Proton and Ion Conductivity in Microporous Materials

Citation

McKeen, John Charles (2009) Proton and Ion Conductivity in Microporous Materials. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/N0GH-3B50. https://resolver.caltech.edu/CaltechETD:etd-05272009-144416

Abstract

Two separate but related investigations on proton and ion conductivity in microporous materials are presented. In the first study, the effect of hydroxyl group density on the proton conductivity in a family of sulfonic-acid functionalized microporous materials with the *BEA framework topology is investigated. Pure silica, aryl sulfonic acid-functionalized microporous materials are synthesized with high concentration of hydroxyl groups from tetraethylammonium hydroxide and aluminum-containing gels and by post synthetic modification of zinc containing CIT-6, and they exhibit proton conductivities of ~5*10⁻³ S/cm. Pure-silica materials with nearly defect free structures (low hydroxyl group density) are synthesized from tetraethylammonium fluoride containing gels and by post synthetic modification of CIT-6, and exhibit proton conductivities an order of magnitude lower, ~5 x 10⁻⁴ S/cm, than the samples with a high –OH density. Propyl sulfonic acid, ethyl phosphoric acid, and carboxylic acid containing pure-silica zeolite beta, MCM-41, and MCM-48 are also prepared and investigated for use as solid proton conductors. Strong acids are necessary for fast proton transport in hydrated systems. The proton conductivities of the functionalized solids decrease accordingly with the strength of the organic acids in solutions, and little difference is observed between microporous and mesoporous solids when functionalized to the same level.

In the second study, the ion conducting properties of the microporous, zincosilicate VPI-9 (Si/Zn = 4.0) containing the alkali cations Li⁺, Na⁺, K⁺, Rb⁺, and Cs⁺, and the alkaline earth cations Mg²⁺, Ca²⁺, and Sr²⁺ are studied using impedance spectroscopy. Monovalent cation exchanged samples Li- and Na-VPI-9 lose X-ray crystallinity upon vacuum dehydration at 450 °C, while K-, Rb-, and Cs-VPI-9 remain crystalline and exhibit conductivities of 1.7 x 10⁻⁴, 3.5 x 10⁻⁴, and 4.9 x 10⁴ S/cm, respectively, at 450 °C in vacuum. While K-VPI-9 is less conductive than K-X, Rb- and Cs-VPI-9 are more conductive than corresponding zeolite X samples. Divalent cation exchanged sample Mg-VPI-9 also loses X-ray crystallinity, while Ca-, and Sr-VPI-9 remain crystalline and exhibit conductivities of 2.3 x 10⁻⁶ S/cm and 7.7 x 10⁷ S/cm, respectively, at 450 °C, greatly exceeding the conductivity of correspondingly divalent exchanged zeolite X materials. Dense, crystalline zincosilicate samples with the compositions K₂ZnSi_xO₂_(x=1) (x = 2 - 5), Rb₂ZnSi₅O₁₂, and Cs₂ZnSi₅O₁₂ are also prepared and exhibit much lower conductivities than their microporous counterparts.

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