Pressure-induced hydration and cation migration in a Cs~+ exchanged gallosilicate zeolite LTL: Synchrotron X-ray powder diffraction study at ambient and high pressures

摘要

The ambient and high pressure structures of a partially cesium-exchanged K-gallosilicate with a zeolite LTL framework topology (Cs_(0.65)K_(0.35)-GaSi-LTL) was studied using synchrotron X-ray powder diffraction. In contrast to the cation distribution present in the aluminosilicate analogues, the larger cesium cations replace part of the potassium cations in the narrow 8-ring channel as well as in the main 12-ring channel of the gallosilicate LTL. Under hydrostatic pressures mediated by a pore-penetrating alcohol and water mixture, anomalous compression behavior is observed with a slight increase in the unit cell volume upon initial compression, i.e., 0.15% expansion at 0.44 GPa. This is the result of the continuous increase of the a-axis length up to 2 GPa and is found to be related to the gradual pressure-induced hydration (PIH) occurring inside the main 12-ring channel, where the water content increases from 15.9(1) H2O at ambient conditions to 26.7(1) H2O per formula unit at 2.83 GPa. During PIH, part of the cesium cations along the main 12-ring channel migrate into the narrow 8-ring channel. Compared to the structural changes observed in K-GaSi-LTL, the degree of pressure-induced hydration and the accompanying cation migration is found to be diminished in Cs_(0.65)K_(0.35)-GaSi-LTL.