Thermochemical Study of the Relative Stability of Dense and Microporous Aluminophosphate Frameworks

To understand why AlPO_4 exhibits the structural diversity observed, the relative stability of a series of dense and microporous AlPO_4 frameworks, e.g., berlinite, tridymite, cristobalite, AlPO_4-5, -8, -11, -42, and VPI-5, has been examined by the combination of DSC and high-temperature calorimetric experiments with molten lead borate (2PbO*B_2O_3) 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); AlPO_4-5, 14.0 (2.2); AlPO_4-8,113 (1.4); AlPO_4-11,12.4 (1.2); AlPO_4-42, 15.6 (1.9); and VPI-5, 16.7 (2.3). All microporous AlPO_4 frameworks are only 11-17 kJ/mol less stable in enthalpy than berlinite. The calcined and then fully rehydrated microporous AlPO_4's, AlPO_4•nH_2O, 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 AlPO_4, 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 AlPO_4 frameworks and explain the structural diversity observed for AlPO_4. These results are compared to those of a previous study on microporous high-silica zeolites