Heat capacities, third-law entropies and thermodynamic functions of SiO₂ molecular sieves from T = 0 K to 400 K

Four zeolitic polymorphs of SiO₂, *BEA, FAU, MFI, and MTT, have been studied by adiabatic heat capacity calorimetry in the temperature interval from appproximately 20 < (T/K) ≤ 400 K. From numerical fits of the heat capacities, thermodynamic functions including the entropy, enthalpy increment, and Gibbs free energy function of all four phases have been obtained. At T = 298.15 K, the standard molar heat capacities of the four phases are C_{p, m}^o = (44.21 ± 0.08) J·K⁻¹middot;mol⁻¹, (45.34 ± 0.08) J·K⁻¹·mol⁻¹, (45.70 ± 0.08) J·K⁻¹·mol⁻¹, (45.97 ± 0.08) J·K⁻¹·mol⁻¹ for *BEA, FAU, MFI, and MTT, respectively. A maximum at T = 365 K was observed in the heat capacity of MFI that has been attributed to the monoclinic-orthorhombic structural phase transition previously studied by x-ray and solid state nmr experiments. The enthalpy of transition Δ_trH was found to be (134.8 ± 0.5) J·mol⁻¹ while the entropy of transition Δ_trS was (0.385 ± 0.001) J·K⁻¹·mol⁻¹. These small values are consistent with the subtle, displacive nature of the transition. The heat capacities of three of the polymorphs (FAU, MFI, and MTT) are greater than that of crystalline quartz over the entire temperature region of this study, while that of *BEA drops below that of crystalline quartz for T > 240 K. In addition, the excess heat capacity relative to crystalline quartz of all four polymorphs is greater than that exhibited by amorphous quartz for T < 200 K. Since amorphous forms of a substance have higher heat capacities at low temperatures than their crystalline counterparts, this result is unexpected.