Resolving the puzzle of solvated electrons’ location in alkali metal doped zeolites

Doping faujasite Y (FAU-Y), a nanoporous aluminosilicate zeolite, with alkali metal atoms (M) is a promising way to produce outstanding catalysts. The dopants, along with extra-framework metal atoms, often form M₄⁺³ clusters inside zeolite pores, leaving the valence electron of the dopant solvated and available for catalysis. Despite extensive experimental efforts^1,2, the distribution of dopants and solvated electrons remains debated to date. Combining a cluster-expansion model, parameterized with density-functional theory calculations, with ab initio atomistic thermodynamics, we address this issue. The electronic structure for low-energy configurations is calculated with hybrid functional HSE06. We find that even at room temperature, Na atoms in NaY zeolites with two extra-framework atoms per unit cell on average redistribute such that areas with lower and higher local concentrations emerge. The redistribution is driven by increased configurational disorder, mainly at higher concentrations. This explains why solvated electrons can be located inside both small and large cages in NaY, reconciling experiments that assign the solvated electrons to a particular pore type.
1. A. R. Armstrong et al., J. Am. Chem. Soc. 117, 9087(1995).
2. W. Louisfrema et al., Mol. Simul. 41, 1371(2015).