New insight into cation relocations within the pores of zeolite rho: In situ synchrotron X-ray and neutron powder diffraction studies of Pb- and Cd-exchanged rho

Upon heating, certain cations exchanged into zeolite RHO undergo large shifts in their positions within the pores. In several of these materials, negative thermal expansion is observed in conjunction with these cation relocations. Rather than being the purely temperature-driven effects presumed in previous reports, a combination of in situ time-resolved synchrotron X-ray and neutron powder diffraction studies indicates that the cation relocations and framework distortions observed in Pb- and Cd-exchanged zeolite rho are mediated by the presence of water in specific sites in the pores of RHO. Rietveld refinements using these data reveal that the initial unit cell contraction (50 °C ≤ T ≤ 100 °C) is due to the loss of unbound water in the α-cages of rho. Water molecules in the double eight-ring (D8R) building units persist after this step, bound to the extraframework cations. The framework then contracts as water molecules are gradually removed (200 °C ≤ T ≤ 400 °C). During this period, the extraframework cations migrate from the single eight-ring (S8R) site to the double eight-ring (D8R) sites in a concerted manner with the dehydration at the D8R. Upon complete removal of bound waters (400 °C ≤ T ≤ 500 °C), lead and cadmium cations experience different rearrangements; Cd²⁺ ions relocate from the D8R and S8R sites to the single six-ring (S6R) site, while all Pb²⁺ ions migrate from the S8R site to the D8R site. Neither transition is reversible upon cooling to room temperature in vacuo although both are reversible in the presence of water vapor. The role of water in these samples appears to determine the coordination environment of the extraframework cations uniquely since other sorbates, such as Kr, methanol, and CO, do not cause significant changes in either extraframework cation or framework atomic positions.