Synthesis, characterization, and catalytic applications of metallic nanoparticles in Tetraalkylphosphonium ionic liquids
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In recent years, ionic liquids have emerged as one of the most promising alternatives to traditional volatile organic solvents when it comes to catalytic reactions. Stable metal nanoparticles suspended in ionic liquids, are catalytic systems that mimic aspects of nanoparticles on solid supports, as well as traditional metal-ligand complexes used in organometallic catalysis. While alkylimidazolium ionic liquids, with or without appended functionalities, have been earmarked as the media of choice for the dispersal of nanoparticles, the tetraalkylphosphonium family of ionic liquids has largely been overlooked, despite their facile synthesis, commercial availability, chemical resemblance to surfactants traditionally used for nanoparticle stabilization, stability under basic conditions, and wide thermal as well as electrochemical windows. It is only recently that a number of research groups have given this family of novel alternative solvents the recognition it deserves, and used metal NPs dispersed in these ILs as catalysts in reactions such as hydrogenations, oxidations, C-C cross-couplings, hydrodeoxygenations, aminations, etc. This thesis investigates the synthesis, characterization, and catalytic applications of transition metal nanoparticles in tetraalkylphosphonium ionic liquids. The ionic liquids described in this thesis functioned as the reaction media as well as intrinsic nanoparticle stabilizers during the course of the catalytic processes. Metallic nanoparticles synthesized in these ionic liquids proved to be stable, efficient and recyclable catalytic systems for reactions of industrial significance, such as hydrodeoxygenations, hydrogenations, and oxidations. It was demonstrated that stability and catalytic activity of these systems were profoundly dependent on the properties of the ionic liquids, such as the nature of the alkyl chains attached to the phosphonium cation, and the coordination ability of the anion. Since heat-induced nanoparticle sintering was a problem, a procedure was devised to redisperse the aggregated and/or sintered nanoparticles so as to restore their initial sizes and catalytic activities. The presence of halides as counter-ions in tetraalkylphosphonium ionic liquids was seen to facilitate the oxidative degradation of agglomerated metal nanoparticles, which was a key step in our redispersion protocol. It was demonstrated that this redispersion protocol, when applied to heat-sintered nanoparticles, produces nanostructures that resemble the freshly made nanoparticles not only in size but also in catalytic activities. The presence of by-products from the borohydride reduction step used to generate the nanoparticles in the ionic liquids actually facilitated multistep reactions such as hydrodeoxygenation of phenol, where a Lewis Acid was necessary for a dehydration step. Finally, an attempt was made to utilize nanoparticles of an earth-abundant metal (iron) as a hydrogenation catalyst in a variety of alternative solvents (including tetraalkylphosphonium ionic liquids) in order to enhance the “green”ness of the catalyst systems. X-ray absorption spectroscopy (XAS) of the iron- nanoparticles/ionic liquid systems at the Canadian Light Source revealed significant details about the chemical interaction between iron and the ionic liquid matrices, which added to our understanding of this neoteric family of catalysts.
DegreeDoctor of Philosophy (Ph.D.)
SupervisorScott, Robert W.
CommitteeFoley, Stephen; Müller, Jens; Dalai, Ajay; Baker, Tom
Copyright DateMay 2015