Reticular Chemistry toward Stable Frameworks and Their Use in Practical Carbon Capture

The unprecedented accumulation of anthropogenic carbon dioxide (CO2) in Earth’s atmosphere has been the cause of multiple aggravating global climate problems and posed severe challenges to our delicate ecosystem. With the entire society’s heavy reliance on fossil fuel combustion, it is of immediate importance to employ efficient and cost-effective CO2 capture techniques globally to control the emission and alleviate the already-emerging environmental problems. Whether capturing from point sources or directly capturing from the air, existing methods exhibit different advantages and disadvantages, and the central challenge is the reduction of the cost per unit capture of CO2, both in terms of energy and matter, which lays the fundament of their scalability and economic feasibility. We believe that the fundamental solution toward the next-generation CO2 capture solution is the use of chemical tools to develop sorbent materials with overall optimized performances, especially in terms of uptake capacity, selectivity, regeneration energy consumption, and stability, to allow for rapid, efficient capture and collection of CO2 from flue gas or air repeatedly over a long time. Chapter 1 summarized major classes of existing sorbent materials for post-combustion capture and direct air capture of CO2 based on chemical looping and cycling capture protocols, elaborating on their advantages and disadvantages in the above-mentioned aspects.The development of reticular chemistry has yielded powerful tools holding high potential in resolving this challenge. By linking judiciously designed molecular building blocks into predetermined crystalline structures through strong coordinative or covalent bonds, a vast variety of compounds including metal-organic frameworks (MOFs) and covalent organic frameworks (COFs) have been developed and studied. Their unique construct combines atomically well-defined structures, extensively exposed surfaces, and the ability to host unparalleled variabilities within one structure while retaining precise tunability. With these limitless possibilities, in this thesis, I focus on the development of stable framework structures toward the design and synthesis of effective amine-functionalized chemisorbents for scalable, cost-effective carbon capture sorbents. Chapter 2 presents the design and synthesis of a class of unsubstituted olefin-linked covalent organic framework, exemplified by COF-701. The central challenge lies in translating the small-molecular organic chemistry into COF linkage chemistry through solving the crystallization problem. I showed that COF-701 can be synthesized through acid catalyzed Aldol condensation reaction starting from molecular reactions. Isotope-labeling experiments using Fourier Transform Infrared Spectroscopy (FT-IR) and solid-state NMR proved the formation of the unsubstituted olefin linkage, which endowed the COF to maintain its crystallinity and composition in the presence of strong acid/base, organolithium reagents, and Lewis acids. Chapter 3 presents the development of a class of amine-functionalized chemisorbents for post-combustion capture through coordinatively functionalizing a Zr-based MOF with amino acids. The sorbent exhibited enhanced uptake of CO2 in the presence of water, which was further probed using solid-state NMR. It was identified that in the presence of humidity and abundance of CO2 (flue gas conditions), the framework uptakes CO2 into bicarbonate species, which can be removed with vacuum at room temperature. The MOF was subjected to a vacuum-swing adsorption process to capture and release CO2 from a simulated coal flue gas without observed capacity decrease. Chapter 4 includes the ongoing progress of developing the next generation chemisorbents for direct air capture using MOFs, which require higher reactivity, low energy requirements, and profound stability. The sorbent development was based on expanding the chemistry of base-stable Fe-hydroxamate MOFs and covalently functionalizing the framework with aliphatic amines, in an effort to withstand the basicity necessary for achieving abundant CO2 affinity for direct air capture. Chapter 5 provides the discoveries of covalently functionalizing stable COFs toward chemisorbents for CO2 capture from flue gas and air. Based on the chemistry established in Chapter 2, I discuss two approaches to introduce amine species into olefin-linked COFs — through pre-crystallization linker amination, and post-synthetic modification. Both directions are currently in progress, and the findings were summarized to discuss the advantages and disadvantages of both approaches. The strategy of functionalizing stable COFs for carbon capture opens up the possibilities to allow a much wider range of chemistry to be developed into the framework without being limited by the stability, which holds high promise for the achievement of the next level of overall optimization, and contribute to resolving the carbon capture challenge fundamentally.

Advisor: Yaghi, Omar M.