DESIGN OF HETEROGENEOUS PHOTOCATALYSTS FOR SELECTIVE OXIDATION OF ALCOHOLS

My doctoral activity deals with oxidative photocatalysis. This is based on the use of photosensitive materials, it employes molecular oxygen as oxidant and it operates under mild temperature and pressure conditions. In particular, I worked on the preparation and characterization of photocatalytic heterogeneous systems containing the decatungstate anion W10O324- or the complex Fe(III) meso-tetrakis (2,6-dichlorophenyl) porphyrin. The photocatalytic properties of these photocatalysts were evaluated in the selective oxidation of aliphatic alcohols. The chosen polyoxoanion and porphyrin have many similarities because both of them absorb in the near UV (λ > 300 nm) and their primary photochemical processes involve the reduction of the metal centre (tungsten or iron), the simultaneous oxidation of the alcohol and the closure of photocatalytic cycle by molecular oxygen. The (nBu4N)4W10O32 was incorporated into silica matrix by sol-gel procedure and two heterogeneous systems were obtained with loadings of 10% and 30%. Their photocatalytic properties were compared in the oxidation of primary and secondary aliphatic alcohols (1-pentanol, 2-pentanol, 3-pentanol, 1-heptanol, 4-heptanol, 2,6-dimethyl-4-heptanol). Photoexcitation (λ > 300 nm) led to the selective conversion of alcohols to the corresponding carbonylic products, with a mass balance higher than 90%. The system with 10% of loading, which is characterized by a pronounced micro- and mesoporous structure, favored the selective adsorption of primary aliphatic alcohols with respect to the homologous secondary compounds, so allowing the reaction between the photoexcited decatungstate and the less hindered OH group. The system with loading of 30% was less selective; that is attributed to its lower porosity and to the fact that part of the immobilized decatungstate was not caged inside silica but weakly adsorbed on its surface and, therefore, easily accessible to all substrates. Photoexcitation of Na4W10O32 dissolved in water led to the formation of OH• radicals which, in turn, were able to oxidize glycerol. This substrate was chosen as a model of polyfunctional alcohol and because its selective transformation is a process of great industrial interest. While the photocatalytic process in solution was scarcely selective, incorporation of Na4W10O32 in a silica matrix provided a heterogeneous system with good selectivity and stability: 90% of the products were carbonylic compounds and CO2 was formed only in negligible amounts. The surface played a fundamental role, favoring adsorption of glycerol, increasing its local concentration near the decatungstate and, thus, facilitating its reaction with the photogenerated OH• radicals. Iron (III) meso-tetrakis (2,6- dichlorophenyl)porphyrin, properly silanized, was linked covalently on the surface of a mesoporous MCM-41 material. The possible effects of the morphology of the solid support on the selectivity of the photooxidation of 1,4-pentandiol were considered. The mesoporous photocatalyst, due to its large surface area, is able to disperse the iron porhyrin. The diol coordinates to iron, but contrary to what occurs in solution, can not act as a bridge between two complexes. This prevents the formation of inactive oligomers. The photocatalytic system presented a good selectivity in the oxidation of the diol in the primary position, with accumulation of 4-hydroxypentanal. It is noteworthy that there is no evidence of formation of products of further oxidation.