Innovative catalytic Formulations for photo-assisted Chemical reactions

Abstract

Heterogeneous photocatalytic oxidation has been extensively studied for environmental applications such as purification processes aimed to the removal of organic substances at room temperature both in water and in air. However, in recent years, photocatalytic processes have also been extended to the synthesis of organic compounds in mild conditions. This Ph.D. work has been focused on the application of heterogeneous photocatalysis for the removal of contaminants (azo-dyes and pesticides) and for the selective oxidation of benzene to phenol to explore its potential application in an industrial process. In particular, visible light

active photocatalysts in powder form have been formulated, optimized through the coupling with E- cyclodextrins and studied in the degradation of water pollutants. Subsequently, the optimized photocatalysts

were employed in the direct oxidation of benzene to phenol. Among the investigated photocatalysts, N doped TiO2 (N-TiO2) photocatalyst showed the highest benzene conversion than the other photocatalytic formulations. Such photocatalyst was coupled with syndiotactic polystyrene in aerogel form (sPS) having a high affinity towards hydrophobic compounds (benzene) and a very low affinity towards hydrophilic compounds (phenol) in order to maximize the selectivity to phenol favoring its desorption from photocatalyst surface. Therefore, the selective photocatalytic oxidation of benzene in liquid phase is studied using visible active N-TiO2 photocatalyst embedded into a monolithic syndiotactic polystyrene aerogel (N-TiO2/sPS, 10/90 w/w) under UV or visible light irradiation with the aim to tune the selectivity towards the desired product. The experimental results evidenced that the presence of sPS allows to achieve a benzene conversion higher than 50% under both irradiation conditions at the spontaneous pH of the solution. In addition, N-TiO2/sPS is able to enhance the phenol selectivity compared to N-TiO2 in powder form that shows a very low selectivity to phenol in presence of UV light and no phenol production with visible light. A remarkable enhancement of both selectivity and phenol yield is achieved under visible light in acidic conditions because of the low affinity of phenol to the hydrophobic structure of sPS, facilitating the desorption of the produced phenol in the aqueous medium from N-TiO2/sPS, and, consequently, inhibiting at a certain extent the phenol overoxidation reactions. The final phase of the PhD work was devoted to the formulation of transition metal oxides (V2O5, Fe2O3 and CuO) supported on N-TiO2 and their dispersion in polymeric aerogels to increase both the benzene consumption rate and the production rate of phenol, maximizing, at the same time, selectivity and yield to the desired product under visible light. Cu/N-TiO2 in powder form allowed to achieve a phenol yield after 10 hours of visible light, significantly higher than that observed with Fe/N-TiO2 and V/N-TiO2. Therefore, to further increase the yield and selectivity to phenol and the phenol production rate,

Cu/N-TiO2 photocatalyst was embedded in sPS aerogel. Then, the operating conditions (dosage of Cu/N- TiO2/sPS, solution pH) were optimized. The best result so far not found in literature was obtained using

Cu/N-TiO2/sPS photoreactive aerogel in acidic conditions which reveals both benzene conversion and a phenol yield higher than 95% and a selectivity to phenol greater than 99 %, values achieved in a very short time of visible light irradiation (180 min) with very little formation of by-products. A possible reaction mechanism was also proposed, The developed photoreactive (Cu/N-TiO2/sPS) solid phase represents a "proof of concept" that could allow a significant leap forward in the development of innovative green processes for the selective oxidation of aromatic hydrocarbons under mild conditions. [edited by Author]