Intrusion-extrusion experiments of MgCl2 aqueous solution in pure silica ferrierite: Evidence of the nature of intruded liquid by in situ high pressure synchrotron X-ray powder diffraction

Abstract

Experimental intrusion-extrusion isotherms of MgCl2•21H2O solution were recorded at room temperature on pure silica FER-type zeolite (Si-FER). The intrusion occurs at 195 MPa and the phenomenon is completely reversible with a slight hysteresis. The “Si-FER - MgCl2 aqueous solution” system behaves like a spring. The material was deeply characterized before and after intrusion-extrusion experiments and no significant changes were observed. The unit cell parameters were refined – on the basis of the in situ synchrotron X-ray powder diffraction data - up to 1.47 GPa and then at Pamb upon pressure release. The Rietveld refinement of the data collected at 0.28 GPa (280 MPa), a pressure close to the intrusion value, shows that both ions and water molecules present in the MgCl2 aqueous solution were intruded in the porosities. However, the solvation degree of the intruded ions differs from the initial solution, revealing a partial desolvation of both magnesium and chloride ions. As a whole, the nature and amount of the intruded species correspond to a MgCl2•10H2O composition. Moreover, at a higher pressure (0.68 GPa), a phase transition from the orthorhombic Pmnn to the monoclinic P21/n s.g. is observed in Si-FER. At 1.47 GPa, the zeolite maintains this monoclinic symmetry, while another phase transition, to the monoclinic P21 s g., is argued from the analysis of the pattern of the sample compressed to 2.6 GPa and then collected upon pressure release to ambient conditions.