UC Berkeley

This dissertation describes the design and synthesis of metal-organic frameworks for applications in high-density hydrogen storage for mobile applications, and in post-combustion carbon dioxide capture from coal- or gas-fired power plants.

Chapter One introduces the area of metal-organic frameworks, which are a new class of porous coordination solids, with an emphasis on the parameters requiring optimization in order to achieve applications in hydrogen storage and carbon dioxide capture. The current state-of-the-art is briefly discussed, and strategies for the development of next-generation materials exhibiting enhanced performance are presented in the context of metal-organic frameworks. Furthermore, the benefit of high-throughput technologies in the synthesis and characterization of metal-organic frameworks is also highlighted as a potential means of accelerating the discovery of high-performance materials.

In Chapter Two, the synthesis and hydrogen storage properties of Be₁₂(OH)₁₂(BTB)₄, the first metal-organic framework based on the lightest divalent metal, Be²⁺, is described. The high surface area resulting from the use of lightweight Be²⁺ cations leads to one of the highest gravimetric hydrogen storage densities observed at both cryogenic and ambient temperatures.

Chapter Three introduces the use of a high-throughput methodology in the synthesis of an Fe²⁺-based, sodalite-type metal-organic framework, Fe₃[(Fe₄Cl)₃(BTT)₈]₂ (Fe-BTT). This material features a high-density of exposed metal cation adsorption sites on the pore surface, which facilitates strong framework-H₂ interactions that are close to the adsorption enthalpy considered optimal for hydrogen storage at ambient temperatures. The resulting hydrogen adsorption properties are discussed, as well as the effect of these adsorption sites on the carbon dioxide capture performance.

The hydrogen storage properties of Mg₂(dobdc), a lightweight metal-organic framework possessing Mg²⁺ adsorption sites, are studied in Chapter Four using a combination of adsorption experiments, infrared spectroscopy, and powder neutron diffraction data. The high affinity of H₂ toward the exposed metal cation sites results in a high enthalpy of adsorption, which is of importance in raising the storage density at ambient temperatures.

Chapter Five describes the hydrogen storage properties of Cr₃(BTC)₂, a metal-organic framework featuring pores decorated with Cr²⁺ adsorption sites. In contrast to Fe-BTT and Mg₂(dobdc), the relatively diffuse nature of the Cr²⁺ cations results in a low adsorption enthalpy at these sites, highlighting the importance of the identity of the metal ion in controlling the thermodynamics of adsorption.

Chapter Six describes a new high-throughput methodology developed for the synthesis and adsorption screening of new metal-organic frameworks for carbon dioxide capture. The use of the workflow is discussed in the context of metal-insertion reactions within the material Al(OH)(bpydc), which features one-dimensional pores lined with 2,2'-bipyridine binding sites. As will be demonstrated, the metal salt employed imparts a considerable impact on the CO₂ adsorption capacity, highlighting the benefit of a high-throughput approach to materials optimization.

The precise control of the opposing wall distribution within metal-organic frameworks is an important aspect in optimizing the adsorption properties for high-density storage and molecular separation applications. In Chapter Seven, a new method for geometrically calculating the wall separation distances from the single-crystal structure is described and employed in studying a variety of known structure types. In this case, the routine is used to analyze metal-organic frameworks for methane storage due to the abundance of high-pressure methane adsorption data in the literature, and it is demonstrated that the optimization of the wall separations is indeed crucial for maximizing the volumetric storage capacity in storage applications.