The design and optimisation of poly(ionic liquid) stabilized metal nanoparticle catalysts

Abstract

Metal nanoparticles have been explored extensively as catalysts for a range of reactions, yet the factors that influence their catalytic activity are ill-defined. Although the employment of surface ligands has become commonplace for nanoparticle synthesis, they are, for the most part, removed prior to catalysis. However, there is evidence that surface ligands can have a beneficial effect on the activity of metal nanoparticles through modification of the metal- surface electronic structure, or by working cooperatively with the nanoparticle to stabilise or activate substrates. Thus, this thesis expands on the synthesis and design of ligand functionalised poly(ionic liquids) as supports for generating metal nanoparticle catalysts for several transformations. Catalysts that facilitate the conversion of carbon dioxide to useful commodities could play a significant role in our energy landscape. Due to their prominence in carbon capture, a series of amines were thus compared as surface ligands for the palladium nanoparticle catalysed hydrogenation of CO2 to formate. In Chapter 2, an aniline functionalised catalyst was found to be more than twice as active as the corresponding unfunctionalised system, and coordination between the amine and the metal precatalyst was shown using XPS and NMR spectroscopy. However, the poly(ionic liquid) support was found to degrade under the conditions of catalysis. Chapter 3 attempts to address this with the development of new, chemically robust ionic liquid monomers and polymer designs. A highly cross-linked amine decorated poly(ionic liquid) was found to be the most stable, and the mechanism of CO2 hydrogenation over palladium was explored at the metal-polymer interface using DRIFTS. Chapter 4, meanwhile, describes how ruthenium nanoparticles immobilized on a phosphine oxide decorated poly(ionic liquid) demonstrate high activity for the selective reduction of nitroarenes to the corresponding N-phenylhydroxylamines. In Chapter 5, the same ruthenium nanoparticles were also found to be active for the hydrolysis of sodium borohydride to hydrogen gas, however, the greatest catalytic activity was achieved in the absence of phosphine oxide and polyethylene glycol groups.