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

Abstract

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 + /AlO 4 − attractive and AlO 4 − /AlO 4 − 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.