Ionicity and framework stability of crystalline aluminophosphates

First-principles quantum chemical techniques are applied to characterize the bonding properties of crystalline aluminophosphates (ALPOs) and to compare them with the isostructural silica-based zeolites. Results of the calculations agree with the available structural and calorimetric experimental data. By population and topological analyses of the calculated electronic density, the bonding in ALPOs is shown to be of molecular- ionic character and comprised of discrete Al3+ and PO43- ions. The latter finding rationalizes the hydrophilicity and the defect chemistry of microporous ALPO frameworks in heterogeneous catalysis.