Nitrate Radical-Mediated Oxidative Cleavage of Olefins Using TiO₂ Photocatalysis

Olefins, key compounds in industrial chemistry, serve as pivotal building blocks for producing an extensive array of materials, including polymers, fine chemicals, and pharmaceuticals. Their versatility stems from their reactive carbon-carbon double bonds, which facilitate numerous chemical transformations. Among these, oxidative cleavage has emerged as a critical process, enabling the conversion of olefins into high-value carbonyl compounds such as aldehydes and ketones. These compounds play an essential role in synthesizing polyamides, polyurethanes, and various other products integral to modern industrial applications. Notably, traditional oxidative cleavage methods heavily rely on hazardous oxidants like potassium permanganate, chromium salts, and ozone. These approaches are not only associated with significant environmental risks but also demand substantial energy inputs and operational costs, making them less suitable for sustainable and scalable industrial processes. Addressing these challenges necessitates the development of innovative routes for oxidative cleavage that minimize hazardous waste and energy consumption while maintaining high efficiency and selectivity. This thesis introduces an advanced approach based on photogenerated nitrate radical for the oxidative cleavage of olefins as a sustainable and scalable alternative to traditional methods. Using limonene as the model substrate, the study provides a green pathway for carbon-carbon double bond cleavage, addressing the limitations of conventional methods that rely on hazardous reagents and energy-intensive conditions. The process operates under mild reaction conditions, using cheap and easily affordable TiO₂-based catalysts, while achieving high conversion and selectivity. In Chapter 2, the oxidative cleavage process was systematically optimized by varying the quantities of nitrate ions, silver, and photocatalyst in the reaction mixture. The influence of surface properties of different TiO₂-based catalysts was investigated, shedding light on the pivotal role of interactions between nitrate ions and the catalyst surface. These findings confirmed that the reaction is driven by nitrate radicals, enabling the proposal of a detailed reaction mechanism. Under optimized conditions, the system achieved for limonene complete conversion within 180 minutes of irradiation with a maximum selectivity of ca. 60% towards the corresponding carbonyl compounds, with broad applicability demonstrated across other cyclic, linear and electron-rich olefins. In Chapter 3 the multifaceted role of silver was explored. Silver ions (Ag⁺) mainly act as fundamental electron scavengers, enhancing nitrate radical generation, while Ag⁰ nanoparticles stabilize photogenerated charges by serving as electron sinks. Under specific conditions, such as an inert atmosphere, these nanoparticles also can promote nitrate-mediated formation of alternative oxidized products, such as epoxides. Computational analyses using DFT and ab initio methodologies corroborate these experimental findings, confirming the proposed reaction mechanism and revealing how the form of silver dictates product distribution and selectivity. These insights emphasize the diverse functionality and complexity of silver in driving and diversifying reaction pathways. In Chapter 4, a comprehensive kinetic model was developed to quantify the dynamic behaviour of the system and validate the proposed mechanism. The rate constant for nitrate radical formation was determined to depend strongly on the presence and form of silver, increasing from 0.002 mM/min with bare TiO₂ to 0.77 mM/min with Ag⁺ ions and 0.35 (1/mM∙min) with Ag⁰ nanoparticles. These findings further underscore silver role, especially in ion form, in influencing nitrate radical formation, thus reaction efficiency. In Chapter 5, in order to address the limitations posed by the stoichiometric usage of silver, an electro-assisted photocatalytic (EA@PC) approach was developed, finding an alternative to the scavenging effect of silver ions. Using styrene as a model substrate, the EA@PC system, under optimized conditions, allowed for comparable performance obtained with silver-assisted approach (conversion 75% within 180 minutes of irradiation and maximum values of selectivity of ca. 37% towards benzaldehyde). The findings demonstrated that an applied bias could effectively substitute effectively silver maintaining the same performance and avoiding, at the same time, the environmental and economic drawbacks associated with sacrificial silver use. Beyond the immediate process improvements, this thesis highlights the potential of nitrate radical photocatalysis for sustainable chemical manufacturing, addressing limitations of traditional oxidative cleavage methods. By providing a cleaner, efficient pathway for producing industrially relevant compounds, it advances the understanding of nitrate radical chemistry while bridging laboratory research and practical applications.