(9f) In silico Structural Analyses of Borosilicate, Aluminosilicate, and Gallosilicate Zeolites Using Effective Tetrahedral Descriptors
We recently rationalized this structure-composition relationship by performing in silico analysis of aluminosilicate zeolites with almost all known zeolite structures and different Si/Al molar ratios . The resulting energy-density landscape of modelled zeolites revealed that the relation between the relative framework energies versus the chemical compositions varies in accordance with the framework topologies. Extension of this approach to other trivalent atoms such as B and Ga is, however, not straightforward due to a lack of reasonable way to offset the energies of zeolites with different trivalent atoms.
Here, we analyze borosilicate, aluminosilicate and gallosilicate zeolites by utilizing a purely structural descriptor developed by Zimmerman et al., namely a tetrahedral ordering parameter, which can evaluate the distortion of TO4 tetrahedra effectively . Analysis of the ordering parameter of structurally optimized zeolites with uniform background charge reveals that B, Al, and Ga distort TO4 tetrahedra themselves as well as the neighboring SiO4 tetrahedra. Furthermore, this general behavior is inherent in the substituting trivalent atoms. The distortion caused by Al is relatively small because AlâO bond length (~1.72 Ã ) is relatively similar to SiâO (~1.61 Ã ). Compared to Al, introduction of B in silicate zeolite framework causes larger overall structural distortion that seems to be mainly taken over by the distortion of surrounding SiO4 due to the lack of flexibility of BO4. Borosilicate zeolites with relatively low Si content (e.g., Si/B < 3) are structurally unfavorable judging from their very low ordering parameter, while aluminosilicates and gallosilicates give reasonably high values at the same composition. The structural distortion induced by Ga seems to be mainly undertaken by GaO4 itself, thereby minimizing the distortion of surrounding SiO4.
Such different characteristics observed in trivalent atoms seemingly control their structure-directing functions to crystallization of zeolites and explain the empirical knowledge of zeolite synthesis such as;
- Aluminosilicate zeolites with high Al content favor the structures with even-numbered ring.
- Borosilicate zeolites tend to be synthesized as relatively high Si/B.
- In several cases, introduction of B and Ga in the synthetic system allows the crystallization of unconventional structures that cannot be synthesized as aluminosilicates and pure silica.
 K. Muraoka, W. Chaikittisilp, and T. Okubo, J. Am. Chem. Soc., 2016, 138 (19), 6184â6193.
 N. E. R. Zimmermann, B. Vorselaars, D. Quigley, and B. Peters, J. Am. Chem. Soc., 2015, 137 (41), 13352â13361.