As a widely used class of solid-acid catalysts, proton-form zeolites and water’s role in their reactivity have been long-standing topics of investigation. Yet, relatively little is understood about the molecular-scale motions and arrangements of confined water molecules and excess protons under varying hydration conditions. We take a collaborative approach to understanding these phenomena, bringing together novel synthesis and sample preparation, 2D IR spectroscopy, and <i>ab initio</i> molecular dynamics simulations to characterize the structuring of water within these pores. Experimental and calculated IR spectra were decomposed to assign O-H stretch frequencies to their molecular environments. Spectral assignments were utilized to quantify the types of waters as double-donor, single-donor, and no-donor, and their statistics were compared with AIMD simulations. Correlations in the 2D IR spectrum indicate that spectral features arise from a common molecular species which includes a water-associated excess proton. This leads to the conclusion that small, protonated water clusters form at Brønsted acid sites, with a hydrogen-bonding structure that is influenced by the local geometry of the zeolite lattice.

Structural characteristics of water confined in HZSM-5 zeolites pores

ACS Fall 2021

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