Structural Characterization of Protonated Water Clusters Confined in HZSM‑5 Zeolites

A molecular description of the structure
and behavior of water
confined in aluminosilicate zeolite pores is a crucial component for
understanding zeolite acid chemistry under hydrous conditions. In
this study, we use a combination of ultrafast two-dimensional infrared
(2D IR) spectroscopy and ab initio molecular dynamics
(AIMD) to study H2O confined in the pores of highly hydrated
zeolite HZSM-5 (∼13 and ∼6 equivalents of H2O per Al atom). The 2D IR spectrum reveals correlations between the
vibrations of both terminal and H-bonded O–H groups and the
continuum absorption of the excess proton. These data are used to
characterize the hydrogen-bonding network within the cluster by quantifying
single-, double-, and non-hydrogen-bond donor water molecules. These
results are found to be in good agreement with the statistics calculated
from an AIMD simulation of an H+(H2O)8 cluster in HZSM-5. Furthermore, IR spectral assignments to local
O–H environments are validated with DFT calculations on clusters
drawn from AIMD simulations. The simulations reveal that the excess
charge is detached from the zeolite and resides near the more highly
coordinated water molecules in the cluster. When they are taken together,
these results unambiguously assign the complex IR spectrum of highly
hydrated HZSM-5, providing quantitative information on the molecular
environments and hydrogen-bonding topology of protonated water clusters
under extreme confinement.