UC Berkeley

The work presented in this dissertation describes the design, synthesis, and characterization of metal-organic frameworks for applications in gas storage and gas separations, with a specific focus on natural gas and hydrogen storage for mobile applications and on post-combustion carbon dioxide capture from coal- or natural gas-fired power plants. A wide variety of techniques and spectroscopic methods are covered, including gas adsorption, x-ray diffraction, infrared and UV-vis-NIR spectroscopies, and calorimetry.

Chapter One provides a brief introduction to metal-organic frameworks as a new class of porous materials for gas adsorption-related applications. The potential of metal-organic frameworks for use in post-combustion carbon dioxide capture and natural gas storage is discussed, and the unique and promising properties of adsorbents with stepped adsorption isotherms for these applications are highlighted.

In Chapter Two, two representative metal-organic frameworks, Zn4O(BTB)2 (BTB3- = 1,3,5-benzenetribenzoate; MOF-177) and Mg2(dobdc) (dobdc4- = 1,4-dioxido-2,5-benzenedicarboxylate; Mg-MOF-74, CPO-27-Mg), are evaluated in detail for their potential use in post-combustion CO2 capture via temperature swing adsorption (TSA). Low-pressure single-component CO2 and N2 adsorption isotherms were measured every 10 °C from 20 to 200 °C, allowing the performance of each material to be analyzed precisely. In order to gain a more complete understanding of the separation phenomena and the thermodynamics of CO2 adsorption, the isotherms were analyzed using a variety of methods. These results show that the presence of strong CO2 adsorption sites is essential for a metal-organic framework to be of utility in post-combustion CO2 capture via a TSA process, and present a methodology for the evaluation of new metal-organic frameworks via analysis of single-component gas adsorption isotherms.

Chapter Three briefly discusses high-pressure adsorption measurements and reviews efforts to develop metal-organic frameworks with high methane storage capacities. To illustrate the most important properties for evaluating adsorbents for natural gas storage and for designing a next generation of improved materials, six metal-organic frameworks and an activated carbon, with a range of surface areas, pore structures, and surface chemistries representative of the most promising adsorbents for methane storage, are evaluated in detail. High-pressure methane adsorption isotherms are used to compare gravimetric and volumetric capacities, isosteric heats of adsorption, and usable storage capacities. Additionally, the relative importance of increasing volumetric capacity, rather than gravimetric capacity, for extending the driving range of natural gas vehicles is highlighted. Other important systems-level factors, such as thermal management, mechanical properties, and the effects of impurities, are also considered, and potential materials synthesis contributions to improving performance in a complete adsorbed natural gas system are discussed.

Chapter Four discusses the design and validation of a high-throughput multicomponent adsorption instrument that can measure equilibrium adsorption isotherms for mixtures of gases at conditions that are representative of an actual flue gas from a power plant. This instrument is used to study 15 different metal-organic frameworks, zeolites, mesoporous silicas, and activated carbons representative of the broad range of solid adsorbents that have received attention for CO2 capture. While the multicomponent results provide many interesting fundamental insights, only adsorbents functionalized with alkylamines are shown to have any significant CO2 capacity in the presence of N2 and H2O at equilibrium partial pressures similar to those expected in a carbon capture process. Most significantly, the amine-appended metal organic framework mmen-Mg2(dobpdc) (mmen = N,N′-dimethylethylenediamine, dobpdc4– = 4,4′-dioxido-3,3′-biphenyldicarboxylate) exhibits a record CO2 capacity of 4.2±0.2 mmol/g (16 wt %) at 0.1 bar and 40 °C in the presence of a high partial pressure of H2O.

In Chapter Five, the flexible metal-organic frameworks M(bdp) (M = Fe, Co; bdp2– = 1,4-benzene-dipyrazolate) are shown to exhibit methane adsorption isotherms that feature a sharp step, giving rise to unprecedented performance characteristics for ambient temperature methane storage. Adsorption measurements combined with in situ powder X-ray diffraction and microcalorimetry experiments performed on Co(bdp) demonstrate a new approach to designing adsorbents for gas storage, wherein a reversible phase transition is used to achieve a high deliverable capacity while providing intrinsic thermal management. Importantly, the energy of the phase transition, together with the adsorption and desorption step pressures, can be controlled through variations in the framework structure, such as replacing Co with Fe, or by application of mechanical pressure. This approach overcomes many of the challenges to developing adsorbents for natural gas storage discussed in Chapter Three and is also relevant to other gas storage applications.

Chapter Six discusses the synthesis and characterization of a new Ti(III) metal-organic framework that is constructed from 1,4-benzenedicarboxylate bridged Ti3O(COO)6 clusters. While many metal-organic frameworks have been synthesized with exposed divalent metal cations, there are comparatively few examples of metal-organic frameworks with coordinatively unsaturated trivalent metal centers. Among other potential applications, frameworks with exposed trivalent metal cations are of particular interest for ambient temperature H2 storage. Additionally, there are also very few reported titanium-based metal-organic frameworks and none that contain all titanium(III). Through a combination of adsorption measurements, diffraction analysis, EPR, infrared, and UV-vis-NIR spectroscopies, and magnetic measurements, this framework is shown to contain five-coordinate Ti3+ cations that irreversibly bind O2 to form titanium(IV)-superoxo and -peroxo species.