Abstract
Loewenstein's rule, which states that Al−O−Al motifs are energetically unstable, is fundamental to the understanding and design of zeolites. Here, using a combination of electronic structure calculations and lattice models, we show under which circumstances this rule becomes invalid and how it can be rationally extended using the chabasite framework for demonstration.
Highlights
Zeolites are aluminosilicate minerals, used in many different industrial applications, including detergents, adsorbents/desiccants, and catalysts.[1,2,3,4] They can occur in a staggering number of frameworks with distinct pore architectures and sizes, which are obtained by different arrangements of the underlying tetrahedral SiO4 building blocks.[5]
Based on density functional theory (DFT) calculations[26,27,28,29] for each site in the dilute limit, we reduced the set of possible sites to four in the case of H+ and three in the case of Na+, K+, and Rb+
In the case of H+– Al+3, the attraction between H+ and the saturated OÀ2 renders the O-sites close to Al+3 energetically preferred. This configuration leads to two levels in the band gap: an empty level near the conduction band minimum (CBM), which exhibits p-like character as in the case of the individual H+ (Figure 2 (i)), and an occupied level near the valence band maximum (VBM), which is comprised of porbitals localized at the four oxygen neighbors of the Al+3 site (Figure 2 (ii))
Summary
Zeolites are aluminosilicate minerals, used in many different industrial applications, including detergents, adsorbents/desiccants, and catalysts.[1,2,3,4] They can occur in a staggering number of frameworks with distinct pore architectures and sizes, which are obtained by different arrangements of the underlying tetrahedral SiO4 building blocks.[5]. While the behavior observed for the alkali ions is largely compatible with Loewensteins rule, hydrogen presents a very different case (Figure 1).
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