Proton-form zeolites are an important class of solid acid catalysts, widely used in chemical reactions in industrial and academic settings. Their small, regular pores and strong Brønsted acid sites make them appealing systems for studying the behavior of water molecules and acidic protons under confinement. From a catalytic perspective, understanding the interactions between water and zeolite acid sites is important for several chemical reactions relevant to bio-renewable synthesis, where water is often present. We take a collaborative approach, bringing together novel sample preparation techniques, FTIR and 2D IR spectroscopy, and <i>ab initio</i> molecular dynamics simulations to characterize the structure and dynamics of confined water molecules. With precise experimental control of the zeolite hydration level, we track the structural evolution of the zeolitic acid site, adsorbed water molecules, and excess proton over the entire accessible range of water loading. Hydration-dependent FTIR and 2D IR spectra are decomposed and assigned to structural configurations with the aid of simulation and spectral calculations. We find that the excess proton is transferred from the zeolite lattice to the water cluster at 2-3 H<sub>2</sub>O / acid site, but is better described in the framework of a shared proton rather than a two-state system. Furthermore, a strong interaction between water and the zeolite framework is observed even at high hydration, suggesting that the protonated water cluster remains tethered to the deprotonated acid site. We comment on the potential implications of this work for zeolite acid-catalyzed reactions in the presence of water.

Tracking the structural evolution of water and protons confined in HZSM-5 zeolite pores

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