UC Berkeley

The work presented in this dissertation outlines the design, synthesis, characterization, and use of metal-organic framework materials for applications in gas storage and separations. The materials herein are studied for use in hydrogen storage for mobile applications, such as light-duty vehicles, and for hydrocarbon separations relevant to the chemical industry. The common theme is that all of the studied materials focus on controlling adsorption within the pores of the metal-organic frameworks by taking advantage of the high level of structural design, synthetic control, and porosity, as well as the unique structural motifs present in these materials to engender them with properties suitable for the desired applications. A variety of techniques are used to gain an understanding of the materials and applications presented herein, including gas adsorption, infrared spectroscopy, powder X-ray and neutron diffraction, single crystal X-ray diffraction, multicomponent gas- and liquid-phase adsorption experiments, and electronic structure calculations.

Chapter One provides an introduction to the field of metal-organic frameworks and some of the relevant design principles which are important in the subsequent chapters. The logical extension of traditional inorganic cluster chemistry to extended three-dimensional metal-organic framework solids is discussed. Furthermore, open metal coordination sites are introduced, which are used in the subsequent work in this dissertation. The potential of using metal-organic frameworks for hydrogen storage, olefin/paraffin separations, xylene isomer separations, and acetylene/ethylene separations is introduced, with previous work in each of these fields being reviewed.

Chapter Two describes the synthesis of a new metal-organic framework, M2(m-dobdc) (M2+ = Mg, Mn, Fe, Co, Ni; m-dobdc4– = 4,6-dioxido-1,3-benzenedicarboxylate), which is a structural isomer of the previously known M2(dobdc) (dobdc4– = 2,5-dioxido-1,4-benzenedicarboxylate). While structurally similar, the altered connectivity of this M2(m-dobdc) material leads to an increased charge density at the open metal coordination sites, which are the key binding site for guests within the pores. This principle was studied in the context of hydrogen storage, with the Ni analog of this material possessing one of the highest physisorptive H2 binding enthalpies of any known adsorbent. A variety of techniques, including powder neutron diffraction, in situ H2-dosed infrared spectroscopy, and electronic structure calculations are used to better understand how and where hydrogen is binding in this class of materials. Importantly, this material was also designed to be produced at scale, with a relatively low cost of synthesis compared to other high-performing MOFs.

Chapter Three extends the work of the previous chapter. The M2(m-dobdc) series of MOFs that previously showed strong binding of H2 within its pores is now studied under conditions relevant to on-board hydrogen storage in fuel cell vehicles. Adsorption isotherms up to 100 bar show that Ni2(m-dobdc) is the top performing porous adsorbent for hydrogen storage of any yet reported, possessing a volumetric H2 capacity of 11.9 g/L at 25 °C and capacities approaching the U.S. Department of Energy targets at lower temperatures. In situ techniques such as powder neutron diffraction and infrared spectroscopy are again used to understand hydrogen binding, revealing that at near-ambient temperatures, the open metal sites control nearly all hydrogen binding and lead to the exhibited high H2 capacity.

In Chapter Four, the utility of the M2(m-dobdc) series is expanded to gas separations. Adsorptive-based olefin/paraffin separations of C2 (ethylene/ethane) and C3 (propylene/propane) hydrocarbon mixtures are studied. Due to the high charge density in these materials at the open metal coordination sites, the olefin is bound more strongly than the paraffin, leading to record adsorptive-based selectivities for each of these separations in the Fe2(m-dobdc) analog and strong performance from other metals in this structure. High capacity and fast kinetics of adsorption are also demonstrated in this class of materials. Finally, multicomponent gas adsorption experiments demonstrate the applicability of this MOF to real gas separations. The combination of high selectivity and capacity, fast adsorption kinetics, easy desorption, and low raw materials cost make this material the best material for adsorptive-based olefin/paraffin separations.

Chapter 5 examines the separation of xylene isomers (o-, m-, p-xylene and ethylbenzene) in the metal-organic frameworks Co2(dobdc) and Co2(m-dobdc) by taking advantage of their open metal coordination sites in a unique way. Rather than having a single guest bind to each open metal site, as in the previous chapters, each xylene molecule interacts with two adjacent metal centers, leading to selectivity based on the ability of each isomer to approach the two-metal binding pocket and pack within the pores. Separation was shown in single-component gas phase isotherms as well as multi-component gas- and liquid-phase adsorption experiments. Furthermore, Co2(dobdc) was shown to separate all four components. Single crystal X-ray diffraction was key in identifying that each xylene interacts with multiple metal centers and was able to corroborate the adsorption data in identifying how strongly each xylene interacted with the two-metal pocket. Interestingly, the Co2(dobdc) material undergoes significant structural distortion upon adsorbing o-xylene and ethylbenzene at lower temperatures, which was surprising given the previously-assumed rigidity of the structure. Lastly, the different symmetries of the Co2(dobdc) and Co2(m-dobdc) isomers of these materials displayed different separation properties and xylene packing, highlighting the structural tunability of metal-organic frameworks.

In Chapter Six, the metal-organic framework Cu[Ni(pdt)2] is used in the separation of acetylene from ethylene. By taking advantage of the small pores and unique structure of this material, it was discovered to have the highest adsorptive selectivity of any know adsorbent for this challenging separation. Through powder X-ray diffraction experiments, the primary binding site was revealed to be a square tetrapyrazine cage in the pores, which can likely interact more strongly with acetylene than ethylene through multiple pi–pi interactions. This material takes advantage of the unique square tetrapyrazine cage this material possesses in order to effect the separation of acetylene and ethylene without the need for open metal coordination sites.