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 1,2 . In the last decades, the scientific community showed a rising interest on the behavior of microporous and mesoporous compounds at high-pressure conditions, and on the crystal-fluid interaction phenomena occurring at extreme conditions3,4 . 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 impact5 . From a geological point of view, zeolites could act as a carrier for H2O and others small molecules or for monoatomic species (e.g., CO2, CH4, H2S, He, Ar, Kr, Xe,…), and any change of the P-induced extraframework population (at ambient to low/high T) can have relevant geochemical and geophysical implications6 . In this experimental thesis, 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 in a diamond anvil-cell, exploring the different crystal-fluid interaction in materials with large or small channels, and with or without extra-framework population. The experiments were conducted at the Earth Science Department of Milan (ESD-MI) and at the beamlines ID15b (at ESRF, Grenoble) and P2.02 (at DESY, Hamburg). The results obtained in the framework of this experimental thesis showed that several variables govern the sorption phenomena at high pressure, among those: the “free diameters” of the framework cavities, the chemical nature and the configuration of the extra-framework population, the partial pressure of the penetrating molecule in the fluid (if mixed with other non-penetrating molecules), the rate of P-increase, the surface/volume ratio of the crystallites and the temperature at which the experiment is conducted. P-induced phenomena at the atomic scale were described on the basis of high-quality structure refinements. Geological and technological potential implications were discussed, e.g. about the H2O load carried by laumontite, leonhardite and phillipsite in relevant geologic environments (e.g., first km of the subducted oceanic crust, oil reservoir, …), along with the potential of synthetic and natural zeolites as energy storage materials.