Study of A Humidity-Swing Carbon Dioxide Sorbent
Creators: Shi, Xiaoyang
Publisher: name: Columbia University
Year: 2017
Subjects: subject: Chemical engineering
subject: FOS: Chemical engineering
subject: Environmental sciences
subject: Sorbents
subject: Carbon dioxide

description: Hydration of neutral and ionic species at interfaces plays an important role in a wide range of natural and artificial, fundamental processes, including in energy systems as well as biological and environmental systems. Owing to the hydration water at the interface, the rate and extent of various types of chemical reactions may be significantly enhanced. The hydration of ions does not only affect the physical structure and dynamics of water molecules, but also chemical energy transfers through the formation of highly structured water complexes that form in the bulk water. Indeed, dehydration could promote the energy levels of aqueous compounds. These shifts in energy states may receive wide applications such as in energy storage with anhydrous salts, enhancement of the free energy of binding ligands to biological systems, and gas separation using a water-modified basicity of ionic sorbents. Of particular interest in this study is a novel technology for direct air capture of carbon dioxide, driven by the free energy difference between the hydrated and dehydrated states of an anionic exchange resin and its effect on the affinity of CO2 to the resin. 
      In this dissertation, we first demonstrate an unconventional reverse chemical reaction in nano-confinement, where changes in the amount of hydration water drive the direction of an absorption/desorption reaction, and apply this novel mechanism of controlling the behavior of a sorbent to air capture of CO2. The reduction of the number of water molecules present in the pore space promotes the hydrolysis of CO32- to HCO3- and OH-. This phenomenon has led to a nano-structured CO2 sorbent that binds CO2 spontaneously in ambient air when the surrounding is dry, while releasing it when exposed to moisture. We name this phenomenon of loading and unloading a sorbent with water a hydration swing.
      Wide application of hydration swings to absorb CO2 requires a detailed understanding of the molecular mechanisms of the hydration induced energy change at the ion hydration/solid interface. Using atomistic simulations, the mechanism of CO2 absorption with respect to water quantity was elucidated via the explorations of the reaction free energy of carbonate ion hydrolysis in a confined nano-environment. Next, based on the understanding of the underlying driving mechanism, a systematic study of the efficiency of effective hydration-driven CO2 capture with respect to different pore sizes, hydrophobic/hydrophilic confined layers, temperatures, and distances of cations may further benefit the optimization of the CO2 capture system, in terms of the energetically favorable states of hydration ions in dry and wet conditions. This part of the research may sheds some insights on future research of designing high efficiency CO2 capture sorbent according to adjust the above described parameters.
       
      This unconventional reverse chemical reaction is not restricted to carbonate ions in nano-confined space. This is an universal phenomenon where hydrated ions carrying several water molecules in nanoscopic pores and in the natural atmosphere under low relative humidity. Such formations of hydrated ions on interfaces with the high ratio of ions to water molecules (up to 1:1) are essential in determining the energetics of many physical and chemical systems. In this dissertation, we present a quantitative analysis of the energetics of ion hydration in nanopores based on computational molecular modeling of a series of basic salts with the different quantities of water molecules. The results show that the degree of hydrolysis of basic salts with several water molecules is significantly different from the conventional degree of hydrolysis of basic salts in bulk water. The reduction of water molecules induces divalent and trivalent basic ions (S2-, CO32-, SO32-, HPO42-, SO42-, PO43-) to hydrolyze water into a larger amount of OH- ions, conversely, it inhibits monovalent basic ions (CN-, HS-)  from hydrolyzing water. This finding opens a vast scope of new chemistry in nanoconfined water.
      Ion hydrations containing interfaces play an important role in a wide range of natural and fundamental processes, but are much less noticeable currently. This thesis sheds some lights on a vast number of chemical processes of hydrated ion pairs containing interfaces, and design possibility for more efficient energy-saving sorbents.