“Holey” Water: Designing Zeolites and Metal–Organic Frameworks for Aqueous Porous Liquids

Abstract

Water is essential to the biological processes required for life. However, the same properties that make water so ubiquitous in biology—namely its polarity and ability to form stable hydrogen bonding networks—also make it a poor solvent for nonpolar gases relative to many organic solvents. As a result, increasing the gas capacity of aqueous systems could have crucial impacts in medicine or for technologies requiring efficient transport of gaseous reactants, such as fuel cells or bioreactors. This dissertation investigates aqueous porous liquids, termed microporous water, as a strategy for improving gas solubility in water. Specifically, it describes two approaches to synthesizing microporous materials for use in microporous water and evaluates the feasibility of using the resulting dispersions in biomedical applications. Chapter One introduces the importance of solvating gases within a liquid medium for industrial applications. Zeolites and metal–organic frameworks are presented as ideal gas sorbents, and field of porous liquids is subsequently discussed to illustrate how these materials have been incorporated into a liquid medium to result in free-flowing liquids with permanent microporosity. Chapter Two proposes a thermodynamic strategy towards creating porous liquids in water whereby microporous nanocrystals with hydrophobic pores resistant to water intrusion and hydrophilic external surfaces can be stably dispersed in water while retaining the porosity of the solid-state porous material. Initial dispersions utilizing pure-silica zeolites and hydrophobic metal–organic frameworks are introduced. Next, we describe the synthesis and characterization of high-silica ZSM-5 nanoparticles for microporous water, yielding a dispersion with record-high oxygen capacities. Finally, we discuss efforts to reproducibly synthesize ZSM-5 with lower Si/Al ratios to probe the effects of framework composition on gas uptake and water intrusion behavior. Chapter Three introduces the utility of encapsulated microbubbles for ultrasound-based medical diagnostics and, in turn, the disadvantages of using these materials compared to more stable microporous frameworks. We discuss our collaborative efforts to quantify the nonlinear cavitation behavior of various microporous water samples and the potential towards using these materials as noninvasive pressure sensors. Afterwards, we describe additional investigations to examine the suitability of aqueous porous liquids as ultrasound contrast agents. Chapter Four addresses the challenge of improving biocompatibility in metal–organic framework nanoparticles. Factors influencing the biocompatibility and hydrophobicity of these materials are considered and we describe our synthetic efforts towards constructing frameworks with biocompatible components. Specifically, we explore strategies to introduce hydrophobicity in an existing magnesium-based framework by utilizing ligands with hydrophobic moieties.