UC Berkeley

Diamine-appended Mg2(dobpdc) (dobpdc4− = 4,4′-dioxidobiphenyl-3,3ʹ-dicarboxylate) and diamine-appended Mg2(olz) (olz4– = (E)-5,5′-(diazene-1,2-diyl)bis(2-oxidobenzoate)) metal– organic frameworks have emerged as promising candidates for carbon capture owing to their exceptional CO2 selectivities, high separation capacities, and step-shaped adsorption profiles, which arise from a unique cooperative adsorption mechanism resulting in the formation of ammonium carbamate chains. The cooperative CO2 adsorption behavior enables the application of CO2 separation using relatively small temperature swings with large CO2 working capacities. More importantly, this cooperativity can be tuned via the critical design of the frameworks and diamine structure. This dissertation discusses the development of new diamine functionalized variants of Mg2(dobpdc) and Mg2(olz) for point source CO2 capture, such as from coal flue gas and biogas, as well as direct air capture. Design and discovery of the novel materials are explored, as well as the construction of the new characterization system.Chapter 1 explores the new frameworks of the type diamine–Mg2(olz) that feature diverse diamines with bulky substituents and display desirable single-stepped CO2 adsorption across a wide range of pressures and temperatures. Previous materials appended with primary,secondary- diamines featuring bulky substituents in Mg2(dobpdc) exhibit excellent stabilities and CO2 adsorption properties. However, these frameworks display double-step adsorption behavior arising from steric repulsion between ammonium carbamate chains, which ultimately results in increased regeneration energies. Chapter 1 also discusses how the basicity of the pore-dwelling amine—in addition to its steric bulk—is an important factor influencing adsorption step pressure; furthermore, the amine steric bulk is found to be inversely correlated with the degree of cooperativity in CO2 uptake. One material, ee-2–Mg2(olz) (ee-2 = N,N-diethylethylenediamine), adsorbs >90% of the CO2 from a simulated coal flue stream and exhibits exceptional thermal and oxidative stability over the course of extensive adsorption/desorption cycling, placing it among the top-performing adsorbents to date for CO2 capture from a coal flue gas. Spectroscopic characterization and van der Waals-corrected density functional theory calculations support that diamine–Mg2(olz) materials capture CO2 via the formation of ammonium carbamate chains. These results point more broadly to the opportunity for fundamentally advancing materials in this class through judicious design. Chapter 2 discusses a new framework, pip2–Mg2(dobpdc) (pip2 = 1-(2- aminoethyl)piperidine), that exhibits two-stepped CO2 uptake and achieves an unusually high CO2 capacity approaching 1.5 CO2 per diamine at saturation. Previous CO2 adsorption mechanisms of diamine-appended Mg2(dobpdc) metal–organic frameworks have been shown to occur predominantly via a chemisorption mechanism involving CO2 insertion at the amine-appended metal sites, a mechanism that limits the capacity of the material to ~1 equivalent of CO2 per diamine. Analysis of variable-pressure CO2 uptake in pip2–Mg2(dobpdc) using solid-state nuclear magnetic resonance (NMR) spectroscopy and in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) reveals that it captures CO2 via an unprecedented mechanism involving initial insertion of CO2 to form ammonium carbamate chains at half of the sites in the material, followed by tandem cooperative chemisorption and physisorption. Powder x-ray diffraction analysis, supported by van der Waals-corrected density functional theory, reveals that the physisorbed CO2 occupies a pocket formed by adjacent ammonium carbamate chains and the linker. Based on breakthrough and extended cycling experiments, pip2–Mg2(dobpdc) exhibits exceptional performance for CO2 capture under conditions relevant to the separation of CO2 from landfill gas. More broadly, these results highlight new opportunities for the fundamental design of diamine–Mg2(dobpdc) materials with even higher capacities than predicted based on CO2 chemisorption alone. Finally, Chapter 3 explore the application of diamine-functionalized Mg2(olz) for direct air capture, which involves capturing CO2 from dilute humid 400 ppm CO2 conditions. We designed a new material, 2-ampd–Mg2(olz) (2-ampd = 2-(aminomethyl)piperidine), that exhibits a cooperative CO2 adsorption profile with full capacity under direct air capture conditions. Analysis of the 1 bar 13CO2-dosed solid-state NMR spectroscopy and in situ DRIFTS data reveal that the material captures CO2 via a cooperative mechanism involving the formation of ammonium carbamate chains in the framework under the direct air capture conditions. In addition, breakthrough and long-term cycling experiments preformed with 2-ampd–Mg2(olz) reveal an exceptionally high capacity and operational stability. The oxidative experiments further confirm the resistance of the material toward degradation in operational environments. These results indicate that 2-ampd–Mg2(olz) is a promising candidate for achieving durable and high CO2 adsorption capacity in direct air capture applications.