Platinum catalysts for the sustainable oxidation of biomass related compounds

Abstract

The project described in this thesis is concerned with the platinum catalysed aerobic oxidation of biomass related model compounds, to explore new clean catalytic routes for the production of relevant intermediates in the field of fine chemicals and materials. This work started with the optimisation of mesoporous and hierarchical TLCT SBA-15 structures through systematic variation of their textural properties, including pore diameter, surface area and metal loading. The successful synthesis has been validated via extensive characterisation of the materials. Pore-expanded mesoporous and macro-mesoporous Pt-TLCT SBA-15 materials have been subsequently applied to the aerobic oxidation of dodecanal. The attentive choice of the type of support architecture conferred significant advantages in terms of internal diffusion and catalytic activity, evidencing the elimination of mass-transport barriers inherent to SBA-15 materials. Furthermore, a green and sustainable alternative for the selective synthesis of cinnamic acid from cinnamaldehyde has been investigated, as cinnamic acid is a promising compound to be developed in the medical field. The complex reaction mechanism has been studied, to identify the optimal conditions that favour the formation of cinnamic acid while minimising the production of benzaldehyde, the main by-product of this reaction. Air as oxidative agent together with a Pt-SiO2 catalyst decreased the activity of the oxidative cleavage mechanism that promotes the formation of benzaldehyde, rendering the reaction more selective towards cinnamic acid. Finally, the aerobic oxidation of 5-hydroxymethylfurfural has been explored with platinum nanoparticles dispersed over fumed silica, a non-porous acidic support that has not been extensively investigated in previous scientific literature, to assess the importance of the solid support in this reaction. A comprehensive study to investigate the catalytic abilities of Pt-SiO2 has been conducted, exploring the effect of different temperatures, pressures and amount of base. The employed catalysts have been prepared with two platinum precursors, in order to examine if different precursors had a remarkable impact on the catalysis. The obtained results led to the conclusion that, between the two precursors used, hydrogen hexachloroplatinate (IV) hexahydrate represents the most suitable one for HMF aerobic oxidation, since it allowed smaller nanoparticle size, which in turn afforded higher platinum content located on the surface of the catalyst, where the reaction occurred.