TY - JOUR
T1 - Thermochemical Study of the Stability of Frameworks in High Silica Zeolites
AU - Petrovic, Ivan
AU - Navrotsky, Alexandra
AU - Davis, Mark E.
AU - Zones, Stacy I.
N1 - Copyright:
Copyright 2017 Elsevier B.V., All rights reserved.
PY - 1993
Y1 - 1993
N2 - 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 (2PbO2·B2O3) 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 enthalpy 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° being the major destabilizing factor. On the other hand, heat contents, (H977-H298), including those for “dense” crystalline polymorphs, show a linear dependence on the framework density or molar volume.
AB - 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 (2PbO2·B2O3) 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 enthalpy 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° being the major destabilizing factor. On the other hand, heat contents, (H977-H298), including those for “dense” crystalline polymorphs, show a linear dependence on the framework density or molar volume.
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U2 - 10.1021/cm00036a019
DO - 10.1021/cm00036a019
M3 - Article
AN - SCOPUS:0001755087
SN - 0897-4756
VL - 5
SP - 1805
EP - 1813
JO - Chemistry of Materials
JF - Chemistry of Materials
IS - 12
ER -