THERMOCHEMICAL STUDY OF THE RELATIVE STABILITY OF DENSE AND MICROPOROUS ALUMINOPHOSPHATE FRAMEWORKS

To understand why AlPO4 exhibits the structural diversity observed, the relative stability of a series of dense and microporous AlPO4 frameworks, e.g., berlinite, tridymite, cristobalite, AlPO4-5, -8, -11, -42, and VPI-5, has been examined by the combination of DSC and high-temperature calorimetric experiments with molten lead berate (2PbO . B2O3) solvent at 979 K. The enthalpies of formation at 298 K, relative to berlinite (quartz structure), are the following (in kJ/mol): tridymite, 5.1 (1.4); cristobalite, 6.1 (1.2); AlPO4-5, 14.0 (2.2); AlPO4-, 11.5 (1.4); AlPO4-11, 12.4 (1.2); AlPO4-42, 15.6 (1.9); and VPI-5, 16.7 (2.3). All microporous AlPO4 frameworks are only 11-17 kJ/mol less stable in enthalpy than berlinite. The calcined and then fully rehydrated microporous AlPO4's, AlPO4 . nH(2)O, show a linear correlation between the degree of hydration (n) and their molar volume. The enthalpy of interaction of adsorbed water with the framework ranges from -11 to -30 kJ/mol of AlPO4, or -10 to -13 kJ normalized per mole of water. Taking the enthalpy of interaction into account, many of these microporous frameworks are energetically comparable to or more stable than berlinite in an aqueous environment. This may imply that there is little or no energy barrier to the formation of microporous AlPO4 frameworks and explain the structural diversity observed for AlPO4. These results are compared to those of a previous study on microporous high-silica zeolites.