Adsorption Structures of Water in NaX Studied by DRIFT Spectroscopy and Neutron Powder Diffraction

Diffuse reflectance infrared Fourier transform spectroscopic (DRIFTS) measurements (4000−1500 cm-1)
and the results of neutron powder diffraction have been combined to study the structure of adsorption complexes
of water in a NaX zeolite at different water loadings (25, 48, 72, and 120 water molecules per unit cell,
respectively). Sharp bands corresponding to non-hydrogen-bonded OH groups of water molecules and broad
associate bands due to hydrogen-bonded molecules are observed in the DRIFT spectra. We observe a remarkable
downshift of the high-frequency associate band in a narrow temperature interval when the water amount
decreases from 120 to 72 molecules per unit cell, which could signify some kind of “phase transition” for the
water inside the zeolite cavities. Neutron powder diffraction results show that water molecules are predominantly
localized in or near the 12-ring windows. Water molecules with hydrogen-bonded and non-hydrogen-bonded
OH groups were found, in agreement with the observation of sharp and broad bands in the DRIFT spectra.
We find strong evidence for the formation of cyclic hexamers of water molecules localized in the 12-ring
windows, which are further stabilized by hydrogen bonds to framework oxygen atoms.