THERMOCHEMICAL STUDY OF THE STABILITY OF FRAMEWORKS IN HIGH-SILICA ZEOLITES

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-degrees 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.