Exploring the Structure and Function of Rare-Earth Elements Incorporated into Zeolite Catalysts

Rare-earth element (REE) incorporation into dealuminated
zeolites
has been shown to catalyze a variety of selective oxygenate transformations,
including ethanol to olefins, yet the structure and function of REE-incorporated
Lewis acid zeotypes remain unclear. In this study, we proposed five
yttrium acid site configurations and evaluated each against experimental
physicochemical characterization techniques including X-ray absorption
spectroscopy and pyridine Fourier transformed infrared spectroscopy
(FTIR). Our analysis identified three fundamental site motifs, defect-open,
dehydrated defect-open, and geminal hydroxyl, stabilized by adjacent
silanol defects and hydroxyl groups that agreed with spectroscopic
characterization. By comparing ethanol dehydration kinetics, we identified
that interconvertible defect-open and dehydrated defect-open sites
are kinetically relevant for catalytic turnovers. The three yttrium
open site structural motifs from Y/deAlBeta were extended to 14 other
REEs (La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu)
to explore trends in Lewis acid strength, assessed via pyridine adsorption
energies and supported by experimentally measured pyridine FTIR. A
linear correlation between Lewis acid strength and highest occupied
molecular orbital + lowest unoccupied molecular orbital energies was
established, offering a predictive framework for understanding structure–function
relationships in REEs incorporated into dealuminated Beta zeotypes.
These findings provide molecular-level insight into REE incorporation
and its role in tuning Lewis acid strength for the selective catalytic
transformation of biomass-derived oxygenates into chemicals and liquid
fuels.