Crystal-fluid interactions at high pressure in natural erionite-K

Investigating the P-induced intrusion of molecules and ions, from the P-transmitting fluids, into the structural nano-cavities of microporous compounds, e.g. zeolites, has experienced a boosted interest in the last two decades. Zeolites have a consolidated history of technological and industrial applications, but the understanding of these phenomena may further expand their utilizations, opening new routes for tailoring functional materials. In this study, we have focused to a natural zeolite: erionite. Its behaviour has been investigated when compressed in non-penetrating and potentially penetrating fluids, i.e. those fluids made by molecules having a kinetic diameter which may allow their P-mediated adsorption into the zeolite structural cavities. Erionites represent a series belonging to the zeolites group, with a wide chemical variability expressed as solid solutions among three end-members: erionite-Ca, erionite-Na and erionite-K. Our sample, classified as erionite-K, has the following average chemical formula: K2.31Na0.02Ca2.15Mg0.69Ba0.04Sr0.02Al9.00Si27.19O72·18.66(H2O). The erionite-framework is characterized by the presence of large cages (23-hedron, called “erionite-cage”), superposed along the c-axis, hosting most of the extra-framework population. Experiments by single-crystal X-ray diffraction under in-situ high-pressure conditions, using an ETH-type diamond anvil cell (DAC) and ruby as P-calibrant, were conducted at the Xpress beamline of the Elettra Synchrotron (Trieste, Italy). Two P-ramps were performed using different P-transmitting media: the first one using the non-penetrating silicone oil, up to 2.60(5) GPa, and the second one with the potentially penetrating methanol:ethanol:H2O = 16:3:1 (hereafter mew) mixture, up to 4.97(5) GPa. A II-order Birch-Murnaghan equation of state was fitted to the P-V data obtained by the silicone oil ramp, yielding the refined isothermal bulk modulus of erionite: KV0 = 44(1) GPa (βV0 = KV0-1 = 0.0227(5), where βV0 is the bulk volume compressibility). P-V data from the mew ramp displays a markedly lower compressibility, which is unambiguously related to the P-induced intrusion of H2O (and possibly alcohols) molecules. The adsorption, which appear irreversible in decompression, seems to occur in three different steps, approximately around 0.2, 1.2 and 2 GPa. The magnitude of the intrusion process appears to be comparable with that previously observed for synthetic zeolites (as e.g. SiO2-ferrierite or AlPO4-5 zeolites), and this is of particular relevance if we consider that it was observed in this study not on a synthetic compound characterized by a controlled crystal chemistry, but on a sample of natural erionite, with structural voids largely filled by the extraframework population (i.e., cations and H2O molecules). Preliminary experiments performed with different classes of potentially penetrating fluids (e.g., 2:2:1 and 1:1:2 mew mixtures), by synchrotron single-crystal X-ray diffraction, seems to corroborate the aforementioned results in terms of compressibility, although an apparently continuous adsorption without discontinuities in the P-V trend was detected. Further experiments will allow a full understanding and constraints of the P-induced adsorption phenomena in erionite.