P-induced crystal-fluid interaction in 6-membered ring zeolites: the case of ERI, OFF and EAB topologies

Pressure (P)-induced intrusion of molecules (or solvated ions) into the structural nano-cavities of microporous materials opened a new route to promote a mass transfer from fluids to structurally-incorporated molecules. A full understanding of this phenomenon in natural or synthetic zeolites might expand the number of their utilizations, e.g. tailoring of new materials, as catalysts in industrial processes [1,2]. On the other hand, from the geological point of view, the study of this phenomenon is unveiling the role played by zeolites as fluid carriers in the upper Earth crust, e.g. during the early subduction of oceanic sediments or altered basalts. We have investigated the high-P behaviour, promoting P-mediated crystal-fluid interaction, of three different zeolites with structural homologies: erionite (ERI framework type, 6-membered ring sequence: AABAAC), offretite (OFF, with AAB seq.), bellbergite (EAB, with AABCCB seq.) and its synthetic counterpart. These studies allowed to 1) a better understanding of the potential role played by erionite as fluid carrier during the early subduction, being this mineral a constituent of ocean floors basaltic alteration [3] and 2) compare the mechanisms adopted by structurally similar 6-mRs frameworks to accommodate the bulk compression and the crystal-fluid interactions. Synchrotron X-ray diffraction experiments have been performed on natural single crystals of erionite, bellbergite and offretite. Additionally, experiments have been performed on powder samples with EAB framework (synthetized according to the Aiello-Barrer protocol [4] and treated in order to obtain Na- and K- forms). Both non penetrating (silicone oil and daphne oil 7575) and potentially penetrating P-transmitting fluids (methanol:ethanol:water 16:3:1 mixture, ethanol:water 1:1 mixture, methanol, H2O, liquid Ne) have been used. Among the natural samples, erionite resulted to be the one with the highest magnitude of adsorption. The new adsorbed molecules act as “pillars” within the framework nanocavities, decreasing the compressibility of the structure. Moreover, the magnitude of the intrusion resulted to be strictly related to the H2O content of the hydrous P-transmitting fluids. Ne atoms were able to penetrate into the 12mRs channel of the offretite framework in response to the applied pressure, with weak Van der Waals interactions with the extra-framework population. Methanol resulted to behave as a non-penetrating fluid for natural bellbergite, while it acts as a penetrating fluid in the synthetic counterparts. This highlighted the role of “secondary factors” on the occurrence of crystal-fluid interaction, e.g. the extra-framework content of the sample and the size of crystallites (single crystal of natural bellbergite vs. synthetic EAB powder). References [1] G.D. Gatta, P. Lotti, G. Tabacchi, (2018), The effect of pressure on open‑framework silicates: elastic behaviour and crystal–fluid interaction, Phys. Chem. Miner., 45, 115–138 [2] D. Comboni, F. Pagliaro, P. Lotti, G.D. Gatta, M. Merlini, S. Milani, M. Migliori, G. Giordano, E. Catizzone, I.E. Collings, M. Hanfland, (2020), The elastic behavior of zeolitic frameworks: The case of MFI type zeolite under high-pressure methanol intrusion, Catal. Today, 345, 88–96. [3] F. Vitali, G. Blanc, P. Larqué, (1995), Zeolite distribution in volcaniclastic deep-sea sediments from the Tonga Trench Margin (SW Pacific), Clays and Clay Miner., 43, 92–104. [4] R.Aiello, R.M. Barrer, (1970), Hydrothermal Chemistry of Silicates.