Sammendrag
The relative stabilities of different proton distributions around a five-atom silicon island in silicoaluminophosphate-34 (SAPO-34) have been investigated by periodic molecular mechanics (MM) energy minimization calculations, and the MM calculations were validated using density functional theory (DFT). SAPO-34 has chabazite topology with only one symmetrically independent tetrahedral site (T-site) and four unique oxygen sites. The preferred position of the proton at isolated acid sites has been the subject of both experimental and computational studies. In previous computational studies of silicon islands, it was assumed that the protons, necessary to keep the silicon island neutral, would occupy the same positions as those preferred for solitary silicon atoms. We have studied all 108 possible proton distributions around a five-atom silicon island in SAPO-34. The results indicate that the proton placement is critical for stability, as the limit deviation in our data set is as high as 90 kJ/mol. Careful analysis of the different structures afforded criteria for stability of the proton configuration around a five-atom silicon island in SAPO-34. Preliminary calculations indicate that these findings are transferable to other topologies and larger islands.