Structure and stability of silica species in SAPO molecular sieves

We have investigated the energetics of silicon incorporation in AlPO4-5 using lattice simulation techniques. The energies of two silicon substitution mechanisms (Al, P --> 2Si; P --> Si, H) have been calculated. Silicon island formation by means of the first mechanism has been found to be energetically unfavorable, and thus 2Si and 4Si islands are not expected to form in SAPO structures. In contrast, the formation of silicon islands by means of a combination of both mechanisms has proved to be energetically favorable, leading to 5Si and 8Si islands being stable in the SAPO structure. These findings correlate well with experimental observation. In addition, the instability of the 2Si and 4Si islands has been related to the presence of [Si-O-P] linkages in the structure. On this basis, the absence of the [Si-O-P] units in SAPOs can be explained. We have also studied the interaction energy between Bronsted acid sites formed by means of one of the mechanisms of silicon substitution (P --> Si, H). The calculations reveal a small binding energy for such centers at close separation.