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Open Access Publications from the University of California

Scholarly Works (3 results)

  • Thesis
  • Peer Reviewed
UC Berkeley Electronic Theses and Dissertations (2022)

Zeolites are crystalline microporous solids composed of corner-sharing, tetrahedrally- coordinated silicate (SiO4) units. The isomorphic substitution of a framework Si atom by an Al atom is charge-compensated by a proton, introducing Brønsted-acidic bridging-hydroxy groups. These proton-exchanged zeolites are used in a large number of processes, including hydrocarbon cracking, isomerization, and alkylation, and conversion of petroleum to trans- portation fuel. In addition to catalytic processes, zeolites can be used as adsorbents for carbon capture, molecular sieving, and pollution control technologies. It is therefore of great interest to predict the impact of zeolite structure and composition on its functional proper- ties to screen new catalysts and improve existing catalysts. Quantum chemical calculations can provide molecular-scale information on zeolite-adsorbate interactions, as well as model the energetic changes and dynamics of important reactions that occur within the channels and pores of zeolite catalysts. However, the application of quantum chemical calculations for the study of chemical reactions occurring in zeolites is made difficult by the lack of reliable methods to generalize the theory beyond zero Kelvin. Modeling adsorption and desorption free energies is particularly troublesome, relying on a subtle balance between enthalpic and entropic terms. While the enthalpic term is becoming ever more accurate through density functional development, the much more temperature-sensitive entropic term remains gener- ally underquantified by frequently-assumed harmonic approximations. The consequence is an inability to computationally replicate experimental observations, such as rate coefficients or equilibrium constants, with chemical accuracy. This work is concerned with a re-examination of harmonic approximations, including quantifying its failures in reaction kinetics of aldol condensation on isolated metal sites, exploring alternative approximation methods for anhar- monic intramolecular motions, and developing new methods for approximating anharmonic external molecular motions for species adsorbed in zeolites.

    Cover page: Ab Initio Methods for Modeling the Thermodynamics of Molecules Adsorbed in Zeolites
    • Article
    • Peer Reviewed
    UC Berkeley Previously Published Works (2019)
    Amination of isobutene with NH3 was investigated over Brønsted acidic zeolites at 1 atm and 453-483 K. To compare catalytic activities over different zeolites, the measured reaction rates are normalized by the number of active sites determined by tert-butylamine temperature-programmed desorption (TPD). Small- A nd medium-pore zeolites with one-dimensional channels exhibit low activity because of pore blockage by adsorbed tert-butylammonium ions. However, turnover frequencies and activation energies are not sensitive to framework identity, as long as the active site is accessible to isobutene and tert-butylamine. Kinetic measurements and FTIR spectroscopy reveal that the Brønsted acid sites in MFI are covered predominantly with tert-butylammonium ions under reaction conditions. The desorption of tert-butylamine is assisted by the concurrent adsorption of isobutene. DFT simulations show that at very low tert-butylamine partial pressures, for example, at the inlet to the reactor, tert-butylamine desorption is rate-limiting. However, at sufficiently high tert-butylamine partial pressures (>0.03 kPa), protonation of isobutene to the corresponding carbenium ion limits the rate of amination.
      Cover page: Zeolite-Catalyzed Isobutene Amination: Mechanism and Kinetics
      • Article
      • Peer Reviewed
      UC Berkeley Previously Published Works (2020)
      We report here the preparation and investigation of isolated Zr centers supported on a high surface area silica for the conversion of carboxylic acids to internal ketones by ketonic decarboxylation (ketonization) and the aldol condensation of ketones to dimeric enones. Catalysts were synthesized by the grafting of Cp2ZrCl2 on the surface of amorphous silica. The connectivity of Zr was characterized by XRD, UV-vis, and Raman spectroscopy. For the lowest Zr loading, Zr is present predominantly as isolated monomeric species. As the Zr loading is increased, a progressively larger fraction of Zr forms oligomeric species and ZrO2 nanoparticles. Measurements of catalytic activity show that the turnover frequency for carboxylic acid ketonic decarboxylation reaction and aldol condensation of ketones decreases monotonically with increasing Zr loading. An H/D kinetic isotope effect was not observed over isolated Zr catalysts, suggesting that α-H abstraction is not the rate-determining step, rather C-C bond forming may be rate limiting for both reactions. This conclusion is supported by computational modeling of the reaction mechanism. The proposed catalytic cycle for ketonization proceeds via a β-keto acid intermediate on isolated Zr sites that are always coordinatively saturated with C-C bond formation as the rate-limiting step. C-C bond formation is also rate-determining for aldol condensation, with an apparent activation energy that is in good agreement with the experiment if the resting state is a saturated ZrOH site with two adsorbed ketone molecules.
        Cover page: Experimental and Computational Studies of Carbon–Carbon Bond Formation via Ketonization and Aldol Condensation over Site-Isolated Zirconium Catalysts
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