Si distribution in silicoaluminophosphate molecular sieves with the LEV topology: a solid-state NMR study.

The solid-state NMR evidence that Si atoms are not randomly distributed in microporous SAPO-35 materials with the LEV topology and their distribution is governed by the Si content in synthesis mixtures is presented. It is also shown that the extraction pattern of Si atoms from the two distinct tetrahedral sites of the SAPO-35 framework during the calcination step at elevated temperatures occurs in a nonrandom manner, which can be rationalized by considering the expected strain on each topologically distinct site. The overall results of this study reveal that, when the level of Si substitution in SAPO-35 materials is high enough to produce various heterogeneous Si environments other than Si(4Al) species having P atoms only as second-nearest T-atom neighbors, the oxide composition of the domain preferentially created is aluminosilicate rather than pure silica in nature.