To Every Rule There is an Exception: A Rational Extension of Loewenstein's Rule

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.

Keywords: Al distribution; Loewenstein rule; catalysis; zeolites.

Figure 1

Variation of the fraction of Al–Al nearest‐neighbor pairs as a function of Al content in a chabasite framework (SSZ‐13) using either ions (H⁺, Na⁺, K⁺, Rb⁺) or free carriers for charge compensation.

Figure 2

Electronic energy levels associated with different defects in SSZ‐13 (top) and their real space representations shown as isosurfaces (bottom). Occupied and empty levels are shown in blue and orange, respectively. Visualized using ovito. ^[35]

Figure 3

Low energy configurations in perspective (top row) and viewed along the c‐axis (bottom row) of (a) H⁺–Al⁺³, (b) Na⁺–Al⁺³, (c) 2×H⁺–Al⁺³, and (d) 4×H⁺–Al⁺³. Yellow and blue isosurfaces show, respectively, negative and positive charge redistribution upon compensation with the different counterions.