Aerobic Photoredox Catalysis for H2O2 Synthesis and Propane Oxidation

The thesis covers two different projects on heterogeneous and homogeneous photocatalysis aimed at the synthesis of valuable chemical feedstocks under aerobic conditions. The first chapter is dedicated to the photo-assisted oxygen reduction reaction to hydrogen peroxide using innovative carbon nitrides known as poly (heptazine imides) (PHIs). The suitability of PHIs as a polymeric scaffold to enable single-atom catalysis is discussed. The synthesis and catalytic screening of different metal-doped PHIs is presented together with a comprehensive characterization of their morphological, optical and electronic properties. It is shown that despite the advantageous modulation of band position, metal doping decreases hydrogen peroxide production by favoring its decomposition. Na-doped and all-protonated PHIs (Na-PHI and H-PHI, respectively), giving the most promising results, are explored in detail for catalytic optimization. In this work, H-PHI is shown to catalyze the synthesis up to 7779 μmol L-1 h-1 H2O2 under 410 nm irradiation in a simple water-oxygen mixture in the presence of glycerol as electron donor. Finally, preliminary mechanistic considerations involving kinetic isotope effect and transient photoluminescence are reported. In the second chapter, propane aerobic photooxidation in acetonitrile solution of iron(III) chloride is investigated. First, the highly superior photocatalytic activity of bare iron(III) chloride over a state-of-the-art iron-poly (heptazine imides) (Fe-PHI) catalyst for hydrocarbon oxidation is proven. The better performance of FeCl3 is also demonstrated by a thorough catalytic screening comprising different metal chlorides and iron halides. Propane is shown to be photooxidized at a very fast rate, achieving 100% conversion with 69% selectivity in acetone after 4 hours of irradiation at 410 nm in a 0.02 mol% FeCl3 solution using oxygen as the final electron acceptor. A consistent section of this work is dedicated to experiments and characterization to probe the underlying reaction mechanism. Strategies explored include kinetic isotope effect, use of quenchers, detection of in-situ generated chlorine radicals and hydrochloric acid, EPR spectroscopy and cyclic voltammetry. The evidence collected in this work point toward a radical-based mechanism, likely based on the well-known radical autooxidation pathway for aerobic hydrocarbon oxidation. It is argued that irradiation induces FeCl3 photolysis with the generation of chlorine radical initiators. The results suggest that commercially available iron(III) chloride might reveal itself as an efficient, cost-effective, visible light-activated radical generator for a variety of purposes.