Monolithic Porous Organic Polymer‐Photocatalyst Composites for Applications in Catalysis

This Review provides a perspective on porous organic polymer‐photocatalyst composites obtained by coupling semiconductors and hydrophilic/hydrophobic polymers which do not modify the properties of the embedded photocatalysts, but can influence the efficiency of the overall catalytic process. Particular attention has been given to polymer composites in the form of monolithic hydrogel/sponge/aerogels obtained by dissolving the polymer in a solvent, which contains the photocatalyst dispersed, inducing gelation or solidification of the solution and subsequently removing the solvent by a drying process. The photocatalytic applications discussed here cover H2 evolution from water splitting, CO2 reduction, and organic synthesis. Indeed, the main aim of this Review is to outline an alternative perspective to the highly studied environmental photocatalytic applications, highlighting the photoactive properties of these composites thanks to the incorporation of semiconductors in the 3D porous structure of organic polymers. Finally the challenges and potential advances associated with the use of porous organic polymer‐photocatalyst composites for future scientific research are outlined.


Introduction
Nowadays, the continuous consumption of fossil fuels has caused and is still causing a sharp increase in environmental pollution and energy depletion. [1]eterogeneous photocatalysis can be seen as one of the most promising strategies to solve these challenges as, through a photocatalytic process, solar energy is converted into chemical energy.In this way, it is possible to activate redox reactions in mild conditions, thanks to suitable illumination sources, such as visible light or sunlight. [2]ver the years, photocatalysis has made significant progress and has emerged as a promising technology for generating chemical fuels, such as H 2 from water splitting reaction and formic acid, formaldehyde and methanol from CO 2 photoreduction, using photons as the only energy input. [3]oreover, in the context of industrial processes, the synthesis of many organic compounds generally requires the use of high temperatures and high operating pressures.Consequently, the development of photocatalytic synthetic reactions that employ the use of suitable light sources to drive chemical processes at significantly lower temperatures is useful for meeting the needs of "green chemistry". [4]Indeed, the use of photocatalysis allows the possibility to achieve reaction pathways and intermediates that are not accessible under thermal conditions [5] Moreover, it is possible to carry out the "one-pot" synthesis of organic compounds, minimizing the number of operating units (in terms of reactors and equipment for purification of the reaction product) necessary to obtain the desired product. [6]4a,7] It is widely reported that nanometer-sized powder photocatalysts guarantee high photocatalytic activity due to their high surface areas. [1]However, it should be emphasized that nanoparticulate powders not only have a strong tendency to agglomerate (especially in the case of photocatalytic reactions performed in a liquid medium) but also require complex operations to recover the photocatalyst for subsequent reuse. [8]o eliminate these disadvantages it is desirable to immobilize the semiconductors on suitable monolithic supports that combine both macro and nanoscale features in order to ensure high photocatalytic efficiency. [1]In particular, this goal could be achieved by entrapping the photocatalysts in a porous macroscopic support.
Generally speaking, natural porous structures (such as sponges belonging to aquatic animals of the phylum Porifera [9] ) could represent a possible inspiration for the design of novel porous materials, and, during the years, new preparation methods for achieving hypercrosslinked porous polymers (HCPs) [10] and porous organic polymers (POPs) were developed. [11]Among them, POPs were recently the object of scientific research because these types of materials can combine the typical properties characterizing porous structures with those typical of polymers. [9]Indeed, the main advantages of POPs with respect to non-porous materials are due to their large specific surface area, well-defined porous structure and the availability of a wide variety of synthetic methods for their preparation. [9]For these reasons, different POPs were prepared and tested in several applications, including adsorption of CO 2 , [12] energy conversion and storage, [13] catalysis [14] as well as drug delivery. [15]ncorporating a photocatalyst into a polymer host matrix is a very common practice.The resulting hybrid-materials are generally defined as a class of polymer based composites.Recently, many reviews dedicated to this material class have appeared in the literature. [16]atalytically active composites based on POPs are among the most studied of this class of composites. [11,17]ithin the large class of photocatalytically active polymerbased composite materials, the polymeric matrix function can be either "active", if the polymer modifies the photocatalyst properties, or "passive", if the matrix only performs the function of immobilizing the photocatalyst.Examples of active matrices are conductive polymers which act as photoelectron transfer pathways and allow to lengthen the charge separation time in the photocatalyst.16d] Furthermore, polymeric matrices hinder the aggregation processes of the catalyst particles [16d,19] and allow their resistance in aggressive environments, and finally they can act as selective filters towards the species on which the catalyst acts (e. g., polar (non-polar) matrices allow the passage of only polar (non-polar) species). [20]he polymer matrices of catalytically active composites are commonly porous materials with a high surface area.In fact, a homogeneously dispersed catalyst on the accessible surfaces of a highly porous matrix, in turn, has a high active surface per unit volume of the composite.Therefore, porous polymer-based composites with embedded photocatalysts can exhibit reaction efficiencies similar to, or better than, those of photocatalytic powders suspended in the reaction medium. [21]20b,d] The 3D architecture of the polymeric matrix can be obtained in many different ways, both during the polymerization process which generates the matrix itself (typical bondforming processes are, for example, radical, anionic, cationic, and condensation reactions), [16f,17d,22] and by suitably treating a commercial or custom-designed polymer.16f,20e] Recently, 3D printing processes have also been employed to obtain porous polymeric matrices. [23]When the formation of the 3D polymer matrix takes place through a bond-forming process, it is necessary to include the catalyst in the reaction environment in which the polymer synthesis takes place.This condition reduces the types of catalysts that can be used for the composite preparation. [1]ven in the case of 3D printing, incorporating the catalyst in the construction phase of the polymeric framework is complex.Conversely, the formation of the polymer porous framework through gelation/solidification processes of polymer solutions, followed by solvent removal, simplifies the formation process of both porous matrix and polymer-catalyst composite, since the catalyst can be incorporated into the polymer solution before gelation/solidification. [16f] The crucial step for 3D porous structure preparation is the solvent removal under conditions to preserve the porosity and integrity of the material.Although an expensive process, solvent removal with supercritical solvents is undoubtedly the best method for obtaining welldefined structures. [24]16f] Several semi-crystalline polymers are capable of forming physical gels and can be converted into porous structures by supercritical drying or freeze-drying. [1,26]The most studied polymers in the literature that give rise to physical gels and aerogels are syndiotactic polystyrene (sPS) and polyimide (PI). [26]f the polymer used as photocatalyst host matrix is hydrophilic, the resulting gel is a hydrogel and the 3D porous structure includes water in the interstitial spaces between the chains.
Conversely, if the polymer is hydrophobic, the polymer gel includes an organic solvent and, after the solvent removal, the polymer chains are simply separated by air over the entire 3D volume.In this case, we speak of aerogels (if the sizes of the pores range from a few nm to a few tens of nm) or sponges (if the size of the pores is between 50 and 500 μm). [27]herefore, the main advantage of hydrophilic polymer/ photocatalyst composites is that they can be used in photocatalytic reactions involving water, electrolytes and organic polar reactants.Conversely, hydrophobic/photocatalyst composites are useful for photocatalytic reactions involving reactants with non-polar character.
When a commercial or custom-designed polymer is used as host matrix to obtain a photocatalytic composite, the catalyst inclusion is also often achieved by simple immersion of the porous polymer framework in a solvent in which the photocatalyst is dispersed, followed by drying at a temperature that depends on the structural and thermal stability of the polymeric support. [28]17d] In this review, we will provide a perspective on porous organic polymer-photocatalyst composites obtained by coupling semiconductors and hydrophilic/hydrophobic polymers which do not modify the photocatalyst properties, but can influence the catalytic process.
Particular attention will be given to polymer composites in the form of monolithic hydrogel/sponge/aerogels obtained by dissolving the polymer in a solvent, which contains the photocatalyst dispersed, inducing gelation or solidification of the solution and subsequently removing the solvent by a drying process.
The applications discussed here cover photocatalytic H 2 evolution, CO 2 reduction, and organic synthesis.The main aim was to outline an alternative perspective to the highly studied environmental photocatalytic applications, highlighting the photoactive properties of monolithic polymer composites thanks to the incorporation of semiconductors in the 3D porous structure of organic polymers.
It is important to underline that graphitic carbon nitrides, metal-organic frameworks (MOFs) and covalent organic frameworks (COFs) can be considered as nanoscale porous polymers but they have not been described in a monolithic form.So these types of materials are beyond the scope of the current review.

Hydrophilic Polymer-Photocatalyst Composites
Hydrophilic polymer/photocatalyst composites are based on the use of hydrogels as supports for photocatalysts.As mentioned in the introductory section, it is possible to talk about hydrogels when the 3D porous structure of the polymer is held upright by the water that occupies the interstitial spaces between the polymer chains.In this section, we focused on hydrogel-photocatalyst composites for application in the field of catalysis.
Hydrogel (gel made by hydrophilic chains that form a 3D cross-linked polymeric network) has 3D macroporous structure that could swell quickly and retain large volumes of water in its swollen structure. [30]The content of water inside a hydrogel is influenced by several factors, such as its chemical composition and the density of cross-linkers.Since the polymeric chains are hydrophilic, it is possible to realize functional soft materials. [31]he classification of hydrogels depends on the polymers involved, the method of crosslinking and their ionic charge.In general, hydrogels can be prepared by polymer crosslinking which can be physical, chemical, or both.The crosslinking process is carried out in different ways, such as simple mixing, solution casting, bulk polymerization, free radical polymerization, polymerization under UV and gamma irradiation, and the interpenetrating network formation method.Hydrogels are also classified according to ionic charge, into cationic, anionic, and neutral hydrogels.The overall charge of the hydrogel depends on the charge of the polymer used during the preparation step. [32]Hydrogels could be considered ideal hosts for embedding photoactive materials to realize novel composites.Indeed, due to their hydrophilic nature, hydrogel-based photoactive composites may show interesting photoactivities in aqueous media.The hydrogel matrix, which acts as a host for the catalytic species, can be used to control the catalytic processes.In particular, the photocatalytic process can be made selective and more efficient if the permeability of the hydrogel is able to differentiate between the molecules involved in the chemical reaction and modulated by external stimuli.Therefore, the strategy of coupling photocatalysts with hydrogels can contribute to more efficient and environmentally friendly catalytic processes.
Figure 1 shows the number of publications for the keywords "hydrogel and photocatalysis".The results indicate an increasing research interest worldwide on this topic.

Preparation of hydrogel-photocatalyst composites
To confer photocatalytic properties to hydrogels, photocatalytic materials are introduced into the 3D cross-linked polymeric networks.The 3D cross-linked polymeric networks provide a porous structure that limits catalyst leakage into the reaction media (air or water) and facilitates the loading of a large amount of catalyst particles.The nanometer-sized photocatalyst provides active sites for the development of catalytic reactions.
Different methods, summarized in Scheme 1, are proposed in the literature for the preparation of hydrogel-photocatalyst composites.These methods can be divided into the following three categories.
a) The embedding of photocatalytic particles in hydrogel networks is realized by combining hydrogel precursors with the colloidal suspension of photocatalyst nanoparticles before hydrogel formation.For instance, Mansurov et al. used N,Ndimethylacrylamide (MDAA) and acrylamide (AA) as precursors for the fabrication of hydrogel, and TiO 2 Degussa/Evonik as the photoactive phase.The hydrogels were synthesized at room temperature by free radical polymerization of AA using MDAA as the cross-linker agent and then different amount of TiO 2 colloidal suspension was added to the system. [31]Another example of TiO 2 particles embedded into a hydrogel was reported by Katzenberg et al.In this case, the authors used, for the hydrogel formation, a copolymerization of hydroxyethyl methacrylate (HEMA) and acrylic acid (AA) with ethylene glycol and dimethacrylate (EGDMA) as a crosslinking agent.HEMA, AA, and EGDMA were mixed in water with an aqueous dispersion of anatase TiO 2 nanoparticles. [33]In the work by Kazem et al., polyacrylamide (PAAm) hydrogel was synthesized under sunlight with commercial TiO 2 particles as the initiator, acrylamide as monomer, and N,N'-methylene bisacrylamide as the crosslinker agent. [34]) Photocatalysts are in situ synthesized in hydrogel networks by combining hydrogel with the photocatalyst precursor solution.The photocatalysts are then obtained by oxidation, reduction, or sulfuration of photocatalyst precursor.In this contest, Yang et al. prepared cadmium sulfide/polyacrylamide hydrogels (CdS/PAM).Monomer Acrylamide (AM) and the crosslinker N,N'-methylene-bis-acrylamide (BIS) were dispersed in aqueous media at ambient temperature and stirred until to reach their complete dissolution.Then initiators ammonium persulfate (APS) and sodium bisulfite (SBS) were added to the above solution, respectively, and the final solution was transferred to a plastic dish to carry out polymerization to obtain the PAM hydrogel.To have CdS/PAM composite, the hydrogel was immersed in a solution containing cadmium chloride to allow interaction between amino groups on the polymer chains and Cd 2 + .Subsequently, the hydrogel was treated with and sodium sulfide solution for several hours.During this step, the colorless PAM hydrogel gradually turned yellow, indicating that CdS nanoparticles were successfully synthesized in the hydrogel matrix.[35] Following almost the same process Li et al. synthetized a cationic hydrogel encapsulating CdS particles.[36] c) Self-assembly hydrogels are prepared by chemical reduction of conductive materials, such as graphene oxide (GO), carbon nanotubes, polypyrrole, and poly(3,4-ethylenedioxythiophene) polystyrene sulfonate followed by H-bond and π-π selfassembly.Chen et al. used this method to prepare polyaniline/ TiO 2 -graphene hydrogel (GH) by chemical reduction of graphene oxide (rGO) followed by H-bond and π-π self-assembly.The rGO and PANI act as a transmitter for the photogenerated e À and h + , enhancing the photocatalytic performance.[37]

Photocatalytic hydrogen evolution
One of the grand challenges of the 21st century is the transition toward more sustainable energy systems.Hydrogen (H 2 ) represents an ideal energy storage medium due to its energy density (142 MJ/kg) and zero-carbon production upon combustion.Hydrogen is also used as a major reactant in important reactions for industrial chemistry, such as carbon dioxide hydrogenation to methanol [38] or ammonia production (Haber-Bosch reaction). [39]However, it must be considered that the extensive use of fossil fuels fed in industrial plants for the production of hydrogen is one of the causes of the sharp decrease in non-renewable sources and the onset of various environmental problems, such as global warming and the greenhouse effect. [40]For these reasons, in the last decade, attempts have been made to develop alternative and effective processes suitable for producing energy from renewable sources such as solar energy.In this context, heterogeneous photocatalysis could be seen as a green method for the production of hydrogen from water splitting reaction.Up to now, different photocatalysts, such as sulfide-, oxide-, and oxynitride-based materials have been proposed for hydrogen production via water splitting under visible and solar irradiation. [41]However, the direct solar-to-hydrogen energy conversion efficiency value for photocatalytic water splitting systems is still very low [42] since photogenerated holes easily recombine with electrons, limiting the reaction efficiency [43] .43a] An ionic hydrogel containing CdS nanoparticles embedded in the 3D hydrogel structure (CdS/HGel PDAM2 ) was prepared and tested by Li et al. in the photocatalytic hydrogen production in presence of triethanolamine or Na 2 SÀ Na 2 SO 3 as sacrificial agent. [36]The experimental results revealed an excellent performance of CdS/HGel PDAM2 composite since a very high hydrogen production rate was achieved (~10 mmol h À 1 g À 1 ) evidencing that the best system was CdS in cationic hydrogel (CdS/HGel PDAM2 ).The excellent photocatalytic performance of CdS/HGelPDAM2 comes from the high swelling performance of the hydrogel structure.So sacrificial agent solution can move freely into the hydrogel.In this way, the molecules of the sacrificial agent can effectively capture the photo-generated holes, inhibiting the electron-hole pairs recombination and improving the reaction activity and stability of the CdS/ HGel PDAM2 photocatalyst. [36]ydrogel-based photocatalyst composed of ZnO/ZnS nanoparticles embedded into the polyvinyl alcohol (PVA) polymeric matrix was prepared by Poliukhova et al. [46] The observed H 2 production rate under UV light was 18.8 μmol • h À 1 from a 0.1 M Na 2 S and 0.1 M Na 2 SO 3 aqueous solution.It has been hypothesized that PVA hydrogel is an excellent support for the photocatalytic production of H 2 due to its high transparency to light, its 3D-type porous structure generated by the interconnection between the polymer chains, and its hydrophilic nature which provides good water transportation inside the hydrogel.For these reasons, PVA-based composite hydrogels have shown good performance in the photocatalytic generation of H 2 and good recyclability. [46]30b] Sai et al. have synthesized functional hybrid polyelectrolyte hydrogels containing self-chromophore amphiphiles, [47] evidencing a hydrogen production rate of 107.4 mol mol cat À 1 h À 1 (sacrificial agent: ascorbic acid) [47] .

Photocatalytic CO 2 reduction
The photocatalytic conversion of CO 2 into renewable fuels in the presence of solar radiation is now considered an ideal strategy to reduce the concentration of carbon dioxide in the atmosphere to overcome the urgent problems causing global warming.Photoreduction of CO 2 is a complex reaction, which can yield numerous products, such as CO, HCOOH, HCHO, CH 3 OH, CH 4 , and other hydrocarbons. [48]An example of a hybrid photocatalyst hydrogel composed of carbon dot (CD)-decorated BiVO 4 (BVO) and a reduced graphene hydrogel (rGH) (CDdecorated BVO/rGH) was prepared by Ma et al. [49] CO 2 photoreduction performances observed in the presence of CDdecorated BVO/rGH are reported in Figure 2. The yields of CH 4 and CO from CO 2 photoreduction were 32.2 μmol g À 1 and 92.3 μmol g À 1 after 180 min of run time, respectively (Figure 2a).In addition, the CH 4 and CO yields remained almost unchanged after six reuse cycles (Figure 2b).It was argued that the photoexcited electrons are transferred from the conduction band of BVO to the surfaces of the CDs and rGH, where they can reduce CO 2 to CO and CH 4 . [49]eanwhile, the photoinduced positive holes in the valence band of BVO are quenched by triethanolamine, used as sacrificial electron donors. [50]espite the interesting results reported in the literature, research papers dealing with the use of organic hydrogelphotocatalyst composites in CO 2 photoreduction are still limited mainly because of the low solubility of CO 2 in water that limits its conversion under light irradiation.Hence, currently, the research activity on CO 2 conversion is based on the use of aerogels, which are obtained from freeze-drying or CO 2 supercritical drying of hydrogels. [49]

Organic synthesis
An example of a hydrogel-photocatalyst composite for organic synthesis was reported by Ma et al. who proposed a freezingthawing strategy for the preparation of an alkaline CuOchitosan hybrid hydrogel (CuO@CSÀ H) for the production of lactic acid under visible light at different operating conditions (i. e. KOH concentration, catalyst dosage, reaction temperature and reaction time) (Figure 3). [51]The authors demonstrated that the synergism between CuO and alkaline chitosan hydrogel promoted a highly efficient photocatalytic reforming of xylose to lactic acid and the highest lactic acid yield was about 82 %. [51] A possible reaction pathway for the synthesis of lactic acid via photocatalytic reforming of xylose by CuO@CSÀ H was also proposed [51] (Scheme 2).
Xylose was first isomerized to xylulose, and then xylulose was further converted to glycidaldehyde by retro-aldol reaction.Subsequently, lactic acid was obtained from glycidaldehyde. [51]ui et al. prepared hydrogel-photocatalyst composites by freeze-induced encapsulation of ultrathin carbon nitride nanosheets in three-dimensionally ordered porous matrices of polyvinyl alcohol (FG-s-PVA@CNNS3) (Scheme 3). [52]he presence of polyvinyl alcohol hydrogel did not affect the visible light absorption properties of carbon nitride nanosheets and enhanced the rate of photoinduced charge separation/transfer.FG-s-PVA@CNNS3 showed excellent performance in the selective oxidation of glucose to lactic acid and the obtained yield was about 93 %.Furthermore, FG-s-PVA@CNNS3 was proved to have good stability and reusability after ten reuse cycles.Interestingly, FG-s-PVA@CNNS3 also showed very good performances in the photocatalytic oxidation of fructose, mannose, rhamnose, arabinose and xylose, producing lactic acid at high yield (89.1 % from fructose, 88.6 % from mannose, 56.0 % from rhamnose, 71.1 % from arabinose and 70.9 % from xylose respectively. [52]

Hydrophobic Polymer-Photocatalyst Composites
In the case of hydrophobic polymers as support for photocatalysts, it is possible to speak of aerogels or sponges when the polymer chains are separated from the air throughout the three-dimensional volume occupied.
The size of the pores distinguishes an aerogel from a sponge.In detail, sponges are characterized by pores having size between 50 and 500 μm, or even larger.Instead, aerogels are characterized by pores ranging in size from a few nm to a few tens of nm.Furthermore, aerogels are ultra-light and highly porous (the porosity can even reach 99 %) compared to sponges.
In this section, we focused on both sponge-photocatalyst and aerogel-photocatalyst composites for photocatalytic reactions.

Sponge-photocatalyst composites
Solid sponges are materials characterized by large specific surface areas and high porosity. [53]n the field of heterogeneous photocatalysis, polydimethylsiloxane (PDMS) sponges are the most suitable materials to be used as support for different types of semiconductors since PDMS sponges have high transparency to the light wavelengths commonly used in photocatalysis. [54]54b]

Preparation of PDMS sponge-photocatalyst composites
The methods used for preparing PMDS sponges have been illustrated and discussed extensively in the scientific literature. [9]ince the purpose of this review paper is the analysis of photocatalytic systems obtained by dispersing photoactive phases within the structure of porous polymeric materials, only the synthesis methods that can be used for this purpose are briefly discussed.a) PDMS photocatalysts/sponges can be obtained by simply adding a certain amount of photocatalyst powder into liquid PDMS.The final composite is obtained following a direct curing process, although intermediate steps and additional processing such as a separate heat treatment may be necessary.Adding photocatalysts to a liquid PDMS system before solidification is an approach that allows for easy management of the final shape of the sponge composite. [56]) As an alternative to the use of an already synthesized powder photocatalyst, the semiconductor particles can be prepared from liquid precursor salts dissolved in the liquid PDMS prepolymer [56c] by the sol-gel method, largely employed in the preparation of photocatalysts.[57] This methodology is able to generate a composite characterized by an optimal dispersion of the photocatalyst particles within the PDMS structure.Several examples of this method are available in the scientific literature.[58] b) An interesting alternative for the preparation of PDMS porous sponges is based on the use of a powder photocatalyst previously mixed with a template which is then immersed in a liquid solution containing PDMS prepolymer.[54b] In detail, the photocatalyst is mixed with template particles (in most cases sugar is used).The mixture thus obtained is kneaded in a mold having dimensions and shapes that can be varied in relation to the final geometric shape of the sponge to be realized.The block thus obtained is then dried and immersed in a solution containing the PDMS prepolymer and a curing agent.In this way, the PDMS fills the spaces within the model due to capillary forces.This stage is usually carried out in a vacuum chamber to remove the solvents used in the preparation of the solutions. The block filled with PDMS is then cured at a temperature of 80 °C for the curing time necessary to achieve the polymerization.Subsequently, the sugar/PDMS model is immersed in water to dissolve the sugar and finally dried to obtain the sponge-photocatalyst composite. The inal material consists of photocatalyst particles well embedded within the PDMS matrix.This method was used for the preparation of a PDMS sponge containing ZnO particles dispersed in the polymer framework [59] (Scheme 4).c) A PDMS/photocatalyst composite can also be achieved by dispersing the catalyst in an already solidified PDMS sponge.[54b] In this case, the photocatalyst particles are embedded in a PDMS sponge by impregnating it with a suspension of the photocatalyst dispersed in suitable low-boiling solvents (such as ethanol).After the impregnation step, the solvent was allowed to evaporate at room temperature.In the obtained spongy material, the photocatalyst particles are deposited on the inner surface of the PDMS pores.The use of a PDMS sponge can provide a high surface area for the photocatalyst, minimizing the intraparticle aggregation phenomena that usually occur when the photocatalyst is deposited on the external surface of non-porous solid substrates (such as glass).
Porous PDMS sponge/TiO 2 prepared with this method are reported in the literature. [60]) In addition to the semiconductor particles, specific organic dyes can be immobilized in the PMDS-based sponges to give the sponges suitable photocatalytic properties.[19a,61] Indeed, the most common materials used in the field of photocatalyzed reactions were typically molecular systems based on transition metal complexes or organic dyes.[62] Some examples of organic dyes employed as photocatalysts are eosin Y (EY) and rose bengal (RB).[63] While the most extensively studied organometallic complexes are based on iridium-based or ruthenium-based polypyridyl.[64] These organic dyes and organometallic complexes are to be understood as homogeneous photocatalysts capable of absorbing visible light.In detail, in the presence of visible light, an electron is promoted from the most occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO), leading to the formation of a positive hole in the HOMO and an excited electron in the LUMO.62] From an economical point of view, the use of organic dyes (i. e.  and RB) is preferable with respect to organometallic complexes.
Organic dyes have very high photocatalytic performance and can homogeneously catalyze a wide range of chemical reactions including water splitting, CO 2 reduction, CÀ C coupling reactions, oxidative coupling of amines, heterocycle formation and enantioselective alpha-alkylation. [62]owever, homogeneous catalysis suffers from several disadvantages.Considering that the photocatalyst is dissolved in the reaction medium, complicated steps of separating the photocatalyst from the reaction products are necessary to recover and recycle the photocatalyst in the photoreactor.Furthermore, the possible presence of the photocatalyst in the final product is often unacceptable due to the toxicity of the photocatalyst.Additionally, many dyes used as photocatalysts have poor solubility in a wide range of solvents, limiting their use. [62]A possible solution to these drawbacks could be to incorporate the organic dyes into macroscopic structures such as PMDS-based sponges.
To immobilize the organic dyes in a PMDS sponge, in many cases some pre-treatment steps of the sponge are necessary which can vary according to the type of dye to be used.For instance, in the case of RB (Scheme 5), the PDMS sponge is subjected to air plasma treatment to induce the formation of hydroxyl groups on the sponge surface, followed by silanization of the hydroxyl groups using vinyltrimethoxysilane.61b] RB is then incorporated in the modified PMDS sponge by ion exchange.61b]

Applications of PDMS based sponges in photocatalytic reactions
In this section, the available scientific papers dealing with the use of PDMS-based sponges as supports for photocatalysts are briefly examined.
It must be underlined that, in the case of inorganic semiconductor particles (e. g.ZnO and TiO 2 ) embedded into PDMS sponge, the totality of the applications concerns the photocatalytic removal of pollutants (mainly in the degradation of organic dyes) and therefore they are, up to now, studied only for environmental applications.For instance, Zhu et al. reported that the photocatalytic activity of the PDMS/TiO 2 composite in the degradation of methyl orange was significantly higher than the powdered TiO 2 . [65]The authors associated the high removal capacity of PDMS/TiO 2 with the high sorption capacity of the PDMS matrix towards methyl orange molecules (and more generally towards all highly soluble pollutants in water).In this way, the pollutants sorbed by the PDMS can come into intimate contact with the surface of the photocatalyst particles incorporated in the PDMS framework, increasing the photocatalytic reaction rate.
Hickman et al. demonstrated that PDMS/TiO 2 composite sponge was able to remove the toxic dye Rhodamine B from an aqueous solution under sunlight, highlighting once again that the removal process is due to synergistic effects between the dye adsorption phenomenon in the sponge matrix and the photocatalytic degradation efficiency of the TiO 2 particles dispersed in the PDMS sponge. [60]The same authors also highlighted that the overall removal efficiency obtained with the composite was comparable to that obtained by dispersing in solution a quantity of TiO 2 equivalent to that embedded in the PDMS sponge.However, the use of the PDMS/TiO 2 sponge avoids the necessity of using post-treatment steps for the separation of TiO 2 particles from the treated solution.
Lee et al. [56a] added gold nanoparticles to a porous PDMS sponge containing TiO 2 particles, showing a significantly higher degradation efficiency of the Rhodamine B dye (both under UV and visible light) than the PDMS sponge containing only TiO 2 particles.Furthermore, the recyclability of the PDMS-based sponge for multiple cycles has been demonstrated.
56b] Furthermore, it was recently reported that the PDMS/ZnO sponge-like photocatalyst showed a significantly better photodegradation performance of methylene blue dye under illumination (UV and UV-Vis) than the ZnO-free PDMS sponge. [59]ased on the above-mentioned works, photocatalytic sponges obtained by incorporating semiconductor particles into the PDMS structure can be considered as efficient, highly stable and easily recoverable photocatalytic systems for the decontamination of wastewater containing organic dyes.
However, no scientific papers are found in the literature in which the efficiency of PDMS sponges functionalized with inorganic semiconductor particles for photo-assisted organic synthesis reactions is shown.

Photocatalytic synthesis of organic compounds under visible light
19a,61] The cross-dehydrogenative coupling (CDC) reaction of the α-CÀ H bond of nitrogen atoms allows to obtain the selective formation of the CÀ C and CÀ X bonds under oxidative conditions. [66]61b] The highest photocatalytic efficiency (Yield = 97 %) was achieved in the presence of ethanol as solvent.
61b] It was argued that RB molecules are excited under visible light irradiation.61b] Li et al. formulated also a modified PDMS-RB sponge.In detail, plasma-treated hydroxyl-rich PDMS sponge was modified with 3-aminopropyltrimethoxysilane, and resulting amino groups were coupled with F moc À Glu(O t Bu)À OH resulting in Glufunctionalized PDMS sponge, which was then subjected to an ion exchange procedure to incorporate RB. [67] The modified PDMS-RB sponge was effective in CPC reactions of tertiary amines with substituted ketones (Scheme 8) but using water as solvent. [67]hese literature papers confirm the efficiency of PDMS-RB sponges as visible light active photocatalytic materials for the synthesis of organic compounds, especially through CDC reactions.
It is important to underline that papers dealing with PDMS sponges functionalized with EY dye for photoassisted synthesis of organic compounds are still lacking.

Applications of non-PDMS based sponges in CO 2 photoreduction
Although PMDS-based sponges are the most used as a support for photocatalysts because of the reasons specified above, a melamine-based sponge has recently been proposed. [68]4a,69] Carbonaceous-based materials (such as graphene and g-C 3 N 4 ) can be easily dispersed in such sponges.Indeed MS/graphene composites have been successfully tested in oilwater separation processes. [70]n the case of heterogeneous photocatalysis applied to processes not aimed at pollutant removal, Yang et al. have formulated a monolithic melamine sponge/g-C 3 N 4 composite (MS/g-C 3 N 4 ) for CO 2 photoreduction. [68]The composite was prepared through an ultrasonic-coating method, In detail, the MS sponge was immersed into a g-C 3 N 4 suspension for 30 min under sonication, and then the excess solution was squeezed out.The obtained sample was finally treated at À 70 °C for 48 h to achieve the g-C 3 N 4 /MS composite with g-C 3 N 4 particles uniformly dispersed within the skeleton of the melamine sponge.
The CO 2 photoreduction test was performed by injecting a fixed volume of gaseous CO 2 into the photoreactor containing the photocatalyst in distilled water.The main detected reaction products were CO and CH 4 with together traces of H 2 (7.48 μmol g À 1 h À 1 CO, 3.93 μmol g À 1 h À 1 CH 4 and 0.26 μmol g À 1 h À 1 H 2 ).Noticeably, the g-C 3 N 4 /MS composite showed higher photocatalytic activity than that of g-C 3 N 4 in powder form probably because of the high ability of melamine in the CO 2 adsorption. [71]In this way, the MS sponge can concentrate CO 2 in its framework, allowing a high number of reactant molecules to enter in contact with the photocatalyst surface, consequently enhancing the photocatalytic performances.

Aerogel-photocatalyst composites
In general, aerogels are porous materials prepared by sol-gel based methods that possess peculiar properties, such as high porosity, extremely low density, high specific surface area and very low thermal conductivity. [72]The first formulations of aerogels were proposed by Kistler [73] who used for the first time the supercritical drying technology as a method of obtaining highly porous structures.Subsequently, thanks to advances in precursor chemistry and drying methods, different types of aerogels characterized by organic, inorganic or even hybrid polymeric chains were prepared. [74]72a,75] Some fields of application of composite aerogels are environmental protection, organic synthesis, [76] biochemical synthesis and biosensors. [77]72a] Indeed, photoactive phases can be embedded in the aerogel framework, providing efficient catalytic materials for photocatalytic oxidation of pollutants.
However, in the case of photocatalytic reactions, to obtain high degradation efficiencies of pollutants on aerogel networks, an optimal affinity between the chemical-physical structure of the aerogel and the chemical nature of the contaminant to be removed is a key aspect to consider.In fact, the photocatalytic efficiency increases when the aerogel is able to ensure a high absorption efficiency of the pollutants.16f,20d]

Preparation of aerogel-photocatalyst composites
Inorganic (mainly based on metal oxide and chalcogenide) aerogel-photocatalyst composites have been studied extensively.It should be noted that most of these composites were prepared using molecule-based sol-gel method, assembly and template methods. [1]Additionally, carbon-based aerogels as support for photoactive phases were also investigated.For example, graphene-based aerogel structures (GA) as a support for photocatalytic particles have been the most investigated due to their high specific surface area, excellent electron mobility, and flexible mechanical properties. [78]ost GA/photocatalyst composites are prepared employing hydrothermal methods [1] (such as hydrothermal self-assembly [79] and in situ hydrothermal growth [80] ).
However, it is worth pointing out that all of the above preparation routes for obtaining inorganic or carbon-based materials aerogel/composites are characterized by somewhat complex and multi-step synthesis procedures which are very energy intensive and are difficult to achieve on a large scale. [1]s an alternative, owing to low-cost and facile synthesis strategies, organic polymer aerogels (OPA) were recently considered to achieve effective photocatalytic composites. [1]or this reason, the methods most commonly used to prepare OPA/photocatalyst composites are here briefly described.
a) Due to the presence of suitable surface functional groups, some organic polymers (such as polyvinylpyrrolidone and polyvinyl alcohol) have the characteristic of being able to solubilize in water.In this manner, an aqueous suspension containing the photocatalyst particles and the dissolved polymer can be easily prepared and directly transformed into hydrophobic aerogel-photocatalyst composites through a simple freeze-drying method, not going through the intermediate formation of a gel.
For example, using this method, polyvinylpyrrolidone/MoS 2 and polyvinyl alcohol/C 3 N 4 composite aerogels were prepared. [81]24a] Moreover, the freeze-drying step requires achieving temperatures below 0 °C (typically down to À 80 °C).Consequently, it is necessary to carry out refrigeration cycles using costly equipment.The freeze-drying step is also timeconsuming since a very high treatment time (up to 48 h in some cases) is required for obtaining the 3D aerogel porous network.
b) Recently, in the field of heterogeneous photocatalysis, aerogels based on thermoplastic hydrophobic polymers characterized by a nanoporous or ultramicroporous crystalline phase, such as syndiotactic polystyrene (sPS), [82] have aroused particular interest as an alternative to water-soluble polymers.
sPS aerogels are easily prepared from a gel obtained by dissolving the polymer in organic solvents (typically chloroform, benzene and toluene [26] ) and cooling the solution to room temperature.16f] Therefore, during the solvent removal process, the collapsing of the aerogel highly porous network can be avoided, allowing to obtain monolithic aerogels.
The preparation procedure for aerogel-photocatalyst composites is the same as that used for the synthesis of pure sPS aerogels but adopting an additional precaution.Specifically, the photocatalyst particles are added to the solution containing the dissolved polymer and maintaining the obtained suspension under sonication to minimize the aggregation of the photocatalyst particles during gel formation at room temperature.20c-f,83]

Photocatalytic hydrogen evolution
Photocatalytic hydrogen production was mainly studied using inorganic or graphene-based aerogels [1,84] and still scarce is the presence of research papers dealing with the use of photocatalytic composites based on organic polymer aerogels for this type of reaction.
Recently, Zhao et al. prepared a polyimide(PI)/Ag composite aerogel by ethanol supercritical drying technique. [85]The PI/Ag aerogel showed a photocatalytic hydrogen production rate of about 166 μmol h À 1 g À 1 from methanol aqueous solution under visible light.The authors also showed that the H 2 production rate observed in presence of PI/Ag aerogel was much higher than that achieved in presence of bare PI aerogel because of the lower charge carriers recombination rate, enhanced visible light optical absorption and large specific area of the PI/Ag aerogel composite. [85]However, it must be considered that the element Ag was introduced into the aerogel structure during the polymerization phase for the generation of the polyimide and not starting from previously synthesized polyimide.
Schreck et al. prepared Pd-TiO 2 based monolithic polymer aerogels starting from 3D printed polymeric scaffolds manufactured from a commercially available acrylate-based resin. [86]In detail, colloidal nanoparticle dispersions of TiO 2 and Pd were inserted into quartz tubes containing the 3D-printed polymer scaffolds.Gels were then obtained by heat treatment at 60 °C for 30 minutes in an ethanol-saturated atmosphere and subsequently cooling to room temperature through a successive series of steps.As a final step, the gels were supercritically dried in CO 2 to generate the final aerogel composites that were tested in the H 2 production from a methanol/water vapor mixture via a gas flow reactor under UV light irradiation. [86]The authors observed that the H 2 evolution rate can be increased up to 1200 μmol h À 1 g À 1 by changing the composite aerogel geometry and underlined that the use of 3D printed polymeric scaffolds is a suitable way to manipulate the monolithic aerogel-photocatalyst composites without altering their composition and morphology. [86]

Photocatalytic synthesis of organic compounds
Despite the presence of some published review articles on the use of porous organic polymers as catalysts for light-driven organic transformations, [87] only a few reports have appeared on organic synthesis reactions employing photocatalysts incorporated into organic polymer aerogels prepared starting from previously synthesized or commercial polymers.
20e] sPS/NÀ TiO 2 composite improved the selectivity and phenol yield compared to NÀ TiO 2 powder.20b] The authors rationalized the obtained results based on the different affinity of benzene (a non-polar compound) and phenol (a polar compound) with sPS aerogel (a non-polar polymer).Indeed, it has been found that the amount of benzene absorbed in the sPS polymer is considerably higher than that of phenol. [88]Therefore, the phenol formed by hydroxylation of benzene by NÀ TiO 2 photocatalyst dispersed in the sPS matrix can easily desorb from the polymer toward the aqueous phase surrounding the sPS/NÀ TiO 2 composite (Scheme 10).In this way, the phenol over-oxidation reactions are prevented.
Furthermore, the monolithic sPS/NÀ TiO 2 aerogel is easily recoverable from the liquid medium.20b] The same authors dispersed FeÀ N codoped TiO 2 photocatalyst in sPS monolithic aerogel using the same preparation method and reported, also in this case, an enhancement of benzene conversion and phenol selectivity under UV and visible light with respect to the photocatalyst dispersed in powder form inside the liquid reaction medium. [89]iven the hydrophobic properties of sPS aerogels (high affinity with non-polar reactants and very low affinity with desired products having polar character), the developed sPS/ photocatalyst composites may represent a viable solution that could allow a significant leap forward in the development of innovative green processes for the selective oxidation of aromatic hydrocarbons under mild conditions.

Summary and Outlook
In this review, we have described the recent developments in the preparation of polymer-photocatalyst composites consisting of photocatalytic active phases embedded in cross-linked porous organic polymers with particular attention to their application in the photocatalytic hydrogen evolution, CO 2 photoreduction and light-driven organic synthesis.
The main aim was to outline an alternative perspective to the highly studied environmental photocatalytic applications, highlighting the photoactive properties of these composites thanks to the incorporation of semiconductors in the 3D porous structure of organic polymers.A drastic improvement of the photocatalytic activity, selectivity and stability can be obtained thanks to the combination of the unique physical properties of porous organic polymers, such as high specific surface areas and optimal accessibility of reactants and products in the porous structure of the polymer.Furthermore, thanks to the high and homogeneous dispersion of the photocatalysts in the 3D polymeric structures, the aggregation phenomena between the photocatalytic particles are minimized, obtaining a photocatalytic activity of the composite better than powder photocatalysts.
We have presented hydrophilic organic polymers (hydrogels) and hydrophobic organic polymers (sponges and aerogels) as host networks and we have mainly focused our attention on the literature papers concerning organic polymer-photocatalyst composites obtained through "gelling/drying" and direct mixing processes (during or after gel preparation), as well as through impregnation or ion exchange processes between photocatalysts and commercial or already prepared polymers.
The following is a summary of some key points about current limits and outlook for catalytic applications of organic polymer/ photocatalyst composites discussed in this review.
1) Thanks to the intrinsic water absorption capacity of hydrogels, the hydrogel-photocatalyst composites could be  seen as a smart reactor (that is a photoreactive solid phase made by a porous polymeric matrix, in which the photocatalytic reactions occur, and the photocatalyst) for photocatalytic hydrogen production from water splitting reaction and for a variety of environmentally friendly selective photocatalytic reactions involving reactants and products highly soluble in an aqueous medium.Some research papers report interesting results but these studies were mainly focused on batch systems.On the other hand, the use of highly translucent hydrogelbased photocatalytic composites with accessible photoactive sites can allow the design and realization of large-scale flow column photocatalytic reactors capable of producing the desired products continuously.Furthermore, most of the research papers have mainly focused on the formulation of the photocatalysts to be dispersed in the hydrogels.On the other hand, studies on the intrinsic properties of the hydrogels to be used as hosts are still scarce.Indeed, it is believed that the regulation of the gel network structure as well as the swelling and adsorption properties of hydrogels can contribute synergistically to the photocatalytic performance of the composites.Therefore, further experimental investigations on these aspects are strongly required.
2) From the literature survey, it emerged that only PMDS sponges functionalized with organic dyes have been studied for photocatalytic organic synthesis reactions (specifically crossdehydrogenative coupling reactions).
For this reason, it is suggested to try these types of reactions also using inorganic photocatalysts (such as ZnO and TiO 2 -based materials) incorporated in the PDMS sponges that, currently, have only been tested in the photocatalytic removal of pollutants.This would allow the preparation of PDMS/ photocatalyst composites by methods much simpler than those necessary for the dispersion of organic dyes in the porous structure of the sponge.However, particular attention must be paid to the formulation of the inorganic photocatalysts to be embedded in such a way as to make them selective towards the desired products.
3 Aerogel composites based on sPS have aroused particular interest, thanks to the easy preparation method and unexpected phenol yield from benzene photocatalytic hydroxylation.Therefore, sPS/photocatalyst composites can be seen as photoreactive solid phases (smart photoreactors) that could allow a significant leap forward in the development of a green process capable of producing phenol in mild conditions.
However, these composites were, at the moment, only tested in the photocatalytic benzene hydroxylation.An in-depth examination of such composites in other photocatalyzed reactions (i.e. selective oxidation reactions involving the activation of CÀ H bonds in aromatic hydrocarbons and CO 2 reduction) to produce oxygenated organic compounds would be very useful and would facilitate more widespread use of these photoreactive solid phases.
Interesting results in the production of hydrogen from a methanol/water vapor mixture in a gas flow reactor were observed using polymer aerogel-photocatalyst composites prepared starting from 3D printed polymeric scaffolds manufactured from a commercially available acrylate-based resin.For these photocatalytic systems, both physical and chemical modifications of the composite aerogels surface to increase the reactants adsorption performances are suggested.Moreover, the development of methods that combine the design of the irradiation source and the aerogel structure to maximize the irradiation efficiency and the assessment of photocatalytic performances in prolonged irradiation time are strongly recommended to allow scale-up considerations.

Vincenzo
Venditto graduated in Chemistry in 1988 at the University of Napoli, where in 1993 he also received his PhD in Chemistry.He is currently Full Professor of Industrial Chemistry at the Department of Chemistry and Biology "A.Zambelli" of the University of Salerno.His main research activities are focused on the physical-chemical and structural characterization of fossil and bio-based polymeric materials.His most recent interest is the design and preparation of photocatalytic composite based on polymeric materials with microporous-crystalline phases for applications in water remediation and catalysis.Vincenzo Vaiano graduated in Chemical Engineering at the University "Federico II" of Napoli in 2000.In March 2006 he received the title of PhD in Chemical Engineering.He is currently Associate Professor of Industrial Chemistry at Department of Industrial Engineering of University of Salerno.In 2005, he conducted research activities at the University of Bradford (UK).Prof. Vaiano is author of several scientific papers dealing with hetero-geneous photocatalysts for different applications, including water pollutants removal and selective synthesis of organic compounds.Olga Sacco is Assistant Professor of Industrial Chemistry at the Department of Chemistry and Biology "A.Zambelli" of the University of Salerno.In 2011, she graduated in Chemical Engineering at the University of Rome "La Sapienza".In February 2014 she received the PhD title in Chemical Engineering.The main research lines are: synthesis and characterization of catalytic materials, phosphors-based nanomaterials, nanostructured photocatalysts and supports, photocatalysis for removing pollutants from water and wastewater, photocatalysts for selective oxidation.

Figure 1 .Scheme 1 .
Figure 1.Number of publications containing the keywords "hydrogel" and "photocatalysis" in the title since 2000 (Search with Web of Science on May 25, 2023).

Figure 4 .
Figure 4. Benzene conversion and phenol yield for five reuse cycles at pH = 2.The photocatalytic reactions were performed in 35 mL aqueous solution containing benzene (initial concentration: 25.6 mM), acetonitrile (2.3 mL) as a co-solvent and 2.8 mL of H 2 O 2 (30 wt % in H 2 O) with 3 g/L of sPS/NÀ TiO 2 aerogel under visible light.Reproduced from ref. [20b] Copyright (2022), with permission from Elsevier.