Experimental Detection of Preferred Lanthanum Siting in Zeolite Y and Its Impact on Catalyst Reactivity

Lanthanum and other rare earth cations are routinely added to commercial zeolite catalysts to increase stability during hydrothermal regeneration steps, as well as to modify catalyst reactivity by changing the distribution of acid sites and operative electric field gradients in working catalysts. Solution ion-exchange procedures are typically used to introduce La cations into zeolite Y, primarily as La³⁺ or its hydroxylated moieties, La^3-n(OH)_n, in the formulation of commercial LaY or ultrastable steam-stabilized Y (USY) catalysts. Within zeolite Y, multiple possible exchange sites exist for La occupation, but quantitative measurement of La siting as a function of La loading in the catalyst is not generally accessible. Specific open questions involve whether La cations reside in both sodalite and supercage sites, which sites are preferred for La incorporation, and whether La can be selectively incorporated into specific site types. In this contribution, a simple quantitative method based on solid-state NMR coupled with the preparation of La-HY catalysts without framework defects reveals that at low La loadings of less than 3 wt %, essentially all La is incorporated into sodalite cages as La³⁺ ions. DFT calculations support these experimental conclusions. Coincident with this incorporation, sodalite Brønsted bridging acid sites (BAS) decrease, but the number of supercage BASs can remain constant depending on the La concentration. Increased La loadings in the catalyst preferentially reduce the number of sodalite BASs compared to supercage acid sites, with both sodalite and supercage BAS amounts, as well as the amount of newly created La^3-n(OH)_n species, quantitatively measured using the methods described here. Flow reactor hexane-cracking experiments, as well as in situ probe reactions, reveal that catalyst reactivity increases relative to that of HY when La resides exclusively in sodalite positions.