Intrusion–Extrusion of Electrolyte Aqueous Solutions in Pure Silica Chabazite by in Situ High Pressure Synchrotron X-ray Powder Diffraction

Abstract

Structural investigation of the high pressure intrusion/extrusion of different electrolyte aqueous solutions (NaCl, NaBr and CaCl2) with different concentrations (2M and 3M) in a pure-silica chabazite was carried out. In situ synchrotron X-ray powder diffraction experiments were performed in the pressure range of 0.12 -2.6 GPa and upon pressure release, in order to unravel the interactions among intruded species and host material. The energetic performance of the systems were determined by porosimetric studies. Results show that cation in the salt seems to influence the intrusion-extrusion pressures, whereas the structural evolutions, undergone by the systems upon pressure-induced intrusion, are essentially independent on the nature of the penetrating media.Moreover, the initial electrolyte concentration seems to influence only the value of the intrusion pressure, but neither the amount nor the interaction mode of the intruded species. Both water and salt molecules enter the pores and the penetration of comparable extra-framework volumes occurs at similar pressure values. However, the composition of intruded species is different from that of initial solution and depends on applied pressure that reinforces the hypothesis on ion desolvation under penetration into the pores. After pressure release, pure-silica chabazite intruded by NaCl and NaBr aqueous solutions does not recover the initial cell volume and partially retains the intruded extra-framework species. On the contrary, the zeosil intruded by CaCl2 recovers the original cell parameters. These differences have been structurally interpreted on the basis of the electrolyte/zeolite interactions. Interestingly, the extrusion behavior results to be mainly determined by the interactions of the anion with silanol defects of chabazite framework, rather than by the coordination bonds of the cation with the framework oxygen atoms.

Differential penetration of ethanol and water in Si-chabazite: High pressure dehydration of azeotrope solution

“…These experimental evidences can be explained by the following considerations: i) X-ray diffraction provides and average view of the crystalline system that, however, could be partially disordered. In particular, a disordered orientation of the EtOH hydroxyls could lead to both guest-guest and hostguest interactions; ii) the experimental evidence of H-bond interactions between EtOH and chabazite framework can be due to the presence of silanol defects, evidenced in this sample by previous NMR studies [21].…”

“…The evolution of the cell parameters of Si-CHA compressed in EtOH96 is reported in Figure 4 and Table S1. Once Si-CHA is contacted with the PTM, even at P amb , a contraction along a axis and an expansion along the c direction with respect to the original value obtained in capillary, is observed [21]. During compression, a parameter only slightly increases up to 1.84 GPa and then slightly decreases remaining, however, close to the original value, while c parameter decreases in the whole P range.…”

“…GPa, due to the low quality of the patterns collected at higher pressure. Since no phase transition occurred, the s.g. was adopted for all the structure refinements, using as starting model that proposed by Confalonieri and coauthors [21]. The profile fittings were performed between 2.5 and Figure S2, S3, S4 and S5.…”

“…In fact, as observed by [63] The inspection of the Fourier difference map confirms the intrusion of some molecules of the PTM in the zeolite porosities even at P amb . The comparison of Si-CHA reference structure at P amb [21] Figure 5, where the three EtOH molecules interact each other through the hydroxyl groups (dashed line in Figure 5). In addition, also a new bond is established between W1…”

“…shows the structure of reference Si-CHA at ambient conditions[21], and of Si-CHA in DAC at P amb , 0.20 GPa, 1.84 GPa and 0.37 GPa (rev). From the figure, it is possible to appreciate the P-induced modifications of the average structure in term of both amount and arrangement of the extra-framework species penetrated in the CHA cage.…”