Abstract
The entropies of a series of pure-silica molecular sieves (structural codes ^*BEA, FAU, MFI, and MTT) are obtained by calorimetric measurements of low-temperature heat capacity. The third-law entropies at 298.15 K are (on the basis of 1 mol of SiO_2): ^*BEA, 44.91 ± 0.11 J·K^(-1)·mol^(-1); FAU, 44.73 ± 0.11 J·K^(-1)·mol^(-1); MFI, 45.05 ± 0.11 J·K^(-1)·mol^(-1); MTT, 45.69 ± 0.11 J·K^(-1)·mol^(-1); while the corresponding entropies of transition from quartz at 298.15 K are ^*BEA, 3.4 J·K^(-1)·mol^(-1); FAU, 3.2 J·K^(-1)·mol^(-1); MFI, 3.6 J·K^(-1)·mol^(-1); MTT, 4.2 J·K^(-1)·mol^(-1). The entropies span a very narrow range at 3.2−4.2 J·K^(-1)·mol^(-1) above quartz, despite a factor of 2 difference in molar volume. This confirms that there are no significant entropy barriers to transformations between SiO_2 polymorphs. Finally, the Gibbs free energy of transformation with respect to quartz is calculated for eight SiO_2 phases and all are found to be within twice the available thermal energy of each other at 298.15 K.
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