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Dyes Encapsulated Within Porous Aluminosilicates as Photocatalysts
C H A P T E R
10 Dyes Encapsulated Within Porous Aluminosilicates as Photocatalysts Josep Albero and Hermenegildo Garcı´a Instituto Universitario de Tecnologı´a Quı´mica CSIC-UPV, Universitat Polite`cnica de Vale`ncia, Valencia, Spain
10.1 INTRODUCTION 10.1.1 Inorganic Photocatalysts The most widely used inorganic materials as photocatalysts are metal oxide semiconductors such as TiO2, ZnO, CeO2, and WO3, among others.1 While some of these inorganic materials are photocatalytically and chemically very robust, allowing to work in a wide range of conditions including aqueous solutions at extreme pH values, they have as a major limitation derived from their wide bandgap that they are photoresponsive only under UV light.2 Considering that natural sunlight only contains about 4% of the total energy in the UV region and about 44% in the visible, there has been a growing interest in developing inorganic photocatalysts exhibiting photoresponse in the visible region and, even more recently, under visible and NIR light. In light of the above, consideration for one obvious field of research has been to introduce visible light response in semiconducting metal oxides.3 Over the years several strategies to
Chemistry of Silica and Zeolite-Based Materials DOI: https://doi.org/10.1016/B978-0-12-817813-3.00010-9
reach the goal of visible light photoresponse in inorganic metal oxides have been developed. Scheme 10.1 illustrates some of these approaches.
SCHEME 10.1 List of some of the most typical strategies to introduce visible light photoresponse in inorganic metal oxide semiconductors.
In two of these strategies metal or nonmetal doping of inorganic metal oxides has been reported to introduce some visible light
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photoresponse. The rationale of this approach is to introduce in the material some intrabandgap new electronic states by the dopant element, either below the conduction band (metal doping) or above the valence band which typically corresponds to oxygen atoms orbitals (nonmetallic doping). However, this approach has reached limited success due to difficulty to dope highly crystalline metal oxides with large lattice energy, besides the lack of reliability of the doping procedure. Another strategy that has been developed for TiO2 consists in the amorphization of the most external layers of this inorganic metal oxide that results in the generation of trapping states on the external surface giving a black visual appearance on the material.4 However, also the photocatalytic activity of black materials under visible light photoirradiation is frequently unsatisfactory since the efficiency and stability of the solids is still far from optimum.
into context the potential interest in the use of organic dyes in photocatalysis.5 In contrast to inorganic materials, organic chemistry allows the preparation of a multitude of molecules with strong visible absorption. The most widely used application of these strongly colored organic compounds is for dying textiles and other domestic objects. Scheme 10.2 illustrates the structure of some of the dye families, several of which will be the subject of the present chapter. Organic dyes also have some drawbacks that have severely limited their use in photocatalysis. One of the major problems of organic dyes is their lack of photostability, undergoing self-degradation upon extensive use. There are several general photochemical pathways that can lead to the degradation of organic dyes, the major one being interaction with the ambient, undergoing organic transformations. Upon light absorption and generation of electronic excited states, the energy of the promoted electron of the dye is generally enough to reduce oxygen to superoxide (O2 2), resulting in the generation of the radical cation of the organic dye and highly aggressive oxygen species. Due to the opposite Coulombic charge, the radical cation of the dye reacts very easily with the
10.1.2 Organic Photocatalysts The aforementioned considerations about the situation of inorganic photocatalysts serve to put
SCHEME 10.2
Structure of some dyes that will be commented on in the chapter.
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10.2 ZEOLITES AND RELATED REGULAR POROUS ALUMINOSILICATES
superoxide, triggering oxidative pathways that eventually lead to the decomposition of the chromophore responsible for light absorption. These self-degradation processes result in the deactivation of the photocatalyst. Moreover, while molecular oxygen in the ground state is a triplet, O2 can be converted into highly reactive single oxygen (1O2) by energy transfer with the triplet excited states of organic molecules. Accordingly, upon excitation of organic dye in the presence of oxygen, another degradation pathway that could operate is the reaction of excited singlet oxygen generated in the process with the dye molecule in the ground state. One strategy to stabilize organic dyes is their inclusion inside the rigid matrix of an inert porous material. Confinement of the dye in a restricted space may make the approach impossible through certain directions of reagents that promote degradation. In that way, by confinement in a constraint reaction cavity, the excited states of the dye could still interact with some molecules and substrates by energy or electron transfer, but the reaction of these active species with the dye could be hindered. Under these circumstances, these hybrid hostguest composite materials can be used as durable photocatalysts with distinctive photophysical properties compared to the behavior of the same dye in solution. The present chapter provides general considerations to understand the properties and applications of dye encapsulated into zeolites and other aluminosilicates. First, a brief description of zeolites and related porous materials, particularly with regard to their ability to include organic guests, will be presented. In a different section the general preparation procedures for zeolite-encapsulated dyes and the used characterization techniques will be briefly described. The main part of this chapter will focus on the description of selected examples of zeolite-encapsulated dyes, their properties, features, and the types of photocatalytic
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reactions that can be promoted with these materials. The final section will provide a summary of the chapter and our views on the future development of the field, especially considering new porous materials alike to zeolites which are currently being investigated.
10.2 ZEOLITES AND RELATED REGULAR POROUS ALUMINOSILICATES Zeolites are crystalline porous aluminosilicates in which the lattice is constituted by SiO42 or AlO42 tetrahedra sharing corners and edges. These tetrahedra are organized in the space-forming channels and cavities of strictly constant dimensions in the nanometric scale. The main structure difference in respect to dense solids is the presence in the zeolite lattice of voids and empty spaces that are generally termed as micropores, because their size is smaller than 2 nm. The micropores are open to the exterior of the crystal and, therefore, mass transfer between the exterior and the interior of the crystal is possible for those molecules that are smaller than the pore diameter. Depending on the pore diameter, zeolites can be classified as small, medium, and large pore size for those zeolites whose pore openings are defined by 8, 10, or 12 oxygen atoms, respectively. In the case of the small pore zeolites, only a few molecules such as N2, O2, and H2O can gain access to the interior of the particles. These small pore zeolites are used for gas separation and as water softener by exchanging Ca21 ions present in hard waters by Na1 ions. In the case of medium pore size zeolites some aromatic organic molecules, such as benzene, toluene, and p-xylene can gain access to the interior of the pores. Interestingly, these medium pore-sized zeolites can exhibit exquisite shape selectivity discrimination between p-xylene accessing the pores and the ortho and
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meta isomers that are excluded from entering the intracrystalline space. The number of organic molecules that can be incorporated inside large pore zeolites, having a typical pore diameter around 0.74 nm, is significantly larger, particularly in the case of tridirectional large pore zeolites. One example of the last type of zeolites is zeolite Y (Scheme 10.3).
SCHEME 10.3 Typical view of the internal pores of zeolite Y and conventional preparation procedure of the acid form of zeolite Y which is an important industrial catalyst.
One particular case that has considerable industrial relevance due to the use of these materials in petrochemistry is when the charge balancing cation is a proton, because acid zeolites having strong acidity can be prepared. Scheme 10.3 illustrates the preparation of the acid form of zeolite Y which is probably among the most important materials from the point of view of industrial production. Another case that is very important for the present chapter is when the cation is organic and, particularly has photoresponse, since in this way a hybrid organic/inorganic material with photoresponse can be developed. Although there are some zeolites that can be found in nature as minerals, most of the known zeolites are synthetic materials that are prepared in the lab by hydrothermal crystallization of aluminosilicate gels under appropriate conditions of temperature, pressure, and stirring.
Zeolites have many remarkable properties including a high thermal and chemical stability. Thus, most of the zeolite structures can stand temperatures above 1000 C without undergoing collapse of the structure. Also, zeolites are stable against many aggressive chemical reagents and solvents. As already commented, zeolites are widely used as catalysts in petrochemical processes, where they are consumed in multiton scale for cracking, reforming, and alkylation among many other processes.6 In comparison with this application other uses of zeolite, particularly those related to the preparation of optoelectronic materials have much less importance. With regard to the purpose of the present chapter focused on zeolites and other aluminosilicates for dye encapsulation, due to their chemical composition, zeolites are transparent to most of the electromagnetic radiations. This transparency of most aluminosilicates goes from the UV and visible region to the infrared. Therefore, it is possible to excite with photon’s guests that are included into the porous matrix. This transparency in the UVvis region is the base of the use of these porous materials as hosts to include photoresponsive organic dyes and the use of the resulting composite material in photocatalysis. In the present chapter we will focus on the use of zeolites as matrix to include organic or metal complex dyes for the purpose of developing photocatalytic and photoresponsive materials. Advantages and disadvantages of using these mesoporous aluminosilicates will also be briefly commented on. In general terms, the tight fit between the organic guest and the walls of the inorganic matrix results in a more significant change in the properties of the encapsulated guest, than when the space in which the organic guest is occluded becomes larger and there are no confinement effects. As a general rule, the properties of the guests incorporated in a loose environment are similar to those that can be observed when these organic guests are adsorbed on the external
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10.3 INCORPORATION PROCEDURES
surface of large surface area solids or even in solution. The influence on the spectral properties caused by adsorption on the external surface of silica observed in the photochemistry organic fluorophores such as pyrene, can be observed when these molecules are incorporated inside large pore-sized mesoporous aluminosilicates.7 In contrast, if the dimensions of the cavities are similar to those of the guests, due to the orbital confinement, polarity effects and other factors, the photochemistry of the guests inside the solid matrix can be different to that observed in solvents either apolar, polar, or having hydrogen bonds (Scheme 10.4). During the present chapter an emphasis will be made on those reported cases where confinement has been claimed as one of the factors responsible for the photochemical response of the incarcerated chromophore.
SCHEME 10.4 Factors affecting the photochemistry of dyes encapsulated inside zeolites.
10.3 INCORPORATION PROCEDURES Absorption of organic dyes, either from a solution or in the vapor phase, is the most widely used procedure to incorporate these molecules inside the zeolite micropores.8 Previous to the inclusion step, the zeolite has to be submitted to exhaustive dehydration, since otherwise ambient-equilibrated materials have all the micropore space occupied by water. There are examples of hydrophilic zeolites in where the water content can be as high
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as 30% of the zeolite weight. Hydrophilicity in zeolites is related to the presence of aluminum, particularly, occupying framework positions.9 There are examples of all-silica zeolites, particularly those that are highly crystalline without defects that are remarkably hydrophobic due to the absence of Coulombic charges in the lattice and the low density of silane groups. The dehydrated zeolites are, then, ready to be used as hosts to include organic molecules (Scheme 10.5). In the case of volatile organic molecules, adsorption can be simply performed by exposing the dehydrated zeolite to vapors of the organic molecule. In the cases in where the vapor pressure is low and the boiling point of the organic molecule is high, gas-phase absorption is an inefficient procedure. In these cases, for this type of molecules, the preferred adsorption procedure consists in preparing a solution of the guest and stirring a suspension of the dehydrated zeolite in this solution. It is well known that the nature of the solvent exerts a strong influence on the amount of the guest that is incorporated into the zeolite.10 As a general rule of thumb apolar solvents that have a weak interaction with the zeolite interior are always much better to incorporate higher guest loadings. The main exception to the previous rule is the case in where the adsorption mechanism is an ion exchange in which case water can be used. Because some organic dyes such as methylene blue and tricyclic heterocyclic dyes are cationic and water soluble, they can be adsorbed by replacing some of the inorganic metal ions, typically Na1, by the cationic organic dye (Scheme 10.5B). Adsorption in the gas or liquid phases is based on diffusion of the guest through the internal voids of the zeolites, and this migration requires that the dimensions of the guest have to be smaller than the pore opening of the zeolites. One particular case in which diffusion is impeded, but the organic guest could be accommodated inside the zeolite micropores, is tri-directional large pore zeolites such
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SCHEME 10.5 (A) Dye adsorption process after zeolite Y dehydration upon thermal treatment. (B) Adsorption mechanism of positive organic dyes through ion exchange. MB1, methylene blue.
SCHEME 10.6
Synthetic procedure of triphenylpyrylium ion (TP1).
as faujasites. Therefore, there are some organic molecules that can be accommodated inside the large supercages, but they cannot enter through the window pores. For these cases, a synthetic strategy denoted as “ship-in-a-bottle” synthesis could be possible. This strategy relies on the diffusion of small reagents that will condense inside the large zeolite cavities forming the final bulky product. Due to the large size of these bulky molecules, once formed, they become permanently immobilized inside
the zeolite cavities due to mechanical immobilization that impedes diffusion from the interior to the exterior of the cavity. An example of ship-in-a-bottle synthesis bulky triphenylpyrylium ion (TP1) was encapsulated inside zeolite Y. The synthesis TP1 is based on the presence of zeolite in Bro¨nsted acid sites that promote the condensation and cyclization of chalcone and benzaldehyde. Scheme 10.6 illustrates the process of the TP1 synthesis.
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The presence of dyes inside the zeolite can be determined by UVvis absorption spectroscopy. In most of the cases the color of the zeolite powder corresponds to the color of the dye in solution. However, polarity effects and aggregation can result in some shifts in the absorption maximum when these dyes are incorporated in the zeolite in comparison to other solvents, particularly apolar medium. Other spectroscopic techniques, particularly infrared spectroscopy can also detect the presence of dyes inside the zeolite matrix. This is due to the fact that zeolites are mostly transparent, also in a large size of the IR wavelength range. Particularly, zeolites are transparent in the characteristic region of organic functional groups that is important for the characterization of organic molecules. Fig. 10.1 illustrates the use of IR spectroscopy to characterize incarcerated organic guests.11 The amount of organic dye present in a zeolite can be easily quantified by combustion chemical analysis. Thermogravimetry also offers valuable information about the weight of an organic compound located in the zeolite matrix, which should be coincident with that of chemical analysis. One important issue that is constant when studying guest molecules incorporated inside porous hosts is how to determine and prove
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the internal location of the guest. One of the simplest techniques that is generally presented to discuss the internal versus external location of the guest is surface area and porosity measurements. When the guest is located inside the pores, a decrease in surface area and pore volume should be observed as consequence of the space occupied by the guest. However, it could also happen as such that guests located on the external surface and at the pore entrances can block the access to the micropores and this situation would also result in a decrease in pore volume and surface area values. Other indirect evidences are related to the disappearance of acidic sites that are sacrificed during the adsorption and incorporation of the guest. However, not all the guests are adsorbed by ion exchange and, in addition, ion exchange could occur by replacing the alkali metal ions by protons or other cations that could be present in the solution, particularly when using water as solvent. Another technique that has been used to support the internal location of guests inside zeolites is XPS analysis combined with ion bombardment to probe the internal locations of the zeolite crystals. XPS is a surface specific technique that provides chemical analysis of only a few nanometer penetration in the
FIGURE 10.1 Cartoon showing the transparency of zeolite matrix for certain IR regimes.
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material surface. In the case of zeolites, this corresponds to the composition of the first supercages closer to the external surface. By ion bombardment, it is possible to penetrate deeper into the zeolite crystal by destroying some of the most external layers of the crystals and in doing this consecutive XPS analyses should monitor deeper inside the crystal.
10.4 MODIFICATION OF PHOTOPHYSICAL PROPERTIES BY ZEOLITE ENCAPSULATION Organic photochemistry was initially developed in homogeneous solution trying to obtain solutions as homogeneous and isotropic as possible.12 Soon it was realized that photochemistry in solution is typically characterized by the simultaneous occurrence of competitive photophysical pathways taking place at different rates. The result is frequently a lack of selectivity and stability. After reaching maturity and as a general strategy to gain selectivity, organic photochemistry was moving from solution toward anisotropic and solid media, in which the properties of the microenvironment experienced by the electronic excited states can exert an influence resulting in higher selectivity of the photophysical process. As such, adsorption or incorporation of organic compounds on solid matrices of different nature is nowadays considered as a general way to control and alter the reactivity of excited states. The potential and advantages of zeolites to modify the photophysics of organic dyes have been exhaustively demonstrated by Douhal and coworkers that have adsorbed a series of organic dyes on zeolites of different structure and composition.1314 Scheme 10.2 illustrates the structure of some of these dyes such as NR and 7HQ. These authors have shown how the composition, polarity, and hydrogen bonding ability of the microenvironment experienced
by the dyes hosted within zeolites may change their photophysical properties. These changes in the association constant and mesomeric equilibrium can be generally determined by monitoring the diffuse reflectance UVvis absorption spectrum of the dyes incorporated in the zeolite matrix compared to transmission UVvis absorption spectrum of the dye in different solvents. These variations in the absorption spectrum are also paralleled in emission spectroscopy (Scheme 10.7A).
SCHEME 10.7 (A) Emission spectra of ND dye interacting with Y zeolite at three different Na/Al ratios (solid line: Na/Al 5 0.01; dashed line: Na/Al 5 0.17; and dotted line: Na/Al 5 1.2). (B) Molecular structures of the different isometric forms of HBA-4NP dye upon excitation inside the cavities of zeolite. ESIPT: Excited state intramolecular proton transfer.
Ultrafast measurements in the femtopicosecond time scale monitoring various wavelengths in the emission spectrum can provide kinetic data on the evolution of the excited states and particularly in the interconversion of the various mesomeric forms in the excited state.13 Among the various cases that have been reported,1416 those related to intramolecular charge transfer and/or proton
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transfer are representative examples that illustrate the capability of the technique to follow the photophysics of the incorporated guests. Scheme 10.7B illustrates general behavior. These studies show that there is a remarkable influence of the zeolite composition on the equilibrium among the various possible mesomeric forms and that the photochemistry of the incorporated guest is strongly dependent on the nature of the zeolite host. This can be easily rationalized based on our previous description of the zeolite structure, in where increasing the Al content in the framework should make the interior of the cavities more polar due to the intense electrostatic fields near unsolvated ions. Another factor to be considered is pore dimension. As a general rule, the smaller the pore diameter is, the larger the polar effects experienced by the guest should be, since electrostatic interactions depend inversely on the distance in a quadratic manner. Polarity effects in zeolites can be considered analogous to perform the photochemistry in highly polar solvents. In this regard initial studies on photoinduced electron transfer in the 1990s clearly showed that single electron transfer from an electronically excited molecule to an acceptor or vice versa cannot occur in apolar solvents and the process is increasingly favored as solvent polarity increases.1719 In light of the previous considerations, it is not surprising that the general observation that incorporation of an organic dye inside zeolites, particularly those with large aluminum content, favors photoinduced electron transfer of the guest versus other alternative photophysical processes. This is illustrated, for instance, with the behavior of cyanoaromatics that can act as electron acceptors when they are incorporated inside a zeolite one.2021 Besides favoring photoinduced electron transfer, another general effect of incorporation of molecules inside zeolites in the
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photoresponse is an increased photostability. This photostability is particularly remarkable when the size of the cavity is commensurate with the size of the organic molecule. As an example of this photostability that can be gained by inclusion inside zeolite pores combined with the preference of photoinduced electron transfer versus other processes, it has been reported that TP1 inside zeolite Y (TPNaY) can be used as photocatalyst for the degradation of organic pollutants in water.22 Besides stability of the ground state, incorporation of organic dyes inside zeolites also has influence on the lifetime of electronically excited states, particularly the lifetime of the triplets. After light excitation and photon absorption, organic molecules reach excited electronic states, the first singlet state being, according to the spin conservation rule, the one that is formed in first instance. Singlet excited states typically decay in solution in a few nanoseconds. In the case of fluorescent dyes, the lifetime of the first singlet excite state can be followed by monitoring the fluorescence signal. According to the Jablonsky diagram, singlet excited states can decay either going back to the ground state by photon emission, by radiation less decay pathways, or by spin flip to the triplet excited state (intersystem crossing). Immobilization inside zeolites, generally disfavors rotation of substituents and other conformational changes either due to geometrical restrictions caused by the confinement or due to the interaction of substituents with the zeolite walls. Accordingly, intersystem crossing to the triplet is an open deactivation process that could still occur in the zeolite cavities, becoming favored respect to an analogous situation in solution. Moreover, generation of the triplets inside the zeolite cavities can even be enhanced by heavy atom effects that promote intersystem crossing by spin orbit coupling, allowing the spin flip from singlet to triplet of the organic dye.
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The enhancement of the lifetime of the triplet excited states is generally much more remarkable than that of the singlet excited states, since triplets can only decay by phosphorescence or vibrational relaxation. This is why in solution triplet excited states can live typically for hundreds of nanoseconds or even a few microseconds. When incorporated in zeolites, these triplet excited states generally decay in the millisecond or longer time scale, what is an enlargement of three orders of magnitude of the lifetime or even longer. After having briefly summarized some of the general features that are frequently encountered in the photochemistry of guests adsorbed in porous aluminosilicates, in the next three sections, selected relevant examples of zeolite-encapsulated dyes with respect to their use as photoresponsive materials or photocatalysts will be presented. Rather than trying to be exhaustive in the coverage of the existing literature, the purpose of the following sections is to present common features of existing examples of zeolite-encapsulated organic dyes.
10.5 RUTHENIUM(II) TRISBIPYRIDYL [RU(BPY)321] COMPLEX ENCAPSULATED INSIDE MICROPOROUS ZEOLITES As commented briefly on in the introduction, the main use of zeolites is as heterogeneous catalysts.6,9 In one of the possibilities, besides the intrinsic activity of the zeolite framework providing acid or basic sites, catalytically active guest can be embedded inside the zeolite pores.6,9 In this way the catalytic activity of the guest can be maintained, while the system is converted into a heterogeneous catalyst, making the easy recovery of the catalyst possible by filtration and recycling the material in consecutive reactions as well as the
design of continuous flow processes by immobilizing the active site. Metallic complexes are widely used in homogeneous catalysis, due to their ability to promote oxidation or reduction reactions, as well as additions, couplings, and rearrangements.23 One of these metal complexes are those containing Ru(II) as active metal. For the purpose of the present chapter, it happens that Ru(bpy)321 is one type of metallic dye whose photochemistry has been widely studied in solution as well as in many other anisotropic media.24 The photochemistry of Ru(bpy)321 is well understood.24 This metal complex exhibits an absorption band at about 460 nm in the visible region that is responsible for the orange visual appearance of this metallic complex. Upon absorption of a photon, a metal-toligand electron jump occurs forming a triplet excited state with intramolecular chargetransfer nature. Typical lifetime of these metal-to-ligand charge-transfer triplets in many solvents is about 500 ns or even longer. These triplet excited states can be quenched by typical triplet quenchers such as aromatic ketones, oxygen, and conjugated dienes.24 In addition, room temperature phosphorescence is commonly observed for this Ru complex due to the efficiency of intersystem crossing, resulting in a high population of triplets and the difficulty of this complex to relax the triplet through radiation less decay pathways due to the rigidity of the complex. The triplet excited state of Ru(bpy)321 can also be quenched by electron donors and acceptors, leading to the generation of Ru(I) or Ru(III) complexes, respectively, as transient states. The most widely studied case that has attracted considerable attention is the quenching of Ru(bpy)321 by electron donors, tertiary amines being the most common ones. Scheme 10.8A illustrates the general photochemical reactivity of Ru(bpy)321 in its excited state.
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10.5 RUTHENIUM(II) TRISBIPYRIDYL [RU(BPY)32 1 ] COMPLEX ENCAPSULATED INSIDE MICROPOROUS ZEOLITES
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SCHEME 10.8 (A) Quenching of Ru(bpy)321 complexes by electron donors (tertiary amines, right hand side), leading
to Ru (I) transient that can act as electron donor against an electron acceptor such as methyl viologen (MV11). (B) Photocatalytic H2 evolution through zeolite-encapsulated Ru(II) polypyridyl dyes.
Thus, in the presence of tertiary amines, a photoinduced electron transfer between the amine in its ground state as electron donor to Ru(bpy)321 in its triplet excited state occurs leading to the radical cation of the tertiary amine and Ru(bpy)31 as a transient complex. The radical cation of the tertiary amine undergoes prompt decomposition by giving a hydrogen atom and forming an iminium cation. The tertiary amine is generally denoted as sacrificial agent because it decomposes in the process. Ru (bpy)31 is a reducing agent that can, for instance, transfer one electron to methyl viologen (MV21) forming the corresponding radical cation MV 1. The process can be visually followed by the characteristic intense blue color of MV 1 radical cation that increases in
intensity as the photocatalytic reaction progresses in the complete absence of oxygen. This cyclic photoinduced electron transfer based on the Ru(II)/Ru(III) or Ru(II)/Ru(I) redox pairs has been proposed as a way to convert solar light into chemical energy by, for instance, increasing the concentration of water-stable MV1 cation which is a chemical species that can react later providing chemical energy. One of the main limitations of the Ru (bpy)321 for energy storage and solar light conversion is the solubility of this complex in water and the difficulty to recover the expensive Ru complex from the aqueous solution. One way to circumvent this limitation is by adsorbing or embedding this Ru complex onto an insoluble solid and in this way, the
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photochemical reaction is converted from homogeneous to heterogeneous process. With this target in mind, Dutta and other researchers2526 have reported the ship-in-abottle synthesis of this large Ru(bpy)321 complex inside the pores of zeolites. Regarding photocatalysis, one of the targets of zeolite-encapsulated Ru(II) polypyridyl dyes has been their use as visible light photosensitizers, able to reduce H2O or H1 to generate H2 in the process. For this purpose MV21 has been frequently employed as efficient quencher of the Ru(bpy)321 triplet excited states, resulting in MV 1 and Ru(bpy)331. The later Ru(III) complex is reduced by a sacrificial electron donor, typically a tertiary amine or an alcohol. Subsequently, MV1 could transfer the electron to H2O or H1 forming H2, either directly or using colloidal Pt nanoparticles as relays. This process is illustrated in Scheme 10.8B. In another example in which zeoliteencapsulated Ru(II) complex contains another component coadsorbed into the zeolite, Garcia and Scaiano27 followed the pioneer work of Anpo preparing TiO2 clusters inside zeolites.28 Upon encapsulation, small clusters of TiO2 exhibit a wider bandgap than conventional TiO2 anatase nanoparticles due to the operation of quantum size effects. However, Anpo’s work has shown that the efficiency of the photochemical events can be larger for these small TiO2 clusters inside zeolites, upon appropriate excitation or photosensitization, than analogous steps in TiO2 nanoparticles. As such, the combination of Ru(bpy)321 and TiO2 inside zeolite cavities resembles, in a certain way, the composition previously described of the photoactive components of dye-sensitized solar cells. Steady-state emission spectroscopy has shown that the excited state of Ru(bpy)321 lifetime is sensitive to the presence and loading of TiO2 clusters inside the zeolite, becoming shorter as the loading of TiO2 increases.29 Although this field of dyes encapsulated in zeolites has lost impetus in the last few years
due to the lack of practical application so far, it is clear that in the context of solar fuels production, it would be of interest to revisit these systems trying to exploit the potential of having a cocktail of complementary dyes in terms of light absorption and charge separation, and to determine the efficiency of these photocatalysts from the point of view of solar-tochemical energy conversion.
10.6 TRIPHENYLPYRYLIUM ION ENCAPSULATED INSIDE ZEOLITE Y Continuing with the previous section, in a series of studies Garcia and coworkers have shown the photocatalytic activity of TP@NaY to promote photoinduced electron transfer processes in aqueous solutions and in other solvents. The use of TP@NaY can serve, for instance, to promote the photocatalytic isomerization of cis-stilbene to transstylbene in organic solvents. In an elaborate development of the TP@zeolite system, TP1 was synthetized inside the oval cages of zeolite Beta also containing framework Ti atoms (TP1@TiBeta). Upon irradiation of this TP1@TiBeta material in aqueous media with visible light, cyclohexene was converted into a mixture of cyclohexenol/cyclohexanone. Control experiments have shown that oxidation does not take place in the absence of light or with light in the absence of Ti atoms in the Beta zeolite. The process is illustrated in Scheme 10.9A. Besides the interest in organic synthesis to develop photocatalytic processes, the most general application of TP@NaY is for the degradation of organic pollutants in aqueous media.30 In a series of studies it has been shown that TP@NaY is able to degrade a range of pesticides and other toxic compounds used in agriculture. Scheme 10.9B contains the structure of some of these compounds.
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10.7 PHTHALOCYANINES ENCAPSULATED IN ZEOLITES AS SOLID SINGLET OXYGEN PHOTOSENSITIZERS
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SCHEME 10.9 (A) Conversion of cyclohexene in a mixture of cyclohexenol and cyclohexanone by photocatalytic oxidation in water promoted by TP1@TiBeta. (B) Structure of some pesticides used in agriculture that can be degraded by irradiation using TP@TiBeta as photocatalyst.
10.7 PHTHALOCYANINES ENCAPSULATED IN ZEOLITES AS SOLID SINGLET OXYGEN PHOTOSENSITIZERS In the previous two examples, the photocatalysts, consisting in an organic dye or a metal complex inside the zeolite cages, undergo preferentially photoinduced electron transfer processes. As commented previously, another general reaction of excited organic molecules exposed to the ambient is reaction with molecular oxygen.31 It has been already indicated that besides oxygen-centered radicals, the photochemical process can lead to the generation of singlet oxygen (1O2). One of these cases is MPcs, such as those of Zn21 and Cu21, encapsulated within zeolite Y as photosensitizers. In general, porphyrins are photochemically labile compounds and degrade by autoxidation at the meso-carbon. In contrast, Pcs are notably more stable than porphyrins and for
this reason, they are preferred for their use as catalysts for oxidation reactions with organic peroxides and as photocatalysts. One of the main problems of metal porphyrins and MPcs is their insolubility as a consequence of their large molecular weight and their tendency to aggregation by ππ stacking due to its planarity. For this reason, site isolation of MPcs and modification of the periphery has been one area of interest in the synthesis of Pcs. Pcs are among the most stable pigments for many applications, including their use in car paints. MPcs also exhibit activity as catalysts and photocatalysts. Particularly in photocatalysis the most important MPcs are those in where the metal has an electronic configuration of d0, like Ti41, V51, or d10, like Zn21 or Cu1. When the transition metal in the Pc contains partially filled d orbitals, light absorption results generally in a very fast decay involving dd orbital transitions that are a waste of energy. In contrast, MPcs of d0 and d10 metals with empty or filled d orbitals exhibit long-lived
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electronic excited states that are generally of the ligand-to-metal (LMCT) or metal-to-ligand (MLCT) charge-transfer nature. Trying to overcome solubility limitations and trying to achieve molecule isolation, in a seminal contribution showing the possibility of ship-in-a-bottle synthesis, SchultzEkloff and Wo¨rhle prepared for the first time MPcs inside the cavities of zeolite Y.32 It appears that molecular modeling indicates that the diameter of MPcs is a little bit larger (1.4 nm) than that of zeolite supercages (1.3 nm) and that MPc inside the zeolite Y supercages should be distorted from the planar configuration. In any case, spectroscopic evidence has shown that MPcs included inside zeolites exhibit similar physical and photophysical properties as those of nonaggregated MPc complexes in liquid phase. Singlet oxygen is a transient species that can live milliseconds or even a shorter time depending on the medium. In this regard, observation of an influence in the lifetime upon changing from H2O to D2O, where 1O2 lives longer, is a good indication that 1O2 is being formed. 1O2 can also be detected by its characteristic phosphorescence that appears about 1100 nm in the near IR region. This 1O2 phosphorescence has been used to prove that the formation of 1O2 takes place inside the pores of the zeolite, and thereafter, this small molecule diffuses outside the zeolite particle into the solution.33 Overall these studies have proved that zeolite-encapsulated MPcs can be excellent and stable solid singlet oxygen photosensitizers, exhibiting very high efficiencies and making the photocatalyst easily recoverable from the reaction mixture and recyclable.
10.8 CONCLUSIONS AND FUTURE PROSPECTS As commented in the precedent sections, general effects observed upon encapsulation of
dyes in zeolites are an increasing stability, higher efficiency for photoinduced chargetransfer processes, and longer lifetime of electronic excited states. However, the main limitation of these materials is the low loading of photoresponsive dye that can be achieved, generally much below 10 wt%. Also the insulating nature of the zeolite framework and the micrometric crystal size are other factors that can play adverse effects for optoelectronic properties. For this reason, research in this area has decreased considering that the main application of these zeolite-encapsulated dyes is as high added value pigments exhibiting a notable stability against fading. This situation, however, has changed, and may change again in the future with the synthesis of novel porous materials with framework similar to zeolites, but with more tunable and flexible synthesis, composition, and properties. One example of these novel types of materials are metal organic frameworks (MOFs). MOFs are constituted by metal nodes coordinated to rigid bipolar or multipolar organic linkers, defining structures that are similar in some cases to those of zeolites. In addition to the possibility of hosting an organic dye inside the pores, MOFs offer also the ability to use as linker a dye molecule or to attach covalently the dye to the linker. Compared to zeolites, MOFs have a much lower framework density and in this regard less protection to the embedded guest. However, as an advantage, the larger porosity allows to include much higher loadings (much above 10 wt%) that can be even higher if the dyes play the role of linkers as in the case of PCN-224 where metal porphyrins act as linkers for the metal nodes. Therefore, novel applications can be expected, expanding the research that was done using zeolites and aluminosilicates as matrices to theses MOFs. Similarly, covalent organic frameworks (COFs), are also porous polymers that should be allowed inclusion and protection of organic
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REFERENCES
dyes. Both COFs and MOFs can exhibit advantages derived from the possibility of being cast as thin films on conductive transparent electrodes and that the framework can exhibit some electric conductivity, thus expanding and complementing the research commented on in this chapter about the use of zeolites as host. Therefore, it can be expected that most of the effects and photocatalytic systems reported for zeolite-encapsulated dyes can be somehow translated to any other porous material.
References 1. Byrne, C.; Subramanian, G.; Pillai, S. C. Recent Advances in Photocatalysis for Environmental Applications. J. Environ. Chem. Eng. 2018, 6 (3), 35313555. 2. Schneider, J.; Matsuoka, M.; Takeuchi, M.; Zhang, J.; Horiuchi, Y.; Anpo, M.; Bahnemann, D. W. Understanding TiO2 Photocatalysis: Mechanisms and Materials. Chem. Rev. 2014, 114 (19), 99199986. 3. Banerjee, S.; Pillai, S. C.; Falaras, P.; O’Shea, K. E.; Byrne, J. A.; Dionysiou, D. D. New Insights into the Mechanism of Visible Light Photocatalysis. J. Phys. Chem. Lett. 2014, 5 (15), 25432554. 4. Bergmann, A.; Martinez-Moreno, E.; Teschner, D.; Chernev, P.; Gliech, M.; de Arau´jo, J. F.; Reier, T.; Dau, H.; Strasser, P. Reversible Amorphization and the Catalytically Active State of Crystalline Co3O4 During Oxygen Evolution. Nat. Commun. 2015, 6, 8625. 5. Prier, C. K.; Rankic, D. A.; MacMillan, D. W. C. Visible Light Photoredox Catalysis with Transition Metal Complexes: Applications in Organic Synthesis. Chem. Rev. 2013, 113 (7), 53225363. 6. Primo, A.; Garcia, H. Zeolites as Catalysts in Oil Refining. Chem. Soc. Rev. 2014, 43 (22), 75487561. 7. Bauer, R. K.; De Mayo, P.; Ware, W. R.; Wu, K. C. Surface Photochemistry. The Photophysics of Pyrene Adsorbed on Silica Gel, Alumina, and Calcium Fluoride. J. Phys. Chem. 1982, 86 (19), 37813789. 8. Kohno, Y.; Shibata, Y.; Oyaizu, N.; Yoda, K.; Shibata, M.; Matsushima, R. Stabilization of Flavylium Dye by Incorporation Into the Pore of Protonated Zeolites. Microporous and Mesoporous Mater. 2008, 114 (1), 373379. 9. Kanellopoulos, N. Nanoporous Materials: Advanced Techniques for Characterization, Modeling, and Processing, CRC Press, 2016.
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10. Ramamurthy, V. Controlling Photochemical Reactions Via Confinement: Zeolites. J. Photochem. Photobiol. C Photochem. Rev. 2000, 1 (2), 145166. 11. Mohan, S. C.; Vijay Solomon, R.; Venuvanalingam, P.; Jothivenkatachalam, K. Encapsulation of a Hexaaza Macrocyclic Nickel(ii) Complex in Zeolite Y: An Experimental and Theoretical Investigation. New Journal of Chemistry 2017, 41 (17), 95059512. 12. Turro, N. J. Modern Molecular Photochemistry, University Science Books, 1991. 13. di Nunzio, M. R.; Caballero-Mancebo, E.; Martı´n, C.; Cohen, B.; Navarro, M. T.; Corma, A.; Douhal, A. Femto-to Nanosecond Photodynamics of Nile Red in Metal-Ion Exchanged Faujasites. Microporous Mesoporous Mater. 2018, 256, 214226. 14. Alarcos, N.; Sa´nchez, F.; Douhal, A. Interrogating Ultrafast Dynamics of a Salicylideneaniline Derivative Within Faujasite Zeolites. Chem. Phys. Lett. 2017, 683, 145153. 15. Alarcos, N.; Sa´nchez, F.; Douhal, A. Spectroscopy and Relaxation Dynamics of Salicylideneaniline Derivative Aggregates Encapsulated in MCM41 and SBA15 Pores. Microporous and Mesoporous Mater. 2016, 226, 3443. 16. Alarcos, N.; Sa´nchez, F.; Douhal, A. Confinement Effect on Ultrafast Events of a Salicylideneaniline Derivative Within Mesoporous Materials. Microporous and Mesoporous Mater. 2017, 248, 5461. 17. Mohapatra, H.; Umapathy, S. Influence of Solvent on Photoinduced Electron-Transfer Reaction: TimeResolved Resonance Raman Study. J. Phys. Chem. A 2009, 113 (25), 69046909. 18. Heitele, H.; Finckh, P.; Weeren, S.; Poellinger, F.; Michel-Beyerle, M. E. Solvent Polarity Effects on Intramolecular Electron Transfer. 1. Energetic Aspects. J. Phys. Chem. 1989, 93 (13), 51735179. 19. Maroncelli, M.; Macinnis, J.; Fleming, G. R. Polar Solvent Dynamics and Electron-Transfer Reactions. Science 1989, 243 (4899), 16741681. 20. Hashimoto, S.; Mutoh, T.; Fukumura, H.; Masuhara, H. Diffuse Reflectance Laser Photolytic Studies of Naphthalene, Biphenyl and Some Aromatic Hydrocarbons Adsorbed in the Cavities of Faujasitic Zeolites. J. Chem. Soc. Faraday Trans. 1996, 92 (19), 36533660. 21. Brancaleon, L.; Brousmiche, D.; Rao, V. J.; Johnston, L. J.; Ramamurthy, V. Photoinduced Electron Transfer Reactions within Zeolites: Detection of Radical Cations and Dimerization of Arylalkenes1. J. Am. Chem. Soc. 1998, 120 (20), 49264933. 22. Scaiano, J. C.; Garcı´a, H. Intrazeolite Photochemistry: Toward Supramolecular Control of Molecular Photochemistry. Acc. Chem. Res. 1999, 32 (9), 783793.
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10. DYES ENCAPSULATED WITHIN POROUS ALUMINOSILICATES AS PHOTOCATALYSTS
23. Bayo´n, J. C.; Claver, C.; Masdeu-Bulto´, A. M. Homogeneous Catalysis With Transition Metal Complexes Containing Sulfur Ligands. Coord. Chem. Rev. 1999, 193-195, 73145. 24. Caspar, J. V.; Meyer, T. J. Photochemistry of Tris(2,20 Bipyridine)ruthenium(2 1 ) Ion (Ru(bpy)32 1 ). Solvent Effects. J. Am. Chem. Soc. 1983, 105 (17), 55835590. 25. Bossmann, S. H.; Turro, C.; Schnabel, C.; Pokhrel, M. R.; Payawan, L. M.; Baumeister, B.; Wo¨rner, M. Ru(bpy) 32 1 /TiO2-Codoped Zeolites: Synthesis, Characterization, and the Role of TiO2 in Electron Transfer Photocatalysis. J. Phys. Chem. B 2001, 105 (23), 53745382. 26. Incavo, J. A.; Dutta, P. K. Zeolite Host-Guest Interactions: Optical Spectroscopic Properties of Tris (bipyridine)ruthenium(II) in Zeolite Y Cages. J. Phys. Chem. 1990, 94 (7), 30753081. 27. Corrent, S.; Cosa, G.; Scaiano, J. C.; Galletero, M. S.; Alvaro, M.; Garcia, H. Intrazeolite Photochemistry. 26. Photophysical Properties of Nanosized TiO2 Clusters Included in Zeolites Y, β, and Mordenite. Chem. Mater. 2001, 13 (3), 715722. 28. Yamashita, H.; Ichihashi, Y.; Anpo, M.; Hashimoto, M.; Louis, C.; Che, M. Photocatalytic Decomposition of NO at 275 K on Titanium Oxides Included within Y-Zeolite Cavities: The Structure and Role of the Active Sites. J. Phys. Chem. 1996, 100 (40), 1604116044.
´ lvaro, M.; Chre´tien, M. N.; Forne´s, V.; Galletero, 29. A M. S.; Garcı´a, H.; Scaiano, J. C. Multicomponent Donor 2 Acceptor Relay System Assembled within the Cavities of Zeolite Y. Photoinduced Electron Transfer Between Ru(bpy)32 1 and 2,4,6-Triphenylpyrylium in the Presence of Interposed TiO2. J. Phys. Chem. B 2004, 108 (43), 1662116625. 30. Sanjua´n, A.; Aguirre, G.; Alvaro, M.; Garcı´a, H. 2,4,6-Triphenylpyrylium Ion Encapsulated Within Y Zeolite as Photocatalyst for the Degradation of Methyl Parathion. Water Res. 2000, 34 (1), 320326. 31. Khan, A. U.; Kearns, D. R. Energetics of the Interaction of Molecular Oxygen with Organic Molecules. J. Chem. Phys. 1968, 48 (7), 32723275. 32. Prochnow, P.; Wark, M.; Schulz-Ekloff, G.; Wo¨hrle, D.; Zukal, A.; Rathousky, J. Cobalt(II)-Phthalocyanine Encapsulated in NaY Faujasite: Aggregation of Co(II)Phthalocyanine Encapsulated in NaY Zeolite Due to the Acidity of Ti(IV) Oxide Species Mediated by Pyridine. J. Porphyrins Phthalocyanines 2002, 06 (08), 494501. 33. Cojocaru, B.; Laferrie`re, M.; Carbonell, E.; Parvulescu, V.; Garcı´a, H.; Scaiano, J. C. Direct Time-Resolved Detection of Singlet Oxygen in Zeolite-Based Photocatalysts. Langmuir 2008, 24 (9), 44784481.
CHEMISTRY OF SILICA AND ZEOLITE-BASED MATERIALS
10 Dyes Encapsulated Within Porous Aluminosilicates as Photocatalysts Josep Albero and Hermenegildo Garcı´a Instituto Universitario de Tecnologı´a Quı´mica CSIC-UPV, Universitat Polite`cnica de Vale`ncia, Valencia, Spain
10.1 INTRODUCTION 10.1.1 Inorganic Photocatalysts The most widely used inorganic materials as photocatalysts are metal oxide semiconductors such as TiO2, ZnO, CeO2, and WO3, among others.1 While some of these inorganic materials are photocatalytically and chemically very robust, allowing to work in a wide range of conditions including aqueous solutions at extreme pH values, they have as a major limitation derived from their wide bandgap that they are photoresponsive only under UV light.2 Considering that natural sunlight only contains about 4% of the total energy in the UV region and about 44% in the visible, there has been a growing interest in developing inorganic photocatalysts exhibiting photoresponse in the visible region and, even more recently, under visible and NIR light. In light of the above, consideration for one obvious field of research has been to introduce visible light response in semiconducting metal oxides.3 Over the years several strategies to
Chemistry of Silica and Zeolite-Based Materials DOI: https://doi.org/10.1016/B978-0-12-817813-3.00010-9
reach the goal of visible light photoresponse in inorganic metal oxides have been developed. Scheme 10.1 illustrates some of these approaches.
SCHEME 10.1 List of some of the most typical strategies to introduce visible light photoresponse in inorganic metal oxide semiconductors.
In two of these strategies metal or nonmetal doping of inorganic metal oxides has been reported to introduce some visible light
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photoresponse. The rationale of this approach is to introduce in the material some intrabandgap new electronic states by the dopant element, either below the conduction band (metal doping) or above the valence band which typically corresponds to oxygen atoms orbitals (nonmetallic doping). However, this approach has reached limited success due to difficulty to dope highly crystalline metal oxides with large lattice energy, besides the lack of reliability of the doping procedure. Another strategy that has been developed for TiO2 consists in the amorphization of the most external layers of this inorganic metal oxide that results in the generation of trapping states on the external surface giving a black visual appearance on the material.4 However, also the photocatalytic activity of black materials under visible light photoirradiation is frequently unsatisfactory since the efficiency and stability of the solids is still far from optimum.
into context the potential interest in the use of organic dyes in photocatalysis.5 In contrast to inorganic materials, organic chemistry allows the preparation of a multitude of molecules with strong visible absorption. The most widely used application of these strongly colored organic compounds is for dying textiles and other domestic objects. Scheme 10.2 illustrates the structure of some of the dye families, several of which will be the subject of the present chapter. Organic dyes also have some drawbacks that have severely limited their use in photocatalysis. One of the major problems of organic dyes is their lack of photostability, undergoing self-degradation upon extensive use. There are several general photochemical pathways that can lead to the degradation of organic dyes, the major one being interaction with the ambient, undergoing organic transformations. Upon light absorption and generation of electronic excited states, the energy of the promoted electron of the dye is generally enough to reduce oxygen to superoxide (O2 2), resulting in the generation of the radical cation of the organic dye and highly aggressive oxygen species. Due to the opposite Coulombic charge, the radical cation of the dye reacts very easily with the
10.1.2 Organic Photocatalysts The aforementioned considerations about the situation of inorganic photocatalysts serve to put
SCHEME 10.2
Structure of some dyes that will be commented on in the chapter.
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10.2 ZEOLITES AND RELATED REGULAR POROUS ALUMINOSILICATES
superoxide, triggering oxidative pathways that eventually lead to the decomposition of the chromophore responsible for light absorption. These self-degradation processes result in the deactivation of the photocatalyst. Moreover, while molecular oxygen in the ground state is a triplet, O2 can be converted into highly reactive single oxygen (1O2) by energy transfer with the triplet excited states of organic molecules. Accordingly, upon excitation of organic dye in the presence of oxygen, another degradation pathway that could operate is the reaction of excited singlet oxygen generated in the process with the dye molecule in the ground state. One strategy to stabilize organic dyes is their inclusion inside the rigid matrix of an inert porous material. Confinement of the dye in a restricted space may make the approach impossible through certain directions of reagents that promote degradation. In that way, by confinement in a constraint reaction cavity, the excited states of the dye could still interact with some molecules and substrates by energy or electron transfer, but the reaction of these active species with the dye could be hindered. Under these circumstances, these hybrid hostguest composite materials can be used as durable photocatalysts with distinctive photophysical properties compared to the behavior of the same dye in solution. The present chapter provides general considerations to understand the properties and applications of dye encapsulated into zeolites and other aluminosilicates. First, a brief description of zeolites and related porous materials, particularly with regard to their ability to include organic guests, will be presented. In a different section the general preparation procedures for zeolite-encapsulated dyes and the used characterization techniques will be briefly described. The main part of this chapter will focus on the description of selected examples of zeolite-encapsulated dyes, their properties, features, and the types of photocatalytic
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reactions that can be promoted with these materials. The final section will provide a summary of the chapter and our views on the future development of the field, especially considering new porous materials alike to zeolites which are currently being investigated.
10.2 ZEOLITES AND RELATED REGULAR POROUS ALUMINOSILICATES Zeolites are crystalline porous aluminosilicates in which the lattice is constituted by SiO42 or AlO42 tetrahedra sharing corners and edges. These tetrahedra are organized in the space-forming channels and cavities of strictly constant dimensions in the nanometric scale. The main structure difference in respect to dense solids is the presence in the zeolite lattice of voids and empty spaces that are generally termed as micropores, because their size is smaller than 2 nm. The micropores are open to the exterior of the crystal and, therefore, mass transfer between the exterior and the interior of the crystal is possible for those molecules that are smaller than the pore diameter. Depending on the pore diameter, zeolites can be classified as small, medium, and large pore size for those zeolites whose pore openings are defined by 8, 10, or 12 oxygen atoms, respectively. In the case of the small pore zeolites, only a few molecules such as N2, O2, and H2O can gain access to the interior of the particles. These small pore zeolites are used for gas separation and as water softener by exchanging Ca21 ions present in hard waters by Na1 ions. In the case of medium pore size zeolites some aromatic organic molecules, such as benzene, toluene, and p-xylene can gain access to the interior of the pores. Interestingly, these medium pore-sized zeolites can exhibit exquisite shape selectivity discrimination between p-xylene accessing the pores and the ortho and
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meta isomers that are excluded from entering the intracrystalline space. The number of organic molecules that can be incorporated inside large pore zeolites, having a typical pore diameter around 0.74 nm, is significantly larger, particularly in the case of tridirectional large pore zeolites. One example of the last type of zeolites is zeolite Y (Scheme 10.3).
SCHEME 10.3 Typical view of the internal pores of zeolite Y and conventional preparation procedure of the acid form of zeolite Y which is an important industrial catalyst.
One particular case that has considerable industrial relevance due to the use of these materials in petrochemistry is when the charge balancing cation is a proton, because acid zeolites having strong acidity can be prepared. Scheme 10.3 illustrates the preparation of the acid form of zeolite Y which is probably among the most important materials from the point of view of industrial production. Another case that is very important for the present chapter is when the cation is organic and, particularly has photoresponse, since in this way a hybrid organic/inorganic material with photoresponse can be developed. Although there are some zeolites that can be found in nature as minerals, most of the known zeolites are synthetic materials that are prepared in the lab by hydrothermal crystallization of aluminosilicate gels under appropriate conditions of temperature, pressure, and stirring.
Zeolites have many remarkable properties including a high thermal and chemical stability. Thus, most of the zeolite structures can stand temperatures above 1000 C without undergoing collapse of the structure. Also, zeolites are stable against many aggressive chemical reagents and solvents. As already commented, zeolites are widely used as catalysts in petrochemical processes, where they are consumed in multiton scale for cracking, reforming, and alkylation among many other processes.6 In comparison with this application other uses of zeolite, particularly those related to the preparation of optoelectronic materials have much less importance. With regard to the purpose of the present chapter focused on zeolites and other aluminosilicates for dye encapsulation, due to their chemical composition, zeolites are transparent to most of the electromagnetic radiations. This transparency of most aluminosilicates goes from the UV and visible region to the infrared. Therefore, it is possible to excite with photon’s guests that are included into the porous matrix. This transparency in the UVvis region is the base of the use of these porous materials as hosts to include photoresponsive organic dyes and the use of the resulting composite material in photocatalysis. In the present chapter we will focus on the use of zeolites as matrix to include organic or metal complex dyes for the purpose of developing photocatalytic and photoresponsive materials. Advantages and disadvantages of using these mesoporous aluminosilicates will also be briefly commented on. In general terms, the tight fit between the organic guest and the walls of the inorganic matrix results in a more significant change in the properties of the encapsulated guest, than when the space in which the organic guest is occluded becomes larger and there are no confinement effects. As a general rule, the properties of the guests incorporated in a loose environment are similar to those that can be observed when these organic guests are adsorbed on the external
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10.3 INCORPORATION PROCEDURES
surface of large surface area solids or even in solution. The influence on the spectral properties caused by adsorption on the external surface of silica observed in the photochemistry organic fluorophores such as pyrene, can be observed when these molecules are incorporated inside large pore-sized mesoporous aluminosilicates.7 In contrast, if the dimensions of the cavities are similar to those of the guests, due to the orbital confinement, polarity effects and other factors, the photochemistry of the guests inside the solid matrix can be different to that observed in solvents either apolar, polar, or having hydrogen bonds (Scheme 10.4). During the present chapter an emphasis will be made on those reported cases where confinement has been claimed as one of the factors responsible for the photochemical response of the incarcerated chromophore.
SCHEME 10.4 Factors affecting the photochemistry of dyes encapsulated inside zeolites.
10.3 INCORPORATION PROCEDURES Absorption of organic dyes, either from a solution or in the vapor phase, is the most widely used procedure to incorporate these molecules inside the zeolite micropores.8 Previous to the inclusion step, the zeolite has to be submitted to exhaustive dehydration, since otherwise ambient-equilibrated materials have all the micropore space occupied by water. There are examples of hydrophilic zeolites in where the water content can be as high
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as 30% of the zeolite weight. Hydrophilicity in zeolites is related to the presence of aluminum, particularly, occupying framework positions.9 There are examples of all-silica zeolites, particularly those that are highly crystalline without defects that are remarkably hydrophobic due to the absence of Coulombic charges in the lattice and the low density of silane groups. The dehydrated zeolites are, then, ready to be used as hosts to include organic molecules (Scheme 10.5). In the case of volatile organic molecules, adsorption can be simply performed by exposing the dehydrated zeolite to vapors of the organic molecule. In the cases in where the vapor pressure is low and the boiling point of the organic molecule is high, gas-phase absorption is an inefficient procedure. In these cases, for this type of molecules, the preferred adsorption procedure consists in preparing a solution of the guest and stirring a suspension of the dehydrated zeolite in this solution. It is well known that the nature of the solvent exerts a strong influence on the amount of the guest that is incorporated into the zeolite.10 As a general rule of thumb apolar solvents that have a weak interaction with the zeolite interior are always much better to incorporate higher guest loadings. The main exception to the previous rule is the case in where the adsorption mechanism is an ion exchange in which case water can be used. Because some organic dyes such as methylene blue and tricyclic heterocyclic dyes are cationic and water soluble, they can be adsorbed by replacing some of the inorganic metal ions, typically Na1, by the cationic organic dye (Scheme 10.5B). Adsorption in the gas or liquid phases is based on diffusion of the guest through the internal voids of the zeolites, and this migration requires that the dimensions of the guest have to be smaller than the pore opening of the zeolites. One particular case in which diffusion is impeded, but the organic guest could be accommodated inside the zeolite micropores, is tri-directional large pore zeolites such
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SCHEME 10.5 (A) Dye adsorption process after zeolite Y dehydration upon thermal treatment. (B) Adsorption mechanism of positive organic dyes through ion exchange. MB1, methylene blue.
SCHEME 10.6
Synthetic procedure of triphenylpyrylium ion (TP1).
as faujasites. Therefore, there are some organic molecules that can be accommodated inside the large supercages, but they cannot enter through the window pores. For these cases, a synthetic strategy denoted as “ship-in-a-bottle” synthesis could be possible. This strategy relies on the diffusion of small reagents that will condense inside the large zeolite cavities forming the final bulky product. Due to the large size of these bulky molecules, once formed, they become permanently immobilized inside
the zeolite cavities due to mechanical immobilization that impedes diffusion from the interior to the exterior of the cavity. An example of ship-in-a-bottle synthesis bulky triphenylpyrylium ion (TP1) was encapsulated inside zeolite Y. The synthesis TP1 is based on the presence of zeolite in Bro¨nsted acid sites that promote the condensation and cyclization of chalcone and benzaldehyde. Scheme 10.6 illustrates the process of the TP1 synthesis.
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10.3 INCORPORATION PROCEDURES
The presence of dyes inside the zeolite can be determined by UVvis absorption spectroscopy. In most of the cases the color of the zeolite powder corresponds to the color of the dye in solution. However, polarity effects and aggregation can result in some shifts in the absorption maximum when these dyes are incorporated in the zeolite in comparison to other solvents, particularly apolar medium. Other spectroscopic techniques, particularly infrared spectroscopy can also detect the presence of dyes inside the zeolite matrix. This is due to the fact that zeolites are mostly transparent, also in a large size of the IR wavelength range. Particularly, zeolites are transparent in the characteristic region of organic functional groups that is important for the characterization of organic molecules. Fig. 10.1 illustrates the use of IR spectroscopy to characterize incarcerated organic guests.11 The amount of organic dye present in a zeolite can be easily quantified by combustion chemical analysis. Thermogravimetry also offers valuable information about the weight of an organic compound located in the zeolite matrix, which should be coincident with that of chemical analysis. One important issue that is constant when studying guest molecules incorporated inside porous hosts is how to determine and prove
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the internal location of the guest. One of the simplest techniques that is generally presented to discuss the internal versus external location of the guest is surface area and porosity measurements. When the guest is located inside the pores, a decrease in surface area and pore volume should be observed as consequence of the space occupied by the guest. However, it could also happen as such that guests located on the external surface and at the pore entrances can block the access to the micropores and this situation would also result in a decrease in pore volume and surface area values. Other indirect evidences are related to the disappearance of acidic sites that are sacrificed during the adsorption and incorporation of the guest. However, not all the guests are adsorbed by ion exchange and, in addition, ion exchange could occur by replacing the alkali metal ions by protons or other cations that could be present in the solution, particularly when using water as solvent. Another technique that has been used to support the internal location of guests inside zeolites is XPS analysis combined with ion bombardment to probe the internal locations of the zeolite crystals. XPS is a surface specific technique that provides chemical analysis of only a few nanometer penetration in the
FIGURE 10.1 Cartoon showing the transparency of zeolite matrix for certain IR regimes.
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material surface. In the case of zeolites, this corresponds to the composition of the first supercages closer to the external surface. By ion bombardment, it is possible to penetrate deeper into the zeolite crystal by destroying some of the most external layers of the crystals and in doing this consecutive XPS analyses should monitor deeper inside the crystal.
10.4 MODIFICATION OF PHOTOPHYSICAL PROPERTIES BY ZEOLITE ENCAPSULATION Organic photochemistry was initially developed in homogeneous solution trying to obtain solutions as homogeneous and isotropic as possible.12 Soon it was realized that photochemistry in solution is typically characterized by the simultaneous occurrence of competitive photophysical pathways taking place at different rates. The result is frequently a lack of selectivity and stability. After reaching maturity and as a general strategy to gain selectivity, organic photochemistry was moving from solution toward anisotropic and solid media, in which the properties of the microenvironment experienced by the electronic excited states can exert an influence resulting in higher selectivity of the photophysical process. As such, adsorption or incorporation of organic compounds on solid matrices of different nature is nowadays considered as a general way to control and alter the reactivity of excited states. The potential and advantages of zeolites to modify the photophysics of organic dyes have been exhaustively demonstrated by Douhal and coworkers that have adsorbed a series of organic dyes on zeolites of different structure and composition.1314 Scheme 10.2 illustrates the structure of some of these dyes such as NR and 7HQ. These authors have shown how the composition, polarity, and hydrogen bonding ability of the microenvironment experienced
by the dyes hosted within zeolites may change their photophysical properties. These changes in the association constant and mesomeric equilibrium can be generally determined by monitoring the diffuse reflectance UVvis absorption spectrum of the dyes incorporated in the zeolite matrix compared to transmission UVvis absorption spectrum of the dye in different solvents. These variations in the absorption spectrum are also paralleled in emission spectroscopy (Scheme 10.7A).
SCHEME 10.7 (A) Emission spectra of ND dye interacting with Y zeolite at three different Na/Al ratios (solid line: Na/Al 5 0.01; dashed line: Na/Al 5 0.17; and dotted line: Na/Al 5 1.2). (B) Molecular structures of the different isometric forms of HBA-4NP dye upon excitation inside the cavities of zeolite. ESIPT: Excited state intramolecular proton transfer.
Ultrafast measurements in the femtopicosecond time scale monitoring various wavelengths in the emission spectrum can provide kinetic data on the evolution of the excited states and particularly in the interconversion of the various mesomeric forms in the excited state.13 Among the various cases that have been reported,1416 those related to intramolecular charge transfer and/or proton
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transfer are representative examples that illustrate the capability of the technique to follow the photophysics of the incorporated guests. Scheme 10.7B illustrates general behavior. These studies show that there is a remarkable influence of the zeolite composition on the equilibrium among the various possible mesomeric forms and that the photochemistry of the incorporated guest is strongly dependent on the nature of the zeolite host. This can be easily rationalized based on our previous description of the zeolite structure, in where increasing the Al content in the framework should make the interior of the cavities more polar due to the intense electrostatic fields near unsolvated ions. Another factor to be considered is pore dimension. As a general rule, the smaller the pore diameter is, the larger the polar effects experienced by the guest should be, since electrostatic interactions depend inversely on the distance in a quadratic manner. Polarity effects in zeolites can be considered analogous to perform the photochemistry in highly polar solvents. In this regard initial studies on photoinduced electron transfer in the 1990s clearly showed that single electron transfer from an electronically excited molecule to an acceptor or vice versa cannot occur in apolar solvents and the process is increasingly favored as solvent polarity increases.1719 In light of the previous considerations, it is not surprising that the general observation that incorporation of an organic dye inside zeolites, particularly those with large aluminum content, favors photoinduced electron transfer of the guest versus other alternative photophysical processes. This is illustrated, for instance, with the behavior of cyanoaromatics that can act as electron acceptors when they are incorporated inside a zeolite one.2021 Besides favoring photoinduced electron transfer, another general effect of incorporation of molecules inside zeolites in the
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photoresponse is an increased photostability. This photostability is particularly remarkable when the size of the cavity is commensurate with the size of the organic molecule. As an example of this photostability that can be gained by inclusion inside zeolite pores combined with the preference of photoinduced electron transfer versus other processes, it has been reported that TP1 inside zeolite Y (TPNaY) can be used as photocatalyst for the degradation of organic pollutants in water.22 Besides stability of the ground state, incorporation of organic dyes inside zeolites also has influence on the lifetime of electronically excited states, particularly the lifetime of the triplets. After light excitation and photon absorption, organic molecules reach excited electronic states, the first singlet state being, according to the spin conservation rule, the one that is formed in first instance. Singlet excited states typically decay in solution in a few nanoseconds. In the case of fluorescent dyes, the lifetime of the first singlet excite state can be followed by monitoring the fluorescence signal. According to the Jablonsky diagram, singlet excited states can decay either going back to the ground state by photon emission, by radiation less decay pathways, or by spin flip to the triplet excited state (intersystem crossing). Immobilization inside zeolites, generally disfavors rotation of substituents and other conformational changes either due to geometrical restrictions caused by the confinement or due to the interaction of substituents with the zeolite walls. Accordingly, intersystem crossing to the triplet is an open deactivation process that could still occur in the zeolite cavities, becoming favored respect to an analogous situation in solution. Moreover, generation of the triplets inside the zeolite cavities can even be enhanced by heavy atom effects that promote intersystem crossing by spin orbit coupling, allowing the spin flip from singlet to triplet of the organic dye.
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The enhancement of the lifetime of the triplet excited states is generally much more remarkable than that of the singlet excited states, since triplets can only decay by phosphorescence or vibrational relaxation. This is why in solution triplet excited states can live typically for hundreds of nanoseconds or even a few microseconds. When incorporated in zeolites, these triplet excited states generally decay in the millisecond or longer time scale, what is an enlargement of three orders of magnitude of the lifetime or even longer. After having briefly summarized some of the general features that are frequently encountered in the photochemistry of guests adsorbed in porous aluminosilicates, in the next three sections, selected relevant examples of zeolite-encapsulated dyes with respect to their use as photoresponsive materials or photocatalysts will be presented. Rather than trying to be exhaustive in the coverage of the existing literature, the purpose of the following sections is to present common features of existing examples of zeolite-encapsulated organic dyes.
10.5 RUTHENIUM(II) TRISBIPYRIDYL [RU(BPY)321] COMPLEX ENCAPSULATED INSIDE MICROPOROUS ZEOLITES As commented briefly on in the introduction, the main use of zeolites is as heterogeneous catalysts.6,9 In one of the possibilities, besides the intrinsic activity of the zeolite framework providing acid or basic sites, catalytically active guest can be embedded inside the zeolite pores.6,9 In this way the catalytic activity of the guest can be maintained, while the system is converted into a heterogeneous catalyst, making the easy recovery of the catalyst possible by filtration and recycling the material in consecutive reactions as well as the
design of continuous flow processes by immobilizing the active site. Metallic complexes are widely used in homogeneous catalysis, due to their ability to promote oxidation or reduction reactions, as well as additions, couplings, and rearrangements.23 One of these metal complexes are those containing Ru(II) as active metal. For the purpose of the present chapter, it happens that Ru(bpy)321 is one type of metallic dye whose photochemistry has been widely studied in solution as well as in many other anisotropic media.24 The photochemistry of Ru(bpy)321 is well understood.24 This metal complex exhibits an absorption band at about 460 nm in the visible region that is responsible for the orange visual appearance of this metallic complex. Upon absorption of a photon, a metal-toligand electron jump occurs forming a triplet excited state with intramolecular chargetransfer nature. Typical lifetime of these metal-to-ligand charge-transfer triplets in many solvents is about 500 ns or even longer. These triplet excited states can be quenched by typical triplet quenchers such as aromatic ketones, oxygen, and conjugated dienes.24 In addition, room temperature phosphorescence is commonly observed for this Ru complex due to the efficiency of intersystem crossing, resulting in a high population of triplets and the difficulty of this complex to relax the triplet through radiation less decay pathways due to the rigidity of the complex. The triplet excited state of Ru(bpy)321 can also be quenched by electron donors and acceptors, leading to the generation of Ru(I) or Ru(III) complexes, respectively, as transient states. The most widely studied case that has attracted considerable attention is the quenching of Ru(bpy)321 by electron donors, tertiary amines being the most common ones. Scheme 10.8A illustrates the general photochemical reactivity of Ru(bpy)321 in its excited state.
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SCHEME 10.8 (A) Quenching of Ru(bpy)321 complexes by electron donors (tertiary amines, right hand side), leading
to Ru (I) transient that can act as electron donor against an electron acceptor such as methyl viologen (MV11). (B) Photocatalytic H2 evolution through zeolite-encapsulated Ru(II) polypyridyl dyes.
Thus, in the presence of tertiary amines, a photoinduced electron transfer between the amine in its ground state as electron donor to Ru(bpy)321 in its triplet excited state occurs leading to the radical cation of the tertiary amine and Ru(bpy)31 as a transient complex. The radical cation of the tertiary amine undergoes prompt decomposition by giving a hydrogen atom and forming an iminium cation. The tertiary amine is generally denoted as sacrificial agent because it decomposes in the process. Ru (bpy)31 is a reducing agent that can, for instance, transfer one electron to methyl viologen (MV21) forming the corresponding radical cation MV 1. The process can be visually followed by the characteristic intense blue color of MV 1 radical cation that increases in
intensity as the photocatalytic reaction progresses in the complete absence of oxygen. This cyclic photoinduced electron transfer based on the Ru(II)/Ru(III) or Ru(II)/Ru(I) redox pairs has been proposed as a way to convert solar light into chemical energy by, for instance, increasing the concentration of water-stable MV1 cation which is a chemical species that can react later providing chemical energy. One of the main limitations of the Ru (bpy)321 for energy storage and solar light conversion is the solubility of this complex in water and the difficulty to recover the expensive Ru complex from the aqueous solution. One way to circumvent this limitation is by adsorbing or embedding this Ru complex onto an insoluble solid and in this way, the
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photochemical reaction is converted from homogeneous to heterogeneous process. With this target in mind, Dutta and other researchers2526 have reported the ship-in-abottle synthesis of this large Ru(bpy)321 complex inside the pores of zeolites. Regarding photocatalysis, one of the targets of zeolite-encapsulated Ru(II) polypyridyl dyes has been their use as visible light photosensitizers, able to reduce H2O or H1 to generate H2 in the process. For this purpose MV21 has been frequently employed as efficient quencher of the Ru(bpy)321 triplet excited states, resulting in MV 1 and Ru(bpy)331. The later Ru(III) complex is reduced by a sacrificial electron donor, typically a tertiary amine or an alcohol. Subsequently, MV1 could transfer the electron to H2O or H1 forming H2, either directly or using colloidal Pt nanoparticles as relays. This process is illustrated in Scheme 10.8B. In another example in which zeoliteencapsulated Ru(II) complex contains another component coadsorbed into the zeolite, Garcia and Scaiano27 followed the pioneer work of Anpo preparing TiO2 clusters inside zeolites.28 Upon encapsulation, small clusters of TiO2 exhibit a wider bandgap than conventional TiO2 anatase nanoparticles due to the operation of quantum size effects. However, Anpo’s work has shown that the efficiency of the photochemical events can be larger for these small TiO2 clusters inside zeolites, upon appropriate excitation or photosensitization, than analogous steps in TiO2 nanoparticles. As such, the combination of Ru(bpy)321 and TiO2 inside zeolite cavities resembles, in a certain way, the composition previously described of the photoactive components of dye-sensitized solar cells. Steady-state emission spectroscopy has shown that the excited state of Ru(bpy)321 lifetime is sensitive to the presence and loading of TiO2 clusters inside the zeolite, becoming shorter as the loading of TiO2 increases.29 Although this field of dyes encapsulated in zeolites has lost impetus in the last few years
due to the lack of practical application so far, it is clear that in the context of solar fuels production, it would be of interest to revisit these systems trying to exploit the potential of having a cocktail of complementary dyes in terms of light absorption and charge separation, and to determine the efficiency of these photocatalysts from the point of view of solar-tochemical energy conversion.
10.6 TRIPHENYLPYRYLIUM ION ENCAPSULATED INSIDE ZEOLITE Y Continuing with the previous section, in a series of studies Garcia and coworkers have shown the photocatalytic activity of TP@NaY to promote photoinduced electron transfer processes in aqueous solutions and in other solvents. The use of TP@NaY can serve, for instance, to promote the photocatalytic isomerization of cis-stilbene to transstylbene in organic solvents. In an elaborate development of the TP@zeolite system, TP1 was synthetized inside the oval cages of zeolite Beta also containing framework Ti atoms (TP1@TiBeta). Upon irradiation of this TP1@TiBeta material in aqueous media with visible light, cyclohexene was converted into a mixture of cyclohexenol/cyclohexanone. Control experiments have shown that oxidation does not take place in the absence of light or with light in the absence of Ti atoms in the Beta zeolite. The process is illustrated in Scheme 10.9A. Besides the interest in organic synthesis to develop photocatalytic processes, the most general application of TP@NaY is for the degradation of organic pollutants in aqueous media.30 In a series of studies it has been shown that TP@NaY is able to degrade a range of pesticides and other toxic compounds used in agriculture. Scheme 10.9B contains the structure of some of these compounds.
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SCHEME 10.9 (A) Conversion of cyclohexene in a mixture of cyclohexenol and cyclohexanone by photocatalytic oxidation in water promoted by TP1@TiBeta. (B) Structure of some pesticides used in agriculture that can be degraded by irradiation using TP@TiBeta as photocatalyst.
10.7 PHTHALOCYANINES ENCAPSULATED IN ZEOLITES AS SOLID SINGLET OXYGEN PHOTOSENSITIZERS In the previous two examples, the photocatalysts, consisting in an organic dye or a metal complex inside the zeolite cages, undergo preferentially photoinduced electron transfer processes. As commented previously, another general reaction of excited organic molecules exposed to the ambient is reaction with molecular oxygen.31 It has been already indicated that besides oxygen-centered radicals, the photochemical process can lead to the generation of singlet oxygen (1O2). One of these cases is MPcs, such as those of Zn21 and Cu21, encapsulated within zeolite Y as photosensitizers. In general, porphyrins are photochemically labile compounds and degrade by autoxidation at the meso-carbon. In contrast, Pcs are notably more stable than porphyrins and for
this reason, they are preferred for their use as catalysts for oxidation reactions with organic peroxides and as photocatalysts. One of the main problems of metal porphyrins and MPcs is their insolubility as a consequence of their large molecular weight and their tendency to aggregation by ππ stacking due to its planarity. For this reason, site isolation of MPcs and modification of the periphery has been one area of interest in the synthesis of Pcs. Pcs are among the most stable pigments for many applications, including their use in car paints. MPcs also exhibit activity as catalysts and photocatalysts. Particularly in photocatalysis the most important MPcs are those in where the metal has an electronic configuration of d0, like Ti41, V51, or d10, like Zn21 or Cu1. When the transition metal in the Pc contains partially filled d orbitals, light absorption results generally in a very fast decay involving dd orbital transitions that are a waste of energy. In contrast, MPcs of d0 and d10 metals with empty or filled d orbitals exhibit long-lived
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electronic excited states that are generally of the ligand-to-metal (LMCT) or metal-to-ligand (MLCT) charge-transfer nature. Trying to overcome solubility limitations and trying to achieve molecule isolation, in a seminal contribution showing the possibility of ship-in-a-bottle synthesis, SchultzEkloff and Wo¨rhle prepared for the first time MPcs inside the cavities of zeolite Y.32 It appears that molecular modeling indicates that the diameter of MPcs is a little bit larger (1.4 nm) than that of zeolite supercages (1.3 nm) and that MPc inside the zeolite Y supercages should be distorted from the planar configuration. In any case, spectroscopic evidence has shown that MPcs included inside zeolites exhibit similar physical and photophysical properties as those of nonaggregated MPc complexes in liquid phase. Singlet oxygen is a transient species that can live milliseconds or even a shorter time depending on the medium. In this regard, observation of an influence in the lifetime upon changing from H2O to D2O, where 1O2 lives longer, is a good indication that 1O2 is being formed. 1O2 can also be detected by its characteristic phosphorescence that appears about 1100 nm in the near IR region. This 1O2 phosphorescence has been used to prove that the formation of 1O2 takes place inside the pores of the zeolite, and thereafter, this small molecule diffuses outside the zeolite particle into the solution.33 Overall these studies have proved that zeolite-encapsulated MPcs can be excellent and stable solid singlet oxygen photosensitizers, exhibiting very high efficiencies and making the photocatalyst easily recoverable from the reaction mixture and recyclable.
10.8 CONCLUSIONS AND FUTURE PROSPECTS As commented in the precedent sections, general effects observed upon encapsulation of
dyes in zeolites are an increasing stability, higher efficiency for photoinduced chargetransfer processes, and longer lifetime of electronic excited states. However, the main limitation of these materials is the low loading of photoresponsive dye that can be achieved, generally much below 10 wt%. Also the insulating nature of the zeolite framework and the micrometric crystal size are other factors that can play adverse effects for optoelectronic properties. For this reason, research in this area has decreased considering that the main application of these zeolite-encapsulated dyes is as high added value pigments exhibiting a notable stability against fading. This situation, however, has changed, and may change again in the future with the synthesis of novel porous materials with framework similar to zeolites, but with more tunable and flexible synthesis, composition, and properties. One example of these novel types of materials are metal organic frameworks (MOFs). MOFs are constituted by metal nodes coordinated to rigid bipolar or multipolar organic linkers, defining structures that are similar in some cases to those of zeolites. In addition to the possibility of hosting an organic dye inside the pores, MOFs offer also the ability to use as linker a dye molecule or to attach covalently the dye to the linker. Compared to zeolites, MOFs have a much lower framework density and in this regard less protection to the embedded guest. However, as an advantage, the larger porosity allows to include much higher loadings (much above 10 wt%) that can be even higher if the dyes play the role of linkers as in the case of PCN-224 where metal porphyrins act as linkers for the metal nodes. Therefore, novel applications can be expected, expanding the research that was done using zeolites and aluminosilicates as matrices to theses MOFs. Similarly, covalent organic frameworks (COFs), are also porous polymers that should be allowed inclusion and protection of organic
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REFERENCES
dyes. Both COFs and MOFs can exhibit advantages derived from the possibility of being cast as thin films on conductive transparent electrodes and that the framework can exhibit some electric conductivity, thus expanding and complementing the research commented on in this chapter about the use of zeolites as host. Therefore, it can be expected that most of the effects and photocatalytic systems reported for zeolite-encapsulated dyes can be somehow translated to any other porous material.
References 1. Byrne, C.; Subramanian, G.; Pillai, S. C. Recent Advances in Photocatalysis for Environmental Applications. J. Environ. Chem. Eng. 2018, 6 (3), 35313555. 2. Schneider, J.; Matsuoka, M.; Takeuchi, M.; Zhang, J.; Horiuchi, Y.; Anpo, M.; Bahnemann, D. W. Understanding TiO2 Photocatalysis: Mechanisms and Materials. Chem. Rev. 2014, 114 (19), 99199986. 3. Banerjee, S.; Pillai, S. C.; Falaras, P.; O’Shea, K. E.; Byrne, J. A.; Dionysiou, D. D. New Insights into the Mechanism of Visible Light Photocatalysis. J. Phys. Chem. Lett. 2014, 5 (15), 25432554. 4. Bergmann, A.; Martinez-Moreno, E.; Teschner, D.; Chernev, P.; Gliech, M.; de Arau´jo, J. F.; Reier, T.; Dau, H.; Strasser, P. Reversible Amorphization and the Catalytically Active State of Crystalline Co3O4 During Oxygen Evolution. Nat. Commun. 2015, 6, 8625. 5. Prier, C. K.; Rankic, D. A.; MacMillan, D. W. C. Visible Light Photoredox Catalysis with Transition Metal Complexes: Applications in Organic Synthesis. Chem. Rev. 2013, 113 (7), 53225363. 6. Primo, A.; Garcia, H. Zeolites as Catalysts in Oil Refining. Chem. Soc. Rev. 2014, 43 (22), 75487561. 7. Bauer, R. K.; De Mayo, P.; Ware, W. R.; Wu, K. C. Surface Photochemistry. The Photophysics of Pyrene Adsorbed on Silica Gel, Alumina, and Calcium Fluoride. J. Phys. Chem. 1982, 86 (19), 37813789. 8. Kohno, Y.; Shibata, Y.; Oyaizu, N.; Yoda, K.; Shibata, M.; Matsushima, R. Stabilization of Flavylium Dye by Incorporation Into the Pore of Protonated Zeolites. Microporous and Mesoporous Mater. 2008, 114 (1), 373379. 9. Kanellopoulos, N. Nanoporous Materials: Advanced Techniques for Characterization, Modeling, and Processing, CRC Press, 2016.
193
10. Ramamurthy, V. Controlling Photochemical Reactions Via Confinement: Zeolites. J. Photochem. Photobiol. C Photochem. Rev. 2000, 1 (2), 145166. 11. Mohan, S. C.; Vijay Solomon, R.; Venuvanalingam, P.; Jothivenkatachalam, K. Encapsulation of a Hexaaza Macrocyclic Nickel(ii) Complex in Zeolite Y: An Experimental and Theoretical Investigation. New Journal of Chemistry 2017, 41 (17), 95059512. 12. Turro, N. J. Modern Molecular Photochemistry, University Science Books, 1991. 13. di Nunzio, M. R.; Caballero-Mancebo, E.; Martı´n, C.; Cohen, B.; Navarro, M. T.; Corma, A.; Douhal, A. Femto-to Nanosecond Photodynamics of Nile Red in Metal-Ion Exchanged Faujasites. Microporous Mesoporous Mater. 2018, 256, 214226. 14. Alarcos, N.; Sa´nchez, F.; Douhal, A. Interrogating Ultrafast Dynamics of a Salicylideneaniline Derivative Within Faujasite Zeolites. Chem. Phys. Lett. 2017, 683, 145153. 15. Alarcos, N.; Sa´nchez, F.; Douhal, A. Spectroscopy and Relaxation Dynamics of Salicylideneaniline Derivative Aggregates Encapsulated in MCM41 and SBA15 Pores. Microporous and Mesoporous Mater. 2016, 226, 3443. 16. Alarcos, N.; Sa´nchez, F.; Douhal, A. Confinement Effect on Ultrafast Events of a Salicylideneaniline Derivative Within Mesoporous Materials. Microporous and Mesoporous Mater. 2017, 248, 5461. 17. Mohapatra, H.; Umapathy, S. Influence of Solvent on Photoinduced Electron-Transfer Reaction: TimeResolved Resonance Raman Study. J. Phys. Chem. A 2009, 113 (25), 69046909. 18. Heitele, H.; Finckh, P.; Weeren, S.; Poellinger, F.; Michel-Beyerle, M. E. Solvent Polarity Effects on Intramolecular Electron Transfer. 1. Energetic Aspects. J. Phys. Chem. 1989, 93 (13), 51735179. 19. Maroncelli, M.; Macinnis, J.; Fleming, G. R. Polar Solvent Dynamics and Electron-Transfer Reactions. Science 1989, 243 (4899), 16741681. 20. Hashimoto, S.; Mutoh, T.; Fukumura, H.; Masuhara, H. Diffuse Reflectance Laser Photolytic Studies of Naphthalene, Biphenyl and Some Aromatic Hydrocarbons Adsorbed in the Cavities of Faujasitic Zeolites. J. Chem. Soc. Faraday Trans. 1996, 92 (19), 36533660. 21. Brancaleon, L.; Brousmiche, D.; Rao, V. J.; Johnston, L. J.; Ramamurthy, V. Photoinduced Electron Transfer Reactions within Zeolites: Detection of Radical Cations and Dimerization of Arylalkenes1. J. Am. Chem. Soc. 1998, 120 (20), 49264933. 22. Scaiano, J. C.; Garcı´a, H. Intrazeolite Photochemistry: Toward Supramolecular Control of Molecular Photochemistry. Acc. Chem. Res. 1999, 32 (9), 783793.
CHEMISTRY OF SILICA AND ZEOLITE-BASED MATERIALS
194
10. DYES ENCAPSULATED WITHIN POROUS ALUMINOSILICATES AS PHOTOCATALYSTS
23. Bayo´n, J. C.; Claver, C.; Masdeu-Bulto´, A. M. Homogeneous Catalysis With Transition Metal Complexes Containing Sulfur Ligands. Coord. Chem. Rev. 1999, 193-195, 73145. 24. Caspar, J. V.; Meyer, T. J. Photochemistry of Tris(2,20 Bipyridine)ruthenium(2 1 ) Ion (Ru(bpy)32 1 ). Solvent Effects. J. Am. Chem. Soc. 1983, 105 (17), 55835590. 25. Bossmann, S. H.; Turro, C.; Schnabel, C.; Pokhrel, M. R.; Payawan, L. M.; Baumeister, B.; Wo¨rner, M. Ru(bpy) 32 1 /TiO2-Codoped Zeolites: Synthesis, Characterization, and the Role of TiO2 in Electron Transfer Photocatalysis. J. Phys. Chem. B 2001, 105 (23), 53745382. 26. Incavo, J. A.; Dutta, P. K. Zeolite Host-Guest Interactions: Optical Spectroscopic Properties of Tris (bipyridine)ruthenium(II) in Zeolite Y Cages. J. Phys. Chem. 1990, 94 (7), 30753081. 27. Corrent, S.; Cosa, G.; Scaiano, J. C.; Galletero, M. S.; Alvaro, M.; Garcia, H. Intrazeolite Photochemistry. 26. Photophysical Properties of Nanosized TiO2 Clusters Included in Zeolites Y, β, and Mordenite. Chem. Mater. 2001, 13 (3), 715722. 28. Yamashita, H.; Ichihashi, Y.; Anpo, M.; Hashimoto, M.; Louis, C.; Che, M. Photocatalytic Decomposition of NO at 275 K on Titanium Oxides Included within Y-Zeolite Cavities: The Structure and Role of the Active Sites. J. Phys. Chem. 1996, 100 (40), 1604116044.
´ lvaro, M.; Chre´tien, M. N.; Forne´s, V.; Galletero, 29. A M. S.; Garcı´a, H.; Scaiano, J. C. Multicomponent Donor 2 Acceptor Relay System Assembled within the Cavities of Zeolite Y. Photoinduced Electron Transfer Between Ru(bpy)32 1 and 2,4,6-Triphenylpyrylium in the Presence of Interposed TiO2. J. Phys. Chem. B 2004, 108 (43), 1662116625. 30. Sanjua´n, A.; Aguirre, G.; Alvaro, M.; Garcı´a, H. 2,4,6-Triphenylpyrylium Ion Encapsulated Within Y Zeolite as Photocatalyst for the Degradation of Methyl Parathion. Water Res. 2000, 34 (1), 320326. 31. Khan, A. U.; Kearns, D. R. Energetics of the Interaction of Molecular Oxygen with Organic Molecules. J. Chem. Phys. 1968, 48 (7), 32723275. 32. Prochnow, P.; Wark, M.; Schulz-Ekloff, G.; Wo¨hrle, D.; Zukal, A.; Rathousky, J. Cobalt(II)-Phthalocyanine Encapsulated in NaY Faujasite: Aggregation of Co(II)Phthalocyanine Encapsulated in NaY Zeolite Due to the Acidity of Ti(IV) Oxide Species Mediated by Pyridine. J. Porphyrins Phthalocyanines 2002, 06 (08), 494501. 33. Cojocaru, B.; Laferrie`re, M.; Carbonell, E.; Parvulescu, V.; Garcı´a, H.; Scaiano, J. C. Direct Time-Resolved Detection of Singlet Oxygen in Zeolite-Based Photocatalysts. Langmuir 2008, 24 (9), 44784481.
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