UC Irvine

With the increasing levels of anthropogenic carbon dioxide emissions, the need for climate change mitigation has never been more urgent. Existing methods for CO2 removal are facilitated by thermally driven systems and thus suffer from being energy-intensive and inefficient. Joint efforts between computational and experimental chemistry fields can help steer research toward uncovering valuable insight into CO2 reactivity with the ideal sorbent. This dissertation presents an investigation into various chemical systems studied, from which understanding the design principles and the limitations of the carbon capture systems came to be illuminated.Chapter 1 details the study of 1,4-bis(tetramethylgaunidino)benzene (1,4-btmgb) as a redox-active sorbent and pH swing mediator. Spectroscopic and crystallographic studies demonstrate that 1,4-btmgb reacts with CO2 in water to form the ion-paired bicarbonate adduct, [1,4-btmgbH2](HCO3–)2. The synthesis and characterization of the mono- and diprotonated forms were performed, and their pKa values were determined using differential pulse voltammetry (DPV) experiments. From electrochemical pH swing experiments, a degradation pathway was identified and proposed based on the observation of a radical species. Chapter 2 describes experimental and computational work expanding on the promising CO2 reactivity of a Ru-PNP pincer complex. Deprotonated Ru[(PNP)(Cl)(CO)(H)] was previously found to bind to CO2 via metal-ligand cooperation. Computational values confirmed by experimental results demonstrate the redox behavior for Ru-PNP in the CO2 bound and unbound forms. However, cycling CO2 with this chemical system proved difficult due to the observed decomposition of the complex after attempts to oxidize to release CO2. Chapter 3 considers carbenes as a sorbent for the direct air capture of CO2. A computational survey of a diverse library of carbenes presents a linear relationship between the calculated pKa of the carbene and CO2 binding energy in water and organic solvents. Experimental evaluation of a selection of N-heterocyclic carbenes demonstrated an affinity to bind CO2 at low concentrations of 1% and below from gas streams. A trend between 13C NMR spectroscopic signals of the varied substituted imidazolium carboxylate compounds and corresponding CO2 binding energy finds that the less bulky electron-donating substituents correlate with a more negative (stronger) binding energy to CO2.