Development of photocatalysts for selective and efficient organic transformations

Accepted Manuscript Short review Development of photocatalysts for selective and efficient organic transformations Shamsa Munir, Dionysios D. Dionysiou, Sher Bahadar Khan, Syed Mujtaba Shah, Bimalendu Adhikari, Afzal Shah PII: DOI: Reference:

S1011-1344(15)00143-8 http://dx.doi.org/10.1016/j.jphotobiol.2015.04.020 JPB 10017

To appear in:

Journal of Photochemistry and Photobiology B: Biology

Received Date: Revised Date: Accepted Date:

12 February 2015 4 April 2015 19 April 2015

Please cite this article as: S. Munir, D.D. Dionysiou, S.B. Khan, S.M. Shah, B. Adhikari, A. Shah, Development of photocatalysts for selective and efficient organic transformations, Journal of Photochemistry and Photobiology B: Biology (2015), doi: http://dx.doi.org/10.1016/j.jphotobiol.2015.04.020

This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Development of photocatalysts for selective and efficient organic transformations Shamsa Munir1, Dionysios D. Dionysiou 2, Sher Bahadar Khan3, Syed Mujtaba Shah1, Bimalendu Adhikari4 and Afzal Shah1,4*

1

Department of Chemistry, Quaid-i-Azam University, 45320, Islamabad, Pakistan 2

Department of Biomedical, Chemical and Environmental Engineering Cincinnati, Ohio 45221-0012, USA

3

Center of Excellence for Advanced Materials Research (CEAMR) and Chemistry

Department, Faculty of Science, King Abdulaziz University, P. O. Box 80203, Jeddah 21589, Saudi Arabia. 4

Department of Physical and Environmental Sciences, University of Toronto Scarborough, 1265 Military Trail, Toronto, M1C 1A4, Canada

*To whom correspondence should be addressed E-mails: [email protected] and [email protected] (Dr. Afzal Shah) [email protected] (Prof. Dionysiou)

1

Abstract One of the main goals of organic chemists is to find easy, environmentally friendly, and cost effective methods for the synthesis of industrially important compounds. Photocatalysts have brought revolution in this regard as they make use of unlimited source of energy (the solar light) to carry out the synthesis of organic compounds having otherwise complex synthetic procedures. However, selectivity of the products has been a major issue since the beginning of photocatalysis. The present article encompasses state of the art accomplishments in harvesting light energy for selective organic transformations using photocatalysts. Several approaches for the development of photocatalysts for selective organic conversions have been critically discussed with the objective of developing efficient, selective, environmental friendly and high yield photocatalytic methodologies. Keywords: Photocatalysts; Selective organic transformation; Efficiency, Environmental friendliness

2

1.

Introduction

The depletion of fossil fuels and the impact of global warming contributed to the promotion of renewable energy sources as alternative sustainable options for solving a variety of problems in modern life. The utilization of solar energy, an abundant and renewable source, is one of the most promising of such sustainable approaches and can play a major role in future developments [1-5]. The existence of life on earth depends upon the conversion of solar energy into chemical energy by photosynthesis. This has prompted researchers to develop a plethora of photosynthetic systems [6-11]. Photocatalysts can, in principle, mediate the decomposition of organic materials and achieve elimination of various pathogenic or problematic microorganisms such as certain types of algae, fungi, bacteria and molds [12, 13]. Environmental pollutants, including volatile organics and harmful toxins, can also be efficiently degraded into environmentally friendly products by using photocatalysts [14]. Besides its ability as a process to destroy harmful and non-biodegradable compounds, photocatalysis is currently gaining growing attention for the selective synthesis of a variety of organic compounds, currently synthesized in industrial scale using complex synthetic procedures. A large body of work concerning semiconductor-based photocatalysts has been reported in the domain of photochemistry, electrochemistry, inorganic, organic, physical, polymer and environmental chemistry [15-19]. The practical application of semiconductor photocatalysts in organic synthesis is an attractive and significant research target [20-25]. Other equally important areas include solar-energy conversion and storage [26-29], reductive fixation of carbon dioxide [30], and mineralization and/or detoxification of organic compounds [31-35]. The present article is focused on the development and importance of photocatalysts for selective and efficient organic transformations.

3

2. Semiconductors as photocatalysts When a semiconductor surface is irradiated with photons of energy equal to or greater than the band gap of the semiconductor, an electron from the valence band is excited to the conduction band, thus, creating a hole, h+ in the valence band. The resulting h+ and e─ photogenerated species are capable of oxidizing and reducing other species by various mechanistic. Many semiconductors, including TiO2, ZnO, CdS, WO3 and NiO are commonly used as photocatalysts [36]. The photocatalytic activity of semiconductors can be modulated by doping with various metals or non-metals (i.e., N, S, P, C) and thus shifting the absorbed radiation to the desired wavelength [37]. The catalysts can also be supported on various surfaces with the objective of increasing surface area for enhancing reaction rates [38-40]. The activity of a photocatalyst is a function of band gap and redox potential. The technology of employing polycrystalline semiconductors as photocatalysts is largely used to degrade toxic and non-biodegradable organic/inorganic pollutants. Many synthetic routes in organic synthesis have been tested using beneficial effects of photocatalysts; however, certain limitations especially low selectivity do exist [41]. Hence, one of the main goals of researchers exploring semiconductor photocatalysts in organic synthesis in recent years is the modification of photocatalysts to achieve high selectivity and yield. In particular, the focus of this contribution is on the most widely employed photocatalysts for the selective synthesis of valuable organic compounds starting from the commonly utilized TiO2 photocatalysts.

2.1. TiO2 in photocatalytic reactions A landmark in the history of photocatalysis is the discovery of the photocatalytic properties of titanium dioxide. Fujishima and Honda et al [35, 42, 43] originally reported water splitting into its components upon irradiation of TiO2 catalyst. The nanostructural modifications of TiO2 subsequently opened a new chapter in the history of photocatalysis. One of the most 4

important phenomena entails ●OH radical generation (one of the strongest oxidizing agents found in nature) which can be utilized in the photocatalytic oxidation of organics and the process has been of high interest in wastewater treatment, air purification, solid remediation, decomposition of crude oil,

sterilization of surgical instruments and self-cleaning

windows [44, 45]. Rutile, brookite and anatase are the three most common allotropic forms of TiO2. Anatase and rutile belong to tetragonal crystal system while brookite is orthorhombic. The principal forms employed as photocatalysts in organic synthesis are anatase and rutile [46]. The commercially available nano scale anatase is about 30 nm in size with a band gap of 3.2 eV which absorb UV radiation at 385 nm and below [46]. In general, the affinity for adsorption of organic compounds on anatase is higher than on rutile [47]. In addition, the recombination rate in anatase is comparably less than that in rutile due to ten-fold greater hole trapping capacity of the former [48]. Rutile is different from anatase since it has a band gap of 3.0 eV and can absorb radiation at 410 nm and below. Considering the combined effects of lower recombination rates and greater adsorption capacity of most organic compounds, anatase is considered as the best TiO2 phase in photocatalytic reactions. The utility of TiO2 as photocatalyst to drive various organic reactions is presented in the subsequent sections: 2.1.1. Photocatalytic hydrogenation processes Boonstra and Mutsaers were the first to report the hydrogenation of ethene and ethyne using TiO2 ( P25) photocatalyst in 1975 [49]. They found that UV illumination i.e. 320 – 390 nm of TiO2 by a Phillip P 500W spectrum lamp in the presence of ethene and ethyne is responsible for their hydrogenation. For these hydrogenation reactions, the surface Ti-OH groups was reported as the hydrogen source, as no hydrocarbons were detected on the surface of titania after post analysis. Further investigations of the photocatalytic hydrogenation of various

5

alkenes and alkynes were performed by Anpo and co-workers using TiO2 photocatalyst in the presence of water vapors. The UV photogenerated electrons not only produced hydrogenated products but also the bond fission products due to C=C or C≡C bond cleavage. The aldehydes and ketones are susceptible to hydrogenation owing to the presence of reactive carbonyl group [50]. The first reported case in this regard is the photocatalytic hydrogenation of pyruvate to lactate using aqueous suspension of TiO2 under irradiation by a solar simulator delivering 750 W/m2 [51]. The hydrogenation of benzaldehyde to benzyl alcohol was investigated later on by Li and co-workers using P25 TiO2 reporting ca. 80% yield of the product [52]. This reaction was recently applied to a micro-reaction system by Matsushita et al. [53, 54]. P25 TiO2 powder extensively investigated by the research team of Kohtani established that this catalyst has excellent potential to hydrogenate aromatic ketones to the corresponding secondary alcohols using UV light of a 300 W xenon arc lamp in the absence of oxygen [55]. The desired secondary alcohols were produced in good quantitative yields which were calculated for more than ten species. The reduction potential of the substrates was found to be the key controlling factor of the reaction rates [51-54]. The photocatalytic hydrogenation of 1,2-diketones such as camphorquinone, 1-phenyl-1,2propanedione and benzil to corresponding α-hydroxyketones using P25 TiO2 (irradiation source was 350 nm Hg lamp) can be seen in Scheme 1 [56, 57]. The yields and stereo selectivity increase in the presence of water as a sacrificial electron donor in methanol solvent. This suggests that water might be a better hole-scavenger than CH3OH due to high affinity for TiO2 surface. In case of camphorquinone, endo-hydroxycamphors are preferentially formed as compared to exo-products though there is little selectivity between the 2 and 3 positions.

6

The photocatalytic properties of titanium oxide can be modified by loading various metals such as Pt, Au, Ag etc. In fact Apno and co-workers investigated the photocatalytic hydrogenation of alkenes and alkynes by using bare and Pt-loaded titania, illuminated with 75 W high pressure mercury lamp and found that hydrogenation occurs mainly without C≡C bond fission (photohydrogenolysis) in alkynes [58]. This change of photocatalytic mechanism from photohydrogenation to photohydrogenolysis after loading bare titania with Pt can be seen in Scheme 2 [58-60]. The effect of particle size of titania TiO2 (rutile and anatse) was also investigated for both bare and Pt loaded photocatayst by the research group of Anpo using CH3C≡CH as a model molecule in the presence of water. The quantum yield of photocatalytic reactions was found to decrease with the increase in particle size of titania and 4 wt% Pt-loaded titania. The resulting high yield in the presence of small sized photocatalyst was attributed to size quantization as diminution in particle size can increase the band gap thus, resulting in a more stabilized photo excited state. Moreover, the selectivity i.e. the ratio of hydrogenation and hydrogenolysis products (i.e. C3H8/C2H6 = 99) was also found higher with Pt-loaded catalyst [61]. Yamataka et al also investigated the hydrogenation of alkenes and alkynes by using Pt-loaded TiO2 [62]. They reported the formation of saturated alkanes with yield of >50% in some cases after 24 hrs of irradiation. The alcoholic solvents were found to oxidize to the corresponding carbonyl compounds. Ohtani and coworkers studied Pt/TiO2 as a photocatalyst for the hydrogenation of the C=N bond of imine intermediates in one pot either by intermolecular or intramolecular deamino condensation reactions by irradiating the photocatalyst with 500 W high pressure Hg lamp [63, 64]. The primary amine was oxidized by two holes to imine (R1CH=NH) in water [63] which further converted to the corresponding aldehyde upon hydrolysis. The condensation of aldehydes later with an amine produced imine intermediate (R1CH=NCH2R1). The hydrogenation of imine resulted in the formation of symmetrical secondary amine (R1CH2NHCH2R1) as the

7

final product. In the alcoholic solvent where alcohol served as the sacrificial hole scavenger [64], alcohol molecule was oxidized by two holes yielding the corresponding carbonyl compound which was condensed with an amine to produce an imine intermediate (R2R3C=NCH2R1). The hydrogenation of the imine finally produced asymmetrical secondary amine (R2R3CHNHCH2R1). The synthesis and photocatalytic application of bimetal-loaded TiO2 i.e. Pd/TiO2/Ni or Pt/TiO2/Cu using 500W high pressure Hg lamp was reported by Baba et al for ethene hydrogenation [65]. The bimetallic TiO2 served as an efficient photocatalyst for the hydrogenation reactions. The results revealed the role of metals such as Ni and Cu to suppress the hydrogen production occurring as a side reaction. In the documented photocatalytic hydrogenation reaction of 1-heptyne using Pd–Au/TiO2 [66], the catalyst was prepared by a combination of incipient wetness impregnation and deposition–precipitation methods. The authors reported that in the synthesis of Au/Pd/TiO2 photocatalysts, the dispersion state of Pd changes during Au loading and results in the formation of small alloy (Pd-Au) particles that modulates the electronic properties of Pt. However, in case of Pd/Au/TiO2 the alloy formation was not clear and electronic properties of Pd were not changed significantly when compared with the monometallic Pd/TiO2. The selectivity was observed to be greater than 95% for the conversion of 1-heptyne to 1-heptene but it decreased to 0% in 120 min using Au/Pd/TiO2 due to further hydrogenation to heptane while it was still >60% in case of Pd/Au/TiO2 and monometallic Pd/TiO2. This behavior was attributed to the enhancement of the rate of the second step, i.e., 1-heptene hydrogenation in case of Au–Pd alloy particles where the Pd species become electron-rich in contrast to Pd/Au/TiO2 [66]. Citral hydrogenation over IrAu/TiO2 and TiO2–SiO2 catalysts has been recently reported by Diaz et al. [67] and Bidaoui et al. [68]. IrAu/TiO2 catalyst was prepared by

8

deposition–precipitation and sequential incorporation of the metals using a combined DPUincipient wetness impregnation approach. Samples were reduced in H2 at 573 K and 773 K. The liquid phase hydrogenation of citral was studied at 363 K over IrAu/TiO2 using Ir/TiO2 as reference catalyst. High selectivity was found for unsaturated alcohols. The only reaction products were geraniol and nerol for Ir/TiO2 DPU and IrAu/TiO2 catalysts. The IrAu/TiO2 catalyst showed higher conversion. The binary oxides (Ti/Si oxides) were employed (using 75 W high pressure Hg lamp) for the catalytic hydrogenation of alkenes and alkynes in aqueous media using CH3C≡CH as a model molecule for the evaluation of reactivity and selectivity of the photocatalysts by varying the Ti content [69-71]. The low Ti content favored hydrogenation with bond fission producing C2H6 and CH4 as the major products whereas the high Ti content produced C3H6. A high selectivity for hydrogenation accompanied with bond fission was attributed to tetrahedrally coordinated titanium oxide while aggregated and octahedrally coordinated titanium dioxide favored hydrogenation without bond fission thus selectively producing C3 H6. Kuntz reported a study of photocatalytic hydrogenation of ethyne by 200 W super pressure Hg lamp using TiO2 supported with molybdenum oxide or sulfide complexes [72, 73]. The sulfur based systems were found somewhat more efficient with the formation of ethene (a 2 electrontransferred product) rather than ethane (4 electron transfer product).

2.1.2. Reduction of nitrobenzenes The photocatalytic reduction of nitrobenzene in suspension of ethanol and titanium dioxide produce aniline and acetaldehyde as the main products [74]. Although the reduction process using photocatalyst is not significantly selective for the production of aniline yet the process is a useful source for other pharmaceutically valuable compounds such as quinoline and indoles. TiO2 nanoparticles are used for the selective photoreduction of nitrobenzenes to

9

anilines under sunlight irradiation [75]. High yield (i.e., greater than 90%) and efficiency of the method are two of the many merits associated with the reported procedure [75]. The method revolves around synthesizing the bicrystalline (consisting of anatase and brookite) TiO2 nanoparticles under high intensity ultrasound irradiation with no thermal treatment. Aromatic amines are employed as intermediates in the synthesis of many dyes and antioxidants. These also find use in the production of pharmaceuticals, photographic, and agricultural chemicals. The photocatalytic reduction of nitrobenzene proceeds stepwise with nitrosobenzene and phenyl hydroxylamine as intermediates [76]. The application of TiO2, Ndoped TiO2 and dye-sensitized TiO2 under UV, green and blue light irradiations for the reduction of nitro aromatic compounds to anilines with appreciable yield has been reported by several research groups [77-80]. 2.1.3. Synthesis of thio-organic compounds Literature survey reveals only a few examples of the photocatalytic synthesis of thio organic compounds. Synthesis of 2-mercaptopyridine by reducing bis(2-dipyridyl)disulfide using TiO2 catalyst irradiated by 400 W high pressure Hg lamp has been reported by the research group of Tada [81]. The loading of 0.24 wt% Ag nanoclustrers (particle size <1nm) on TiO2 was found to significantly enhance the reduction of bis(2-dipyridyl)disulfide to 2mercaptopyridine due to increased saturated adsorption amount of about 17.9-fold [81]. Various photocatalytic systems such as CdS, ZnS and TiO2 have been tested for the photo oxidation of H2S [82] and production of hydrogen [83, 84]. Photocatalytic synthesis of propane-1-thiol using TiO2 photocatalyst [85] via the following proposed steps: i.

Generation of electron–hole pair by the irradiation of TiO2

ii.

Adsorption of alkene (propene) and dissociative adsorption of H2S on the surface of TiO2 providing HS─(ads)

iii.

Abstraction of e- from HS─ by photogenerated holes to give HS●(ads)

10

iv.

Reaction of HS● with propene

Although the photocatalytic activity of TiO2 was found to be three times higher than that of CdS, yet both showed high selectivity (> 99%). 2.1.4. Synthesis of phenol Phenol is an important intermediate in the production of phenolic resins, caprolactam, aniline, alkylphenols, diphenols, and salicylic acid. It is mainly produced from benzene through the cumene process (Hock process). Molinari and co-workers have reported the direct conversion of benzene to phenol in a hybrid photocatalytic membrane reactor using TiO2 Degussa P25 as catalyst [86]. The reaction set up consisted of a batch reactor with a pyrex glass surrounding and a 500 W medium pressure Hg lamp having emission intensity in UV-visible range. The reaction mixture comprised of 500 mL pure water in which the catalyst and substrate were suspended and this was constantly stirred using a magnetic stirrer. They studied the effect of pH and concentration of catalyst on the catalytic conversion of benzene to phenol and observed that lowering the pH and increasing the concentration of titania increase the rate of transformation. The selective conversion of benzene to phenol using titanium oxide nanoparticles was also reported by Zhang et al. [87]. The photocatalyst was incorporated in mesocellular siliceous foams (MCF) in order to develop hydrophobic character in it. Generally, the hydroxyl radicals generated during the photo-illumination of TiO2 crystals are highly reactive but not selective as large numbers of secondary by-products are formed. The main characteristic of this method is the induction of selectivity due to hydrophobic nature of MCF which causes encapsulation of benzene preferentially into the hydrophobic mesopores and rapid release of hydrophilic phenol from the cavity before its oxidation. The authors have compared the catalytic activity of three materials i.e. titania incorporated in MCF (TiO2@MCF), in MCF modified with methylsilyl groups (TiO2@MCF/CH3) and UV treated titania incorporated in

11

MCF and modified with methylsilyl (TiO2@MCF/CH3/UV) with the corresponding activity of TiO2 in the anatase form for the oxidation of benzene in water. The authors found that TiO2@MCF has better performance than TiO2 in connection with phenol yield (YP) and phenol selectivity (SP, %). In particular YP increased from 10 mmol/g TiO2 for TiO2 to about 35 mmol/g TiO2 for TiO2@MCF. Similarly, an increase of SP from 15.8 to 22.2 % was also observed. Upon silylating, the catalytic performance of TiO2@MCF was reduced as compared to TiO2. This behavior was ascribed to the blocking of active sites of TiO2 by methylsilyl groups in TiO2@MCF/CH3. The removal of these groups by UV irradiation restored the catalytic activity. In fact TiO2@MCF/CH3/UV gave a YP above 50 mmol/g TiO2 and SP of about 35 %. The research group of Zheng [88] modified the photocatalytic activity of TiO2 by depositing nanoparticles of noble metals on its surface. The authors developed a noble-metal plasmonic photocatalyst which was effective for the oxidation of benzene to phenol under visible light irradiation. 2.1.5. Photocatalytic oxidation of toluene Toluene is converted into benzene and xylenes by catalytic dealkylation and transalkylation [89]. Different authors have investigated the oxidation of toluene to benzaldehyde via photocatalytic reactions using TiO2. Rezala et al. investigated the photo oxygenation of toluene and xylenes using Ti-Pillared montmorillonite clays (Ti-PILCs) in the presence of oxygen [90]. Toluene was oxidized to benzaldehyde whereas ortho and para xylenes were converted to the respective tolualdehyde with significantly high selectivity (90%). Oxygen was proved to be involved in the oxygenation process demonstrated in Scheme 3. Photochemical excitation of TiO2 results in the formation of h+ and e-. The electrons are promoted to the conduction band. O2 acting as electron scavenger in the conduction band results in the formation of O2●- and ●OH radicals. In addition, the O2

12

molecule can also react with alkyl radicals giving peroxy radicals. The photogenerated peroxy radicals produced in this way lead to the final oxygenated product. In another method described by Cao et al. [91] TiO2 hollow spheres prepared by the hydrothermal reaction between TiF4 and H2O was used for the photocatalytic oxidation of toluene to benzaldehyde in water using light radiation of 310 nm wavelength by a 6 W UV lamp. Toluene conversion efficiency increased from 9.0% to 21% by increasing the hydrothermal treatment duration from 20 min to 6 hrs. The results revealed selectivity of about 90% with the order of catalytic activity varying in the sequence TiO2-20 min < TiO2-40 min << TiO2-6 hrs < TiO2-12 hrs < TiO2-72 hrs The TiO2-72 h microspheres showed two times higher activity than commercial Degussa P25 due to wider [001] facets. 2.1.5. Selective photocatalytic oxidation of alcohols Heterogeneous photocatalysis is a promising route for the selective conversion of alcohols to aldehydes. Feng and co-workers have demonstrated the utilization of heterogeneous TiO2 photocatalyst for the oxidation of benzyl alcohol [92]. They used commercial TiO2 as a support with surface modified by the photo deposition of different metals such as Ag, Au, Pd, Pt and Ir. The photocatalysts prepared in this way favored the selective oxidation of benzyl alcohol into banzaldehyde. The metal deposited TiO2 was found to have high photocatalytic activity as compared to bare titania. The deposition of a series of transition metals on the surface of TiO2 for enhancing its photocatalytic activity has been reported. As the work functions of noble metals are higher than TiO2 so the photogenerated electrons tend to migrate from TiO2 to metal clusters and get trapped therein. Hence, the deposition of metal clusters on the surface of TiO2 promotes the separation of photogenerated electron–hole pairs and thus boosts the photocatalytic activity of TiO2. Ir/TiO2 especially used for the selective photocatalytic oxidation of benzyl alcohol is

13

prepared by a variety of methods including photo deposition (p), co-instantaneous wet impregnation (i), and chemical reduction method (c) [84]. Amadelli and coworkers [93] developed vanadyl grafted photocatalyst, (VOx)n/TiO2 and examined its photocatalytic activity by monitoring the oxidation of benzyl alcohol to benzaldehyde under medium pressure Hg lamp. The resultant benzaldehyde was subsequently photo oxidized to benzoic acid. The results of their experiments revealed that vanadium-modified catalyst favors the formation of benzaldehyde as compared to Degussa P25 TiO2. Moreover, the extent of benzaldehyde formation was found to depend largely on the concentration of vanadium. Precious metal – semiconductor composites have been studied in selective synthesis of organic molecules [94]. Gold nanoparticles (<5 nm) supported on anatase-rutile interphase (Evonik P-25 photocatalyst) by a deposition-precipitation method could take advantage of the plasmonic effects to achieve the photocatalytic selective production of several aromatic aldehydes from their corresponding aromatic alcohols. This photocatalytic process can be induced via plasmon activation of the gold clusters by visible light followed by consecutive electron transfer in the Au/rutile/anatase interphase contact. Activated Au particles transfer their conduction electrons to rutile and then to adjacent anatase which catalyzes the oxidation of substrates by the positively charged Au particles along with reduction of oxygen by the conduction band electrons on the surface of anatase titania [94]. 2.1.6. Photocatalytic oxidation of cyclohexene, cyclohexane and benzene Photo-oxidation of cyclohexene to cyclohexenone and cyclohexenol in non-aqueous medium was reported by Amadelli et al., using (VOx)n/TiO2 as a photocatalyst irradiated by medium pressure Hg lamp [93]. The alcohol to ketone ratio was found 0.84 as compared to 0.48 for bare TiO2 P-25 indicating that cyclohexenol is the major product when using vanadium modified TiO2. Thus (VOx)n/TiO2 can be singled out for increasing the alcohol to ketone

14

ratio. Shimizu and co-authors reported the selective photo-oxidation of benzene and cyclohexane by using TiO2 pillared clays using 500 W xenon lamp [95]. Three different groups of clays namely mica, montmorillonite and saponite prepared by the previously reported methods [96, 97] were employed to pillar TiO2. The effect of various solvents and mixture of solvents was investigated on different catalyst samples and TiO2 mica showed quite diverse selectivity of the products of photo-oxidation of benzene. The partial oxidation of benzene in aqueous environment was associated with higher selectivity and activity of the products. Photocatalytic activity found to vary in the sequence TiO2-mica > TiO2-sapo > TiO2(P25) > TiO2-mont The activity of TiO2 mica was analyzed for the oxidation of benzene in acetonitrile, water and acetonitrile having 10% water. Several oxygenates were formed and their amount varied with the solvent system; being highest in water and lowest in acetonitrile. Selectivity was observed to increase with rise in percentage of water content in solvent mixture. Although the highest activity was observed for TiO2-mica, yet the selectivity of the products depended on the type of catalyst. The difference in photocatalytic activity and selectivity of various forms of the catalyst was attributed to the clay host rather than the structure of TiO2 pillars because the structure of pillared clays was found similar by various characterization techniques. The photo-oxidation of cyclohexane on TiO2-mica and P-25 was also compared. TiO2-mica brought about the selective production of cyclohexanol and cyclohexanone whereas, on TiO2 P-25 the main product of oxidation was CO2 [95]. It was concluded that selectivity for the partial oxidation of liquid hydrocarbons increased due to the hydrophobic nature of the tested pillared clays. 2.1.7. Photocatalyzed oxidation of adamantane Different types of TiO2 powders (e.g., anatase, rutile) were employed by Ohno et al., for the photocatalytic oxidation of adamantane in a mixed solvent containing acetonitrile and

15

butyronitrile [98]. 1-Adamantanol, 2-adamantanol, and 2-adamantanone were found as the main oxidation products. Anatase displayed higher photoactivity than rutile; however, the addition of hydrogen peroxide to rutile solution enhanced its photoactivity even more than anatase and the production rate of 1-adamantanol increased 10 times. The quantum efficiency of 1-adamantanol raised from 6.4% to 25%after the addition of hydrogen peroxide to rutile. Thus oxidation of adamantane to hydroxylated products in the presence of hydrogen peroxide using light energy provides a green route in organic synthesis. 2.1.9. Selective transformation of carboxylic acids and sugars Shorter chain acids [99] can be produced from aliphatic carboxylic acids or can be decarboxylated by means of photo-Kolbe-type processes [100]. In the absence of oxygen and presence of Pt/TiO2, aliphatic carboxylic acids (especially C4-C5 acids) are decarboxylated to the corresponding reduced hydrocarbons. The use of guanidine remarkably decreases the band gap of the titania semiconductor, and in the presence of iron-based materials can result in the synthesis of efficient, visible-active and magnetically separable photocatalyst TiO2guanidine-(Ni,Co)-Fe2O4. In this case, selectivity close to 80% could be achieved in less than 2 hours of reaction [99]. The efficiency of heterogeneous nano-titania photocatalysts in photocatalytic selective oxidation of sugars into important chemicals has also been reported [101]. This reaction was found to be highly selective (>70%) to glucaric and gluconic acids. The best product selectivity was achieved with titania synthesized by the sonication-induced sol–gel procedure (TiO2(US)) [101]. Solvent composition and duration of irradiation considerably affected the photocatalysts activity/selectivity. The total selectivity to organic compounds was found to be 39% and 71% for liquid phase reactions using 1:9 = water: acetonitrile and 1:1 = water: acetonitrile, as solvent mixtures, respectively, for the most selective photocatalyst TiO2. An improvement in selectivity to glucaric and gluconic acids can be achieved by supporting

16

nano-titania systems on a zeolite type Y (SiO2:Al2O3 = 80) [102]. Homogeneously distributed TiO2 on zeolite Y provided improved selectivity for glucose oxidation towards glucaric and gluconic acids. Total selectivity was ca. 68% after 10 min illumination time using a 1:1 H2O/acetonitrile solvent composition. Results were comparably superior to those of unsupported TiO2 and commercially available Evonik P-25 photocatalyst. Further photocatalyst optimization via development of transition-metal containing supported nanotitania materials [103, 104] provided advanced systems incorporating Fe or Cr able to achieve improved selectivity to carboxylic acids. No metal (Fe, Cr, Ti) leaching was observed after photoreaction, with Fe-TiO2 systems being the most selective (94 % after 20 min of illumination under similar conditions previously reported).

2.2. Zn based photocatalysts in organic synthesis The photocatalytic potential of ZnO is comparable to that of TiO2 as both have quite close band-gap. ZnO nanoparticles have been reported as better photocatalyst for the degradation of common organic contaminants as compared to bulk ZnO and commercial TiO2 Degussa P25 [105]. Nanostructured ZnO particles due to good catalytic activity have also been used for the removal of organic water pollutants [106]. 2.2.1. Photocatalytic hydrogenation The investigations of ZnS catalyzed addition of 3,4-dihydropyran to azobenzene revealed that photocatalytic hydrogenation of azobenzene to hydrazobenzene also occurs as a side reaction (Scheme 4) [107, 108]. The formation of hydrazobenzene was considerably favored when Ptloaded ZnS was employed in the process. This is ascribed to the preferentially favorable twoelectron transfer process on Pt particles. The reason of using ZnS in the hydrogenation of aliphatic aldehydes and ketones is due to its sufficiently negative conduction band as compared to TiO2 [55]. Yanagida and co-workers [109] employed ZnS nano-crystallites with

17

particle size 2–5 nm to investigate the photoreaction of acetaldehyde in water under UV irradiation and reported that acetaldehyde is both reduced and oxidized under these conditions. The main reduction products were ethanol and H2 and the apparent quantum yield of ethanol (main product) formation was 0.25 at 313 nm. A similar behavior for propionaldehyde was observed on ZnS nano-crystallites [109]. Aliphatic ketones such as acetone, 2-butanone, 3-pentanone, 2-hexanone, cyclopentanone, and cyclohexanone were also investigated for photocatalytic hydrogenation using ZnS nano-crystallites in the presence of sacrificial hole scavengers namely S2- and SO32- (Oxidation reaction: S2- + SO32- + 2h+ → S2O32-) [110]. The photocatalytic hydrogenation of ketones produced alcohols at reasonable rates and in good yields except that of acetone.

2.3. Decatungstate (DT) as an efficient photocatalyst Decatungstate, a polyoxometalate consisting of at least three transition metal oxyanions linked together by shared oxygen, is gaining significant attention as a robust and versatile photocatalyst. DT has been emerged as an efficient photocatalyst due to its ability of catalyzing many organic reactions of considerable synthetic importance. It was first utilized for the oxidation of organic substrates such as alkanes, alkenes, and alcohols. More recent developments in this field have been centered on the immobilization of decatungstate on a solid support (e.g. silica, alumina or polymeric membranes) thus providing an efficient and recyclable photocatalytic system, especially for oxidation reactions. 2.3.1. Functionalization of alkanes using DT Many research efforts have been directed towards the utilization of decatungstate catalyst in organic synthesis under anaerobic conditions. Tzirakis and co-workers have reported photochemical functionalization of alkanes using decatungstate as photocatalyst (in the absence of O2) to produce useful alkyl derivatives with high yield and good selectivity [111].

18

Their work demonstrated anaerobic conditions as prerequisite for avoiding the trapping of alkyl radicals by molecular oxygen. The successful dehydrogenation of alkanes using decatungstate as a catalyst in anaerobic environment giving a corresponding alkene isomer in reasonably high selectivity was reported by Hill et al. [112]. The same research group effectively applied decatungstate for the catalytic radical addition reactions of unactivated C– H bonds to acetylene (vinylation) and ethylene (ethylation). For selective radical carbonylation of alkanes with CO, the alkyl radicals were generated from alkanes via decatungstate photocatalysis followed by carbonylation under CO atmosphere. The reaction exhibited high turnover rate and selectivity for the production of the corresponding aldehydes; however, facile decarbonylation of the products under the same conditions was found to be a yield-limiting factor. In late 90s, Hill’s group developed a convenient method for the preparation of nitriles and α-iminoesters [113]. The method consisted of decatungstate photo-mediated cleavage of unactivated C–H bonds of a variety of alkanes followed by coupling with methyl cyanoformate affording the corresponding nitriles and α-imino esters in good yields at high and low temperatures, respectively. Following these early studies, Albini and co-workers investigated the alkylation of electrophilic alkenes by alkanes using decatungstate photocatalysts (Scheme 5) [114, 115]. 2.3.2. Synthesis of unsymmetrical ketones Inter and intramolecular addition of nucleophilic acyl radicals to olefins or alkynes is one of the most useful approaches for the synthesis of cyclic and acyclic ketones. Thus, numerous methods of generating acyl radicals have been developed and applied in organic synthesis. In these processes the photocatalytic role of decatungstate is to produce acyl radicals by activating C(O)–H bond of the corresponding aldehydes. Trapping of such acyl radicals by electrophilic alkenes has led to the synthesis of unsymmetrical ketones in moderate to good yields [116]. The competing decarbonylation of acyl radicals prior to acylation of secondary

19

or tertiary aldehydes at room temperature initially proved to be a limiting factor for the scope of this reaction. However, at low reaction temperatures (i.e. at -20ºC) the decarbonylation pathway of secondary aldehydes decreased dramatically with a simultaneous increase in the acylation products [117]. More recently, direct acylation of [60] fullerene with acyl radicals has been achieved via decatungstate catalysis [117]. 2.3.3. Amidation of olefins Angioni and co-workers reported that tetra butyl ammonium decatungstate (TBADT) can be utilized for the functionalization of C–H bond of amides [118]. In their study, different carbon-centered radicals depending on the amide structure were generated via decatungstate catalysis and subsequently trapped by the electrophilic alkenes. In particular it was found that in a tertiary amide (i.e. DMF or N, N -dimethylacetamide) the N-methyl C–H bond is chemoselectively cleaved with no competition by the C–H bond of formyl group or αhydrogen of the carboxamide group. On the contrary, a complete change in chemoselectivity was observed in case of a secondary amide (i.e. N-methyl-formamide) in which only the C–H bond of the formyl group is homolytically cleaved as shown in Scheme 6. The same method was extended to carbamates thus affording the corresponding alkylation products in 26–59% yields. 2.3.4. TBADT mediated activation of 1,3-benzodioxole moiety 1,3-Benzodioxole ring containing compounds have broad range biological activities. The reactivity of this ring under TBADT-mediated photocatalytic conditions was examined by Ravelli and co-workers. The results revealed this moiety is an excellent substrate allowing access to a variety of α,α-dioxy radicals that were successfully trapped with isolated yields

ranging

between

46

and

77%.

The

presence

of

different

substituents

(-OR, -COOMe, -Me or -CHO) was well tolerated by the system. Moreover, the use of 5chloro-1,3-benzodioxole derivative allowed the synthesis of a safrole (5-allyl-1,3-

20

benzodioxole) derivative via a one-pot procedure consisting of two consecutive irradiations at two different wavelengths [119]. 2.3.5. TBADT-mediated radical addition to vinyl sulfones Ravelli and co-workers carried out detailed investigations of vinyl sulfones, since the -SO2R group (with R mostly a phenyl ring) is convertible into a variety of other functional groups. The attention was mainly devoted to phenyl vinyl sulfone which is a proven excellent trap [120]. However, the system also allowed the use of β-substituted olefins. It was found that the introduction of a substituent cause the addition to the double bond less efficient (without inhibiting it). On the contrary, when the double bond is inserted into a cyclic skeleton, an enhanced reactivity is observed, especially in case of overall strained structure. 2.3.6. TBADT-mediated functionalization of single walled carbon nanotubes The research team of Ravelli extended the photocatalytic approach to the field of nanomaterials by assessing the feasibility of single-walled carbon nanotubes functionalization via direct addition of photo-generated radicals to the surface [121]. Polyethylene glycol chains were chosen as the donors since these impart an increased bio-compatibility to the resulting material in view of the possible conjugation with a bioactive molecule. The extent of

functionalization

process

was

qualitatively

and

quantitatively

evaluated

by

thermogravimetric analysis and Raman spectroscopy. The functionalization caused partial dissolution of the resulting carbon nanotubes in the reaction medium. 2.3.7. Oxidation of aliphatic alcohols and alkanes Aliphatic alcohols and alkanes are the first among organic compounds that have been oxidized via decatungstate catalysts in the presence of molecular oxygen. The oxidation mechanism of aliphatic secondary alcohols and alkanes to corresponding ketones and hydroperoxides using a laser flash photolysis technique was first studied by the Tanielian research team [122]. It was found that the reduced form of decatungstate (W10O325-) can be

21

reoxidized to W10O324- in the presence of O2 with the parallel formation of peroxy compounds. For example, propan-2-ol was converted to acetone and hydrogen peroxide upon decatungstate photo-catalysis whereas oxidation of adamantane resulted in the formation of the corresponding hydroperoxide (Scheme 7). The decatungstate-catalyzed oxidation of aliphatic alkanes was also investigated by Giannotti’s group [123]. The oxidation of adamantane occurred selectively to 1- and 2-adamantanol in low to medium conversions after the reduction of the so formed hydroperoxides with the aid of trimethylphosphite. However, the tertiary alcohol (adamantanol-1) was formed in almost 80% relative yield. On the other hand, at conversion higher than 70% (based on the starting material) the accumulation of poly-oxygenated products lowered the efficiency of oxidation. Similarly, Maldotti and coworkers studied the homogeneous oxidation of cyclohexane. The only products obtained were the corresponding cyclohexanol and cyclohexanone. Moreover, the O2 pressure was found to have a pronounced effect on the cyclohexanone to cyclohexanol ratio. In particular, high O2 pressure was found to favor the formation of cyclohexanone whereas low O2 pressure led to the selective formation of cyclohexanol [124]. 2.3.8. Oxidation of aromatic alkanes and alcohols Decatungstate has been found as an efficient catalyst for the selective oxidation of aromatic hydrocarbons (i.e., cumene, ethylbenzene, and methyl fluorene). The major products obtained from the oxidation of these aromatic hydrocarbons were the corresponding alcohols as shown in Scheme 8. For example considering para-substituted cumene, tertiary alcohol was formed as a final product after the reduction of initially formed hydroperoxide with triphenylphosphine. Kinetic isotope effect studies evidenced the abstraction of hydrogen atom from the aromatic substrate as the rate-determining step of this oxidative transformation [125]. Apart from decatungstate-catalyzed oxidation of aromatic alkanes, the oxidation of benzyl alcohols under similar conditions has also been documented [126]. The corresponding

22

aromatic ketones are selectively formed as the only products in good yields. On the basis of results obtained from several investigations, including time-resolved techniques, kinetic isotope effect, Hammett kinetics, and product analysis, hydrogen atom abstraction mechanism has been proposed for this reaction. 2.3.9. Oxidation of alkenes The research of Maldotti and co-workers showed that the oxidation of cyclohexene and cyclooctene in the presence of W10O324- results in the formation of secondary hydroperoxides and α,β-unsaturated cycloketones in moderate to high yields. Moreover, the presence of FeIII-[meso-tetrakis-(2,6-dichlorophenyl)porphyrin]chloride as co-catalyst in the oxidation of these cycloalkenes was found to affect both the efficiency and chemoselectivity of the reaction. The decomposition of allylic hydroperoxides to the corresponding allylic alcohols is also catalyzed by this co-catalyst [127]. The selectivity of the decatungstate catalyzed oxidation of various alkyl and phenyl substituted cycloalkenes (e.g., 1-alkyl- or 1aryl-substituted cycloalkenes) in the presence of O2 has been reported [128]. The results suggest that a hydrogen atom on less hindered side of the double bond is preferentially abstracted by the decatungstate thus, affording the formation of the corresponding allylic hydroperoxides and enones as the major products. The reasonably high yield makes this catalytic reaction quite useful for synthesis. 2.3.10. Oxidation of organic compounds by heterogenized decatungstate Maldotti and co-workers were the first to report the application of heterogeneous (nBu4N)4W10O32 photocatalysis [129]. For the preparation of such a catalyst, an impregnation procedure was followed by adsorbing (nBu4N)4W10O32 on silica through electrostatic interactions [130]. The as prepared catalyst was tested for the oxidation of cyclohexane to cyclohexanol and cyclohexanone. In 2002, Maldotti et al., reported the heterogeneous catalysis using (nBu4N)4W10O32 immobilized on amorphous and mesoporous MCM-41 (pore

23

size 20–100 Å, surface area=1000 m2g-1) silicas [129]. The ketone/alcohol ratio was found to increase by the oxidation using decatungstate supported on amorphous as well as MCM-41 silicas. Decatungstate when used with amorphous silica oxidized cyclododecane at efficiency higher than that of homogeneous phase and with the added advantage of using the loaded photocatalysts again and again (at least three times) without any significant loss of activity. The investigations about the effect of the nature of cations (in the tetralkylammonium and sodium decatungstate supported on silica) on the photocatalytic oxidation of organic substrates [131] revealed that the efficiency of cyclohexane photo oxidation is substantially enhanced by the use of tetralkylammonium cations rather than their ammonium or sodium analogues. Silica supported (n-Bu4N)4W10O32 was tested for the photocatalytic oxidation of cyclohexene and cyclooctene by Maldotti and co-workers. They also examined the photocatalytic properties of (n-Bu 4N)4W10O32–SiO2 in the presence of a co-catalyst i.e., FeIII [meso-tetrakis(2,6-dichloro-phenyl)porphyrin] chloride (Fe(TDCPP)Cl). The major oxidation products were allylic hydroperoxide and cyclooctene epoxide and the co-catalyst (CH2Cl2) addition affected the process positively by enhancing the photocatalytic activity. The results revealed that the epoxide/hydroperoxide ratio is increased by the use of co-catalyst in the oxidation of cyclooctene. The work was further extended by using the same catalyst supported on silica, i.e. (n-Bu 4N)4W10O32–SiO2 with a different co-catalyst for the oxidation of cyclohexene and cyclooctene [132]. The co-catalyst significantly altered the photoactivity of (n-Bu4N)4W10O32–SiO2. The product distribution and the overall yield were found to be quite different. The important outcome was that the photo-oxidation of both cyclohexene and cyclooctene gave corresponding epoxides. Thus, the phocatalytic activity of decatungstate and selectivity of the product can be altered by the use of suitable co-catalyst. These heterogeneous photocatalysts in combination with a co-catalyst can be employed to obtain valuable organic compounds in a greener way.

24

The deposition of decatungstate on the surface of γ-alumina and silica by wet impregnation method at various pH values has been investigated [133]. The structure and dispersion state of catalyst was found to depend strongly on the impregnation pH and nature of the support used. The heterogeneous catalyst supported on silica showed more effectiveness than its homogeneous form. However, Al2O3 supported catalyst exhibited lower photoactivity as compared to its homogeneous and silica supported forms. The catalysts were tested to determine their efficiency and selectivity for the catalysis of the photo-oxidation of a series of primary and secondary benzyl alcohols. The oxidation resulted in the selective and quantitative formation of para-substituted benzoic acids and aryl ketones with both alumina and silica supported catalysts as depicted in Scheme 9. Heterogeneous catalysis using zirconia-supported decatungstate, Na4W10O32–ZrO2 was documented by Farhadi and co-workers [134]. The catalyst was utilized for the oxidation of primary and secondary benzyl alcohols under oxygenated atmosphere. It was observed that Na4W10O32–ZrO2 can successfully oxidize these substrates to carbonyl compounds with yields varying from 60-90%. In addition, the catalyst was found to prevent the over oxidation of benzaldehydes to the corresponding acids. Like other supported catalysts, this heterogeneous system was also found more efficient as compared to bare Na4W10O32. Despite the conventional impregnation or sol–gel technique to support decatungstate on silica, a modified method was employed, i.e. the surface was first functionalized with different ammonium cations to establish covalent bonds with silica [135]. The immobilization of polyoxoanion on the solid surface was done through an exchange reaction by mixing the selected surface bound alkylammonium salt and an aqueous solution of Na4W10O32. By adopting this procedure, Bigi et al. were able to link the catalyst to solid silica support by chemical bonding as compared to simple electrostatic interactions. These catalyst systems were proved effective in photo-oxidizing various sulfides with high efficiency and selectivity.

25

Maldotti et al. utilized the same catalytic systems to investigate photocatalysed regioselective oxidation of diols [136]. 1,3-Butanediol was 90% photo-oxidized to 4-hydroxy-2-butanone and 1,4-pentanediol was selectively converted to 4-hydroxy-pentanal. The terminal –OH group in diols was selectively oxidized with high efficiency. Photocatalytic activity of sodium decatungstate supported on sol–gel silica was examined for the oxidation of glycerol by Molinari et al. [137]. Entrapment of Na4W10O32 inside a silica matrix by a sol–gel procedure provided a new heterogeneous photocatalyst characterized by the presence of micropores (7 and 13 Å) and mesopores (30 Å). The results revealed that the interaction between polyoxoanion and protonated silanol groups is strong enough to prevent any kind of leaching. Silica surface was found to enhance alcohol adsorption, hence favoring its reaction with photogenerated hydroxyl radicals that lead to the formation of glyceraldehyde and dihydroxyacetone. On the other hand, using non heterogenized Na4W10O32, photooxidation of glycerol was characterized by low selectivity and degradation to CO2. No formation of carbon dioxide in case of employing heterogenized Na4W10O32 indicated the possibility of controlling the high oxidizing power of sodium decatungstate through its heterogenization over a suitable support [137]. Apart from inorganic materials, organic polymeric substances can also serve as support to heterogenize decatungstate for photocatalysis. In this regard, the immobilization of decatungstate by ion-exchange method onto an organic ion exchange resin has been studied by Fornal and Giannotti [138]. The photocatalytic activity of the resin–decatungstate composite was evaluated from the oxidation of cyclohexane by molecular oxygen. The selectivity of this oxidation reaction depended strongly on the loading of decatungstate on support rather than its concentration in the irradiated solution. Thus, lower loading of decatungstate promotes cyclohexanone production whereas higher loading of decatungstate favors cyclohexyl hydroperoxide generation. (n-Bu 4N)4W10O32 supported on macroreticular

26

styrene–divinylbenzene copolymer bearing –N(CH3)3+ functional group was found to promote the conversion of phenol and anisole to the corresponding mono-brominated derivatives and cycloalkenes to corresponding bromohydrins and dibromides in the presence of a bromide source. The active species “Br+” is formed as a consequence of a two-electron oxidation of Br− by the photogenerated hydroperoxides [139]. Bromohydrins were quantitatively transformed into the corresponding epoxides (Scheme 10) by simply adjusting the pH value. These catalytic transformations are of considerable synthetic interest due to the otherwise difficult mono bromination of activated arenes and the diverse role of epoxides as important intermediates in organic synthesis. The entrapping of decatungstate in membranes offers new possibilities for heterogeneous catalysis in the sense that selective transport properties of membranes can be exploited for enhancing the yield and selectivity of reactions [140]. Poly-vinylidene fluoride (PVDF) and polydimethylsiloxane (PDMS) based systems showing resistance to self-induced degradation upon irradiation in water were tested for the oxidation of several water soluble alcohols as shown in Scheme 11. Both membrane-based systems exhibited good performance although in all cases the corresponding homogeneous oxidation was faster. The preparation of new hybrid photocatalysts by embedding the fluorous-tagged decatungstate (RfN)4W10O32 (RfN = [CF3(CF2)7(CH2)3 ]3CH3N+ ) within fluoropolymeric films has also been reported [141]. Perfluoropolymers such as hyflons are preferred over other polymeric materials due to their outstanding resistance (thermal and oxidative) and molecular oxygen preferential permeability. The resulting hybrid materials exhibited a remarkable improvement in morphology and performance in comparison with PVDF or hyflons membranes embedding the fluorous-free (n-Bu4N)4W10O32.

2.4. Other photocatalysts in organic synthesis

27

Phenol hydroxylation previously reported on TiO2 photocatalysts was tried by Huixian et al. using a new catalyst combination of Fe-Al-silicate [142]. The catalyst was obtained by ion adsorption process and phenol hydroxylation was carried out in the presence of H2O2 using UV irradiation of 365 nm via 125 W high pressure Hg lamp. The oxidation process was explained in terms of ●OH generation by the reaction of H2O2 with conduction band electrons and its subsequent reaction with phenol to produce catechol and hydroquinone. The presence of Fe-Al silicate as a photocatalyst was found necessary for significant conversion as the control experiment without catalyst showed negligible catechol and hydroquinone production. Without catalyst, even a large amount of H2O2 could not cause any significant photocatalytic reaction. The effect of co-catalyst (acetonitrile) addition and reaction time on phenol oxidation was also examined. The results revealed the addition of acetonitrile and longer reaction time to favor phenol conversion. For instance, under the reaction condition of using 0.5 g phenol, 15 mL water, 4 mL acetonitrile and 1mL of H2O2, 95% selectivity was achieved with 39.3 and 22.3% yields of catechol and hydroquinone, respectively. Among many other investigated photocatalysts, the one highly mentioned in the past since 1970s is [Ru(bpy)3]2+. Recently reported organic transformations using Ru(bpy)3]2+ include the oxyfunctionalization of alpha position of aldehydes e.g. α-oxyamination of aldehydes [143], intermolecular addition of glycosyl halides to alkenes to form saturated C-glycosides exclusively with α-selectivity [144], and halogenation of alcohols to form halogenated products under mild conditions [145].

3.

Conclusions and prospects

The successful applications of photocatalysts for environmental monitoring in connection with the degradation of non-biodegradable organic compounds spurred the scientists to utilize their photocatalytic properties in organic conversions. By the use of photocatalysts, more

28

complex synthetic procedures have been replaced by relatively simple and safe routes. Suitable modifications in photocatalysts can improve the efficiency of reactions and control the selectivity of the products. Among the photocatalysts employed in organic transformations, TiO2 has been extensively utilized because of its environmental friendly nature, low cost, and good catalytic properties. After TiO2, many other photocatalysts including CdS, ZnO, ZnS and compounds of organic and inorganic nature have been found effective in some particular reactions. The objectives of high efficiency and selectivity are achievable by using various supports, employing a variety of co-catalysts, doping of different metals, and optimizing reaction conditions.

Acknowledgments The authors gratefully acknowledge the financial support of Higher Education Commission of Pakistan through project number 20-3070, Quaid-i-Azam University and University of Cincinnati, USA .

29

References [1] [2] [3] [4] [5] [6] [7] [8]

[9] [10] [11] [12] [13] [14]

[15]

[16]

[17]

H. B. Gray, "Powering the planet with solar fuel," Nature chemistry, vol. 1, pp. 7-7, 2009. J. L. Dempsey, B. S. Brunschwig, J. R. Winkler, and H. B. Gray, "Hydrogen evolution catalyzed by cobaloximes," Accounts of chemical research, vol. 42, pp. 1995-2004, 2009. N. S. Lewis and D. G. Nocera, "Powering the planet: Chemical challenges in solar energy utilization," Proceedings of the National Academy of Sciences, vol. 103, pp. 15729-15735, 2006. V. Balzani, A. Credi, and M. Venturi, "Photochemical conversion of solar energy," ChemSusChem, vol. 1, pp. 26-58, 2008. D. L. Royer, R. A. Berner, and J. Park, "Climate sensitivity constrained by CO2 concentrations over the past 420 million years," Nature, vol. 446, pp. 530-532, 2007. M. R. Wasielewski, "Self-assembly strategies for integrating light harvesting and charge separation in artificial photosynthetic systems," Accounts of chemical research, vol. 42, pp. 1910-1921, 2009. S. Fukuzumi, "Development of bioinspired artificial photosynthetic systems," Physical Chemistry Chemical Physics, vol. 10, pp. 2283-2297, 2008. S. Fukuzumi, K. Saito, K. Ohkubo, V. Troiani, H. Qiu, S. Gadde, F. D'Souza, and N. Solladié, "Multiple photosynthetic reaction centres using zinc porphyrinic oligopeptide–fulleropyrrolidine supramolecular complexes," Physical Chemistry Chemical Physics, vol. 13, pp. 17019-17022, 2011. S. Fukuzumi and K. Ohkubo, "Assemblies of artificial photosynthetic reaction centres," Journal of Materials Chemistry, vol. 22, pp. 4575-4587, 2012. S. Fukuzumi, K. Ohkubo, F. D'Souza, and J. L. Sessler, "Supramolecular electron transfer by anion binding," Chemical Communications, vol. 48, pp. 9801-9815, 2012. M. A. Fox and M. T. Dulay, "Heterogeneous photocatalysis," Chemical Reviews, vol. 93, pp. 341-357, 1993. Z. Tribelsky and M. Ende, "Disinfection through packaging," ed: US Patent 20,040,219,056, 2004. C. A. Linkous, "Photocatalytic surfacing agents with varying oxides for inhibiting algae growth," ed: Google Patents, 1999. Y. Zhang, Z.-R. Tang, X. Fu, and Y.-J. Xu, "TiO2 − graphene nanocomposites for gasphase photocatalytic degradation of volatile aromatic pollutant: is TiO2− graphene truly different from other TiO2− carbon composite materials?," ACS nano, vol. 4, pp. 7303-7314, 2010. T. Honda, T. Nakanishi, K. Ohkubo, T. Kojima, and S. Fukuzumi, "Structure and photoinduced electron transfer dynamics of a series of hydrogen-bonded supramolecular complexes composed of electron donors and a saddle-distorted diprotonated porphyrin," Journal of the American Chemical Society, vol. 132, pp. 10155-10163, 2010. F. D’Souza, A. N. Amin, M. E. El-Khouly, N. K. Subbaiyan, M. E. Zandler, and S. Fukuzumi, "Control over photoinduced energy and electron transfer in supramolecular polyads of covalently linked azaBODIPY-bisporphyrin ‘molecular clip’hosting fullerene," Journal of the American Chemical Society, vol. 134, pp. 654664, 2011. S. Fukuzumi, "New development of photoinduced electron-transfer catalytic systems," Pure and applied chemistry, vol. 79, pp. 981-991, 2007.

30

[18] [19] [20] [21] [22] [23] [24] [25] [26] [27]

[28] [29]

[30] [31] [32] [33] [34]

[35]

B. Ohtani, Y. Ogawa, and S.-i. Nishimoto, "Photocatalytic activity of amorphousanatase mixture of titanium (IV) oxide particles suspended in aqueous solutions," The Journal of Physical Chemistry B, vol. 101, pp. 3746-3752, 1997. H. Kisch, "Preparative photoreactions catalyzed by semiconductor powders," Journal für Praktische Chemie/Chemiker‐Zeitung, vol. 336, pp. 635-648, 1994. M. Kaneko and I. Okura, Photocatalysis: Science and Technology: Springer, 2002. Y. Shiraishi and T. Hirai, "Selective organic transformations on titanium oxide-based photocatalysts," Journal of Photochemistry and Photobiology C: Photochemistry Reviews, vol. 9, pp. 157-170, 2008. D. Ravelli, D. Dondi, M. Fagnoni, and A. Albini, "Photocatalysis. A multi-faceted concept for green chemistry," Chemical Society Reviews, vol. 38, pp. 1999-2011, 2009. M. Fagnoni, D. Dondi, D. Ravelli, and A. Albini, "Photocatalysis for the formation of the CC bond," Chemical Reviews, vol. 107, pp. 2725-2756, 2007. A. Maldotti and A. Molinari, "Design of heterogeneous photocatalysts based on metal oxides to control the selectivity of chemical reactions," Top Curr Chem, vol. 303, pp. 185-216, 2011. I. N Lykakis, E. Evgenidou, and M. Orfanopoulos, "Photo-catalysis and Polyoxoanion Decatungstate in Organic Chemistry: A Manifold Concept for Green Chemistry," Current Organic Chemistry, vol. 16, pp. 2400-2414, 2012. A. J. Bard, "Photoelectrochemistry," Science, vol. 207, pp. 139-144, January 11, 1980 1980. Y. Inoue, Y. Asai, and K. Sato, "Photocatalysts with tunnel structures for decomposition of water. Part 1.—BaTi4O9, a pentagonal prism tunnel structure, and its combination with various promoters," J. Chem. Soc., Faraday Trans., vol. 90, pp. 797-802, 1994. K. Domen, M. Hara, J. Kondo, T. Takata, A. Kudo, H. Kobayashi, and Y. Inoue, "New aspects of heterogeneous photocatalysts for water decomposition," Korean Journal of Chemical Engineering, vol. 18, pp. 862-866, 2001/11/01 2001. K. Sayama, K. Mukasa, R. Abe, Y. Abe, and H. Arakawa, "A new photocatalytic water splitting system under visible light irradiation mimicking a Z-scheme mechanism in photosynthesis," Journal of Photochemistry and Photobiology A: Chemistry, vol. 148, pp. 71-77, 2002. J. Irvine, B. Eggins, and J. Grimshaw, "Solar energy fixation of carbon dioxide via cadmium sulphide and other semiconductor photocatalysts," Solar energy, vol. 45, pp. 27-33, 1990. B. Ohtani, S. Adzuma, S.-i. Nishimoto, and T. Kagiya, "Photocatalytic degradation of polyethylene film by incorporated extra-fine particles of titanium dioxide," Polymer degradation and stability, vol. 35, pp. 53-60, 1992. A. Mills, R. H. Davies, and D. Worsley, "Water purification by semiconductor photocatalysis," Chem. Soc. Rev., vol. 22, pp. 417-425, 1993. M. R. Hoffmann, S. T. Martin, W. Choi, and D. W. Bahnemann, "Environmental applications of semiconductor photocatalysis," Chemical Reviews, vol. 95, pp. 69-96, 1995. H. Kominami, M. KOHN, Y. KERA, and B. OHTANI, "Photocatalytic mineralization of acetic acid in aerated aqueous suspension of ultra-highly active titanium (IV) oxide prepared by hydrothermal crystallization in toluene," Chemistry letters, vol. 1996, pp. 1051-1052, 1996. A. H. K. W. T. Fujishima, TiO2 photocatalysis : fundamentals and applications. Tokyo: Bkc, 1999. 31

[36]

[37]

[38] [39] [40]

[41] [42] [43] [44] [45] [46] [47]

[48] [49] [50] [51] [52]

H.-i. Kim, J. Kim, W. Kim, and W. Choi, "Enhanced photocatalytic and photoelectrochemical activity in the ternary hybrid of CdS/TiO2/WO3 through the cascadal electron transfer," The Journal of Physical Chemistry C, vol. 115, pp. 97979805, 2011. A. Kongkanand, K. Tvrdy, K. Takechi, M. Kuno, and P. V. Kamat, "Quantum dot solar cells. Tuning photoresponse through size and shape control of CdSe-TiO2 architecture," Journal of the American Chemical Society, vol. 130, pp. 4007-4015, 2008. S. Fukuzumi and K. Ohkubo, "Metal ion-coupled and decoupled electron transfer," Coordination Chemistry Reviews, vol. 254, pp. 372-385, 2010. F. D'Souza and O. Ito, "Photoinduced electron transfer in supramolecular systems of fullerenes functionalized with ligands capable of binding to zinc porphyrins and zinc phthalocyanines," Coordination Chemistry Reviews, vol. 249, pp. 1410-1422, 2005. S. Fukuzumi, I. Amasaki, K. Ohkubo, C. P. Gros, R. Guilard, and J.-M. Barbe, "Photoinduced electron transfer in supramolecular complexes of a π-extended viologen with porphyrin monomer and dimer," RSC Advances, vol. 2, pp. 3741-3747, 2012. G. Palmisano, V. Augugliaro, M. Pagliaro, and L. Palmisano, "Photocatalysis: a promising route for 21st century organic chemistry," Chemical Communications, pp. 3425-3437, 2007. T. Inoue, T. Watanabe, A. Fujishima, K. i. Honda, and K. Kohayakawa, "Suppression of surface dissolution of CdS photoanode by reducing agents," Journal of The Electrochemical Society, vol. 124, pp. 719-722, 1977. A. Fujishima and K. Honda, "Photolysis-decomposition of water at the surface of an irradiated semiconductor," Nature, vol. 238, pp. 37-38, 1972. S. Rajagopalan, "Nanostructured Materials: Industrial," Handbook of Industrial Chemistry and Biotechnology: Volume 1 and 2, vol. 1, p. 265, 2013. C. Kutal and N. Serpone, Photosensitive metal-organic systems: American Chemical Society, 1993. A. L. Linsebigler, G. Lu, and J. T. Yates Jr, "Photocatalysis on TiO2 surfaces: principles, mechanisms, and selected results," Chemical Reviews, vol. 95, pp. 735758, 1995. A. Sclafani and J. Herrmann, "Comparison of the photoelectronic and photocatalytic activities of various anatase and rutile forms of titania in pure liquid organic phases and in aqueous solutions," The Journal of Physical Chemistry, vol. 100, pp. 1365513661, 1996. M. D. Hernández-Alonso, F. Fresno, S. Suárez, and J. M. Coronado, "Development of alternative photocatalysts to TiO2: challenges and opportunities," Energy & Environmental Science, vol. 2, pp. 1231-1257, 2009. A. Boonstra and C. Mutsaers, "Photohydrogenation of ethyne and ethene on the surface of titanium dioxide," The Journal of Physical Chemistry, vol. 79, pp. 20252027, 1975. Y. Shiraishi, M. Ikeda, D. Tsukamoto, S. Tanaka, and T. Hirai, "One-pot synthesis of imines from alcohols and amines with TiO2 loading Pt nanoparticles under UV irradiation," Chemical Communications, vol. 47, pp. 4811-4813, 2011. P. Cuendet and M. Graetzel, "Direct photoconversion of pyruvate to lactate in aqueous titanium dioxide dispersions," Journal of Physical Chemistry, vol. 91, pp. 654-657, 1987. C. Joyce-Pruden, J. K. Pross, and Y. Li, "Photoinduced reduction of aldehydes on titanium dioxide," The Journal of Organic Chemistry, vol. 57, pp. 5087-5091, 1992. 32

[53] [54] [55] [56] [57] [58] [59] [60]

[61] [62] [63]

[64]

[65] [66]

[67]

Y. Matsushita, S. Kumada, K. Wakabayashi, K. Sakeda, and T. Ichimura, "Photocatalytic reduction in microreactors," Chemistry letters, vol. 35, pp. 410-411, 2006. Y. Matsushita, N. Ohba, S. Kumada, K. Sakeda, T. Suzuki, and T. Ichimura, "Photocatalytic reactions in microreactors," Chemical Engineering Journal, vol. 135, pp. S303-S308, 2008. S. Kohtani, E. Yoshioka, K. Saito, A. Kudo, and H. Miyabe, "Photocatalytic hydrogenation of acetophenone derivatives and diaryl ketones on polycrystalline titanium dioxide," Catalysis Communications, vol. 11, pp. 1049-1053, 2010. J. W. Park, M. J. Hong, and K. K. Park, "Photochemical Reduction of 1, 2-Diketones in the Presence of TiO2," Bulletin-Korean Chemical Society, vol. 22, pp. 1213-1216, 2001. J. W. Park, E. K. Kim, and K. K. Park, "Photochemical Reductions of Benzil and Benzoin in the Presence of Triethylamine and TiO2 Photocatalyst," Bulletin-Korean Chemical Society, vol. 23, pp. 1229-1234, 2002. M. Anpo, S. Dohshi, M. Kitano, Y. Hu, M. Takeuchi, and M. Matsuoka, "The preparation and characterization of highly efficient titanium oxide-based photofunctional materials," Annu. Rev. Mater. Res., vol. 35, pp. 1-27, 2005. M. Anpo, N. Aikawa, S. Kodama, and Y. Kubokawa, "Photocatalytic hydrogenation of alkynes and alkenes with water over titanium dioxide. Hydrogenation accompanied by bond fission," The Journal of Physical Chemistry, vol. 88, pp. 2569-2572, 1984. C. Yun, M. Ampo, S. Kodama, and Y. Kubokawa, "Uv irradiation-induced fission of a C [double bond, length half m-dash] C or C [triple bond, length half m-dash] C bond adsorbed on TiO2," Journal of the Chemical Society, Chemical Communications, pp. 609-609, 1980. M. Anpo, T. Shima, S. Kodama, and Y. Kubokawa, "Photocatalytic hydrogenation of propyne with water on small-particle titania: size quantization effects and reaction intermediates," Journal of Physical Chemistry, vol. 91, pp. 4305-4310, 1987. H. Yamataka, N. Seto, J. Ichihara, T. Hanafusa, and S. Teratani, "Reduction of C–C multiple bonds using an illuminated semiconductor catalyst," Journal of the Chemical Society, Chemical Communications, pp. 788-789, 1985. S. I. Nishimoto, B. Ohtani, T. Yoshikawa, and T. Kagiya, "Photocatalytic conversion of primary amines to secondary amines and cyclization of polymethylene-diamines by an aqueous suspension of TiO2/Pt," Journal of the American Chemical Society, vol. 105, pp. 7180-7182, 1983. B. Ohtani, Y. Goto, S.-I. Nishimoto, and T. Inui, "Photocatalytic transfer hydrogenation of Schiff bases with propan-2-ol by suspended semiconductor particles loaded with platinum deposits," J. Chem. Soc., Faraday Trans., vol. 92, pp. 42914295, 1996. R. Baba, S. Nakabayashi, A. Fujishima, and K. Honda, "Photocatalytic hydrogenation of ethylene on bimetal-deposited semiconductor powders," Journal of the American Chemical Society, vol. 109, pp. 2273-2277, 1987. P. Kittisakmontree, B. Pongthawornsakun, H. Yoshida, S.-i. Fujita, M. Arai, and J. Panpranot, "The liquid-phase hydrogenation of 1-heptyne over Pd–Au/TiO2 catalysts prepared by the combination of incipient wetness impregnation and deposition– precipitation," Journal of Catalysis, vol. 297, pp. 155-164, 2013. G. Díaz, A. Gómez-Cortés, O. Hernández-Cristobal, J. J. Murcia, G. Borda, and H. Rojas, "Hydrogenation of citral over IrAu/TiO2 catalysts. Effect of the preparation method," Topics in Catalysis, vol. 54, pp. 467-473, 2011.

33

[68]

[69] [70]

[71]

[72] [73] [74] [75] [76] [77]

[78] [79] [80] [81] [82]

M. Bidaoui, C. Especel, N. Bouchenafa-Saib, D. Duprez, O. Mohammedi, and S. Royer, "Citral hydrogenation on high surface area mesoporous TiO2–SiO2 supported Pt nanocomposites: Effect of titanium loading and reduction temperature on the catalytic performances," Applied Catalysis A: General, vol. 445–446, pp. 14-25, 2012. S. Kodama, H. Nakaya, M. Anpo, and Y. Kubokawa, "A common factor determining the features of the photocatalytic hydrogenation and isomerization of alkenes over TiSi oxides," Bulletin of the Chemical Society of Japan, vol. 58, pp. 3645-3646, 1985. M. Anpo, H. Yamashita, Y. Ichihashi, Y. Fujii, and M. Honda, "Photocatalytic reduction of CO2 with H2O on titanium oxides anchored within micropores of zeolites: effects of the structure of the active sites and the addition of Pt," The Journal of Physical Chemistry B, vol. 101, pp. 2632-2636, 1997. H. Yamashita, S. Kawasaki, Y. Ichihashi, M. Takeuchi, M. Harada, M. Anpo, C. Louis, and M. Che, "Characterization of Ti/Si binary oxides prepared by the sol-gel method and their photocatalytic properties: The hydrogenation and hydrogenolysis of CH3CCH with H2O," Korean Journal of Chemical Engineering, vol. 15, pp. 491-495, 1998/09/01 1998. L. Lin and R. R. Kuntz, "Photocatalytic hydrogenation of acetylene by molybdenumsulfur complexes supported on titania," Langmuir, vol. 8, pp. 870-875, 1992. R. R. Kuntz, "Comparative Study of Mo2OxSy(cys)22- Complexes as Catalysts for Electron Transfer from Irradiated Colloidal TiO2 to Acetylene," Langmuir, vol. 13, pp. 1571-1576, 1997/03/01 1997. S. Flores, O. Rios-Bernij, M. Valenzuela, I. Córdova, R. Gómez, and R. Gutiérrez, "Photocatalytic reduction of nitrobenzene over titanium dioxide: by-product identification and possible pathways," Topics in Catalysis, vol. 44, pp. 507-511, 2007. Z. Zand, F. Kazemi, and A. Safari, "Photocatalytic Reduction of Nitroaromatic Compounds Using TiO2 Nanoparticle by Solar Light." J. Kumar, M. M. Ho, and T. Toyokuni, "Simple and chemoselective reduction of aromatic nitro compounds to aromatic amines: reduction with hydriodic acid revisited," Tetrahedron Letters, vol. 42, pp. 5601-5603, 2001. K. Imamura, S.-i. Iwasaki, T. Maeda, K. Hashimoto, B. Ohtani, and H. Kominami, "Photocatalytic reduction of nitrobenzenes to aminobenzenes in aqueous suspensions of titanium (IV) oxide in the presence of hole scavengers under deaerated and aerated conditions," Physical Chemistry Chemical Physics, vol. 13, pp. 5114-5119, 2011. H. Wang, J. Yan, W. Chang, and Z. Zhang, "Practical synthesis of aromatic amines by photocatalytic reduction of aromatic nitro compounds on nanoparticles N-doped TiO< sub> 2," Catalysis Communications, vol. 10, pp. 989-994, 2009. S. Füldner, R. Mild, H. I. Siegmund, J. A. Schroeder, M. Gruber, and B. König, "Green-light photocatalytic reduction using dye-sensitized TiO2 and transition metal nanoparticles," Green Chemistry, vol. 12, pp. 400-406, 2010. S. Füldner, T. Mitkina, T. Trottmann, A. Frimberger, M. Gruber, and B. König, "Urea derivatives enhance the photocatalytic activity of dye-modified titanium dioxide," Photochemical & Photobiological Sciences, vol. 10, pp. 623-625, 2011. H. Tada, K. Teranishi, Y.-i. Inubushi, and S. Ito, "Ag nanocluster loading effect on TiO2 photocatalytic reduction of bis (2-dipyridyl) disulfide to 2-mercaptopyridine by H2O," Langmuir, vol. 16, pp. 3304-3309, 2000. M. C. Canela, R. M. Alberici, and W. F. Jardim, "Gas-phase destruction of H2S using TiO2/UV-VIS," Journal of Photochemistry and Photobiology A: Chemistry, vol. 112, pp. 73-80, 1998.

34

[83] [84]

[85]

[86] [87] [88]

[89] [90] [91]

[92] [93] [94]

[95] [96] [97]

C. Linkous, N. Muradov, and S. Ramser, "Consideration of reactor design for solar hydrogen production from hydrogen sulfide using semiconductor particulates," International Journal of Hydrogen Energy, vol. 20, pp. 701-709, 1995. N. Serpone, E. Borgarello, and M. Grätzel, "Visible light induced generation of hydrogen from H2S in mixed semiconductor dispersions; improved efficiency through inter-particle electron transfer," Journal of the Chemical Society, Chemical Communications, pp. 342-344, 1984. K. Schoumacker, C. Geantet, M. Lacroix, E. Puzenat, C. Guillard, and J.-M. Herrmann, "Photocatalytic synthesis of thio-organic compounds: case study of propan-1-thiol," Journal of Photochemistry and Photobiology A: Chemistry, vol. 152, pp. 147-153, 2002. R. Molinari, A. Caruso, and T. Poerio, "Direct benzene conversion to phenol in a hybrid photocatalytic membrane reactor," Catalysis Today, vol. 144, pp. 81-86, 2009. G. Zhang, J. Yi, J. Shim, J. Lee, and W. Choi, "Photocatalytic hydroxylation of benzene to phenol over titanium oxide entrapped into hydrophobically modified siliceous foam," Applied Catalysis B: Environmental, vol. 102, pp. 132-139, 2011. Z. Zheng, B. Huang, X. Qin, X. Zhang, Y. Dai, and M.-H. Whangbo, "Facile in situ synthesis of visible-light plasmonic photocatalysts M@TiO2 (M= Au, Pt, Ag) and evaluation of their photocatalytic oxidation of benzene to phenol," Journal of Materials Chemistry, vol. 21, pp. 9079-9087, 2011. M. Weber, M. Weber, and M. Kleine-Boymann, "Phenol," in Ullmann's Encyclopedia of Industrial Chemistry, ed: Wiley-VCH Verlag GmbH & Co. KGaA, 2000. H. Rezala, H. Khalaf, J. L. Valverde, A. Romero, A. Molinari, and A. Maldotti, "Photocatalysis with Ti-pillared clays for the oxofunctionalization of alkylaromatics by O2," Applied Catalysis A: General, vol. 352, pp. 234-242, 2009. F.-L. Cao, J.-G. Wang, F.-J. Lv, D.-Q. Zhang, Y.-N. Huo, G.-S. Li, H.-X. Li, and J. Zhu, "Photocatalytic oxidation of toluene to benzaldehyde over anatase TiO2 hollow spheres with exposed {001} facets," Catalysis Communications, vol. 12, pp. 946-950, 2011. W. Feng, G. Wu, L. Li, and N. Guan, "Solvent-free selective photocatalytic oxidation of benzyl alcohol over modified TiO2," Green Chemistry, vol. 13, pp. 3265-3272, 2011. R. Amadelli, L. Samiolo, A. Maldotti, A. Molinari, and D. Gazzoli, "Selective Photooxidation and Photoreduction Processes at surface modified by vanadyl" International Journal of Photoenergy, vol. 2011, 2011. D. Tsukamoto, Y. Shiraishi, Y. Sugano, S. Ichikawa, S. Tanaka, and T. Hirai, "Gold nanoparticles located at the interface of anatase/rutile TiO2 particles as active plasmonic photocatalysts for aerobic oxidation," Journal of the American Chemical Society, vol. 134, pp. 6309-6315, 2012. K.-I. Shimizu, T. Kaneko, T. Fujishima, T. Kodama, H. Yoshida, and Y. Kitayama, "Selective oxidation of liquid hydrocarbons over photoirradiated TiO2 pillared clays," Applied Catalysis A: General, vol. 225, pp. 185-191, 2002. Y. Kitayama, T. Kodama, M. Abe, H. Shimotsuma, and Y. Matsuda, "Synthesis of titania pillared saponite in aqueous solution of acetic acid," Journal of Porous Materials, vol. 5, pp. 121-126, 1998. T. Kaneko, M. Fujii, T. Kodama, and Y. Kitayama, "Synthesis of titania pillared mica in aqueous solution of acetic acid," Journal of Porous Materials, vol. 8, pp. 99-109, 2001.

35

[98] [99]

[100] [101] [102] [103]

[104]

[105] [106] [107] [108] [109]

[110]

[111] [112]

T. Ohno, T. Mitsui, and M. Matsumura, "TiO2-photocatalyzed oxidation of adamantane in solutions containing oxygen or hydrogen peroxide," Journal of Photochemistry and Photobiology A: Chemistry, vol. 160, pp. 3-9, 2003. A. M. Balu, B. Baruwati, E. Serrano, J. Cot, J. Garcia-Martinez, R. S. Varma, and R. Luque, "Magnetically separable nanocomposites with photocatalytic activity under visible light for the selective transformation of biomass-derived platform molecules," Green Chemistry, vol. 13, pp. 2750-2758, 2011. T. Sakata, T. Kawai, and K. Hashimoto, "Heterogeneous photocatalytic reactions of organic acids and water. New reaction paths besides the photo-Kolbe reaction," The Journal of Physical Chemistry, vol. 88, pp. 2344-2350, 1984. J. C. Colmenares, A. Magdziarz, and A. Bielejewska, "High-value chemicals obtained from selective photo-oxidation of glucose in the presence of nanostructured titanium photocatalysts," Bioresource Technology, vol. 102, pp. 11254-11257, 2011. J. C. Colmenares and A. Magdziarz, "Room temperature versatile conversion of biomass-derived compounds by means of supported TiO2 photocatalysts," Journal of Molecular Catalysis A: Chemical, vol. 366, pp. 156-162, 2013. J. C. Colmenares, A. Magdziarz, K. Kurzydlowski, J. Grzonka, O. Chernyayeva, and D. Lisovytskiy, "Low-temperature ultrasound-promoted synthesis of Cr–TiO2supported photocatalysts for valorization of glucose and phenol degradation from liquid phase," Applied Catalysis B: Environmental, vol. 134–135, pp. 136-144, 2013. J. C. Colmenares, A. Magdziarz, O. Chernyayeva, D. Lisovytskiy, K. Kurzydłowski, and J. Grzonka, "Sonication‐Assisted Low‐Temperature Routes for the Synthesis of Supported Fe–TiO2 Econanomaterials: Partial Photooxidation of Glucose and Phenol Aqueous Degradation," ChemCatChem, vol. 5, pp. 2270-2277, 2013. B. Dindar and S. Içli, "Unusual photoreactivity of zinc oxide irradiated by concentrated sunlight," Journal of Photochemistry and Photobiology A: Chemistry, vol. 140, pp. 263-268, 2001. C. Hariharan, "Photocatalytic degradation of organic contaminants in water by ZnO nanoparticles: Revisited," Applied Catalysis A: General, vol. 304, pp. 55-61, 2006. H. Kisch, "Semiconductor photocatalysis for organic synthesis," Advances in Photochemistry, Volume 26, pp. 93-143, 2007. R. Künneth, C. Feldmer, F. Knoch, and H. Kisch, "Semiconductor-Catalysed Photoaddition of Olefins and Enol Ethers to 1,2-Diazenes: A New Route to Allylhydrazines," Chemistry – A European Journal, vol. 1, pp. 441-448, 1995. S. Yanagida, Y. Ishimaru, Y. Miyake, T. Shiragami, C. Pac, K. Hashimoto, and T. Sakata, "Semiconductor photocatalysis. 8. Zinc sulfide-catalyzed photoreduction of aldehydes and related derivatives: two-electron-transfer reduction and relationship with spectroscopic properties," The Journal of Physical Chemistry, vol. 93, pp. 25762582, 1989. S. Yanagida, M. Yoshiya, T. Shiragami, C. Pac, H. Mori, and H. Fujita, "Semiconductor photocatalysis. I. Quantitative photoreduction of aliphatic ketones to alcohols using defect-free zinc sulfide quantum crystallites," Journal of Physical Chemistry, vol. 94, pp. 3104-3111, 1990. M. D. Tzirakis, I. N. Lykakis, and M. Orfanopoulos, "Decatungstate as an efficient photocatalyst in organic chemistry," Chemical Society Reviews, vol. 38, pp. 26092621, 2009. C. L. Hill, "Introduction of Functionality into Unactivated Carbon‐Hydrogen Bonds. Catalytic Generation and Nonconventional Utilization of Organic Radicals," ChemInform, vol. 26, 1995.

36

[113] C. Hill, "Alkanes to nitriles and α-iminoesters. Polyoxotungstate photocatalytic radical chain initiation," Chemical Communications, pp. 2467-2468, 1998. [114] D. Dondi, M. Fagnoni, A. Molinari, A. Maldotti, and A. Albini, "Polyoxotungstate photoinduced alkylation of electrophilic alkenes by cycloalkanes," Chemistry-A European Journal, vol. 10, pp. 142-148, 2004. [115] D. Dondi, M. Fagnoni, and A. Albini, "Tetrabutylammonium Decatungstate‐Photosensitized Alkylation of Electrophilic Alkenes: Convenient Functionalization of Aliphatic C-H Bonds," Chemistry-A European Journal, vol. 12, pp. 4153-4163, 2006. [116] S. Esposti, D. Dondi, M. Fagnoni, and A. Albini, "Acylation of Electrophilic Olefins through Decatungstate‐Photocatalyzed Activation of Aldehydes," Angewandte Chemie, vol. 119, pp. 2583-2586, 2007. [117] M. D. Tzirakis and M. Orfanopoulos, "Acyl Radical Reactions in Fullerene Chemistry: Direct Acylation of [60]Fullerene through an Efficient DecatungstatePhotomediated Approach," Journal of the American Chemical Society, vol. 131, pp. 4063-4069, 2009/03/25 2009. [118] S. Angioni, D. Ravelli, D. Emma, D. Dondi, M. Fagnoni, and A. Albini, "Tetrabutylammonium Decatungstate (Chemo) selective Photocatalyzed, Radical C-H Functionalization in Amides," Advanced Synthesis & Catalysis, vol. 350, pp. 22092214, 2008. [119] D. Ravelli, A. Albini, and M. Fagnoni, "Smooth Photocatalytic Preparation of 2‐Substituted 1, 3‐Benzodioxoles," Chemistry-A European Journal, vol. 17, pp. 572579, 2011. [120] D. Ravelli, S. Montanaro, M. Zema, M. Fagnoni, and A. Albini, "A Tin‐Free, Radical Photocatalyzed Addition to Vinyl Sulfones," Advanced Synthesis & Catalysis, vol. 353, pp. 3295-3300, 2011. [121] D. Ravelli, S. Montanaro, C. Tomasi, P. Galinetto, E. Quartarone, D. Merli, P. Mustarelli, and M. Fagnoni, "One‐Step Decatungstate‐Photomediated PEGylation of Single‐Walled Carbon Nanotubes," ChemPlusChem, vol. 77, pp. 210-216, 2012. [122] C. Tanielian, K. Duffy, and A. Jones, "Kinetic and mechanistic aspects of photocatalysis by polyoxotungstates: a laser flash photolysis, pulse radiolysis, and continuous photolysis study," The Journal of Physical Chemistry B, vol. 101, pp. 4276-4282, 1997. [123] C. Giannotti and C. Richter, "Aerobic photolysis of saturated C-H bond with decatungstate anion, mechanism and products," Trends in photochemistry & photobiology, vol. 4, pp. 43-54, 1997. [124] A. Maldotti, R. Amadelli, V. Carassiti, and A. Molinari, "Catalytic oxygenation of cyclohexane by photoexcited (nBu4N)4W10O32: the role of radicals," Inorganica Chimica Acta, vol. 256, pp. 309-312, 1997. [125] I. N. Lykakis and M. Orfanopoulos, "Deuterium kinetic isotope effects in homogeneous decatungstate catalyzed photooxygenation of 1, 1-diphenylethane and 9-methyl-9-H-fluorene: evidence for a hydrogen abstraction mechanism," Tetrahedron Letters, vol. 46, pp. 7835-7839, 2005. [126] C. Tanielian, I. N. Lykakis, R. Seghrouchni, F. Cougnon, and M. Orfanopoulos, "Mechanism of decatungstate photocatalyzed oxygenation of aromatic alcohols: Part I. Continuous photolysis and laser flash photolysis studies," Journal of Molecular Catalysis A: Chemical, vol. 262, pp. 170-175, 2007. [127] A. Molinari, R. Amadelli, V. Carassiti, and A. Maldotti, "Photocatalyzed Oxidation of Cyclohexene and Cyclooctene with (nBu 4N)4W10O32 and (nBu4N)4W10O32/FeIII[meso-Tetrakis(2,6-dichlorophenyl)porphyrin] in 37

[128] [129]

[130]

[131]

[132] [133]

[134]

[135] [136] [137] [138] [139]

[140] [141]

Homogeneous and Heterogeneous Systems," European Journal of Inorganic Chemistry, vol. 2000, pp. 91-96, 2000. I. N. Lykakis and M. Orfanopoulos, "Lone selectivity of the decatungstate-sensitized photooxidation of 1-substituted cycloalkenes," Synlett, vol. 2004, pp. 2131-2134, 2004. A. Maldotti, A. Molinari, G. Varani, M. Lenarda, L. Storaro, F. Bigi, R. Maggi, A. Mazzacani, and G. Sartori, "Immobilization of (nBu4N)4W10O32 on mesoporous MCM-41 and amorphous silicas for photocatalytic oxidation of cycloalkanes with molecular oxygen," Journal of Catalysis, vol. 209, pp. 210-216, 2002. A. Molinari, R. Amadelli, L. Andreotti, and A. Maldotti, "Heterogeneous photocatalysis for synthetic purposes: oxygenation of cyclohexane with H3PW12O40 and (nBu 4N)4W10O32 supported on silica," Journal of the Chemical Society, Dalton Transactions, pp. 1203-1204, 1999. A. Molinari, R. Amadelli, A. Mazzacani, G. Sartori, and A. Maldotti, "Tetralkylammonium and sodium decatungstate heterogenized on silica: effects of the nature of cations on the photocatalytic oxidation of organic substrates," Langmuir, vol. 18, pp. 5400-5405, 2002. A. Maldotti, R. Amadelli, I. Vitali, L. Borgatti, and A. Molinari, "CH2Cl2-assisted functionalization of cycloalkenes by photoexcited (nBu4N)4W10O32 heterogenized on SiO2," Journal of Molecular Catalysis A: Chemical, vol. 204–205, pp. 703-711, 2003. M. D. Tzirakis, I. N. Lykakis, G. D. Panagiotou, K. Bourikas, A. Lycourghiotis, C. Kordulis, and M. Orfanopoulos, "Decatungstate catalyst supported on silica and γ alumina: Efficient photocatalytic oxidation of benzyl alcohols," Journal of Catalysis, vol. 252, pp. 178-189, 2007. S. Farhadi and Z. Momeni, "Zirconia-supported sodium decatungstate (Na4 W10O32/ZrO2: An efficient, green and recyclable photocatalyst for selective oxidation of activated alcohols to carbonyl compounds with O2," Journal of Molecular Catalysis A: Chemical, vol. 277, pp. 47-52, 2007. F. Bigi, A. Corradini, C. Quarantelli, and G. Sartori, "Silica-bound decatungstates as heterogeneous catalysts for H2O2 activation in selective sulfide oxidation," Journal of Catalysis, vol. 250, pp. 222-230, 2007. A. Maldotti, A. Molinari, and F. Bigi, "Selective photooxidation of diols with silica bound," Journal of Catalysis, vol. 253, pp. 312-317, 2008. A. Molinari, A. Maldotti, A. Bratovcic, and G. Magnacca, "Photocatalytic properties of sodium decatungstate supported on sol–gel silica in the oxidation of glycerol," Catalysis Today, vol. 206, pp. 46-52, 2013. E. Fornal and C. Giannotti, "Photocatalyzed oxidation of cyclohexane with heterogenized decatungstate," Journal of Photochemistry and Photobiology A: Chemistry, vol. 188, pp. 279-286, 2007. A. Molinari, G. Varani, E. Polo, S. Vaccari, and A. Maldotti, "Photocatalytic and catalytic activity of heterogenized W10O324-in the bromide-assisted bromination of arenes and alkenes in the presence of oxygen," Journal of Molecular Catalysis AChemical, vol. 262, pp. 156-163, 2007. M. Bonchio, M. Carraro, G. Scorrano, E. Fontananova, and E. Drioli, "Heterogeneous photooxidation of alcohols in water by photocatalytic membranes incorporating decatungstate," Advanced Synthesis & Catalysis, vol. 345, pp. 1119-1126, 2003. M. Carraro, M. Gardan, G. Scorrano, E. Drioli, E. Fontananova, and M. Bonchio, "Solvent-free, heterogeneous photooxygenation of hydrocarbons by Hyflon® membranes embedding a fluorous-tagged decatungstate," Chem. Commun., pp. 45334535, 2006. 38

[142] H. Shi, T. Zhang, B. Li, X. Wang, M. He, and M. Qiu, "Photocatalytic hydroxylation of phenol with Fe–Al-silicate photocatalyst: A clean and highly selective synthesis of dihydroxybenzenes," Catalysis Communications, vol. 12, pp. 1022-1026, 2011. [143] T. Koike and M. Akita, "Photoinduced Oxyamination of Enamines and Aldehydes with TEMPO Catalyzed by [Ru (bpy)(3)](2+)," Chemistry letters, vol. 38, pp. 166167, 2009. [144] R. S. Andrews, J. J. Becker, and M. R. Gagné, "Intermolecular addition of glycosyl halides to alkenes mediated by visible light," Angewandte Chemie International Edition, vol. 49, pp. 7274-7276, 2010. [145] C. Dai, J. M. Narayanam, and C. R. Stephenson, "Visible-light-mediated conversion of alcohols to halides," Nature chemistry, vol. 3, pp. 140-145, 2011.

39

P25 TiO2

O

+ 2e + 2H

O

+

+

MeOH/H2O (4/1)

O

O

Camphorquinone

OH

2- endo 35% O

OH

3-endo 46% OH

P25 TiO2

CH3 O

+

2e + 2H+

CH3

0.5 M TEA in MeOH

+

O

1-phenyl-1,2-propanedione 56 %

OH

O

P25 TiO2

+ O

Benzil

2e + 2H+

CH3 CN/CH3OH/H2O/TEA (88/7/2/3)

O

85 %

Scheme 1. Photocatalytic hydrogenation of diketonic compounds on P25 TiO2 powder [56, 57].

40

Pt /TiO2

Unloaded TiO2

e- + H+

e- + H+

H (On Pt)

h+ + OH

h+ + OH-

OH (On Pt)

H3C

C

CH

H

(Close existence near photon absorption)

OH

Photohydrogenolysis C C Cleavage

2H

CH4 + H3C

CH3

Photohydrogenation H3C

C H

CH2

2H

H3C

H2 C

CH3

Scheme 2. Photocatalytic hydrogenation on bare and Pt loaded TiO2 [58-60] .

41

RH2 + h+ (OH)

RH + H+(H2O)

O2 + e-(+H+)

O2 (HO2)

RH + O2

RHOO

RHOO + e- + H+

RO + H2O

RHOO + O2 + H+

RO + O2 + H2O

2RHOO RO + RHOH + O2 Scheme 3. Photocatalytic oxidation of C–H bonds of hydrocarbons [90].

N

N

hv, ZnS

Azobenzene

N

N

O

MeOH

NH N

+

NHNH

5%

90%

O

1-(3,4-dihydro-2H-pyran-4-yl)-1,2-diphenylhydrazine

hv, Pt/ZnS NHNH

O

+

NH N

MeOH 90% Hydroazobenzene

5%

O

Scheme 4. Photocatalytic hydrogenation of azobenzene to hydrazobenzen using ZnS or CdS [107, 108].

42

RCN Cynation

O

R

CO2CH3

alpha ketoesters

R Vinylation

NCCO2CH3 High Temp. CH

NCCO2CH3 low Temp.

RCOCH3 Acetylation CH3CN, H2O

HC

H2C

RH

CH2

R Ethylation

EWG

CO

EWG

Less substituted alkenes RCHO Carbonylation

R

Alkylation of electrophilic alkenes

Scheme 5. Functionalization of unactivated C–H bonds in alkanes photocatalyzed by decatungstate under anaerobic conditions [111, 114, 115].

43

O

hv W10O32-4

CH3

C

+

N

H

R1

CH3

R2

O

47-86% H

N

R

CH3

O

C H

hv W10O32-4

H N

+ CH3

46%

COOMe

O H N

COOMe CH3

R1 = R2 = COOMe R1 = H, R2 = COOEt, CN, COMe

Scheme 6. Decatungstate-mediated amidation of electron-poor olefins [111, 118].

44

OH H3C

C H

O

hv, O2

CH3

W10O32-4

H3C

C

H

+

H2O2

OOH OOH

hv, O2

+

W10O32-4

(CH3O)3P

OH

OH

+

1- adamantol 80%

2-adamantol 20%

O

hv, O2

OH

+

W10O32-4

Scheme 7. Decatungstate-catalyzed oxidation of aliphatic alkanes and alcohols [122, 123].

45

hv, O2

H

OOH(OH)

W10O32-4 R

R

3 h, 90-100%

R

R

OH CH

O R

hv, O2

W10O32

C

R

-4

20 mints, 20-47%

R = H, Me

Scheme 8. Decatungstate-catalyzed oxidation of aromatic alkanes and alcohols [125].

46

OH

O R3

R3

hv, O2 R2

W10O32-4/silica or Al2O3 Upto 99%

R1 H

R1

R1 = H, CH3, Ph R2 = H, CH3, Ph R3 = CH3, Ph OH

R2

H O

hv, O2 W10O32-4/silica or Al2O3 > 99%

R

OH

R

R - H, CF3, CH3, OCH3

Scheme 9. Heterogeneous decatungstate-photocatalyzed oxidation of secondary and primary benzyl alcohols [133].

47

RH O

Amb/W10O32-4 O2. hv ROOH Br

-

Amb/W10O32-4

Br+

Br

OH (or -OCH3) OH (or OCH3)

Scheme 10. Bromide assisted bromination of arenes and alkenes via heterogeneous decatungstate catalysis in the presence of oxygen [139].

48

OH

O

hv, O2 -4 W10O32 / PVDF or PDMS upto 99%

n

PVDF =

F

H

C

C

F

H

n

i

CH3

PDMS =

CH2

Si CH3

O

Si

CH3 O

H

Si

CH3 O

Si

CH3 CH2 CH2

CH3

Si CH3

Scheme 11. Heterogeneous photo oxidation of alcohols in water by photocatalytic membranes incorporating decatungstate [140].

49

Highlights •

Methodologies for the development of photocatalysts have been critically discussed

•

Focus is on the selective organic transformations using photocatalysts

•

The objective is to overcome the existing limitations of photocatalysis

•

More complex and unsafe procedures can be made simple and safe by photocatalysts

•

Photocatalysts for efficient, selective, environmental friendly and high yield synthesis

50

Graphical Abstract

O

RH

W10O32-4*

N O

e-

Conduction band R TiO2

NH2

W10O32-5H+

W10O32-4

h+ Valence Band

EWG

hv hv

EWG

EWG

Sun light

hv n-(Bu4N)4W10O32 O

O +

R

C

H

(R = alkyl or aryl)

C R H

Photocatalysts using renewable solar light have brought revolution in the synthesis of organic compounds. The activity and selectivity of photocatalysts can be enhanced by suitable modifications.

51