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

Abstract

The ambient and high pressure structures of a partially cesium-exchanged K-gallosilicate with a zeolite LTL framework topology (Cs 0.65K 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) H 2O at ambient conditions to 26.7(1) H 2O 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 0.65K 0.35–GaSi–LTL.