Orbital Interactions in Metal–Organic Frameworks for Gas Separations and Ambient Hydrogen Storage

The work presented herein describes the synthesis and characterization of metal–organic frameworks (MOFs) and their application for gas separations and hydrogen storage. Metal–organic frameworks are crystalline, porous materials that offer a high degree of synthetic tunability and can be thought of as designer zeolites. With an upfront investment in material design, followed by discovery of the appropriate synthetic conditions, new materials with specific properties optimized for a target application can be realized. Herein a new framework, V2Cl2.8(btdd), is synthesized and its unprecedented properties for gas separations and hydrogen storage is described. Other frameworks are also discussed, and overall designing orbital interactions to engender specific adsorption behavior is discussed as a relatively underexplored frontier to develop adsorbents for various important use cases.Chapter 1 introduces metal–organic frameworks, highlights how bulk properties can be controlled at the molecular level, and notable examples are provided. Key concepts of adsorption, gas separations, and hydrogen storage are discussed. Emphasis is placed on the various mechanisms of gas binding and the implications for gas separations and storage applications. The limitations of adsorbents today are discussed, and some of the outstanding challenges in gas separations are outlined. The idea of expanding the functionality of adsorbents based on orbital interactions is established. Chapter 2 describes the first example of a porous material with coordinatively unsaturated vanadium(II) sites, V2Cl2.8(btdd). The material is characterized by powder X-ray diffraction (PXRD), K-edge X-ray absorption spectroscopy, infrared spectroscopy, solid-state NMR, and gas adsorption experiments. The exposed vanadium sites are capable of backbonding with industrially- relevant π-acids, such as N2. This orbital-mediated binding and high-density of binding sites gives rise to record N2 uptake capacities and importantly enables selective capture of N2 in the presence of CH4. This work expands the functionality available to adsorbents to now include the ability to discriminate gas mixtures based on π-acidities. This has immensely important implications for industrial use, as many industrial processes utilize π-acidic gases. Chapter 3 explores the hydrogen adsorption properties of V2Cl2.8(btdd). While hydrogen is a promising zero-emission transportation fuel, its implementation is limited in part by the difficulty in its storage. Current approaches require high pressures or cryogenic temperatures to reach sufficiently high storage densities. An adsorbent with an optimal binding enthalpy for ambient-temperature hydrogen storage could transform the transportation sector. In this chapter, we show that V2Cl2.8(btdd) is the first porous material that exhibits an optimal binding enthalpy, −21kJ/mol. The orbital-mediated binding, or Kubas-type complexation, is characterized by neutron diffraction, infrared spectroscopy, and electronic structure calculations. Ultimately, the pursuit of MOFs containing high densities of weakly π-basic metal sites may enable storage capacities under ambient conditions that far surpass those accessible with compressed gas storage Chapter 4 details a spectrochemical walk of relevant industrial adsorbates (H2, C3H8, C2H6, CH4, CO2, N2, C3H6, and C2H4) with V2Cl2.8(btdd). Thermodynamic binding values are obtain and electronic structure calculations are used to understand orbital interactions and their contributions to binding. Notably, V2Cl2.8(btdd) exhibits a higher enthalpy for methane than ethane and propane, in contrast to traditional adsorbents with open metal sites. This surprising result is explained by the unique orbital-mediated binding as well as steric effects. Importantly, this binding mechanism establishes a new design principle to realize selective capture of methane over other paraffins. Methane binding is characterized further by high pressure gas adsorption experiments. Chapter 5 presents a perspective on oxygen-selective adsorbents for air separations. The need for small-scale, low-capital air separation units is described. Limitations of current adsorbents are discussed and molecular principles for O2 capture are highlighted. The bulk of the chapter details with key considerations for designing oxygen-selective adsorbents that can greatly outperform incumbent technologies. A thermodynamic analysis is described which is used to benchmark materials based on the ∆G of O2 binding. The importance of entropy of O2 binding and its effect on working capacities is emphasized. Promising research direction are described, and calculation results are included which substantiate some of the proposed directions.

Advisor: Long, Jeffrey R.