Characterization of pyrolysis and oxidation processes of biomaterials, biomasses and biofuels

The urgent need for alternative solutions mitigating the impacts of human activities on the environment has strongly opened new challenges and opportunities in view of the energy transition. Indeed, the automotive industry is going through a revolutionary moment in its quest to reduce its carbon footprint, with biofuels being one of the viable alternatives. The use of different classes of biofuels as fuel additives/standalone components has attracted the attention of many researchers. Despite their beneficial effects, biofuel’s combustion can also result in the production of undesirable pollutants, requiring complete characterization of the phenomena occurring during their production and consumption. Industrial scale-up of biomass conversion is challenging owing to the complexity of its chemistry and transport phenomena involved in the process. In this view, the role of solid-phase and gas-phase chemistry is paramount. Thus, this study is devoted to detailed analysis of physical-chemical phenomena characterizing biomass pyrolysis and biofuel oxidation. The pyrolysis mechanism has been represented by 20 reactions whereas, the gas-phase kinetic models; manually upgraded model (KiBo_MU) and automated model (KiBo_AG), comprises 141 species and 453 reactions, and 631 species and 28329 reactions, respectively. The accuracy of the kinetic models was tested against experimental data and the models captured experimental trends very well. While the development and validation of detailed kinetic mechanisms is the main deliverable of this project, the realized procedure integrating schematic classifications with methodologies for the identification of common decomposition pathways and intermediates represents an additional source of novelty. Besides, the fundamentally oriented nature of the adopted method allows the identification of most relevant reactions and species under the operating conditions different industrial applications, paving the way for reduced kinetic mechanisms. Ultimately, the resulting detailed mechanisms can be used to integrate with more complex fluid dynamics model to accurately reproduce the behavior of real systems and reactors.

Radical chemistry of the catechol/quinone system and its role in the control of lipid peroxidation and melanin biosynthesis

The catechol (1,2-dihydroxybenzene) is a privileged structural motif among natural antioxidants like flavonoids, owing to its reactivity with alkylperoxyl radicals due to the stability of the semiquinone radical. The exploration of the relevance and mechanism of this non-conventional antioxidant chemistry in heterogenous biomimetic systems (aqueous micelles and unilamellar liposomes) is explored for the first time in Chapter 1. Results show antioxidant behaviour that surpasses that of nature’s premiere antioxidant α-tocopherol and relies on the cross-dismutation of alkylperoxyl and hydroperoxyl radicals at the water-lipid interface with regeneration of the catechol function from the oxidized quinone. The design and synthesis of new biomimetic catechol-type antioxidants by conjugation of thiols (e.g. cysteine) with quinones highlighted an unusual 1,6-type regioselectivity, which had been previously reported but never fully rationalized. Owing to its importance both in nature and in the development of new antioxidants, we investigated it in detail in Chapter 2. We could prove the onsetting of a radical-chain mechanism intermediated by thiyl and thiosemiquinone radicals at the basis of the “anomalous nucleophilic addition” of thiols to ortho-quinones, which paves the way to better understanding of the chemistry of such systems. The oxidation of catechols to the corresponding quinones is also a key reaction in the biosynthesis of melanins, mediated by enzyme Tyrosinase.