Photocatalytic process is a well-known reaction in photosynthesis by plants and algae. Artificial photosynthesis is a chemical process that mimics the natural plant photosynthesis to make important chemicals by using man-made materials. One of the most promising methods of artificial photosynthesis is synthesis of organic chemicals, including biodegradable plastics, pharmaceutical drugs, liquid fuels and intermediates for valuable chemicals, etc. In 1972, Fujishima and Honda discovered photocatalytic process using TiO2 semiconductor oxide electrodes to generate hydrogen from water. Researchers have achieved a single-step system that uses semiconductor particles for organic fine chemical synthesis under UV or visible radiation. This chapter summarizes the recent research trends on artificial photosynthesis by photocatalytic process for organic fine chemical synthesis on selected photocatalytic organic transformations, especially photocatalytic transformations by oxidation, carbon-carbon and carbon-heteroatom coupling, cyclization, etc.
- artificial photosynthesis
- organic synthesis
- UV light
- visible light
- C-C bond formation
The Earth receives energy of about 3 ×1024 joules in a year from sun, which is higher than the global energy consumption. Plants utilize the solar energy to fix atmospheric carbon dioxide and produce carbohydrates and oxygen by photosynthesis. Warburg and Negelein disclosed the photosynthesis, in which reaction occurs between CO2 and H2O in the presence of chlorophyll and form O2 and carbohydrates . It is worth to utilize the solar energy for various energy-consuming processes, since it is clean, sustainable and abundant resource. Artificial photosynthesis is a process in which fundamental scientific principles of natural photosynthesis are applied to design a solar energy conversion to make important chemicals by using man-made materials. Many approaches are reported. One of the successful reports provides an outline for solar energy storage in fuels . Another important approach is finding a new route for the chemical production and synthesis by utilizing solar energy, which is an energy-decisive industrial process. The method to synthesize organic chemical by photochemical process was first reported by Giacomo Ciamician in 1912 . Presently, photochemistry is a well-developed field of chemistry and the majority of the experiments use direct excitation of molecules by UV light . To activate the photoreaction under visible light, sensitizers are applied, which transfer energy or an electron from the excited state to the molecule to be converted. Various homogeneous sensitizers are successfully used for various organic transformations . The homogeneous sensitizers are substituted by heterogeneous photosensitizers using photocatalytic process and which are easily separable and therefore recycled. In 1972, Fujishima and Honda reported a photocatalytic process using TiO2 semiconductor oxide e1ectrodes to generate hydrogen from water . Semiconductors can act as photocatalysts for light-induced chemical transformations because of their unique electronic structure, which is characterized by a filled valence band and an empty conduction band. When a photon with an energy of hν matches or exceeds the band gap energy (Eg) of the semiconductor (SC), an electron in the valence band (VB) is promoted to the conduction band (CB), leaving a positive hole (h+) in VB. The photo-excitation can be written as :
Where SC is semiconductor, vb and cb represent the valance band and the conduction band, respectively. The reactive species, h+ and e− are powerful oxidizing and reducing agents, respectively. Subsequently, various oxidizing species such as •OH , O2•− , various forms of active oxygen species, such as HO2, H2O2 and O, are produced [8,9].
The photo-excitation process over semiconductor photocatalyst is presented in Figure 1.
Different semiconductors (e.g., TiO2, ZnO, α-Fe2O3 and WO3) are considered for their potential use as photocatalysts. In early 1980s, great effort was placed on organic synthesis by semiconductor photocatalysis . Photocatalysis in synthetic route has attracted many researchers, because this method presents a greener approach to organic synthesis [11,12]. Recent studies have revealed that highly selective redox reactions could be achieved by visible light irradiation. The studies employed in photocatalytic organic transformation includes, oxidation, reduction, carbon-carbon and carbon-hetero atom coupling, cyclization, isomerization, etc. This chapter deals with various studies performed in oxidation, carbon-carbon and carbon-heteroatom coupling, cyclization by UV and visible light-induced photocatalysis.
In photocatalytic process, photo-oxidation is the most studied because the VB edge in most of the semiconductor catalysts have more positive than the oxidation potential of the functional group in the organic compounds. The oxidation reactions are mainly focused on the oxidation of alcohols, amines, cyclohexane and aromatic alkanes. The conversion and product selectivity could be controlled by tuning the reaction conditions, such as nature of solvent, excitation wavelength of the light, interface engineering such as surface modification to change the adsorption mode or electron transfer pathway.
2. Oxidation of hydrocarbon
Selective partial oxidation of hydrocarbon into its oxidation products, such as aldehyde, ketone, and carboxylic acid, by photocatalytic process is of great importance for chemical industries. Selective oxidation of methane into its useful oxygenates such as methanol and formaldehyde by photocatalytic method is of great challenge. Gondala et al. , for the first time, reported the photocatalytic transformation of methane into methanol over WO3, TiO2 (rutile) and NiO semiconductor photocatalysts, at room temperature, under the irradiation of a strong UV laser beam at 355 nm. The methanol yield in all three catalysts observed was low due to the degradation of methanol immediately after its formation in the aqueous suspension. Methane activation at low temperature and at atmospheric pressure using supported molybdena TiO2 catalyst  excited by band gap illumination motivated the researchers to test this reaction with various catalysts [15,16]. However, the conversion of methane was low and an advancement in the source of methane such as methane hydrates was used. Methane hydrates available in the ocean at depths between ~280 and 4000 m have higher concentration of methane in water, which is higher than that of water–methane pressurized systems. Comparison of photocatalytic conversion of methane dissolved in water and methane hydrate showed that the conversion of methane dissolved in water was observed at temperatures below 70°C. On the other hand, the photocatalytic conversions of the methane hydrate occurred at temperatures below −5°C . The comparison of the photocatalytic reaction under visible and the full spectrum of UV-visible showed that the production of methanol was 50% greater in visible compared to that with the full spectrum lamp of UV-visible. Complete oxidation of methane could be avoided by changing the atmospheric condition. V-MCM-41 (acid) catalyst prepared under acidic conditions resulted in the selective formation of methanol with NO oxidant compared to O2 as oxidant under UV radiation accompanied by the formation of trace amounts of CO2 and acetaldehyde. Metal doping is used to improve the photocatalytic activity by avoiding charge recombination and to achieve visible-driven photocatalysis. Silver-impregnated WO3 was studied for the conversion of methane to methanol under laser illumination (100 mJ) in the aqueous medium . The overall photonic efficiency of photocatalytic conversion process based on hydroxyl radicals is ∼8%. The remaining portion of photogenerated hydroxyl radicals is consumed in oxygen formation, interaction with methanol, interaction with methanol by-products such as formaldehyde and recombination.
Photocatalytic oxidation of cyclohexane to cyclohexanol and cyclohexanone is an important commercial reaction, used as precursors in the synthesis of adipic acid and caprolactam, in turn used in the manufacture of nylon 66 and nylon 6, respectively. The selective oxidation of cyclohexane to cyclohexanol and cyclohexanone both under liquid and gaseous phase at room temperature and pressure by photocatalytic process was studied [10,19–21]. The product selectivity depends on the nature of the catalyst, irradiation wavelength and presence of solvent and O2, etc. The comparison of oxidation of cyclohexane with and without TiO2 photocatalyst showed that under photolytic oxidation at
3. Oxidation of alcohol
Selective oxidation of various primary and secondary alcohols in a gas-phase photochemical reactor using immobilized TiO2 catalyst at 463 K showed that in the absence of UV light radiation 8% conversion was obtained . Irradiation of the catalyst with the UV light increased the conversion dramatically to 48%. In the presence of UV light, benzaldehyde was formed in preference to phenyl acetaldehyde, in addition to small amounts of other secondary and tertiary reaction products. Benzylic alcohols gave higher conversions, however, with more secondary reaction products. The presence of oxygen was found to be critical for photo-oxidation. The conversion increased from 13 to 20% when a very small quantity of oxygen (O2/alcohol = 1) was added to the nitrogen carrier gas. There was significant improvement in the conversion, from 13 to 36%, when nitrogen was replaced by air as the carrier gas. Aldehyde is formed only in the presence of O2. Mohamed et al.  studied the photocatalytic oxidation of selected aryl alcohols, such as benzyl alcohol, 1-phenylethanol, benzhydrol, 4-chlorobenzhydrol, hydrobenzoin, 4,4'-dichlorohydrobenzoi and 4,4'-dimethoxyhydrobenzoin, in a polar, nonhydroxylic solvent (CH3CN). The main products formed are aldehydes or ketones; the carboxylic acids are formed in small amount. The selective oxidation of alcohol to aldehyde by photocatalytic process was demonstrated for the conversion of 4-methoxybenzyl alcohol (MBA) into 4-methoxybenzaldehyde (p-anisaldehyde, PAA) in organic-free water containing aqueous suspensions of commercial and home-prepared TiO2 . The home-prepared catalysts, obtained under mild conditions, showed to be much more selective than TiO2 Merck and TiO2 Degussa P25 with highest selectivity to PAA. The nanostructured anatase TiO2 samples synthesized by simply boiling the aqueous solutions of titanium tetrachloride (TiCl4) showed a yield much higher (42% mol for conversions of ca. 65%) than the commercial TiO2 samples. The least crystalline sample showed higher quantum efficiency (0.116%) . Wang et al.  showed that the photocatalytic oxidation of alcohol with O2 is accelerated by Brønsted acids adsorbed onto TiO2 or TiO2/SiO2 photocatalysts.
The photocatalytic oxidation of benzyl alcohol and its derivatives, such as 4-methoxybenzyl alcohol, 4-chlorobenzyl alcohol, 4-nitrobenzyl alcohol, 4-methylbenzyl alcohol, 4-(trifluoromethyl)benzyl alcohol and 4-tertiary-butylbenzyl alcohol, into corresponding aldehydes was achieved at stoichiometric conversion and selectivity (>99%) on a TiO2 photocatalyst under irradiation of light from a blue LED (
4. Hydroxylation of aromatics
Conversion of benzene to phenol is an industrially important reaction. With TiO2 based photocatalysis, the yield and selectivity of phenol production from benzene were about ca. 0.5 and 80%, respectively , which are lower than the conventional Fenton process (5% and 80–90%) . The by-product formed in the photocatalytic oxidation of benzene is biphenol. The efficiency of the photocatalytic phenol synthesis can be enhanced by structural modification of the catalyst, and by using additives. Noble metal deposits on TiO2 surface often enhance the photocatalytic reactivity because they trap CB electrons with reducing charge pair recombination and promoting interfacial electron transfer. Pt/TiO2 exhibited an enhanced yield (1.7 times, i.e., 4.4%) and selectivity (96%). The addition of various electron acceptors such as O2, Fe3+, H2O2, Ag+ and N2O further enhanced the yield and selectivity. TiO2 along with polyoxometalate PW12O403− increased the phenol production yield (i.e., 11 and 70% selectivity) . Here, polyoxometalate performs dual roles of both as a photocatalyst and as an electron shuttle. 2 atom % vanadium incorporated into the lattice of disordered mesoporous titania, 1 wt% Au incorporated into Ti0.98V0.02O2 (TV2), showed two times higher activity than bare TV2 . Zheng et al.  compared three photocatalysts M@TiO2 (M = Au, Pt, Ag) for the oxidation of benzene to phenol in aqueous phenol under visible light (
5. Epoxidation of alkenes
Epoxidation of alkenes to epoxide is a very useful transformation in organic synthesis. A series of highly dispersed transition metals such as Ti4+, Cr6+, Mg2+ and Zn2+, in SiO2 matrix, exhibited photocatalytic activity for propene oxide formation . Various studies reported on silica supported that metal oxide catalysts are effective photocatalysts for propylene epoxidation in a closed reactor system. Studies on silica supported on V, Ti and Cr showed that the conversion rate of propylene and the formation rate of propylene oxide were increased in the order Cr < Ti < V oxides on silica . Gradual deactivation was observed for CrO3/SiO2 with the course of the photoreaction, although the initial activity was almost same as that of TiO2/SiO2. Shiraishi et al.  prepared Cr-SiO2 catalyst containing highly dispersed chromate species by a sol-gel method and when subjected to photocatalytic oxidation reaction under visible light irradiation (
6. Oxidation of amines to imines
In organic synthesis, selective oxidation of amines to imines are considered as important reactions, because imines and their derivatives have immense applications in the synthesis of nitrogen heterocycles, especially alkaloid synthesis, which are biologically important compounds. Lang et al.  have studied extensively and achieved highly selective photocatalytic conversion of various amines into its corresponding imines on TiO2 using 1 atm of air as the oxidant in acetonitrile under UV irradiation. It is documented that it is a challenging task to achieve high selectivity in water medium as complete degradation is potential to occur by over oxidation. The reaction rate in terms of conversion of amine is much higher in water medium than in CH3CN with high selectivity of imine formation . However, the selectivity decreased to some extent because of free radical intermediates, which are comparable to photocatalytic oxidation of amine in CH3CN. Studies on substituents’ effect on the catalytic activity of amine to imine showed that substituents on the benzylic amines did not affect the selectivity significantly both in CH3CN and in water medium . The presence of electron-donating substituents (CH3– and CH3O–) and electron withdrawing substituents (F– and Cl–) on the phenyl ring had little effect on the reaction rate and product selectivity of the oxidation reaction. The halo-substituted benzylamines provided good selectivity, as the halo-substituted positions along with the imine functionality are useful for further transformations. Comparison of benzylic and nonbenzylic amines showed that no imines formation occurred in nonbenzylic amines such as cyclohexylamine and n-hexylamine and only fragmentation products were detected by unselective auto-oxidation at multiple reactive sites. Utilization of visible/solar energy is the prime concept to make the synthesis a economically and environmentally viable process. Nb2O5 converts amine to imine with high selectivity under visible light >390 nm to about 460 nm in benzene . Studies on the photocatalytic activity and selectivity in the aerobic oxidation of benzylamine over various metal oxides illustrate that TiO2 exhibited higher yield than Nb2O5 and ZnO. However, the selectivity to N-benzylidene benzylamine was comparatively low because benzaldehyde was formed as a by-product. In V2O5, benzylamine-N-carbaldehyde was formed as a main by-product. Hiroaki Tada and his research group have used the "plasmon photocatalysts", in which noble metal nanoparticles are dispersed into semiconductor photocatalysts, which possesses two prominent features: a Schottky junction and localized surface plasmonic resonance (LSPR) . Au NPs supported on various metal oxides (anatase and rutile TiO2, SrTiO3, ZnO, WO3, In2O3, Nb2O5) under visible-light irradiation (
7. Oxidation of polycyclic aromatic hydrocarbons
Polycyclic aromatic compounds (PHA) are compounds containing multiple aromatic rings which are fused together. The highly efficient photocatalytic method was adopted to various PAHs and it was found that it can be degraded under UV or visible light irradiation . The reaction mechanism illustrated that the toxic PAHs degraded through various useful intermediates. The selectivity of the product depends on the type of catalysts used. Other factors which influence the catalytic activity are solvent medium, atmospheric condition and wave length of light radiation, etc. Photocatalytic oxidation of naphthalene in water medium with Degussa P25 under UV radiation (345 nm, 500 Watt) produced 2-formyl-3-hydroxycinnamaldehyde and 5,8-dihydroxy-2,4-naphthaquinone as stable intermediates . Bahnemann and coworkers found 15 different intermediates when naphthalene is in water medium with Degussa P25 under UV radiation (366 nm, 50 Watt) . The main intermediates observed are coumarin, 1,4-naphthalenedione, 2,3-dihydro-2,3-epoxy-1,4-naphthalenedione, 2-formyl-Z-cinnamaldehyde, 1,2-benzenedicarboxaldehyde and phthalic acid. Karam et al. synthesized 1,4-naphthaquinone in a closed system reactor under UV radiation . Solvent plays an important role in the photocatalytic degradation of naphthalene. In water containing 1% MeCN produced significant amounts of three intermediates: isomeric 2-formylcinnamaldehydes and 1,4-naphthoquinone. In organic solvent, the oxidation rate was almost an order of magnitude slower than in water and produced significant amount of naphthoquinone and phthalic anhydride . In acetonitrile and water mixture, with UV light (
8. C-C bond formation
The C-C bond formation between an electrophilic alkene and adamantane was achieved by the irradiation of TiO2 suspensions in the presence of isopropylydenmalonitrile (IPMN, 0.02 M, with adamantane likewise 0.02 M) and under nitrogen forms a new product as an adduct, 2-[1-(1-adamantyl)-1-methyl)]ethylpropanedicarbonitrile (35% yield), together with traces of oxygenated products, such as 1-adamantanol, 2-adamantanol and 2-adamantanone . By adding silver sulfate as a sacrificial electron acceptor, the yield increased to 75%. The single-electron transfer oxidation of adamantane is followed by deprotonation leading to the 1-adamantyl radical that couples with isopropylydenmalonitrile.
9. Cyclization reaction
Intramolecular cyclization of N-(β-hydroxypropyl)-ethylenediamine in the presence of a semiconductor (TiO2 or CdS or ZnO)-zeolite composite catalysts in O2 atmosphere produces 2-methylpiperazine and piperazine . The yield of 2-methylpiperazine and piperazine depends on the type of semiconductor and zeolite. Zeolites modified with TiO2 (5 wt% TiO2–Hβ) composite considerably facilitated the intramolecular cyclization with a yield of 31.9%. Zeolites modified with semiconductors ZnO and CdS showed lower activity. This is due to moderate hydrophobicity and acid site strength offered by TiO2-zeolite composite for the cyclization reaction. Selvam and Swaminathan have shown one-pot synthesis of quinaldines from nitroarenes by combined redox-cyclization reaction assisted by photocatalytic method using pure TiO2 and Au-loaded TiO2 catalyst in absolute ethanolic solution under UV radiation (
There is an extensive research and dramatic growth going on in the field of heterogeneous photocatalysis. This method has been commercialized in various aspects of environmental detoxification. Photocatalysis method can be successfully applied for fine organic chemical synthesis. The advantages of photocatalysis method in organic synthesis include the possibility of utilizing clean and abundant renewable energy source, harmless chemicals used as catalysts, reaction can be carried out at room temperature, product type and its selectivity can be tuned by varying the nature of solvent, atmospheric condition, and wavelength of light source used, catalyst reusability, etc. Various challenging aspects have to be considered before implementing in industrial scale. Most commonly, simulated sunlight or UV sources are used in the laboratory. Experiments need to be devised to monitor the performance of the catalyst in sunlight. In future, more catalysts need to be devised that can trap the visible radiations as well. Since organic solvents are utilized in organic transformations, the band gap of the metal oxide photocatalysts in organic medium has to be explored. Systematic study has to be conducted to optimize the reaction parameters and understand the reaction mechanism, thereby the product yield could be improved.