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

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.