Comparative Study of Adsorbed Complexes inside Pores and Cavities of Acid Zeolites with Different Topology: Experimental and Theoretical Insights into Confinement Effects

The distinctive topology of pores and cavities of zeolites
plays
a key role in several applications including catalysis, adsorption,
and ion exchange, where confinement effects are significant and can
affect the selectivity, stability, and reactivity of catalytic processes.
In this study, the effect of confinement on the adsorption of acetic
acid on zeolites H-ZSM-5, H-Beta, H-Y, and H-MOR was evaluated by
means of thermogravimetric analysis coupled with Fourier Transform
Infrared (TG-FTIR) spectroscopy and density functional calculations,
including electron density analysis by quantum theory of atoms in
molecules. The catalysts were characterized using X-ray powder diffraction
(XRD), attenuated total reflection (ATR), and Fourier transform infrared
(FTIR) spectroscopy of adsorbed Pyridine (FTIR-Py). TG-FTIR for the
first mass loss process, adsorption energy calculations, and electron
density results show that the strength of the adsorption of acetic
acid between the different acidic zeolites decreases in the order
of H-MOR > H-ZSM-5 > H-Beta > H−Y. Adsorption of acetic
acid
within H-ZSM-5 and H-MOR allows to maximize the interactions with
the zeolite framework and thus the confinement, resulting in an additional
stabilization in the adsorption process. In contrast, for H-Beta and
H−Y zeolites, the confinement effect on the acetic acid molecule
is small, and the adsorption energy is mostly related to the interactions
at the active site (BAS and basic site).