Infrared spectroscopic characterization of dehydration and accompanying phase transition behaviors in NAT-topology zeolites

Abstract

Relative humidity (P H 2 O , 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 P H 2 O conditions by in situ diffuse-reflectance infrared Fourier transform spectroscopy and parallel X-ray powder diffraction. Dehydration was characterized by the disappearance of internal H 2 O vibrational modes. The loss of H 2 O molecules caused a sequence of structural transitions in which the host framework transformation path was coupled primarily via the thermal motion of guest Na ? /Ca 2? cations and H 2 O molecules. The observation of different interactions of H 2 O molecules and Na ? /Ca 2? cations with host aluminosilicate frameworks under highand low-P H 2 O conditions indicated the development of different local strain fields, arising from cation-H 2 O 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-H 2 O-Al-Si framework interactions in NAT-type channels, and these bands can be used to evaluate the energetic evolution of Na ? /Ca 2? 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 ? /Ca 2? order-disorder that occur when mesolite loses its Ow4 H 2 O molecules.