Mechanism of Si Island Formation in SAPO-34

With
the aim of understanding the Si island formation in SAPO-34, we have
carried out a computational mechanistic study. Briefly, the Si island
formation in SAPO-34 is explained by three successive reactions. First,
the framework Si atom is removed from the framework through the action
of four water molecules. Second, the hydrogarnet defect generated
by the desilication is healed by an available H3PO4 molecule. Third, the extra framework Si­(OH)4 species
inserts in the framework position of a phosphorus atom while, in a
concerted fashion, “kicking out” the phosphorus atom
as a H3PO4 extra-framework species. When these
exchanges of framework and extra-framework species are repeated, the
isolated Si atoms may eventually cluster into Si islands.