# Encapsulated Non‐Exchangeable Na+ Ions Determining the Upper Limit of Al Inclusion in FAU—A Multiscale Simulation

## TL;DR

This study shows how non-exchangeable Na+ ions control the maximum amount of aluminum in FAU zeolites, affecting their acid strength and catalytic performance.

## Contribution

The study identifies non-exchangeable Na+ ions as the key factor determining the upper limit of Al inclusion in FAU zeolites.

## Key findings

- Na+ ions prefer larger Al separations due to electrostatic interactions, unlike H+ ions.
- The upper limit of Al inclusion is thermodynamically favorable at low Si/Al ratios (≤ 3).
- Zeolites with non-exchangeable Na+ ions show stronger acid strength and higher catalytic activity.

## Abstract

Na+ ion as charge‐balance agent controlling the distribution of framework aluminum atoms in Y‐zeolite from 1 to 14 Al atoms (Si/Al = 47‐2.4) and the acid strength have been investigated based on DFT computations, descriptors from machine learning, AIMD simulations, and experimental analysis. Different from previous results with proton (H+) as charge‐balancing agent preferring next‐nearest‐neighbor Al separation (3N‐Al), the framework Al atoms, in the presence of Na+ ions, prefer larger separations (4N‐Al and 5N‐Al) due to the balance between Na+/AlO4
− attractive and AlO4
−/AlO4
− repulsive interaction and the preferential occupancy of Na+ ions following the order of six‐membered ring (6MR) > double six‐membered ring (D6R) > four‐membered ring (4MR). The computed sequential substitution enthalpy for 1‐14 Al shows the thermodynamically favorable upper limit of rather low Si/Al ratio (≤ 3) and explains the difficult synthesis of Y‐zeolite with higher Si/Al ratios. Y‐zeolite with non‐exchangeable Na+ ions has stronger acid strength based on the adsorption energy of pyridine and ammonia and exhibits higher catalytic activity in propane cracking.

DFT computation, AIMD simulations, machine learning, statistical analysis, and experimental characterization reveal that the upper limit of Al inclusion in FAU zeolite synthesized under hydrothermal conditions is determined by the encapsulated non‐exchangeable Na+ ions. The balanced repulsive‐attractive electrostatic interaction among Na+ ions and multiple AlO4
− units is the driving force to control the location and distribution of Al inclusion. This affects the acid strength and catalytic activity.