Thermochemical study of the stability of frameworks in high silica zeolites
- Princeton Univ., NJ (United States)
- California Institute of Technology, Pasadena, CA (United States)
- Chevron Research and Technology, Richmond, CA (United States)
The series of high-silica zeolites ZSM-5, ZSM-11, ZSM-12, SSZ-24, cubic and hexagonal faujasite has been studied to understand the relation between crystal structure and stability in open silicate frameworks. High-temperature solution calorimetry using lead borate (2PbO[sub 2][center dot]B[sub 2]O[sub 3]) solvent at 977 K measured enthalpies of solution and transposed temperature drop calorimetry obtained heat contents at 977 K. Data have been compared with those for quartz and other dense', crystalline polymorphs of silica. The enthalpies of formation at 298 K are as follows: ZSM-12, -(902.0 [+-] 1.3); ZSM-5, -(902.5 [+-] 1.3); ZSM-11, (902.5 [+-] 1.4); SSZ-24, -(903.5 [+-] 1.3); hexagonal faujasite, -(900.2 [+-] 1.3); and cubic faujasite, -(897.1 [+-] 1.2) kJ/mol. The value for quartz is -(910.70 [+-] 1.00) kJ/mol. All zeolitic silicas are only 7-14 kJ/mol less stable in enthalphy than quartz. This implies an entropic or kinetic rather than a large stabilizing energetic role of the template in zeolite synthesis. The small variations in energy among these structures cannot be directly related either to the degree of openness' (framework density or molar volume) or to the mean Si-O-Si angle. Rather, the overall distribution of bond angles seems to dictate the energy of these structures, with Si-O-Si angles below 140[degrees] being the major destabilizing factor. On the other hand, heat contents, (H[sub 977]-H[sub 298]), including those for dense' crystalline polymorphs, shows a linear dependence on the framework density or molar volume. 55 refs., 7 figs., 4 tabs.
- DOE Contract Number:
- FG02-85ER13437
- OSTI ID:
- 5040263
- Journal Information:
- Chemistry of Materials; (United States), Vol. 5:12, Issue 12; ISSN 0897-4756
- Country of Publication:
- United States
- Language:
- English
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Related Subjects
ZEOLITES
STABILITY
CHEMICAL PREPARATION
CUBIC LATTICES
HEXAGONAL LATTICES
SILICA
SOLUTION HEAT
CHALCOGENIDES
CRYSTAL LATTICES
CRYSTAL STRUCTURE
ENTHALPY
INORGANIC ION EXCHANGERS
ION EXCHANGE MATERIALS
MATERIALS
MINERALS
OXIDE MINERALS
OXIDES
OXYGEN COMPOUNDS
PHYSICAL PROPERTIES
SILICATE MINERALS
SILICON COMPOUNDS
SILICON OXIDES
SYNTHESIS
THERMODYNAMIC PROPERTIES
360204* - Ceramics
Cermets
& Refractories- Physical Properties