DFT-Derived Location and Hydroxide Coordination of Lanthanum Ions in Zeolite Y Structures

Abstract

Fluid Catalytic Cracking is a process of great interest in oil refining. This process is governed by the stability and activity of acid sites held within zeolite frameworks. Rare earth exchanged, specifically lanthanum exchanged, zeolite Y is known to have increased resistance to framework dealumination. This study examines lanthanum exchanged zeolite Y (Si/Al = 3) through the use of density functional theory calculations with the purpose of elucidating the location and nature of La species held within the zeolite framework. This is accomplished through calculation of silicon chemical shifts to determine the arrangement of Al atoms in the framework. Followed by stability calculations for the position and hydroxide coordination of La(OH)X (X is the number of OH groups and ranges from 0 to 2) species at each of the seven unique ion exchange position in zeolite Y. The most stable single La position is found to be at the center of the hexagonal prism (site I) as a bare La3+ cation, followed closely by LaOH located atop the hexagonal prism (site I’). Lanthanum clusters of three are not preferred. However, La clusters of two inside the sodalite cage are preferred over all other lanthanum orientations. After finding the most stable positions, proton chemical shifts were calculated for lanthanum containing structures and compared to corresponding deprotonation energies, however, no trend between the two is found. Values of deprotonation energy for H connected to La molecules are too high to be strong Bronsted acid sites, though they could potentially act as strong Lewis acid sites.