INVESTIGATION OF PHASE TRANSITION BEHAVIORS AND TRANSITION MECHANISMS OF NAT-TOPOLOGY ZEOLITES IN T-P(H2O) SPACE

Abstract

NAT-topology zeolites, namely natrolite (Na-bearing), mesolite (Na/Ca-bearing), and scolecite (Ca-bearing), consist of a three-dimensional host framework of (Al2Si3)O10 chains, with structural "pores" that contain reactive guest cations and H2O molecules. The nature of structural interactions between a host framework and guest cations and H2O molecules defines the character of phase transitions, which depend on the chemistry of the guests and rely on the symmetry (topology) of the host framework. X-ray diffraction and synchrotron X-ray total scattering experiments coupled with appropriate modeling methods showed the existence of different dehydration (heating)/phase transition behaviors under high- and low-P(H2O) conditions in these zeolites. Infrared spectroscopic studies of the dynamic interactions among the guest cation-H2O complexes and host frameworks allowed characterization of host-guest interactions as a function of T and P(H2O). Structural analyses reconciled these data and showed that different structural evolutions result from the cooperative processes in which the thermal behaviors of cations-H2O complexes are coupled with the framework symmetry constraints through direct bonding interactions. These results showed for the first time that the extent of coupling under specific T and P(H2O) conditions influences the flexibility of the structure and ultimately drives structural changes. The determined bulk thermodynamic behaviors in the NAT system were consistent with the observed structural behaviors. The computed apparent enthalpies of dehydration in these zeolites reflect strong dependence not only on guest cation chemistry but also on the local hydrogen bonding configurations.