Entropy of Pure-Silica Molecular Sieves
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.
Additional Information
© 2001 American Chemical Society. Received 7 February 2001. Published online 31 May 2001. Published in print 1 June 2001. Financial support for this work was provided by the Chevron Research and Technology Co. P.M.P. thanks Dr. Stacey Zones (Chevron) for providing the ZSM-23 (MTT) sample. J.B.G. and B.F.W. thank Rebecca Stevens and B. Hom for assistance with the heat capacity measurements and B. Lang for calculations. A.N. thanks the National Science Foundation (Grant DMR/97-31782) for support.Additional details
- Eprint ID
- 77545
- Resolver ID
- CaltechAUTHORS:20170518-071319729
- Chevron Research and Technology Co.
- DMR 97-31782
- NSF
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2017-05-18Created from EPrint's datestamp field
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2021-11-15Created from EPrint's last_modified field