Finding more sustainable ways to create nanoparticles
Hello! My name is Lanja Karadaghi, and I am a fifth year Ph.D. Candidate working with Professor Richard Brutchey in the Department of Chemistry at the University of Southern California. Throughout my Ph.D., one of my main research focuses has been on sustainably manufacturing catalytically active colloidal nanoparticles using ionic liquids (ILs) as a green solvent alternative to traditional volatile organic solvents. I am so honored to be a Wrigley Graduate Fellow this summer!
Volatile organic compound (VOC) solvents, such as traditional organic solvents, are employed in many industrial chemical reactions and processes. VOCs are components of many commonly used products including paint thinners, dry cleaning agents, petroleum fuels, and hydraulic fluids. However, VOCs are detrimental hazards to the environment because of their toxic emissions, ranking as the second highest contributor in the total national emissions in 2020, according to the United States Environmental Protection Agency Air Pollutant Emissions Trends Data. Not only are VOC solvents environmental and workplace toxins due to their high vapor pressures and flammability, but they are also extremely challenging to recycle, or reuse, resulting in considerable amounts of waste, ultimately contributing to CO2 emissions.
One alternative class of solvents that has been widely studied as a sustainable replacement to VOC solvents is ionic liquids (ILs). ILs are molten salts, usually comprised of an organic cation and an inorganic or organic anion, that melt below 100 °C. ILs have garnered significant attention over the last 20 years because of their many advantages over traditional organic solvents. Specifically, ILs are non-flammable, have negligible vapor pressures (~10-10 Pa at 25 °C), possess high thermal and chemical stability, and have the potential to be more easily recovered and recycled. The tunability of ILs is owed to the vast number of combinations of anions and cations that can be generated, allowing for the tailoring of properties including solubility, density, hydrophobicity, and viscosity. This make ILs very attractive as solvents in chemical syntheses because of their design ability for a wide range of applications.
More recently, ILs have been explored as alternative solvents for the fabrication of colloidal inorganic nanoparticle catalysts. Well-defined nanoparticles are important catalytic materials because of their large surface-area-to-volume ratio, which allows for an increase in exposed active sites compared to their bulk counterparts. Additionally, nanoparticle catalysts have unique structure-function relationships, based on their size, shape, and compositions, that can be controlled for specific catalytic applications. Nanoparticle catalysts have an important role in many renewable and sustainable energy generating processes including energy storage, biofuel production, and fuel cell operation. However, current methods for synthesizing these catalysts include the use of large amounts of VOC solvents, ultimately leading to waste and exacerbating toxic emissions and contributing to pollution. Additionally, when used in nanoparticle syntheses, the low interfacial tension in ILs helps facilitate rapid nucleation, while their high dielectric constant and ionic charge help to stabilize nanoparticles and support high colloidal concentrations, enabling process intensification and improvement. This is important for future implementation of nanoparticles in industrial processes. Nanoparticle catalysts produced in ILs also retain higher catalytic activity compared to those synthesized in traditional organic solvents.
Although ILs represent a sustainable alternative for nanoparticle syntheses, the major challenge in adapting and ultimately scaling these solvents lie in their high cost. The cost of ILs often exceeds $800/kg, making industrial-scale applications impractical, especially because organic solvents are generally pretty cheap. The most promising pathway to economically feasible large-scale applications is through recycling and reusing the ILs. Achieving cost competitiveness with ILs compared to traditional organic solvents would greatly reduce the environmental footprint of nanoparticle synthesis.
My research focuses on determining how the recycling of ILs affects the product characteristics in a colloidal platinum (Pt) nanoparticle synthesis. I am interested in assessing the chemical stability of the IL through each recycle, and the overall cost of nanoparticle fabrication using a techno-economic analysis. The product characteristics that I am most interested in are morphology (size and shape), because that has the largest impact when these materials are used as catalysts. We recently reported that a standard IL, 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (BMIM-NTf2) can be recovered and reused through multiple successive reactions without degradation of product quality or change in nanoparticle morphology. Through a techno-economic analysis (CatCostTM) of this system, we found that recycling the IL can achieve a cost that is not only competitive but also potentially lower than that of a compared conventional VOC solvent used in nanoparticle syntheses. This study suggests that IL recycling is an effective and valuable technique for lowering IL costs, unlocking the technical and environmental benefits of these solvents for commercial applications of colloidal nanoparticle synthesis.
I would like to give a huge thank you to the Wrigley Institute for their support this summer with this research!