Aluminium distribution in ZSM-5 revisited: The role of Al–Al interactions

We present a theoretical study of the distribution of Al atoms in zeolite ZSM-5 with Si/Al=47, where we focus on the role of Al–Al interactions rather than on the energetics of Al/Si substitutions at individual sites. Using interatomic potential methods, we evaluate the energies of the full set of symmetrically independent configurations of Al siting in a Si94Al2O192 cell. The equilibrium Al distribution is determined by the interplay of two factors: the energetics of the Al/Si substitution at an individual site, which tends to populate particular T sites (e.g., the T14 site), and the Al–Al interaction, which at this Si/Al maximises Al–Al distances in general agreement with Dempsey’s rule. However, it is found that the interaction energy changes approximately as the inverse of the square of the distance between the two Al atoms, rather than the inverse of the distance expected if this were merely charge repulsion. Moreover, we find that the anisotropic nature of the framework density plays an important role in determining the magnitude of the interactions, which are not simply dependent on Al–Al distances.

Graphical abstractRole of Al–Al interactions in high silica ZSM-5 is shown to be anisotropic in nature and not dependent solely on Coulombic interactions. Figure optionsDownload full-size imageDownload as PowerPoint slideHighlights► Si–Al distribution in ZSM-5 is revisited, stressing the role of the Al–Al interaction. ► Coulomb interactions are not the key factors controlling the Al siting. ► Anisotropy of the framework is identified as a source of departure from Dempsey’s rule.