Synthesis and Gas-Expanded Liquid (GXL) Processing of Metal and Metal Oxide Nanoparticles: Fundamentals and Application

Abstract

This dissertation involves a detailed discussion into the design of metal and metal oxide nanoparticles along with associated gas-expanded liquid (GXL) processing technologies for use in various industrially relevant applications. Nanoparticles have unique size-dependent physicochemical properties which need to be harnessed for their efficient use in the aforementioned applications. These size-dependent properties necessitate the presence of monodisperse nanoparticles in any nanoparticle-based system being investigated. The typical way to obtain monodisperse nanoparticles is through the use of specialized synthesis methods or through the post-synthesis processing of polydisperse nanoparticles. However, the methods to synthesize monodisperse nanoparticles are usually tedious and expensive, while the processing methods are often solvent intensive and lack large throughputs. To overcome these issues, our lab previously designed a process to effectively fractionate metal nanoparticles based upon their size using gas-expanded liquids. GXLs are tunable solvents whose properties can be manipulated by varying applied gas pressure. While the GXL nanoparticle fractionation process can successfully separate metal nanoparticles based upon their size, it still suffers from a few drawbacks such as requiring relatively high CO2 operating pressures and an inability to separate large quantities of nanoparticles in a scalable manner. The studies described in this dissertation aim to eliminate these drawbacks in the GXL nanoparticle processing system through the use of various nanoparticle systems and through the design of a completely new fractionation apparatus which is scalable in nature. The GXL system is also then extended to metal oxide nanoparticles and their use in industrially relevant applications such as catalysis and environmental remediation. To eliminate the drawback of high operating pressures required in the GXL fractionation process, this dissertation contains a study which shows that the use nanoparticle systems with sterically hindered solvent/ligand combinations can reduce operating pressures significantly. Through the investigation of gold nanoparticles coated with dodecanethiol and its tertiary isomer along with various isomers of hexane, it was shown that the system using gold nanoparticles coated with tert-dodecanethiol dispersed in 2,2-dimethylbutane (the most sterically hindered isomer of hexane) resulted in six-fold reduction in operating pressures (41.4 bar to 6.9 bar). Furthermore, the fractionation of iron oxide nanoparticles coated with oleic acid was also successfully attempted in the cascaded-vessel apparatus, thus proving that the GXL process can be extended to metal oxide nanoparticles. The GXL fractionation system was then modified using a Parr reactor to enable the fractionation of large quantities of metal and metal oxide nanoparticles. Oleic acid-coated iron oxide nanoparticles, synthesized using the coprecipitation method, were successfully fractionated into various distinct size-fractions using this scalable apparatus. The design of this apparatus enables the fractionation and processing of large quantities of nanoparticles, thereby allowing application-based studies on the size effects of nanoparticle systems. These fundamental studies into the synthesis and processing of metal and metal oxide nanoparticles pave the path forward towards the use of the GXL system in various relevant applications. In this dissertation the possible use of the GXL system in the fields of environmental remediation and catalysis is investigated. While an attempt to use GXL system to study the size effect of iron oxide nanoparticles in the formation of oil-in-water (O/W) emulsions for oil spill remediation was unsuccessful due to the inherent nature of hydrophobic nanoparticles required for the GXL fractionation process, a system to effectively stabilize O/W emulsions using stearoyl lactylate (SL) bilayer coated iron oxide nanoparticles was developed using a coprecipitation-based synthesis technique. The Parr-reactor apparatus was also used to deposit iron oxide nanoparticles onto various support materials in a GXL to generate supported iron catalysts for Fischer Tropsch Synthesis (FTS). The catalysts generated using this method were then used along with catalysts synthesized using traditional techniques in a FTS system to study their effectiveness. It was observed that the catalysts generated using the GXL method gave stable performance and higher conversion than their traditional counterparts in the FTS system. Analysis of the GXL catalyst using various characterization techniques indicated lower support-metal interactions as the cause of this higher activity. Hence, as a whole, the research presented in this dissertation improves and increases the versatility the GXL nanoparticle fractionation process. It has demonstrated that the initial fundamental studies into the GXL fractionation process resulted into significant improvements in the process, which was in turn used to generate nanomaterials for fields such as catalysis. Eventually, the GXL nanoparticle processing technique which provides several advantages over conventional solvent techniques can be used in the generation of advanced nanoparticle systems and nanomaterials