Abstract

We employ computational modeling to elucidate the distribution, transformation, and framework-protecting role of various La species in zeolite Y (Si/Al = 3). Energy analysis reveals that La3+ ions preferentially occupy the hexagonal prisms (D6R) in the dehydrated framework, while hydration drives their transformation into highly coordinated species, such as [La(H2O)3]3+, [La(OH)(H2O)2]2+, [La(OH)2(H2O)]+, and [La(OH)3] which predominantly reside in the sodalite cages (SOD) and, to a less extent, in the supercages (SUP). Examination of the dynamic evolution process upon hydration shows that La3+ converts into [La(H2O)]3+ in the D6R cage; while it transforms into more stable [La(H2O)3]3+, [La(OH)(H2O)2]2+ and [La(OH)2(H2O)]+ species in the larger SOD cage. However, in the largest SUP cage, [La(OH)2]+ and La(OH)3 can readily convert into [La(OH)]2+ through protonation by nearby Brønsted acid sites, resulting in [La(OH)(H2O)2]2+ as the most stable species. Ab initio thermodynamics calculations reveal that reaction conditions critically control the thermodynamic stability distribution of La species in the SOD framework. Our investigation demonstrates that the hydroxylated La species, specifically [La(OH)]2+ and [La(OH)2]+ within SOD cage, exhibit a much stronger stabilizing effect on the zeolite Y framework than La3+, indicating the critical role of the hydroxide ligands in fortifying the zeolite framework against dealumination.

Link:https://pubs.acs.org/doi/full/10.1021/acs.jpcc.5c03931