Metal–Organic Frameworks for Permanent Microporosity in Aqueous Media

Abstract

Aqueous media with high gas solubility are critical to the development of many emerging biomedical and energy technologies. From a biological standpoint, most physiological processes depend on cellular interaction of gases with water, while the design and synthesis of sustainable energy materials often require efficient gas-liquid mass transfer in an aqueous medium. However, water lacks the ability to store and transport significant amounts of gas due to its high energetic penalty against cavity formation required to solubilize gas molecules. To overcome this limitation, herein, we establish a generalizable method towards instilling permanent microporosity in aqueous media for increased gas carrying capacities. Specifically, we utilize metal–organic frameworks (MOFs) to instill stable, hydrophobic pores in aqueous media that can readily adsorb gas while excluding water from their pores. This novel approach towards creating aqueous gas carriers allows for not only unprecedented gas solubility in water but also provides a model platform upon which the interfacial effects between water and the micropore can be investigated. Chapter One introduces gas solubilization in water, and the associated thermodynamic challenges. The concept of instilling permanent free volume towards increased gas solubility is introduced in the context of porous liquids, and the limits of its steric exclusion approach in aqueous systems discussed. The grounds for establishing thermodynamic exclusion of water molecules from hydrophobic MOF micropores are proposed instead to establish permanent microporosity in water. Chapter Two outlines the design and synthesis of aqueous gas carriers via stably dispersing hydrophobic microporous solids – termed “microporous water” – and the development of in situ analytical methods for directly probing the gas solubility and release of these microporous water systems. Through this study, we report that these microporous water systems can concentrate gases to densities magnitudes higher than what is possible for other aqueous gas carriers, which has exciting implications for biomedical and energy applications. Chapters Three and Four investigate the fundamental factors that govern water intrusion into hydrophobic micropores through a 1-dimensional (1D) pore channel MOF. We find that pore size, in addition to ligand hydrophobicity, plays a critical role in determining water intrusion into 1D micropores and establish design principles towards de novo synthesis of 1D channel MOFs with microporous water behavior. In addition to structural factors that influence water intrusion, we also explore the effects of pore chemical environment on water intrusion behavior, in which the ligands of a fully water-intruded MOF are systematically functionalized to understand the effects of varying degrees of hydrophobicity and steric bulk. Chapter Five extends the microporous water concept towards microporous hydrogels for applications in controlled gas delivery. Through leveraging the surface hydrophobicity of aqueous MOF dispersions, we can achieve colloidal MOF hydrogels without the addition of extensive polymer matrices that may infiltrate the pores at the detriment of gas capacity. This proof-of-concept system opens a new avenue for incorporating dry porosity in hydrogels, enabling the sustained delivery of gaseous species previously considered elusive within a water-spanned matrix.