**Recent Progress in Semiconductor Photocatalysis for Organic Fine Chemical Synthesis**

Suzan A. Khayyat, Rosilda Selvin and L. Selva Roselin

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/62220

#### **Abstract**

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and Dr. Avi Rotem2

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146 Applied Photosynthesis - New Progress

**Author details**

, Tali Goldman2

1 Greenonyx Technologies, Israel

2 Beta-O2 Technologies Ltd., Israel

Yoav Evron1

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Dimitry Azarov2

Photocatalytic process is a well-known reaction in photosynthesis by plants and algae. Artificial photosynthesis is a chemical process that mimics the natural plant photosyn‐ thesis 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.

**Keywords:** artificial photosynthesis, photocatalysis, organic synthesis, oxidation, cyc‐ lization, UV light, visible light, C-C bond formation

#### **1. Introduction**

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 chloro‐ phyll and form O2 and carbohydrates [1]. It is worth to utilize the solar energy for various energyconsuming processes, since it is clean, sustainable and abundant resource. Artificial

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

photosynthesis is a process in which fundamental scientific principles of natural photosynthe‐ sis are applied to design a solar energy conversion to make important chemicals by using manmadematerials.Manyapproachesarereported.Oneofthesuccessfulreportsprovidesanoutline for solar energy storage in fuels [2]. 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 [3]. Presently, photochemistry is a well-devel‐ oped field of chemistry and the majority of the experiments use direct excitation of molecules by UV light [4]. 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 [5]. The homogeneous sensitizers are substituted by heterogeneous photosensitizers using photocata‐ lytic process and which are easily separable and therefore recycled. In 1972, Fujishima and HondareportedaphotocatalyticprocessusingTiO2 semiconductoroxidee1ectrodestogenerate hydrogen from water [6]. Semiconductors can act as photocatalysts for light-induced chemi‐ cal transformations because of their unique electronic structure, which is characterized by a filledvalencebandandanemptyconductionband.Whenaphotonwithanenergyofhνmatches 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 photoexcitation can be written as [7]:

$$\text{SC} + h\nu \rightarrow \text{SC} \left( h^{+} \right)\_{\left( \text{v} b \right)} + \text{SC} \left( e^{-} \right)\_{\left( c b \right)} \tag{1}$$

Where SC is semiconductor, vb and cb represent the valance band and the conduction band, respectively. The reactive species, h+ and e<sup>−</sup> 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**.

**Figure 1** Semiconductor photocatalysed reaction.

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 semicon‐ ductor photocatalysis [10]. 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 irradia‐ tion. 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 carbonheteroatom 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.
