Modeling of Silicon Substitution in SAPO-5 and SAPO-34 Molecular Sieves

We investigate the energetics of Si island formation in Si-substituted aluminophosphate molecular sieves. Lattice energy calculations confirm the stability of clusters (or islands) comprising Si−O−Si bridges within both SAPO-5 and SAPO-34. The influence of the framework topology is apparent in larger islands containing eight Si atoms, where effects of second-neighbor T shells become operative; the formation of the larger island is calculated to be energetically more favorable in SAPO-5 compared with SAPO-34 as observed experimentally. Estimates of deprotonation energies indicate greater acidity at the edges of islands and a correlation between acidity and island size, indicating that the acidity of these systems is directly related to the concentration of Si islands.