Heterogeneous & Homogeneous Photocatalysis Based on Titania and Ruthenium Complex for Selective Conversion and Synthesis

Abstract

Studies on the selective conversion and synthesis with photocatalysts in heterogeneous & homogeneous systems in terms of environmental remediation and organic synthesis are presented in this thesis. Therefore, in this work, the inorganic titania and organic ruthenium complex photocatalysts were immobilized onto silica based materials and further investigated for their activities and selectivity on the decomposition of organic pollutants in the water, conversion of benzene to phenol and process of free radical polymerization..1. The selective photocatalytic degradation of charged pollutants in water was achieved on titania encapsulated into FAU-type zeolites. The electrostatic attraction of cationic substrates and repulsion of anionic substrates by the negatively charged zeolite framework facilitated the selective photocatalytic degradation of charged substrates. The hybrid zeolite–titania photocatalysts were prepared through the ion-exchange method. The titania clusters were mainly well distributed within the cavities of FAU-type zeolites whereas no TiO2 nanoparticles aggregates were observed on the external surface of zeolite crystals. The hybrid zeolite–titania photocatalysts were characterized by diffuse reflectance UV–visible spectroscopy, transmission electron microscopy, energy-dispersive X-ray analysis and X-ray photoelectron spectroscopy. The selective degradation of charged pollutants was investigated by employing three pairs of oppositely charged substrates. The comparison between the cationic and anionic substrates clearly showed that the degradation rates for the cationic substrates on the hybrid photocatalysts are markedly higher than those for the anionic substrates. Among the cationic substrates, the smaller cations such as tetramethylammoniums were preferentially degraded. This enabled the selective removal of cationic substrates among the mixture. Such a selective photocatalytic degradation of water pollutants may provide a useful strategy for the development of economical photocatalytic process by targeting only the most recalcitrant pollutant.2. The selective photocatalytic conversion of benzene to phenol was achieved on titania incorporated in hydrophobically modified mesocellular siliceous foam (MCF). Titanium oxide nanoparticles entrapped into mesocellular siliceous foam (TiO2@MCF) were prepared by “co-condensation” method using block copolymer as a template. TiO2@MCF was further modified by surface organo-grafting with silylation agent to make it hydrophobic (TiO2@MCF/CH3) and the grafted organic functional groups on the surface of TiO2@MCF/CH3 were selectively removed by post UV-irradiation to obtain the catalyst (TiO2@MCF/CH3/UV) that has a higher selective activity for benzene oxidation. By modifying the hydrophobicity of the mesoporous siliceous cage environment, both the adsorption of reactants (benzene) and the desorption of the desired products (phenol) on photocatalytic sites can be facilitated with an increase in selectivity and yield of phenol production. The titania nanoparticles incorporated into MCF cage were characterized by X-ray diffraction, nitrogen adsorption-desorption, high-resolution transmission electron microscopy, energy-dispersive X-ray analysis, and UV-visible diffuse reflectance spectroscopy. The hydrophobic modification of the catalysts was confirmed by Fourier transform infrared spectroscopy and water contact angle measurement. The adsorption isotherms of benzene and phenol on various catalysts suspended in aqueous solution were measured to investigate the effect of hydrophobic MCF cage. The hydrophobically modified TiO2@MCF/CH3/UV sample showed the highest selectivity and yield for the photocatalytic hydroxylation of benzene to phenol in aqueous solution. This could serve as an efficient and green photocatalyst for the selective oxidative conversion of a hydrophobic reactant to a hydrophilic product.3. A new method of free radical polymerization is developed on the basis of visible light photocatalysis using Ru(bpy)3Cl2 that initiates and controls the polymerization at ambient temperature. The α-haloester and benzylic halide as radical initiators that can be activated through the Ru(bpy)3+ photoredox cycle under visible light irradiation. Successful free radical polymerizations of various methacrylates were realized using Xe- arc lamp as well as household florescent lamp as light source. The polymerization is initiated with light on and immediately terminated upon turning the light off. In addition, the molecular weight of polymer can be varied by changing the ratio of monomer and initiator. The present method has merits of the mild reaction conditions with weak light irradiation, ambient temperature and lower catalyst loading, which could be an alternative to the traditional thermal or photo-based free radical initiation methods.4. Ruthenium complex catalyst that is immobilized on Nafion coated silica, SiO2/Nf/RuL, shows recyclable and stable photocatalytic activity for free radical polymerization at ambient temperature under mild visible light irradiation. The adsorption of ruthenium complex is greatly enhanced by coating a layer of Nafion onto silica surface. The stable SiO2/Nf/RuL photocatalyst can be easily prepared and recycled by simple washing without chemical treatment. The SiO2/Nf/RuL exhibits high catalytic performance in the FRP of methacrylates and no notable decrease in catalytic efficiency after five times test. The present heterogeneous photocatalytic FRP method has merits of the mild reaction conditions with weak light excitation and ambient temperature, easy operation and high efficiency, which could be applied for economic polymer production, and also in a variety of radical mediated organic transformations