Catalytic and photoelectrochemical oxidations processes: a multifaceted sustainable approach

Over the years, the public awareness about the urgent need to preserve our planet resources and consequently next generation welfare rapidly increased. In particular, within the chemistry community, both at industrial level and in academia research, the imperative necessity to shift from a waste treatment solution to a comprehensive policy, led to the development of the Sustainable Chemistry. In such context, the aim is to design a green chemical process that reduces the environmental impact (following the Green Chemistry Principles). Furthermore, a careful evaluation of the cost/benefits in the economicenvironmental-social relationship has to be carried out. Catalytic oxidation processes have a crucial role in the sustainable chemistry approach. Indeed, the design of catalytic one-pot reactions to obtain added value products from raw materials, using earth-abundant metals combined with environmental oxidant, has been pursued in the whole chemistry community. In this framework, in our research group, vanadium-based catalysts coupled with hydrogen peroxide have been studied as powerful oxidising system in environmental friendly conditions with a broad substrate scope. The selective and sustainable oxidation of toluene to benzaldehyde represents a very hottopic because of its massive use in our everyday life. However, in the modern processes, harsh conditions usually are required and selectivity is very low. Hence, in chapter 2 a vanadium-based oxidising system has been investigated in the selective oxidation of toluene to benzaldehyde. A safe and cheap vanadium precursor has been used to activate hydrogen peroxide leading to the formation of V(V)-monoperoxido complex, that is the active oxidant, in solution. To increase the sustainability of the process, the use of chlorinated co-solvents has been excluded. In fact, in this work, toluene is used as substrate and solvent. An ample screening of the reaction conditions has been carried out. Noteworthy, KF or O2 have a promoting effect on the toluene oxidation mechanism. Indeed, in their absence no benzaldehyde has been detected in the reaction mixture. After 24 hours a 30% of benzaldehyde has been obtained using 20% of the V(V)-salt with respect to hydrogen peroxide under O2 atmosphere at 60°C. Furthermore, taking advantage from the biphasic system it has been possible to recharge with H2O2 and, consequently, to recycle the mixture to continuously produce benzaldehyde. The model system has been able to oxidise toluene with a constant round yield over four round, obtaining an overall yield of 37%. This feature is the distinctive aspect of this system opening the possibility to oxidise toluene with a sustainable process. Another aspect of the modern development is to increase the use of the renewable energy in order to slow down climate change. Hydrogen and hydrogen peroxide can be used as efficient energy carriers. Their production from solar energy, that is the most promising renewable energy, has received increasing attention over the years. In this context, one of the most promising technologies is the settling of efficient photoelectrochemical (PEC) devices. In our research group, since 2012, we have employed KuQuinone as photosensitizer for the development of PEC cell: the main chemical features of such compound being the highly conjugated pentacyclic core, the intra-molecular hydrogen bond and the quinone moieties. They confer it an intense absorption in the visible region and a first reduction potential near to 0 V vs. SCE. KuQuinone deposited on ITO surface have been able to produce photocurrent in the presence of an electron donor. To the aim of investigating its electrochemical properties and the nature of its reduced species in the Chapter 3 an electrochemical, spectroelectrochemical and theoretical study has been carried out. In particular, KuQuinone have highly customizable redox chemistry and thus it is possible to tune its electrochemical behaviour by changing solvent polarity as well as by using hydrogen bond donor and Lewis acid additives. Such experimental evidence allows its application in sensor, as well as in photoelectrochemical devices. In the chapter 4, KuQuinone has been employed as photoelectrocatalyst in PEC device for the oxidation of water to hydrogen peroxide. Accordingly, an amphiphilic derivative has been deposited onto ITO surface and it has been used as working electrode in PEC cell. The device has been able to immediately produce photocurrent and accumulate 50 μC after 30 minutes of irradiation. Yet, the product of water oxidation, likely hydrogen peroxide, has to be determined. To this aim, two novel platinum nanoparticle based electrochemical sensors, i.e. NpPt/PDDA/SPE and NpPt/SPE, have been developed using a screen printed electrode platform. Such sensors are able to work at alkaline pH (water oxidation condition) and to perform real-time measurements. NpPt/SPE sensor showed a good reproducibility and high sensitivity. Measurements of the KuQuinone based PEC cell showed production up to (2.4 ± 0.1) μM of hydrogen peroxide after 30 minutes, but an overestimation of the H2O2 concentration by the sensor has been experienced. In Chapter 4, a KuQuinone derivative with a carboxylic acid in its lateral chain has been grafted onto nanostructured SnO2 surface and then coupled with a water oxidation catalyst such as ruthenium polyoxometalate. In this work, carried out in collaboration with the University of Padua and Ferrara, the ad hoc prepared SnO2|KuQ(O)3OH|Ru4POM photoelectrode has been able to produce dioxygen from water with a Faradaic efficiency of 70 ± 15%. Furthermore, through fluorescence, electrochemical and transient absorption spectroscopy experiments, the mechanism of the generated photocurrent has been determined. Such results open the possibility to design and realize novel DS-PEC.