The work herein describes an investigation of metal-organic frameworks as adsorbents for selectively removing carbon dioxide (CO2) from low-pressure gas mixtures. Metal-organic frameworks are permanently porous, crystalline solid phase materials composed of organic molecules connected together by metal-based nodes into ordered structures. Generally exhibiting very high gravimetric surface areas, the pore surfaces of metal-organic frameworks can be rationally designed to allow for highly specific interactions between the adsorbent and guest species. Through chemical modifications of the pore surfaces, metal-organic frameworks were designed to adsorb CO2 over other small molecules.
Chapter 1 begins with background information on carbon capture and sequestration (CCS) and the role it can potentially play in slowing anthropogenic CO2 emissions. An analysis of desirable metal-organic frameworks properties is presented along with a summary of the most significant work in the field of developing new metal-organic frameworks as CO2 adsorbents. Finally, a summary of amine-functionalized solid adsorbents that have directly influenced the synthesis and characterization methods reported in this investigation is presented.
Chapter 2 reports the synthesis and characterization of the metal-organic framework mmen-CuBTTri. At the time it was first synthesized, mmen-CuBTTri exhibited some of the best CO2 adsorption properties of any metal-organic framework, including the highest selectivity for CO2 over N2 yet measured. The sorbent was the first to demonstrate that aliphatic amines could significantly improve the CO2 adsorption properties of metal-organic frameworks with open metal sites. Furthermore, despite an enthalpy of CO2 adsorption of nearly –100 kJ/mol at zero coverage, it was shown that the sorbent could be effectively cycled with modest temperature swings.
Chapter 3 reports the original synthesis and characterization of mmen-Mg2(dobpdc). Utilizing the same diamine as the sorbent in Chapter 2, it was demonstrated that the nature of metal-organic framework support, and not just the amine functional groups, affects the CO2 adsorption properties. In this case, the high density of amines within the pores resulted in a material that could effectively remove CO2 at very low concentrations; it was the first metal-organic framework studied for its ability to remove CO2 directly from air. Furthermore, mmen-Mg2(dobpdc) was the first amine-functionalized solid sorbent to exhibit steps in its pure component CO2 isotherm. Finally, it was shown that the adsorption properties of the material, especially the regeneration energy, make it competitive with aqueous amine solutions.
Chapter 4 builds upon the work of Chapter 3. The adsorption mechanism of mmen-Mg2(dobpdc), which was studied by infrared spectroscopy, solid state NMR spectroscopy, and in situ powder X-ray diffraction measurements was revealed to be a previously unprecedented cooperative insertion mechanism. The origin of the unusual isotherm steps was revealed to be a phase transition of the amines attached to the pore surface. In Chapter 4, a method of controlling the position of isotherm steps is described. Finally, the superior carbon capture characteristics of phase change adsorbents are enumerated.
Chapter 5 is a departure from the previous chapters and describes a simple and convenient method of utilizing a commercially available thermogravimetric analyzer to assess the porosity and activation conditions of metal-organic frameworks. The importance of identifying proper activation is discussed and a suggested protocol for researchers to use is given. Lastly, the ability of the method to improve the reported gas adsorption properties of the metal-organic framework Mn-BTT is reported.