HIGH-PRESSURE BEHAVIOUR AND CRYSTAL-FLUID INTERACTION OF THE ABC-6 ZEOLITES: THE CASE OF ERIONITE, OFFRETITE AND BELLBERGITE

Zeolites represent an important class of hydrated aluminosilicates, characterized by an open-framework structure formed by interconnection of channel and cage systems, hosting alkali/alkaline-earth cations and small molecules. The framework is composed of TO4 tetrahedral units. The International Zeolite Association (IZA) recognises the existence of more than 250 compounds with zeolitic structures, among those more than 50 found as natural species, highlighting the considerable crystallographic diversity and the scientific and industrial interest on zeolites. In the last decades, the scientific community showed a rising interest on the high-pressure behaviour (ranging from few kbar to GPa) of zeolites, both natural and synthetics, particularly concerning the crystal-fluid interactions mediated by pressure. Given the zeolites capacity to host and exchange H2O, other small molecules or monoatomic species (e.g., nobles gases), experiments involving hydrostatically compressed zeolites in potentially-penetrating mixtures of molecules can provide new insights about their potential role as carrier of geological fluids in the first kilometres of the upper Earth crust. On the other hand, these experiments may open new routes for tailoring novel functional materials through specific host-guest interaction, or enhancing the performance of specifics industrial catalytic processes, potentially leading to a relevant technological impact. In this PhD thesis, after a general review about the zeolites applications and their high-pressure techniques and experimental findings reported so far (Chapters 1, 2 and 3), the high-pressure behaviour and the crystal-fluid interaction of three zeolitic species, belonging to the ABC-6 group, are resumed. In detail, natural erionite (ERI topology,), offretite (OFF,) and bellbergite (EAB) were investigated, along with a synthetic counterpart of bellbergite with EAB topology. The protocol involved a first crystallo-chemical characterization of the samples, using a multi-methodological approach, followed by in-situ high-pressure experiments aimed to assess the intrinsic compressibility of these species and the pressure-mediated crystal-fluid interaction. In details, the high-pressure characterization involved: 1. Experiments using ‘non-penetrating’ P-transmitting fluids, such as daphne oil or silicone oil, aimed at determine the compressibility path and the elastic parameters of the materials without any interference of the P-transmitting fluid, setting a benchmark for the second-step. 2. Experiments using ‘potentially-penetrating’ P-transmitting fluids, such as alcohols-water mixtures or noble gases (in their liquid state), to describe the P-mediated crystal-fluid interaction. The results of the experiments, along with their protocols, are detailed in the Chapter 4, 5 and 6, for erionite, offretite and bellbergite (and its synthetic analogues), respectively. Chapter 7 provides an overall discussion with a comparative analysis of their P-response, unveiling similarities and differences of the high-pressure behaviour of the three frameworks and the parameters governing the crystal-fluid interaction.