Structure and Stability of Zeolite Offretite under Si4+/(Al3+, M+) Substitution (M = Na, K): A First Principles Molecular Dynamics Study

Abstract

A local density functional study of the structure and energetics of offretite, with a Si4+ ion substituted by (A13+, M+) (M = Na, K), is presented. The calculations have been performed within the framework of the first principles molecular dynamics method using a periodically repeated unit cell with 55 atoms. It is found that the preferred site for K+ cations lies inside the cancrinite cage, in agreement with experiment. This site is related to A1 substituting a Si atom at a TI site, which is one of the two inequivalent tetrahedral sites of offretite, and belongs to the hexagonal prism, Site selectivity is quite pronounced in the case of K+, as the large size of this cation gives rise to important steric effects in the offretite framework. For Na+, instead, the lowest energy site is located in the channel, near a window of the cancrinite cage, and corresponds to an A1 in Tz. For this cation the spread in the relative substitution energies of the different binding sites is very small, the four sites of lower energies being in a range of -2.5 kcal/mol

Energetics of sodium–calcium exchanged zeolite A

“…14 In turn, the extra-framework cations also significantly influence the energetics and structural stability of the zeolite frameworks. 15,16 The cations may also act as precursors to obtain new zeolite frameworks. 17 Therefore, understanding the underlying thermodynamic properties defining the structure, stability, phase evolution and cation-water-framework interactions is fundamentally important for the synthesis and industrial development of zeolites.…”

Modeling of Adsorption Properties of Zeolites: Correlation with the Structure

“…During the last decade, the main effort has been devoted to H-zeolites, mainly due to their widespread applications in acid catalysis. [5][6][7] Only recently have some dynamical studies of Na and Ca A-zeolite, [8][9][10] Na Y-zeolite, 11 and Na and K offretite 12 proposed detailed structures including cation positions. In recent papers, we have shown that the nature of the cation and its location in the framework are essential factors for N 2 and O 2 adsorption.…”

The role of density functional theory in chemistry: Some historical landmarks and applications to zeolites

“…[89][90][91][92][93][94]). In the mid-1990s, she investigated the structural parameters and the energetic of offretite [89][90][91] using the local density approximation within the framework of Car-Parrinello method [65]. In the meantime, the substitution of silicon by aluminum at different tetrahedral sites of zeolite-b and the influence of the counter ion were performed for the first time at a correlated level of theory [92].…”