Hydronium ion and water interactions with SiOSi, SiOAl, and AlOAl tetrahedral linkages

Abstract

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 2M1/nn+AlO₂ + 2SiO₂ = 2M1/nn+AlSiO₄. For this reaction ΔHmix = -100.4 kJ mol⁻¹ for M = Na, and ΔHmix = -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.