Thermodynamics of formation of molecular sieves

Abstract

Molecular sieves, porous, crystalline frameworks with pore sizes of molecular dimensions, are of great industrial importance as detergents, catalysts and absorbants. Despite their technological importance, the syntheses of these materials are still not well understood and typically rely on extensive series of trials to produce new framework structures. 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. Si0_2 molecular sieves are shown to be only modestly (6.8-14.4 kJ/mol) metastable with respect to quartz. A strong linear correlation between enthalpy and molar volume is observed, implying that the overall packing quality determines the relative enthalpies of Si0_2 molecular sieves. Silanol (Si-O-H) defect sites lead to an additional destabilization of no more than 2.4 kJ/mol. The entropies of four pure-silica molecular sieves spanning the entire range of molar volumes available to Si0_2 frameworks are determined by the integration of heat capacity measurements from 5 to 400 K. The entropies of these structures are almost identical (3.2-4.2 J∙K^(-1)∙mol^(-1) above quartz), hence the empty pore volume and cages do not contribute appreciably to the vibrational density of states. The enthalpy and entropy data are combined to calculate the Gibbs free energies of transition from quartz to eight other silica polymorphs, including four molecular sieves as well as silica glass. At typical synthesis conditions, the available thermal energy is RT = 3.5 kJ/mol. The molecular sieve Gibbs free energies are only slightly larger than RT at 5.5-12.6 kJ/mol above quartz and lie in the same energetic region as the amorphous precursors used for molecular sieve preparation. There are therefore no significant thermodynamic barriers to transformations among silica polymorphs. Thus the role of SDA in molecular sieve syntheses is not the stabilization of 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 SiO_2), as expected if the predominant interactions are van der Waals contacts between the hydrophobic silica frameworks and the hydrocarbon portions of the SDAs. Interaction entropies can be estimated for three framework/SDA pairs, and, when used in combination with the interaction enthalpies, allow the calculation of the Gibbs free energies of interaction between these three inorganic/organic pairs. The latter values range from -2.0 to -5.4 kJ/mol SiO_2, smaller in magnitude than twice the available thermal energy at molecular sieve synthesis temperatures. This energy range is comparable to the range observed for the molecular sieve frameworks alone, showing that energetics of both the frameworks and of the molecular sieve/SDA interactions must be considered in order to adequately describe molecular sieve synthesis. The energetics of the synthesis of molecular sieves (considering all components present in the synthesis mixture) are examined here and also reveal small differences between various molecular sieve/SDA combinations. The energetic contribution of the effective dilution experienced by the SDA upon occlusion is similar in magnitude to the other energetic effects. The strong selectivity of organic SDAs experimentally observed in the face of the comparatively small energetic differences suggests that kinetic factors dominate in molecular sieve preparation.