Hill, Gemma Louise
(2010).
High pressure diffraction studies of gallosilicate natrolite: applications to pressure-induced cation trapping.
University of Birmingham.
Ph.D.
Abstract
The natrolite family of zeolites undergoes volume expansion under pressure. Natrolite itself is a three dimensionally porous zeolite with 8-ring channels. It shows an unusual unit cell volume expansion when under hydrostatic pressure where it undergoes two distinct, reversible phase transitions to paranatrolite and then super-hydrated natrolite. The behaviour of gallosilicate natrolite, a synthetic analogue of the aluminosilicate mineral, was investigated in this thesis. Two forms exist, orthorhombic (as per the mineral) and tetragonal. These two forms have been synthesised and structurally characterised by powder neutron diffraction and synchrotron X-ray diffraction techniques at ambient pressure to elucidate details of the ordering. In addition, high pressure neutron and synchrotron X-ray diffraction data have been collected on both the tetragonal and the orthorhombic forms. These data confirm pressure-induced hydration and pore swelling in both the gallosilicate materials, as seen in the aluminosilicate form but at much lower pressures. The neutron data has also allowed a full study of the changes in hydrogen-bonding in the normal and superhydrated states. The natrolite family will not normally undergo ambient ion-exchange due to the small pores and the strongly bound framework cations. Exploitation of the pressure-induced state offered a high pressure ion-exchange route, with trapping of large exchanged cations upon pressure release. Ion exchange investigations using the aluminosilicate and gallosilicate natrolites have provided the first evidence of high pressure cation trapping. Exposure to a saturated CsCl solution at a pressure of 1 GPa and a temperature of 100°C led to cesium trapping within the framework upon pressure release. Back-exchange treatment in a sodium chloride reflux did not leach the trapped cesium. For the aluminosilicate natrolite up to 33% Na/Cs exchange was trapped within the framework via this high pressure exchange method.
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