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Gas Adsorption in Flexible Metal–Organic Frameworks


The work presented herein describes the synthesis and investigation of structurally flexible metal–organic frameworks for gas adsorption applications. These highly crystalline, porous solids are promising materials for the storage and separation of gases such as CH4, CO2, and H2. Furthermore, metal–organic frameworks are amenable to straightforward synthetic modification, due to their modular nature, and thus provide fertile ground for structure-property relationship studies.

Chapter 1 begins with a description of gas adsorption in porous materials, including activated carbons and zeolites. Metal–organic frameworks are introduced as an emerging alternative class of adsorbents, and their synthesis and hallmark structural features are discussed. Chapter 1 then provides an overview of flexible metal–organic frameworks in the literature and discusses their structural features and mechanisms of flexibility. Finally, the flexible metal–organic framework Co(bdp) is described, along with its record-setting CH4 storage properties, to set the stage for the research presented herein.

Chapter 2 reports the synthesis of a library of Co(bdp) derivatives with varying functionalizations of the bdp2- ligand, resulting in synthetic control over CH4 pressure at which Co(bdp) undergoes a phase change. Notably, fluorination of bdp2- disrupts π– π interactions that stabilize the collapsed phase of Co(bdp), while methylation of bdp2- strengthens these interactions, thus lowering or raising the phase change pressure, respectively. The structure-property relationship between ligand functionalization and CH4-induced phase change pressure is supported by in situ powder X-ray diffraction experiments.

Chapter 3 reports the effect of the identity of the adsorbate molecule on this phase change pressure (using CO2, CH4, N2, and H2) and the highly effective separation of CO2 and CH4 in Co(bdp). The mechanism of this selectivity is investigated with in situ powder X-ray diffraction studies, which reveal that Co(bdp) expands to CO2-templated phases even in the presence of CO2/CH4 mixtures. Multicomponent equilbrium adsorptions measurements, supplemented by dynamic breakthrough experiments, probe the limits of the remarkable CO2/CH4 selectivity shown by Co(bdp).

Chapter 4 describes the discovery of novel synthetic routes to the organic molecule 4,4’-bipyrazole, via the previously unknown homocoupling of pyrazoleboronic esters. This molecule is then used to synthesize the metal–organic framework Co(bpz), which is shown to be structurally flexible. CO2 adsorption and N2 adsorption are shown to induced structural phase changes in this framework, in contrast to H2 adsorption, which does not trigger a phase change, and this adsorbate-dependent phase change behavior can potentially be leveraged to accomplish gas separations in Co(bpz).

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