Thermodynamics of formation of molecular sieves

Thermodynamic investigations are undertaken to better understand the energetic differences amongst molecular sieve frameworks and the mechanisms and interactions important in molecular sieve self-assembly. The enthalpies relative to quartz at 298.15 K are determined by high-temperature solution calorimetry for a collection of calcined pure-silica molecular sieves with diverse structural features. SiO₂ molecular sieves are shown to be only modestly (6.8–14.4 kJ/mol) metastable with respect to quartz. The available thermal energy at typical synthesis conditions is RT = 3.5 kJ/mol. A strong correlation between enthalpy and molar volume is observed. The entropies of four pure-silica molecular sieves with a wide range of molar volumes are determined by heat capacity measurements from 5 to 400 K. The entropies of these structures are almost identical (3.2–4.2 J · K−1mol−1 above quartz). The enthalpy and entropy data are combined to calculate the Gibbs free energies of transition from quartz to eight other silica polymorphs. The molecular sieve Gibbs free energies are only 5.5–12.6 kJ/mol less stable than quartz. Therefore, there are no significant thermodynamic barriers to transformations between silica polymorphs. This result suggests that structure-directing agents (SDAs) in molecular sieves syntheses do not serve to stabilize otherwise very unstable phases.; Interaction enthalpies between inorganic frameworks and organic SDAs are measured by HF solution calorimetry for six molecular sieve/SDA pairs. The enthalpies are only moderately exothermic (−1.1 to −5.9 kJ/mol SiO₂), as expected if the predominant interactions are silica/hydrocarbon van der Waals contacts. Interaction entropies are estimated for three framework/SDA pairs, and together with the interaction enthalpies allow the calculation of the Gibbs free energies of interaction. The latter values range from −2.0 to −5.4 kJ/mol SiO₂. This energy range is comparable to that observed for the SiO₂ frameworks alone, showing that energetics of both the frameworks and of the inorganic/organic interactions must be considered to adequately describe molecular sieve synthesis. The energetics of the synthesis of molecular sieves (considering all components present in the synthesis mixture) are examined and reveal small differences between various molecular sieve/SDA combinations. The strong selectivity of organic SDAs experimentally observed in the face of comparatively small energetic differences suggests that kinetic factors are of major importance in molecular sieve preparation.