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).