Application-Focused Design and Development of Amine-Appended Metal–Organic Frameworks for CO2 Capture

Amine-appended Mg2(dobpdc) (dobpdc4− = 4,4′-dioxidobiphenyl-3,3′-dicarboxylate) metal–organic frameworks are a novel class of CO2 capture materials that exhibit a unique cooperative CO2 adsorption mechanism, in which the entire material saturates with CO2 when the CO2 pressure reaches a specific threshold. This behavior enables access to large CO2 working capacities with relatively small temperature or pressure swings, and importantly, this threshold pressure can be tuned via altering the structure of the appended amine to target CO2 capture from a variety of streams covering a wide range of CO2 partial pressures. This dissertation discusses the development of these materials for two specific applications, CO2 capture from coal flue gas and CO2 removal from space vessels for air regeneration. Stability to relevant contaminants and conditions is explored, requiring the design, construction, and validation of new sorbent characterization systems, as well as discovery of novel materials and establishment of structure-property relationships. Chapter 1 explores the stability of diamine–Mg2(dobpdc) materials to SO2, a contaminant present in coal flue gas which is known to degrade the majority of CO2 capture materials, even at low concentrations. Previous work has established dmpn–Mg2(dobpdc) (dmpn = 2,2-dimethyl-1,3-propanediamine) as the optimal diamine–Mg2(dobpdc) material for coal flue gas CO2 capture based on the position of its CO2 uptake step pressure. The results in Chapter 1 further support the potential of dmpn–Mg2(dobpdc) for use in coal flue gas capture as it exhibited the highest stability to SO2 under relevant industrial conditions of all diamine–Mg2(dobpdc) materials tested. Furthermore, a structure-property relationship describing how the appended diamine structure affects SO2 stability was established, explaining the robust stability of dmpn–Mg2(dobpdc) and allowing for more informed design of future materials. Chapter 2 describes the synthesis and characterization of a novel material in this class of sorbents, 6-6–Mg2(dobpdc), (6-6 = bis(hexamethylene)triamine) which exhibits unprecedented stability to direct steam contact. Low-grade steam (≤120 °C) steam provides both temperature and pressure swing driving forces to remove bound CO2 from the material and is often available at low cost on industrial sites such as power plants. Thus, materials capable of withstanding direct steam contact and taking advantage of this effective and inexpensive regeneration method are of great interest. The 6-6–Mg2(dobpdc) material possesses a CO2 uptake threshold pressure applicable to CO2 capture from coal flue gas, yet carefully investigating the structure of this material should help enable design of steam-stable amine–Mg2(dobpdc) variants for other applications as well. Finally, Chapter 3 assesses the viability of employing diamine–Mg2(dobpdc) materials to remove CO2 from space vessel air to prevent buildup of unhealthy CO2 levels in these enclosed spaces. Through screening of the vast library of diamine–Mg2(dobpdc) materials previously characterized in the Long group, a promising material was identified and subjected to various multicomponent, dynamic tests and long-term stability studies. These results demonstrated the high CO2 capacity of this material under realistic conditions and over a wide range of relative humidity values. Custom instrumentation constructed for this study enabled novel characterization of the H2O co-adsorbed by these materials under humid conditions. These results established the high H2O co-adsorption capacity of this material, enabling its potential for simultaneous CO2 and H2O removal from an enclosed space and replacement of two separate air regeneration units currently employed on space vessels. This type of technology is of great interest as it can save valuable space, weight, and energy aboard a space vessel. However, long-term oxygen stability proved to be a weakness of the top material candidate, and this has sparked further investigation of the interaction between amine–Mg2(dobpdc) and oxygen in the group.

Advisor: Long, Jeffrey R