| Popis: |
XRD, IR and 29Si MAS NMR are used to characterize three SAPO's(5, 34 and 37). The changes in structure or arrangement of T atoms are shown to strongly depend on heating conditions(muffle oven, vacuum or flow of gas). Even when the initial crystalline structure is maintained at 1173 K, a solid state transformation is shown to occur in the three compounds, gathering the Si atoms in Si islands. The order of stability of the three structures with regard to rehydration or phase transformation to tridymite + cristobalite structures is shown to be SAPO-5 >SAPO-34 ≥ SAPO-37. This is parallel to the rank of topological densities. The most stable structures are the denser. |
