Investigating patterns in aluminum distribution and stability in mordenite zeolite structure: Analysis and theoretical conclusions from existing experimental data

Abstract

Building upon our prior theoretical study, this work focuses on determining the position of the seventh, eighth, and ninth aluminum atoms, along with their respective exchange ▪ cations, within the unit cell of mordenite zeolite. The main objective was to identify the positions of these atoms and cations that correspond to the minimum energy state. As a result, the ground state sequence that describes the order in which aluminum atoms enter in the framework as their number increases was completed. Through our investigation, we were able to derive an analytical expression, based on the Madelung energy, which describes the stability of mordenite zeolite as a function of its Si/Al ratio. This expression provides valuable insights, revealing that as the aluminum content within the framework increases, the stability of the zeolite exhibits an asymptotic decrease. It was found that for the mordenite zeolite, Löwenstein's rule is valid for the full range of Si/Al ratios. Interestingly, the restriction that only one aluminum atom can be accommodated per ζ-cage justifies the nature of the aluminum saturation limit (Si/Al=5) in the mordenite zeolite framework. Furthermore, we conducted an evaluation of the aluminum distribution across various configurations and compared it to different experimental observations. This analysis allowed us to make a comprehensive comparison and draw meaningful insights regarding the distribution patterns of aluminum in the mordenite zeolite.