Environmental friendly photocatalytic synthesis of p-anisaldehyde, 2,5-furandicarbaldehyde and vanillin by nitrogen doped Tio2 and Tio2

This thesis consists of 3 parts. All home-prepared (HP) TiO2 photocatalysts in the thesis were prepared by using TiCl4 as precursor by a sol–gel method. Commercial TiO2 photocatalysts were used for comparison aims. HP photocatalysts were characterized by BET specific surface area, XRD, ESEM, UV-reflectance, TGA, FT-IR, and XPS methods. All catalysts were used for synthesis of some industrially important compounds in green conditions; by using water as solvent, oxygen as oxidant source, and sun or UV light as light source. The properties of photocatalysts for green synthesis were determined. In addition, the preparation condition of catalysts, light source and substrate kind on the effect of reaction activity and selectivity were determined by using characterization and experiment results. In the first part of thesis, in order to increase the efficiency of TiO2 under sun light, Ndoped anatase-rutile biphasic photocatalysts (N-HP) were prepared by using urea as Ndoping source. TiO2 was synthesised in crystalline biphasic anatase-rutile forms aiming to get a synergic effect. The influence of doping amount, calcination temperature and light source has been studied. The samples photocatalytic activity and selectivity were tested for the partial oxidation of MBA to PAA in water under near-UV, visible, UV-visible and simulated solar light. The results show that less crystalline, amorphous N-TiO2 samples are much more selective than that of Degussa P25. Especially, from these amorphous catalysts N-HP5-1%-100 and N-HP5-2%-100 showed high activity and selectivity (ca. 85%) under UV, UV-vis and simulated sun light, but not under visible light, in water. Anatase-rutil ratio and hydrophobicity of catalyst surface were increased by N doping amount on TiO2. The obtained catalyst characterization results show that the catalysts needed a thermal treatment temperature higher than 300 °C for real nitrogen doping; this treatment allowed obtaining crystalline and hydrophobic samples. The results show that Ndoping improves visible light absorption of catalysts whereas reaction rate does not iv increase. Selectivity was higher in the presence of badly crystallized catalysts, whereas doping seems to have a slightly improving effect. It was also found that XPS analysis could be used for determining crystallinity of TiO2. Moreover, XPS analysis must be used together with DRS and FT-IR to determining whether N is incorporated on TiO2 lattice or N species are just adsorbed on TiO2 surface. In the second part of thesis, 5-(Hydroxymethyl)-2-furaldehyde (HMF) was selectively oxidized to 2,5-furandicarbaldehyde (FDC) in aqueous medium by using HP anatase, rutile, and brookite TiO2 nanoparticles. Crystalline ratios of synthesized and characterised HP samples were also determined. Commercial TiO2 catalysts (SA, Merck, Degussa P25) were also used for comparison aim. The HMF oxidation activity and FDC selectivity of catalysts in different properties were compared each other and the properties of catalysts for active and selective HMF oxidation were determined. The reactivity results showed that HP catalysts are predominantly amorphous and give rise to selectivities toward FDC more than twice that of commercial and well-crystallized catalysts (ca. 25% toward ca. 12%). With the aim of improving the selectivity toward partial oxidation reaction, trace amount of methanol respect to water as an efficient hole trap was added to the reacting suspension. The results indicate that in the methanol containing photocatalytic system methanol acts as an efficient hole trap by decreasing the overall degradation rate, but it affects in a more significant way the mineralization reaction as the selectivity increases. In the third part of thesis, the influence of pH on the photocatalytic partial oxidation of MBA and vanillyl alcohol (VA) to their corresponding aldehydes in aqueous suspension was investigated by using amorphous HP and crystalline commercial TiO2 photocatalysts. Selectivities of 100% in MBA oxidation have been obtained at low pH's, whereas selective (28%) VA oxidation has been fulfilled at high pH values by using both HP and commercial photocatalyts. The results clearly show as tuning pH can strongly impart selectivity to photocatalytic processes which are often quite unselective in aqueous suspensions. The obtained results show that pH efficiently effects to photocatalytic processes. Notably, the reaction rates in the presence of crystalline catalysts are generally high at very high pH's by oxidising both MBA and VA; conversely, the use of amorphous HP samples give rise to high reaction rates and selectivities at low pH's in MBA oxidation. 2014, xvi + 94 pages Keywords: Heterojenous photocatalysis, TiO2, N doped TiO2, green synthesis, panisaldehyde, 2,5-furandicarbaldehyde, vanillin, pH effect, solar irradiation, X-ray photoelectron spectroscopy.