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Open Access Publications from the University of California

Scholarly Works (19 results)

  • Thesis
  • Peer Reviewed
UC Berkeley Electronic Theses and Dissertations (2015)

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.

    Cover page: Metal-Organic Frameworks for Gas Storage and Separation
    • Article
    • Peer Reviewed
    UC Berkeley Previously Published Works (2015)
    We describe a robust screening technique that correlates the surface area of metal-organic frameworks to the proton T2 relaxation behavior of imbibed solvent at low field (13 MHz). In frameworks with small pore sizes (<1 nm) or strong solvent-framework interactions, diffusional exchange between the pore-confined and inter-particle solvent populations remains slow compared to the T2 of the pore-confined solvent, allowing for a direct porosity analysis of the T2 spectrum obtained from Laplace inversions. Increases in framework pore-size (>1 nm) lead to corresponding increases in the rate of solvent exchange, as confirmed by T2 relaxation exchange (REXSY) experiments; increases in the pore size also increases the T2 of the pore-confined solvent. The combination of these two effects results in comparable rates of relaxation and exchange, which precludes the direct analysis of Laplace inversions. Thus, two- and three-site kinetics models were applied to extract porosity from relaxation decays, thereby improving the utility of the porosity screening tool.
      Cover page: NMR relaxation and exchange in metal–organic frameworks for surface area screening
      • Article
      • Peer Reviewed
      UC Berkeley Previously Published Works (2016)
      To store natural gas (NG) inexpensively at adequate densities for use as a fuel in the transportation sector, new porous materials are being developed. This work uses computational methods to explore strategies for improving the usable methane storage capacity of adsorbents, including metal-organic frameworks (MOFs), that feature open-metal sites incorporated into their structure by postsynthetic modification. The adsorption of CH4 on several open-metal sites is studied by calculating geometries and adsorption energies and analyzing the relevant interaction factors. Approximate site-specific adsorption isotherms are obtained, and the open-metal site contribution to the overall CH4 usable capacity is evaluated. It is found that sufficient ionic character is required, as exemplified by the strong CH4 affinities of 2,2'-bipyridine-CaCl2 and Mg, Ca-catecholate. In addition, it is found that the capacity of a single metal site depends not only on its affinity but also on its geometry, where trigonal or "bent" low-coordinate exposed sites can accommodate three or four methane molecules, as exemplified by Ca-decorated nitrilotriacetic acid. The effect of residual solvent molecules at the open-metal site is also explored, with some positive conclusions. Not only can residual solvent stabilize the open-metal site, surprisingly, solvent molecules do not necessarily reduce CH4 affinity, but can contribute to increased usable capacity by modifying adsorption interactions.
        Cover page: A computational study of CH 4 storage in porous framework materials with metalated linkers: connecting the atomistic character of CH 4 binding sites to usable capacity
        • Article
        • Peer Reviewed
        UC Berkeley Previously Published Works (2022)
        Natural gas constitutes a growing share of global primary energy due to its abundant supply and lower CO2 emission intensity compared to coal. For many natural gas reserves, CO2 contamination must be removed at the wellhead to meet pipeline specifications. Here, we demonstrate the potential of the diamine-appended metal-organic framework ee-2-Mg2(dobpdc) (ee-2 = N,N-diethylethylenediamine; dobpdc4- = 4,4'-dioxidobiphenyl-3,3'-dicarboxylate) as a next-generation CO2 capture material for high-pressure natural gas purification. Owing to a cooperative adsorption mechanism involving formation of ammonium carbamate chains, ee-2-Mg2(dobpdc) can be readily regenerated with a minimal change in temperature or pressure and maintains its CO2 capacity in the presence of water. Moreover, breakthrough experiments reveal that water enhances the CO2 capture performance of ee-2-Mg2(dobpdc) by eliminating "slip" of CO2 before full breakthrough. Spectroscopic characterization and multicomponent adsorption isobars suggest that the enhanced performance under humid conditions arises from preferential stabilization of the CO2-inserted phase in the presence of water. The favorable performance of ee-2-Mg2(dobpdc) is further demonstrated through comparison with a benchmark material for this separation, zeolite 13X, as well as extended pressure cycling. Overall, these results support continued development of ee-2-Mg2(dobpdc) as a promising adsorbent for natural gas purification.
          Cover page: A cooperative adsorbent for the switch-like capture of carbon dioxide from crude natural gas
          • Article
          • Peer Reviewed
          UC Berkeley Previously Published Works (2015)
          Metal-organic frameworks featuring pores lined with exposed metal cations have received attention for a wide range of adsorption-related applications. While many frameworks with coordinatively unsaturated M(II) centers have been reported, there are relatively few examples of porous materials with coordinatively unsaturated M(III) centers. Here, we report the synthesis and characterization of Ti3O(OEt)(bdc)3(solv)2 (Ti-MIL-101; bdc(2-) = 1,4-benzenedicarboxylate; solv = N,N-dimethylformamide, tetrahydrofuran), the first metal-organic framework containing exclusively Ti(III) centers. Through a combination of gas adsorption, X-ray diffraction, magnetic susceptibility, and electronic and vibrational spectroscopy measurements, this high-surface-area framework is shown to contain five-coordinate Ti(III) centers upon desolvation, which irreversibly bind O2 to form titanium(IV) superoxo and peroxo species. Electronic absorption spectra suggest that the five-coordinate Ti(III) sites adopt a distorted trigonal-bipyramidal geometry that effectively shields nuclear charge and inhibits strong adsorption of nonredox-active gases.
            Cover page: Synthesis and O2 Reactivity of a Titanium(III) Metal–Organic Framework
            • Article
            • Peer Reviewed
            UC Berkeley Previously Published Works (2017)
            Small-molecule binding in metal-organic frameworks (MOFs) can be accurately studied both experimentally and computationally, provided the proper tools are employed. Herein, we compare and contrast properties associated with guest binding by means of density functional theory (DFT) calculations using nine different functionals for the M2(dobdc) (dobdc4- = 2,5-dioxido,1,4-benzenedicarboxylate) series, where M = Mg, Mn, Fe, Co, Ni, Cu, and Zn. Additionally, we perform Quantum Monte Carlo (QMC) calculations for one system to determine if this method can be used to assess the performance of DFT. We also make comparisons with previously published experimental results for carbon dioxide and water and present new methane neutron powder diffraction (NPD) data for further comparison. All of the functionals are able to predict the experimental variation in the binding energy from one metal to the next; however, the interpretation of the performance of the functionals depends on which value is taken as the reference. On the one hand, if we compare against experimental values, we would conclude that the optB86b-vdW and optB88-vdW functionals systematically overestimate the binding strength, while the second generation of van der Waals (vdW) nonlocal functionals (vdw-DF2 and rev-vdW-DF2) correct for this providing a good description of binding energies. On the other hand, if the QMC calculation is taken as the reference then all of the nonlocal functionals yield results that fall just outside the error of the higher-level calculation. The empirically corrected vdW functionals are in reasonable agreement with experimental heat of adsorptions but under bind when compared with QMC, while Perdew-Burke-Ernzerhof fails by more than 20 kJ/mol regardless of which reference is employed. All of the functionals, with the exception of vdW-DF2, predict reasonable framework and guest binding geometries when compared with NPD measurements. The newest of the functionals considered, rev-vdW-DF2, should be used in place of vdW-DF2, as it yields improved bond distances with similar quality binding energies.
              • Article
              • Peer Reviewed
              UC Berkeley Previously Published Works (2015)
              Despite the large number of metal-organic frameworks that have been studied in the context of post-combustion carbon capture, adsorption equilibria of gas mixtures including CO2, N2, and H2O, which are the three biggest components of the flue gas emanating from a coal- or natural gas-fired power plant, have never been reported. Here, we disclose 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 presented in this work 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, dobpdc (4-) = 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.
                Cover page: Application of a High-Throughput Analyzer in Evaluating Solid Adsorbents for Post-Combustion Carbon Capture via Multicomponent Adsorption of CO2, N2, and H2O
                • Article
                • Peer Reviewed
                UC Berkeley Previously Published Works (2015)
                A thorough experimental and computational study has been carried out to elucidate the mechanistic reasons for the high volumetric uptake of methane in the metal-organic framework Cu3(btc)2 (btc(3-) = 1,3,5-benzenetricarboxylate; HKUST-1). Methane adsorption data measured at several temperatures for Cu3(btc)2, and its isostructural analogue Cr3(btc)2, show that there is little difference in volumetric adsorption capacity when the metal center is changed. In situ neutron powder diffraction data obtained for both materials were used to locate four CD4 adsorption sites that fill sequentially. This data unequivocally shows that primary adsorption sites around, and within, the small octahedral cage in the structure are favored over the exposed Cu(2+) or Cr(2+) cations. These results are supported by an exhaustive parallel computational study, and contradict results recently reported using a time-resolved diffraction structure envelope (TRDSE) method. Moreover, the computational study reveals that strong methane binding at the open metal sites is largely due to methane-methane interactions with adjacent molecules adsorbed at the primary sites instead of an electronic interaction with the metal center. Simulated methane adsorption isotherms for Cu3(btc)2 are shown to exhibit excellent agreement with experimental isotherms, allowing for additional simulations that show that modifications to the metal center, ligand, or even tuning the overall binding enthalpy would not improve the working capacity for methane storage over that measured for Cu3(btc)2 itself.
                  Cover page: Critical Factors Driving the High Volumetric Uptake of Methane in Cu₃(btc)₂.
                  • Article
                  • Peer Reviewed
                  UC Berkeley Previously Published Works (2016)
                  We present accurate force fields developed from density functional theory (DFT) calculations with periodic boundary conditions for use in molecular simulations involving M2(dobdc) (M-MOF-74; dobdc4- = 2,5-dioxidobenzenedicarboxylate; M = Mg, Mn, Fe, Co, Ni, Zn) and frameworks of similar topology. In these systems, conventional force fields fail to accurately model gas adsorption due to the strongly binding open-metal sites. The DFT-derived force fields predict the adsorption of CO2, H2O, and CH4 inside these frameworks much more accurately than other common force fields. We show that these force fields can also be used for M2(dobpdc) (dobpdc4- = 4,4′-dioxidobiphenyl-3,3′-dicarboxylate), an extended version of MOF-74, and thus are a promising alternative to common force fields for studying materials similar to MOF-74 for carbon capture applications. Furthermore, it is anticipated that the approach can be applied to other metal-organic framework topologies to obtain force fields for different systems. We have used this force field to study the effect of contaminants such as H2O and N2 upon these materials' performance for the separation of CO2 from the emissions of natural gas reservoirs and coal-fired power plants. Specifically, mixture adsorption isotherms calculated with these DFT-derived force fields showed a significant reduction in the uptake of many gas components in the presence of even trace amounts of H2O vapor. The extent to which the various gases are affected by the concentration of H2O in the reservoir is quantitatively different for the different frameworks and is related to their heats of adsorption. Additionally, significant increases in CO2 selectivities over CH4 and N2 are observed as the temperature of the systems is lowered.
                    • Article
                    • Peer Reviewed
                    UC Berkeley Previously Published Works (2017)
                    The crystallographic characterization of framework-guest interactions in metal-organic frameworks allows the location of guest binding sites and provides meaningful information on the nature of these interactions, enabling the correlation of structure with adsorption behavior. Here, techniques developed for in situ single-crystal X-ray diffraction experiments on porous crystals have enabled the direct observation of CO, CH4, N2, O2, Ar, and P4 adsorption in Co2(dobdc) (dobdc4- = 2,5-dioxido-1,4-benzenedicarboxylate), a metal-organic framework bearing coordinatively unsaturated cobalt(ii) sites. All these molecules exhibit such weak interactions with the high-spin cobalt(ii) sites in the framework that no analogous molecular structures exist, demonstrating the utility of metal-organic frameworks as crystalline matrices for the isolation and structural determination of unstable species. Notably, the Co-CH4 and Co-Ar interactions observed in Co2(dobdc) represent, to the best of our knowledge, the first single-crystal structure determination of a metal-CH4 interaction and the first crystallographically characterized metal-Ar interaction. Analysis of low-pressure gas adsorption isotherms confirms that these gases exhibit mainly physisorptive interactions with the cobalt(ii) sites in Co2(dobdc), with differential enthalpies of adsorption as weak as -17(1) kJ mol-1 (for Ar). Moreover, the structures of Co2(dobdc)·3.8N2, Co2(dobdc)·5.9O2, and Co2(dobdc)·2.0Ar reveal the location of secondary (N2, O2, and Ar) and tertiary (O2) binding sites in Co2(dobdc), while high-pressure CO2, CO, CH4, N2, and Ar adsorption isotherms show that these binding sites become more relevant at elevated pressures.
                      Cover page: Structural characterization of framework–gas interactions in the metal–organic framework Co 2 (dobdc) by in situ single-crystal X-ray diffraction
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