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 | |
| 1. | Pb/ZnIn2S4, TiO2 |
| 2. | TiO2 |
| 3. | In2S3 |
| 4. | CdS/CdOx |
| 5. | Pt/ZnS-ZnIn2S4 |
| 6. | Mesoporous graphitic nitride |
| 7. | Carbazolic copolymers |
| 8. | Ligand-controlled CdS Quantum Dots |
| 9. | Ir(ppy)2(bpy)-MCFs |
| 10. | CuOx/CeO2/A-NTs |
| 11. | g-C3N4 |
| 12. | Au-HYT |
| 13. | Ag-P25 |
| 14. | Ru-LaFeO3/Fe2O3 |
| 15. | NiO/NaTaO3 |
| 16. | CoPz/g-C3N4 |
| 17. | Nb2O5 |
| 18. | WO3/g-C3N4 |
| 19. | Sn-Beta zeolite |
| 20. | Acid hydrolysate |
| 21. | TiO2/Pt |
| 22. | La/TiO2 |
| 23. | Pt-F-TiO2 |
| 24. | Cr/TiO2/zeolite |
| 25. | Rh/TiO2 |
