Abstract
Relative humidity (PH2O, partial pressure of
water)-dependent dehydration and accompanying phase
transitions in NAT-topology zeolites (natrolite, scolecite,
and mesolite) were studied under controlled temperature
and known PH2O conditions by in situ diffuse-reflectance
infrared Fourier transform spectroscopy and parallel X-ray
powder diffraction. Dehydration was characterized by the
disappearance of internal H2O vibrational modes. The loss
of H2O molecules caused a sequence of structural transitions
in which the host framework transformation path
was coupled primarily via the thermal motion of guest
Na?/Ca2? cations and H2O molecules. The observation of
different interactions of H2O molecules and Na?/Ca2?
cations with host aluminosilicate frameworks under highand
low-PH2O conditions indicated the development of
different local strain fields, arising from cation–H2O
interactions in NAT-type channels. These strain fields
influence the Si–O/Al–O bond strength and tilting angles
within and between tetrahedra as the dehydration temperature
is approached. The newly observed infrared bands (at
2,139 cm-1 in natrolite, 2,276 cm-1 in scolecite, and 2,176
and 2,259 cm-1 in mesolite) result from strong cation–
H2O–Al–Si framework interactions in NAT-type channels,
and these bands can be used to evaluate the energetic
evolution of Na?/Ca2? cations before and after phase
transitions, especially for scolecite and mesolite. The 2,176
and 2,259 cm-1 absorption bands in mesolite also appear
to be related to Na?/Ca2? order–disorder that occur when
mesolite loses its Ow4 H2O molecules.