnetically separated, after the photocatalytic reaction, from the suspension system thanks to their magnetic properties.

Vanillin can be produced also by photooxidation of vanillyl alcohol. Very recently Martin-Perales et al. [141] studied this synthesis by using TiO₂-based materials as photocatalysts [141]. The absorption of visible light, together with the electronic properties and with the photocatalytic efficiency of the synthesized titania, was improved by incorporation of Ce and Zr oxides on the titania surface in comparison to pure TiO₂. The best catalytic behavior was obtained for 1Ce/TiO₂-BS and 5Zr/TiO₂-BS samples, with a maximum conversion of 39.7% and 52.4% for ceria and zirconia modified samples, respectively, and selectivity values of 99% towards vanillin and carbon balance higher than 98%.

3.3.4. Benzene Oxidation to Phenol

Another important oxidation reaction widely studied is the direct benzene oxidation to phenol [142,143,144,145,146,147,148,149,150,151,152]. Phenol is an important chemical intermediate for the synthesis of petrochemicals, agrochemicals, and plastics [1,2,3,4]. The choice of a suitable photocatalyst is especially important to control the reaction progress and increase the phenol selectivity [145]. So far, several studies were reported on the visible light-induced catalytic oxidation reaction of benzene to phenol in the presence of hydrogen peroxide [145,148,149,150,151,152]. In the following some types of photocatalysts tested in the benzene oxidation to phenol and some experimental data are discussed. A summary of the photocatalysts and the main results are reported in

Table 8. Summary of some photocatalysts tested in benzene oxidation to phenol and main results.

Photocatalyst	Main Results	Ref.
[RuII(Me2phen)3]2+ and [CoIII(Cp*)(bpy)(H2O)]2+	Yield of phenol about 30% and quantum yield (QY) 1.7%	[145]
Pt-WO3	Selective hydroxylation of benzene to phenol 300 < λ < 500 nm	[150]
Pt-WO3	Selectivity of phenol over 70% 420 < λ < 540 nm	[149]
10 wt% Fe-modified g-C3N4	Phenol yield 4.8% and phenol selectivity 3.2%	[151]
Au−Pd/g-C3N4	Benzene conversion 26% and phenol selectivity 100%	[152]

.

Han et al. [145] reported the photocatalytic hydroxylation of benzene to phenol in a O₂-saturated acetonitrile solution containing [RuII(Me₂phen)₃]²⁺ as a photocatalyst, [CoIII(Cp*)(bpy)(H₂O)]²⁺ as an efficient photo-catalyst activator for both the water oxidation and benzene hydroxylation reactions, and water as an electron source in the presence of Sc(NO₃)₃. During the photocatalytic oxidation of benzene by O₂ the yield of PhOH remained constant while increased the by-product formation (p-benzoquinone) caused by further oxidation of phenol in the reaction environment. The TON for the production of both phenol and benzoquinone was determined to be 500 (where the TON of p-benzoquinone is counted three times). By increasing the concentration of the catalyst and decreasing the concentration of the benzene substrate, it was obtained a yield of phenol based on benzene of about 30%. The quantum yield (QY) was determined to be 1.7% from the amount of PhOH produced during the photocatalytic reaction under photoirradiation (λ = 440 nm) for 1 h.

Tungsten trioxide (WO₃) is one of the promising visible-light photocatalysts having a direct band-gap excitation at ca. 2.7 eV. Tomita et al. [150], reported that the Pt-deposited WO₃ (Pt-WO₃) photocatalyst exhibited selective hydroxylation of benzene to phenol in the presence of O₂ and H₂O under light irradiation of both UV light and visible light (300 < λ < 500 nm). Instead, Kurikawa et al. [149], focused on understanding the photocatalytic activities for hydroxylation of benzene on the Pt-WO₃ photocatalyst under irradiation of only visible light (420 < λ < 540 nm).

Another photocatalyst with absorption setting at wavelengths of 460 nm is g-C₃N₄. Chen et al. [151] reported that its absorption is extendable to ~650 nm by iron complexation. The authors reported that Fe-g-C₃N₄ was able to oxidize directly benzene to phenol using hydrogen peroxide. With 10 wt% Fe-modified g-C₃N₄ catalysts, the phenol yield based on benzene was 1.8% in the dark and 4.8% in visible light. However, the phenol selectivity based on H₂O₂ was 3.2% in the dark and 8.3% under visible light.

Under optimized conditions, a mesoporous photocatalyst based on Au−Pd nanoparticles incorporated into g-C₃N₄ showed benzene conversion of 26% at a phenol selectivity of 100%, giving no dihydroxylated by-products [152]. This photocatalyst was prepared by a coassembly method using melamine as the carbon and nitrogen source, polyvinyl pyrrolidone as the dispersing agent, and pulse laser ablation in liquid technique for preparing gold nanoparticles and subsequent decoration with Pd nanoparticles. At the final stage, Au−Pd/g-C₃N₄ nano-photocatalyst was obtained via low-ramping pyrolysis in an argon atmosphere.

3.3.5. Water Treatment

The growing increase of pharmaceutical and agricultural pesticide products as environmental pollutants require an enhancement of wastewater treatment to limit potential risks to aquatic organisms and human health [1,153]. Photocatalysis has emerged as a foremost reliable application with high efficiency for organic pollutants degradation due to its benefits of good photocatalytic activity, nontoxicity, chemical inertness, and low cost [1,153,154,155,156]. Pesticides are the most harmful for humans and environment among various organic pollutants [157]. Currently, the conventional methods used for pesticide removal are membrane separation, surface adsorption, biodegradation, and photocatalytic degradation. The most studied photocatalyst for the degradation of a wide range of dyes, pharmaceuticals, and organic compounds is TiO₂ [158] but, also ZnO has received much attention by some authors [159]. The major influential parameters for the efficient degradation of organic contaminants are the structural and surface characteristic of semiconducting materials such as surface area, average particle size, crystalline structure, bandgap, porosity and surface density [160]. The surface of most of the metal oxides such as ZnO, TiO₂, Al₂O₃ is also a good adsorbent of organic contaminants in addition to the photocatalytic property. In the following some types of photocatalysts tested in water treatment and some experimental data are discussed. A summary of the photocatalysts and the main results is reported in

Table 9. Summary of some photocatalysts tested in water treatment and main results.

Photocatalyst	Main Results	Ref.
ZnO–Bi2O3	Lambda-Cyhalothrin (L-CHT) degradation percentage 85.7% achieved within 120 min	[161]
Cu-ZnO	MCP removal efficiency about 90%	[162]
G2.0FNTO	MB removal amount about 97%	[163]
GQDs	Methyl Orange degraded to 52% while Methylene Blue degraded 79.4% both within two hours	[164]
Black TiO2	Reduction rate of Cr(VI) 96.2%	[165]

.

Premalatha and Miranda [161] studied the degradation of λ-cyhalothrin (L-CHT) under visible light by using ZnO–Bi₂O₃ as photocatalyst. The results showed that, compared to pristine ZnO, a degradation percentage of 85.7% was achieved within 120 min of the photocatalytic process.

Thi et al. [162] studied the photocatalytic degradation of monocrotophos pesticide (MCP) under visible radiation by using ZnO doped with Cu that prevented the recombination of photo-excited e− and h+ of ZnO. The synthesized photocatalyst showed photocatalytic activity for the degradation of MCP under visible radiation.

The photocatalytic abilities of the introduction of ions Fe, Gd, and N on TiO₂ for the degradation of methylene blue (MB) under visible light were evaluated by Li et al. [163]. The introduction of ions Fe, Gd, and N reduced the particle size of doped TiO₂ and enhanced its surface area and pore volume. The impurity energy levels formed by N and Fe³⁺ doping, extended the absorption of TiO₂ into the visible light region, resulting in the reduction of the bandgap. Doping with N and Gd³⁺ ions resulted in the formation of defect sites and oxygen vacancies in the TiO₂ lattice. The authors tested a series of Gd–Fe–N–TiO₂ samples synthesized by changing the dosage of gadolinium nitrate at 1, 1.5, 2, 2.5, 3, and 3.5 wt%, further named as G_1.0FNTO, G_1.5FNTO, G_2.0FNTO, G_2.5FNTO, G_3.0FNTO, and G_3.5FNTO. The co-doped TiO₂ sample showed the best photocatalytic activity when the doped amount of Gd reached 2 wt% (G_2.0FNTO). The first-order reaction constant of MB photodegradation was 1.10 h⁻¹, which was ten times higher than that of un-doped TiO₂.

Very recently Mandal et al., studied the degradation of some dyes by using Graphene quantum dots (GQDs) as photocatalyst synthesized from pyrocatechol with a homogenous spherical shape at a size of ~8 nm. The results showed that methyl orange was degraded by 52% while methylene blue was degraded by 79.4%, both within two hours [164]. Another problem, in many types of wastewaters, is metals removal and this can be accomplished with transformations using visible light photocatalysts [165]. As an example, hexavalent chromium (Cr(VI)) reduction to Cr(III) has been attracted an extensive attention to limit its concentration in drinking water because the toxicity of Cr(III) is 2–3 orders of magnitude lower than that of Cr(VI). The tanning process usually uses a large quantity of sodium formate, which results in production of end-of-pipe wastewater containing high-concentration hexavalent chromium (Cr(VI)) and formate [165,166]. However, the maximum tolerable concentration of Cr(VI) is 0.05 mg/L in drinking water and 0.5 mg/L in industrial effluents [167]. In addition, the high mobility of Cr(VI) makes it quite easier to pollute both surface water and groundwater [168]. To remove oxidative pollutants in water, the advanced reduction process (ARP) has drawn an increasing interest, even more if this reduction can be obtained by using CO₂-produced from formate originally existing in wastewater. Liu et al. [165] demonstrated the possibility of in-situ reduction of Cr(VI) by using CO₂ with black TiO₂ photocatalyst under visible-light. The prepared black TiO₂ with nanotube structure enhanced the reduction rate of Cr(VI) as high as 96.2% in the presence of formate. The formate, the black TiO₂ concentration and the pH influenced the Cr(VI) reduction rate.

4. Analysis of Vis-Light Photocatalysts and Types of Conversion of Interest in Photocatalytic Membrane Reactors (PMRs)

There is growing concern for the development of efficient PMRs using renewable and sustainable energy sources, such as solar irradiation [1,2,3,4]. Moreover, the use of visible light in photocatalysis allows the use of renewable solar energy, which is an important feature in terms of global sustainability [9,14]. Furthermore, photocatalysis can be combined with other physical and chemical technologies such as membrane separations and membrane reactors [1,2,3,4,15,169,170]. The advantage of coupling the visible light activity of the photocatalyst and the separation process in a single unit is mainly the possibility of employment of solar light avoiding UV radiation, thus elongating lifetime of polymeric membranes owing to their lower degradation [57,59].

The conversions using vis-light photocatalysts, described in the previous Section 3, can be deeply studied and, eventually, can be carried out in PMRs to take advantage of their peculiar characteristics. In the following paragraphs, some transformations already present in literature using PMRs working under visible-light are reported describing also the main advantages of the membrane role.

4.1. Water Splitting in PMRs

Several reactors have been proposed for solar photocatalytic hydrogen production including the parabolic trough concentrator (PTC), column reactor, compound parabolic concentrator (CPC), surface uniform concentrator (SUC), membrane reactor etc. [171]. In the case of membrane reactors, some materials, used in ultrafiltration or microfiltration membranes, are hydrophobic such as polypropylene, polyvinylidene fluoride, or polytetrafluoroethylene (PTFE) [172]. These materials are prone to fouling and, in particular, to biofouling, because they are highly hydrophobic and thus, do not meet the fouling resistance criteria. Membrane fouling generally occurs in forms of foulant adhesion/deposition and thermodynamic filtration resistance of gel/cake layer. The accumulation of foulants on the membrane surface can block membrane pores resulting in a low flux through the membrane and a decrease of the membrane life, which means, consequently, an increase of the cost of membrane technology. Therefore, the interest to fabricate membranes with ability to degrade foulants is constantly increasing [173]. Another application of photocatalysis is pre-treatment prior to membrane filtration to eliminate bacteria from the feed solution for biofouling reduction. Lin et al. [172], reported a study on S-doped g-C₃N₄ nanosheet as a photocatalyst to be used for both water splitting and biofouling reduction. The photocatalytic activity in the generation of H₂ and O₂ was examined in the combination of Ru/SrTiO₃:Rh with the addition of [Co(bpy)₃]^3+/2+ as electron mediator to improve the charge transfer in a Z-scheme system. The H₂ and O₂ evolution rates in the system were 24.6 and 14.5 µmol⁻¹h⁻¹, respectively. In addition, S-doped g-C₃N₄ was incubated with a solution of Escherichia coli to determine its antibacterial effect. This test revealed that S-doped g-C₃N₄ has a high activity in reducing biofouling on a membrane.

Moreover, the possibility of producing hydrogen from organic matter contained in wastewater by photocatalysis with solar energy is a very attractive way. Rodriguez et al. [174] studied a system combining a photoreactor for hydrogen production by photocatalytic methanol dehydrogenation and a proton exchange membrane fuel cell (PEMFC) obtaining a system to directly convert solar irradiance into electricity. The photocatalytic system consisted of four photoreactors and an aluminum reflector (Figure 6) that was used to optimize irradiation of the slurry with solar light and it was continuously oriented to the sun. Photoreactors were linked in parallel with a recirculating pump and shared their gas phase. The system was purged with helium at 20 mL min⁻¹ for thirty minutes before each experiment to remove air/O₂ from the photoreactor and the hydrogen stream. The fuel cell contained a 25 cm² active area of a membrane separating two compartments: one filled at the anode by photocatalytic hydrogen and the other at the cathode filled by oxygen from the air in a self-breathing mode. Fuel cell performance was followed at constant voltage. Without any purification of hydrogen and without any optimization of the design, the system successfully provided an electric power density of 1 mW cm⁻² of photoreactor optical surface area exposed to the sunlight.

4.2. Reductions in PMRs

To our knowledge very few syntheses on reduction reactions in liquid phase in PMR under visible light irradiation are present in scientific literature. In the following some reduction reactions are described. Only in the case of CO₂ conversion many papers are present in literature but here only the very recent ones are reported, more references and details can be found elsewhere [1,2,3,4].

4.2.1. CO₂ Conversion in PMRs

In the lasts years the global energy demand of CO₂ conversion into desirable and sustainable solar fuels, such as methanol (CH₃OH), methane (CH₄), and formic acid (HCOOH) is increasing [1]. Recently, some works are reported on the use of solar energy in PMRs [175,176,177,178,179].

An interesting natural material that can be used to synthesize membranes is a biopolymer such as chitosan, because of the abundant presence of amine and carboxyl groups, which are favorable for CO₂ adsorption. Zhao et al. [175] prepared and tested chitosan in the photocatalytic conversion of CO₂, under visible light irradiation. The membrane matrix was: CdS/NH₂-UiO-66 hybrid membrane where UiO-66 is a metal–organic framework (MOF) with a very high surface area as well as high thermal stability. By incorporating MOFs and semiconductors into membranes, the synergistic activity accelerated the electrons transfer limiting the recombination of electron–hole pairs in the hybrid membrane resulting in an improvement of the CO₂ photocatalytic reduction reaction.

Very recently, Pomilla et al. [176], studied the CO₂ conversion into liquid fuels such as methanol and ethanol by using a catalytic membrane. The photocatalytic membrane (exfoliated C₃N₄ incorporated into a Nafion matrix) was used in a continuous photocatalytic reactor by using H₂O as reducing agent. The photocatalytic tests were conducted by irradiating the membrane with both UV and visible light in the range from 360 nm to 600 nm. The authors reported that alcohol production was promoted by the low contact time obtained thanks to the fast removal of the reaction mixture from the reacting volume, which permeated through the membrane, thus limiting the oxidation and/or secondary reactions.

Some authors reported that photocatalytic conversion of CO₂ with H₂O should promote the reduction of CO₂ by the photogenerated electrons and protons and simultaneous oxidation of H₂O by the photogenerated holes [177]. These simultaneous reactions are difficult for most of the reported photocatalysts and they need the presence of organic hole scavengers (e.g., triethylamine (TEA), trimethylamine (TMA), or ethylenediaminetetraacetic acid (EDTA)) [178,179]. To limit this problem, Wang et al. [177], reported the preparation of Cu₂O–Pt/SiC/IrOx hybrid photocatalyst, composed by a photo-oxidation unit (IrOx) and a photoreduction unit (Cu₂O–Pt) on SiC surface. This photocatalyst can increase the photocatalytic efficiency by improving the CO₂ adsorption and the lifetime of photogenerated charges. Additionally, to facilitate H₂O oxidation half reaction and to eliminate the backward reaction of the products, the authors performed this photocatalytic reduction in a spatially separated reaction system consisting of two reaction chambers divided by a Nafion membrane that allowed Fe²⁺ and Fe³⁺ ions to permeate through (Figure 7). The first chamber was loaded with the Cu₂O–Pt/SiC/IrOx photocatalyst and Fe²⁺ for CO₂ reduction, while the second chamber with Pt/WO₃ and Fe³⁺ for H₂O oxidation.

This artificial system exhibited photocatalytic performance for CO₂ reduction to HCOOH with a yield of 896.7 μmol g⁻¹ h⁻¹ for the long-term reaction and simultaneous H₂O oxidation to O₂ with a stoichiometric ratio. These results can be due to both the direct Z-scheme electronic structure of Cu₂O-Pt/SiC/IrOx and the indirect Z-scheme spatially separated reduction and oxidation units, which greatly prolong lifetime of photogenerated electrons and holes and prevents the backward reaction of products.

4.2.2. Conversion of Acetophenone to Phenylethanol in PMRs

Only few works have appeared till now on the use of a PMR in reduction reactions such as photocatalytic transfer hydrogenation of ketones [57,59]. The reduction of carbonyl compounds, in the corresponding alcohols, plays an important role in organic synthesis [1,2,3,4,57,59]. Molinari et al. [57], to develop a sustainable process, studied the photocatalytic hydrogenation of acetophenone to phenylethanol using commercial TiO₂ and homemade Pd/TiO₂ photocatalysts under UV and visible light, with water and formic acid, in batch and membrane reactor. They tested different methods for the substrate addition in the membrane photoreactor finding the best performance when the acetophenone was used as both solvent and reactant (substrate) with improved performance (productivity 4.44 mg g⁻¹ h⁻¹ vs. 2.96 mg g⁻¹ h⁻¹ PMR vs batch reactor). The photocatalytic activity in the PMR under visible light irradiation was improved five times by using Pd/TiO₂ (productivity 22.0 mg g⁻¹ h⁻¹). Recently, Lavorato et al. [59] studied the photocatalytic properties of TiO₂-loaded faujasite (FAU) zeolite and Pd/TiO₂/FAU in the heterogeneous transfer hydrogenation of AP under UV and visible light in batch and in a membrane reactor. A highly homogeneous dispersion of TiO₂ catalyst particles on the zeolite surface was obtained in the sample TF10P, prepared by using the lowest amount of TiO₂ precursor and isopropanol as solvent. The results showed that Pd doping of TF10P sample gave the photocatalyst Pd_TF10P active in the visible light. The productivity obtained in the PMR was higher by using Pd_TF10P [59] (productivity 99.6 mg g_TiO2 ⁻¹ h⁻¹ vs. 22 mg g_TiO2 ⁻¹ h⁻¹) with an extraction percentage of phenylethanol of ca. 25%.

4.3. Partial and Total Oxydations in PMRs

Membrane separation has been studied in partial (e.g., benzene to phenol) or total oxidation (e.g., water treatment) due to its excellent performance, relatively low energetic cost, and nontoxicity [1,2,3,4,180,181,182].

4.3.1. Partial Oxidation of Benzene to Phenol in PMRs

Phenol production is mainly obtained by the three-step cumene process. An interesting alternative is direct benzene oxidation. Many studies have been achieved on this topic, also in membrane reactor, but in general under UV light irradiation [2,37,181]. The photocatalytic approach consists of the unselective oxidation by generation of OH^• radicals with formation of by-products because phenol is more reactive than benzene [37,150]. To improve the selectivity for phenol production, some authors reported photocatalytic oxidations combined with a membrane to achieve phenol separation during the photocatalytic reaction thus avoiding further oxidation [37]. These studies use UV light, instead of a greener approach that can be achieved by irradiating the photocatalyst with visible or solar light. For this reason, some authors studied various modification of the photocatalyst to improve its activity under visible light but, until now, only studies in batch reactors (see Section 3.3.4) are reported. In the near future, studies on PMRs using visible light are expected.

4.3.2. Chemical Conversions for Water Treatment in PMRs

Membranes for water purification can offer easily clean water, but some disadvantages are membrane fouling and the necessity of the successive removal, from permeate water, of some contaminants, still contained, with further treatment [183,184,185]. Various authors proposed the coupling of membrane filtration with photocatalysis to overcome these problems [1,181,185,186].

For example, Ma et al. [185], and Zhang et al. [186], fabricated a membrane via the immobilization of TiO₂ on the surface of a ceramic membrane. This photocatalytic system, with integrated membrane, not only presented improved removal efficiency, but also showed an enhanced fouling resistance during the filtration process, furthermore, the contaminants in water can be degraded by photocatalysis. The majority of reported photocatalytic membranes are applied under UV light irradiation [1,2,3,4], but, nowadays, studies on systems operating under solar or visible light are found in literature [180,187,188]. In the following, very recent papers on oxidation reactions and water treatment in photocatalytic membrane reactors under visible light are reported. Roso et al. [187], investigated the degradation of volatile organic compounds, such as acetaldehyde and methanol, using PVDF electrospun membranes modified with TiO₂, Ag₂CO₃ and GO. The results showed a complete degradation of 1600 ppm of acetaldehyde within 15 min and 600 ppm of methanol within 40 min under visible light irradiation employing the PVDF/TiO₂–Ag₂CO₃–GO system. The authors explained the high photoactivity with the presence of the heterojunction formed between Ag₂CO₃ and TiO₂, which effectively separated the charge carriers.

Zhang et al. [180], reported the preparation of g-C₃N₄ quantum dots (QDs) assembled into TiO₂ nanotube array (TNA) membranes to obtain a visible-light-driven g-C₃N₄/TNA membrane. They reported more than 60% of rhodamine B removed from water under visible light irradiation thanks to the synergistic effect of membrane filtration and photocatalysis. Moreover, this type of membrane integrated process showed also an improved anti-fouling ability during filtration of water containing Escherichia coli, under visible light irradiation, with a permeate flux 2 times higher than the filtration alone.

The growing increase of the presence of pharmaceutical products and their metabolites as environmental pollutants require advances in wastewater treatment to limit the potential risk they pose to aquatic organisms and human health [153]. Photocatalysis has emerged as a foremost reliable application with high efficiency for degradation of pharmaceuticals due to its ability to destroy (mineralize) organic pollutants, nontoxicity, chemical inertness, and low cost [15,153,170,171,180,188]. Among the various pharmaceuticals, diclofenac (DCF) is a very common synthetic non-steroidal anti-inflammatory drug found in wastewaters. It is an organic pollutant hardly biodegradable, and thus difficult to completely remove by conventional wastewater treatment processes [153]. To enhance DCF removal, very recently, Nguyen et al. [153], studied the performance of visible/N-doped TiO₂ photocatalyst (with and without H₂O₂ addition to the reaction environment) using a submerged photocatalytic membrane reactor (SMPR) with suspended photocatalyst obtaining best results by adding hydrogen peroxide as oxidant. N-TiO₂ is a promising photocatalyst for wastewater treatment because N is more effective than other dopants (C, S, P) in narrowing the optical bandgap of TiO₂ because of closing energy between N 2p state and O 2p state. The SMPR, Figure 8, was a cylindrical photoreactor (2 L volume) with an immersed tube of a MF ceramic membrane surrounded by five visible lamps of 50 W (420–720 nm). The membrane was connected to a suction pump to collect the treated water and its sampling. The oxygen was continuously fed under the UF membrane.

The results indicated that by using a higher DCF initial concentration the efficiency of the process decreased, but, by adding H₂O₂, the system performance enhanced. The experimental data were found to fit well a pseudo-first-order kinetic model.

Hu et al. [189], designed another PMR system integrated with photocatalyst for wastewater treatment. They used P-doped g-C₃N₄ (PCN) as photocatalyst, coated on an Al₂O₃ substrate followed by integration with an inorganic Al₂O₃ hollow fiber membrane module for use as a PMR. In Figure 9 the PMR configuration is showed: it was built by collecting a Pyrex reactor for batch-type photocatalytic degradation with a 300 W Xe lamp used as the light source to simulate solar irradiation and connecting it to the UF membrane. In this PMR system the photocatalyst, PCN@S is separated from the Al₂O₃ hollow fiber membrane module. The wastewater (1 L) contained methylene blue (MB), methyl orange (MO) or phenol. The photocatalyst amount of 10 wt% PCN exhibited the highest degradation activity for MB removal under visible irradiation. The PMR exhibited higher efficiency and stability in the removal of MB, MO and phenol. The total organic carbon (TOC) analysis revealed that more than 92% of the phenol was decomposed and mineralized in the PMR, which also had a MB removal efficiency greater than of 90% when repeatedly used for four times.

Athanasekou et al. [190] studied the efficacy of a hybrid process that included photocatalysis and ultrafiltration to reduce the presence of synthetic dyes in water. They prepared some ceramic UF membranes with deposition of various photocatalysts (TiO₂, graphene oxide-TiO₂ composites) based nanomaterials on the external and internal (pore) surface of UF mono-channel monoliths. The photocatalytic filtration experiments took place in a patented water purification device in continuous flow conditions, using MB and MO as azo-dye model pollutants, under near-UV/vis and visible light irradiation.

Ashar et al. [191], studied the photocatalytic degradation of RB5 reactive dye under artificial sunlight by using ZnO and Fe³⁺@ZnO nano discs in PMRs. The results showed 88.89% of dye degradation by using ZnO/PMR and 98.34% by using Fe³⁺@ZnO PMR in 180 min. The photocatalytic activity of Fe³⁺@ZnO PMR gradually decreased after the PMR reuse for eight reaction times.

It is known that one of the main problems in membrane purification is the fouling. To this aim, Sun et al. [192], prepared a photocatalytic membrane with good antifouling and self-cleaning ability. They reported the use of an external magnetic field to arrange the magnetic TiO₂@Ni particles (MNPs) onto the polymeric polyether sulfone (PES) membrane surface that acquired a self-cleaning property under UV light and sunlight radiation. The optimal membrane, after self-cleaning, presented a flux recovery ratio (FRR) of 75.4%, 99.56%, 92.11% and 98.26% for bovine serum albumin (BSA), yeast extract fermentation (YEF), ammonium alginate (SA) and humic acid (HA) solutions, respectively. The group of Lv et al. [193] fabricated a self-cleaning membrane using a mussel-inspired method, that was used with high photocatalytic efficiency in dye degradation under visible light and with hydrogen peroxide. The membrane consisted of a polydopamine (PDA)/polyethyleneimine (PEI) intermediate layer casted on a UF membrane support and a photocatalytic layer made of β-FeOOH nanorods. The wettability enhanced after this treatment (the dynamic water contact angle decreased from 60° to 20°). During the photocatalytic test the authors observed a slight decrease of permeate flux in the initial 2 h, while it recovered to nearly the original value after 6 h of filtration.

Graphene oxide (GO) is a material of particular interest to make membranes for water purification thanks to its hydrophilic surface properties and the special interconnected 2D nanofluidic channels for ion/molecule transport. To improve the photocatalytic response of the photocatalytic membrane, eliminating also the fouling under visible light irradiation, Liu et al. doped the surface of GO and titanate nanotubes (TNTs) with Ag nanoparticles [194]. Ag/GO/TNT membranes exhibited good ability on photocatalytic degradation of MB dye under visible light. They reported that 90% of MB could be degraded after 120 min irradiation. The flux of Ag/GO/TNT membranes, with a ratio of GO and TNT equal to 1:3 and 6% of Ag content, was 34.7 L m⁻² h⁻¹ which was double compared to some membrane filtration tests without visible light irradiation. Four types of membranes prepared by using a non-toxic solvent and GO nanosheets as a metal-free catalyst, to photoactivate the membrane, were investigated by Alyarnezhad for the degradation of methylene blue (MB⁺) under visible light irradiation (>420 nm) [195]. The incorporation of GO enhanced the mechanical strength of the membranes prepared and their wettability. Moreover, the presence of one hydration layer on the membrane surface limited the fouling decreasing the attachment of pollutants and microorganisms. The results showed a dye removal efficiency of 83.5% under simulated solar light irradiation by using M8 membrane sample, prepared with PVP, PEG and GO (0.125 wt%) in the dope solution, and exposed for 2.5 min to moisture during the VIPS step.

5. Design of Novel PMRs

The use of magnetic materials in the preparation of photocatalyst composites and coating semiconductors on optical fibers can give a significant advancement in potable water treatment. In the last years the development of new materials that incorporate graphene-based semiconductors have been increased for their adsorptive ability towards pharmaceuticals for waste water treatment [196]. Advancements in photocatalytic materials research have allowed the development of various materials, e.g.: adsorbents incorporated in membrane technologies, photocatalysts combined with magnetic material and coated on optical fibers. Magnetic composites can make the removal of the photocatalysts from water easier and more effective reducing the chance that they may accidentally end up in the environment. Moreover, the introduction of magnetic materials in the composite improved their degradative properties [196,197,198,199]. Some authors reported the use of magnetic FeNi₃/SiO₂/CuS to remove tetracycline [199] while magnetic fluorinated mesoporous graphitic carbon nitride and a magnetic TiO₂-GO-Fe₃O₄ [112,197] were used to remove amoxicillin from waste water. Recently, some authors reported various photocatalytic materials loaded on optical fibers [196]. The immobilization of semiconductors on optical fibers is an interesting method for recovering photocatalysts; furthermore, this method allows light to better reach the nanoparticles as less light is absorbed by other particles present in the solution.

A type of composite semiconductor as TiO₂-rGO coated optical fibers was able to degrade pharmaceutical compounds such as sulfamethoxazole and ibuprofen [200]. Modification of existing potable water treatment plant equipment or processes, including photocatalysts or graphene-based materials in membrane technologies, can significantly enhance the functionality of the membranes, reduce fouling and degrade contaminants. Indeed, the use of TiO₂ alone in polyacrylonitrile membranes was reported to be unsuitable for long-term use [201].

Another type of hollow fiber membrane was studied by Kamaludin et al. [202], for the removal of bisphenol A (BPA) from water under visible light. Today a treatment for the removal of BPA is greatly required because its presence in various water sources has potentially led to numerous adverse effects in humans. The authors reported the preparation of a photocatalytic dual-layer hollow fiber (DLHF) membrane fabricated via co-spinning phase inversion. The resultant N-doped TiO₂ exhibited good optical properties with an obtained band gap of 2.64 eV which presented excellent photocatalytic activity under ultraviolet and visible irradiation. DLHF membranes showed a 90% BPA removal under UV light irradiation while N-doped TiO₂ DLHF removed 81.6% of BPA under visible light irradiation.

An optical-fiber reactor was employed to photocatalytically reduce CO₂ with H₂O to fuels under UVA artificial light and concentrated natural sunlight [203]. The optical fiber was prepared by coating it with a gel-derived TiO₂–SiO₂ mixed oxide-based photocatalyst. The insert of Fe atom into the TiO₂–SiO₂ lattice during the sol–gel process, influenced the visible light absorption and product selectivity. Under UVA, by using Cu–Fe/TiO₂, the main product was ethylene with the quantum yield of 0.0235%, instead, by using Cu–Fe/TiO₂–SiO₂ catalyst, methane production was favored with the quantum yield of 0.05%. Under natural sunlight only methane was produced by using both TiO₂–SiO₂ and Cu–Fe/TiO₂–SiO₂ catalysts with the production rates of 0.177 and 0.279 mmol g_cat⁻¹ h⁻¹, respectively.

Some recent systems, reported in the following, despite work under UV light, can help to develop novel PMRs working under visible light. Tugaoen et al. reported the development of a compact reactor, for photocatalytic water treatment, in which TiO₂ was coated on optical fibers by using light-emitting diodes [201]. The TiO₂/optical fiber flow reactor, schematized in Figure 10, consisted of a Near Clear PVC cylinder with an inner diameter of 1.9 cm and a total length of 18 cm. A peristaltic pump circulated solutions at 5 mL min⁻¹, resulting in a 10 min hydraulic retention time within the reactor. This one was connected in series with a 100 mL reservoir in which solutions containing 0.1 mM of pCBA at pH 4.0 were introduced and recirculated through the system. The coated optical fibers LED couple (OF/LED) were inserted into the reactor at the same distance (1 cm). Three configurations OF/LED units connected to a single LED source were examined: (1) an individual fiber, (2) a bundle of three fibers, and (3) a bundle of fifteen fibers. Moreover, the number of OF/LED units inside the reactor was varied from 1 to 5. The use of TiO₂ coated optical fiber bundles reduced the energy requirements to deliver photons and increased available surface area enhancing the oxidative removal performance of the chlorinated pollutant para-chlorobenzoic acid (pCBA).

Cheng et al. [204] developed a photocatalytic reactor to improve the performance of the photocatalytic reduction of CO₂. They used an optofluidic planar microreactor irradiated by a 100 W LED (365 nm) with the light intensity changed by adjusting the distance between the microreactor. The reaction environment was constituted by an aqueous alkaline solution. During tests CO₂ (99.99% purity) was continuously provided to the NaOH solution for 1 h to saturate with CO₂ and to remove dissolved oxygen. Then the CO₂ saturated solution was pumped into the microreactor by a syringe pump. The microreactor was made by a transparent rectangular reaction chamber as the top cover and a porous TiO₂ film coated glass as the bottom substrate. The porous TiO₂ film on the glass slide was formed by the wet spray method. This system presented the following advantages: large surface-area-to-volume ratio, enhanced mass and photon transfer and more uniform light distribution. The performances were evaluated by measuring the concentration of produced methanol to estimate the methanol yield. At high light intensity and NaOH concentration, both the methanol concentration and yield increased. Also, improving the catalyst loading, the performance of the system increased with better results (methanol yield of 454.6 mmol g_cat⁻¹ h⁻¹) using a liquid flow rate of 50 mL min⁻¹, 0.2 M NaOH, and light intensity of 8 mW cm⁻².

6. Summary and Future Perspective

In the present overview, recent literature on photocatalytic organic synthesis and water treatment under visible light is reported. Previous studies on photocatalytic reduction as well as partial and total oxidation of organics on membrane reactors have shown the great potential of this technology but they ostly involve the use of UV light. In recent years, solar-driven photocatalytic conversion has become very attractive as a means to convert solar energy in a clean and effective way for chemical reactions. For this reason, in this paper we focus on the photocatalytic organic synthesis and water treatment under visible light. To utilize more efficiently the solar energy, several semiconductor materials possessing visible-light- activity have been designed and prepared by many authors. The key point in the design is to expanse the light-harvesting region from UV to visible obtaining photocatalysts active under visible light. The reported recently studies on the innovation and quality enhancement of photocatalytic materials for photocatalytic reactions show the crucial role of the suitable design of the photocatalytic system. In particular: choice of the photocatalyst, photocatalyst design, combination of photocatalysts, doping with noble metals, dye sensitization, etc. to allow to improve the visible light absorption of photocatalyst thus limiting photogenerated charge-carrier problems and increasing yield and selectivity. Photocatalysts based on TiO₂, CdS, WO₃, ZnFe₂O_4, graphene, graphitic carbon nitride (g-C₃N₄), GQDs and MOFs etc., further modified or doped to improve their visible light absorption seems of particular interest in synthesis and water treatment. Combining photocatalysis with membrane separations, hybrid systems named photocatalytic membrane reactors (PMR) are obtained. Their main advantages are: synergic effect, catalyst reuse, selectivity enhancement, improved anti-fouling ability, lower degradation rate and longer lifetime of polymeric membranes, thanks to the lower energy of visible light. The use of magnetic materials in the preparation of photocatalyst composites, the coating of semiconductors on optical fibers, the use of membrane microreactors, can be promising in synthesis and potable water treatment for the easier removal of the photocatalysts from water with the future perspective to employ visible light. A sustainable process can be obtained by operating PMRs with visible-LED or solar light as clean non-limited energy source and with green photocatalysts. On this aspect, the development of new photocatalysts with high activity under visible light and their application in the various research fields in suitable designed PMRs is an expanding research area.