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MFI-Type Zeolite Nanosheets Laminated Membranes for Ion Separation in Aqueous Solutions

Cao, Zishu
http://rave.ohiolink.edu/etdc/view?acc_num=ucin1593269786201009

Abstract Details

2020, PhD, University of Cincinnati, Engineering and Applied Science: Chemical Engineering.
Zeolite membranes possess tremendous potentials for efficient molecular separation via size-exclusion effects. However, practical realizations of this great potential have been hindered by nonselective permeation through intercrystalline spaces and high resistance to intracrystalline diffusion in the conventional polycrystalline zeolite membranes. Zeolite nanosheets with nanometer-scale thickness and very large aspect ratios offer new opportunities to control crystal orientation and minimize thickness of zeolite-composite membranes for achieving enhanced selectivity and flux in molecular and ion separations. However, there is currently a lack of success in synthesizing zeolite nanosheets with low Si/Al ratios in the framework, which is desired for high-performance ion separations or proton-selective ion conduction in aqueous solutions. This dissertation presents studies on the synthesis of alumina containing zeolite nanosheets, formation of zeolite nanosheet laminated membranes (ZNLM), and mechanisms of mass transport in the ZNLM for important applications in high concentration brine desalination and ion separations in redox flow battery (RFB). A continuous hydrothermal crystallization method with a scheduled change of precursor composition has been established for the synthesis of ZSM-5 (i.e. Al-containing MFI type) zeolite nanosheets. ZSM-5 nanosheets with micrometer-scale lateral dimensions (1.5 – 4 µm) and nanometer thickness (~6.7 nm) in the preferred straight channel direction (i.e. b-orientation) have been achieved for the first time. The ZSM-5 nanosheets have been used to fabricate ultrathin (<500nm) laminated membranes on macroporous a-alumina substrates. This ultrathin b-oriented ZSM-5 ZNLM with high surface hydrophilicity has demonstrated significantly higher water flux than the pure-silica MFI zeolite (silicalite) ZNLM. It also showed extraordinary water flux combined with high salt rejection in pervaporation desalination for brines containing up to 24 wt.% of dissolved NaCl. The ZNLM may offer practical solutions to the issues of inherent intercrystalline spaces along the thickness and the long and tortuous intracrystalline pathways that lower the selectivity and permeability for conventional polycrystalline zeolite membranes. Synthesis of the ZNLM on practically meaningful polymer supports has been investigated and the ZNLM-polymer composite membranes have been evaluated for application as ion separators in RFB to simultaneously reduce the metal ion crossover rates and proton transport resistance for enhancing the energy efficiency. To enable the fabrication of ZNLM on polymer substrates, a room temperature UV-irradiation method has been developed for template removal from the zeolite nanosheets that avoids the difficulty of zeolite activation after the composite membrane formation. The composite membrane fabrication is accomplished by synthesis of a ~350 nm-thick free-standing UV-activated ZSM-5 ZNLM, which is subsequently sandwiched in between two Nafion thing films to form a layered structure with an overall thickness of ~120 µm. This zeolite nanosheet-Nafion composite membrane has exhibited drastically improved proton selectivity over the vanadium ions and reduced proton transport resistance as compared to the benchmark Nafion-117 membrane. The composite membrane has achieved higher Coulombic and voltage efficiencies in all-vanadium RFB operations as compared to the Nafion-117 membrane. Future research efforts are needed for further understanding the mass transfer behaviors through the multi-layered nanosheet structures and developing scalable methods for membrane fabrication.
Junhang Dong, Ph.D. (Committee Chair)
Anastasios Angelopoulos, Ph.D. (Committee Member)
Jianbing Jiang, Ph.D. (Committee Member)
Vesselin Shanov, Ph.D. (Committee Member)
George Sorial, Ph.D. (Committee Member)
134 p.
Chemical Engineering
zeolite nanosheets; laminated membranes; desalination; redox flow batteries; transport mechanisms

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Citations

  • Cao, Z. (2020). MFI-Type Zeolite Nanosheets Laminated Membranes for Ion Separation in Aqueous Solutions [Doctoral dissertation, University of Cincinnati]. OhioLINK Electronic Theses and Dissertations Center. http://rave.ohiolink.edu/etdc/view?acc_num=ucin1593269786201009

    APA Style (7th edition)

  • Cao, Zishu. MFI-Type Zeolite Nanosheets Laminated Membranes for Ion Separation in Aqueous Solutions. 2020. University of Cincinnati, Doctoral dissertation. OhioLINK Electronic Theses and Dissertations Center, http://rave.ohiolink.edu/etdc/view?acc_num=ucin1593269786201009.

    MLA Style (8th edition)

  • Cao, Zishu. "MFI-Type Zeolite Nanosheets Laminated Membranes for Ion Separation in Aqueous Solutions." Doctoral dissertation, University of Cincinnati, 2020. http://rave.ohiolink.edu/etdc/view?acc_num=ucin1593269786201009

    Chicago Manual of Style (17th edition)

ucin1593269786201009
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This open access ETD is published by University of Cincinnati and OhioLINK.