The minimum energy structures of H₆Si₂O₇, H₇SiAlO₇, and H₈Al₂O₇ have been calculated using quantum mechanical molecular orbital techniques. The calculated bond lengths and angles of H₆Si₂O₇ and H₇SiAlO₇ agree with those found in silicate and aluminosilicate minerals, but no such comparison is possible for H₈Al₂O₇ since we know of no aluminates having such an underbonded bridging oxygen (Pauling bond strength sum of 1.5). The total energies of the three molecules were used to model the stability of the SiOAI unit relative to the SiOSi and AIOAI units in framework aluminosilicates such as the feldspars and the zeolites. The calculated electronic energy for the reaction
H₆Si₂O₇ + H₈Al₂O₇ = 2H₇SiAlO₇
is positive (ca. + 20 kJ mol⁻¹). This result docs not adequately model the "aluminum avoidance rule," but the value is closer than previous calculations performed on energy optimized molecules (which gave ΔE = - 484 kJ moI⁻¹) to experimental enthalpies of mixing for the reaction
2M<sub>1/n</sub><sup>n+</sup>AlO₂ + 2SiO₂ = 2M<sub>1/n</sub><sup>n+</sup>AlSiO₄.
For this reaction ΔH<sub>mix</sub> = -100.4 kJ mol⁻¹ for M = Na, and ΔH<sub>mix</sub> = -75.6 kJ mol⁻¹ for M = Ca.
The calculated relative order of stability for reactions of water and hydronium ion with the hydroxyacid molecules used in this study was found to be
H₃O⁺ + SiOSi > H₃O⁺ + SiOAl
> H₂O + SiOAl
> H₃O⁺ + AlOAl
≃ H₂O + AlOAl
> H₂O + SiOSi
The results model the hydrophilic nature of aluminosilicate zeolites and the hydrophobic nature of the silicate zeolite silicalite. / M.S.
| Identifer | oai:union.ndltd.org:VTETD/oai:vtechworks.lib.vt.edu:10919/104320 |
| Date | January 1986 |
| Creators | Foley, Jeffrey Arthur |
| Contributors | Geological Sciences |
| Publisher | Virginia Polytechnic Institute and State University |
| Source Sets | Virginia Tech Theses and Dissertation |
| Language | English |
| Detected Language | English |
| Type | Thesis, Text |
| Format | v, 30 leaves, application/pdf, application/pdf |
| Rights | In Copyright, http://rightsstatements.org/vocab/InC/1.0/ |
| Relation | OCLC# 15062054 |
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