Using zeolites to create microporous water with high gas carrying capacities

Abstract

Solutions to many important global challenges, including capturing industrial pollutants, synthesizing mRNA & antibody therapeutics, and developing better treatments for cardiovascular emergencies, often rely on the transport of gases in water. Indeed, sufficient delivery of O2 is critical to many biomedical technologies, such as bioreactors and artificial blood substitutes. Absorption and transport of gases like H2 and CO2 could promote alternative energy and climate friendly solutions, such as amine-free CO2 scrubbing. Problematically however, the density of solubilized gas molecules in water is very low—up to 60-fold lower than in the gaseous phase. Alternative solvents are often considerably more toxic, expensive, and in many scenarios, simply non-existent. Herein, we show that it is possible to disperse dry, water-resistant pockets of microporosity throughout an aqueous medium, thereby significantly enhancing the corresponding gas carrying capacity of the solution. The work presented in this dissertation describes the design, synthesis, properties, and practical utility of these aqueous gas carriers. Chapter One describes the challenges of gas transport in water, existing solutions. It also introduces the concept of liquids containing permanent porosity – known as “porous liquids” – and discusses the fundamental incompatibility between existing porous liquid design strategies and the use of water as a solvent. Chapter Two demonstrates our work on circumventing these challenges and developing high gas carrying aqueous dispersions. Using an amphiphilic pure silica zeolite known as “silicalite-1”, we show that it is possible to disperse and maintain permanent microporosity in water. Known as “microporous water”, these solutions can possess gas densities that are over an order of magnitude greater than that of pure water. These solutions display rapid and facile gas release, very high reusability, and extremely high stability for months. Chapter Three describes the use of this strategy to synthesize an aqueous fluid possessing an O2 density approximately double that of pure O2 gas at equivalent temperature and pressure – while still maintaining high fluidity. Chapter Four demonstrates how the high gas solubilities of microporous water translate to significantly improved mass transport even under continuous operation. Using the oxygen reduction reaction (ORR) as a model system, we show how the addition of silicalite-1 to an aqueous electrolyte can dramatically enhance the reaction rate in a stable fashion even at pH 1. Furthermore, we show how microporous water can facilitate calculation of the intrinsic activity of electrocatalysts. Chapter Five describes our work in understanding how polymers interact with silicalite-1. Specifically, we investigate the infiltration of commonly used polymers such as poly(ethylene glycol) and poly(propylene glycol) into silicalite-1 when dispersed in water. We further demonstrate how to tune polymer size and backbone to mitigate infiltration. Chapter Six discusses our attempts at passivating the surface of silicalite-1 with poly(ethylene glycol) and zwitterionic molecules. We show how tuning of the size and polarity of functionalizing molecules can disfavor infiltration and thus maintain high gas carrying capacities.