The effect of pressure on open-framework silicates: elastic behaviour and crystal-fluid interaction

The physical behavior of microporous materials (i.e., materials with structural voids  2 nm) compressed in a fluid is strongly affected by the potential crystal-fluid interaction, with a penetration of new molecular species through the structural cavities in response to the applied pressure. Recent experimental findings and computational modeling show that, when no crystal-fluid interaction occurs, the effects of pressure are mainly accommodated by tilting of the (quasi-rigid) tetrahedra, around the bridging oxygen atoms that act as hinges. Tilting of tetrahedra is the dominant mechanism at low-mid P-regime, followed by distortion and compression of polyhedra which become dominant at the mid-high P-regime. The mechanisms of deformation, accommodating the bulk compression, are governed by the topology of the tetrahedral framework. One of the most common deformation mechanisms in zeolitic frameworks is the increase of channels ellipticity. However, the compressibility of the cavities (in the form of channels or cages) is governed by the ionic and molecular extraframework content, with different unit-cell volume compressibility in isotypic structures. Only a few zeolites experience a P-induced intrusion of new monoatomic species or molecules from the Ptransmitting fluids. Natural zeolites, in particular, have well-stuffed channels at room conditions, which tend to hinder the penetration of new species through the structural cavities. A comparative analysis of experimental findings allow us to provide an overview of the intrusion phenomena, which are diverse for monoatomic species (e.g., He, Ar, Kr), small (e.g., H2O, CO2) or complex molecules, along with the recently observed P-induced polymerization phenomena (e.g., C2H2, C2H4, C2H6O, C2H6O2, BNH6, electrolytic MgCl2·21H2O solution), with potential technological and geological implications. Several variables control the sorption phenomena at high pressure: the “free diameters” of the framework cavities, nature and bonding configuration of the extra-framework population, the kinetic diameter of the potentially penetrating molecules, 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 under investigations, and the temperature at which the experiment is conducted.