When Si at the tetrahedral site (T site) of the zeolite frameworks is substituted by di- or trivalent atoms, negative charges at oxygen surrounding the substituting atoms are generated, thereby yielding anionic silicate-based frameworks. These negative tetrahedra require cationic charge-compensators that are typically active species for adsorption and catalysis. With respect to utilization of zeolites, as a result, the locations of substitutes are of great importance because they are directly related to active sites, for example, Brönsted acid sites when negative charges are counter-balanced by protons. Di- and trivalent substituting T atoms include Be, Zn, B, Al, and Ga. The contents and locations of substitutes in the zeolite frameworks are one of the key factors determining the physicochemical properties of zeolites. Systematic evaluation of the characteristics of zeolites with a wide variety of framework topologies, a wide range of substituting contents, and various locations of substitutes is of great significance, but very challenging due to the limited ranges of the realizable chemical compositions in the existing zeolites as well as the limitation of the current analytical techniques.
Here, we present the systematic computational evaluation of the energetics of trivalent atom-substituted zeolites with 218 existing framework topologies at different substituting contents using the lattice-energy minimization technique with the aid of the Monte Carlo sampling. The results coincide well with the structural knowledge obtained experimentally. The relation between the relative framework energies versus the substituting contents varies in accordance with the framework topologies, suggesting that the relative thermodynamic stability of zeolites depends not only on the framework topologies, but also on the contents of substituting atoms [1]. The effects of types of the substituting atoms (i.e., B, Al, and Ga) and the counterions (i.e., H+ and Na+) on the energetics of zeolites are also discussed in a context of their structure-directing functions
[1] K. Muraoka, W. Chaikittisilp, T. Okubo, J. Am. Chem. Soc., doi: 10.1021/jacs.6b01341.