Two-Step Pressure-Induced Superhydration in Small Pore Natrolite with Divalent Extra-Framework Cations

In situ high pressure X-ray powder diffraction studies of natrolite (NAT) containing the divalent extra-framework cations (EFC) Sr²⁺, Ca²⁺, Pb²⁺, and Cd²⁺ reveal that they can be superhydrated in the presence of water. In the case of Ca-NAT, Sr-NAT, and Pb-NAT pressure-induced hydration (PIH) inserts 40 H₂O/unit cell into the zeolite compared to 32 in superhydrated natrolites containing monovalent EFC. Cd-NAT is superhydrated in one step to a zeolite containing 32 H₂O/unit cell. PIH of Ca-NAT and Sr-NAT occurs in two steps. During PIH of Pb-NAT three distinct steps have been observed. The excess H₂O in natrolites with divalent EFC are accommodated on sites no longer required for charge compensation. Two distinct families with ordered and disordered EFC–water topologies have been found. Our work established the importance of both size and charge of the EFC in PIH

Pressure-Induced Metathesis Reaction To Sequester Cs

We report here a pressure-driven metathesis reaction where Ag-exchanged natrolite (Ag₁₆Al₁₆Si₂₄O₈₀·16H₂O, Ag-NAT) is pressurized in an aqueous CsI solution, resulting in the exchange of Ag⁺ by Cs⁺ in the natrolite framework forming Cs₁₆Al₁₆Si₂₄O₈₀·16H₂O (Cs-NAT-I) and, above 0.5 GPa, its high-pressure polymorph (Cs-NAT-II). During the initial cation exchange, the precipitation of AgI occurs. Additional pressure and heat at 2 GPa and 160 °C transforms Cs-NAT-II to a pollucite-related, highly dense, and water-free triclinic phase with nominal composition CsAlSi₂O₆. At ambient temperature after pressure release, the Cs remains sequestered in a now monoclinic pollucite phase at close to 40 wt % and a favorably low Cs leaching rate under back-exchange conditions. This process thus efficiently combines the pressure-driven separation of Cs and I at ambient temperature with the subsequent sequestration of Cs under moderate pressures and temperatures in its preferred waste form suitable for long-term storage at ambient conditions. The zeolite pollucite CsAlSi₂O₆·H₂O has been identified as a potential host material for nuclear waste remediation of anthropogenic ¹³⁷Cs due to its chemical and thermal stability, low leaching rate, and the large amount of Cs it can contain. The new water-free pollucite phase we characterize during our process will not display radiolysis of water during longterm storage while maintaining the Cs content and low leaching rate

Spontaneous Ionic Polarization in Ammonia-Based Ionic Liquid

Ionic liquids and gels have attracted attention for a variety of energy storage applications, as well as for high-performance electrolytes for batteries and supercapacitors. Although the electronic structure of ionic electrolytes in these applications is of practical importance for device design and improved performance, the understanding of the electronic structure of ionic liquids and gels is still at an early stage. Here we report soft X-ray spectroscopic measurements of the surface electronic structure of a representative ammonia-based ionic gel (DEME-TFSI with PS-PMMA-PS copolymer). We observe that, near the outermost surface, the area of the anion peak (1s N^– core level in TFSI) is relatively larger than that of the cation peak (N⁺ in DEME). This spontaneous ionic polarization of the electrolyte surface, which is absent for the pure ionic liquid without copolymer, can be directly tuned by the copolymer content in the ionic gel, and further results in a modulation in work function. These results shed new light on the control of surface electronic properties of ionic electrolytes, as well as a difference between their implementation in ionic liquids and gels

Pressure-Dependent Structural and Chemical Changes in a Metal–Organic Framework with One-Dimensional Pore Structure

Pressure-dependent structural and chemical changes of the metal–organic framework (MOF) compound MIL-47­(V) have been investigated up to 3 GPa using different pore-penetrating liquids as pressure transmitting media (PTM). We find that at 0.3(1) GPa the terephthalic acid (TPA) template molecules located in the narrow channels of the as-synthesized MIL-47­(V) are selectively replaced by methanol molecules from a methanol–ethanol–water mixture and form a methanol inclusion complex. Further pressure increase leads to a gradual narrowing of the channels up to 1.9(1) GPa, where a second irreversible insertion of methanol molecules leads to more methanol molecules being inserted into the pores. After pressure release methanol molecules remain within the pores and can be removed only after heating to 400 °C. In contrast, when MIL-47­(V) is compressed in water, a reversible replacement of the TPA by H₂O molecules takes place near 1 GPa. The observed structural and chemical changes observed in MIL-47­(V) demonstrate unique high pressure chemistry depending on the size and type of molecules present in the liquid PTM. This allows postsynthetic nonthermal pressure-induced removal and insertion of organic molecules in MOFs forming novel and stable phases at ambient conditions