(260d) A Molecular Mechanics Study of the Zeolites with Uniform and Random Al Distributions

Authors: 
Muraoka, K., The University of Tokyo
Chaikittisilp, W., The University of Tokyo
Okubo, T., The University of Tokyo

Zeolite, a class of (alumino)silicate crystalline microporous material, has been intensively studied due to its uniform pore size and high surface area. TO4 tetrahedra (T = Si, Al, etc.), the constituent element of zeolites, construct various framework topologies. Besides the framework structures, the essence of physicochemical properties of zeolites is an aluminum (Al) atom at the frameworked, tetrahedral sites (T-sites). It makes a zeolite negatively charged and thereby requiring charge-compensating cations, generally being alkali cations or protons. These negative charges and their compensating cations enable industrial applications of zeolites as ion exchangers, adsorbents, catalysts, and catalyst supports. In addition, even if the framework structure and the amount of Al are identical, physicochemical properties can be altered due to the difference in the distribution of Al T-site. One of the ultimate goals of the control of Al T-site would be the synthesis of zeolites that contain Al atom only at a specific T-site, while other T-sites are occupied by Si atoms. From the viewpoint of symmetry, such zeolites with uniform Al distribution seem to be energetically more stable than those with random distribution, but such ideal crystal is hard to be synthesized. This study screens the zeolite frameworks that are possible to realize uniform Al distribution under the verified Lowenstein’s rule (forbidding Al-O-Al bonds) 1 and generates ideal aluminosilicate structures. Structural optimization was performed using a force field termed SLC potential 2. Comparison among the structures with uniform and random Al distributions revealed that a specific T-site is energetically more stable than random one, but other T-sites are rather unfavorable.

[1] W. Loewenstein, Am. Mineral. 39, 92, 1953.
[2] K. P. Schröder, et al., Chem. Phys. Lett. 188, 320, 1992.