Intrusion-extrusion of electrolyte aqueous solutions in pure silica chabazite by in situ high pressure synchrotron X-ray powder diffraction

We have performed a structural investigation of the high pressure intrusion/extrusion of a series of electrolyte aqueous solutions (NaCl, NaBr, and CaCl2) with different concentrations (2 and 3 M) in a pure-silica chabazite. In situ synchrotron X-ray powder diffraction experiments were performed in the pressure range of 0.12−2.6 GPa and upon pressure release, to unravel the interactions among intruded species and host material. The energetic performances of the systems were determined by porosimetric studies. Results show that the chemical nature of saline solution influences the intrusion−extrusion pressure, whereas the structural evolutions undergone by the systems upon pressure-induced intrusion are essentially independent of the nature of the penetrating media. Moreover, the initial electrolyte concentration influences 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 the intruded species is different from that of the initial solution and depends on the applied pressure, thus reinforcing 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 zeolite 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 was found to be mainly determined by the interactions of the anion with silanol defects of the chabazite framework, rather than by the coordination bonds of the cation with the framework oxygen atoms.