and water into glucose together with nutrients. In brevity, photocatalysis in plants is a photochemical reaction which provides oxygen with the help of a photocatalyst, released by plants or trees into air. A schematic diagram for this process in which facilitation of chemical reaction from solar light radiation due to photocatalyst is given is shown in Figure 1.3 [13].
In short, photocatalysis is the activity in which light radiation intersects on the surface of a specific substance to carry out chemical reactions such as oxidation and reduction reactions.
Here, specific substance is known as “Photocatalyst” which is quite responsible to attain enough energy level to absorb those incident heat waves for modifying the state of reacting molecules into valuable chemical products. Photocatalysis has two types:
1 (A) Homogeneous photocatalysis/photochemical reactions
2 (B) Heterogeneous photocatalysis/photochemical reactions.
Figure 1.3 Photosynthesis process in plants [13].
In homogeneous photocatalysis, the reactants and photocatalysts are available in the same phase. Acid catalysis, organo-metallic catalysis, and enzymatic catalysis are more common homogeneous photocatalysis. Ozone and photo-Fenton systems are prominent homogeneous photocatalysts in nature.
In heterogeneous photocatalytic reaction, the reactant and photocatalyst are present in different phases. These reactions include dehydrogenation, metal depositing, removal of gaseous pollutants, water detoxification, oxidation hydrogen atom transfer, etc. Most commonly heterogeneous photocatalysts are semiconductors and transition metal oxides. In heterogeneous photocatalysis, titanium dioxide TiO2 is the most recognized and studied photocatalyst. It is highly efficient for removing severely toxic and non-biodegradable organic contaminates of air or water. TiO2 is well-known as superior photocatalysts compared to others due to its versatile characteristics, such as cost effectivity, safety in use, highly stability, high photocatalytic activity at ambient conditions, i.e. temperature.
1.3.1 Mechanism for Photocatalytic Conversion of Biomass
Usually, photocatalysis depends on the substrate type present on the surface of semiconductor. In primary steps of photocatalysis, several observations are important such as generation of charge carrier, solar incident photon radiation or its absorption, charge separation and charge trapping. Many researchers reported that the separation of product and less stability of photons in homogeneous systems are tough to analyze. Although the predictability about the interaction between reactants and heterogeneous photocatalysts is easy than interaction of the heterogeneous photocatalysts with reactants the efficiency and applicability are higher in heterogeneous systems [5]. So, the focus has been created on heterogeneous systems which consist of rare metal ions with organic materials for a wide range of applications in technical ways. Heterogeneous surfaces are chosen for handling multiple and complex reaction pathways in which a variety of reactions such as oxidation, reduction, and redox-neutral reactions. Moreover, photocatalysis is pre-treatment process for converting biomass substrates as cellulose and lignin into high value added chemicals such as alcohols, diols, and acids by fermentation process; HMF and Furfural by hydrolysis; and alkanes by aqueous phase reforming [14].
Lignocellulosic materials consist of dry waste obtained from plants and trees which have three main ingredients as lignin, cellulose, and hemicelluloses. Before processing into valuable chemicals, purification and separation processes are quite challenging tasks. A classic way to utilize lignocellulosic biomass material is the photocatalytic pre-treatment to achieve simple structured products and photo-reforming for the production of hydrogen. Basically, the conversion of lignocellulosic materials provide a variety of products such as Arabinose, Erythrose, HMF, hydrogen, ethanol, carbon dioxide, glucose, syringaldehyde pyrocatechol raspberryketon, vanillic acid, guaiacol, and vanillin 4-phenyl-1-buten-4-ol.
Many carbohydrates are converted into high value added chemicals under ultraviolet and visible lights with fine selectivity of products such as glucaric acid, gluconic acid, Arabitol, Erythrose, glyceraldehydes, formic acid, hydrogen, fructose, xylitol, formate, etc. In this process of converting carbohydrates into products, mostly TiO2 catalyst with combination of other materials is used.
The conversion of HMF provides FDC and FDCA with basic and acidic attributes through photocatalytic valorization under ultraviolet, visible, and natural solar lights. Moreover, glycerol, methanol, ethanol, and toluene are converted into hydrogen and other chemical products under specific operating conditions via photocatalytic reforming.
1.3.2 TiO2 as a Significant Photocatalyst
The selection of suitable photocatalyst depends on the compatibility between band position of oxidation and reduction potentials of the reactants in a reaction. There are two types of band positions: conduction band position and valence band position. To carry out reduction reactions with electrons from the conduction band of photocatalyst, the position of conduction band should have more negative value than the reduction potential of reagents. In opposite of this, for the occurrence of oxidation reactions in photocatalysis with holes, the position of valence band should have more positive value than the oxidation potential of the reagents. The search of optimal photocatalysts for the photocatalytic reactions is based on band gap which initiate redox reactions with over potentials. TiO2 is the more chosen photocatalyst by researchers because it is non-toxic, stable, biological and chemically inert [5, 14, 15]. TiO2 is a very active photocatalyst under ultraviolet and visible light radiation especially for the conversion of biomass substrates. UV-photocatalysts can be divided into oxide (Ti4+, Sn4+, Ge4+, Zr4+, etc.) and non oxide groups. Many UV active photocatalysts can be improved by increasing the range of visible light so that more absorption of light can be done. Nowadays, the development of new catalysts for maximum absorption of visible light is in great attention with mixed metal oxides, sulfides, and nitrides for biomass utilization. Table 1.3 provides a list of important homogeneous and heterogeneous catalysts as follows in which mostly heterogeneous catalysts are associated with TiO2.
1.3.3 Factors Affecting Photocatalytic Efficiency
There are so many operation parameters in photocatalytic process which are listed below in brevity [16].
Table 1.3 List of important photocatalysts for biomass conversion [14, 15].
| Homogeneous photocatalysts | |
|---|---|
| S. no. | Name of catalysts |
| 1. | Vanadium complexes |
| 2. | [Ir(ppy)2-(dtbbpy)]PF6 |
| 3. | [Ir(dF(CF3)ppy]2(dtbbpy)]PF6/Na2S2O8/Pd(OAc)2 |
| 4. | [PMim][NTf2][PrSO3HMim][OTf] |
| 5. | N-hydroxyphthalimide(NHPI) |
|
Heterogeneous photocatalysts
| |