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

To understand why AIPO₄ exhibits the structural diversity observed, the relative stability of a series of dense and microporous AIPO₄ frameworks, e.g., berlinite, tridymite, cristobalite, AIPO₄-5, -8, -11, -42, and VPI-5, has been examined by the combination of DSC and hightemperature calorimetric experiments with molten lead borate (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); AIPO₄-5, 14.0 (2.2); AIPO₄- 8,11.5 (1.4);AlPO₄-11, 12.4 (1.2);AlPO₄42,15.6 (1.9); and VPI-5,16.7 (2.3). All microporous AIPO4 frameworks are only 11-17 kJ/mol less stable in enthalpy than berlinite. The calcined and then fully rehydrated microporous AlPO₄'s, AlPO₄·nH₂O, 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 AIPO₄, 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 AIPO₄ frameworks and explain the structural diversity observed for AIPO₄. These results are compared to those of a previous study on microporous highsilica zeolites.