The structure and dynamics of water and aqueous protons under confinement has implications for many catalytic and electrochemical processes. Zeolites are widely used as acid catalysts in reactions including dehydration, steam reforming, and biofuel synthesis where water is present. Understanding the water structure inside the zeolite pores, interactions between water and the catalytic acid site, and the ultrafast motions of water are thus crucial to characterizing these catalytic processes. In this study, we use ultrafast IR spectroscopy to study H2O confined in the pores of zeolite HZSM-5 (pore diameter ~5.5 Å) prepared under high hydration (8 H2O / Al site). The IR spectrum displays features characteristic of both hydrogen-bound and free water OH-stretches and the continuum absorption of the aqueous proton. These features are anharmonically coupled at the earliest measurable waiting time of 100 fs, suggesting the formation of protonated water clusters. The excitation of the hydrogen-bonded OH-stretch relaxes rapidly, with a timescale of 230 fs, to a feature that resembles the hot ground state observed in bulk H2O. Furthermore, the polarization anisotropy of the hydrogen-bonded OH-stretch decays with a timescale of 160 fs, which is assigned to rapid energy transfer between degenerate stretches. Both of these observations are consistent with a delocalized OH-stretch feature similar to the OH-stretch delocalization in bulk water. Experimental FTIR and 2D IR spectra were compared to AIMD simulations of protonated water clusters ( H+(H2O)8 ) confined in HZSM-5 pores. Estimated populations of water in different hydrogen-bonding configurations were qualitatively consistent with the 8 H2O simulation. Taken together, our results suggest the formation of protonated water clusters of approximately 8 H2Os in the pores of HZSM-5 zeolites.

Structural insights on protonated water clusters confined in HZSM-5 zeolites

ACS Spring Meeting 2021

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