Two-Step Pressure-Induced Superhydration in Small Pore Natrolite with Divalent Extra-Framework Cations

In situ high pressure X-ray powder diffraction studies of natrolite (NAT) containing the divalent extra-framework cations (EFC) Sr2+, Ca2+, Pb2+, and Cd2+ reveal that they can be superhydrated in the presence of water. In the case of Ca-NAT, Sr-NAT, and Pb-NAT pressure-induced hydration (PIH) inserts 40 H2O/unit cell into the zeolite compared to 32 in superhydrated natrolites containing monovalent EFC. Cd-NAT is superhydrated in one step to a zeolite containing 32 H2O/unit cell. PIH of Ca-NAT and Sr-NAT occurs in two steps. During PIH of Pb-NAT three distinct steps have been observed. The excess H2O in natrolites with divalent EFC are accommodated on sites no longer required for charge compensation. Two distinct families with ordered and disordered EFC–water topologies have been found. Our work established the importance of both size and charge of the EFC in PIH.