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Open Access Publications from the University of California

Scholarly Works (785 results)

  • Thesis
  • Peer Reviewed
UC Berkeley Electronic Theses and Dissertations (2019)

In this thesis, I have used computational methods to the study of flexibility in metal-organic frameworks (MOFs) and the behavior of adsorbed gases and liquids in MOFs. Among nanoporous materials, MOFs are of particular interest for adsorption-based applications for reasons that include (1) their huge chemical space, which raises the possibility of tuning them for specific industrial processes and (2) the ability of the under-coordinated metals present in some MOFs to endow their respective materials with impressive adsorbate binding energies.

Chapters 2 and 4 relate to analogs of a series of MOFs with under-coordinated metals (the M-MOF-74 series). In Chapter 2, I report the discovery of a novel argon adsorbate-induced deformation pattern for this framework series. This result was arrived at using a flexible framework model, and is presented as an explanation for an intriguing signal observed in experimental small-angle X-ray scattering profiles upon argon adsorption. This hypothesis is supported by a complementary investigation of an alternative explanation for the X-ray signal, in which argon atoms are proposed to adsorb at different densities in adjacent pores.

In Chapter 3, I studied the dynamics of adsorbed xylene isomers in MOF-5. I compare the translational and rotational motion of the xylene isomers, and explain the differences based on molecular geometries. The most rod-like molecule, para-xylene, is more rotationally constricted in the pore due to its intermolecular interactions with the aromatic group of the MOF linker. This finding has implications for the rational design of MOFs, as a process that can take advantage of the MOF-induced variations in xylene isomer dynamics could be used for lucrative liquid-phase xylene separations.

In Chapter 4, I present a method for parameterizing the type of flexible framework model used in Chapter 2 from ab initio quantum chemistry calculations. The goal of this work is to facilitate the rapid development of flexible, versatile MOF models that can capture changes in the coordination bonding of metals in MOFs. This type of model can be used to study structural transitions and may be extended to study MOF formation. We demonstrated that our approach yields models for the M-MOF-74 series that are stable and have simulated structural properties in good agreement with quantum chemistry calculations.

    Cover page: Computational Investigations of Coordination Bonding and Adsorbate Properties in Metal-Organic Frameworks
    • Article
    • Peer Reviewed
    UC Berkeley Previously Published Works (2017)
    IRMOF-74 analogues are among the most widely studied metal-organic frameworks (MOFs) for adsorption applications because of their one-dimensional channels and high metal density. Most studies involving the IRMOF-74 series assume that the crystal lattice is rigid. This assumption guides the interpretation of experimental data, as changes in the crystal symmetry have so far been ignored as a possibility in the literature. Here, we report a deformation pattern, induced by the adsorption of argon, for IRMOF-74-V. This work has two main implications. First, we use molecular simulations to demonstrate that the IRMOF-74 series undergoes a deformation that is similar to the mechanism behind breathing MOFs, but is unique because the deformation pattern extends beyond a single unit cell of the original structure. Second, we provide an alternative interpretation of experimental small-angle X-ray scattering profiles of these systems, which changes how we view the fundamentals of adsorption in this MOF series.
      • Article
      • Peer Reviewed
      UC Berkeley Previously Published Works (2017)
      For applications of metal-organic frameworks (MOFs) such as gas storage and separation, flexibility is often seen as a parameter that can tune material performance. In this work we aim to determine the optimal flexibility for the shape selective separation of similarly sized molecules (e.g., Xe/Kr mixtures). To obtain systematic insight into how the flexibility impacts this type of separation, we develop a simple analytical model that predicts a material's Henry regime adsorption and selectivity as a function of flexibility. We elucidate the complex dependence of selectivity on a framework's intrinsic flexibility whereby performance is either improved or reduced with increasing flexibility, depending on the material's pore size characteristics. However, the selectivity of a material with the pore size and chemistry that already maximizes selectivity in the rigid approximation is continuously diminished with increasing flexibility, demonstrating that the globally optimal separation exists within an entirely rigid pore. Molecular simulations show that our simple model predicts performance trends that are observed when screening the adsorption behavior of flexible MOFs. These flexible simulations provide better agreement with experimental adsorption data in a high-performance material that is not captured when modeling this framework as rigid, an approximation typically made in high-throughput screening studies. We conclude that, for shape selective adsorption applications, the globally optimal material will have the optimal pore size/chemistry and minimal intrinsic flexibility even though other nonoptimal materials' selectivity can actually be improved by flexibility. Equally important, we find that flexible simulations can be critical for correctly modeling adsorption in these types of systems.
        Cover page: The Influence of Intrinsic Framework Flexibility on Adsorption in Nanoporous Materials
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        • Peer Reviewed
        UC Berkeley Previously Published Works (2017)
        We combined nuclear magnetic resonance (NMR) and molecular dynamics (MD) simulation to study xylene behavior in MOF-5, probing the effects of adsorbate geometry in a weakly interacting model isotropic metal organic framework (MOF) system. We employed NMR diffusometry and relaxometry techniques at low field (13 MHz) to quantify the self-diffusion coefficients (Ds) and the longitudinal relaxation times (T1) of xylenes in MOF-5 as a function of temperature at the saturated loading for each xylene. These experiments reveal the translational motion activation energies to be 15.3, 19.7, and 21.2 kj mol-1 and the rotational activation energies to be 47.26, 12.88, and 11.55 for the (p-, m-, o-) xylene isomers, respectively. Paraxylene exhibits faster translational motion, yet shows four times the activation energy barrier for rotational motion vis-à-vis the other isomers. MD simulations performed on these model systems corroborate the findings for paraxylene and suggest that paraxylene has the lower free energy barrier for hopping away from its binding sites, yet has the slowest rotational motion in the plane of the xylene molecule.
          Cover page: Translational and Rotational Motion of C8 Aromatics Adsorbed in Isotropic Porous Media (MOF-5): NMR Studies and MD Simulations
          • Article
          • Peer Reviewed
          UC Berkeley Previously Published Works (2018)
          We present a comprehensive investigation of the CO2 adsorption properties of an isostructural series of metal-organic frameworks, M-BTT (M = Cr, Mn, Fe, Cu; BTT3- = 1,3,5-benzenetristetrazolate), which exhibit a high density of open metal sites capable of polarizing and binding guest molecules. Coupling gas adsorption measurements with in situ neutron and X-ray diffraction experiments provides molecular-level insight into the adsorption process and enables rationalization of the observed adsorption isotherms. In particular, structural data confirms that the high initial isosteric heats of CO2 adsorption for the series are directly correlated with the presence of open metal sites and further reveals the positions and orientations of as many as three additional adsorption sites. Density functional theory calculations that include van der Waals dispersion corrections quantitatively support the observed structural features associated with the primary and secondary CO2 binding sites, including CO2 positions and orientations, as well as the experimentally determined isosteric heats of CO2 adsorption.
            Cover page: An experimental and computational study of CO 2 adsorption in the sodalite-type M-BTT (M = Cr, Mn, Fe, Cu) metal–organic frameworks featuring open metal sites
            • Article
            • Peer Reviewed
            UC Berkeley Previously Published Works (2021)
            Significant advances have been made in the synthesis of chemically selective environments within metal-organic frameworks, yet materials development and industrial implementation have been hindered by the inability to predictively control crystallite size and shape. One common strategy to control crystal growth is the inclusion of coordination modulators, which are molecular species designed to compete with the linker for metal coordination during synthesis. However, these modulators can simultaneously alter the pH of the reaction solution, an effect that can also significantly influence crystal morphology. Herein, noncoordinating buffers are used to independently control reaction pH during metal-organic framework synthesis, enabling direct interrogation of the role of the coordinating species on crystal growth. We demonstrate the efficacy of this strategy in the synthesis of low-dispersity single-crystals of the framework Co2(dobdc) (dobdc4-= 2,5-dioxido-1,4-benzenedicarboxylate) in a pH 7-buffered solution using cobalt(II) acetate as the metal source. Density functional theory calculations reveal that acetate competitively binds to Co during crystallization, and by using a series of cobalt(II) salts with carboxylate anions of varying coordination strength, it is possible to control crystal growth along the c-direction. Finally, we use zero length column chromatography to show that crystal morphology has a direct impact on guest diffusional path length for the industrially important hydrocarbon m-xylene. Together, these results provide molecular-level insight into the use of modulators in governing crystallite morphology and a powerful strategy for the control of molecular diffusion rates within metal-organic frameworks.
              Cover page: Buffered Coordination Modulation as a Means of Controlling Crystal Morphology and Molecular Diffusion in an Anisotropic Metal–Organic Framework
              • Article
              • Peer Reviewed
              UC Berkeley Previously Published Works (2018)
              Metal-organic frameworks are promising materials for energy-efficient gas separations, but little is known about the diffusion of adsorbates in materials featuring one-dimensional porosity at the nanoscale. An understanding of the interplay between framework structure and gas diffusion is crucial for the practical application of these materials as adsorbents or in mixed-matrix membranes, since the rate of gas diffusion within the adsorbent pores impacts the required size (and therefore cost) of the adsorbent column or membrane. Here, we investigate the diffusion of CO2 within the pores of Zn2(dobpdc) (dobpdc4- = 4,4'-dioxidobiphenyl-3,3'-dicarboxylate) using pulsed field gradient (PFG) nuclear magnetic resonance (NMR) spectroscopy and molecular dynamics (MD) simulations. The residual chemical shift anisotropy for pore-confined CO2 allows PFG NMR measurements of self-diffusion in different crystallographic directions, and our analysis of the entire NMR line shape as a function of the applied field gradient provides a precise determination of the self-diffusion coefficients. In addition to observing CO2 diffusion through the channels parallel to the crystallographic c axis (self-diffusion coefficient D∥ = (5.8 ± 0.1) × 10-9 m2 s-1 at a pressure of 625 mbar CO2), we unexpectedly find that CO2 is also able to diffuse between the hexagonal channels in the crystallographic ab plane (D⊥ = (1.9 ± 0.2) × 10-10 m2 s-1), despite the walls of these channels appearing impermeable by single-crystal X-ray crystallography and flexible lattice MD simulations. Observation of such unexpected diffusion in the ab plane suggests the presence of defects that enable effective multidimensional CO2 transport in a metal-organic framework with nominally one-dimensional porosity.
                Cover page: Unexpected Diffusion Anisotropy of Carbon Dioxide in the Metal–Organic Framework Zn2(dobpdc)
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                • Peer Reviewed
                UC Irvine Previously Published Works (1999)
                The central drift chamber for the BaBar detector at the SLAC B-factory is based on a hexagonal cell design with 7104 cells arranged in 40 layers and drift gas Helium:isobutane (80%:20%). Performance optimization and integration requirements led to an electronics design that mounts the amplifier-discriminator and digitizing circuitry directly on the endplate. High channel density is achieved using a 4-channel custom amplifier-discriminator IC and an 8-channel custom CMOS TDC/FADC IC on a single circuit board. Data read from the front ends are multiplexed on 4 fiber optic links, and prompt trigger data are sent out continuously on 24 links. Analysis of cosmic ray data demonstrates that the electronics design meets the performance goals for the BaBar drift chamber. The final electronics were installed on the drift chamber in July, 1998. Installation of BaBar on beamline is scheduled for March, 1999. © 1999 IEEE.
                  Cover page: Electronics for the BaBar central drift chamberCreative Commons 'BY' version 4.0 license
                  • Article
                  • Peer Reviewed
                  LBL Publications (1989)
                  The branching ratio for inclusive ψ production in ψ(1S) decays is measured to be (0.11 ± 0.04 ± 0.02)%. © 1989.
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                    • Peer Reviewed
                    LBL Publications (1989)
                    Using the CLEO detector, we have measured the branching ratios for the exclusive decays of the Ds+ meson into K*0K+, K0K+, and K*+K0 relative to the decay Ds+→φπ+ to be close to unity. We have also measured the mass diference between the Ds+ and D+ meson to be 98.5±1.5 MeV/c2. © 1989.
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