Thermodynamics of Pure-Silica Molecular Sieve Synthesis

Abstract

The thermodynamics of pure-silica molecular sieves (denoted by their structural codes ^*BEA, MFI, MTW, and STF) are investigated by solution calorimetry at 323.15 K using 25% aqueous HF as solvent. The enthalpies of solution are determined for both calcined (silica frameworks) and organic structure-directing agent (SDA) occluded samples (SDAs:  tetraethylammonium (TEA), tetrapropylammonium (TPA), trimethylenebis(N-methyl,N-benzylpiperidinium) (BISPIP), and 2,6-dimethyl-1-aza-spiro[5.4]decane (SPIRO)). These measurements provide data that enable the determination of the following interaction enthalpies between the molecular sieve frameworks and SDAs at 323.15 K: ^*BEA/TEA, −3.1 ± 1.4 kJ/mol SiO_2 (−32 ± 15 kJ/mol SDA); ^*BEA/BISPIP, −5.9 ± 0.7 kJ/mol SiO_2 (−181 ± 21 kJ/mol SDA); MFI/TEA, −1.1 ± 1.4 kJ/mol SiO_2 (−27 ± 33 kJ/mol SDA); MFI/TPA, −3.2 ± 1.4 kJ/mol SiO_2 (−81 ± 34 kJ/mol SDA); MTW/BISPIP, −1.6 ± 1.3 kJ/mol SiO_2 (−124 ± 97 kJ/mol SDA); and STF/SPIRO, −4.9 ± 0.9 kJ/mol SiO_2 (−83 ± 16 kJ/mol SDA). Interaction entropies are estimated, and when used in combination with the measured interaction enthalpies, provide the following Gibbs free energies of interaction between the SDAs and the molecular sieve frameworks: ^*BEA/TEA, −5.4 ± 1.5 kJ/mol SiO_2; MFI/TEA, −2.0 ± 1.4 kJ/mol SiO_2; and MFI/TPA, −4.9 ± 1.4 kJ/mol SiO2. 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. Assuming complete transformation of the starting amorphous silica into a molecular sieve, the Gibbs free energy changes for the crystallization are ^*BEA/TEA, −8.5 ± 2.9 kJ/mol SiO_2; MFI/TEA, −4.9 ± 2.8 kJ/mol SiO_2; and MFI/TPA, −8.1 ± 2.8 kJ/mol SiO_2. No single factor (enthalpy, entropy, etc.) dominates the overall Gibbs free energies, and these small energetic differences suggest that kinetic factors are of major importance in molecular sieve preparation.

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