UC Berkeley

The work presented in this dissertation describes the conceptualization, validation, and characterization of crystalline molecular polyamines that undergo phase transitions upon carbon dioxide (CO2) capture to assemble porous ammonium carbamate network solids. This emerging class of materials for CO2 capture is contextualized with other sorbent materials classes alongside the modern needs of at-scale carbon dioxide capture from air and industrial emissions. Structural, spectroscopic, and sorption techniques are covered and contextualized in the answers they can provide specifically towards understand molecular polyamine systems. Throughout this dissertation, several projects are highlighted to provide a holistic understanding of the current state of molecular polyamines for CO2 capture.Chapter One provides an overview of the current state of CO2 capture. First, several CO2 capture separation types, especially direct air capture and natural gas carbon capture, are each discussed based on target CO2 capture goals, solved problems within each separation, and persistent challenges that new sorbents can breach. Second, a variety of materials classes are introduced and contextualized based on the persistent challenges that each material class does or does not address. Lastly, molecular polyamines as CO2 absorbents are discussed in their advantages and limitations relative to other materials classes. The competitive advantage of molecular polyamines are detailed to rationalize their merit as a new materials class of interest for CO2 separations.If Chapter One serves to outline the motivation for pursuing molecular polyamines, then Chapter Two exists as the manual that outlines the key design principles and characterization methods of molecular polyamines. Several synthetic strategies towards isolating high-symmetry molecular polyamines are identified and evaluated based on their synthetic viability. Key experimental techniques to structurally and spectroscopically verify phase transition behavior, such as powder x-ray diffraction, diffuse reflectance infrared Fourier Transform spectroscopy, and solid-state nuclear magnetic resonance spectroscopy are discussed. Additional gas sorption techniques such as thermogravimetric analysis, differential scanning calorimetry, and breakthrough experiments are also discussed. Third, several crystallographic techniques such as single-crystal x-ray diffraction, powder x-ray diffraction, microcrystal electron diffraction, and serial crystallography are compared for the crystallographic insight that each technique could provide towards understanding polyamine and ammonium carbamate structure. Throughout Chapter Two, several molecular polyamines are discussed to provide examples for synthetic viability, experimental design, and structural characteristics.In Chapter Three, the molecular triamine 1,3,5-tris(aminomethyl)benzene (C6H3(CH2NH2)3, TriH) has shown to rapidly absorb CO2 from air to form the porous ammonium carbamate network solid TriHCO2. The transformation from TriH to TriHCO2 occurs via a phase change, which was verified by in situ powder X-ray diffraction, solid-state nuclear magnetic resonance spectroscopy, and diffuse reflectance infrared Fourier Transform spectroscopy. Importantly, the all-organic composition and high amine functional group density of TriH enables CO2 absorption with remarkably high gravimetric CO2 capacities. When exposed to simulated air at a range of different global temperatures and relative humidities, TriH exhibits gravimetric CO2 capacities as high as 8.9 mmol/g, which represents a record for any solid sorbent under direct air capture conditions. These findings establish a new approach to designing absorbents for CO2 capture and suggests the broad applicability of molecular polyamines for CO2 capture from a range of point source emissions.In Chapter Four, the molecular hexamine 2,3,6,7,14,15-hexakis-(aminomethyl)triptycene (C20H8(CH2NH2)6, TriptH) demonstrates the ability to capture and release CO2 while retaining porosity in both the amine and ammonium carbamate network solid. Importantly, extrinsic porosity is established in TriptH due to the irregular packing of molecular polyamines that give rise to void spaces that are aggregated by intermolecular hydrogen bonding interactions to form sorbate-accessible one-dimensional channels. Importantly, imparting porosity in both the amine network solid and the ammonium carbamate solid product was anticipated to reduce the activation barriers for the absorption and desorption of CO2, facilitating its uptake and release under mild conditions. Indeed, TriptH demonstrates the unprecedented ability to capture CO2 from humid, low-concentration streams and release it with little or no temperature change and vacuum pressures as high as 100 mbar. Additionally, capture and release of CO2 under mild conditions supported long-term oxidative and thermal stability through the course of 660 absorption–desorption cycles, and the high amine functional group density of TriptH enabled a high CO2 capacity of 5.3 mmol/g under humid air. It is anticipated that the design of new polyamine structures with greater porosity will enable significant further improvements in isothermal CO2 capture performance.