Understanding the Structure of Cationic Sites in Alkali Metal-Grafted USY Zeolites

This contribution describes the structure
of cationic sites in
alkali metal-grafted high-silica USY zeolites, a novel class of zeolite
base catalysts, through the combination of computational studies and
experimental evidence. Based on density functional theory calculations,
we investigate the geometry various defects that coordinate sodium
or potassium, and their interaction with adsorbed CO. The predicted
CO-stretching frequencies are verified experimentally by infrared
spectroscopy, which indicates the existence of a distribution of different
site geometries rather than a single site type. The corresponding
bands lie in the range of 2180–2160 and 2170–2145 cm–1 for Na+ and K+ cations, respectively.
While CO primarily interacts with the grafted metal cation (effect
from the bottom), the CO stretching frequency is strongly modulated
by neighboring silanol groups (effect from the top). Based on a good
agreement of experimental and theoretical results, the deprotonation
of silanol groups to silanolates followed by an ion exchange appears
to be a realistic mechanism for alkali metal incorporation in silica-rich
USY zeolites. The developed insights on the cation site structure
represent an important step to understand the catalytic performance
of these basic zeolites in aldol-type reactions, and pave the way
for future studies investigating transition states to resolve their
role in catalysis.