Supported sulphonic acids: solid catalysts for eco-compatible oxidation reactions

Abstract

My PhD dealt with the preparation and characterization of effective and efficient heterogenization of polymeric perfluorosulphonic acid (PFSA) Aquivion resin into a silica network. The material proved to be an active organocatalyst for redox processes, allowing selective oxidation of toluene to benzaldehyde with hydrogen peroxide under solvent- and metal-free conditions. Remarkably, the oxidation of benzylic C-H bonds with an organocatalyst was not previously reported. Preliminary results with the perfluorinated resin delivered good selectivity but low conversions. Therefore, we decided to heterogeneize Aquivion on silica. The Aquivion-silica composites (10wt% of Aquivion) were prepared by Sol-Gel method with the aid of different types of surfactants, obtaining materials with either mesoporous structure or as nanoparticles. Under optimized conditions, the hybrid materials Aquivion-silica showed good catalytic activity and excellent selectivity. The nanoparticle catalyst is fully reusable, maintaining stable performances for up several consecutive cycles without any intermediate treatment. The complex catalytic cycle likely involves -SO3H groups and the morphology of the support can influence the activity and selectivity of this radical reaction. Anilines could be selectively oxidized too, enabling one to access diphenyldiazenes via oxidative dimerization. Also in this case it was investigated the reaction mechanism, proving that the complex catalytic cycle likely involves –SO3H groups and the morphology of the support can influence the activity and selectivity of these radical reactions. My studies was focused on the scope of these novel catalytic methods and under respectively optimized conditions the hybrid material Aquivion-silica showed good catalytic activity and excellent selectivity, which occurs under mild conditions on gram scale and allows broad functional group tolerance.

Another topic dealt during my PhD was the oxidation of naphthalene with hydrogen peroxide to 1,4-naphthoquinone. The best results was obtained with Aquivion powder as catalyst and I optimized this reaction for 2 mmol of naphthalene (92% of conversion and 95% of selectivity). Later I monitored the reaction and I observed that the reaction time was 7 hours. Then I studied the mechanism of reaction through some tests thanks to which I can affirm that the reaction mechanism is radical. Finally I supposed that 1,4-dihidroxynaphthalene is an intermedium. Once the oxidation reaction of naphthalene was optimized, I then studied the scope of this reaction, trying to oxidize different kinds of substituted naphthalenes and polycyclic aromatic compounds.

Last topic of my PhD thesis was the effective and efficient synthesis of a titanium oxide-based catalyst on a commercial silica network. The material proved to be an active catalyst for batch organosulfur oxidation processes of liquid fuels, allowing the selective oxidation of dibenzothiophene to the corresponding sulphone using diluted hydrogen peroxide solution as oxidant, in acetonitrile under metal-free conditions. Preliminary tests led to a very good conversion of the substrate. However, we noticed a rapid deactivation of the catalyst probably due to the adsorption of the polar product on the catalyst surface or for strong interactions between catalyst and water molecules. Replacing hydrogen peroxide with TBHP, we finally reached complete conversion and demonstrated that in these conditions the catalyst could be reused for several times in batch without any intermediate treatment. More studies are focused on inorganic support modification and on the upscale of this catalytic method, screening a real fuel containing different organosulfur groups as substrates.