Entropy of pure-silica molecular sieves

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 SiO2): *BFA, 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 SiO2 polymorphs. Finally, the Gibbs free energy of transformation with respect to quartz is calculated for eight SiO2 phases and all are found to be within twice the available thermal energy of each other at 298.15 K.