HIGH-PRESSURE BEHAVIOR OF MICROPOROUS MATERIALS: CRYSTAL-FLUID INTERACTIONS AND DEFORMATION MECHANISMS AT THE ATOMIC SCALE

Zeolite are crystalline, hydrated aluminosilicates characterized by a tetrahedral framework of TO4 units connected in such a way that sub-nanometric channels and cages occur. These structural cavities host the so-called extra-framework population, which mainly consists of alkali and alkaline-earth cations and small molecules, such as H2O. In the last decades, the scientific community showed a rising interest on the behavior of microporous and mesoporous compounds (e.g., zeolites) at high-pressure conditions, and in particular on the crystal-fluid interaction phenomena occurring at extreme conditions. As zeolites could act as an ideal carrier of H2O and others small molecules or monoatomic species (e.g., CO2, CH4, H2S, He, Ar, Kr, Xe,…), experiments on zeolites compressed (and ambient to low/high T) in aqueous mixtures have important implications in the Earth Sciences. Furthermore, high-pressure experiments on synthetic zeolites may pave the way for new routes of tailoring new functional materials (made by hybrid host-guest architecture), bearing a potentially relevant technological impact. In this experimental thesis, after an overall introduction and a section on the high-pressure experimental techniques (Chapter 1 and 2 , respectively), the high-pressure behavior and the crystal-fluid interaction at the atomic scale of a selected series of natural and synthetic zeolites (i.e., AlPO4-5, leonhardite, laumontite, phillipsite) and a zeolites-like mineral (i.e., armstrongite) have been investigated by means of in-situ single-crystal X-ray diffraction, using “penetrating” and “non-penetrating” pressure-transmitting fluids. Into details: 1. AlPO4-5 (Chapter 3): the high-pressure behavior of AlPO4-5 has been studied by single crystal XRD using synchrotron radiation and a diamond anvil cell (DAC), with crystals compressed in silicone oil and methanol:ethanol:water =16:3:1 (m.e.w.) mixture. The high-pressure evolution of the crystalline structure and the deformation mechanism at atomic scale have been described on the basis of high-quality structure refinements, revealing adsorption phenomena of H2O (and likely methanol) already at 2 Kbar. Moreover, evidence of an incommensurately modulated structure of AlPO4-5 have been found. 2. Leonhardite and laumontite (Chapter 4): the H2O adsorption kinetics, at ambient pressure and temperature, of leonhardite to give laumontite has been investigated using single crystal XRD techniques. In-situ high-pressure XRD experiments, using synchrotron radiation and a DAC, have been performed in order to obtain the bulk moduli of the two minerals (previously unknown). A detailed description of the atomic deformation mechanisms has been addressed. 3. Phillipsite (Chapter 5): the pressure-induced deformation mechanisms, at the atomic scale, have been studied via single crystals XRD-experiments, using synchrotron radiation and a DAC. Despite no pressure-induced adsorption was observed, the experimental findings suggest a change in the deformation mechanisms induced by a re-arrangement of the extra-framework population. 4. Armstrongite (Chapter 6): the high-pressure evolution of this zeolite-like mineral has been studied in the m.e.w. as nominally penetrating fluid. A first-order phase transition has been detected between 4 and 5 GPa. In the Chapter 7 a detailed discussion of the aforementioned experimental findings has been addressed, along with their technological and geological implications. The results of the present studies have been published in peer-reviewed journals.