Crystal-fluid interactions in erionite-group zeolites under compression

In the last two decades many studies showed that hydrostatic compression is able to enhance or induce the intrusion of molecules (or solvated ions) into the structural nano-cavities of microporous materials, pointing out that this is a viable way 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]. In addition, this phenomenon bears an intrinsic relevance also in Earth Sciences, as zeolites may act as fluid carriers in the upper Earth crust, e.g. during the early subduction of oceanic sediments or altered basalts. In this scenario, we focused on three natural zeolites, structurally characterized by six-membered rings of tetrahedra and belonging to the large group of ABC-6 open-framework materials: erionite, offretite and bellbergite. Erionite is a quite common zeolite in nature, where it forms in basaltic vugs, crystallizing from hydrothermal fluids. It shows an ERI-type framework, made by the repetition of AABAAC sequences of 6-membered rings of tetrahedra layers. Offretite (OFF framework type) shows an AAB sequence and is commonly intergrown with erionite, due the easy occurrence of stacking faults at B and C positions of the 6-membered rings layers. Bellbergite is a rather uncommon zeolite in nature, more famous for its synthetic counterparts [3], and shows an EAB framework with ABBACC sequence. The crystal-fluid interactions during compression were investigated by means of in situ single-crystal X-ray diffraction, which allows to focus the study on the effects that interaction has on the crystal structure of zeolites. The experiments were performed at the ID15B beamline of the European Synchrotron Radiation Facility, using diamond anvil cells to apply hydrostatic pressures on the investigated samples and using different pressure-transmitting fluids: namely, the non-penetrating silicone oil and daphne oil 7575 and potentially penetrating methanol:ethanol:water 16:3:1 mixture, ethanol:water 1:1 mixture, methanol, distilled H2O and liquid Ne. As non-penetrating are intended those fluids which molecules have a kinetic diameter larger than the free diameter of the open-framework of the zeolite and, therefore, cannot be pressure-intruded into the crystal structure. The compressional experiments in non-penetrating fluids provide, therefore, a benchmark to which compare the behavior of the same microporous compound in a potentially penetrating fluid. Among the investigated natural samples, erionite resulted to be the one with the highest magnitude of adsorption, as shown by Figure 1. The new adsorbed molecules act as “pillars” within the framework nanocavities, decreasing the compressibility of the structure, as it is clear comparing the unit-cell vs. pressure evolution of erionite compressed in silicone oil and methanol:ethanol:water (16:3:1) mixture, respectively (Figure 1). The obtained results also allow to conclude that the magnitude of the intrusion for a given zeolite is strictly related to the H2O content of the hydrous P-transmitting fluids, where the largest is the water fraction, the highest the magnitude of the intrusion and (sometimes) the lower the pressure at which it occurs. A comparison of the crystal-fluid interactions under pressure in natural erionite and in other synthetic zeolites (e.g. SiO2-ferrierite [4]), points out that the observed magnitude of intrusion in this study is surprisingly high for a natural zeolite, characterized by channels and cages already filled by extraframework cations and molecules. These results suggest that natural zeolites, despite being intrinsically less inclined to show pressure-induced crystal-fluid interaction with respect to synthetic ones, should not be a priori excluded as targets for the tailoring of new materials by exploiting hydrostatic compression, especially when a modest temperature is also applied. Moreover, the obtained results also suggest that the role of zeolites as fluid carriers or fluid moderators in the geological processes occurring in the upper Earth crust deserves a more comprehensive characterization for a full understanding. Acknowledgements: ESRF is acknowledged for the provision of beamtime. The Italian Ministry of Education (MUR) is acknowledged for the support through the projects “PRIN2017—Mineral reactivity, a key to understand large-scale processes” (2017L83S77) and “Dipartimenti di Eccellenza 2023-2027”. References (up to five): [1] G.D. Gatta, P. Lotti, G. Tabacchi, Physics and Chemistry of Minerals 45, 2018, 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, Catalysis Today 345, 2020, 88–96. [3] R. Aiello, R.M. Barrer, Journal of the Chemical Society A, 1970, 1470-1475. [4] P. Lotti, R. Arletti, G.D. Gatta, S. Quartieri, G. Vezzalini, M. Merlini, V. Dmitriev, M. Hanfland, Microporous and Mesoporous Materials 218, 2015, 42-54.