Towards sustainable heterogenous catalysis in molecular solar thermal energy storage (MOST) systems
This doctoral thesis focuses on triggering the back-reaction in MOlecular Solar Thermal (MOST) energy storage systems using heterogenous catalysts. The MOST system consists of a pair of organic switchable molecules which can capture solar radiation, can store the energy within its chemical bonds and release the energy when required in the form of heat. The MOST molecules explored belong to a well-investigated isomeric couple known as norbornadiene (NBD) and quadricyclane (QC). Despite the recent resurgence in MOST systems, triggering the back-reaction utilising heterogenous catalysts remains scarcely investigated. This thesis is divided into two introductory chapters, four principal chapters and a concluding chapter. The first chapter introduces the present state of affairs, the climate scenario, the sun’s influence on proceedings and the ways we are attempting to harness sunlight and transform it into renewable energy. In the second chapter concentrates on the concept of photoswitchable molecules and their integration within the MOlecular Solar Thermal (MOST) energy storage concept. A review of the most relevant literature of photoswitches, especially relating to the class of NBD/QC molecules is undertaken, concerning their history, synthetic design strategies applied and their applications. This review is intended to serve as a reference for the subsequent chapters. In the third chapter, the development of catalytic materials for the eventual heat release occurring from the QC to NBD back-reaction is embarked upon. The synthesis, characterization and experimental screening of sets of heterogenous catalysts are conducted on a well-investigated NBD/QC pair. The design and evaluation of an efficient catalyst trigger for the QC to NBD back-reaction is discussed. In the fourth chapter, catalyst materials and the NBD/QC photoswitch are synthesised in larger scales to perform catalytic reactions in flow conditions. The aim of this chapter was to determine the ideal physical properties the catalyst requires towards the ultimate scale-up of a MOST reactor. Alongside the key experimental properties determined including site time yield and TON values, computational fluid dynamic (CFD) simulations applying Lattice Boltzmann methods were carried out to view fluid dynamic behaviour at the solid-liquid interface. In the fifth chapter, the solvent’s impact on the NBD/QC photoswitch reactions is investigated with an emphasis placed on finding a green solvent suitable towards domestic applications. Reactions were performed in various solvent media to determine which don’t interfere but actually promotes the chemical photoisomerisation and catalytic reactions of the NBD/QC photoswitch. In the sixth chapter, a host of NBD/QC photoswitches will be synthesized and tested in conjunction with a previously confirmed active heterogenous catalyst. The aim was to discover how the substitution of different functional groups on the NBD/QC core affects the catalytic back-reaction. A comparison of the catalytic rates of reaction was accomplished along with first-time experimentally reported catalytic back-reaction barriers of NBD/QC photoswitches. In the final chapter, an outlook of the research results within the domain of MOST systems and concluding remarks for future research are deliberated.
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